Napoleon Enteria Aliakbar Akbarzadeh Solar Energy Sciences and Engineering Applications Solar Energy Sciences and Engineering Applications This page intentionally left blank Solar Energy Sciences and Engineering Applications Napoleon Enteria Enteria Grün Energietechnik, Davao, Philippines Aliakbar Akbarzadeh RMIT University, Melbourne, Australia Cover illustration: Photovoltaic Installation in Demonstration Building,Yonsei University, Incheon, South Korea. Photographer:Napoleon Enteria CRC Press/Balkema is an imprint of the Taylor & Francis Group, an informa business © 2014 Taylor & Francis Group, London, UK Typeset by MPS Limited, Chennai, India Printed and Bound by CPI Group (UK) Ltd, Croydon, CR0 4YY All rights reserved. No part of this publication or the information contained herein may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, by photocopying, recording or otherwise, without written prior permission from the publisher. Although all care is taken to ensure integrity and the quality of this publication and the information herein, no responsibility is assumed by the publishers nor the author for any damage to the property or persons as a result of operation or use of this publication and/or the information contained herein. Library of Congress Cataloging-in-Publication Data Enteria, Napoleon. Solar energy sciences and engineering applications / Napoleon Enteria, Enteria Grün Energietechnik, Davao, Philippines, Aliakbar Akbarzadeh, RMIT University, Melbourne, Australia. pages cm Includes bibliographical references and index. ISBN 978-1-138-00013-1 (hardback) 1. Solar energy. I. Akbarzadeh,Aliakbar. II. Title. TJ810.E58 2013 621.47—dc23 2013041799 Published by: CRC Press/Balkema P.O. Box 11320, 2301 EH Leiden,The Netherlands e-mail: Pub.NL@taylorandfrancis.com www.crcpress.com – www.taylorandfrancis.com ISBN: 978-1-138-00013-1 (Hbk) ISBN: 978-0-203-76205-9 (eBook PDF) Table of contents Preface xv About the editors xvii 1 Physics of solar energy and its applications 1 1.1 Introduction 1 1.2 Solar energy and energy demand 1 1.3 Solar energy utilizations 3 1.4 Perspective 5 2 Exergy analysis of solar radiation processes 7 2.1 Introduction 7 2.2 Exergy 8 2.2.1 Definition of exergy 8 2.2.2 Exergy annihilation law 10 2.2.3 Exergy of substance 12 2.2.4 Exergy of photon gas 17 2.2.5 Exergy of radiation emission 19 2.2.6 Exergy of radiation flux 25 2.3 Thermodynamic analysis 31 2.3.1 Significance of thermodynamic analysis 31 2.3.2 Energy balance equations 32 2.3.3 Exergy balance equations 36 2.3.4 Process efficiency 41 2.4 Solar radiation processes 45 2.4.1 Conversion of solar radiation into heat 45 2.4.2 Solar cylindrical-parabolic cooker 62 2.4.3 Solar chimney power plant 71 2.4.4 Photosynthesis 84 2.4.5 Photovoltaic 91 3 Exergy analysis of solar energy systems 97 3.1 Introduction 97 3.2 Energy and exergy aspects and analyses 98 vi Table of contents 3.3 Case studies 100 3.3.1 Case study 1: Exergy analysis of an integrated solar, ORC system for power production 100 3.3.2 Case study 2: Exergy analysis of solar photovoltaic/thermal (PV/T) system for power and heat production 105 3.3.3 Case study 3: Exergy assessment of an integrated solar PV/T and triple effect absorption cooling system for hydrogen and cooling production 111 3.4 Concluding remarks 116 4 Solar energy collection and storage 119 4.1 Solar thermal energy collectors 119 4.1.1 Overview 119 4.1.2 Flat plate solar energy collectors 120 4.1.3 Evacuated tube collectors 121 4.1.4 Collector components 122 4.2 Integral collector storage systems 124 4.2.1 Integral passive solar water heaters 124 4.2.2 Salt gradient solar ponds 124 4.3 Concentrators 126 4.3.1 Introduction 126 4.3.2 Concentration systems 126 4.4 Solar water heating 128 4.4.1 Overview 128 4.4.2 Applicability of particular collector types to specific outlet temperatures and diffuse fractions 129 4.4.3 Freeze protection methods 131 4.4.4 Sensible and latent heat storage 133 4.4.5 Analytical representation of thermosyphon solar energy water heater 134 4.4.6 Solar water heater design 137 4.5 Solar energy collection and storage for drying crops 140 4.6 Solar energy collector and storage for thermal power generation 142 4.7 Overall system optimization 142 5 Basics of the photovoltaic thermal module 149 5.1 Introduction 149 5.2 PV/T devices 151 5.2.1 Liquid PV/T collector 153 5.2.2 Air PV/T collector 154 5.2.3 Ventilated PV with heat recovery 157 5.2.4 PV/T concentrator 159 5.3 PV/T module concepts 160 5.3.1 Different types of PV/T modules 161 5.4 Techniques to inprove PV/T performance 162 5.5 Conclusion 165 Table of contents vii 6 Thermal modelling of parabolic trough collectors 171 6.1 Introduction 171 6.2 The energy model 176 6.2.1 Convection heat transfer between the HTF and the receiver pipe 178 6.2.2 Conduction heat transfer through the receiver pipe wall 179 6.2.3 Heat transfer from the receiver pipe to the glass envelope 180 6.2.4 Conduction heat transfer through the glass envelope 182 6.2.5 Heat transfer from the glass envelope to the atmosphere 182 6.2.6 Solar irradiation absorption 184 6.3 Code testing 187 6.4 Conclusions 191 7 Salinity gradient solar ponds 195 7.1 Introduction 195 7.2 Solar pond – design philosophy 197 7.2.1 Sustainable use of resources 197 7.2.2 Best site characteristics 198 7.2.3 Performance and sizing 198 7.2.4 Liner, salt and water 199 7.2.5 Transient performance prediction 201 7.3 Solar pond – construction and operation 202 7.3.1 Set-up and maintenance 202 7.3.2 Turbidity control 204 7.3.3 Heat extraction 205 7.3.4 Performance monitoring 206 7.3.5 EEE (Energy, Environmental and Economic) benefit evaluation 206 7.4 Solar ponds – worldwide 209 7.4.1 Solar ponds – Israel 209 7.4.2 Solar ponds – Australia 209 7.4.3 Solar ponds – USA 210 7.4.4 Solar ponds – Tibet, China 212 7.4.5 Solar ponds – India 213 7.5 Solar ponds – applications 214 7.5.1 Heating 214 7.5.2 Aquaculture 214 7.5.3 Desalination 215 7.5.4 Power production 215 7.6 Future directions 215 8 The solar thermal electrochemical production of energetic molecules: Step 219 8.1 Introduction 219 8.2 Solar thermal electrochemical production of energetic molecules: An overview 221 8.2.1 STEP theoretical background 221 viii Table of contents 8.2.2 STEP solar to chemical energy conversion efficiency 225 8.2.3 Identification of STEP consistent endothermic processes 230 8.3 Demonstrated step processes 233 8.3.1 STEP hydrogen 233 8.3.2 STEP carbon capture 233 8.3.3 STEP iron 239 8.3.4 STEP chlorine and magnesium production (chloride electrolysis) 244 8.4 Step constraints 246 8.4.1 STEP limiting equations 246 8.4.2 Predicted STEP efficiencies for solar splitting of CO2 247 8.4.3 Scaleability of STEP processes 249 8.5 Conclusions 250 9 Solar hydrogen production and CO2 recycling 257 9.1 Sustainable fuels with solar-based hyrogen production and carbon dioxide recycling 257 9.2 Solar-based hydrogen production with water splitting methods 259 9.2.1 Solar-to-hydrogen efficiency of water splitting processes 259 9.2.2 Matching the temperature requirements of solar-based hydrogen production methods 261 9.2.3 Thermolysis, thermal decomposition and thermochemical methods 262 9.2.4 Water electrolysis 267 9.2.5 Photoelectrolysis and photoelectrochemical water splitting 270 9.2.6 Photochemical, photocatalytic, photodissociation, photodecomposition, and photolysis 272 9.2.7 Hybrid and other hydrogen production methods 275 9.3 Solar-based CO2 recycling with hydrogen 277 9.4 Summary 281 10 Photoelectrochemical cells for hydrogen production from solar energy 293 10.1 Introduction 293 10.2 Photoelectrochemical cells systems overview 293 10.2.1 Solar water-splitting arrangements 293 10.2.2 Working principles of photoelectrochemical cells for water-splitting 297 10.2.3 Materials overview 299 10.2.4 Stability issues – photocorrosion 304 10.2.5 PEC reactors 306 10.3 Electrochemical impendance spectroscopy 311 10.3.1 Fundamentals 312 10.3.2 Electrical analogues 315 10.3.3 EIS analysis of PEC cells for water-splitting 318 Table of contents ix 10.4 Fundamentals in electrochemistry applied to photoelectrochemical cells 320 10.4.1 Semiconductor energy 321 10.4.2 Continuity and kinetic equations 328 10.5 Pec cells bottlenecks and future prospects 333 11 Photobiohydrogen production and high-performance photobioreactor 343 11.1 Introduction 343 11.2 General description of photobiohydrogen production 344 11.2.1 Photoautotrophic hydrogen production 344 11.2.2 Photoheterotrophic hydrogen production 347 11.2.3 Critical issues in photobiohydrogen production 348 11.3 Genetic and metabolic engineering 349 11.4 High-performance photobioreactor 352 11.4.1 Modification of photobioreactor configurations 352 11.4.2 Optimization of the operating parameters 357 11.4.3 Application of cell immobilization 361 11.5 Challenges and future directions 367 12 Decontamination of water by combined solar advanced oxidation processes and biotreatment 375 12.1 Introduction 375 12.2 Solar photo-fenton 376 12.2.1 Solar photo-Fenton hardware 378 12.3 Strategy for combining solar advanced oxidation processes and biotreatment 382 12.3.1 Average oxidation state 383 12.3.2 Activated sludge respirometry 384 12.3.3 Zahn-Wellens test 386 12.3.4 Factors to be considered in designing a combined system 388 12.4 Combining solar advanced oxidation processes and biotreatment: Case studies 389 12.4.1 Case study A: An unsuccessful AOP/biological process 389 12.4.2 Case study B: A successful AOP/biological process 389 13 Solar driven advanced oxidation processes for water decontamination and disinfection 395 13.1 Introduction 395 13.2 Solar radiation collection for AOPs applications 396 13.3 Solar homogenous photocatalysis 398 13.3.1 Degradation of organic pollutants by solar driven photo-Fenton processes 399 13.3.2 Microorganisms inactivation by solar driven photo-Fenton processes 400 x Table of contents 13.4 Solar heterogenous photocatalysis 403 13.4.1 Degradation of organic pollutants by solar driven heterogeneous photocatalysis 405 13.4.2 Microorganisms inactivation by solar driven heterogeneous photocatalysis 406 13.5 Challenges and perspectives 406 13.5.1 Photorreactor design 406 13.5.2 Suspended vs. immobilized photocatalyst 407 13.5.3 Visible light active photocatalyst materials 408 13.6 Conclusions 408 14 Solar energy conversion with thermal cycles 413 14.1 Introduction 413 14.2 Solar concentration concept in thermal systems 414 14.3 Concentrating solar technologies 417 14.3.1 Linear focus 420 14.3.2 Parabolic trough 422 14.3.3 Reflectors 424 14.3.4 Heat collection element 425 14.3.5 Structure 427 14.3.6 Parabolic trough performance 428 14.3.7 Linear fresnel 430 14.3.8 Heat collection element 432 14.3.9 Reflectors 433 14.3.10 Linear Fresnel performance 434 14.3.11 Cost comparison of linear focus technologies 438 14.3.12 Point focus 438 14.3.13 Central receiver systems 439 14.3.14 Collector field 440 14.3.15 Central receiver 442 14.3.16 Solar dish 445 14.3.17 Receiver 446 14.3.18 Power system 447 14.4 Heat transfer fluids and storage 448 14.4.1 Heat transfer fluids 449 14.4.2 Storage 452 14.5 From heat to power 459 14.5.1 Rankine cycle 461 14.5.2 Rankine cycle performance 466 14.5.3 Stirling cycle 466 14.5.4 Stirling configurations 468 14.5.5 Stirling working fluids 471 14.6 Economics and future perspectives 472 15 Solar hybrid air-conditioning design for buildings in hot and humid climates 485 15.1 Introduction 485 Table of contents xi 15.2 Design approaches of solar air-conditioning 486 15.2.1 The solar-electric approach 486 15.2.2 The solar-thermal approach 486 15.2.3 A hybrid approach to system design 490 15.2.4 A hybrid approach to energy sources and system design 491 15.3 Performance evaluation of various solar air-conditioning systems 492 15.3.1 Principal solar-thermal air-conditioning systems 493 15.3.2 SHAC with load sharing 494 15.3.3 SHAc with radiant cooling 495 15.3.4 SHAC coordinated with new indoor ventilation strategies 497 15.3.5 SHAC for premises with high latent load 499 15.4 Application potential of SHAC in various hot and humid cities in southeast asia 501 15.5 Conclusion and future development 502 16 Solar-desiccant air-conditioning systems 507 16.1 Introduction 507 16.1.1 Energy and environment 507 16.1.2 The building environment 508 16.2 The basic concept 510 16.2.1 Thermodynamic processes 510 16.2.2 Advantages of the open systems 512 16.2.3 Desiccant materials 513 16.3 Solid-based system 515 16.3.1 Basic concept 515 16.3.2 Typical systems 516 16.3.3 Modified systems 517 16.3.4 Hybrid systems 520 16.4 Liquid-based system 522 16.4.1 Basic concept 522 16.4.2 Typical systems 522 16.4.3 Modified systems 523 16.4.4 Hybrid systems 523 16.5 System application 525 16.5.1 Countries 525 16.5.2 Temperate regions 526 16.5.3 Sub-temperate regions 529 16.5.4 Hot and humid regions 531 16.6 Future and perspectives 536 17 Building integrated concentrating solar systems 545 17.1 Introduction to building integration of solar energy systems 545 17.1.1 Solar thermal systems and building integration requirements 546 17.1.2 Solar photovoltaic systems and building integration requirements 550 xii Table of contents 17.2 Building integrated concentrating systems 556 17.2.1 Physics of concentrating solar system 556 17.2.2 Types of concentrators 557 17.2.3 Building integrated concentrating photovoltaics 561 17.2.4 Building integrated solar thermal (concentrating) 575 17.2.5 Concentrating systems and building integration requirements 578 17.3 Conclusions 579 18 Solar energy use in buildings 589 18.1 Introduction 589 18.2 Passive solar gains in cold and moderate climatic regions 590 18.2.1 Passive solar gains by glazing 592 18.3 Total energy transmittance of glazing 592 18.4 New glazing systems 596 18.5 Transparent thermal insulation (TTI) 597 18.6 Operational principle of transparent thermal insulation 597 18.7 Materials used and construction 601 18.8 Heat storage by interior building elements 602 18.9 Component temperatures for sudden temperature increases 605 18.10 Solar gains, shading strategies and air conditioning of buildings 609 18.11 Influence of the urban form on solar energy use in buildings 614 18.12 Residential buildings in an urban context 614 18.13 Site density effect and urban shading in moderate climates 614 18.14 Climate effect 617 18.15 Solar gains and glazing 618 18.16 Office buildings in an urban context 620 19 The contribution of bioclimatic architecture in the improvement of outdoor urban spaces 623 19.1 Introduction 623 19.2 Mitigation strategies 625 19.2.1 Planted areas 626 19.2.2 Cool materials 627 19.2.3 Shadings 629 19.2.4 Thermal sinks 629 19.2.5 Combination and interplay of mitigation strategies 629 19.3 Experimental analysis of outdoor spaces 630 19.3.1 Assessment of outdoor comfort conditions 630 19.3.2 Assessment of bioclimatic technologies 634 19.4 Conclusions and future prospects 638 20 Legislation to foment the use of renewable energies and solar thermal energy in building construction: The case of Spain 643 20.1 Introduction 643 20.2 European regulatory framework for renewable energy sources in the context of the energy performance of buildings 643 Table of contents xiii 20.3 Application of EU regulations in member states: The case in spain 648 20.3.1 National action plan for renewable energies 649 20.3.2 Basic procedure for the certification of energy efficiency 651 20.3.3 The spanish technical building code 652 20.3.4 Spanish regulations for thermal installations in buildings 653 20.4 The solar thermal system 654 20.5 The spanish technical building code as a legal means to foment the use of renewable energies in building construction 657 20.6 Measures to foment the use of renewable energies: Government incentives 659 20.7 Economic impact of solar thermal energy 660 20.8 Conclusions 662 Subject index 665 This page intentionally left blank Preface As the world’s conventional energy supply nears its peak, and with the demand for that energy increasing year on year, it is expected that balancing supply and demand will become increasingly challenging. Consequently it is expected that non-conventional energy sources and renewable energy resources are likely to play a greater role in addressing the imbalance between supply and demand. Many experts advocate increased harnessing of renewable energy as an important alternative energy source. Utilization of renewable energy resources is sometimes expensive and difficult to apply fully in particular sectors of society because of the location, intensity and nature of the applications. Therefore specific matching of the renewable energy source to the application is a very important aspect of maximizing the utilization of renewable energy. Solar energy is available in differing intensities in different parts of the planet. Maximization of its potential as a primary alternative renewable energy source depends however on the specific usage made of it. Hence, this book was conceived to serve the purpose of identifying primary and secondary applications of solar energy in order to maximize their potential. As solar energy applications can span almost the entire spectrum of human activity, including for example biological processes, chemical processes, mechanical processes and other aspects of our daily lives, preparation of a book that considers all these facets is very important in determining how existing sciences and technologies can further refine and expand solar energy utilization and applications. Many experts in solar energy were invited to contribute to this book, with content ranging from basic to higher concepts of solar radiation, the thermodynamics of solar energy processes and applications, the application of solar energy to producing an alternative, renewable secondary energy source through hydrogen production, thermochemical processes to separate some greenhouse gases, how to apply solar energy in thermo-mechanical processes, maximization of solar energy use in energy-efficient housing and other buildings, and the role of solar energy in planning the outdoor environment. International experts from the many different fields of science and technology to which solar energy has feasible application have collaborated in the preparation of this book. Consequently there is wide coverage of solar energy as an alternative energy source which can also offer low greenhouse gas emissions. In this context, firstly the editors acknowledge with gratitude each of the global experts in solar energy who have fully supported and contributed chapters to this xvi Preface book and who are individually listed in the book chapters. Secondly, we are grateful to Janjaap Blom of the Taylor Francis-CRC Press for the support given from conceptualization through to the publication of this book. Thirdly, we thank our families for their support during the entire process of production of the book with our aim of supporting the prospect of sunnier and clearer skies in the future of our planet. Napoleon Enteria Aliakbar Akbarzadeh About the editors Napoleon Enteria is the Managing Consultant of the Enteria Grün Energietechnik, Philippines. At the same time, he is a Visiting Researcher of the Faculty of Engineering, Tohoku University, Japan. He was a Research Staff of the Faculty of Engineering, Tohoku University, Japan, for the Industry-Academia-Government Collaboration. He was doing research in collaboration with different Japanese universities and companies with the prime support of Japanese government agencies in the area of solar energy, HVAC systems and building sciences. In addition, he provides technical and scientific advice to graduate and undergraduate students. He was a scientist with the Solar Energy Research Institute of Singapore, a component of the National University of Singapore, performing collaborative research with the Fraunhofer Institute of Solar Energy Systems in Germany, a German company and the Department of Mechanical Engineering of the National University of Singapore in the field of solar thermal energy, HVAC systems and membrane heat exchangers; the latter was supported by the Singaporean government agency during his stay in Singapore. Before going to Singapore, he was a Global Center of Excellence Researcher in the Wind Engineering Research Center of Tokyo Polytechnic University doing research in natural ventilation and air-conditioning systems in collaboration with Japanese universities, companies and the Global Center of Excellence Program of the Japan Ministry of Education, Culture, Sports, Science and Technology. In addition, he was a guiding instructor to two undergraduate students for theses research. Napoleon has authored several scientific and engineering papers in books, review journals, research journals and conference proceedings. He has presented and submitted dozens of technical reports for collaborative projects with research institutes, universities and companies in different countries. He is regularly invited as reviewer for several international journals in the field of air handling systems, energy systems and building sciences. On occasion, he is invited to review research funding applications and gives technical and scientific comments on international scientific and engineering activities. He is a member of the American Society of Mechanical Engineers (ASME), the International Solar Energy Society (ISES) and an associate member of the International Institute of Refrigeration (IIR). He was awarded his Doctor of Philosophy (2009) in engineering, specializing in Building Thermal Engineering at the xviii About the editors Tohoku University, Japan, as Japanese Government Scholar; and his Master of Science (2003) and Bachelor of Science (2000) in the field of mechanical engineering from Mindanao State University at Iligan Institute of Technology, Philippines, as Philippine Government Scholar. Aliakbar Akbarzadeh was born in Iran in 1944. He received his BSc degree in Mechanical Engineering from Tehran University in 1966. In 1972, he obtained his MSc and in 1975 his PhD, also in Mechanical Engineering and both from the University of Wyoming, USA. From 1975 to 1980 he was an Associate Professor and also Head of the Mechanical Engineering Department at Shiraz University in Shiraz, Iran. Later he worked at the University of Melbourne as a Research Fellow (1980– 1986), primarily doing research on applications of solar energy as well as energy conservation opportunities in thermodynamic systems. Since June 1986, Aliakbar has been working as an academic at RMIT University in Melbourne, Australia. During this period, he also worked as a visiting Fellow for half-a-year at the Nuclear Engineering Department of the University of California at Berkeley, USA, where he did research on passive cooling of nuclear reactors through computer modelling as well as experimental simulations. At present, Aliakbar is a Professor in the School of Aerospace, Mechanical and Manufacturing Engineering at RMIT University, and also the Leader of the Energy CARE (Conservation and Renewable Energy) Group in the same school. Aliakbar lectures in thermodynamics as well as remote Area power supply systems. He is the Principal Supervisor of ten full-time PhD postgraduate research students on energy conservation and renewable energy systems. He has also one post-doctoral research fellow working with him on geothermal energy utilization for power generation. Aliakbar is a specialist in thermodynamics of renewable energy systems. His industry oriented research projects enrich his teachings and makes them relevant. He spends about half of his time in supervising industry supported research in energy conservation and renewable energy area, which also form a vehicle for postgraduate training of his PhD students. He has been the first supervisor of about 30 PhD candidates who have completed their degrees. Aliakbar has over 100 refereed publications and two books all in his area of specialization which is solar energy applications. One of his publications on solar energy won the ASME Best Paper of the year award in 1996. Aliakbar’s industry-oriented research on energy systems has resulted in a number of Australia National Energy Awards for him, as well as a number of products, such as the Heat Pipe-based Heat Exchanger for waste heat recovery in bakeries, the Temperature Control of solar water heaters using thermo-syphons and an innovative system for simultaneous power generation and fresh water production using geothermal resources. Aliakbar has also been working on salinity gradient solar ponds as a source of industrial process heat and also for power generation. In the last 35 years he has developed several concepts related to salinity gradient maintenance, as well as efficient methods of heat extraction from solar ponds. At present, his research group is the world leader on applications of solar ponds. Chapter 1 Physics of solar energy and its applications Napoleon Enteria1 & Aliakbar Akbarzadeh2 1Enteria Grün Energietechnik, Davao, Philippines 2School of Aerospace, Manufacturing and Mechanical Engineering, RMIT University, Melbourne, Australia 1.1 INTRODUCTION Solar energy has existed for millions of years and has been used by many non-living and living things for physical, chemical and biological changes and processes. For generation after generation, solar energy has been the main source of daily energy in many ways. The start of human modernity and civilization changed the utilization of solar energy. Previous human civilizations utilized solar energy for lighting, food drying and personal care. Modern humans utilize solar energy for everyday existence, work purposes and generally for living. As human demand for modern energy supply increases, attention to solar energy becomes more intense. Because of the increasing population, demand for better comfort, urbanization and industrialization, the pressure on conventional energy sources is increasing. The rapid increase of energy demand rattles the chain of energy supply which has difficulty in meeting the rapid rise in energy demand. This rapid demand not only makes the world more volatile in terms of energy politics but also accentuates the environmental hazards associated with emission of greenhouse gases, particularly from chemical processes involved in utilizing the thermal energy obtained from conventional energy sources. Consequently there are active plans to utilize solar energy for different processes to minimize energy demand from conventional energy supply sources. Conventional energy supplies are based on fossil fuel or carbon-based energy sources, based on the liberation of stored energy through combustion. Through combustion processes or chemical processes, thermal energy is generated. At the same time, radiant energy or solar energy is an available energy which can be used as replacement for the energy liberated/generated during combustion processes. Therefore, several methods, concepts and ideas are being pursued to maximize the utilization of solar energy as an alternative form of energy to minimize the usage of carbon-based energy sources. This chapter introduces the concept of solar energy and its applications as alternatives for conventional energy supply and demand. 1.2 SOLAR ENERGY AND ENERGY DEMAND The Sun is the nearest and only star around which the Earth and other planets are constantly rotating. The energy generated by the Sun is utilized on Earth for the support of living organisms. As the Sun is radiating and transmitting energy in the form of 2 Solar energy sciences and engineering applications radiant energy with a range of wavelengths and intensities, a nuclear fusion-fissionfusion reaction is happening inside the core of the Sun. The nuclear processes of the Sun create tremendous amounts of energy which are sometimes devastating due to the power of nuclear processes. The power generated by the Sun is almost endless and the human benefit from it is expected to last for countless generations to come. The energy generated in the Sun is transmitted to its surface through convection and radiation. From the surface, the energy is transmitted to its surroundings through solar radiation. This is a consequence of space being a vacuum from which both conductive and convective modes of energy transmission are eliminated. The solar energy available in the upper atmosphere of the Earth is almost constant and depends on the motions and distance between the Earth and the Sun associated with the constant rotation and revolution of the Earth around the Sun and the nuclear activities of the Sun. As the Earth has a layer of atmosphere, the available solar energy in the upper atmosphere of the Earth is reduced in transmission before it reaches the surface of the Earth, depending on the weather and climatic conditions, surface locations and local activities such as heavy smokes from wildfires. The net energy of the Sun, reaching the surface of the Earth in the form of radiant energy, has different intensities. The intensities depend on the radiant energy wave spectrum. The radiant energy wave spectrum is generally classified as short wave (0 to 300 nanometer), infrared (300 to 750 nanometer) and long wave (750 nanometer and above). With these different radiant energy wave spectrum energy intensities, utilization of solar energy is very interesting and for different applications depends on a variety of techniques and methods. The radiant energy from the Sun has been naturally utilized by different terrestrial living organisms for millions of years. Early humans utilized solar energy in different applications including food production, cloth making and others. As modern civilization demands more energy, alternative sources can be investigated and attention to solar energy and its provider, the Sun, becomes an interesting topic for the modern world. There are many concepts, methods, ideas and practical solutions both for simple and for more advanced utilization of radiant energy from the Sun. As global demand is for clean energy sources which are potentially renewable and inexhaustible, solar energy becomes the center of attention. Demand for conventional energy sources has increased tremendously since the start of the industrial revolution. Conventional energy sources are carbon-based, including coal, oil and gas, and burning these sources creates large amounts of carbon dioxide which is a so-called greenhouse gas. The greenhouses gases are responsible for the increase of the Earth’s temperature through trapping the Sun’s radiant energy as it reflects from the surface of the Earth. Hence, massive consumption of carbon-based conventional energy sources has major future effects – depletion of the energy stock resulting in increased prices, political issues related to energy shortage and increase of the Earth temperature as human demand for better comfort increases. Moreover there are other effects on biological and agricultural production. As the population, urbanization, industrialization and demand for better human comfort increase, it is expected that the demand for energy consumption will increase. The rapid increase in demand for energy is associated with rapid industrialization of the developing world. China and India are expected to contribute a bigger share in the rapid increase of energy consumption. Latin America and South East Asia Physics of solar energy and its applications 3 are also expected to contribute a bigger part of the increase of energy demand. The rapid demand for conventional energy sources has created a geopolitical energy tension due to the sizable amount of energy supply from major energy producing countries and regions instead. It is expected that the Middle East and Russia will play a major role as global energy producers. The increase of greenhouse gas emissions is attributed to the increase of carbonbased energy consumption. With rapid demand for coal for power plants and oil and gas for transportation and other sectors, greenhouse gas emissions are increasing. The report of the Intergovernmental Panel on Climate Change (IPCC) shows that the increase of the global temperature is caused by the increase of greenhouse gas emissions (IPCC). As global warming and climate change have tremendous and complex effects, they will have greater significance for human survival. Therefore, minimization of greenhouse gas emissions without stifling the demand for progress and human comfort is crucial. The reduction of greenhouse gas emissions is possible through reduction of the consumption of conventional energy resources. The reduction of the use of conventional energy resources is achievable by the means of energy efficiency, energy conservation and utilization of renewable energy sources. Existing processes, devices and operations can be energy efficient through application and development of new and novel technologies with the aid of basic sciences. The energy conservation measures are possible through the combined application of the passive (natural) and active (artificial) methods, of which in previous and present times active methods have dominated. Utilization of renewable energy sources, particularly solar energy which is available in most parts of the planet, can be advanced through investigation of the existing equipment, processes and sources of energy and energy requirements which can be alternatively sourced from solar energy. In this scenario, demand for and consumption of conventional energy sources or carbon-based energy sources will gradually decrease, resulting in the reduction of greenhouse gas emissions. 1.3 SOLAR ENERGY UTILIZATIONS In the modern world, cities and other modern facilities are operated using electrical energy. This is because of easy and simple transmission and storage of electrical energy in different forms. In this situation it is expected that demand for electrical energy will increase in future generations. Electricity production through solar energy is being achieved in two major processes – solar thermal and solar-photovoltaic. Solar thermal power plants employ concentrating solar collectors to produce high temperatures by focusing solar energy in order to produce steam for power plants. Solar-photovoltaic power plants employ semiconductor materials to convert radiant energy photons to electrons to produce electricity. The two technologies have both advantages and disadvantages depending on the point of view of the user. However, as solar energy is free and clean, the utilization of solar energy for electricity production is compelling and offers both energy security and environmental benefits. There are many biological processes in which thermal energy is a requirement and which are called endothermic processes. In endothermic processes, the thermal 4 Solar energy sciences and engineering applications energy is utilized to accelerate the biological processes and thus reduce the waiting time or increase the production cycle. In endothermic biological processes, application of solar energy directly or indirectly is possible using different solar energy collectors. Production of bio-fuels with the support of solar energy enhances the processes. Hydrogen production from water and other sources has been demonstrated as an alternative clean energy source. However, production of hydrogen through thermochemical or molecular breaking or through electro-chemical processes are the main methods and require external energy input. Hydrogen production or any thermochemical processes can be done with the support of solar energy either as direct thermal energy or using electricity generated from solar energy. There are several large scale research facilities for the production of hydrogen through thermal energy. It has also been demonstrated that the production of hydrogen through electrochemical processes is feasible. In agricultural industries, application of thermal energy for drying is most important to increase the shelf life of the products. Solar energy can also be used for the bio-chemical processes in agricultural production. There are many existing and under-demonstration technologies for agricultural applications. The most common and typical application is the solar dryer which has been demonstrated to be feasible and practical using simple design, local materials and unskilled workers. In addition, solar cookers and other food processing applications of solar energy have been demonstrated. Machines minimize human effort with increase of production and operations in many different applications. There are many thermally operated mechanical processes including heating, ventilating and air-conditioning systems, heat engines, pumps and fans. Application of solar energy to support the thermal requirements of these machines and equipment is feasible. Thermally operated air-conditioning systems, pumps and fans have been shown and demonstrated to be feasible and practical. There are many concepts, designs and technologies readily available and being conceptualized for applications. The occupants of buildings always demand thermal comfort conditions during summer time and winter time, both daytime and nighttime. The maintenance of indoor thermal comfort conditions in buildings consumes large amounts of energy. Also, the reduction of humidity in buildings consumes large amounts of energy in tropical climates. Indoor cooling and heating both in tropical and temperate climates consumes considerable amounts of energy. Furthermore, the lighting for buildings is another main consumer of energy particular for office buildings. Solar energy which is readily available can be utilized to support the day-to-day operation of buildings and to support the indoor comfort conditions for occupants. For instance, solar energy can be utilized to support the air flow rate requirement through natural ventilation. Solar energy can be used to support the thermal energy requirement of buildings through solar thermal collection. Solar energy can be used to support the electricity requirements of buildings through photovoltaic installations. Solar energy can be collected through thermal storage to support the nighttime thermal energy requirement of buildings. The effect of a heat island is felt in big cities because of the increased utilization of air-conditioning systems and application of urban materials which absorb solar energy. Proper design through urban planning can minimize the effect of solar radiation including sun shading, alternative materials, air movement for natural ventilation and the Physics of solar energy and its applications 5 general utilization of solar energy for building applications. There are conceptualizing technologies to capture the solar energy in urban areas through photovoltaic technologies and solar thermal technologies to reduce the solar energy contribution to the heat island effect. Application of technologies, which can effectively collect the solar energy in the different areas of cities or urban areas, minimizes its contribution to the heat island effect. There are several day-to-day applications of solar energy which we are using that can be alternatively sourced. Day-to-day applications of light to illuminate us such as during reading or other indoor activities are common. Washing of clothes and drying can consume energy. The daily usage activities and personal usage of energy can be sourced alternatively using solar energy in the form of day lighting and thermal energy which can make us minimize the use of conventional and existing energy sources. 1.4 PERSPECTIVE Solar energy is available in every part of the planet; however, utilization of the energy depends on our desire and needs and in most cases on economic factors. Depleting conventional energy sources and the issue of global energy politics have become very serious. Global warming and climate change present a serious situation which endangers humanity. Therefore, serious consideration of alternative approaches to the issue of energy resources is vital. REFERENCE IPCC Second Assessment Report: Climate Change 1995 (SAR).Working Group I: The Science of Climate Change. http://www.ipcc.ch/publications_and_data/publications_and_data_reports. shtml#1 This page intentionally left blank Chapter 2 Exergy analysis of solar radiation processes Ryszard Petela Technology Scientific Ltd., Calgary, Alberta, Canada 2.1 INTRODUCTION Radiation energy can be converted to work, heat, chemical energy or electricity. Direct conversion to work is so far not well developed, but potential examples are the idea of sailing in space via photon wind, a combination of gravity and buoyancy in a solar chimney power plant or utilization of radiation pressure. By conversion to heat the enthalpy of any operating fluid is usefully increased. Conversion to chemical energy is photosynthesis and conversion to electricity occurs in photovoltaic. The present chapter contains problems of engineering thermodynamics of thermal radiation and thus is mainly based on the book by Petela (2010), until now the only one written on this subject. The chapter outlines the fundamentals of examining processes in which radiation takes place. Beside traditional methods of energy analysis of such processes, the full thermodynamic analysis, including the exergy analysis, is discussed and illustrated by examples of some typical utilization processes of solar radiation. The analysis is preceded by a basic description of exergy which is a property of any matter. Everything which has mass is called matter. The matter appears in substantial or non-substantial forms. Mass is a property of matter which determines momentum and the gravitational interactions of bodies. Substance is matter for which the rest mass is not equal to zero. Thus, the substance is the macroscopic body composed of elemental particles (atoms, molecules). The matter for which the rest mass equals zero (e.g. radiation photons) appears in the form of different fields; e.g. fields of electromagnetic waves (radiation), gravity fields, surface tension fields, etc. Substance can be the object of the conservation equation. Non-substantial matter (called sometimes field matter) can also be considered as the component in processes of energy conversion; however it does not fulfill the matter conservation equation. Processes considered here are composed of substance and field matter. The chapter develops a methodology of examining thermodynamic processes under the assumption that the reader is familiar with the fundamentals of engineering thermodynamics and heat and mass transfer. The details of mechanisms of the considered processes and installations are discussed in other parts of the book together with relevant references. 8 Solar energy sciences and engineering applications 2.2 EXERGY 2.2.1 Definition of exergy Exergy is a thermodynamic concept and is one of several thermodynamic properties (functions of thermodynamic parameters) of states of matter which can appear as a substance or any field matter, e.g., as radiation. The functions are defined to make consideration easier, allow for interpretation of phenomena and, most important, most of them have practical applications in thermodynamic calculations. For example, for a substance the enthalpy is used to determine the energy of an exchanged substance with the considered system, whereas the internal energy expresses the energy of a substance remaining within the system during the time of the system’s consideration. Entropy determines the thermodynamic probability of a given matter state. Exergy was introduced to express the practical energetic value of matter relative to the environment given by nature. This practical value is determined by the ability of matter to perform mechanical work. Work was selected as the measure not only due to the human inclination to laziness, but first of all, because work represents the energy exchange at the unlimited level. Work is a process in which the energy does not degrade. However, the full utilization of energetic value of matter to perform work within the determined environment could not occur without the cooperation with this environment. For example, to utilize the energetic value of natural gas by its combustion, a certain amount of oxygen contained in the environmental air has to be taken. Another example: to fully utilize a compressed air at the environment temperature, the depressurizing of the air has to occur at constant temperature, and to keep this temperature steady, heat from the environment has to be taken. The full definition of exergy was given by Szargut which after modification, i.e. not only for substance but for any matter, including radiation, is as follows: Exergy of matter is the maximum work the matter could perform in reversible process in which the environment is used as the source of worthless heat and worthless substances, if at the end of the process all the forms of participating matter reach the state of thermodynamic equilibrium with the common components of the environment. The environment is the natural reference state given by nature which consists of the worthless components available in arbitrary amounts. The environmental components of apparent energetic value, appearing however in limited amounts, are the exceptions and are recognized as natural resources, e.g. natural fuels. The key component of exergy results from the values of the thermodynamic parameter of the considered matter, and such component is called the thermal exergy. The formula for thermal exergy can be derived as follows. The considered matter of a given temperature T and entropy S is delivered into the system from which this matter is exiting at the state of equilibrium with the human environment, i.e. at environmental temperature T0 and appropriate entropy S0. To obtain the maximum work W, which according to the exergy definition would be equal to exergy B of the matter (W =B), the process in the system has to be ideal (reversible) and appropriately adjusted by heat Q exchanged with a heat source, which is the environment at temperature T0. In the energy balance equation for the system, the energy E of the considered matter Exergy analysis of solar radiation processes 9 delivered to the system is equal to the energy E0 of this matter leaving the system, to performed work W and to exchanged heat Q: E=E0 + B +Q (2.2.1) The algebraic overall entropy growth for the reversible process is zero: S0 - S + Q T0 =0 (2.2.2) When equation (2.2.2) is used to eliminateQfrom equation (2.2.1) then the general formula for the exergy B of matter is: B=E - E0 - T0(S - S0) (2.2.3) For example when the considered matter is a substance then energy E is interpreted as the enthalpy of the substance. In the case of radiation the appropriate radiation energy has to be taken for E as discussed in the next paragraphs. The thermodynamic value of a matter determined by energy E is significantly different from the value determined by exergy B. This fact enables the interpretation of the value of a matter, as well as the process, from two viewpoints and the deeper understanding of the considered thermodynamic problem. However any engineering designing should be based on the results from energy calculation. The exergy concept is also applied to the phenomena of energy exchange which are work and heat. Exergy of work, by definition, is directly equal to the work. Work, beside mechanical work, can also appear as work performed by an electrical current or magnetic field, etc. Regarding heat, the thermodynamic concept of a heat source is applied. A heat source is defined as the body, at given temperature, which can absorb or release infinitely large amounts of heat without a change in the body temperature. Exergy BQ of heat Q at temperature T is measured by the change of exergy of the heat source at temperature T absorbing heat Q. The maximum work which could be performed by heat Q is determined as the work performed in the ideal Carnot engine cooperating with two heat sources: one at temperature T and other at environment temperature T0. Using the definition of Carnot efficiency: BQ =  Q T - T0 T  (2.2.4) For example, heat Q at temperature T =T0 has practically zero value and from formula (2.2.4) the exergy BQ of such heat is zero. This is a typical illustration of interpretation advantages of exergy over energy (heat). The value BQ can be recognized either as a positive input to the system at T >T0 or a positive output from the system if T 0) in the course of the process. For any theoretical model of reversible phenomenon d=0; if however d<0 then the whole phenomenon is impossible. For example, during design of a heat exchanger, care should be taken about the so-called pinch point for which, if locally d<0, then the whole process of heat exchange is impossible. Thus, the entropy is very useful in verification of designed new processes from a reality viewpoint. The larger the overall entropy growth, the more irreversible is the considered process. If the substance remains unchanged during a process (e.g. during physical process), then only the respective change in the substance entropy exiting and entering the system are taken into calculation of . If a substance disappears (chemical reaction) then its absolute entropy has to be taken with a negative algebraic sign. If a substance appears, the positive sign of entropy should be used. The exergy loss expressed by formula (2.2.10) is called the internal exergy loss, because it occurs within the considered system. This loss is totally non-recoverable. Internal loss of exergy for a multi-component system is calculated by summing up the internal losses of exergy occurring in the particular system components. Each exergy loss contributes to the increase in the consumption of the energy carrier which sustains the process or to the reduction of the useful effects of the process. One of the main engineering tasks is operating the processes in the way at which the exergy loss is at a minimum. However, most often, the reduction of the exergy loss is possible only by increasing the capital costs of the process. For example the reduction of exergy loss in a heat exchanger is reachable by costly increases of the surface area of heat exchange. Therefore, the economics of such reduction of exergy loss can be verified by economic calculations. The exergy analysis explains the possibilities of improvement of thermal processes; however only the economic analysis can finally motivate an improvement. Usually, from a thermal process there is released one, or more, waste thermodynamic media (e.g. combustion products), of which the parameters are still different from the respective parameters of such a medium being in equilibrium with environment. The waste medium represents certain exergy unused in the process. Such exergy, if released to the environment, is destroyed due to irreversible equalization of parameters of the waste medium with the parameters of environmental components. The 12 Solar energy sciences and engineering applications exergy loss of the system, caused by such a way, is called external exergy loss and its numerical value is equal to the exergy of the waste medium released by the system. External exergy loss is recoverable, at least in part, e.g., by utilization in other systems. The irreversibility of radiation processes occurs due to such basic phenomena as emission and adsorption. The irreversibility mechanism of many radiation processes, e.g., diluting or attenuation of propagating radiation, can be explained based on the irreversibility of emission and absorption. Obviously, in the combined processes, in which substance and radiation take place, all the sources of irreversibility should be considered; those for substance together with those for radiation. The radiation entering or absorbed in the considered system has a negative sign of radiation entropy, whereas radiation leaving the system, or emitted, has positive entropy. The radiation entropy is recognized as absolute. The problem of radiation irreversibility was considered for the first time by Petela (1961b) and later edited in the book (Petela 2010). Based on the overall entropy growth for considered processes it was proved that the emission alone (not accompanied by any adsorption) is possible (>0), whereas the absorption alone is irreversible, and without accompanying emission of the considered surface is impossible (=0). The simultaneous emission and absorption is always possible (>0). The exergy of radiation reaching any surface can be reflected (re-radiated) and the reflected radiation has its exergy at the temperature of the original radiation, which was not utilized by the absorbing surface. If the reflection process does not change the radiation temperature then this process is reversible, and not generating any exergy loss. However, the radiation emitted by the absorbing surface has its own exergy determined by the emissivity and temperature of the absorbing surface. This is the problem of the efficiency of the absorbing surface, or any other device utilizing the radiation somehow, in how much of the whole incident exergy b the surface, or the other device, can be grasped and utilized. The efficiency of the absorbing device or surface is an entirely different thing and does not depend on the theoretical potential represented by b. Acceptance of such interpretation is very important in correct reasoning on the theory of radiation exergy, because if not noticed by some researchers, it can mislead to strange conclusions. 2.2.3 Exergy of substance 2.2.3.1 Traditional exergy A total exergy of substance is composed of some components as shown schematically in Figure 2.2.1. Usually, only these components are used, which vary during the analysis. Most often is used the thermal exergy, which is the sum of physical and chemical exergies. The physical exergy results from different temperature and pressure of the considered substance in comparison to its temperature and pressure in equilibrium with the environment (dead state). The chemical exergy results from the different chemical composition of the considered substance in comparison to the common substance components of the environment. If the considered substance has significant velocity, then the kinetic exergy can be recognized as equal to the kinetic energy calculated for the velocity relative to the environment. Potential exergy is equal to the potential energy if it is calculated for the Exergy analysis of solar radiation processes 13 Figure 2.2.1 Exergy components. reference level, which is the surface of the Earth. The “other’’ possible components, e.g., nuclear, or interfacial tension, are rarely used and are excluded from the present consideration. The sum (Bph + Bch =B) of most important components for substance considerations; physical exergy Bph and chemical exergy Bch is called thermal exergy B and according to the general formula (2.2.3) is determined as: B=H - H0 - T0(S - S0) (2.2.12) where B is the thermal exergy of the considered substance at enthalpy H and entropy S, and H0 and S0 are the enthalpy and entropy of this substance in eventual state of thermodynamic equilibrium with the environment parameters. The thermal exergy B expressed by equation (2.2.12) is for a substance passing through the system boundary. The exergy B of substance can be positive or negative (e.g. for each medium flowing through the pipeline there could be a sufficiently low pressure at which thermal exergy B is smaller than zero). Especially for air, such negative exergy can happen for example when the air temperature is not much higher relative to environment, and the air pressure is lower than atmospheric. However always positive is the exergy Bs of any part of the system which remains within the system boundary under pressure p and occupying volume V: Bs =B - V(p - p0) (2.2.13) where p0 is the environment pressure. The concepts B and Bs suggest an analogy to enthalpy and internal energy. In a particular case if a space of volume V is empty, then the pressure p=0, so B=0 (because there is no substance), then from formula (2.2.13) the exergy of the lack of substance is: (B)p=0 =Vp0 (2.2.14) As shown later, a similar effect regarding lack of radiation (for T.0) in formula (2.2.31) shows the finite exergy value bb,S =a · T4 0 /3 in the vessel and the value from formula (2.2.42) for emissions. 14 Solar energy sciences and engineering applications In practice, the formula (2.2.3) can be rearranged for example to the frequently applied form for the exergy b, J/kg, of an ideal gas of temperature T and pressure p: b=cp(T - T0) - T0  cp ln T T0 - Rln p p0  (2.2.15) where cp is the specific heat of gas at constant pressure, R is the individual gas constant, T0 is the absolute temperature of environment and p0 is the partial pressure of the considered gas in equilibrium with environment. 2.2.3.2 Gravitational interpretation of exergy Solar heating of a surface on Earth, combined with the gravity field, creates specific effects, e.g., a driving force for solar chimney power plant. The full role of the Earth’s gravity field in such processes could be better analyzed by introducing the additional component of mechanical exergy bm (called shortly ezergy) and by additional terms of gravity input G in the exergy balance, both proposed by Petela (2010). The mechanical exergy concept bm is derived from the difference between density . of considered substance and density .0 of environment. Regardless of the temperature T and pressure p of the substance under consideration, the buoyancy of the substance is unstable, and thus the ability to work in the environment at respective parameters T0 and p0 is sensed if removed as either an anchor (.<.0) or a supports (.>.0). In the first case, the substance moves upwards, in other case, the substance sinks. The altitude of the considered substance is measured from the actual level x=0. In both the above cases the substance tends to achieve an equilibrium altitude (x=H), at which the density of local environment .0,x is equal to density of the considered substance; .=.0,x. The motion of substance (remaining at constant T and p) to the equilibrium altitude would generate work which is called the buoyant exergy bb, which does not depend on the kind of the substance. During repositioning of the substance, from actual altitude x=0 to x=H, the gravity acceleration gx is changing, e.g. decreasing with growing altitude x, thus: bb = x=H x=0 gx  .0,x . - 1  dx (2.2.16) The solution of equation (2.2.16) is discussed by Petela (2010). At level H the substance could be allowed to generate additional work, denoted by bH, which would occur during a reversible process of equalization of parameters T and p with the respective local environment parameters T0,H and p0,H. In case of a gas, during equalizing of the gas parameters T and p with the parameters T0,H and p0,H at the altitude H, based on Formula (2.2.15), the following work (exergy bH) can be done: bH =cp(T - T0,H) - T0,H  cp ln T T0,H - Rln p p0,H  (2.2.17) Exergy analysis of solar radiation processes 15 Figure 2.2.2 Exergy components including the mechanical exergy (ezergy). On the other hand, the gas at the actual altitude (x=0) has the traditional physical exergy b, expressed by formula (2.2.15), equal to the work which could be done by the gas during the equalizing of its parameters, T and p, with respective environment parameters T0 and p0. Exergy definition postulates the exergy to be the maximum possible work. Therefore, the larger work of the two, bb + bH or b, is the true exergy called the mechanical exergy. bm =max[(bb + bH), b] (2.2.18) Including the mechanical exergy (eZergy) into consideration, the scheme of the all components of the exergy of substance is now shown in Figure 2.2.2. The eZergy is applied only for the substance (e.g. not for heat or radiation) and it replaces the two traditional exergy components: physical (Bph) and potential (Bp). To better distinguish exergy of the substance from the eZergy of the substance, different symbols could be used: B for eXergy and Z for eZergy, (e.g. in paragraph 2.4.3: Bm =Z=f (Bph,Bp). 2.2.3.3 Chemical exergy of substance In a chemical process, in contrast to a physical process, substances change and only the chemical elements remain unchanged. Therefore, to calculate the chemical energy (or exergy) of substances the reference substances have to be assumed appropriately. The most common methods for determination of the chemical energy of substances are enthalpy formation and devaluation enthalpy, which differ mainly by the definition of reference substances. In the enthalpy formation method, the reference substances are the chemical elements at standard temperature and pressure. In the devaluation reaction method the number of reference substances is the same; however, they are not the chemical elements but the devaluated substances (compounds or chemical elements most commonly appearing in the environment). For example, the reference substance of C is gaseous CO2, for H it is gaseous H2O, and for O it is just O2. Therefore, in any particular case, when a substance is composed only of C, O, H, N, and S, then the devaluation enthalpy of the substance is equal to its calorific value. Contrary to the devaluation enthalpies, the values of the enthalpy of formation are a little illogical. For example, the enthalpy of formation for valuable pure carbon C is zero and the enthalpy of formation of not valuable CO2 is significantly different from zero (-394 MJ/kmol). However, for comparison, the devaluation enthalpy of C 16 Solar energy sciences and engineering applications is equal to its calorific value ~394 MJ/kmol, whereas the calorific value of CO2 is zero (as it is the reference substance for C). The reference substances for the devaluation enthalpy and chemical exergy are the same. Also, the reference temperature and pressure are the same. Thus, only the devaluation enthalpy method, contrary to the formation enthalpy method, allows for fair comparison of the values of chemical energy and chemical exergy. For example, the chemical exergy of C is ~413 MJ/kmol and devaluation enthalpy (calorific value) of C is only ~394 MJ/kmol. Therefore, only the devaluation enthalpy method should be used in thermodynamic analysis, because then the comparative energy and exergy analyses are simultaneously included. More details on the devaluation method is discussed by Szargut et al. (1988). Only the significance of the concept of devaluation reaction and the resulting concept of devaluation enthalpy, used for calculation of chemical energy, is outlined here. The method allows calculation of the following quantities: enthalpy devaluation of a substance (appearing in the energy conservation equation), standard entropy of devaluation reaction (in the entropy considerations), and, consequently, chemical exergy of substance (appearing in the exergy balance equation). Devaluation enthalpy is determined based on the stoichiometric devaluation reaction for a substance. The devaluation reaction has to be a combination only of the considered substance and the various reference substances. A good example of a devaluation reaction is reaction of photosynthesis: 6H2O + 6CO2.C6H12O6 + 6O2 (2.2.19) in which, besides the considered substance of sugar (C6H12O6), only the reference substances appear; CO2, H2O and O2. The devaluation enthalpy, dn, is calculated from the energy conservation equation for the chemical process in which substrates are supplied, and products are extracted, all at standard temperature and pressure. As shown, in comparison to the physical exergy bph of a substance, the calculation of the chemical exergy bch of a substance is more complex, depending on its composition, and is based on the devaluation reaction. The calculation procedure is discussed by Szargut et al. (1988), where the calculated standard values of the devaluation enthalpy (dn) and chemical exergy (bn) are tabulated for the standard environment temperature T0 =Tn. If the environment temperature T0 differs from the standard environment temperature Tn, then, when using the standard data on dn and bn, the formula for the chemical exergy of condensed substances (solid or liquid), should be corrected as shown, e.g., for the specific chemical exergy bchSU of sugar: bch,SU =bnSU + Tn - T0 Tn (dnSU - bnSU) (2.2.20) where bn,SU and dn,SU are the standard tabulated values of the chemical exergy and devaluation enthalpy of sugar, respectively. If a substance has a temperature different from the surrounding environment, then also a physical component of energy or exergy has to be included as shown, e.g., again for the physical exergy bph,SU of the sugar: bph,SU =cSU(T - T0) - T0cSU ln T T0 (2.2.21) where cSU is the specific heat of sugar. Exergy analysis of solar radiation processes 17 Note as well that based on the devaluation reaction, the so-called standard entropy sn of the devaluation reaction can be determined. For example, again for the photosynthesis reaction, based on Equation (2.2.19), the standard entropy of the devaluation reaction, sn,SU, is: sn,SU =6(sH2O + sCO2 )n - 6(sO2 )n - sn,SU (2.2.22) where sH2O, sCO2 , and sO2 are the absolute standard entropies of the respective gases. The stoichiometric factor 6 results from Equation (2.2.19). 2.2.4 Exergy of photon gas Radiation has two meanings: it could be the process of a radiating body or it could be the product of this process. This process can be considered twofold – either as the propagating magnetic field or as the population of photons traveling in space. Then the photon populations can be imagined as the photons trapped in a system of limited space, or as the freely traveling flux of photons. The states of trapped or traveling photons are similar to the substance concepts of thermodynamic functions of state which are respectively the internal energy or enthalpy. The traveling photons in a form, e.g. of emitted radiation from a substance body (emission), or as a bundle of rays from many bodies of different temperatures (arbitrary radiation flux), are considered respectively in the next paragraph 2.2.5 and 2.2.6. In the present paragraph the large population of photons trapped in a space is considered. The temperature concept in radiation problems can be applied only to a photon batch and the temperature T of thermal radiation can only be determined indirectly, i.e. by measuring the temperature of the substance with which the radiation is in equilibrium. It can be derived that the internal energy U, in J, of the photon gas within a system of volume V, is: U =aVT4 (2.2.23) where a is the universal constant (a=7.564 · 10-16 J/(m3 K4)). The rest mass of photon gas is zero, therefore, energy U cannot be related to the mass of this gas but rather to its volume. Thus the photon gas energy density u, J/m3, is: u= U V =aT4 (2.2.24) The entropy density sS, J/(Km3), of black radiation in the system is derived as: sS = 4 3 aT3 (2.2.25) Radiation transports a linear momentum and may exert radiation pressure on an object by irradiating it. Radiation pressure can be considered either as the effect of interaction between radiation and substance, or as the effect only within the internal structure of the radiation. In the first case, radiation pressure exerted on a substance 18 Solar energy sciences and engineering applications of transmissivity t =0 is zero, however this pressure grows with the growing transmissivity and is maximum for t =1. In the second case, the photons can transfer momentum to other particles upon impact and such possibility of potential pressure exists regardless of the properties (e.g. transmissivity) of any target. As in the consideration of substance pressure, the equipartition theorem can be applied to the energy in a three-dimension system and accordingly the pressure p of black radiation is: p= u 3 (2.2.26) and using (2.2.24) in (2.2.26): p= a 3 T4 (2.2.27) One of the possible processes of photon gas is the isentropic process during which the photon gas does not exchange heat with the surroundings. For example such a process can be imagined for a photon gas trapped in the space surrounded with an expandable wall of perfect reflectivity. The isentropic process occurs reversibly and the entropy in each elemental process stage remains constant. The entropy in J/K of the gas occupying volume V is determined based on formula (2.2.25) and the condition of constant entropy in the process is: V 4 3 aT3 =const. (2.2.28) or eliminating temperature T by pressure p with use of formula (2.2.27): pV4/3 =const. (2.2.29) Exergy of black radiation was determined for the first time by Petela (1964), by consideration of the isentropic process in which the V =1m3 of radiation of temperature T changes the initial pressure p to the final pressure p0 at temperature of the environment. The final state of the considered radiation is in equilibrium with the environment and the exergy of this radiation is zero. Therefore, according to the exergy definition, the initial density bb,S, J/m3, of black radiation within the system is equal to the useful work performed during the process: bb,S = V0 V pdV - p0(V0 - V) (2.2.30) Using (2.2.27) and (2.2.29) in (2.2.30): bb,S = a 3 (3T4 + T4 0 - 4T0T3) (2.2.31) Exergy analysis of solar radiation processes 19 Figure 2.2.3 Exergy of photon gas as function of temperature. For example, Figure 2.2.3 shows bb,S as a function of temperature T at T0 =300 K. The exergy of radiation is always non-negative and the zero value is achieved only at T =T0. 2.2.5 Exergy of radiation emission During radiation of substantial bodies (solid, liquid or some gaseous), a part of their energy (e.g. internal energy or enthalpy) transforms into the energy of electromagnetic waves of a length theoretically from 0 to 8. These waves can travel in a vacuum because they do not require any medium for their propagation. Independently one can also imagine this radiation process as the energy non-continuously emitted in form of the smallest indivisible energy portions called photons. If the energy of a body is not simultaneously supplemented from an external source, then temperature of the radiating body decreases. The phenomenon of such radiation is called emission, which is the key problem in study of radiation, especially its simplified models. Temperature of radiation is always equal to the temperature of the emitter. According to the Stefan-Boltzmann law the energy eb of emission of a black surface at temperature T is: eb =sT4 (2.2.32) where s =5.6693 · 10-8 W/(m2 K4) is the Boltzmann constant for black radiation. Emissivity of the emitter, e.g., the emissivity of a solid surface, determines the surface ability measured by the rate at which the black radiation is produced. The perfect gray surface of emissivity e<1 emits black radiation of an amount determined by the emissivity e. If the density of emission eb expresses the amount of the emitted black radiation energy from 1m2 of black surface at e=1, then density e, (e=e · eb) expresses the amount of the emitted black radiation energy from 1m2 of gray surface, at the rate e: e=esT4 (2.2.33) 20 Solar energy sciences and engineering applications The black emission eb has its exergy bb, however as the rate of emission e of a gray surface is smaller (e=e × eb) then the exergy b (of this gray surface emission e), is also reduced by e: b=ebb (2.2.34) However using the energy emissivity e in formula (2.2.34) for exergy calculation is not precise and, as discussed in details by Petela (2010), the smaller is the precision the smaller is the emissivity e. The definition of exergy can be applied to the emitted photon gas. Therefore, the exergy of the emitted photon gas is a function of an instant state of the gas and of the state of the gas in the instant of eventual equilibrium with the environment. Such equilibrium determines the reference state in calculation of the exergy of the photon gas. The environment consists of many bodies of different temperatures and different radiative properties (e.g. emissivities or transmissivities). The dominating temperature of the environment’s bodies can be assumed as the standard (averaged) environmental temperature T0. As discussed previously the surface always emits black radiation, thus the environment surface at temperature T0, regardless of the surface properties, emits black radiation at temperature T0. The properties of the surface determine only the rate at which the emission occurs. Thus, the environment space permanently contains the black radiation at temperature T0 and this radiation is in equilibrium with the environmental surfaces at T0. Therefore, the exergetic reference state for a photon gas (black radiation) is its state at temperature T0, and such reference state depends only on the temperature T0 and does not depend on diversified values of emissivities of the environmental bodies. The black emission exergy bb appearing in formula (2.2.33) is always a function only of temperature T of the considered surface and of the environmental temperature T0, bb =f (T,T0). No pressure has to be considered for establishing the exergy reference state for radiation because the pressure of radiation is determined only by the radiation temperature and any pressure of environmental substance does not affect the radiation, which is not a substance. Detailed analyses confirming the independency of radiation exergy on emissivity of the environment, on the configuration of surrounding environment and on the presence of other surfaces of different temperatures, are gathered in the book by Petela (2010). The exergy bb,S (J/m3) of the photon gas enclosed within a system was discussed in Paragraph 2.2.4. However the exergy of the photon gas as being the propagating product of the emitting process, i.e., the exergy bb (W/m2) of emission density of a black surface is: bb = s 3 (3T4 + T4 0 - 4T0T3) (2.2.35) The simplest way of obtaining formula (2.2.35) is multiplication of bb,S by factor c0/4, making bb =c0 · bb,S/4, based on purely geometric considerations and keeping in mind that s =a × c0/4, where c0 is the light velocity. Another way to derive formula (2.2.35) is by application of the exergy definition formula (2.2.3) with interpretation of energy E and entropy S as respective emission eb and its entropy s. However for the confirmation of both ways and for the purpose of the well teaching demonstration of the analysis of radiation processes, an additional way can be Exergy analysis of solar radiation processes 21 Figure 2.2.4 Radiating parallel surfaces. also discussed as follows. Simple derivation of the emission exergy of a black surface, published for the first time by Petela (1961b) in Polish and then repeated in English, Petela (1964), is based on the balance of the emitting surface according to the model shown in Figure 2.2.4. The two surfaces A and A0 which are black, flat, infinite, parallel, facing each other, enclose a space without substance (vacuum) and interchange heat by means of radiation. The model of such two-surfaces-only is often selected for consideration because the space is enclosed by the simplest possible geometry involving only two plane surfaces. Each surface is maintained at uniform and constant temperatures due to exchange of the compensating heat with the respective external heat sources. Surface A0 at temperature T0 represents the emitting environment whereas surface A, at arbitrary temperature T, emits the considered radiation. The simplicity of the model with the black surfaces is that there is no reflected radiation to be considered. In order to derive the formula on the emission exergy density bb of a black surface, the following exergy balance for surface A, is considered: bb0 + bq =bb + db (2.2.36) where the terms in equation (2.2.36) or in Figure 2.2.4, all in W/m2, are: bb, bb0 – exergy of emission density of surfaces A and A0, respectively, bq, bq0 – change in exergy of respective heat source, db, db0 – exergy loss due to irreversibility of simultaneous emission and absorption on the respective surface. From the definition of exergy the radiation of a surface at environment temperature bb0 =0. The change in exergy of heat source, based on formula (2.2.4): bq =q T - T0 T (2.2.37) where q, W/m2, is the heat delivered by the heat source of temperature T. This is the amount of heat which allows surface A to emit and maintain its constant temperature T. This is also the heat exchanged by radiation between surfaces A and A0, which with use of formula (2.2.32) can be calculated from the energy balance of surface A: q=(eb - e0)=s · (T4 - T4 0 ). 22 Solar energy sciences and engineering applications The algebraic overall entropy growth  due to simultaneous emission and absorption of heat taking place at surface A: =-q T + s - s0 (2.2.38) where-q/T is the decrease of entropy of heat source at temperature T, and the entropy s, J/(m2 K), of emission density s= 4 3sT3 (2.2.39) is used for determination of s and -s0, respectively for surface A and A0. The exergy loss db is determined by the Gouya-Stodola law (2.2.10). Making use of (2.2.37), (2.2.38) and (2.2.39) in equation (2.2.36), after some rearranging and using (2.2.34), the formula for the exergy of emission density b, W/m2, of a perfectly gray surface of emissivity e is obtained: b=e s 3 (3T4 + T4 0 - 4T0T3) (2.2.40) The mathematical analysis of formula (2.2.40) (Petela (1964)), reveals first of all that exergy b is always nonnegative and it has the lowest value zero when T =T0. The exergy b reaches also zero if the considered surface is white (i.e. perfectly reflecting, e=0). Keeping in mind formula (2.2.33), it follows that the exergy of emission of black surface (e=1), for the environment temperature approaching absolute zero, becomes equal to the energy of emission: lim T0.0 (bb)=sT4 =eb (2.2.41) It could be noticed that the characteristic term in brackets of formula (2.2.31), appearing also in formula (2.2.40) was derived by Petela (1964) from the consideration without using the Stefan-Boltzmann law (2.2.32). The obtained equations (2.2.31) as well as (2.2.40) can be recognized as independent of equation (2.2.32). Therefore, the energy of emission e can be interpreted as the particular case of the exergy of this emission at the theoretical condition T0 =0, or other words, the Stefan-Boltzmann law is a particular case of the emission exergy law expressed by formula (2.2.31). As the surface temperature T approaches absolute zero the exergy of emission expressed by formula (2.2.40) approaches the finite value: lim T.0 (b)=e s 3 T4 o (2.2.42) Based on equation (2.2.40), Figure 2.2.5 illustrates the exergy bb (solid thick line) of emission density of a black surface (e=1) at the constant value of the environment temperature T0 =300 K. For comparison, the energy eb (solid thin line) of emission density according to equation (2.2.32) is presented. For a sufficiently small temperature T the exergy of black emission is larger than energy of such emission. The dashed line Exergy analysis of solar radiation processes 23 Figure 2.2.5 Emission eb, exergy bb, difference (eb - bb), and the exergy-energy ratio . as function of surface temperature T, at T0 =300 K. in Figure 2.2.5 shows the difference (eb - bb) which with the growing temperature T from the negative values grows indefinitely. The emission exergy of a surface of temperature lower than the environment temperature (so-called cold radiation), the effects of varying environmental temperature and the suggestions regarding calculation of radiation exergy of a surface of non-uniform temperature, are discussed in Petela (2010). A real energy conversion efficiency .E of thermal radiation into work can be defined as the ratio of work W, performed due to utilization of the radiation, to the energy e of this radiation: .E = W e (2.2.43) The maximum work Wmax can be obtained from radiation energy in the ideal (reversible) conversion process. Such work in an ideal process is equal to the exergy of the radiation; Wmax =b, and its efficiency .E changes to the maximum conversion efficiency .E,max called exergy/energy radiation ratio ., defined for the first time by Petela (1964): b e =.E,max =. (2.2.44) which can be larger (.>1), equal (.=1), or smaller than unity (.<1), (Fig. 2.2.5). 24 Solar energy sciences and engineering applications If the emission density eb from formula (2.2.32) and exergy bb of emission density from formula (2.2.35) are used in (2.2.44) then: .=1 + 1 3  T0 T 4 - 4 3 T0 T (2.2.45) where T is the temperature of the considered radiation. This characteristic ratio . in thermodynamics of radiation has a significance similar to that of the Carnot efficiency for heat engines. The ratio . represents the relative potential of maximum energy available from radiation. However, the real exergy conversion efficiency .B of thermal radiation into work can be defined as the ratio of the work W, performed due to utilization of the radiation, to the exergy b of this radiation: .B = W b (2.2.46) Introducing (2.2.43) to (2.2.46) to eliminate the workW, and then using equation (2.2.44) to eliminate the exergy b, one finds that the real exergy efficiency of conversion of radiation exergy to work is equal to the ratio of the real and the maximum energy efficiencies: .B = .E .E,max =1 (2.2.47) Using equation (2.2.44) in (2.2.47) to eliminate .E,max, the ratio . becomes also the ratio of energy-to-exergy efficiency of the radiation conversion to work: .= .E .B (2.2.48) Figure 2.2.5 presents the example of the ratio . (dotted line) for T0 =300 K.With the growing temperature T from zero to infinity the value . decreases from infinity to the minimum value zero for T =T0 and then increases to unity. However in spite of . approaching unity for infinite temperature T, the difference (eb - bb) does not approach the expected zero but approaches infinity. Although the . is not defined as efficiency, it can be recognized like an efficiency of a maximum theoretical conversion of radiation energy to radiation exergy. For example, for any arbitrary radiation of the known energy and at certain presumable temperature T, the exergy of this radiation could be approximately determined as the product of the considered energy and the value . taken from (2.2.45) for this temperature T. Example 2.2.5.1 The value .=0.2083 for a black emission at temperature T = 473K (200 C) is calculated from formula (2.2.45) at T0 =300 K. In paragraph 2.2.6, example 2.2.6.1, the .wv value for water vapor at T =473K and T0 =300K is calculated based on the radiation spectrum as .wv =0.185. The smaller value of ratio .wv for water vapor, in comparison to black surface radiation (.wv<.) results from a significant difference in spectra of the water vapor and black surface. However, Exergy analysis of solar radiation processes 25 in paragraph 2.4.1.1, example 2.4.1.2, for solar radiation, the difference between calculated .S =0.9326 for the considered solar spectrum and the value .=0.9333, (example 2.4.1.1), for black surface at 6000 K is insignificantly smaller because the solar spectrum is not much different from the black surface spectrum. 2.2.6 Exergy of radiation flux In paragraph 2.2.4 the energy u, J/m3, of trapped radiation residing within a space is discussed. Radiation emission density e, W/m2, of a surface at known temperature is discussed in paragraph 2.2.5. However generally, the radiation flux propagating in space can consist of many emissions from unknown surfaces and of unknown temperatures. Such radiation energy flux, discussed in the present paragraph, can be categorized as a radiosity j, W/m2, of an arbitrary radiation flux of an arbitrary energy spectrum which can be determined, for example, from measurement. Now we know that there are many different methods for derivation of the general formula for exergy of the arbitrary flux of radiation. Each method leads to the same result which could be also achieved simply e.g. by interpretation of equation (2.2.3) as shown by Petela (2010): E.j=  ß  .  . (ib,0,.,max + ib,0,.,min)Td2Cd. (2.2.49) E0.j0 =  ß  .  . (ib,0,.,max + ib,0,.,min)T0 d2Cd. (2.2.50) S.sj =  ß  .  . (Lb,0,.,max + Lb,0,.,min)Td2Cd. (2.2.51) S0.sj,0 =  ß  .  . (Lb,0,.,max + Lb,0,.,min)T0d2Cd. (2.2.52) where: j, j0 – radiosity density of considered radiation and environment, W/m2, sj, sj,0 – entropy of radiosity density of considered radiation and environment, W/(m2 K), T, T0 – absolute temperature of radiating surface and the environment, K. The monochromatic normal directional intensity ib,0,., W/(m2 sr), for linearly polarized black radiation propagating within a unit solid angle and dependent on wavelength ., was established by Planck (1914): ib,0,. = c2 0h .5 1 exp  c0h k.T  - 1 (2.2.53) 26 Solar energy sciences and engineering applications Figure 2.2.6 Geometry scheme for radiation flux (from Petela, 1962). where: c0 =2.9979 · 108 m/s is the speed of propagation of radiation in vacuum, h=6.625 · 10-34 J s is the Planck constant, k=1.3805 · 10-23 J/K is the Boltzmann constant. The entropy Lb,0,., W/(m3 Ksr), of monochromatic directional normal radiation intensity and for linearly polarized black radiation propagating within a unit solid angle and dependent on wavelength . according to Planck (1914) is: Lb,0,. = c0k .4 [(1 + Y) ln(1 + Y) - Y ln Y] where Y = .5ib,0,. c2 0h (2.2.54) The total energy or entropy of radiation is respectively the same regardless whether the spectrum is expressed as function of the wavelength . or frequency .. Therefore, e.g.: 8 0 ib,0,. d.= 8 0 ib,0,. d. (2.2.55) To apply such recalculation formula (2.2.55), the formula (2.2.53) for ib,0,. has to be used together with relation . · .=c0. Based on such a possibility, the formulae on radiation exergy can also be presented as functions of the wavelength . or frequency .. It is assumed that the considered elemental flux propagates between any two control elementary surface areas dA and dA separated by distance R, in a direction determined by the flat angles of ß (called declination) and f (called azimuth), as shown in Figure 2.2.6. The solid angle of propagation d.=dA /R2 =sin ß · dß · d. and the abbreviation: d2C =cos ß sin ß dß d. (2.2.56) Exergy analysis of solar radiation processes 27 used, e.g., in the case of surface radiating to the forward hemisphere is:  ß  . d2C = ß=p/2 ß=0 .=2p .=0 cos ß sin ß dß d.=p (2.2.57) Exergy of arbitrary polarized radiation. The exergy bA , W/m2, of the arbitrary polarized radiation originating from unknown surface A and arriving in point P of the considered surface A per unit time and unit absorbing surface area, can be interpreted in equation (2.2.3) as B=bA . Developing the whole equation (2.2.3) by including also interpretation (2.2.49) – (2.2.52), after rearranging, it yields: bA =  ß  .  . (i0,.,min + i0,.,max) cos ß sin ß dß d. d. -  ß  .  . (L0,.,min + L0,.,max) cos ß sin ß dß d. d. + sT4 0 3p  ß  . cos ß sin ß dß d. (2.2.58) In order to utilize formula (2.2.58) one has to know the solid angle . within which the surface A is seen from point P on surface A, and to know (e.g. from measurements) i0,.,min and i0,.,max as a function of frequency . and direction defined by ß and f. The total exergy BA.A of the considered arbitrary radiation arriving to the all points of surface A is calculated as: BA.A =  A bA dA (2.2.59) Exergy of arbitrary non-polarized radiation. The formula for such radiation is obtained after taking into account in formula (2.2.58), that for a non-polarized radiation i0,.,max =i0,.,min, thus i0,.,max + i0,.,min =2 · i0,.. Additionally also L0,.,max =L0,.,min thus L0,.,max + L0,.,min =2 · L0,.. bA = 2  ß  .  . i0,. cos ß sin ß dß d. d. - 2  ß  .  . L0,. cos ß sin ß dß d. d. + sT4 0 3p  ß  . cos ß sin ß dß d. (2.2.60) In order to utilize formula (2.2.60) one has to know the solid angle . within which the surface A is seen from point P on surface A, and to know i0,. as function of frequency . and direction defined by ß and .. Formulae (2.2.59) can be also useful. 28 Solar energy sciences and engineering applications Exergy of arbitrary, non-polarized and uniform radiation. The formula for such radiation results from (2.2.60) in which i0,. (and L0,v) does not depend on angles ß and .: bA = . .2  . i0,.d. - 2T0  . L0,.d. + sT4 0 3p . .  ß  . cos ß sin ß dß d. (2.2.61) and to utilize formula (2.2.61) the solid angle . within which the surface A is seen from point P on surface A, as well as the radiation spectrum as function of frequency, i0,.(.), (e.g. determined by measurement), has to be known. Again, the formulae (2.2.59) can be applied. Exergy of arbitrary, non-polarized and uniform radiation propagating within solid angle 2p. The formula for such radiation is derived by substituting equations (2.2.57) into (2.2.61): b=2p  . i0,.d. - 2pT0  . L0,.d. + s 3 T4 0 (2.2.62) To utilize formula (2.2.62) the function i0,.(.), has to be known. The total exergy of the considered radiation arriving to the all points of the surface A is calculated as follows: B=bA (2.2.63) Example 2.2.6.1 Figure 2.2.7 shows the measured monochromatic normal radiation intensity i0,., (solid line) of radiation, as a function of wavelength ., for the water vapor layer of equivalent thickness 1.04 m at temperature 200.C according to Jacob (1957). The product of the thickness and the partial pressure for the vapor is 10.4mkPa. The monochromatic normal intensity ib,0,. for black radiation, calculated from equation (2.2.53), is also shown for comparison (dashed line). For approximate calculation, instead of the surface area under the solid line, the area of seven rectangles (dotted line) is taken into account as the integral energy emitted by the vapor upon the hemispherical enclosure. The areas of these rectangles can be recognized as the absorption bands of width . spread symmetrically on both side of wavelength ., of which values are given in Table 2.2.1. Exergy of radiation arriving in 1m2 of the enclosing hemispherical wall can be calculated from formula (2.2.62), in which frequency, with interpretation explained by formula (2.2.55), is eliminated by wavelength and each integral can be replaced by the sum of appropriate products: b= s 3 T4 0 + 2p i0 .. - 2pT0 L0 .. (2.2.64) Exergy analysis of solar radiation processes 29 Figure 2.2.7 Radiation of water vapor layer of thickness 1.04m at temperature 473.15 K and pressure 0.1 MPa (from Jacob, 1957). Table 2.2.1 Radiation data for water vapor layer of thickness 1.04m at temperature 473.15 K and pressure 0.1 MPa. Successive rectangle number . . i0,.× 10-6 i0,.×. L0,.×10-4 L0,.×. µm W m3 sr W m2 K sr 1234567 2.69 6.15 7.95 9.8 14.8 21.0 26.8 0.66 2.8 0.8 2.9 7.1 5.3 6.3 5.0 45.7 17.2 3.7 6.4 5.1 2.2 3.3 128.0 13.8 10.7 45.4 27.0 13.9 1.07 11.72 5.41 1.56 2.38 1.83 0.78 0.0071 0.3282 0.0433 0.0452 0.1690 0.0970 0.0491 Total 242.1 – 0.7389 For the assumed temperature T0 =300K formula (2.2.64) yields: b = 5.6693 × 10-8 3 3004 + 2p × 0.2421 - 2p × 300 × 0.7389 × 10-3 = 0.153 + 1.521 - 1.393=0.281kW/m2 The ratio of the exergy of radiation of the vapor to its energy emission is .wv = b/e=0.281/1.521=0.185. More details of the considered example are discussed by Petela (1961a). 30 Solar energy sciences and engineering applications Exergy of arbitrary, non-polarized, black and uniform radiation, propagating within solid angle 2p. The formula for such radiation is derived from (2.2.62) in which formulae (2.2.53) and (2.2.54) are used with the interpretation explained by formula (2.2.55): bb = s 3 (3T4 + T4 0 - 4T0T3) (2.2.65) To utilize formula (2.2.65) only the temperature of the black radiation is required. The total exergy arriving at surface A can be calculated from formula (2.2.63). It is noteworthy that equation (2.2.65) is identical to equation (2.2.40) derived for the black emission. This similarity is a confirmation that exergy of black radiation is equal to the exergy of emission of black surface. It is the consequence of the radiosity of a black body being equal to its emission. Exergy of non-polarized, uniform, black radiation propagating within solid angle .. The formula for such radiation is: bb. = bb p  ß  . cos ß sin ß dß d. (2.2.66) where solid angle . has to be determined by the appropriate ranges of variation of the flat angles ß (declination) and f (azimuth). The magnitude . is defined as the ratio of exergy and energy of the same radiation. Both the exergy and the energy are functions of state, thus they do not depend on any geometrical configuration parameters. The angles ß and . do not have any geometrical meaning but are only the coordinates determining the solid angle in which the spectrum is considered. For any arbitrary radiation the ratio . could be defined as the ratio of exergy determined by formula (2.2.58) to the first term of the right hand side of this formula which represents the radiation energy. Thus for a polarized radiation: .=1 - sT4 0 3p  ß  . cos ß sin ß dß d. -  ß  .  . (L0,.,min + L0,.,max) cos ß sin ß dß d.  ß  .  . (i0,.,min + i0,.,max) cos ß sin ß dß d. (2.2.67) The exemplary values of . are discussed in this book in examples 2.2.6.1 (for non-polarized, uniform water vapor radiation propagating within a solid angle 2p), and in example 2.4.1.2 (for non-polarized and uniform solar radiation). Exergy analysis of solar radiation processes 31 2.3 THERMODYNAMIC ANALYSIS 2.3.1 Significance of thermodynamic analysis The thermodynamic analysis is the method which can be applied to the examining of any energy conversion phenomenon. In the first step the analysis develops the conservation equations of mass and energy, based on the First Law of thermodynamics, which allow for the traditional energy analysis of the process. Next, the examining can apply the Second Law of thermodynamics, which allows for the entropic evaluation of the process irreversibility, and then it applies the crowning of the whole thermodynamic analysis with exergy analysis. Thermodynamic analysis based on developed balance equations for mass, energy, exergy and on the entropy growth equations, provides different (energy, entropy and exergy) views of the same phenomenon in terms of engineering quantity, probability and quality, respectively. An energy balance, (based on the First Law of Thermodynamics), is developed to better understand any process, to facilitate design, operation and control, to point at the needs for process improvement, and to enable eventual optimization. The degree of perfection of energy utilization in the process, or its particular parts, allows for comparing the degree of perfection, and the related process parameters, to those in other respective processes. Comparison with the currently achievable values in the most efficient systems is especially important. Also the priorities for the required optimization attempts for the systems, or its components, can be established. Such establishing can be carried out either based on the excessive energy consumptions or on the particularly low degree of perfection. Entropy analysis, (based on the Second Law of Thermodynamics), requires the complete data obtained from mass and energy considerations to allow for developing entropy relations to verify the correctness of a mathematical model of mass and energy results. The analysis allows for identification and location of the sources of irreversibility contributing to the overall unavoidable degradation of energy. Entropy can be used for process optimization by minimization of entropy generation. However the entropy has limited application for micro systems containing a denumerable number of independent particles. The smaller the number of particles, the less precisely the Second Law is fulfilled. For example for any microbiological system containing only a few components the Second Law may not be fulfilled. Exergy is the concept derived from joint application of the First and Second Laws of Thermodynamics. Exergy balance is developed according to a similar methodology as for energy analysis, and with the same purposes. Whereas thermodynamic probability is expressed in units of entropy, exergy is expressed in energy units. Consequently exergy data are more practical and realistic in comparison to the respective energy values. Thus, the exergy analysis provides a more realistic view of process, which sometimes dramatically differs in comparison to the standard energy analyses. Exergy analysis can be compared to the energy analysis like the second different projection in a technical drawing disclosing additional details of the subject seen from a different side. The knowledge about nature is continually studied with many methods and observations. The scale of approach may be microscopic (e.g. a microscopic observation or differential calculus) or macroscopic (phenomenological considerations or integral calculus). Usually the studies are organized by focusing attention on the particular 32 Solar energy sciences and engineering applications system appropriately representing the aimed problem. Description and definition of the system is then a very important stage in the investigating approach. However, any analysis not based on the precisely defined system can lead to astonishing but incorrect results. The system has to be precisely determined by separating precisely the elements included from those excluded. This is usually effectively rendered by applying an imaginary system boundary which tangibly separates the system from its surroundings. The best practical way is to draw a scheme of contents of the system indisputably separated from surroundings by the drawn system boundary. Sometimes the investigated problem can be easily solved by introducing sub-systems, also precisely defined. Each balance equation allows for determination of an unknown variable or for establishing a relation between variables. The radiation processes accompanying processing on substances can be nonnegligible and often the systems in which radiation and substance play roles together, have to be considered. As discussed, the variables obtained from mass and energy analyses are very important, thus they have to be carefully prepared. The variables can be measured, assumed or calculated. If the system is over-determined; i.e. the number of unknowns is smaller than the number of available independent equations, then all the variables can be corrected based on the probability reconciliation calculus e.g. like that discussed for radiation by Petela (2010). 2.3.2 Energy balance equations The energy conservation equation is based on the substance balance equation. The principle of conservation of substance claims that the number of molecules in physical processes is constant, or a number of elements in chemical processes or a number of nucleons in the processes of split and synthesis of nuclei is constant. The substance conservation equation does not need to account for radiation or any other form of matter except for substance. Such an equation is developed for the system defined precisely by the system boundary. For the elementary process lasting a very short period: dmin =dmS + dmout (2.3.1) where min and mout, kg, is the elementary amount of substance respectively delivered and extracted from the system, and mS is the elementary increase of amount of substance within the considered system. The equation (2.3.1) can be appropriately modified for steady state (dmS =0), or for a certain instant with use of mass flow rates, or for a certain period of time. The equation can be separately applied for particular compounds (if there is no chemical reaction) or elements. The amount unit can be kg, kmol or the standard m3 of the considered component. A particular form of the substance conservation equation can be e.g. the equation summarizing fractions of components in a considered composite material: fi =1, where fi is the fraction of the i-th component of the material. The energy conservation equation is the result of observations and cannot be proved or derived. From a long view of the history of mankind there are no phenomena recorded occurring in disagreement with the First Law of Thermodynamics. Exergy analysis of solar radiation processes 33 Energy balance is the basic method for solving problems of thermodynamics. If sometimes one wants to start analysis of any problem but does not know how, the general advice is to try to make an energy balance of a system which would represent the problem subject. The energy balance can be applied to diversified problems which, however, require an appropriately well defined system for consideration. The system boundary should be the same for energy and substance balances because the substance balance is the basis for balance of energy. Sometimes only the specific definition of the system and particular tracing of the system boundary allows for the solution of the thermodynamic problem. In other cases the solution can be obtained by defining more different sub-systems. Generally, the energy Ein delivered to system remains partly within the system as the increase ES of the system energy, and the rest is the energy Eout leaving the system. Thus, the general equation of energy balance is: Ein =ES + Eout (2.3.2) Usually, for better illustration of the balance equation, the particular terms of the equation are shown in the bands diagram. The principle of such a diagram is shown by a simple example (Fig. 2.3.1), illustrating equation (2.3.2). In principle, for energy considerations, the reference state for calculation of energy of the matters included in consideration can be defined arbitrarily; however, it is recommended to select this reference as for the exergy consideration, to make fair comparison of both, energy and exergy viewpoints. Generally, application of an energy balance does not require analyzing of processes occurring within the system boundary. It is sufficient only to know (e.g. from measurements) the parameters determining components of the energy delivered and leaving the system as well as to know the parameters determining the initial and final state of the system. Obviously, if only the one unknown magnitude appears in the balance equation then the equation can be used to calculate this magnitude. The energy balance can be differently tailored depending on the considered viewpoint and actual conditions. For example, there are some possibilities to categorize the case under consideration as: a) energy delivered is spent entirely for an increase of system energy at no energy leaving the system, b) energy leaving system comes entirely from the decrease of energy of system at no energy delivered to system, c) there is neither delivered nor leaving energy but only energy exchange (e.g. by work or heat) within the system, d) energy delivered is equal to energy leaving the system at no change of the system energy. Other possibilities are that some components of energy can be neglected either due to relatively small changes, or because they are unchanged at all. The balance equation can be written for the steady or transient systems, for the system considered on macro scale or micro scale for which differential equations are applied, etc. For example, for the elemental process lasting an infinitely short time, the balance equation (2.3.2) can take the form: dEin =dES + dEout (2.3.3) 34 Solar energy sciences and engineering applications In equation (2.3.3) only dES is the total differential and in order to demonstrate it clearly it is better to write equation (2.3.3) as follows: .E in(t)dt =dES + .E out(t)dt (2.3.4) where .E in and .E out are the respective fluxes (e.g. in W) of energy delivered and extracted from the system and t is the time. Determination of dES requires not only accounting for the change of intensive parameters of the system state but also on the eventual change in the amount of matter in the system. If, e.g. the considered system contains only a homogeneous substance then: dES =d(mSeS)=mSdeS + eSdmS (2.3.5) where mS and eS are, respectively, the amount of substance and its specific energy contained within the system. Sometimes the subject of consideration can be recognized as moving in space (e.g. solar vehicle, radiometer vane). Then the simplest energy balance equation is obtained by assuming that the coordinates system determining velocity and location is moving together with the system boundary. However there are some consequences of such an assumption. Kinetic energy should be determined for the velocity relative to the moving system. The useful work done by the system does not appear in the energy balance because the forces acting on the system do not make replacements relative to the coordinates system. The useful work can be determined only for velocity and location relative to earth. The components of an energy balance equation are the energy of the system and energy exchanged with the system. Energy of system (ES) depends on its state. An increase ES of an energy system, changing from its initial to final state, does not depend on the means of change between these states and is a difference of final ES,fin and initial ES,inl energy of the system: ES =ES, fin - ES, inl (2.3.6) Generally, the system energy can consist of macroscopic components like Emacr,i due to velocity (kinetic energy), surface tension (surface energy), gravity (potential energy), or any other energy of field nature (e.g. radiation). The remaining part of the system energy, containing microscopic components Uj, constitutes the internal energy: ES = i Emacri + j Uj (2.3.7) where i and j are the successive numbers of the macro and micro components, respectively, of the system energy. If the kind of substance, before and after process (e.g. physical process), is the same, then the reference state for calculation of the energy of the substance can be established with a certain degree of freedom. For example, the reference state can be assumed as the state of the substance entering the system. Thus, the substance energy entering the system is zero whereas the energy of this substance exiting the system is Exergy analysis of solar radiation processes 35 equal to the energy surplus relative to the reference state. This surplus has to account for the latent heat of eventual change of the substance phase. In addition, the components of negligible or constant value must not be taken into account. For example, the energy of surface tension can be included only in consideration of a fluid mechanics process of liquid atomization or of a mechanical process of solid material comminution. Both processes have been analyzed by Petela (1984a,b). Energy exchange (Ein and Eout) with system can occur on different ways. Electrical energy can be delivered for heating the system, for driving an electric motor, or generating an electromagnetic effect within the system (e.g. a strong electric field affects combustion). In reverse processes the electric energy can be obtained, e.g. with use of an electric generator the energy is obtained from the system. The energy flux of electric energy (power) is measured by a wattmeter. Mechanical work can be exchanged with a system by means of a piston rod of reciprocal motion or with a rotating shaft. The energy balance of a system should comprise the mechanical work performed by all forces acting on system boundary. Therefore, if a substance flux passes through the boundary then the work performed by the force acting in the place of passing should be taken into account. Such work of transportation of substance through the boundary is expressed by enthalpy. For some kind of substance the enthalpy can be calculated with specific formulae, e.g., formulae for plasma are discussed by Petela and Piotrowicz (1977). If the considered system is moving relative to the coordinate system determining the location and velocity, then the work done by the forces causing the system displacement, has to be accounted for. The energy balance should also include the work done by deformation of the system boundary if its shape changes during consideration. Kinetic energy should be accounted if substance passes the system boundary with significant velocity relative to the boundary. Potential energy of a substance exchanged with a system is included in the energy balance if the substance has significant elevation above the reference level. This energy component results from the presence of the gravity field. Energy transferred by heat occurs by direct contact of system with body at a temperature different from the system temperature, or can occur without contact by radiation. The effect of contact during heat exchange appears in heat conduction as well as in heat convection. The model of pure conduction occurs when the particles of the contacted body do not change their location (solids). The energy is then transferred by free electrons and oscillations of atoms in the crystal lattice. Still pseudo pure conduction can be recognized between fluids of very laminar flow; conduction occurs in the direction perpendicular to the ordered motion of particles at component velocity only in the flow direction. In such cases, excluding the possibility of diffusion, there is no perpendicular substance flow and in spite of the medium flow this heat is transferred by conduction. The essence of heat convection is the motion of substance (fluids), during which mixing of hot and cold fluids occur. However, the micro-mechanism of this mode of heat transfer, also depends on direct effective contacts (conduction) between the hot and cold fluids portions being replaced. If mixing is caused by non-uniform distribution of density (temperature profile) then convection is called natural convection. 36 Solar energy sciences and engineering applications In contrary, if the mixing is a result of acting of pump or ventilator, etc., then a forced convection occurs. Energy can be also exchanged with the system due to a diffusive substance flux. Then, the enthalpy of the diffusing substance has to be taken into account. For example, consider a system boundary demarcated over the laminar zone of a mixture of gases of a non-uniform temperature distribution. If it is assumed to be a laminar (no convection) mode of transparent gases (no radiation), then the energy EL, W, exchanged through the boundary due to the heat conduction and enthalpy of diffusing substance, is composed of two respective terms. The first term represents the heat conducted according to Fourier’s law and the second term expresses enthalpy of diffusing gas according to Fick’s law. Thus: EL=-A  k .T .y + T cp iDL i .ci .y  (2.3.8) where A, m2, is the surface area, k, W/(m K), is the overall conductivity of gas mixture, T, K, is the temperature of the gas at the boundary, y, m, is the space coordinate perpendicular to the system boundary surface and perpendicular to the gas flow direction, cp,i, J/(kg K), Di, m2/s, and ci, kg/m3, are respectively the specific heat at constant pressure, the laminar diffusion coefficient, and the concentration of gas component, where i is the successive number of the gas mixture component. Real processes occur with friction on which a friction work has to be spent. The friction work increases the energy of system due to absorption of heat in amount equivalent to friction work. Friction causes dissipation of energy which can be only partly recovered. The friction heat does not appear as a member of the energy balance equation; however it affects the final system energy and the components of exiting energy. Chemical energy is assumed to be the same for the substance considered as the component of the system and for the substance component separately exchanged with the system. The enthalpy and internal energy include generally physical and chemical components. 2.3.3 Exergy balance equations The exergy balance equation is the basis of the exergetic part of thermodynamic analysis. Exergy analysis can be applied to diversified problems which, however, like the energy analysis, require an appropriately well-defined system for the analysis. The system boundary should be the same as for the matter balance. The exergy conservation equation can be applied only to reversibly occurring processes. For real processes the exergy conservation equation is fulfilled only when the unavoidable exergy loss, due to irreversibility of the process, is taken into account. Thus, corresponding to energy equation (2.3.2), the following traditional exergy balance equation is applied: Bin =BS + Bout + dB (2.3.9) Exergy analysis of solar radiation processes 37 Figure 2.3.1 Bands diagram of energy balance. Figure 2.3.2 Bands diagram of the traditional exergy balance. where Bin and Bout are the respective sum of exergy delivered and released from the system, BS is the change in the exergy of the system and dB is the exergy loss due to the process irreversibility, calculated from the Guoy-Stodola law, equation (2.2.10). The bands diagram for exergy balance is shown in Figure 2.3.2. In comparison with the respective diagram for energy balance (Fig. 2.3.1), the exergy diagram shows the exergy dB disappearing within the system. Like the energy balance, the exergy balance can be differently tailored depending on the considered problem and actual conditions. For example, some components of exergy can be neglected either due to relatively small changes, or because they are unchanged at all. The balance equation can be written for steady or transient systems, for system considered on the macro scale or micro scale using differential equations, etc. Obviously, for calculation of exergies, there is no freedom in defining the reference state, which is only the environment, as determined by exergy definition. For any elemental process lasting an infinitely short time, the exergy balance equation can take the form: .B indt =dBS + .B outdt + dB (2.3.10) where .Bin and .B out are the respective fluxes of exergy delivered and extracted from the system and dBS is the total differential exergy growth of the system. 38 Solar energy sciences and engineering applications The differential dBS should be determined analogously to equation (2.3.5). If the subject of consideration is moving in space, then the simplest exergy balance equation is obtained by assuming, as for the energy balance, that the coordinates system determining velocity and location is moving together with the system boundary. An increase .BS of the exergy system, changing from its initial to final state, does not depend on the method of change between these states and is equal to the difference of final and initial exergy components: BS = . . i (B)fin,i - j (B)inl,j . . S (2.3.11) where the sum of initial or the sum of the final components is: (B)S =Bk + Bp + BS + Bb +· · · (2.3.12) and where i and j are the successive numbers of the final and initial (respectively) exergy components, Bk is the kinetic exergy, Bp is potential exergy, BS is the thermal exergy of the system calculated with use of formula (2.2.13), and Bb is the exergy of photon gas (black radiation) calculated for example based on equation (2.2.31). Also the other eventual components in equation (2.3.12), as shown in Figure 2.2.1, can be added if necessary, e.g. exergy of surface tension which is equal to the energy of surface tension, etc. Exergy fluxes (Bin and Bout) exchanged with system can occur on different ways described for the energy balance. Electrical exergy is equal to electrical energy. Exergy of mechanical work is equal to work. Exergy of substance flux is calculated with use of formula (2.2.12), however kinetic exergy should be taken separately, (calculated as the kinetic energy for absolute velocity), and potential exergy (equal to potential energy relative to the Earth surface level). Exergy of heat exchanged with the system is determined by formula (2.2.4). Exergy can be exchanged with the system also due to a diffusive substance flux. Then, the exergies of diffusing substances is taken into account as the exergy determined by formula (2.2.12) interpreted for the partial pressure of the substances. Any internal exergy loss dBF caused by friction is determined e.g., by assumption that the friction heat QF, equal to the friction work, is entirely absorbed by the substance at temperature T. For the heat absorption process, assuming the entropy growth F =QF/T, the exergy loss can be calculated from formula (2.2.10) as follows: dBF =QF T0 T (2.3.13) The exergy loss by friction is the smaller the higher is the temperature T of absorbing substance. The exergy loss dBF can be smaller or larger than the friction heat QF dependently on the temperature ratio T0/T. This observation is particularly important for refrigerating processes where often T 0), (from Petela, 2010). the environment parameters could then be helpful. Fluctuation of environment parameters is potentially one of the natural low-value resources, like e.g., a waste heat at very low temperature. A particular problem is a variation of the effective sky temperature, discussed by Duffie and Beckman (1974), which determines the radiant heat lost from the Earth’s surface. The fluctuation of environment parameters has relatively insignificant influence on the high-values natural resources, e.g. natural fuels. It is also possible to consider the variation of environment parameters with altitude and this effect is taken into account in consideration of the concept of mechanical exergy, discussed in paragraph 2.2.3.2. However, application of mechanical exergy (eZergy) leads to the exergy balance with gravity input term. Petela (2009a) proposed to insert an appropriate term, called gravity input G, as an additional exergy input in the left hand side of the exergy balance equation. Thus equation (2.3.9) for the constant environment parameters becomes: Bin + G=BS + Bout + dB (2.3.15) The gravity input G can be positive, zero or negative. The bands diagram for the exergy balance with included positive gravity input is shown in Figure 2.3.4. The value of G is calculated from exergy balance equation. The gravity input G can appear only when a substance is considered in the exergy balance and if eZergy is applied to the substance. The interpretation of the algebraic sign of G from exergy viewpoint could be proposed as follows. In the case G<0; due to the effect of a gravity field on the considered process, the process product expressed by the total exergy value of the right hand side of the exergy balance equation diminishes and has to be balanced by the negative gravity input G added to the left hand side of the equation. The considered process can be recognized as opposing the effect of the gravity field. In the case of G>0; the presence of a gravity field during the considered process generates a certain “surplus’’ of exergy disclosed by the right hand side of the exergy balance equation. This surplus has to be balanced by a positive gravity input G added to the left hand side of the equation. The gravity field favors the process by contributing with some exergy input. Exergy analysis of solar radiation processes 41 Figure 2.3.4 Bands diagram of exergy balance interpretation including gravity input in case G>0, (from Petela, 2010). In case of G=0; there is no change in the traditional exergy, and it means that the work of substance during theoretical expansion at altitude H (considered in paragraph 2.2.3.2) to obtain the equilibrium of densities has no accountable importance. The exemplary calculation and analysis including gravity input in the case of the waste combustion products in a chimney were carried out by Petela (2009b). The other example of calculation of gravity input, for adiabatic expansion of air in a turbine and for drawing air through a throttling valve followed by a fan are discussed by Petela (2009a). Further application of gravity input interpretation is also discussed in paragraph 2.4.3. 2.3.4 Process efficiency 2.3.4.1 Carnot efficiency By observation of nature we know that the continuous generation of a useful effect (e.g. work or heat) or conversion of energy is possible only in a situation when at least two heat sources of different temperatures are available. Such situation can then be utilized in an installation in which a fluid (e.g. gas, liquid or photon gas) is circulating and cyclically exchanging heat and performing work. In a search for the best effectiveness of the cycle process, which would occur reversibly without any losses, the ideal model was established by Carnot (1824). The real cycles could be then designed possibly close to the model by application of different “carnotization’’ attempts. The model cycle should consist of only ideal (reversible) processes. Thus the cycle processes of releasing and absorbing heat should occur reversibly (at an infinitely small temperature difference between the heat source and circulating fluid) and the flow of fluid should be frictionless. The other cycle processes, during which work is generated or consumed, should also occur reversibly (at constant entropy) which is possible if the fluid does not exchange heat (adiabatic) with the surroundings when with no friction (isentropic) it expands or is compressed. Based on the energy conservation law the net work W performed in the Carnot cycle is equal to heat absorbed QI and released QII by fluid: W =QI -QII . The 42 Solar energy sciences and engineering applications overall entropy growth for such a reversible cycle is zero and takes into account only entropies of exchanged heat at the respective heat source temperatures TI and TII : 0=QII/TII -QI/TI . The efficiency .C of the considered Carnot cycle is the ratio of work W to the cycle input QI ; .C =W/QI , which is: .C =1 - TII TI (2.3.16) The commonly called Carnot efficiency is in fact the efficiency of the Carnot cycle and is the most important efficiency in thermodynamics. All other defined efficiencies are less general, mostly arbitrary or specifically adjusted to the objects or situations. One of the most significant properties of the Carnot efficiency is that it is valid independently of the nature of the working fluid and can be applied to any material or field matter used as the working fluid. The Carnot efficiency can be used as a reference value for calculation of exergy efficiency of a thermal engine. The energetic and exergetic efficiency of an engine are respectively .E,eng =W/QI and .B,eng =W/BQI. Based on formulae (2.3.16) and (2.2.4) the ratio of energetic and Carnot efficiencies is: .B,eng = .E,eng .C (2.3.17) The exergy efficiency .B,eng of engine demonstrates how much the real energy efficiency departs from the ideal efficiency represented by the Carnot efficiency. In the ideal case (.E,eng =.C) the exergy efficiency approach 100%. 2.3.4.2 Perfection degree of process Practically, process efficiency can be defined in different ways. For example, energy or exergy can be used for expressing the numerator and denominator of the efficiency. However, the best method for reviewing the process seems to be the application of the degree of perfection recommended by Szargut et al. (1988) for measuring the thermodynamic perfection of process. The energy and exergy degrees of perfection are defined analogously for convenient comparison. To determine the degree of perfection, all terms of the energy (or exergy) balance equation are categorized either as useful product, or process feeding, or loss. The perfection degree is then defined as the ratio of useful product to the process feeding. The loss is not disclosed in the perfection degree formula because it is a compensation of the perfection degree to 100%. As was discussed in paragraph 2.2.2, the exergy losses can be internal and external. The energy balance can disclose only energy external loss, whereas the exergy balance can contain the terms of the external and internal exergy losses. Internal exergy loss is calculated from the Guoy-Stodola law. External loss is equal to the energetic or exergetic value of the unavoidably released waste heat or matter which however, can be somehow utilized beyond the considered system in an additional process of “waste recovery’’. In multi-processes systems, in contrary to external losses, only the internal exergy losses can be summed. Exergy analysis of solar radiation processes 43 The concept of perfection degree can include also exergy change due to the varying of environment parameters and the specific terms (e.g. gravity input). Thus, in the modified version it can be proposed that the denominator of the degree of perfection represents the feeding terms, gravity input and exergy change due to the environment variation, whereas the numerator expresses the useful products. For example, for the steady process in which numerous fluxes of energy are exchanged the exergy degree .B of perfection could be proposed as follows: .B = i Buse,i + k BQ,use,k +Wuse j Bfeed,j + m BQ,feed,m +Wfeed + G - Be (2.3.18) where i is the number of useful exergy fluxes Buse obtained from the process, including substance and radiation, k is the number of useful exergy fluxes BQ,use of heat j is the number of entering exergy fluxes Bfeed , including substance and radiation, m is the number of entering exergy fluxes BQ,feed of heat, Wuse is the total work produced, Wfeed is the total work consumed, G is the gravity input, considered if eZergy is applied, Be is the exergy gain in case of variation of environment parameters. Formula (2.3.18) can be applied also for combined processes in which more than one intended product is obtained (e.g. the combined generation of heat and power). A particular example of application of the energy and exergy perfection degrees, with no work, G and Be, is discussed in paragraph 2.4.4 for photosynthesis. The exergy balance should be developed possibly with most detailed distribution of the internal losses in order to obtain the most exact information on the possibility of perfection improvement of the considered system. For example the internal exergy loss could be divided into the components corresponding to friction, heat transfer at finite temperature difference, radiation emissions and absorptions, etc. If in any part of the considered system there occur several irreversible phenomena then, in principle, it is possible to calculate only the overall internal exergy loss caused by the phenomena. The splitting of the effects of these irreversible phenomena, occurring simultaneously at the same place and time is impossible because these phenomena interact mutually. The splitting of the exergy loss in such case can be based only on the assumed agreement. For example for combustion process the radiative heat exchange occurs between the flame and surrounding wall. In order to split the effects of irreversible chemical reactions of combustion from the irreversible radiation exchange, it can be assumed that first the combustion occurs and then the heat exchange takes place. However, with such an assumption the temperature differences in heat exchange are larger than the real. Therefore it is better to split exergy losses according to time and location of occurrence, instead of according to the causes, unless the examined causes occur in different locations and different instants. 44 Solar energy sciences and engineering applications 2.3.4.3 Specific efficiencies Generally, the efficiency of a process can be arbitrarily defined to expose the most important aspect. For example, the exergy effect of the hot water heated in pipe by solar radiation can be related either to the exergy of heat q at the temperature of the Sun’s surface TSun; q · (1 - T0/TSun), or to the exergy bSun of the Sun’s radiation, or to the exergy of heat q absorbed at the water pipe temperature TW; q · (1 - T0/TW). The exergy efficiency increases successively through the above three possibilities due to the decreasing values of the denominators in the efficiency formulas: q · (1 - T0/TSun)> bSun >q · (1 - T0/TW). An exergy efficiency which relates the process effect to the decrease of the Sun’s exergy, q · (1 - T0/TSun), is unfair because the exposed surface of pipe obtains only the solar radiation exergy and the pipe is independent of irreversible emissions at the Sun’s surface. Relating the process effect to the exergy of heat absorbed, q · (1 - T0/TW), favors the exposed surface by neglecting its imperfection during the absorption of heat q. Thus, from these three possibilities, the relating the heating water effect to the exergy bSun of the Sun’s radiation is the only best estimation in this analysis. 2.3.4.4 Consumption indices Sometimes instead of efficiency the specially defined indices are used for estimation of processes. For example there are some processes which occur spontaneously due to interaction with the environment. Drying, cooling, vaporizing or sublimation, are the examples of such processes in which the self-annihilation of exergy takes place. Often these processes, especially in industrial practice, are accelerated with use of the appropriate driving input. Exergy application for estimation of perfections of these processes reveals some problems. For example applying the common exergy efficiency definition, effect and input ratio, leads to a negative or infinite value of the efficiency. Therefore instead of the efficiency some specially defined criteria have to be used for the evaluation and comparison of processes perfection. For example for drying processes the unit exergy consumption index is defined as the ratio of the exergy of the used in the drying medium to the mass of the liquid extracted in form of the vapor. In the case of the application of solar energy for drying, the index would express the exergy of absorbed radiation per mass of the vaporized moisture. Another index can be used for process occurring in a water cooling tower. Szargut and Petela (1968) propose the evaluation of the process with the index defined as ratio of the sum of exergy lost in the tower and the heat extracted from water. The typical value of the index for cooling tower of steam power station is about 0.088 kJ of exergy per kJ of heat. Petela (1990) proposes a specific approach to the exergy annihilation due to spontaneous processes. He considers the natural exergy annihilation rate which expresses the ability of the environment to spontaneously reduce the exergy of a substance or radiation. The natural wind velocity, the temperature and composition of environment air, particularly humidity, as well as the solar radiation, the local surrounding surfaces configuration and surfaces’ emissivities taken into account together can determine the available exergy effect for annihilation of exergy in the spontaneous processes of drying, cooling, etc. A so called “windchill’’ factor is the example of the concept expressing certain ability of environment air. Exergy analysis of solar radiation processes 45 2.4 SOLAR RADIATION PROCESSES 2.4.1 Conversion of solar radiation into heat 2.4.1.1 Calculation the exergy of solar radiation Solar energy is the most important renewable source of energy on the Earth. Solar energy is a high temperature source, however it’s harvesting occurs inefficiently due to extensive degradation of the energy. The degradation of solar energy is well demonstrated by exergy analysis. Therefore the engineering thermodynamics of thermal radiation addresses mainly exergy analyses of diversified problems of utilization of solar radiation. Generally, the solar radiation passing through the atmosphere is absorbed, scattered and reflected not only by air molecules but also by e.g. water vapor, clouds, dust, pollutants, smoke from forest fires and volcanoes. These factors cause diffusion (called also dilution) of solar radiation. The portion of solar radiation which reaches the Earth’s surface without being diffused is called direct beam solar radiation. Thus, the global solar radiation (global irradiance) consists of the diffuse and direct solar radiation. For example, during thick cloudy days the atmosphere reduces direct beam radiation to zero. Only a part of scattered sunlight reaches the Earth because some sunlight is scattered back into space. Also some radiation from the Earth, together with sunlight scattered off the Earth’s surface is re-scattered to the atmosphere. This effect can be significant e.g. when the Earth’s surface is covered with snow. Solar radiation energy is difficult to calculate because, as discussed, the radiation energy reaching the surface on Earth, is composed of direct and diluted radiation components, and depends on geographic location, time of day, season of year, local weather and even on local landscape. The relatively effective method of determining of solar radiation is by carrying out spectral measurement and application of the obtained results in the formulae derived in paragraph 2.2.6. A certain basis for the evaluation of solar radiation reaching the Earth’s surface can be the energy of solar radiation incident outside the Earth’s atmosphere which is called extraterrestrial radiation, and its average value is about 1367W/m2. The exergy of extraterrestrial radiation is determined with the following examples. Example 2.4.1.1 The exergy of the extraterrestrial solar radiation, when recognized as non-polarized, black, uniform and propagating within solid angle ., may be approximately calculated by means of equation (2.2.66). The required exergy bb of emission density can be calculated from (2.2.65) for the Sun’s surface temperature T =6000K and for the environment temperature T0 =300K as follows: bb = 5.6693 × 10-8 3 (3 × 60004 + 3004 - 4 × 300 × 60003)=68.5MW/m2 (a) Approximately, the radius of the sun is R=695,500km and the mean distance from the Sun to the Earth is L=149,500,000 km. The integral in formula (2.2.66) 46 Solar energy sciences and engineering applications expresses the solid angle . and is equal to the area of circle of radius R divided by the square distance L, thus:  ß  . cos ß sin ß dß d.= R2p L2 =2.16 × 10-5p sr (2.4.1) By substitution of (a) and (2.4.1) into (2.2.66): bb,. = 68500 p 2.16 × 10-5p=1.48kW/m2 (b) The emission eb,. of black solar radiation arriving in the solid angle . is: eb,. = eb p  ß  . cos ß sin ß dß d. (2.4.2) where eb =73.47MW/m2 is determined by formula (2.2.32). The ratio of exergy to energy of emission is .=bb,./eb,. =68.5/73.47˜0.93. Example 2.4.1.2 More exact computations of the exergy of solar radiation were carried out by Petela (1961a) based on the extraterrestrial radiation spectrum determined experimentally by Kondratiew (1954). Calculations are based on equation (2.2.61) for non-polarized and uniform radiation. Table 2.4.1 presents some exemplary Kondratiew data on intensity of radiation i0,. (column 2) as a function of wavelength . (column 1). The part of the spectrum is shown in Figure 2.4.1 together with three spectra (dashed lines), for comparison, for black radiation at absolute temperatures 6000, 5800 and 5600 K. The i0,. values in Table 2.4.1 are assumed constant for the respective ranges of wavelengths . (column 5). Correspondent ranges of frequency . calculated based on equation d.=c0 · d./.2, for c0 =2.9979 · 108 m/s, are shown in column 6 whereas values i0,. in column 4 were determined from equation i0,. =c0 · i0,./.2. The . values in column 3 were determined from equation . · .=c0. The L0,. values of column 8 are calculated from equation (2.2.54). Columns 7 and 9 are calculated as respective products of columns 4 and 6; (i0,. · .), and 6 and 8; (L0,. · .). Formula (2.2.61) is applied in the following numerical form: bA =  2 i0,.. - 2T0 L0,.. + sT4 0 3p  ß  . cos ß sin ß dß d. (2.4.3) Assuming the environment temperature T0 =300 K, substituting formula (2.4.1) into (2.4.3) and using data from Table 2.4.1: bA =  2 × 10079300 - 2 × 300 × 2263.3 + 5.6693 × 10-8 × 3004 3p  × p × 2.16 × 10-5 = 1367.9 - 92.151 + 0.0033=1275.8W/m2 Exergy analysis of solar radiation processes 47 Table 2.4.1 Spectrum of the extraterrestrial solar radiation, (from Petela, 1962). .·1010 i0,.·10-10 .·10-11 i0,.·1012 . · 1010 .·10-12 i0,. · . L0,.·1013 L0,. · . m W m3sr 1 s J m2sr m 1 s W m2sr J m2s sr W m2K sr 1 2 3 4 5 6 7 8 9 2200 2300 2400 .. 60000 70000 10 26 31 .. 11 13627 13035 12492 .. 500 428 15 47 59 .. 1765 1201 100 100 100 .. 10000 10000 62.0 56.7 52.1 .. 0.714 0.535 960 2650 3090 .. 1260 640 0.03 0.10 0.12 .. 7.19 5.48 0.205 0.540 0.639 .. 0.514 0.293 Total 10079300 2263.306 Figure 2.4.1 Spectrum of extraterrestrial solar radiation (from Petela, 1962). The obtained result 1275.8W/m2 is the exergy of the extraterrestrial solar radiation arriving in a 1m2 surface which is perpendicular to the direction of the sun. The obtained ratio of radiosity (equal to emission) to exergy is .S = 1275.8/1367.9=0.9326. 2.4.1.2 Possibility of concentration of solar radiation exergy Solar energy, although rich, is poorly concentrated and thus it requires a relatively large surface to harvest the Sun’s radiation. From this viewpoint solar radiation is especially valuable for those countries which have lot of unused areas (e.g. deserts). 48 Solar energy sciences and engineering applications Figure 2.4.2 Scheme of concentrated radiation (from Petela, 2010). The poor concentration of energy needs intensive theoretical studies in order to obtain acceptable efficiency of energy utilization. Effective method for such purpose could be exergy analysis. The concentration possibility of radiation can be illustrated with use of a simple model shown in Figure 2.4.2. The imagined plane surface (long dashed line) of area AS represents the black (eS =1) solar irradiance IR at constant temperature TS. The other plane surface (solid line) of area A is gray, at emissivity e, and its temperature T is controlled by the cooling heat Q. The vacuum space between the two surfaces is enclosed by other cone-shape surface (short dash line) which is mirror-like (e0 =0). The surface areas ratio aS =AS/A. The energy balance of the cooled surface A is: aSeIR=esT4 + k(T - T0) (2.4.4) where k is the heat transfer coefficient at which heat Q is extracted from surface A. The heat rate q=k(T - T0) (2.4.5) can be used to express the total heat Q absorbed by surface A: Q= AS aS q (2.4.6) The energy efficiency .E of concentration of solar radiation can be measured as the ratio of absorbed heat Q and the solar irradiance IR: .E = Q IR (2.4.7) For comparison, exergetic efficiency .B can also be considered based on the following definition: .B = BQ IR. (2.4.8) Exergy analysis of solar radiation processes 49 where . is the exergy-energy ratio discussed in paragraph 2.2.5 and the exergy BQ of heat absorbed by surface A is: BQ =Aq  1 - T0 T  (2.4.9) The reality of the discussed effect of concentration of solar radiation can be evaluated by the calculated value of the overall entropy growth , which consists of: the positive entropies of heat Q, the emission of surface A, and the negative entropy of absorbed solar radiation: = Q T + Ae 4 3sT3 - ASeSR (2.4.10) where SR is the entropy of irradiance IR. It has to be noted that using the energy emissivity e in formula (2.4.10) for entropy calculation is not precise and, as discussed in details by Petela (2010), the smaller is the precision the smaller is the emissivity e. The magnitude SR can be evaluated from the assumed ratio SR/IR to be equal the ratio s/e of the black emission entropy and emission energy, SR/IR=s/e. With use of formulae (2.2.32) and (2.2.39), the following relation can be derived: SR= 4 3 IR TS (2.4.11) The overall entropy growth determined from equation (2.4.10) should be positive (>0). Otherwise, (when =0), the concentration of solar radiation is impossible as being against the Second Law of Thermodynamics. Example 2.4.1.3 The concentration of solar radiation can be considered, e.g., at IR=800W/m2 arriving at the imagined surface of area AS =1m2 shown in Figure 2.4.2. Assuming k=3 W/(m2K) and the environment temperature T0 =300K equation (2.4.4) allows for determining the temperature T of surface A as function of the surface ratio aS. As is shown in Figure 2.4.3, with the increase in aS, the temperature T grows (thin solid line) and the heat rate q is also increasing (long-dashed line), determined by formula (2.4.5). However, according to formula (2.4.6), with growing aS the total heatQis varying (short-dash line) with a maximum of about 134 W at about aS ˜2. The maximum appears because with growing aS its effect becomes stronger than the effect of growing heat rate q. The energy efficiency .E of concentration of solar radiation, based on definition (2.4.7) is varying as shown with the thick-dashed line in Figure 2.4.3. The efficiency .E has the maximum of about 16.8% appearing also at about aS ˜2, correspondently to the maximum of Q. Exergy BQ of absorbed heat is determined by (2.4.9) and shown in Figure 2.4.3 with a dash-dot line. The exergy BQ varies and has a maximum of about 45.8W, which appears in the surface area ratio about aS ˜6. The maximum is a result of two factors varying with growing aS: one is growing exergy of heat due to growing temperature T, other is due to diminishing of the absorbed heat Q. 50 Solar energy sciences and engineering applications Figure 2.4.3 Exemplary effects of concentration of solar radiation (from Petela, 2010). Assuming .=0.933 like for the black radiation at temperature TS =6000 K, the efficiency .B can be determined from formula (2.4.8) and shown in Figure 2.4.3 (thick solid line). The efficiency .B has a maximum about 5.34% which also corresponds to value of aS ˜6. The overall entropy growth determined from equation (2.4.10) for the data used in the example is always positive (>0) and with growing aS diminishes to zero (=0) for aS =91,843 corresponding to temperature T =6000 K. For further growing of aS the overall entropy growth becomes negative (<0) i.e. the further concentration of solar radiation is impossible. Based on the calculations the process of “de-concentration’’ of solar radiation, which would correspond to reducing aS below 1, is irreversible and can occur but heat absorbed by the surface A is negative which means that the surface would be heated. The data used in the present example were also used for the computation of the results shown in Table 2.4.2 which illustrates the trends of the output data in response to changes in some input parameters. The values in column 3 of Table 2.4.2 are considered as the reference values for studying the influence of the varying input parameters in the output. Therefore each of the next columns (4 to 6) corresponds to the case in which the input is changed only by the values shown in a particular column, whereas the other input parameters remain at the reference level. For example, column 4 corresponds to a change in the emissivity e, which increases from 0.9 to 1. This 10% e increase causes the increase of: temperature T from 517.8 to 519.9 K, q from 653.3 to 659.5W/m2, Q from 108.9 to 109.9W, .E from 13.61 to 13.74%, BQ from 45.79 to 46.49W and .B from 5.34 to 5.42%. Column 5 and 6 can be similarly interpreted. For example increasing the heat transfer coefficient k from 3 to 5 W/(m2 K) causes increase in exergetic efficiency from 5.34 to 8.04%, which is the result of increased heat rate q from 653.3 to 1021W/m2. Exergy analysis of solar radiation processes 51 Table 2.4.2 Responsive trends of output to change of some input parameters (for IR=800W/m2, TS =6000 K, .=0.933,AS =1m2), (from Petela, 2010) Mono-variant changes of input parameters and Reference resulting outputs Quantity Units value 1 2 3 4 5 6 Input e 0.9 1 k W/(m2 K) 3 5 T0 K 300 270 Output T K 517.8 519.9 504.2 514.8 q W/m2 653.3 659.6 1021 734.6 Q W 108.9 109.9 170.2 122.4 BQ W 45.79 46.49 68.94 58.23 .E % 13.61 13.74 21.28 15.30 .B % 5.34 5.42 8.04 6.79 2.4.1.3 Global warming effect Solar radiation is the energy source for life on the Earth and this radiation establishes the temperatures of the atmosphere and the Earth’s surface. However, human activity seems to change the conditions of the energy exchange between the Sun and Earth and it seems that the observed tendency is a gradual increase of these temperatures. This effect of temperature growth is called the global warming effect. Some exergy insights are discussed in the present paragraph. The atmosphere absorbs some of visible radiation directly from the Sun. From the Earth the atmosphere receives some infrared radiation and exchanges convective heat. The Earth, besides energy exchanged with the atmosphere, receives radiation from the Sun and reflects some radiation to the space. Assuming that the extraterrestrial solar radiation directed to the Earth renders 100%, the energy flow diagram for the thermal equilibrium for any Earth temperature T0, can be shown schematically in Figure 2.4.4a. It is usually estimated that the annual average temperature T0 of the Earth surface is about 14.C (287 K) and at this temperature the Earth’s energy emission is ~192%. The atmosphere, at a certain assumed effective temperature TA, emits energy to the Earth (~138%) and space (~83%). The global warming effect is not a subject well exposed by the exergy concept because the exergy relates to environment temperature T0 with no regards of how high this temperature is. For example, for any Earth temperature T0 the exergy radiated from the Earth’s surface will be always zero, as shown in Figure 2.4.4b in which 100% is assumed for the exergy of extraterrestrial solar radiation directed to the Earth. Example 2.4.1.4 To illustrate the global warming effect a very rough consideration of exchanged radiative and convective heat fluxes are analyzed in a simplified model of a polluted air layer between the space and the Earth’s surface. The air layer of transmissivity t is assumed as a body of reflectivity .=0 and of absorptivity equal to 52 Solar energy sciences and engineering applications Figure 2.4.4 Simplified scheme of radiation energy (a) and exergy (b), (from Petela, 2010). emissivity a=e, thus the body emissivity is e=1 - t. The air layer has temperature TA, transmissivity tS =0.71 for the high temperature solar radiation and t0 =0.17 for a low temperature radiation. The yearly average solar irradiance S=100W/m2 arriving in the air layer is assumed. The Earth’s surface is black, does not transfer energy to the ground, and has temperature T0 =287.16K (14.C). The air layer absorbs: solar energy (1 - tS)·S, radiation from sky and radiation from earth surface, whereas it releases the energy by radiation and convection to the sky and the Earth surface: (1 - tS)S + (1 - t0)s(T4 sky + T4 0 )=2(1 - t0)sT4A + k[(TA - T0) + (TA - Tsky)] (2.4.12) and the Earth’s surface absorbs: solar energy, radiation from the air layer and radiation from sky, whereas it releases the energy by radiation and convection: tSS + (1 - t0)sT4A + t0sT4 sky =sT4 o + k(T0 - TA) (2.4.13) where Tsky is the sky temperature representing black space above the air layer, s is the Boltzmann constant for black radiation, and k is the convective heat transfer coefficient assumed equal for all convections. For k=3 W/(m2 K), from the two equations (2.4.12) and (2.4.13) the two unknown temperature can be calculated: TA =279.6K and Tsky = 267.4K. The global warming effect could be considered based on the influence of changing the factor t0 describing pollution of air, on the change of environment temperature T0. From equation (2.4.12) the partial derivative .T0 .t0 = T4 0 + T4 sky - 2T4A 4T3 0 (1 - t0) + k s =44.087 K/% (2.4.14) Exergy analysis of solar radiation processes 53 Figure 2.4.5 Effect of combustion of hydrocarbons on CO2 generation. For example, due increased pollution, the relatively significant increase from 17% do 17.5% of the transmissivity t0 of the air for the low temperature radiation, causes, based on formula (2.4.14), a relatively small increase in environment temperature T0 from 287.16 K to: 44.087 · (0.175 - 0.17) + 287.16=287.38 K. The main factor considered in global warming is the concentration of CO2 in the atmosphere. The CO2 is a product of combustion of substances containing carbon. It is noteworthy to emphasize that regardless which kind of compound (fuels or biomass, etc.), when carbon is combusted there is always CO2 released into the atmosphere. Therefore combustion of any compounds containing carbon (e.g. hydrocarbons) is releasing CO2 proportionally to the content of carbon in the compound. The effect of combustion of some exemplary hydrocarbons is illustrated in Figure 2.4.5. Data were obtained by the assumption that the calorific value of combusted hydrocarbon is utilized by 40% for power generation. Generally, the larger the molar content of C in combusted material, the smaller the energy per released amount of CO2. The smallest value corresponds to pure carbon C. However, for example, pure hydrogen cannot be shown in the diagram because the corresponding potential energy amount is infinity. This should be emphasized that such infinity value corresponds also to utilization of solar energy in production of power, heat or photosynthesis. 2.4.1.4 Canopy effect The global warming effect often is compared to the effect of a greenhouse in which the solar radiation is trapped at the Earth by using transparent canopy over the surface 54 Solar energy sciences and engineering applications Figure 2.4.6 The three typical situations considered in study of the canopy effect (from Petela, 2010). absorbing solar radiation. The real greenhouse is built of transparent walls through which solar radiation penetrates and heats the ground inside and then the confined air gets heat from this ground. Based on the rough analogy the term ‘greenhouse effect’ is referred also to the process in which the infrared radiation is exchanged between the atmosphere and the Earth’s surface. These two processes differ because in a real greenhouse air is trapped whereas the environment air, when warmed from the ground, rises and mixes with cooler air aloft. The analogy could be eventually recognized in the fact that the glass roof of the greenhouse traps the infrared radiation to warm the greenhouse air and the greenhouse roof plays the role of the huge and thick layer of the atmosphere. It is estimated that in the absence of the greenhouse effect the Earth’s surface temperature would be decreased from about 14 to about -19.C. It is believed that the recent warming of the lower atmosphere is the result of an enhancing of the greenhouse effect by an increasing of amount of gaseous, liquid and solid ingredients of different radiative properties than air. Using exergy, the effect of the canopy located above a considered surface to screen the surface from the direct solar radiation can be considered, based on the three situations presented schematically in Figure 2.4.6. A black horizontal plate, of surface area A, located on the Earth’s surface can be exposed to direct solar radiation as shown schematically in Figure 2.4.6a. In the thermodynamic equilibrium state the irradiance S is spent on heat Q extracted at constant plate temperature Tp and on the convective (Ep-0) and radiative (Ep-sky) heat fluxes from the plate to the surroundings. The plate temperature Tp is controlled by the appropriately arranged amount of heat Q. The energy balance equation for the plate is: S=Q+ Ep-sky + Ep-0 (2.4.15) where Ep-sky =As(T4 p - T4 sky) (2.4.16) Exergy analysis of solar radiation processes 55 Figure 2.4.7 Plate exposed to solar radiation, S=700W/m2 (left) and S=1000W/m2 (right), (from Petela, 2010). Ep-0 =Akp-0(Tp - T0) (2.4.17) and where kp-0 is the convective heat transfer coefficient. The harvest of the solar energy can be determined by the energetic efficiency .E: .E = Q S (2.4.18) or by the exergetic efficiency .B: .B = Q .cS (1 - T0 Tp ) (2.4.19) where .c is the exergy/energy ratio discussed in paragraph 2.2.5. It can be shown that, for the direct radiation of the Sun at its surface temperature 6000 K, the theoretical value of the ratio is .=0.933. According to Gueymard (2004), the irradiance of the direct solar radiation arriving at the Earth is 1366W/m2. As the irradiance values applied in the present canopy consideration are smaller then the exergy/energy ratio, they could be taken as for a smaller irradiance temperature, e.g., .c =0.9. For example, assuming A=1m2, kp-0 =5 W/(m2 K) and determined by the Swinbank (1963) formula: Tsky =0.0552 · T1.5 0 in which T0 =287.16K (14.C), Figure 2.4.7 shows the calculation results for the two different values of irradiance, S=700W/m2 and S=1000W/m2. With the increasing plate temperature Tp, (t. pC), the energetic efficiency .E decreases whereas the exergetic efficiency .B is maximum. In the second situation, shown in Figure 2.4.6b, the plate of the surface area 1m2 is a fragment of a very large and flat surface of the same uniformly distributed values of temperature Tp and radiative properties. The plate is screened from the solar radiation with a very large, flat and horizontal canopy. Material of the canopy can transmit the whole solar radiation to the plate (canopy transmissivity tSOL =1), although the low temperature emission from the plate is entirely absorbed by the canopy (canopy transmissivity tPLA =0). The extreme values of these two transmissivities are assumed to show better the effect of the canopy on the exchanged radiative heat. Due to a very small thickness of the canopy the both its surfaces, that exposed to the Sun as well as that exposed to the plate, have the same canopy temperature Tc. 56 Solar energy sciences and engineering applications Figure 2.4.8 Plate under a canopy in the environment air, S=700W/m2 (left) and S=1000W/m2 (right), (from Petela, 2010). In the thermodynamic equilibrium state of the situation shown in Figure 2.4.6b, the irradiance S is spent on heat Q extracted at constant plate temperature Tp and on the convective (Ep-a) and radiative (Ep-c) heat fluxes from the plate to the canopy. The plate temperature Tp is controlled by the appropriately arranged amount of heat Q. The canopy temperature Tc is constant for the given plate temperature Tp and distributed uniformly over the surfaces of the canopy. The energy balance equation for the plate is: S=Q+ Ep-c + Ep-a (2.4.20) where Ep-c =s(T4 p - T4 c ) (2.4.21) Ep-a =Akp-a(Tp - Ta) (2.4.22) and where kp-a is the respective convective heat transfer coefficient and Ta is the temperature of air between the plate and the canopy. Simplifying, it is assumed Ta =T0. The energy balance of the canopy is: Ep-c =Ec-sky + Ec-0 + Ec-a (2.4.23) where Ep-a =Ec-0 =kp-a(Tp - T0) (2.4.24) and where kp-a =kc-a are the respective convective heat transfer coefficients. The harvest of the solar energy in the considered situation can be again determined by the energetic efficiency .E and exergetic efficiency .B determined respectively from formulae (2.4.18) and (2.4.19). For example, assuming kp-a =kc-a =5 W/(m2 K), Figure 2.4.8 shows the calculation results for the two different values of irradiance, S=700W/m2 and S=1000W/m2. As in situation (a) also in situation (b), with the increasing plate Exergy analysis of solar radiation processes 57 Figure 2.4.9 Plate under a canopy with a vacuum between the canopy and the plate, S=700W/m2 (left) and S=1000W/m2 (right), (from Petela, 2010). temperature Tp, the energetic efficiency .E decreases whereas the exergetic efficiency .B is at maximum. The third possible situation, shown in Figure 2.4.6c, is the same as in the previous situation (b), except that between the plate and canopy is a vacuum, thus in this space heat convection does not occur. The energy balance equations for the plate and the canopy are: S=Q+ Ep-c (2.4.25) Ep-c =Ec-sky + Ec-0 (2.4.26) The energetic efficiency .E and exergetic efficiency .B are determined respectively also from formulae (2.4.18) and (2.4.19). Figure 2.4.9 shows the calculation results for the two different values of irradiance, S=700W/m2 and S=1000W/m2. As in situations (a) and (b), also in situation (c), with the increasing plate temperature Tp the energetic efficiency .E decreases whereas the exergetic efficiency .B is at maximum. The comparative discussion of the three models (Fig. 2.4.6) can be summarized as follows. The irradiated black plate (a), the plate under the canopy (b) and the plate under the vacuum and canopy (c), were considered under simplifying assumptions of extreme values of surface properties to better emphasize the canopy idea. The comparison of Figures 2.4.7, 2.4.8 and 2.4.9 illustrates benefits of application of canopy for increasing effect of trapping solar radiation. The amount of exergy (practical value) of absorbed heat grows gradually through the three considered situations from (a) to (c). 2.4.1.5 Evaluation of solar radiation conversion into heat Solar radiation can be converted to heat in many various applications. Generally, in each application the solar radiation is absorbed by certain designed surfaces at a temperature controlled by appropriate amount of heat extracted. Unfortunately the high quality of solar energy, e.g., measured by exergy, is significantly degraded during a conversion to heat. A simple introduction to evaluation of the non-concentrated solar radiation potential for heating and determination of heat temperature can be considered with use of the model of 1m2 of absorbing surface shown in Figure 2.4.10. The considered surface 58 Solar energy sciences and engineering applications Figure 2.4.10 Scheme of the fluxes in energy balance of the surface on earth absorbing radiation from sun, (from Petela, 2003). is on the Earth and is perpendicular to the Sun’s direction. From the Sun, the black (e=1) radiation of exergy b., energy e. and entropy s., all within the solid angle ., arrives in the absorbing surface. (For simplification the subscript “b’’ for black is omitted in this paragraph). These three fluxes are absorbed by the absorbing surface at temperature Ta and emissivity ea. The absorbing surface, in the solid angle 2p, emits its own radiation fluxes of exergy ba, energy ea entropy sa and obtains, in the solid angle 2p - ., the radiation fluxes of exergy b0, energy e0 and entropy s0 from the environment at temperature T0 (assumed to be equal to the sky temperature, Tsky =T0) and at assumed emissivity e0 =1. For the conversion of solar radiation the energy efficiency .E and the exergy efficiency .B are: .E = q e. (2.4.27) .B = bq b. (2.4.28) In the present considerations the solar radiation is considered as non-polarized, uniform and black, at temperature T =6000 K, arriving in the Earth within the solid angle .. Exergy b. of such radiation can be calculated from formulas (2.2.66) and (2.4.1) in which the double integral represents angle .=p · R2/L2 with use of the radius of the Sun R and the mean distance L from the Sun to the Earth: b. = b p pR2 L2 =b R2 L2 (a) Exergy analysis of solar radiation processes 59 Analogously, the energy emission e. arriving from the Sun at the absorbing surface within the solid angle ., is: e. =e R2 L2 (b) where the Sun density emission e and the exergy b of the black radiation density emitted by the Sun are: e=sT4 (c) b= s 3 (3T4 + T4 0 - 4T0T3) (d) The heat q, absorbed by the surface at temperature Ta, is extracted in the amount determined from the following energy conservation equation for the absorbing surface: q=eae. - ea + e0 (e) and the exergy bq of this heat, absorbed by the heat source temperature Ta, is: bq =q Ta - T0 Ta (f) Determination of ea and e0 in equation (e) is required. The absorbing surface of emissivity ea and temperature Ta, radiates its own emission ea to the whole hemisphere: ea =easT4 a (g) and the respective exergy ba which is: ba =ea s 3 (3T4 a + T4 0 - 4T0T3 a) (h) The considered absorbing surface, beside emission from the Sun, obtains from the remaining part of environmental hemisphere the black (e0 =1) radiation energy e0, (e0 =e0,(2p.)), at temperature T0, which is absorbed in amount determined by emissivity ea of the adsorbing surface: e0 =easT4 0  2 - R2 L2  (i) However, regarding exergy, according to the definition, the exergy of environment radiation is zero: b0 =0 (j) 60 Solar energy sciences and engineering applications Using equations (b), (c), (e), (g) and (i) in (2.4.27): .E =ea . .... 2 - T4 a - T4 0  1 - R2 L2  T4R2 L2 . .... (2.4.29) Using formulae (a)–(i) in (2.4.28), the exergy conversion efficiency of solar radiation into heat can be determined as follows: .B =3ea Ta - T0 Ta T4 - T4 0 - (T4 a - 2T4 0 ) L2 R2 3T4 + T4 0 - 4T0T3 (2.4.30) The larger the ratio L/R, the smaller are both efficiencies. The increasing emissivity ea of the absorbing surface will increase the conversion efficiencies. The larger the Ta, the smaller is the energy efficiency .E, however, the exergy conversion efficiency .B is at maximum. The optimal temperature Ta,opt can be calculated based on (2.4.30) from the condition: ..B .Ta =0 (2.4.31) For example, if the solar radiation is considered at ea =1, T0 =300 K, T = 6000 K, R=6.955 · 108 m and L=1.495 · 1011 m, then Ta,opt ˜363K (90.C). If the environment temperature drops to T0 =273K then Ta,opt ˜383K (110.C). The Ta optimum, at the unchanged exergy b. of solar radiation, results from the fact that with increasing Ta, which increases the heat quality (bq), the amount of this heat decreases. The emissivity value ea does not appear in equation (2.4.31) so this emissivity has no effect on the optimal temperature Ta,opt. The universal traveling of the human population motivates considering the environment temperature in a wide range, theoretically for 0Ta,max the heat q becomes negative because the radiation of absorbing surface to the environment is larger than the heat received from solar radiation. Some more computation results for analyzing effect of Ta, (taC), are shown in Figure 2.4.12. An increase in Ta, (ta C), will decrease exergy loss db and heat q, whereas .B and bq reveal maxima as results of growing of the Carnot efficiency: .C,a = 1 - T0/Ta. 2.4.2 Solar cylindrical-parabolic cooker The most common devices for utilization of solar radiation are cookers of different types. The simple solar cylindrical-parabolic cooker (SCPC), shown schematically in Figure 2.4.13, is used to demonstrate the methodology of exergy analysis of the cooker and the distribution of the exergy losses. Also explained is the general problem of how the exergy loss at any radiating surface should be determined, if the surface absorbs many radiation fluxes of different temperatures. Additionally a possibility of introduction of an imagined surface to complete the cooker surfaces system is shown. The cylindrical cooking pot filled with water is surrounded with the cylindricalparabolic reflector. The considered system of exchanging energy consists of three long surfaces of length L. The outer surface 3 of the cooking pot has an area A3. The inner Exergy analysis of solar radiation processes 63 Figure 2.4.12 Effect of varying temperatureTa of absorbing surface, at constantT=6000 K,T0 =300 K and ea =0.8 (Petela, 2003). Figure 2.4.13 The scheme of the SCPC, (from Petela, 2005). surface 2 of the reflector has an area A2. The system is completed with the imagined plane surface 1 of area A1. The imagined surface 1, which represents the ambience and the irradiation supplied to the considered system, is defined by transmissivity t1 =1 (and thus reflectivity .1 = 0), absorptivity a1 =0 and emissivity e1 =0. The effective emission of the imagined surface 1, can be determined as the irradiation I calculated as follows: I =2.16 · 10-5A1eSsT4 S (2.4.32) where 2.16 · 10-5 accounts on the solid angle within which the Sun has been seen from the Earth, eS is the emissivity of the Sun surface, (assumed eS =1), s is the Boltzmann 64 Solar energy sciences and engineering applications constant for black radiation, and TS is the absolute temperature of the Sun surface. Formally, it can be assumed that the energy emission of surface 1 is E1 =I. It is assumed that the surfaces 2 and 3 have uniform temperatures T2 and T3, respectively, uniform reflectivities, respectively, .2 and .3, different from zero, and the emissivities of the surfaces, e2 =1 - .2 and e3 =1 - .3. Thus, the emissions of surfaces 2 and 3 are: E2 =A2e2sT4 2 (2.4.33) E3 =A3e3sT4 3 (2.4.34) The geometric configuration of the SCPC can be described by the value .i-j of the nine mutual view factors for the three surfaces 1, 2 and 3. The considerations are carried out only for the 1 m section of the SCPC length which remains in thermal equilibrium. The known input data are: – outer diameter D of cooking pot and its geometric location, – dimensions of the parabolic reflector, – surfaces areas A1, A2, A3, and all view factors, fi-j, – heat transfer coefficients (including conductivity) k2 and k3, for surfaces 2 and 3, – emissivities e2 and e3 of surfaces 2 and 3, – reflectivities .2 and .3 (defined by respective emissivities e2 and e3), – absolute temperature of the Sun’s surface TS =6000 K, – absolute water temperature Tw (average of the inlet and outlet temperatures), – absolute environment temperature T0 =293 K. The unknown output data are: – emissions E2, E3 and irradiation I, (I =E1), – convective heat Q2,c from reflector to environment, – radiative heat Q2,r from outer side of reflector to environment, – convective heat Q3,c from surface 3 to environment, – radiosity of the three surfaces J1, J2 and J3, – absolute temperatures T2 and T3of surface 2 and 3, – energy efficiency of the SCPC, expressed by the water enthalpy change Q3,u, – all respective quantities related to exergy. The energy analysis is based on the following energy conservation equations for each involved surfaces: J1 =.2-1J2 + .3-1J3 +Q2,c +Q2,r +Q3,c +Q3,u (2.4.35) e2(.1-2J1 + .2-2J2 + .3-2J3)=E2 +Q2,c +Q2,r (2.4.36) e3(.1-3J1 + .2-3J2 + .3-3J3)=E3 +Q3,c +Q3,u (2.4.37) The magnitudes J1, J2 and J3 are the radiosity values for surfaces 1, 2 and 3, respectively, and the values of .i-j are the respective view factors. The radiosity expresses the Exergy analysis of solar radiation processes 65 total radiation which leaves a surface and includes emission of the considered surface as well as all reflected radiations arriving from other surfaces of the system. The concept of radiosity is convenient for energy calculation, however it cannot be used for exergy considerations because it does not distinguish the temperatures of the components of the radiosity. The radiosity J1 of the imagined surface 1 equals the irradiation I: J1 =I (2.4.38) and another independent equation on the radiosity is also included into calculations: J2 =E2 + .2(.1-2J1 + .2-2J2 + .3-2J3) (2.4.39) It is assumed that the reflector is very thin so the uniform temperature T2 prevails through the whole reflector thickness and on the inner and outer side of the reflector. Thus the heat Q2,c transferred from both sides of the reflector is: Q2,c =2A2k2(T2 - To) (2.4.40) and heat radiating from the outer side of reflector to the environment is: Q2,r =A2e2s(T4 2 - T4 o ) (2.4.41) where k2 is the convective heat transfer coefficient and T0 is the environment temperature. Heat Q3,c transferred by convection from surface 3 to the environment is: Q3,c =A3k3(T3 - To) (2.4.42) and the useful heat Q3,u transferred through the wall of the cooking pot is: Q3,u =A3k 3(T3 - Tw) (2.4.43) where k3 is the convective heat transfer coefficient, Tw is the absolute temperature of water in the cooking pot and k 3 is the heat transfer coefficient, which takes into account the conductive heat transfer through the cooking pot wall and convective heat transfer from the inner cooking pot surface to the water. The equations system (2.4.32)–(2.4.43) can be solved by successive iterations. The energy analysis of the SCPC can be carried out based on the evaluation of the terms in the following energy conservation equation for the whole SCPC: .2-1J2 + .3-1J3 +Q2,c +Q2,r +Q3,c +Q3,u =I (2.4.44) The first two terms in equation (2.4.44) represent radiation energy escaping from the SCPC due to the radiosities of surfaces 2, (.2,1·J2), and 3 (.3,1·J3). Dividing the both sides of equation (2.4.44) by I, the percentage values ß of the equation terms can 66 Solar energy sciences and engineering applications be obtained, e.g., for heatQ2,c respectively is ß2,c =Q2,c/I, however the term withQ3,u determines the energetic efficiency .E = Q3,u I (2.4.45) Therefore, equation (2.4.44) can be also written as: ß + .E =1 (2.4.46) The exergy analysis enables the additional quality interpretation of the cooker. For the considered SCPC, the exergy of radiating fluxes, overall exergy efficiency of the SCPC process and the exergy losses during irreversible component phenomena occurring in the SCPC are calculated. Sometimes, it is convenient to determine an exergy B of radiation emission at temperature T by multiplying its emission energy E by the characteristic exergy/energy ratio ., defined by formula (2.2.45), e.g., B=E·.. Thus, the exergy efficiency .B of the SCPC is the ratio of the exergy of the useful heat Q3,u, at temperature T3, and of the exergy of solar emission at temperature TS: .B = Q3,u  1 - T0 T3  I.S (2.4.47) where .S is the exergy/energy ratio for the solar emission of temperature TS, which for TS =6000K and T0 =293K is .S =0.9348. Reflection and transmission of radiation are reversible so the exergy losses in the SCPC are considered only for the following component phenomena: – Simultaneous emission and absorption of radiation at surfaces 2, (dB2) and 3, (dB3). There is no exergy loss at the imagined surface 1, (dB1 =0), because neither absorption nor emission occurs but only transmission of radiation which is reversible. Other surfaces, 2 and 3, are solid and thus produce the irreversible effects of radiation. – Irreversible transfer of convection heat Q2c from the both sides of surface 2 to the environment, (dBQ2c). – Irreversible transfer of radiation heat Q2r from the outer side of surface 2 to the environment, (dBQ2r). – Irreversible transfer of heat Q3u from surface 3 to water, (dBQ3u), due to temperature difference T3 - Tw, – Irreversible transfer of convection heatQ3c from surface 3 to environment, (dBQ3c). The exergy dB1-0 escaping through surface 1, results from reflections from the SCPC surfaces to the environment. This loss is sensed only by the SCPC and is not irreversible because theoretically it can be used elsewhere. This loss consists of the radiation exergies B1-1, B2-1, B3-1 at the three different temperatures (TS, T2 and T3) dB1-0 =B1-1 + B2-1 + B3-1 (2.4.48) Exergy analysis of solar radiation processes 67 Analogously to the energy conservation equation, the exergy balance equation can be applied. When relating all the equation terms to the exergy input, which is the exergy I · .S of solar radiation entering the SCPC system, the following conservation equation for the whole SCPC, can be written: .B11 + .B21 + .B31 + .BQ2c + .BQ2r + .Q3u + .Q3c + .B2 + .B3 + .B =100 (2.4.49) where any percentage exergy loss . is calculated as the ratio of the loss to the exergy input, e.g., for the convection heat Q2,c one obtains .Q2c =dBQ2c/(I · .S). In the considered system of non-black surfaces the radiation energy striking a surface is not totally absorbed and part of it is reflected back to other surfaces. The radiant energy can be thus reflected back and forth between surfaces many times. To simplify the effect of further such multi reflections, it is assumed that surface 3 is black, (e3 =1). Thus, as the imagined surface 1 was previously assumed to be black (e1 =1), the only non-black surface in the exergetic analysis of the SCPC system is surface 2, (e2 <1). The nine exergy losses appearing in equation (2.4.49) can be categorized in three groups. The first group contains the exergy losses (external) related to heat transfer, (.BQ2c, .BQ2r, .Q3u, .Q3c). The second group (.B11, .B21, .B31) determines the exergy fluxes (external losses) escaping from the SCPC and the third group contains the exergy losses (internal) due to irreversible emission and absorption on surface (.B2, .B3). First group losses. The exergy loss dBQ2c, due to the convection transfer of heat Q2,c from the each of the two sides of the reflector to the environment, is equal to the exergy of heat Q2,c: dBQ2c =Q2,c  1 - T0 T2  (2.4.50) The exergy loss dBQ2r, due to the radiation transfer of heat Q2,r from the outer side of the reflector to the environment, is equal to the exergy of heat Q2,r: dBQ2r =Q2,r  1 - T0 T2  (2.4.51) The external exergy loss dBQ3u, due to the transfer of the useful heat Q3,u from surface 3 through the cooking pot wall to water, is equal to the difference of the exergy of this heat at temperature T3 and at temperature Tw: dBQ3u =Q3,u  T3 - T0 T3 - Tw - T0 Tw  (2.4.52) The exergy loss dBQ3c, due to convective heat transfer from surface 3 to the environment, is determined similarly: dBQ3c =Q3,c  T3 - T0 T3 - T0 - T0 T0  (2.4.53) where, obviously, the second fraction in the brackets of equation (2.4.53) is zero. 68 Solar energy sciences and engineering applications Second group losses. As results from formula (2.4.48) the external exergy loss dB1-0 is equal to the escaping exergy of three unabsorbed emissions at temperatures T1 =TS, T2 and T3 reflected to the environment. By multiplying these emissions respectively by the exergy/energy ratio the three losses can be expressed as follows: B1-1 =Q1-1.S (2.4.54) B2-1 =Q2-1.2 (2.4.55) B3-1 =Q3-1.3 (2.4.56) where .S, .2 and .3 are calculated from formula (2.2.45) for TS, T2 and T3, respectively, and Q1-1, Q2-1 and Q3-1 are the sums of the unabsorbed portion of the respective emissions of surfaces 1, 2 and 3. Thus, heatQ1-1 represents energy portions from many reflections of emission E1 of temperature TS, at the concave surface 2, and arriving in surface 1: Q1-1 =E1.1-2.2.21 + E1.12.2.2-2.2.2-1 + E1.12.2.2-2.2.2-2.2.2-1 +· · · (2.4.57) The portions in equation (2.4.57) can be expressed as the sum of the terms of the infinite geometric progression with the common ratio f2-2·.2, thus Q1-1 =E1.1-2.2.2-1 1 1 - .2-2.2 (2.4.58) Heat Q2-1(T2) represents the portion E2·f2-1 of emission E2 of surface 2 which directly arrives at surface 1 and the portions in results of many reflections of emission E2 at surface 2, arriving at surface 1: Q2-1 =E2.2-1 + E2.2-2.2.2-1 + E2.2-2.2.2-2.2.2-1 + . . . (2.4.59) and Q2-1 =E2.2-1 1 1 - .2-2.2 (2.4.60) Heat Q3-1(T3) represents the portion E3·f3-1 of emission E3 of surface 3 which arrives at surface 1 as the direct radiation, and the portions as a result of many reflections of emission E3 at surface 2: Q3-1 = E3.3-1 + E3.3-1.2.2-1 + E3.3-1.2.2-2.2.2-1 +E3.3-1.2.2-2.2.2-2.2.2-1 + . . . (2.4.61) and Q3-1 =E3  .3-1 + .3-2.2.2-1 1 1 - .2-2.2  (2.4.62) Exergy analysis of solar radiation processes 69 Third group losses can be calculated based either on the determination of the overall entropy growth used in the Guoy-Stodola equation (2.2.10) or determined from the exergy balance equation for the considered surface in the steady state. The latter method will be used. To develop the analysis, an imagined heat source connected to each considered surface has to be assumed. For each surface, 2 or 3, there are the arriving emissions at three different temperatures to be taken into account. It is assumed that these emissions are absorbed by the surface and transferred as heat to the imagined heat source. Then immediately this heat is taken from the source to generate the emission of the surface at its temperature. Thus the exergy balance equation for surface 2 can be interpreted as including: the input exergy entering the surface 2 and represented by terms due to: – emission .S ·Q1-2 arriving from surface 1, – emission .2 ·Q2-2 arriving from surface 2, – emission .3 ·Q3-2 arriving from surface 3, – heat E2 · (1 - T0/T2) needed for emission of surface 2 and delivered from the heat source. the output exergy leaving the surface 2 and represented by terms due to: – emission Q1-2·(1 - T0/T2) of surface 1 converted as the heat absorbed by the heat source, – emission Q2-2·(1 - T0/T2) of surface 2 converted as the heat absorbed by the heat source, – emission Q3-2·(1 - T0/T2) of surface 3 converted as the heat absorbed by the heat source, – emission E2·.2 of surface 2. The exergy balance equation can be written in form of the exergy loss dB2 equal to the difference of the exergy input and output: dB2 =.3Q3-2 + .SQ1-2 + .2(Q2-2 - E2) - (Q3-2 +Q1-2 +Q2-2 - E2)  1 - T0 T2  (2.4.63) Analogically, the exergy loss dB3 is: dB3 =.2Q2-3 + .SQ1-3 + .3(Q3-3 - E3) - (Q1-3 +Q2-3 +Q3-3 - E3)  1 - T0 T3  (2.4.64) Applying respectively again the formula for the sum of the terms of the infinite geometric progression, the values or required heat can be determined as follows. 70 Solar energy sciences and engineering applications Heat Q1-2, at temperature TS, is the sum of the portions of emission of surface 1 reaching surface 2, thus: Q1-2 =E1.1-2e2 1 1 - .2-2.2 (2.4.65) Heat Q2-2 at temperature T2, is the sum of the portions of emission of surface 2 reaching surface 2, thus: Q2-2 =E2.2-2e2 1 1 - .2-2.2 (2.4.66) Heat Q3-2 at temperature T3, is the sum of the portions of emission of surface 3 reaching surface 2, thus: Q3-2 =E3.3-2e2 1 1 - .2-2.2 (2.4.67) HeatQ1-3, at temperature TS, which is the sum of the totally absorbed irradiation which reaches surface 3 at view factor .1-3, and the totally absorbed irradiation parts reflected from surface 2, can be determined as follows: Q1-3 =I.1-3 + I.1-2.2.2-3 1 1 - .2-2.2 (2.4.68) Heat Q2-3 at temperature T2, is the sum of the portions of emission of surface 2 reaching surface 3, thus: Q2-3 =E2.2-3 1 1 - .2-2.2 (2.4.69) Heat Q3-3 at temperature T3, is the sum of the portions of emission of surface 3 reflected from surface 2 to surface 3: Q3-3 =E3.3-2.2.2-3 1 1 - .2-2.2 (2.4.70) As shown in the exergy part of the present paragraph even the assumption on the black surfaces 1 and 3 for exergetic consideration required far more equations in comparison to the respective energetic considerations developed for the system in which only one black surface 1 was assumed. Obviously, exergetic consideration of the system with only one black surface 1 would require developing of significantly more equations in comparison to the considerations presented in this paragraph. Comparison of the energy and exergy balances for the considered SCPC, at the assumed e3 =1, is presented in Table 2.4.4. In the both analyses the radiation escaping from the SCPC is estimated at a relatively high level, (energy: 68.34 + 4.43=72.77% and exergy 57.069 + 0.026 + 0.132=57.226%). The energy analysis allows for splitting the escaping radiation loss according to the radiosity of surface 2 and 3, whereas Exergy analysis of solar radiation processes 71 Table 2.4.4 The comparison of the energy and exergy balance terms for the considered SPC, (e3 =1), (from Petela, 2005). Energy Exergy Description expression % expression % Input: Irradiation (radiosity J1 =I) (I=1586.4W) 100 (I ·.S =1483.1W) 100 Total 100 100 Output: Escaping radiosity from surface 2 ß.21J2 68.34 Escaping radiosity from surface 3 ß.31J3 4.43 Escaping fraction of emission E1, (E1 =I) .B11 57.069 Escaping fraction of emission E2 .B12 0.026 Escaping fraction of emission E3 .B13 0.132 Radiation irreversibility on surface 2 .B2 20.433 Radiation irreversibility on surface 3 .B3 20.528 Transfer of convective heat from surface 2 ßQ2c 13.41 .BQ2c 0.619 Transfer of radiative heat from surface 2 ßQ2r 1.36 .BQ2r 0.063 Transfer of convective heat from surface 3 ßQ3c 3.22 .BQ3c 0.292 Irreversibility of transferred useful heat Q3u .BQ3u 0.004 Useful heat Q3u delivered to water . 9.24 .B 0.834 Total 100 100 the exergy analysis makes this split according to the temperature (TS, T2 and T3) of the escaping emissions. According to the energy analysis the heat losses to the environment are relatively high; by convection 13.41% (Q2,c), 3.22% (Q3,c), and by radiation 1.36% (Q3,r), whereas the exergy estimation of these losses is relatively very low; (0.619% and 0.292% and 0.063%), respectively. As shown in Table 2.4.4, the energy analysis does not reveal any degradation losses at surfaces 2 and 3 and during transfer of the useful heat, in contrast to the exergy analysis which, respectively, estimates the first two losses relatively high 20.433%, 20.528%, and the third loss as a very small; 0.004%. More details of the energy and exergy balances, as well as optimization possibilities, were analyzed by Petela (2010). 2.4.3 Solar chimney power plant The considered solar chimney power plant (SCPP) is one of many possible examples of the power plant driven by solar radiation. The overall process is very complex and, up to date, only some selected aspects of the SCPP have been studied. The outline of the methodology of simplified exergy analysis applied to the SCPP and the possible different thermodynamic interpretations of processes occurring in the SCPP were developed including the following characteristic elements: – formulation of energy balance of total SCPP, – application of exergy balance for interpretation of component processes, 72 Solar energy sciences and engineering applications Figure 2.4.14 Scheme of the considered SCPP, (from Petela, 2009). – application of eZergy balance for estimation of effect of gravity, – involving exchange of radiation energy and exergy between chimney and deck, – distinguishing the energy, exergy and eZergy losses to the environment and sky, – proposing the convective-radiative effective temperature concept for the surfaces. As shown in Figure 2.4.14, the considered SCPP consists of a circular greenhouse type collector and a tall chimney at its centre. Air flowing radially inwards under the collector deck is heated from the collector floor and deck, and through a turbine enters the chimney. A draft-driven environmental air (point 0) enters the collector through the gap of height He. The collector floor of diameter Df is under the transparent deck which declines appropriately to ensure a constant radial cross-section area for the radially directed flow of the air. The assumption of constant cross-section area in the collector means that p · Df · He =p · D1 · H1 =p · D21 /4, and so, the assumed value He allows for calculation of the inlet turbine diameter D1 =(4 · He · Df )0.5 and height H1 =D1/4. The collector floor preheats air from state 0 to state 1 (state 1 prevails in the zone denoted with a dashed line). Preheated air (state 1) expands in the turbine to state 2. The turbine inlet and outlet diameters are D1 and D2, respectively. The height of turbine is HT; (H1 + HT =H2). Expanded air leaves the SCPP (at point 3) through the chimney at height H3. For the established geometrical parameters of the collector-turbine-chimney system, and for the constant thermodynamic input data, like solar radiation intensity and environment parameters, the system spontaneously self-models in response to the actual situation. This means that the buoyancy effect determines the flow rate of air Exergy analysis of solar radiation processes 73 through the system and all the air parameters; temperature and pressure along the air flow path. The present study attempts to develop analysis of total SCPP process. The complexity of such a thermodynamic object enforced many simplifying assumptions and the main are: (i) Floor has no heat loss to the soil; is perfectly insulated, and is perfectly black (emissivity ef =1). Thus, there is no solar energy reflected from the floor. (ii) Deck material is prepared in such a way that it is almost perfectly transparent for solar radiation (transmissivity td =0.95) and the remaining part (5%) of solar radiation arriving at the deck is reflected. However, the deck material absorbs perfectly (absoptivity a=1) low temperature radiation from the floor. Thus, consideration of multi-reflected radiation fluxes is simplified. In addition, the deck is thin enough that heat conducted through the deck occurs at zero temperature gradient. (iii) A certain “effective temperature’’ Teff of floor or deck is applied, which expresses potential to the heat transfer by conduction and radiation: A(kaverageTeff + sT4 eff )=  A klocalT dA+ s  A T4dA (2.4.71) where kaverage and klocal are the average and local convective heat transfer coefficients, respectively, s is the Boltzmann constant for black radiation, A is the surface area and T is the local surface temperature. (iv) The chimney material is perfectly black. The chimney wall is thin thus there is no temperature gradient along the wall thickness and both sides of chimney (inner and outer has the same temperature (average) constant along the chimney height. (v) Distribution of air temperature is represented by a certain effective temperature defined according to equation (2.4.71), however with excluded radiative heat transfer. (vi) Air is considered as an ideal gas whose parameters fulfill the state equation; p=. · R · T, and the specific heat is assumed constant, (average, not varying with temperature). (vii) Air is almost perfectly transparent for radiation, (transmissivity ta ˜1 and emissivity ea ˜0). Air can exchange heat only by convection and conduction. (viii) Air flow in the whole SCPP is frictionless. According to investigation by von Backström and Fluri (2006), the relative air pressure drop rT during expansion in turbine: “for maximum fluid power, the optimum ratio’’ is 2/3, thus p1 - p2 p1 - p3 =rT = 2 3 (2.4.72) The exemplary distribution of environment pressure and the pressure of air along its flow within the SCPP is shown in Figure 2.4.15. The air pressure of environment (solid thin line) drops from p0, at the zero level, to p3, at the level H3 of chimney inlet. The air pressure inside the SCPP drops from p0 to p1 at 74 Solar energy sciences and engineering applications Figure 2.4.15 Distribution of the absolute pressure in the considered SCPP, (from Petela, 2009). the collector outlet (dashed line) and it is assumed that the same pressure p1 prevails also at the inlet to the turbine. Then, within the turbine, air pressure (thick solid line) drops from p1 to p2 during adiabatic (isentropic) expansion generating power. Air from the turbine flows upward and its pressure (dotted line) achieves value p3 at the chimney exit. (ix) Using average values of gravitational acceleration and air density along the height H3: p3 =p0 - g0 + gH3 2 .0 + .H3 2 H3 (2.4.73) where the following approximations, used by Petela (2008), (Table 1), were applied: gH3 =g0 - 3.086 · 10-6 · H3 and .H3 =.0 - 9.973 · 10-5 · H3. At the Earth’s surface the atmospheric pressure p0 =101.235 kPa and gravity acceleration g0 =9.81 m/s2. (x) Momentum conservation equation for the air flow within collector is derived as: p0 - p1 =.a1w2 1 (2.4.74) where .a1 and w1 are the density and flow velocity of air at point 1. (xi) Deck and chimney radiate to the space of sky temperature Tsky determined by the Swinbank (1963) formula: Tsky =0.0552 · T1.5 0 for a clear sky. (xii) In order to obtain a fair comparison basis, the reference state for calculation of energy is the same as for exergy: environment temperature T0 =288.14 K, (15 C), and environment pressure p0 =101.235 kPa. Exergy analysis of solar radiation processes 75 Energy analysis is based on the energy conservation equations. The energies E are used in six equations written successively for: floor surface, air in collector, collector (including floor, air and deck), turbine, chimney and chimney surface: ES-f =Ef -a + Ef -d (2.4.75) Ef -a + Ed-a =Ea1 + Ew1 + Ep1 (2.4.76) ES-f =Ea1 + Ew1 + Ep1 + Ed-sky + Ed-0 + Ed-ch (2.4.77) Ea1 + Ew1 + Ep1 =Ea2 + Ew2 + Ep2 + EP (2.4.78) Ea2 + Ew2 + Ep2 + Ed-ch =Ea3 + Ew3 + Ep3 + Ech-0 + Ech-sky + Ech-gr (2.4.79) Ea-ch + Ed-ch =Ech-0 + Ed-sky + Ech-gr (2.4.80) Energies E have the following subscripts: S-f – solar radiation arriving at the floor, f -a – convection heat from floor to air, f -d – energy exchanged by radiation between floor and deck, d-a – convection heat from deck to air, d-sky – energy exchanged by radiation between deck and sky, d-0 – convection heat from deck to atmosphere, d-ch – energy exchanged by radiation between deck and chimney, ch-0 – convection heat from chimney surface to atmosphere, ch-sky – energy exchanged by radiation between chimney surface and sky, ch-gr – energy exchanged by radiation between chimney surface and ground, a-ch – heat transferred from chimney air to the chimney surface, a1, a2, a3 – enthalpy of air at points 1, 2 and 3, w1, w2, w3 – kinetic energy due to the air flow velocity w1, w2 and w3, p1, p2, p3 – potential energy of air at points 1, 2 and 3, P – turbine power. Kinetic energies are calculated as Ew =m · w2/2, where m is the air mass flow rate; m=0.25 · p · D21 · w1 · .a1. Enthalpy of air is Ea =m · cp · (Ta - T0) where cp is the specific heat of air at constant pressure. The potential energy of the considered air, at its constant density ., depends on the altitudinal variation of atmospheric air density and gravity acceleration. The solution of the differential formula (2.2.16) on potential energy Ep, J/kg, equal to potential exergy Bp, is determined by Petela (2010): Ep =m  - 1 a4.  a2 6a4 (. - a3)3 + a1 2 (. - a3)2  (a) where the constant values are a1 =9.7807 m/s2, a2=-3.086 × 10-6 1/s2, a3 = 1.217 kg/m3, and a4=-9.973 × 10-5 kg/m4. Total solar energy received by the floor is: ES-f =tdef SAd (b) 76 Solar energy sciences and engineering applications where S, W/m2, is the solar radiosity at the Earth’s surface, td is the transmissivity of deck, and ef =1 is the floor emissivity. Energy exchanged by radiation between deck and chimney: Ed-ch =ed.d-ch p 4 [D2f - (cDD2)2]s(T4 d,eff - T4 ch) (c) where s is the Boltzmann constant for black radiation, Td,eff is the effective temperature of the deck, and cD is the factor to account on thickness of the chimney wall. The view factor .d-ch can be calculated from reciprocity relation: .d-ch p 4 [D2f - (cDD2)2]=.ch-dpcDD2(H3 - H2) (d) It can be derived that .ch-d =0.5 · (90 - ß)/90 where the angle ß is determined by tan ß=2 · H3/Df . Energy exchanged by radiation between floor and deck: Ef -d =Ads(T4 f ,eff - T4 d,eff ) (e) where Tf ,eff is the effective temperature of the floor and surface area Ad =p· (D2f - D21 )/4. The following formulae are applied for convection heat transfer from: floor to air: Ef -a =Adkf -a(Tf ,eff - Ta,eff) (f) deck to air: Ed-a =Adkd-a(Td,eff - Ta,eff) (g) deck to environment: Ed-0 =Adkd-0(Td,eff - T0) (h) chimney to environment: Ech-0 =Achkch-0(Tch - T0) (i) and chimney air to chimney wall: Ea-ch =pD2(H3 - H2)ka-ch  Ta,2 + Ta,3 2 - Tch  (j) where k is the respective coefficient and the chimney surface Ach =p · cD · D2· (H3 - H2). It is assumed that the coefficient ka-ch =Nu · ./D2 where .=0.0267 W/(m K) is thermal conductivity of air and the Nusselt number Nu=0.023 · Re0.8 · Pr0.4 and where the Prandtl number for air is Pr=0.7 and the Reynolds number Re=w2 · D2/., (kinematic viscosity coefficient for air .=1.6 · 10-5 m2/s). Exergy analysis of solar radiation processes 77 The coefficient kf -a is determined in a similar way. Although the air flow is driven by buoyancy effect, the forced convection mechanism of the air flow is assumed. Thus the calculations are based on the Reynolds number instead of the Grashof number. In calculations the average flow velocity of air is assumed. The effective diameter Deff for the air flow can be assumed as the average ratio of the respective flow crosssection area A1 multiplied by four, to the respective average perimeter lengths; Deff = (4/p) · A1/(Df + D2). It was assumed that ka-d =kf -a. The following formulae are applied for energy exchange by radiation between: floor and deck: Ef -d =Ads(T4 f ,eff - T4 d,eff) (k) deck and chimney: Ed-ch =.d-chAds(T4 d,eff - T4 ch) (l) deck and sky: Ed-sky =.d-skyAds(T4 d,eff - T4 sky) (m) chimney and sky: Ech-sky =.ch-skyAchs(T4 ch - T4 sky) (n) chimney and ground beyond the floor: Ech-gr =.ch-grAchs(T4 ch - T4 gr) (o) where the view factors fulfill the following relations: .d-sky + .d-ch =1 (p) .ch-sky + .ch-d + .ch-gr =1 (q) The view factors .ch-d and .d-ch are determined based on equation (d), whereas the configuration of chimney relative to sky determines view factor .ch-sky =0.5. Calculation of temperature Ta,2 is based on the equation for the isentropic expansion in the turbine at assumed isentropic exponent . for air and the internal efficiency of the turbine .T. Conversion of the energy of air into electric power occurs at an overall efficiency .o which would include additionally mechanical and electric efficiencies of the turbine-generator unit. Additionally it was assumed that the air temperature distribution in the collector is linear and thus Ta,eff =(T0 + Ta1)/2. The diameter ratioD1/D2 =0.95. The air temperature drop in the chimney can be tentatively estimated as proportional to the chimney surface and inversely proportional to the air mass rate; Ta,2 - Ta,3 ˜0.154 · D2 · H3/m. The presented mathematical model of SCPP is illustrated with exemplary computation results in which, for comparison to the 36-kW pilot SCPP plant in Manzares, near 78 Solar energy sciences and engineering applications Madrid, Spain, the floor diameter is Df =240m and the chimney height H3 =195 m. Other data are as follows: S=800W/m2 Tgr =T0 cD =1.015 cp =1000 J/(kg K) .=1.4 .T =0.7 R=287.04 J/(kg K) HT =1m kch-0 =7 W/(m2 K) kd-0 =5 W/(m2 K) He =0.3m The computation results are shown in the bands diagram (Figure 2.4.16) in which the values e are expressed in %, and the solar radiation energy arriving at the deck ES =39.05MW is assumed as 100%. This amount, reduced by the 5% reflection, is distributed between five SCPP components; collector air, floor, deck, turbine and chimney. The floor (black body) fully absorbs the solar radiation (95.00%) transmitted through the deck and converts this radiation energy to the energy at the level of temperature Tf ,eff . Part of this energy (ef -d =77.19%) radiates to the deck and the rest ef -a =17.81% is transferred by convection to heated air in the collector. The power performed by the turbine is relatively small (EP =0.23MW) mostly due to the small mass flow rate of air (m=276 kg/s) and due to small pressure drop during the air expansion. The percentage power of the turbine eP =0.64% represents the energy efficiency .E of the SCPP. The exhausted energy (enthalpy) of air from chimney is ea3 =20.75% whereas the exhausted potential and kinetic energies are small; ep3 =0.52% and ew3 =3.87 × 10-4%, respectively. The other SCPP energy losses are by radiation and convection heat transferred from deck and chimney to the sky and environment. Solar energy reflected from the deck is assumed eR =5.00%. Exergy analysis is based on the exergy balance equations. Exergy B in these equations has the subscripts respectively to E in equations (2.4.76)–(2.4.80) for energy analysis. The five separate exergy equations can be written for floor, deck, air in collector, turbine and chimney. The exergy equations are analogical to energy equations and differ by the additional members, B, representing the respective irreversible exergy losses: BS-f =Bf -a + Bf -d + Bf (2.4.81) Bf -d =Bd-a + Bd-sky + Bd-0 + Bd-ch + Bd (2.4.82) Bf -a + Bd-a =Ba1 + Bw1 + Bp1 + Ba (2.4.83) Ba1 + Bw1 + Bp1 =Ba2 + Bw2 + Bp2 + BP + BT (2.4.84) Ba2 + Bw2 + Bp2 + Bd-ch =Ba3 + Bw3 + Bp3 + Bch-0 + Bch-sky + Bch-gr + Bch (2.4.85) Exergy of solar radiation can be estimated for the radiation temperature slightly smaller than 6000 K, e.g., BS ˜. · ES, where .˜0.9. Exergy analysis of solar radiation processes 79 Figure 2.4.16 ENergy balance of the SCPP, (from Petela, 2009). Generally, in determined geometrical configuration, the radiation exergy B of a surface at its temperature T, emissivity e and surface area A, is determined based on formulas e.g., (2.2.33), (2.2.40) and (2.2.63) applied for the whole area A: B=.Ae s 3 (3T4 + T4 0 - T0T3) (2.4.86) 80 Solar energy sciences and engineering applications where . is the view factor accounting for geometrical configuration of the considered surface in relation to an eventual surface at which the considered radiation would arrive. Based on equation (2.4.86) the exergy of radiation Bx-y exchanged between any two different surfaces at different temperature Tx and Ty can be determined according Petela (2010): Bx-y =Bx - By =.x-yAxex-y s 3 [3(T4 x - T4 y ) - 4T0(T3 x - T3 y )] (2.4.87) where Ax is the surface area of one of two considered surfaces, .x-y is the view factor for configuration of surfaces x and y, ex-y is the effective emissivity depending on emissivities ex and ey of respective surfaces and calculated as for radiation energy exchange. The effective emissivity simplifies to ex-y =1 when the emissivities ex =ey =1. Formula (2.4.87) is used appropriately for calculations of the five radiation exergies: Bf -d, Bd-sky, Bd-ch, Bch-sky, and Bch-gr. The physical exergy of air (Ba1, Ba2 and Ba3 in W), is calculated for the air mass flow rate m with use of formula (2.2.15) for specific exergy, (J/kg): Ba =m  cp(Ta - T0) - T0  cp ln Ta T0 - Rln p p0  (s) where cp and R are the specific heat and individual gas constant. Obviously, exergy of air entering the collector is zero, (Ba0 =0), because air is taken from environment. Exergy B of convective heat E transferred from a surface at temperature T to air (environmental or heated) is calculated based on formula (2.2.4): B=E  1 - T0 T  (t) Formula (t) is used appropriately for calculations of the four exergies Bf -a, Bd-a, Bd-0, and Bch-0. Potential exergies of air are equal to potential energies, (Bp1 =Ep1, Bp2 =Ep2 and Bp3 =Ep3). Kinetic exergies of air are equal to kinetic energies, (Bw1 = Ew1, Bw2 =Ew2 and Bw3 =Ew3). The computation results are shown in the bands diagram (Figure 2.4.17). The solar radiation exergy arriving at the deck BS =32.41MW, assumed as 100%, is distributed between five SCPP components; collector air, floor, deck, turbine and chimney. In the diagram the exergy streams B, W, are represented by their percentage values b related to the solar radiation exergy BS. Exergy considerations disclose large degradation of solar radiation. The floor fully absorbs the received high temperature radiation exergy and converts it to the exergy at the lower temperature Tf ,eff . Part of this Tf ,eff exergy (bf -d =17.24%) radiates to the deck and another part bf -a = 5.10% is transferred by convection to heated air in the collector. The remaining large part (bf =72.16%) is lost during irreversible processes of absorption and emission at the floor surface. The power Bp performed by turbine is the same as in the energy balance; BP = EP =0.23MW. The percentage power of the turbine bP =0.70% represents the exergy efficiency .B of the SCPP. Exergy efficiency is slightly higher than the energy efficiency because the same power is related to the radiation exergy which is smaller than the Exergy analysis of solar radiation processes 81 Figure 2.4.17 EXergy balance of the SCPP, (from Petela, 2009). radiation energy. The exhausted exergy of air from the chimney is negative ba3 = -0.61% whereas the exhausted potential and kinetic exergies are small; bp3 =0.01% and bw3 =ew3, respectively. The SCPP is losing the exergy due to irreversibility and by radiation and convection heat transferred from deck and chimney to the sky and 82 Solar energy sciences and engineering applications environment. Solar exergy reflected from the deck is bR =eR =5%. The possibility of negative value of physical exergy (ba) of air was discussed in paragraph 2.2.3.1. EZergy analysis uses mechanical exergy component for air. Exergy balance equations for floor and deck do not contain terms for substance. These two equations, (2.4.81) and (2.4.82), remain unchanged because they contain only terms of the radiation and convection heat for which the gravitational effect is not considered. However, the exergy balance equations, (2.4.83), (2.4.84) and (2.4.85), considered for heating air in collector, turbine and chimney, are modified by adding the gravity input. Ga + Bf -a =Z1a + Bw1 + Ba-d + Ba (2.4.88) GT + Z1a + Bw1 =Z2a + Bw2 + BP + BT (2.4.89) Gch + Z2a + Bw2 + Bf-ch =Z3a + Bw3 + Bcv + Bch-0 + Bch (2.4.90) where Ga, GT and Gch are the gravity inputs in eZergy balance for the collector air, turbine and chimney, respectively. Note that in eZergy balance equation the potential exergy does not appear as a separate term because this exergy is interpreted by eZergy of substance. To more easily distinguish it from the traditional exergy (B or b), the eZergy is denoted by Z, W, or z,%. Denotations of other exergy magnitudes remain unchanged because their values are unchanged. However, eZergy of air generally differs from exergy of air; Za =Ba: Za =max(Bp + BH,Ba) (2.4.91) where Bp is the potential exergy, (Bp =Ep), Ba is the traditional physical exergy of air calculated from formula (s). Magnitude BH is the physical exergy calculated also based on equation (s), however, for the environment parameters (temperature TH and pressure pH) prevailing at the altitude H: BH =m  cp(Ta - TH) - TH  cp ln Ta TH - Rln p pH  (2.4.92) According to the interpolation formulae given by Petela (2009b): H =1.215485 · 106 - 1.214 · 106.6.02353·10-3 a (2.4.93) and the interpolated atmospheric parameters at altitude H: TH =288.16 - 0.0093H + 3.2739 · 10-7H2 - 2.9861 · 10-12H3 (2.4.94) pH =101235e1.322·10-4H (2.4.95) The computation results of exergy balances with use of eZergy are shown in the bands diagram (Figure 2.4.18). The solar radiation exergy BS =ZS =32.41MW, assumed as 100%, arriving at the deck is distributed between five SCPP components in the case of using substance eZergy. The eZergy streams Z, W, are represented by Exergy analysis of solar radiation processes 83 Figure 2.4.18 EZergy balance of the SCPP, (from Petela, 2009). their percentage values z, related to the solar radiation exergy ZS. The part of the diagram (Fig. 2.4.18) related to floor and deck is the same as in Figure 2.4.17, because substance does not appear in balances of the floor and deck. Also degradations of solar radiation and convective heat are the same like shown in Figure 2.4.17 and also the power performed by turbine is unchanged (0.23 MW). The percentage power of the 84 Solar energy sciences and engineering applications turbine zP =bP =0.70% represents the eZergy efficiency of SCPP. Specificity of the diagram in Figure 2.4.18 is showing the relatively large ezergies of air za1 =8.96%, za2 =8.82% and za3 =8.24%. As a result, the gravity inputs are: zGa =7.28% for the air in collector, smaller for turbine zGT =0.89% and the smallest for chimney zGch =0.67% More details of the energy and exergy balances, especially for different input parameters, were analyzed by Petela (2010). 2.4.4 Photosynthesis The very simplified threefold study of photosynthesis process was developed by Petela (2008a) including a) an energy analysis (the energy conservation equation developed to estimate the energy effects of the process); b) entropy analysis (the changes of entropy were used to estimate the irreversibility of the component processes); and c) exergy analysis (developed for thermodynamic evaluation of involved matters). In the present paragraph only the outline of energy (a) and exergy (c) analyses is discussed based on engineering thermodynamics to propose the methodology of exergetic consideration of photosynthesis. Photosynthesis is the process by which the energy of the photosynthetically active radiation (PAR), i.e. within the wavelength range 400–700 nm, is used to split gaseous carbon dioxide and liquid water and recombine them into gaseous oxygen and a sugar called glucose. The photo-chemical reaction of photosynthesis cannot occur without the presence of chlorophyll and is a complex two stage process. For the present analyses only the following endothermic overall reaction of the photosynthesis is considered: 6H2O + 6CO2.C6H12O6 + 6O2 (2.4.96) A simplified scheme of the considered system shown in Figure 2.4.19 is defined by the system boundary and contains a leaf surface layer in which biomass is created at temperature T. Diffusion of gaseous substances and convective heat transfer occurs through the gaseous boundary layer at the leaf surface. The boundary layer is not considered for radiation fluxes because it is assumed that air in this layer is transparent to radiation. The leaf surface absorbs part of the incident solar radiation and emits its own leaf radiation of temperature T. The absorbed radiation is expended on the metabolism processes of the leaf and on maintaining the leaf temperature T above the environment temperature T0. Liquid water, at temperature T, from the leaf body enters the considered system. A relatively small amount of this water is used for the assimilation of the CO2, which diffuses into the leaf from the external environment. The large excess of water is transpired in the form of vapor diffusing from the leaf to environment. Oxygen produced during the photosynthesis also diffuses into the environment. The water vapor and oxygen exiting the boundary layer, as well as CO2 entering the boundary layer, have environment temperature T0 at the respective environment mole concentrations zH2O,0, zO2,0, and zCO2,0. Only the chemical and physical components of the energy and exergy of the substances are considered. Also only the overall effects described by equation (2.4.96), Exergy analysis of solar radiation processes 85 Figure 2.4.19 Simplified scheme of substances and radiation fluxes in photosynthesis, (from Petela, 2008a). and the matter fluxes observed around the leaf (Figure 2.4.19), are analyzed based on the following main assumptions: (i) The considered area is a conventional horizontal unitary (1m2) surface of the leaf in a certain instant at the determined constant conditions during which the input is equal to output and change in the system. (ii) To determine the actual energy arriving at the leaf surface, the solar radiation energy of the spectrum measured at the highest layer of atmosphere, is multiplied by a certain weakening factor .; the larger the ., the smaller the weakening, (. =1). In the proposed simplified model the weakening factor is not studied, nor is it concretely defined. Only certain possible values of . are used in calculations. The radiation arriving at the leaf could be accurately determined by measuring of the radiation spectrum directly at the leaf surface so the factor . would be given. However the purpose of the present considerations is analysis of photosynthesis for a given .. (iii) Cloudy situations are not analyzed. (iv) The solar radiation arrives, directly from the Sun in the zenith (solar radiation is perpendicular to the horizontal surface of the considered leaf), within the solid angle determined by the diameter of the Sun and its distance from the Earth. The reduced effect due to the non-perpendicular radiation could be expressed e.g. by an appropriate value of factor .. (v) Sufficient chlorophyll necessary for the photosynthesis process is available. Any change in the chlorophyll concentration during photosynthesis, and the thermodynamic effect of such change, are neglected. 86 Solar energy sciences and engineering applications (vi) The surroundings of the considered leaf consists only of the surfaces at temperature T0 and of absorptivity a0 =1, (the Sun’s surface area is neglected as being seen within a relatively very small solid angle). (vii) Mixtures of substances in the system are ideal; the components do not mutually interact. Therefore, mixture properties are the respective sums of the component properties. For example, the biomass contained within the leaf structure is an ideal solution of solid C6H12O6 and water. (viii) The environment air contains onlyN2, O2, CO2 andH2O. The dry environment air contains 79.07% N2, 20.9% O2, and 0.03% CO2. The sum of all mole fractions of the air components is: zN2,0 + zO2,0 + zCO2,0 + zH2O,0 =1, where zH2O,0is determined by the relative humidity .0 of the air and the saturation pressure ps0 for the environment temperature T0: zH2O,0 =.0 · ps0. In terms of radiation it is assumed that the diatomic gases have transmissivity 100% and the concentration of the triatomic components (CO2 and H2O) is relatively small and they also have transmissivity 100%. (ix) The considered leaf has uniform temperature T; there is no heat transfer within the considered surface layer. According to the evaluation by Jørgensen and Svirezhev (2004) there is a several degree difference T between the leaf temperature and environment temperature. (x) The liquid water required for photosynthesis is available in sufficient amount. (xi) In the considered conditions the rate of sugar production is limited by the effectiveness of diffusion of gases; not by the reaction kinetics depending on temperature. (xii) The generated sugar has only chemical exergy bch resulting from chemical reaction (2.4.96). The component of the sugar exergy gained as a result of ordering (structure of the biomass in according to genetic plan) is neglected. It could be expected that some simplifications may not qualitatively affect the final conclusions although the quantitative results could be affected remarkably. The considered substances in the system (Fig. 2.4.19) are gaseous CO2, O2, H2O (assumed to be ideal), liquid water and the leaf substance (biomass). The enthalpies of the gases are zero because at the system boundary they have environment temperature T0. However, for the liquid H2O the water vapor is the reference phase. Therefore, the enthalpy of liquid water is equal to the sum of the negative value of the latent heat of vaporization at temperature T0 and of the temperature difference T=T - T0 multiplied by the specific heat of water. The generated biomass is assumed to be a mixture of liquid water and sugar. The enthalpy of this mixture is calculated as the sum of the component enthalpies. (For liquid and solid bodies the enthalpy is practically equal to the internal energy). The enthalpy of sugar is the sum of the physical enthalpy and devaluation enthalpy. The physical enthalpy of sugar is calculated for the temperatures range from T0 to T at the constant specific heat cSU =430.2 kJ/(kmol K). The values of devaluation enthalpies dn for the standard parameters (pressure pn = 101.325 kPa and temperature Tn =298.15 K) are tabulated by Szargut et al. (1988). The varying of the devaluation enthalpy within the considered temperatures range is negligible. Exergy analysis of solar radiation processes 87 Due to lack of data, the devaluation enthalpy dn,C6H12O6 =dn,SU =2, 529, 590 kJ/ (kmol of sugar) is assumed as for the a-D-galactose, predicting that the devaluation enthalpy of the real substance generated in the leaf differs insignificantly. Such an assumption can be supported by the fact that the devaluation enthalpies tabulated for the substances of the same chemical formula (a-D-galactose and L-sorbose), differ insignificantly. The physical bph and chemical bch exergy, are taken into account to calculate the total exergy b=bch + bph of any substance. The exergy of each gas (CO2, H2O and O2) is zero because in the considered case their states are in full equilibrium with environment. The exergy of liquid water bw, kJ/kmol, is the sum of the physical part bw,ph and chemical part bw,ch, where bw,ch =R · T0·ln(1/.0). Using the Szargut and Petela (1965b) diagrams the interpolation formula for calculation of the physical exergy bw,ph of liquid water is bw,ph =a + b · t + c · t2, where a=-23.22 + 2.718 · t0 + 0.0675 · t2 0, b=2.689 - 0.5787 · t0 + 0.00767 · t2 0, and c=0.117 - 1.05 · 10-3 · t0 + 2.7 · 10-4 · t2 0 - 7.5 · 10-6 · t3 0 and where t0 =T0 - 273. The exergy of the generated biomass is the sum of the exergy of the components (liquid water and sugar). The specific chemical exergy of sugar is determined based on the standard tabulated value bn,SU =2, 942, 570 kJ/kmol, to which the correction on the difference of temperatures Tn and T0 is added according to formula (2.2.20). The specific physical exergy bph,SU is determined from formula (2.2.21). The radiation arriving at the leaf surface from the Sun is recognized as nonpolarized and uniformly propagating within the solid angle under which the Sun is seen from the Earth. The radiosity jS of such solar radiation of the real spectrum as function of wavelength . is: jS =2 . ..  ß  . cos ß sin ß dß d. . ..  . i0,.d. (2.4.97) The double integral in the bracket of equation (2.4.97) was calculated in Example 2.4.1.1; formula (2.4.1), and if the single integral in equation (2.4.97) is presented in a numerical form, then: jS =4.329 · 10-5p n (i0,..)n (2.4.98) where i0,. is the measured monochromatic intensity of radiation depending on the wave length ., and n is the successive number of the wavelength interval within the considered wavelength range. For the 0 to 8 wavelength range jS =1.3679 kW/m2, as shown in Example 2.4.1.2. For the PAR arriving only within the wavelengths range (400–700 nm) the radiosity of the PAR calculated from equation (2.4.98), is jV =0.5446 kW/m2. The energy emission of the leaf surface propagates in all directions of hemisphere and it is assumed that the radiation of the environment arrives at the leaf surface from all directions of the hemisphere. Therefore, the energy eL exchanged between the leaf and the environment is: eL =aL,a · s · (T4 - T4 0 ), where aL,a is the average 88 Solar energy sciences and engineering applications absorptivity of the leaf surface, s is the Boltzmann constant for black radiation and T0 is the environment temperature. To simplify the consideration the sky temperature is assumed to be equal to the environment temperature. At small temperatures (T or T0) the energy of PAR is relatively small e.g. in comparison to the case of radiation at the Sun temperature. Therefore, the assumption that the average absorptivity aL,a equals the leaf absorptivity aL for the non-PAR wavelengths range: aL,a ˜aL, slightly affects the value of the calculated energy eL. Exergy bS for a non-polarized, uniform and direct solar radiation arriving in the Earth’s atmosphere, is calculated based on formula (2.4.3) in which frequency . is replaced by wavelength . and the double integral value is according to (2.4.1): bS =4.329 · 10-5p  n (i0,..)n - T0 n (L0,..)n + 9.445 · 10-12 p T4 0  (2.4.99) Using T0 =293K in equation (2.4.99), the exergy of total solar radiation bS = 1.2835 kW/m2 and the exergy of PAR, bV =0.5155 kW/m2, are calculated. Emission exergy bL of leaf surface at temperature T can be determined by the formula (2.2.40) with introduction of absorptivity aL (assumed equal to emissivity eL): bL =aL s 3 (3T4 + T4 0 - 4T0T3) (2.4.100) whereas the exergy of environmental radiation arriving at the leaf surface is zero. Mass conservation equations are the basis for further considerations. The mass fluxes, kmol/(m2 s), of CO2 and O2 are determined by the stoichiometric factors of equation (2.4.96): nCO2 =6nSU, nO2 =6nSU, where nSU is the amount of produced sugar (kmol) within period of 1 s and per 1 m2 of irradiated leaf surface. The mass flux nw of water entering the leaf contains: a) water nwL within the generated biomass, b) water 6·nSU entering the chemical reaction, and c) water nH2O vaporized into environment. Thus, nw =nwL + 6nSU + nH2O where nwL =nSU(1 - zSU)/zSU and where zSU is the mole fraction of sugar in the biomass composed of sugar and water. As discussed by Jørgensen and Svirezhev (2004), an important factor in the determination of the effectiveness of photosynthesis is the mole ratio r=nH2O/nHCO2 of the water vapor and carbon dioxide rates. Water vapor diffuses from the internal surface of the leaf, through the stomata and intercellular space, towards the external surface of the leaf, and then diffuses through the boundary layer to the atmosphere. The water rate, nH2O, is proportional to the generalized coefficient DH2O of diffusion and to the difference (zH2O,L - zH2O,0) where zH2O,L is the initial mole concentration of vapor at the inner surface and zH2O,0 is the final mole concentration in the environment. Diffusion of carbon dioxide occurs in the opposite direction and is also proportional to the generalized CO2 diffusion coefficient DCO2, and to the respective difference of mole concentrations zCO2,0 and zCO2,L. Therefore the rates ratio is: r= DH2O DCO2 zH2O,L - zH2O,0 zCO2,0 - zCO2,L MH2O MCO2 (2.4.101) Exergy analysis of solar radiation processes 89 whereMH2O andMCO2 are the molecular masses of H2O and CO2. The diffusion coefficients ratio was estimated by Jørgensen and Svirezhev (2004) as DH2O/DCO2 ˜1.32 and according to Budyko (1977): zCO20 - zCO2L ˜0.1zCO20. It is also assumed that the concentration of water vapor within the leaf corresponds to the saturation pressure ps,T at temperature T: zH2OL =ps,T/p0. Thus equation (2.4.101) can be written as: r=5.4 psT - .0ps0 p0zCO20 (2.4.102) The ratio r is determined by the diffusion processes which control the rate of reaction (2.4.96), accordingly to assumption (xi). Energy conservation equation for the system at instant steady state shown schematically in Figure 2.4.19 includes the energy delivered of absorbed solar radiation and the enthalpies of carbon dioxide and liquid water. The energy increase of the system is determined by the rates of the sugar substance and liquid water in the produced biomass. The extracted energy consists of the enthalpies of oxygen and water vapor as well as convective heat and emission exchanged by the leaf surface: .[aVjV + aL(jS - jV)] + nCO2hCO2 + nwhw =nSUhSU + nwLhw + nO2hO2 + nH2OhH2O + qk + eL (2.4.103) where aV and aL are the absorptivities of the leaf within and beyond the PAR wavelength range, respectively. According to assumption (vii), the biomass is an ideal solution of sugar and water and the total enthalpy of biomass is the sum of the respective components; nSU · hSU and nwL · hw. The heat transferred by convection from the leaf surface to the environment is: qk =k · (T - T0), where k is the convective heat transfer coefficient. Equation (2.4.103) is used to calculate the unknown rate nSU. The leaf temperature T is higher than the environment temperature T0 by the difference T, i.e. T =T0 + T, according to assumption (ix). Exergy balance equation according to the scheme (Fig. 2.4.19) is: .[aVbV + aL(bS - bV)] + nCO2bCO2 + nwbw =nSUbSU + nwLbw + nO2bO2 + nH2ObH2O + bqk + beL + db (2.4.104) where db is the total exergy loss due to the sum of every irreversibility occurring within the system and is determined by formula (2.2.10). Again as for enthalpy, the total exergy of biomass is the sum of the respective components; nSU · bSU and nwL · bw. Perfection degrees of photosynthesis are calculated based on the assumption that the produced sugar represents the useful product and the feed is determined by radiation, CO2 and liquid water. Other components of the balance equations are categorized 90 Solar energy sciences and engineering applications as waste. Based on the definition discussed in paragraph 2.3.4.2, the energy degree of perfection .E, of the considered photosynthesis: .E = nSUhSU .[aVjV + aL(jS - jV)] + nCO2hCO2 + nwhw (2.4.105) whereas the exergy degree of perfection .B, of the photosynthesis based on formula (2.3.18): .B = nSUbSU .[aVbV + aL(bS - bV)] + nCO2bCO2 + nwbw (2.4.106) Example 2.4.4.1 The following input values have been used in the exemplary computations for the system presented in Figure 2.4.19: – Environment temperature T0 =293 K, – Temperature difference T =5K, – Relative humidity of environment air .0 =0.4, – Environment pressure p0 equal to the standard pressure p0 =pn =101.325 kPa, – Weakening radiation factor . =0.7, – Leaf absorptivity within PAR wavelength range aV =0.88, – Leaf absorptivity beyond the PAR range aL =0.05, – Convective heat transfer coefficient k=0.003 kW/(m2 K), – Mole fraction of sugar in biomass zSU =0.08. The leaf temperature is T =T0 + T=298K and from equation (2.4.104) the sugar production rate nSU =3.21 · 10-9 kmol/(m2 s). The percentage terms of energy and exergy equations (2.4.103) and (2.4.104), respectively, are shown in Fig. 2.4.20. The 100% reference for the output terms is assumed as the input sum due to the absorbed radiation and the substances of CO2 and liquid water. Thus the perfection degree value .E =35.4% is larger than .B =2.608% (by about 35.4/2.608˜14 times) mainly because of the denominators in equations (2.4.105) and (2.4.106). The exergy of liquid water, in the denominator of equation (2.4.106), is positive whereas the energy of this water in the denominator of equation (2.4.105) is negative, (the water vapor is assumed as the reference phase for enthalpy calculation). The energy terms (Fig. 2.4.20, energy balance) show that the input consists of the positive radiation energy (1459.8 + 125.4=1585.2%) and the negative (-1485.2%) liquid water enthalpy. The energy of the consumed carbon dioxide is zero because it enters the system at the reference temperature T0. The output energy terms show no irreversible loss and the zero energy of both; the produced oxygen and released water vapor leave the system at the reference temperature. Heat transferred by convection and radiation are 65.3% and 6.4%, respectively. The energy of liquid water contained in the produced biomass is negative (-7.1%) also because the vapor phase was assumed as the reference substance for water. The exergy input terms (Figure 2.4.20, exergy balance) are the absorbed radiation (87.76 + 7.43=95.19%) and the water of positive value 4.81%. The exergies of the delivered CO2, released O2 and water vapor are zero because these gases, at the Exergy analysis of solar radiation processes 91 Figure 2.4.20 Band diagram of energy and exergy balances for the considered photosynthesis process shown in Figure 2.4.19 (from Petela, 2010). external side of the system boundary layer, have the parameters equal to the environment parameters. The exergy of convective heat is zero because it is released to the environment. The exergy of the leaf radiation (0.003%) is small due to relatively low temperature of the leaf and is significantly smaller than the respective energy (6.4%). The exergy of liquid water contained in the produced biomass is 0.023%, (positive). Unlikely the enthalpy and energy analyses, exergy analysis shows the irreversibility loss, which in this case is relatively very large (97.366%). More aspects of the energy and exergy balances and some inspired problems were analyzed by Petela (2010). 2.4.5 Photovoltaic The present paragraph gives an outline of the simple energy and exergy analysis of the simultaneous generation of heat and power by photovoltaic (PV) technology. This double conversion of radiation energy can categorize the PV technology into the general systems of cogeneration of power and heat. In the considerations only direct solar radiation is accounted for the case of a clear sky (at temperature Tsky assumed as environment temperature T0, Tsky =T0), and assuming the Sun is a black surface. As there is no motion of substance in the gravitational field, the eZergy consideration is not introduced. The 1 m2 surface of a solar cell at temperature TC is shown in Figure 2.4.21. Generally, the heat qS transferred from the Sun’s surface at temperature TS to the outer surface of the solar cell on the Earth is distributed to the generated electrical energy E, reflected solar radiation qr, useful heat qC absorbed by the solar cell, and to convection and radiation heat 92 Solar energy sciences and engineering applications Figure 2.4.21 Energy streams of solar cell (from Petela, 2010). qk and q0, respectively, both transferred to the environment. Therefore, the energy conservation equation for the considered system, defined by the system boundary is: qS =qr + qk + q0 + qC + E (2.4.107) where qS =2.16 × 10-5sT4 S (i) qr =.CqS (ii) qk =k(TC - T0) (iii) q0 =eCs(T4C - T4 0 ) (iv) and where 2.16 × 10-5 is the view (Sun-Earth) factor, s is the Boltzmann radiation constant of a black surface, k is the convective heat transfer coefficient and T0 is the environment temperature. The solar cell surface is assumed to be perfectly gray at emissivity eC, (reflectivity .C =1 - eC). The useful heat qC can be determined from equation (2.4.107) if the electrical energy E is known, e.g., from the measurement. The solar cell can be evaluated by the energy electrical efficiency: .E,el =E/qs, by the energy heating efficiency: .E,q =qc/qs, or by the energy cogeneration efficiency: .E,cog =(E + qc)/qs According to exergetic interpretation the exergy bS incoming to the considered surface from the Sun is split into the exergy br of reflected solar radiation, the exergy of heat b0 radiating to the environment, exergy of heat bk transferred to the environment by convection, exergy of useful heat bC transferred from the solar cell to its interior, electric energy E and the exergy loss db due to the irreversibility of the considered system. Thus, the exergy equation for the system shown in Figure 2.4.21 is: bS =br + bk + b0 + bC + E + db (2.4.112) where: bS =2.16 × 10-5 s 3 (3T4 S + T4 0 - 4T0T3 S) (v) Exergy analysis of solar radiation processes 93 Table 2.4.5 Results of the energy and exergy calculations of the solar cell. Term Related symbol Energy % Exergy % INPUT: solar radiation eS, bS 100 100 Subtotal 100 100 OUTPUT: reflection (.C) 5 5 convection qk, bk 6.21 0.62 radiation qr, br 12.43 0.67 useful heat qc, bc,(.E,q,.B,q) 65.88 6.61 electricity E,(.E,el, .B,el) 10.48 11.16 irreversibility loss db – 75.94 Subtotal 100 100 br =.CbS (vi) bk =qk  1 - T0 TC  (vii) b0 =eC s 3 (3T4C + T4 0 - 4T0T3C ) (viii) bC =qC  1 - T0 TC  (ix) The exergy loss db can be calculated by completion of equation (2.4.112). From the exergetic viewpoint the solar cell can be evaluated by: the exergy electric efficiency .B,el =E/bs, the exergy heating efficiency .B,q =bc/bs, or the exergy cogeneration efficiency .B,cog =(E + bc)/bs. As the solar radiation energy is always larger than the solar radiation exergy (qS >bS), and the electrical exergy and electrical energy are equal, thus the energy electrical efficiency of the solar cell is always smaller than the exergy electrical efficiency, (.B,el >.E,el ). Example 2.4.5.1 A 1m2 surface of the photovoltaic cell at temperature Tc =318 K, emissivity eC =0.95, (reflectivity .C =0.05), generates E=152W of electrical energy. The temperature of the Sun is assumed TS =5800 K. The convective heat transfer coefficient k=3 W/(m2 K). The results calculated with the formula of the present paragraph are shown in Table 2.4.5. The values of the solar radiation energy and exergy: eS =1386W/m2 and bS =1294W/m2, were respectively assumed as the 100% in the energy and exergy balances. The instant energy electric efficiency .E,el =10.48% is smaller from the exergy electric efficiency .B,el =11.16%, however the energy cogeneration efficiency .E,cog = 10.48 + 65.88=76.36% is significantly larger than the exergy cogeneration efficiency .B,cog =11.16 + 6.61=17.77%. Table 2.4.5 illustrates also that the low temperature heat (convection, radiation and useful heat) has small exergy value. 94 Solar energy sciences and engineering applications Nomenclature A surface area, m2 a universal constant a=7.564 · 10-16 J/(m3 K4) B exergy, J b exergy of emission, W/m2 c0 velocity of light in vacuum c0 =2.9979 · 108 m/s cp specific heat at constant pressure, J/(kg K) D diffusion coefficient, m2/s or diameter, m dn enthalpy of devaluation, J/kmol E energy, J or W/m2 e density of emission energy, W/m2 G gravity input, J g gravity acceleration, m/s2 H enthalpy, J or height, m h Planck’s constant h=6.625 × 10-34 J s or enthalpy, J/kg I irradiance, W/m2 IR irradiance (par. N.4.1.2), W/m2 i. monochromatic directional radiation intensity, W/(m2 m sr) J radiosity, W j radiosity density, W/m2 k Boltzmann constant k=1.3805 × 10-23 J/K k convective heat transfer coefficient, W/(m2 K) L mean distance from the Sun to the Earth L=149,500,000 km. L. entropy of i., W/(m2 Ksr) m mass, kg n mass, kmol PAR photosynthetically active radiation p pressure, Pa Q heat, J q heat flux, W/m2 R individual gas constant, J/(kg K), or radius of the Sun R=695, 500km r mole ratio of the water vapor and carbon dioxide rates rT relative air pressure drop S entropy, J/K or irradiance (par. N.4.1.3 and N.4.1.4), W/m2 SR entropy of irradiance, W/(Km2) s entropy, J/(Km2) or J/K kmol) sj entropy of radiosity density, W/(m2 K) t time s, or temperature, C T absolute temperature, K U internal energy, J u specific internal energy, J/m3 V volume, m3 W work, J x distance, m Z eZergy, J z mole fraction or exergy, % Exergy analysis of solar radiation processes 95 Greek a absorptivity of radiation,% ß declination, deg e emissivity,% . azimuth, deg or view factor . radiation weakening factor . wavelength, m . efficiency,%  overall entropy growth, J/K . exergy/energy radiation ratio . oscillation frequency, 1/s . density, kg/m3 or reflectivity of radiation,% s entropy of devaluation reaction, J/(K kmol) s Boltzmann constant for black radiation s =5.6693 × 10-8 W/(m2 K4) t transmissivity of radiation,% . solid angle, rd Subscripts B exergetic ch chemical E energetic k convection m mechanical n standard (normal) ph physical r reflection S Sun sky sky 0 environment REFERENCES Budyko, M. I. (1977) Global Ecology. Mysl’, Moscow, 328 pp. Carnot, S. (1824) Reflections on the motive power of fire, and on machine fitted to develop that power. Bachelier, Paris. Duffie, J. A.&Beckman,W. A. (1991) Solar Engineering of Thermal Processes, 2nd edn. J.Wiley and Sons, New York. Gueymard, C.A. (2004) The Sun’s total and spectral irradiance for solar energy applications and solar radiation models. Solar Energy, 76, 423–453. Jacob, M. (1957) Heat Transfer. Vol. II, John Wiley, New York. Jørgensen, S.E. & Svirezhev, Y.M. (2004) Towards a thermodynamic theory for ecological systems. Elsevier Amsterdam. Kondratiew, K. Ya. (1954) Radiation energy of the Sun. GIMIS, (in Russian). Petela, R. (1961a) Exergy of heat radiation. PhD. Thesis, Faculty of Mechanical Energy Technology, Silesian Technical University, Gliwice (in Polish). 96 Solar energy sciences and engineering applications Petela, R. (1961b) Exergy of radiation of a perfect gray body. Zesz. Nauk. Pol. Sl. (30), Energetyka 5, 33–45, in Polish. Petela R. (1962) Exergy of radiation radiosity. Zesz. Nauk. Pol. Sl. (56), Energetyka 9, 43–70, (in Polish). Petela, R. (1964) Exergy of heat radiation. ASME Journal of Heat Transfer, 86, 187–192. Petela, R. & Piotrowicz, A. (1977) Exergy of plasma. Archiwum Termodynamiki i Spalania, 3, 381–391. Petela, R. (1984a) Exergetic analysis of atomization process of liquid. Fuel, 3, 419–422. Petela, R. (1984b) Exergetic efficiency of comminution of solid substances. Fuel, 3, 414–418. Petela, R. (1990) Exergy analysis of processes occurring spontaneously. CSME Mechanical Engineering Forum, June 3–9, University of Toronto, Vol. I, pp. 427–431. Petela, R. (2005) Exergy analysis of the solar cylindrical-parabolic cooker. Solar Energy 79, 221–233. Petela, R. (2008) Influence of gravity on the exergy of substance. International Journal of Exergy, 5, 1–17. Petela, R. (2008a) An approach to the exergy analysis of photosynthesis. Solar Energy, 82, 311–328. Petela, R. (2009) Thermodynamic study of a simplified model of the solar chimney power plant. Solar Energy, 83, 94–107. Petela, R. (2009a) Gravity influence on the exergy balance. International Journal of Exergy, 6, 343–356. Petela, R. (2009b) Thermodynamic analysis of chimney. International Journal of Exergy, 6, 868–880. Petela R. (2010) Engineering Thermodynamics of thermal radiation for solar power utilization, McGraw Hill, New York. Planck, M. (1914) The theory of heat radiation. Dover, New York, translation from German by Morton Mausius. Swinbank, W. C. (1963) Long-wave radiation from clear skies. Quarterly Journal of Royal Meteorological Society, 89, 339. Szargut, J. & Petela R. (1968) Exergy. Energija, Moscow, in Russian. Szargut, J., Morris, D.R. & Steward, F.R. (1988) Exergy analysis of thermal, chemical, and metallurgical processes. Hemisphere Publishing Corporation, New York. Von Backström, T.W. & Fluri, T.P. (2006) Maximum fluid power condition in solar chimney power plants – An analytical approach. Solar Energy, 80, 1417–1423. Chapter 3 Exergy analysis of solar energy systems Ibrahim Dincer & Tahir Abdul Hussain Ratlamwala Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, Ontario, Canada 3.1 INTRODUCTION Extensive use of fossil fuels in past decades has led us to the era of global warming and depletion of the stratospheric ozone layer. Fossil fuels such as gasoline, diesel, natural gas etc. when used emit harmful greenhouse gases such as CO, CO2, SO2 , NOx etc. Since the realization of the harmful effect of using fossil fuels, researchers have started looking for alternative sources of energy which are renewable and environmentally friendly. One of the very promising alternative and renewable sources of energy is solar energy. Solar energy converts solar flux entering the Earth’s surface into electricity or heat. The electricity generated by the solar energy system is in direct current format which can be used as it is or can be converted to alternative current based on end user requirements. Major benefits of using a solar energy system include (a) environmentally benign operation, (b) no moving parts, (c) no wearing of parts if the system is carefully protected from the environment, (d) energy output can vary from watts to megawatts based on the size of the system, (e) can be used to power phones or to power a community, and (f) module by module construction so that the size of the system can be altered based on the requirements. Solar energy systems are characterized as passive solar energy systems or active solar energy systems. Solar energy systems are assigned to these categories based on the way they capture, convert and distribute solar energy. Passive solar techniques include (a) designing a building in a way that it uses solar energy for day lighting, (b) selecting materials with favorable thermal mass or light dispersing properties, and (c) designing spaces such that they naturally circulate air. Active solar energy systems are divided in to three categories which are (a) photovoltaic systems which generate electricity, (b) thermal systems which generate heat, and (c) photovoltaic/thermal (PV/T) systems which generate both electricity and heat. There are two types of photovoltaic panels available in the market, which are rigid photovoltaic panels and flexible photovoltaic panels. Rigid photovoltaic panels have higher power to area density and flexible photovoltaic panels have low power to area density. Solar thermal systems are sub-divided into following categories: (a) low temperature such as flat plate collectors, (b) medium temperature such as concentrated collectors or solar dishes, and (c) high temperature such as heliostat fields. 98 Solar energy sciences and engineering applications Low-temperature thermal sources are usually used for direct application such as providing hot water or heat to the building. Medium temperature thermal sources are used for either producing cooling with the help of absorption cooling systems or producing power using Organic Rankine Cycles (ORCs). High temperature thermal sources are used to produce huge amounts of power by running a steam cycle. Integrated solar energy systems provide an attractive way of producing multiple outputs such as power, heat, hydrogen, cooling etc. in an environmentally benign manner. As it is expected that future economy will be dominated by hydrogen fuel, many researchers have studied integrated solar energy systems for hydrogen production. Thomas and Nelson (2010) stated that hydrogen fuel can be produced by using solar electric energy from photovoltaic (PV) modules for the electrolysis of water without emitting carbon dioxide or requiring fossil fuels. The results of analyses conducted by different researchers such as (Ratlamwala et al., 2011; Koroneos et al., 2004; Yilanci et al., 2009) related to solar hydrogen production show that an integrated solar production system is very promising technology as it produces hydrogen in an environmentally friendly and cost effective manner. Also studies have shown that using solar for hydrogen production enhances the efficiency of the overall system. Solar PV/T systems can also be used for multi-generation purposes as studied by Ratlamwala et al. (2011) where energy and exergy analyses show that integrated solar energy systems are suitable for hydrogen and cooling production. In this chapter, the aim is to discuss energy and exergy related aspects of solar energy systems, consider various solar energy based systems for analysis, assessment and comparison, and evaluate them for practical applications from the exergy point of view. 3.2 ENERGY AND EXERGY ASPECTS AND ANALYSES The relationship between energy and economics was a prime concern in the 1970s. At that time, the linkage between energy and the environment did not receive much attention. As environmental concerns, such as acid rain, ozone depletion and global climate change, became major issues in the 1980s, the link between energy utilization and the environment became more recognized. Since then, there has been increasing attention for this connection, as it has become more clear that energy production, transformation, transport and use all impact the Earth’s environment, and that environmental impacts are associated with the thermal, chemical and nuclear emissions which are a necessary consequence of the processes that provide benefits to humanity. Many suggest that mitigating the environmental impact of energy resource utilization and achieving increased resource utilization efficiency are best addressed by considering exergy. The exergy of an energy form or a substance is a measure of its usefulness or quality or potential to cause change. The latter point suggests that exergy may be, or provide the basis for, an effective measure of the potential of a substance or energy form to impact the environment. In practice, the Exergy analysis of solar energy systems 99 authors feel that a thorough understanding of exergy and the insights it can provide into the efficiency, environmental impact and sustainability of energy systems are required for the engineer or scientist working in the area of energy systems and the environment. The need to understand the linkages between exergy and energy, sustainable development and environmental impact has become increasingly significant. The solar PV and PV/T systems are one of the most significant and rapidly developing renewable-energy technologies, and its potential future uses are notable. During the last decade, solar PV and PV/T applications have increased in many countries and are observed throughout the residential, commercial, institutional and industrial sectors. The clean, renewable and in some instances economical features of solar PV and PV/T systems have attracted attention from political and business decision makers and individuals. Advances in solar PV and PV/T technology have also driven the trend to increased usage. Solar energy resources and technologies have three main benefits which are (a) they generally cause no or less environmental impact as compared to other energy sources, (b) they cannot be depleted; in contrast, fossil fuel and uranium resources are diminished by extraction and consumption, and (c) they favor system decentralization and local solutions that are somewhat independent of the national network, thus enhancing the flexibility of the system and providing economic benefits to small isolated populations. Also, the small scale of the equipment often reduces the time required from initial design to operation, providing greater adaptability in responding to unpredictable growth and/or changes in energy demand. Exergy, as a potential tool, has several qualities that make it suitable as a common quantifier of the sustainability of a process (Dincer and Rosen, 2004; Sciubba, 2001). These qualities are (a) exergy is an extensive property whose value is uniquely determined by the parameters of both the system and the reference environment, (b) if a flow undergoes any combination of work, heat and chemical interactions with other systems, the change in its exergy expresses not only the quantity of the energy exchanges but also the quality, (c) the value of a product of a process, expressed in terms of ‘resource use consumption,’ may be obtained by adding to the exergy of the original inputs all the contributions due to the different streams that were used in the process. The higher performance, lower cost and better reliability demonstrated by today’s solar PV and PV/T systems are leading many potential users to consider the value of these systems for particular applications. Together, these applications will likely lead industry to build larger and more cost-effective production facilities, leading to lower solar PV and PV/T costs. Public demand for environmentally benign sources of electricity will almost certainly hasten adoption of solar PV and PV/T. The rate of adoption will be greatly affected by the economic viability of solar PV and PV/T with respect to competing options. Many analysts and researchers believe that it is no longer a question of if, but when and in what quantity, solar PV and PV/T systems will see widespread adoption. In Table 3.2.1, the general thermodynamic quantities and general energy and exergy balance equations as well as energy and exergy efficiencies are listed as they will be employed for system analyses and performance assessment. 100 Solar energy sciences and engineering applications Table 3.2.1 General thermodynamic quantities and balance equations. Specific enthalpy h=u+vP Specific entropy s2 -s1 = c ln T2 T1 - R ln P2 P1 Specific exergy* exi = [(hi - h0) -T0(si - s0)] Thermal exergy .Exth =  1 -  T0 T  × Q Energy balance  (m. ihi)in =  (m. ihi)out + .Q + .W Exergy balance  (m. iexi)in =  (m. iexi)out + .Exde +.Exth Energy efficiency .en = .Enout .Enin Exergy efficiency .ex = .Exout .Exin *Based on changes in chemical formulation specific chemical exergy is added. 3.3 CASE STUDIES In this section several studies are presented which highlight the use of solar energy systems. Detailed energy and exergy analyses are also presented in order to help model integrated solar energy systems. In the first case study a solar energy system is integrated with an Organic Rankine Cycle (ORC) for producing power, in the second study a solar PV/T system is modeled for power and heat production, in the third study a solar PV/T system is integrated with an electrolyzer and absorption system for hydrogen and cooling production. 3.3.1 Case study 1: Exergy analysis of an integrated solar, ORC system for power production In this case study, a detailed energy and exergy model of an integrated solar ORC is presented for power production. Operating parameters such as ambient temperature, area of solar energy system and pressure at state 4 are varied to see their effect on energy and exergy efficiencies. 3.3.1.1 System description An integrated solar thermal ORC system for power production studied in this case study is shown in Figure 3.3.1. Air at state 1 returning from the boiler is passed through the solar thermal collector. In the solar thermal collector, the solar flux hitting the collector is absorbed by the air passing through the collector. The air at high temperature leaving the solar collector at state 2 enters the boiler where it loses heat to the isobutane coming in at state 4. Isobutane coming in at state 4, after gaining heat from state 2 leaves the boiler at state 3 to enter the turbine. In the turbine, the pressure and temperature of the isobutane is dropped and isobutane leaves at state 6 to enter Exergy analysis of solar energy systems 101 Figure 3.3.1 Schematic of solar thermal integrated with binary cycle. the condenser. In the condenser, isobutane at state 6 loses heat to the environment to leave at state 3 as saturated liquid in order to enter the pump where its pressure is increased to that of state 4. The part of the power produced by the turbine is supplied to the pump and remaining power is available for later use. 3.3.1.2 Energy and exergy analyses The rate of heat gained by the air passing through the collector is calculated as .Q so =  I × A 1000  (3.3.1) where .Q so represents rate of heat gained by the air passing through the collector, I represents solar flux, and A represents solar collector area. The exergy destruction rate in the solar thermal collector is calculated as E. x1 + E. xso = E. x2 + E. xde,so (3.3.2) where E. x1 = .m1  (h1 - h0) - T0(s1 - s0)  E. xso =  1 - T0 Tsun  I × A 1000  E. x2 = .m2  (h2 - h0) - T0(s2 - s0)  where E. x1 represents exergy rate at state 1, E. xso represents exergy rate of solar flux, E. x2 represents exergy rate at state 2, and E. xde,so represents exergy destruction rate in the solar collector. The power consumed by the pump is defined as .Wp = .m3  v3 (P4 - P3) .p  (3.3.3) 102 Solar energy sciences and engineering applications where .W p represents power consumed by the pump, m. 3 represents mass flow rate at state 3, v3 represents specific volume at state 3, P4 represents pressure at state 4, P3 represents pressure at state 3 and .p represents isentropic efficiency of the pump which is considered to be 80%. The rate of heat supplied to the boiler is taken to be the same as heat absorbed by the air in the solar collector as shown below .Q bo = .Q so (3.3.4) The exergy destruction rate in the boiler is calculated as E. x4 + E. xbo = E. x5 + E. xde,bo (3.3.5) where E. x4 = .m4  (h4 - h0) - T0(s4 - s0)  E. xbo =  1 - T0 Tbo  .Q bo Tbo = T4 + T5 2 E. x5 = .m5  (h5 - h0) - T0(s5 - s0)  where E. x4 represents exergy rate at state 4, E. xbo represents exergy rate carried by heat entering the boiler, E. x5 represents exergy rate at state 5, and E. xde,bo represents exergy destruction rate in the boiler. The power produced by the turbine is found using .W t = .m5(h5 - h6) (3.3.6) where .W t represents power produced by the turbine, m. 5 represents mass flow rate at state 5, h5 represents specific enthalpy at state 5, and h6 represents specific enthalpy at state 6. The exergy destruction rate in the turbine is calculated as E. x5 = E. x6 + E. xde,t + .W t (3.3.7) where E. x5 = .m5  (h5 - h0) - T0(s5 - s0)  E. x6 = .m6  (h6 - h0) - T0(s6 - s0)  where E. x5 represents exergy rate at stat 5, E. x6 represents exergy rate at state 6, and E. xde,t represents exergy destruction rate in the turbine. Exergy analysis of solar energy systems 103 The rate of heat rejected by the condenser is defined as .Q con = .m6(h6 - h3) (3.3.8) where .Q con represents heat transfer rate rejected by the condenser, m. 6 represents mass flow rate at state 6, h6 represents specific enthalpy at state 6, and h3 represents specific enthalpy at state 3. The exergy destruction rate in the condenser is calculated as E. x6 = E. x3 + E. xcon + E. xde,con (3.3.9) where E. x6 = .m6  (h6 - h0) - T0(s6 - s0)  E. xcon =  1 - T0 Tcon  .Q con Tcon = T6 + T3 2 E. x3 = .m3  h3 - h0  - T0 (s3 - s0)  where E. x6 represents exergy rate at stat 6, E. xcon represents exergy rate carried by heat exiting the condenser, E. x3 represents exergy rate at state 3, and E. xde,con represents exergy destruction rate in the condenser. The energy and exergy efficiencies are calculated as .en = .W t - .W p  I×A 1000  (3.3.10) .ex = .W t - .W p E. xso (3.3.11) where .en represents energy efficiency of the system and .ex represents exergy efficiency of the system. 3.3.1.3 Results and discussion Effects of variation in parameters such as ambient temperature, solar thermal area, and pressure at state 4 on energy and exergy efficiencies are studied in this section. It is observed that rise in ambient temperature doesn’t affect the energy efficiency but does result in a slight increase in exergy efficiency as seen in Figure 3.3.2. The energy efficiency is found to be constant at 16.37% while exergy efficiency increases from 17.19% to 17.3% with a rise in ambient temperature from 275K to 310 K. This result is important as it shows that from the energy perspective changes in ambient temperature have no effect on the performance of the system, but we know that in real 104 Solar energy sciences and engineering applications Figure 3.3.2 Effect of rise in ambient temperature on energy and exergy efficiencies of the system. Figure 3.3.3 Effect of area of solar collector on energy and exergy efficiencies of the system. life ambient temperature does play a role in performance determination of any system as it is shown by exergy efficiency. The effect of an increase in solar thermal area on energy and exergy efficiency of the system is shown in Figure 3.3.3. It can be seen that an increase in solar thermal area results in degrading performance of the system. The energy and exergy efficiencies are found to be decreasing from 16.97% to 16.37% and 17.89% to 17.26%, respectively with increase in solar thermal area from 700 m2 to 1000 m2. Such results are obtained Exergy analysis of solar energy systems 105 Figure 3.3.4 Effect of pressure at state 4 on energy and exergy efficiencies of the system. because working fluid passing through the solar collector, as air in this case, has a certain limit up to which it can get heated and therefore an increase in area does not necessarily mean better performance of the system from the efficiency perspective. The pressure of the working fluid entering the turbine at state 4 plays an important role in the performance of the system as shown by energy and exergy efficiencies in Figure 3.3.4. The energy and exergy efficiencies are found to be increasing from 16.37% to 18.745 and 17.26% to 19.76%, respectively with increase in pressure at state 4 from 500 kPa to 1000 kPa. Increase in pressure of the fluid entering the turbine means that the stream entering the turbine has higher energy content as compared to the low pressure stream and therefore for the same exit pressure the performance of the system is enhanced. Finally, a bar chart is provided to illustrate which component of the system has the greatest amount of exergy destroyed at a constant ambient temperature and pressure of 298K and 101 kPa as shown in Figure 3.3.5. It can be seen that the maximum amount of exergy is destroyed by solar thermal collector followed by condenser, boiler and turbine. This bar chart shows that in order to enhance the energy and exergy efficiencies one should first try to reduce the exergy destruction rate in the solar thermal collector as it is the source of maximum losses. 3.3.2 Case study 2: Exergy analysis of solar photovoltaic/thermal (PV/T) system for power and heat production In this case study, a detailed energy and exergy model of solar PV/T system is presented. Operating parameters such as ambient temperature and solar flux are varied to see their effect on energy and exergy efficiencies. 106 Solar energy sciences and engineering applications Figure 3.3.5 Exergy destruction rate in individual components and overall system. Figure 3.3.6 Schematic of solar PV/T system. 3.3.2.1 System description A solar PV/T system studied is shown in Figure 3.3.6. Solar flux hits the surface of the system which is photovoltaic (PV) panels. As solar flux comes in contact with the PV panels, the molecules inside the panels start to vibrate and they produce power. Due to high solar flux falling on the PV panels, the back surface of the PV panels is heated up and that is the heat which is utilized to produce hot air. The air is allowed to pass through the duct of the PV/T system in order to act as a cooling medium for PV panels. The air passing through the duct gains heat from the PV panels and its temperature Exergy analysis of solar energy systems 107 Table 3.3.1 Solar PV/T parameters. Parameter Values A 100m2 B 0.45m hp1 0.88 L 1.2m ßc 0.83 .c 0.12 tg 0.95 Ub 0.62W/m2 K Ut 2.8W/m2 K ac 0.90 increases. The heated air leaving the duct can be used for different heating and cooling purposes based on the temperature of the air leaving the duct and the operating system. 3.3.2.2 Energy and exergy analyses The equations which are used to solve the mathematical model of the PV/T system are derived from Joshi et al. (2009a, and 2009b). Moreover, the assumptions and constants of solar PV/T system are listed in Table 3.3.1. The equation to calculate power produced by the PV module is given as .W so = .c × I × ßc × tg × A (3.3.12) where .W so represents power produced by the solar panel, .c represents cell efficiency, I represents solar flux, ßc represents the packing factor of the solar cell, tg represents transitivity of glass, and A represents the area of the solar PV/T system The heat transfer rate from solar panels to air passing through the duct is given by .Q so = m. a × cpa UL ×  (hp2G × z × I  - UL × (Tai - T0)) ×  1 - exp -b × UL × L m. a × cpa  (3.3.13) where z = ab × t2 g × (1 - ßc) + hp1G × tg × ßc × (ac - .c) where .Q so represents heat transfer rate from PV panels to air, m. a represents mass flow rate of air, cpa represents specific heat at constant pressure of air, UL represents overall heat transfer coefficient from solar cell to ambient through the top and back surface of the insulation, hp2G represents a penalty factor due to presence of an interface between glass and working fluid through an absorber plate for a glass-to-glass PV/T system, z represents a variable, Tai represents temperature of air entering the duct, T0 represents ambient temperature, b represents breadth of the single PV panel, L represents length of the single PV panel, ab represents absorptance of a painted black surface, hp1G 108 Solar energy sciences and engineering applications represents a penalty factor due to the presence of solar cell material, glass and EVA for a glass-to-glass PV/T system, and ac represents absorptance of the solar cell. The solar exergy rate is calculated by E. xso =  1 - T0 Tsun  I × A 1000  (3.3.14) where E. xso represents solar exergy rate, and Tsun represents temperature of the Sun. The thermal energy and exergy efficiencies of the solar thermal system are defined as .th,en = .Q so  I×A 1000  (3.3.15) .th,ex = E. xPV/T E. xso (3.3.16) where E. xPV/T =  1 - T0 Tc  × .Q so Tc = tg × ßc × I × (ac - .c) + Ut × T0 + ht × Tbs Ut + ht Tbs = z × I + (Ut + Utb) × T0 + hba × Tair Ub + hba + Utb Tair =  T0 + hp2G × z × I UL  × . .1 - 1 - exp  -b×UL×L m. a×cpa  b×UL×L m. a×cpa . . + Tai × . . 1 - exp  -b×UL×L m. a×cpa  b×UL×L m. a×cpa . . where E. xPV/T represents exergy rate of PV/T, Tc represents cell temperature, Ut represents the overall heat transfer coefficient from solar cell to ambient through the glass cover, ht represents the heat transfer coefficient from a black surface to air through glass, Tbs represents back surface temperature, Utb represents the overall heat transfer coefficient from glass to black surface through solar cell, hba represents the heat transfer coefficient from black surface to air, Tair represents temperature of air flowing through the duct, and Ub represents the overall heat transfer coefficient from bottom to ambient. The electrical energy and exergy efficiencies of solar PV system are defined as .el,en = .W so  I×A 1000  (3.3.17) .el,ex = .c × (1 - 0.0045 × (Tc - 25)) (3.3.18) Exergy analysis of solar energy systems 109 The overall energy and exergy efficiencies of the solar PV/T system are defined as .ov,en = .W so + .Q so  I×A 1000  (3.3.19) .ov,ex = .W so + E. xPV/T E. xso (3.3.20) 3.3.2.3 Results and discussion This section highlights the importance of conducting a parametric study to see the effect of variation in operating conditions such as ambient temperature and solar flux on energy and exergy efficiencies of the system. The ambient temperature plays an important role in the performance of any system. Figure 3.3.7 shows how the energy and exergy efficiency of electrical and thermal systems vary with a rise in ambient temperature. The electrical energy efficiency is seen to be a constant at 9.46% whereas electrical exergy efficiency is found to be decreasing from 9.04% to 7.74%, respectively with an increase in ambient temperature from 280K to 310 K. The thermal energy and exergy efficiencies are found to be increasing from 5.89% to 61.65% and 1.28% to 11.6%, respectively with a rise in ambient temperature. The overall energy and exergy efficiencies are found to be increasing with a rise in ambient temperature as displayed in Figure 3.3.8. The overall energy and exergy efficiencies are found to be increasing from 15.36% to 71.11% and 11.22% to 21.59%, respectively with a rise in ambient temperature. These figures show that a rise in ambient temperature affects the performance of the system a lot. It is also observed that having a solar PV/T system is better than having an individual solar PV or thermal system, as a solar PV/T system has higher energy and exergy efficiencies as Figure 3.3.7 Effect of rise in ambient temperature on electrical and thermal energy and exergy efficiencies. 110 Solar energy sciences and engineering applications Figure 3.3.8 Effect of rise in ambient temperature on overall system energy and exergy efficiencies. Figure 3.3.9 Effect of increase in solar flux on electrical and thermal energy and exergy efficiencies. compared to individual systems. It is also observed that electrical energy efficiency is not affected by a rise in ambient temperature but electrical exergy efficiency is affected hence indicating that it is always better to conduct an exergy analysis. The second parameter studied in this case study is solar flux. The effect of increase in solar flux on energy and exergy efficiencies of an electrical and thermal system is shown in Figure 3.3.9. The electrical energy efficiency remains constant at 9.46% whereas electrical exergy efficiency decreases from 8.53% to 5.13%, respectively with Exergy analysis of solar energy systems 111 Figure 3.3.10 Effect of increase in solar flux on overall system energy and exergy efficiencies. an increase in solar flux from 600W/m2 to 1200W/m2. The thermal energy and exergy efficiencies are found to be increasing from 38.73% to 44.57% and 7.23% to 14.06%, respectively. The effect of an increase in solar flux on overall energy and exergy efficiencies is also studied and is illustrated in Figure 3.3.10. The overall energy and exergy efficiencies increase from 48.19% to 54.03% and 17.21% to 24.03%, respectively with an increase in solar flux. The results show that energy and exergy efficiencies are dependent on solar flux received by the solar PV/T system. 3.3.3 Case study 3: Exergy assessment of an integrated solar PV/T and triple effect absorption cooling system for hydrogen and cooling production In this case study, a detailed energy and exergy model of an integrated concentrated solar PV/T and triple effect cooling system is presented. Operating parameters such as solar flux, area, and air inlet temperature are varied to see their effect on overall energy and exergy efficiencies, the power produced by the solar PV panels, the rate of hydrogen produced, and energy and exergy COPs. 3.3.3.1 System description In this case study, we have studied an integrated concentrated solar PV/T absorption cooling system for the cooling and hydrogen production as shown in Figure 3.3.11. In this integrated system, concentrated solar PV/T is used to produce power and heat. Power produced is supplied to the electrolyzer and the pump in the cooling cycle. The electrolyzer is utilized to break the water molecule bond. As the water molecule breaks, it splits into hydrogen and oxygen. The hydrogen molecules are taken out of the electrolyzer and are stored in a tank for later use as an energy source. The high temperature air coming out of solar PV/T is fed into the HTG of the absorption cooling 112 Solar energy sciences and engineering applications Figure 3.3.11 Schematic of integrated system. system and is used as an energy source for the absorption cooling system. A detailed description of this system can be found in Ratlamwala et al. (2011). 3.3.3.2 Energy and exergy analyses Detailed energy and exergy analyses of solar PV/T system are presented in case study 2 or can be found in Ratlamwala et al. (2011). The electrolyzer is used to split water molecules into hydrogen and oxygen molecules, where hydrogen molecules are stored in the tank for later usage as an energy provider to the HTG or as a fuel to produce power using PEMFC. The amount of hydrogen produced depends on the efficiency of the electrolyzer, the heating value of the hydrogen, and power input to the electrolyzer. .elec = m. H2 × HHV .W solar - .W pump (3.3.21) After getting the heat and power output from the PEM fuel cell, equations for HTG were written in order to run the TEACS to achieve the cooling load. The amount of energy provided to the HTG is shown below .Q HTG = .Qso (3.3.22) The mass balance equations for HTG are given as follows m. 21x21 = m. 22x22 + m. 8x8 (3.3.23) Exergy analysis of solar energy systems 113 m. 21 = m. 22 + m. 8 (3.3.24) The energy balance equation for HHX is given below . m17ah17a + .Q HHX = .m21h21 (3.3.25) . m22h22 = .Q HHX + .m23h23 (3.3.26) The mass and energy balance equations for the condenser are given below m. 2 = m. 6 + m. 3 (3.3.27) . m2h2 + .Q con = .m6h6 + .m3h3 (3.3.28) Below mentioned equation is for energy balance of evaporator . m2ah2a + .Q eva = .m1h1 (3.3.29) The following energy balance equation is used to calculate the heat rejected by the absorber. . m11h11 + .Q abs = .m1h1 + .m28h28 (3.3.30) The thermal exergy of evaporator and HTG are defined as E. xth,eva =  1 - T0 Teva  × .Q eva (3.3.31) E. xth,HTG =  1 - T0 THTG  × .Q HTG (3.3.32) The energy and exergy COPs are calculated as COPen = .Q eva .Q HTG + .W P (3.3.33) COPex = E. xth,eva E. xth,HTG + .W P (3.3.34) Overall energy and exergy efficiencies are defined as .ov,en =  m. H2 × HHV + .Q eva I × b × L  × 100 (3.3.35) .ov,ex =  E. xH2 × E. xth,eva E. xso  × 100 (3.3.36) 114 Solar energy sciences and engineering applications Figure 3.3.12 Effect of increase in solar flux on overall system energy and exergy efficiencies. 3.3.3.3 Results and discussion In Figure 3.3.12, the study of the effect of solar radiation on the overall energy and exergy efficiencies has been carried out. It is noticed that the overall energy and exergy efficiencies decrease when the solar radiation is increased while keeping air inlet temperature, area of the PV/T and time for which solar radiation is available constant at 25.C, 10m2, and 12 hr, respectively. The energy and exergy efficiencies drop from 14.44% to 12.13% and 7.33% to 6.15% respectively, as the solar radiation increases. As the solar radiation increases the power production capacity of PV module increases and at the same time heat transfer rate also increases. The increase in power means that more water molecules are broken down to produce hydrogen. On the other hand, increase in the rate of heat results in a higher amount of heat given to the absorption system to provide the fixed amount of cooling. This increase in rate of energy fed into the cooling system results in the degraded performance of the cooling system as the cooling system rejects more heat through the condenser to achieve the desired cooling. These degrading performances of the cooling system result in lower energy and exergy COPs of the system. As the performance of the cooling system degrades the overall efficiency of the system decreases because more energy is being consumed to acquire the required outputs. The increase in area of the PV module results in a higher power output from the PV module and higher hydrogen production. The solar radiation, operating hours, and air inlet temperature are kept constant at 608W/m2, 30.8.C, and 12 hr, respectively. The power output and hydrogen production increase from 0.28kWto 0.86kWand 5.24 kg to 15.7 kg, respectively as shown in Figure 3.3.13. The power output of the PV module is directly related to the solar radiation and the area on which it is concentrated. As the solar radiation increases, the molecules in the PV module vibrate at a higher pace Exergy analysis of solar energy systems 115 Figure 3.3.13 Effect of increase in area of solar cell on amount of power and hydrogen produced. Figure 3.3.14 Effect of increase in air inlet temperature on energy and exergy COPs. and as a result more and more bonds are broken into protons and electrons. As the power produced by the PV module increases, the hydrogen production rate increases because more power is being fed into the electrolyzer in order to break the bonds of water molecules at a higher rate to produce hydrogen. When the water molecules are broken into hydrogen and oxygen, a greater amount of hydrogen is available which can be taken out and stored in a cylinder for later use as an energy provider by burning or using PEMFC. The effect of an increase in air inlet temperature of energy and exergy COPs of the TEACS is showing in Figure 3.3.14. The energy and exergy COP are found to be increasing from 0.95 to 2.51 and 0.89 to 2.36, respectively with increase in air inlet temperature from 20.C to 40.C. This increase is seen because, as the rate of heat input to the cooling system decreases to a certain practical limit for achieving certain cooling load, the performance of the system increases and a lesser amount of heat is being rejected through condenser. 116 Solar energy sciences and engineering applications 3.4 CONCLUDING REMARKS This chapter discusses energy and exergy related aspects of solar energy systems, considers various solar energy-based systems for analysis, assessment and comparison, and evaluates them for practical applications from the exergy point of view. Nomenclature A Area of solar module, m2 b Breadth of PV module, m COP Coefficient of performance c Specific heat, kJ/kg ·K cp Specific heat at constant pressure, kJ/kg ·K .E x Exergy rate, kW exi Specific exergy at state i, kJ/kg ·K exphi Specific physical exergy at state i, kJ/kg ·K exchi Specific chemical exergy at state i, kJ/kg ·K h Specific enthalpy, kJ/kg hba Heat transfer coefficient from black surface to air, W/m2 ·K ht Heat transfer coefficient from black surface to air through glass, W/m2 ·K hp1G Penalty factor due to the presence of solar cell material, glass and EVA for glass to Glass PV/T system, W/m2 ·K hp2G Penalty factor due to presence of interface between glass and working fluid through absorber plate for glass to glass PV/T system, W/m2 ·K I Solar flux, W/m2 L Length of the PV module, m m. Mass flow rate, kg/s P Pressure, kPa .Q Heat transfer rate, kW s Specific entropy, kJ/kg ·K T Temperature, K u Specific internal energy, kJ/kg Ub Overall heat transfer coefficient from bottom to ambient, W/m2 ·K UL Overall heat transfer coefficient from solar cell to ambient through top and back surface of insulation, W/m2 ·K Ut Overall heat transfer coefficient from solar cell to ambient through glass cover, W/m2 ·K Utb Overall heat transfer coefficient from glass to black surface through solar cell, W/m2 ·K v Specific volume, m3/kg .W Work rate, kW z Variable Greek letters ac Absorptance of solar cell ab Absorptance of painted black surface Exergy analysis of solar energy systems 117 ßc Packing factor of solar cell . Efficiency tg Transitivity of glass Subscripts a Air ai Air inlet abs Absorber bo Boiler bs Back surface of photo-voltaic panels c Solar cell con Condenser de Destruction el Electrical elec Electrolyzer en Energy ex Exergy eva Evaporator H2 Hydrogen HHX High temperature heat exchanger HTG High temperature generator i State i ov Overall system p Pump PV/T Photo-voltaic/thermal so Solar t Turbine th Thermal 1. . .23 State numbers 0 Ambient or reference condition Acronyms CHX Condenser heat exchanger con Condenser eva Evaporator HHX High temperature heat exchanger HHV Higher heating value HTG High temperature generator LHX Low temperature heat exchanger LTG Low temperature generator PV Photo-voltaic PV/T Photo-voltaic/thermal MHX Medium temperature heat exchanger MTG Medium temperature generator TEACS Triple effect absorption cooling system 118 Solar energy sciences and engineering applications REFERENCES Dincer, I. and Rosen, M.A. (2007) Exergy, energy, environment and sustainable development. Oxford: Elsevier. Joshi, A.S., Tiwari, A., Tiwari, G.N., Dincer, I. and Reddy B.V. (2009a) Performance evaluation of a hybrid photovoltaic thermal (PV/T) (glass-to-glass) system. International Journal of Thermal Sciences, 48, 154–164. Joshi, A.S., Dincer, I. and Reddy, B.V. (2009b) Performance analysis of photovoltaic systems: A review. Renewable and Sustainable Energy Reviews, 13, 1884–1897. Koroneos, C., Dompros, A., Roumbas, G. and Moussiopoulos, N. (2004) Life cycle assessment of hydrogen fuel production processes. International Journal of Hydrogen Energy, 29, 1443–1450. Ratlamwala, T.A.H., Gadalla, M.A. and Dincer, I. (2011) Performance assessment of an integrated PV/T and triple effect cooling system for hydrogen and cooling production. International Journal of Hydrogen Energy, 36, 11282–11291. Sciubba, E. (2001) Beyond thermoeconomics? The concept of extended exergy accounting and its application to the analysis and design of thermal systems. Exergy-An International Journal, 1, 68–84. Thomas, L.G. and Nelson, A.K. (2010) Predicting efficiency of solar powered hydrogen generation using photovoltaicelectrolysis devices. International Journal of Hydrogen Energy, 35, 900–911. Yilanci, A., Dincer, I. and Ozturk, H.K. (2009) A review on solar-hydrogen/ fuel cell hybrid energy systems for stationary applications. Progress in Energy and Combustion Science, 35, 231–244. Chapter 4 Solar energy collection and storage Brian Norton Dublin Energy Laboratory, Dublin Institute of Technology, Dublin, Ireland 4.1 SOLAR THERMAL ENERGY COLLECTORS 4.1.1 Overview Different types of solar thermal collectors provide energy in the form of heated • water for direct use (Rabl, 1985) • aqueous solutions, usually of glycols for freeze damage prevention (Norton and Edmonds, 1991; Norton et al., 1992), for hot water production • air (Norton, 1992), usually for space heating • specialized heat transfer fluid, mainly in solar thermal power generation (Duffie and Beckman, 1991) • steam, also in solar thermal power generation systems (Kalogirou, 2003) • inherent energy storage as in integral passive solar water heaters (Bainbridge, 1981; Smyth et al., 2006) and solar ponds (Leblanc et al., 2011) • fluid with electricity production from a photovoltaic module (Norton et al., 2011) employed as the absorber • refrigement or volatile fluid (Shreyer, 1981; Ong and Haider-E-Alathi, 2003) • material undergoing solid to liquid phase change (Sion et al., 1979; Rabin et al., 1995). The two principal solar thermal collector designs employed for space heating and hot water supply are the flat-plate solar collector and the vacuum tube solar collector the latter may be employed with line-axis concentrators . Flat-plate solar collectors have now been overtaken in total global numbers by the increasingly popular vacuum tube solar collector. The latter has a higher efficiency at higher temperatures and has become relatively inexpensive due to high production volumes of the all-glass type, particularly in China (Weiss and Mauthner, 2010). Integral collector-storage systems range from small-scale domestic water heaters to very large scale solar ponds. The former were the first mass-produced solar water heaters (Butti and Perlin, 1980) whereas the latter are site-specific designed civil engineering projects (Leblanc et al., 2011). Though most systems produce heated fluids for domestic, industrial (Kulkarni et al., 2008) or power generation applications, more esoteric users of solar heat include sterilization of medical equipment (Bansal et al., 1988) to the passive protection of grape vines from frost damage (Smyth and Skates, 2009). 120 Solar energy sciences and engineering applications 4.1.2 Flat plate solar energy collectors Flat plate collectors can absorb solar energy inident from a direct beam component, a diffuse component and ground reflected albedo of insolation. Flat plate collectors’ inclusion of the latter two insolation components means that in many climates there are only a few instances where any viable long-term performance advantage is gained by tracking a flat plate collector to follow the sun’s daily path across the sky. Most flat plate collectors have south-facing fixed mountings that usually provide a static inclination that is cogniscent of the maximum average diurnal solar energy collection period’s duration, the annual durations of utilizable solar energy (Reddy, 1987), the prevalence of diffuse conditions and any diurnal or annual bias of the predominant times of hot water withdrawal. A flat-plate solar collector usually consists principally of (Lenel and Mudd, 1984) • tubes through which a heat transfer fluid is conveyed connected with good thermal contact with an • an absorber plate that, often solar selectively coated, absorbs incoming solar radiation and • an aperture cover plate that inhibits outgoing long-wave thermal radiation losses and traps an insulating air layer so inhibiting convective heat losses • thermal insulation to maximize heat loss and a casing to provide weather protection together with mechanical integrity to the sides and back A typical example of a flat plate solar energy collector is illustrated in Figure 4.1.1. The tubes are usually integrated fully in the absorber plate by a variety of Figure 4.1.1 A flat plate solar energy collectors in close coupled thermosyphon systems. Solar energy collection and storage 121 manufacturing techniques (Duffie and Beckman, 1991) to ensure good thermal contact between the tubes and the plate. 4.1.3 Evacuated tube collectors Evacuated-tube collectors are fabricated from either concentric glass tubes or a metal tube end-sealed to and within a glass tube. An enclosed evacuated annular space and a selective absorber surface provide a very low overall heat loss particularly when operated at higher temperatures. The evacuated space between the glazing and absorber eliminates convective loss and long-wave thermal radiative heat loss is inhibited by the deposition of the spectrally selective absorber coating on the absorber surface (Morrison, 2001). Evacuated-tube solar collectors generally have low thermal mass. The ability to heat-up rapidly (often from higher maintained overnight temperatures than a flat plate collector) gives low utilizable insolation thresholds providing good low insolation performance. Heat removal in evacuated tube collectors can be indirect often using a volatile fluid in the absorber via a closed heat pipe (Tabassum et al., 1988). More frequently water, as a heat transfer fluid, moving in a thermosyphon through the collector is employed (Morrison, 2001, Budihardjo and Morrison, 2007). Evacuated tube collectors can have copper absorbers or use a selectively coated glass inner glass tube as the absorber. The latter all-glass type is now commonplace in China with over 15 million square metres of collector area installed by 2010 (Norton, 2011). A typical example of an evacuated tube collector is show in Figure 4.1.2. Figure 4.1.2 An all-glass “wet type’’ evacuated tube solar energy collector. 122 Solar energy sciences and engineering applications 4.1.4 Collector components 4.1.4.1 Absorber plate The plate and tubes of both a flat-plate and metal-in-glass evacuated tube solar energy collector absorber are usually made of a metal with high thermal conductivity such as copper or aluminum. Good heat transport is thus provided through the plate to the heat transfer fluid. An ideal absorber plate has a high solar absorbance surface to selectively absorb as much as possible of the incident insolation, together with low emittance to long-wave thermal radiation so that long-wave radiative heat losses are low (Norton, 1992). Such solar selective absorbers often consist of two layers with appropriately different optical properties; often a thin upper layer that exhibits high absortance to solar radiation whilst being relatively transparent to thermal radiation is deposited on an underlying surface whose high reflectance provides low emittance to thermal long-wave radiation. Alternatively a heat selective mirror that has high solar transmittance with high infrared reflectance can be placed on top of a non-selective high absorbtance material. An example of this latter of selective surface combination is “black chrome’’ that consists of microscopic chromium particles deposited on a metal substrate; the chromium particles reflect long-wave thermal radiation, but shorter wavelength insolation passes between the chromium particles. In very low cost low temperature solar energy collectors for applications, such as swimming pool heating (Ruiz and Martinez, 1992), all these guidelines for absorber materials and fabrication are inappropriate as the minimum initial cost usually dominates design choices. Black paint which has a high absorptivity but not being selective is equally high emittance is used on absorbers or is the dark pigment in unglazed often ground-mounted plastic coils through which water is solar heated. 4.1.4.2 Aperture cover Most glasses are almost completely transparent to the shortwave radiation the associated with insolation, but nearly opaque to long-wave thermal radiation. When employed as an aperture cover, glass inhibits successfully radiative heat loss from an absorber plate to ambient or to the lower temperature radiative heat sink that the sky often constitutes. Glass is thus the aperture cover material employed most commonly for solar energy collectors (Norton, 1992). It is desirable that a large part of the incoming direct, diffuse and ground reflected solar radiation is transmitted through the cover and used efficiently to heat the transfer fluid. This means that the transmittance of an aperture cover must be high which requires both low reflectance and absorptance. The reflectance of a material depends on its refractive index and the angle of insolation incidence. It can vary for different wavelengths of insolation transmittance and decreases with increasing angle of insolation incidence (Duffie and Beckman, 1991). An aperture cover glazing with a smaller refractive index exhibits a lower reflectance and higher transmittance. The overall solar transmittance of an aperture cover glazing is the normalized sum of each quantized wavelength of the solar radiation spectrum transmitted by the aperture cover. Since iron absorbs light in the visible part of the spectrum, the transmittance of glass for the solar spectrum decreases with increasing iron concentrations. Thus low-iron glass typically with an iron content below 0.06 percent is preferred for flat plate collector aperature cover glazing. Solar energy collection and storage 123 Glass reflectance can be reduced either by coating the glass with a thin film with a low refractive index material, or by etching the surface to create a porous lower refractive index medium, reducing the reflectance of the glass. For improved insulation of the collector, multiple glasses that trap multiple insulating air layers can be used, though the optical losses associated with each additional glass sheet reduce the insolation transmitted. Many clear plastics, without specific treatments, become yellow and brittle after long-term exposure to ultra-violet part of the insolation spectrum. Many plastics can also be damaged by the high temperatures that can be reached in solar energy collectors, particularly if fluid stagnation occurs in the solar energy collector. Another disadvantage of most plastics is that, compared to glass, transmittance to thermal radiation is high at longer thermal wavelengths. Altering the optical properties of plastics to reject heat gain under the influence of heating or an electric field has been studied as a means of avoiding overheating damage when plastics are used as absorbers in solar energy collectors (Resch and Wallner, 2008). Both glass and plastic aperture covers do, despite their different solar optical properties, will enable absorber heat losses to be ameliorated as they both trap a largely stagnant insulating air layer between the glazing and absorber (Rommel and Wagner, 1992). 4.1.4.3 Choice of heat transfer fluid The choice of the heat transfer fluid is determined by maximum operating temperature, initial and operating costs, toxicity flammability and environmental impact. For high temperature applications water and aqueous solutions are often inappropriate. Hydrocarbon and synthetic-based heat transfer oils may be used up to their maximum temperatures of around 450.C. They are however flammable. The special safety systems together with the environmental issues in their post-use disposal measure the operational costs of the use of synthetic heat transfer rate. Steam has been studied for many central receiver applications with maximum temperature applications 550.C. Water used to generate steam must be deionized in order to prevent scale buildup on the inner surfaces of the receiver. Liquid sodium and nitrate salt mixtures can also be used as both a heat transfer fluid and storage medium, with a maximum operating temperatures of 600.C and 560.C. 4.1.4.4 Combined photovoltaic – thermal collectors A photovoltaic panel may convert typically 10%–15% of the incident insolation to electricity. The rest of the solar energy absorbed heats the panel reducing photovoltaic electrical energy conversion efficiency. Combined photovoltaic-thermal collectors have thus been developed with either water (Ji et al., 2006) or air as the heat transfer fluid. The many such systems that have been proposed all require displacement of both suitable electrical and heat loads if economic viability is to be achieved. Hot water produced by such combined systems can be used for space heating or domestic hot water. When warm air is produced it can be used for pre-heating of ventilation air. The heat can also be used in absorption chillers to cool a building. A typical PV/T collector is shown in Figure 4.1.3. 124 Solar energy sciences and engineering applications Figure 4.1.3 A PV/T solar energy collector. A solar heat pump uses low-temperature, low-quality energy from the outside air, transferring heat from the cold reservoir to a warmer reservoir (Morrison, 1984). The working fluid is alternately evaporated and condensed. A heat pump involves isentropic compression in the compressor, constant-pressure heat rejection in the condenser, throttling in an expansion device and constant-pressure heat absorption in an evaporator (Charters et al., 1980). Solar heat pumps for small-scale heating and cooling applications are referred to as Combi+ systems (Troi et al., 2008). 4.2 INTEGRAL COLLECTOR STORAGE SYSTEMS 4.2.1 Integral passive solar water heaters Integral storage solar collectors are a tank, glazed lying on top of insulation (Bainbridge, 1981; Arthur and Norton, 1988; Smyth et al., 1998; 1999; 2001a; 2001b; 2003; 2004; 2005; 2006). The water inside the collectors is heated by absorbed incident insolation. The heated water may be pumped directly to a demand or a hot storage tank for later use. At night or during periods of low insolation, the water in the collectors may be drained back to tank, thereby conserving the heat collected (Dickinson et al., 1976). A typical example is illustrated in Figure 4.2.1. Many techniques are available to determine the optimal design of such systems for particular climates and hot water loads (Bar-Cohen, 1976; Chauhan and Kadambi, 1976). 4.2.2 Salt gradient solar ponds A salt-gradient solar pond employs a salt concentration gradient to suppress natural convection. The physical processes are illustrated in Figure 4.2.2. Heated water Solar energy collection and storage 125 Figure 4.2.1 An integral collector storage solar water heater. NCZ UCZ LNA LN LC X = LN X = LN kw LCZ HEAT-BALANCE EQUATION CONTROL VOLUME CONSIDERED THRMAL PROPERTIES AND PARAMETERS THRMAL PROPERTIES AND PARAMETERS FOR THE GROUND Tg, kg, .g, sg dT (t) dt T (t) = T – T COS (.t – d) aTN (x.t) ax aTg(x.t) aTN(x,t) ax ax x = LN+Lc THRMAL PROPERTIES AND PARAMETERS FOR NCZ TN(x.t), kw, aw, sw HEAT DIFFUSION EQUATION LU 0 x WATER SURFACE n = 1 n 4 .w LC H (LN – t) –kg a2 TN (x.t) TN (LU.t) = Ta (t) TN (LN.t) = T (t) q (t) = q – q COS (.t – dq) – ˜ Tg (t) Tg (t) = Ta + Ta COS (t –da ) T (t) = H+H COS .t ax2 a TN (x.t) at a H (x.t) ax x = LN – LC kg aTg (x.t) ax = –kw –q (t) = 1 xw – 1 kw AIR H (x.t) = TH (t) . e–µnx H (LN.t) FOR LCZ T (t), kw, .w, sw HEAT DIFFUTION BOUNDARY CONDITIONS Figure 4.2.2 Heat transfer in a non-convecting solar panel (Norton, 1992). dissolves holds more salt than does cooler water. Salty, heated water being heavier remains at the base of a solar pond. Insolation penetrating through the top layers of a pond is absorbed by the layer with heat loss inhibited by the intervening non-convecting layer (Leblanc et al., 2011). Salt-gradient solar ponds consist of three zones: • a surface convecting zone of low-salinity water, typically 0.2 m–0.4m thick; • a non-convecting or salinity-gradient zone beneath the surface zone, which thermally insulates a lower convecting heat-storage stratum in which dissolved salt concentration increases with depth, typically 1.0 m–1.5m thick; and 126 Solar energy sciences and engineering applications • a storage zone at the bottom of the pond of ideally uniformly high dissolved salt concentration that stores heat and is typically 1 m–3m thick. Heat stored at the bottom of the pond, is removed by hot brine being withdrawn from the lowest storage zone of the pond by a pump passed through a heat exchanger and then returned back to the lower of the storage zone. For power production applications that employ an organic Rankine cycle, the engine’s condenser cooling water is withdrawn from the top of the pond, from where it is passed through the condenser before being returned back to the surface layers of the pond. Non-convecting solar ponds are energy collectors with “built-in’’ seasonal heatstorage capabilities that can provide heat at temperatures in excess of 90.C with such a large volume of inherent sensible thermal storage that heat can be collected in summer and stored for use during. The lower convecting zone (LCZ), is where the highest uniform salt concentration occurs. In it the solar radiation will heat the highly-saline water but, because of its high relative density due to its salt content, this heated water will not rise into the lower salinity layers. Thus the heat is stored yet inhibited from being transferred by convection. In order to establish a conventional non-convecting solar pond for power production, it must have a large surface area (i.e. extend over several square kilometres), and so vast excavations and site preparations are inevitably necessary: these operations can usually account for more than 40% of the total capital cost of the nonconvecting solar pond. Construction of economically viable solar ponds requires the ready availability of inexpensive flat land; accessibility to water; and an inexpensive source of salt or brines (Norton, 1992). 4.3 CONCENTRATORS 4.3.1 Introduction Solar energy is concentrated to produce higher output temperatures and/or reduce collector cost by replacing an expensive absorber area with less expensive reflector. Concentration ratio is the factor by which an intervening concentrating mirror, lens or luminescent system increases the insolation flux on a solar energy absorbing surface. A geometric concentrator ratio is the collector aperture area (Aa) is divided by the absorber surface area (AR) (Rabl, 1985): C = Aa Ar (4.3.1) 4.3.2 Concentration systems Thermal losses are dependent largely on the heat loss characteristics of, and proportional to, absorber area. Insolation can be concentrated by a two-dimensional system to a theoretical upper limit given by: Cmax,2D = n sin(e/2) (4.3.2) Solar energy collection and storage 127 where e is the angular size of the suns disc, n is the refractive index of the last material insolation traversed for the refractive index of air Cmax,2D is approximately 216. For a three-dimensional concentrator, the equivalent upper limit is: Cmax,3D = n2 sin2(e/2) (4.3.3) Cmax,3D in typically 46000 for concentrators operating in the refractive index of air. To maintain high concentration high concentration ratio two-dimensional concentrators need accurate solar tracking systems as concentration decreases sharply at off-normal insolation incidence. As they have much larger maximum concentration ratios threedimensional systems need less-precise tracking accuracy to yield acceptable optical performance. Both the earth’s diurnal rotation about its axis and annual rotation about the sun mean that a tracking solar energy collector must move continually in two axis to maintain the direct insolation component precisely at normal incidence to the aperture plane (Rabl, 1985). Two-axis solar tracking is essential to achieve concentration ratios from 100–1000. Such concentrators can elevate fluids to temperatures at which it is usually possible to generate electricity using steam turbines or Stirling cycles or to provide high grade thermal energy for industrial processes. Single axis trackers are usually oriented either horizontal east-west, or inclined north-south achieving normal incidence once a day or twice a year respectively. Stationary concentrating systems with a concentration ratio in the range from 1 to 3 can be integrated into buildings as they obviate the need for moving parts, complex mounting and associated mechanical systems. Most systems are based on two-dimensional non-imaging compound parabolic concentrators (CPCs). Building integration of high temperature solar thermal applications is attractive as a means of lowering installation cost. The initial investment cost will be reduced if lower cost reflector materials replaced more extensive use of evacuated tube collectors. The two dimensional CPC is termed an ideal concentrator, with a concentration ratio of 1/sin.max, since all the light incident at angles less than the angle of acceptance will arrive at the absorber. As can be seen in Figure 4.3.1, the CPC is deep in comparison with the width of the absorber. The arrangement is both impractical and costly in the practical fabrication of a concentrator (Tripanagnostopoulos et al., 2002; 2004a; 2004b). Increasing the concentration ratio reduces the angle of acceptance resulting in a considerably deeper trough. Truncating the height of a CPC does not diminish the aperture significantly, if a third of the length of a low concentration ratio CPC trough were truncated it would only reduce the aperture area by about 3%. Thus in most practical solar energy collectors that employ CPCs, they are truncated significantly. As the maximum concentration ratio is n/sin .max it is possible to increase the concentration ratio by replacing the air in the trough with a dielectric medium. When a dielectric medium with a refractive index greater than v 2 is used, total internal reflection will ensue at each reflection. A dielectric-medium concentrator without reflectors can thus be constructed, that will have no reflection losses. This will be of lower cost if eliminating the cost of the reflector is not offset by the cost of the dielectric material. 128 Solar energy sciences and engineering applications Aperture Cover Reflector Horizontal Absorber, with or without selective surface Envelope might or might not be present Vertical Axis of Collector Modified reflector to prevent thermal short circuit .a .a = Acceptance half-angle Ø = Angle of inclination .a Ø Figure 4.3.1 A compound parabolic concentrator. When using a dielectric medium with a refractive index of >1, as the concentrator, light will be refracted to a smaller angle of incidence rendering it possible to accept insolation across a larger acceptance angle. A concentrator filled with a high refractive index low-iron dielectric would increase the concentration ratio by 52%. 4.4 SOLAR WATER HEATING 4.4.1 Overview The diverse different forms of solar energy water heaters that are now available (Norton, 2011) include: • thermosyphon solar water heaters (Norton and Probert, 1986) operate by natural convection, can be direct or indirect and can use a variety of flat-plate or evacuated tube collectors. • pumped-circulation solar water heaters (Reddy, 1987; Duffie and Beckman, 1991) usually indirect and have larger collector area than thermosyphon systems, they also can use either flat plate or evacuated tube collectors. The pump is usually activated by a set collector outlet-inlet temperature difference though photovoltaic modules have also been used to power pumps (Parker, 1976; Al-Ibrahim et al., 1998). Most pumped systems are indirect (Frazer et al., 1995; Parent and Van der Meer, 1990; Yazdanshenas et al., 2008; Too et al., 2009). • combisystems (Weiss, 2003) (a contraction of “combined systems’’) these provide both space heating and hot water. These, invariably indirect, systems require careful design of heat exchanger arrangement and central strategies of the optimal Solar energy collection and storage 129 solar savings fraction is to be provided. (Yazdanshenas and Furbo, 2007; Letz et al., 2009; Yazdanshanas et al., 2008). The annual variations of space and water heating loads can be quite different. • integral collector storage (Smyth et al., 2006; 1998; 1999; 2001a, 2001b; 2003; 2004) in which the heat water store is also the solar energy collector. • Combi+; these are solar assisted heat pumps (Morrison, 1984; Troi et al., 2008). • large scale interseasonal energy storage systems (Schmidt et al., 2004). • swimming pool heating (Ruiz and Martinez, 2010); employing flat plate collectors often unglazed using a low-cost plastic pipe absorber. • photovoltaic solar water heaters (Fanney and Daugherty, 1997; Fanney et al., 1997) where a electrical heating element immersed the water to be treated is powered by a photovoltaic array. In addition to evacuated tube and flat plate collectors, solar water heating can also be accompanied using photovoltaic/thermal collectors. Such latter collectors use the 85–95% of the incident on a photovoltaic array not connected to electricity (Ji et al., 2006; Chow et al., 2008) as heat. 4.4.2 Applicability of particular collector types to specific outlet temperatures and diffuse fractions Solar energy availability can be diurnally or annually out-of-phase with a heat load. When the heating needs are at their peak, the supply of solar energy can be at its lowest. However, domestic and industrial hot water needs often do not vary over the year and solar collectors can be used for producing hot water during the summer. Solar energy water heaters can be categorised as either active or passive. An active system requires a pump to drive the heated fluid through the system, whereas a passive system requires no external power. Distributed systems (Prapas et al., 1993) comprise a solar collector, hot water store and connecting pipework; they may be either active or passive. In the former, a pump actuated by temperature sensors via a control circuit temperature difference between collector inlet and outlet is actuated by the required to convey the fluid from the collector to the store (Prud’homme and Gillet, 2009; Wuestling et al., 1985). The pump may be powered by a photovoltaic module giving automony from mains power (Al-Ibrahim et al., 1988). In a passive thermosyphon solar water heater; fluid flow is due to buoyancy forces occurring in a closed circuit comprising a collector, hot water store and the connecting pipework produced by the difference in densities of the water in the collector and that of the cooler water in the store. Concentrating solar energy collectors require high direct components of insolation to be effective. Extensive use of solar power generation is likely to be most viable where the annual direct fraction is above 60%. This delineates their geographical applicability in Figure 4.4.1. Similarly solar water system productivity is a function of insolation and ambient temperature. For identical load profiles and temperatures solar savings fractions and continuous autonomous provision can be determined (Yohanis et al., 2006a; 2006b). A geographical distribution of the latter is shown in Figure 4.4.2. 50% 40% 50% 50% 40% 70% 50% 60% 40% 50% 50% 40°S 40°N 20°S 20°N 0° 50% 50% 50% 50% 50% 60% 50% 50% 60% 60% Figure 4.4.1 Annual direct fractions of insolation, areas suitable to solar thermal electrical power generation have >60% direct fractions. Solar energy collection and storage 131 Figure 4.4.2 Distribution of continuous days of hot water production at 37.C is across Europe (Yohanis et al., 2006a; 2006b). 4.4.3 Freeze protection methods Freeze protection of a water collector in winter can be gained by either draining the collector circuit using multiple glazings (Bishop, 1983) evacuation to eliminate convective heat loss. (Mason and Davidson, 1993; Hongchuan and Guangming, 2001), internal convection suppression (Smyth, 2001a) or heat transfer fluid choice. The heat transfer fluid used in a solar collector during winter sign should not allow freeze expansion damage. The most common heat carrier in solar collectors is an aqueous solution of water and glycol together with appropriate additives that inhibit corrosion. Regulatory compliance has meant that ethylene glycol has been superseded by less toxic propylene glycol. Glycol-filled indirect system can produce warm water in winter unlike a direct system which would still remain drained down (the latter being usually a biennial operation). The advantage of a longer operating period is counteracted in summer by the lower thermal efficiency of an indirect system with a, more viscous, propylene glycol solution when compared with a direct system in which water is the heat transfer fluid. For thermosyphon solar water heaters, the sources of an indirect system’s relative 132 Solar energy sciences and engineering applications inefficiency are, in order of significance (Norton and Edmonds, 1991; Norton et al., 1992): (1) the higher viscosity of aqueous glycol solutions (compared with water) reducing the natural-circulation flow rate, (2) the additional (compared with a direct system) fluid frictional flow resistance caused by the heat exchanger, (3) heat transfer resistance associated with the, frequently double-walled, in the heat exchanger used, and (4) the lower specific heat capacity of aqueous propylene glycol compared with water. A major concern in the design of indirect systems is the choice of antifreeze heat transfer fluid. Being both readily available and cheap, glycol solutions are used usually. However, even the use of low-toxic propylene glycol is only permissible in many building codes, standards and regulations with a double-walled heat exchanger. The latter, however, further reduces system performance. Nonaqueous liquids, such as silicones and some hydrocarbons, are low to moderately toxic; but as they have higher viscosities (compared with aqueous solutions) together with lower specific heats and thermal conductivities, their use impairs performance. An aqueous-glycol solution may be used with a single-walled heat exchanger in some jurisdictions if it is both effectively nontoxic and stable chemically over the full range of the water heater’s operating conditions, propylene glycol solutions containing the appropriate inhibitors satisfy these requirements. Many solar energy water heaters act as a preheater in series with an auxiliary heater (DGS, 2005). In a preheat system, the mass of water withdrawn from the solar system forms the hot water consumption in the building, so energy is always gained from the solar energy system even when it raises the water temperature only slightly above main supply temperature. An auxiliary heater raises this water to a temperature above 55.C sufficient to eradicate any Legionnella bacteria. The auxiliary outlet water temperature is thus above usual bathing temperatures and is normally mixed with cold at the faucet, shower or bath to achieve the temperature desired. If water is drawn off at temperatures above the auxiliary heater set point, then it can be argued that the draw-off volume should be reduced because a greater proportion of cold water will be used to give the desired temperature at the point of use. In practice this will depend on whether a two-tank system is used and on heat losses from the auxiliary heater tank. The freezing point of commercially available propylene-glycol solutions varies with concentration. The ambient temperature distributions for each of the months when frost is recorded also changes with location. Ensuant critical concentrations for system survival are thus location dependent.It is usually worthwhile to continue operating a solar energy water heater in winter if produces a net heat output. In London, UK, for example there is no month in which a solar energy water heater if designed appropriately containing propylene glycol at the critical concentration necessary for survival, would not produce heat output. It could be preferable to use a direct solar energy hot water system, drain the collector in winter, and forego the energy that is available during the winter months in order to benefit from greater hot water production efficiency during the summer. Solar energy collection and storage 133 To determine if this approach is valid the direct solar energy water heating system performance thus needs to be compared with that of an indirect system. The latter should contain the optimum aqueous propylene-glycol solution concentration that yields the maximum total output during all the months where it can survive the minimum ambient temperature. 4.4.4 Sensible and latent heat storage Energy storage is employed in solar thermal energy systems to enable excess energy produced during high insolation to be available when insolation is low or non-existent (at night). Energy storage may be needed; for where some of the solar thermal energy produced during the day is stored for use later during the night or to provide energy over sequences of cloudy days. There is a broad range of heat storage media for solar thermal energy systems. However, practical design considerations limit thermal energy storage options to sensible and latent heat energy storage in liquids/solids and phase change materials respectively. Thermochemical energy storage has considerable promise but as yet to realise practical deployment. In sensible heat thermal energy storage, cold fluid in an insulated store is heated to a higher temperature by the hot fluid from the solar energy collectors. Colder fluid is withdrawn from the bottom of the store and is heated in the collector. The hot fluid from the collector returns to top of the hot water store. The less dense hot storage fluid will form a stratified layer delineated by a thermocline on top of the cold fluid (Hollands and Lightstone, 1989). Water heated by the solar energy collector can enter the store at a temperature lower than that of the local stored heated water. This can ensure of, for example, high insolation has been succeeded by cloudier skies. Fluid inlet arrangements have been devised to reduce destratification by water solution of colder water to the warmer layers of a store (Lavan and Thompson, 1977; Davidson and Adams, 1994). When the hot fluid is withdrawn from the store, the latter is usually replenished by cold mains water. The introduction of such cold water is best accomplished via a low velocity flow that does not disrupt the stratification of the hot water store (Eames and Norton, 1993; Furbo and Fan, 2008). This can cause an unacceptable rate of hot water delivery in both industrial and domestic (e.g. showers) applications that will require intermediate stores to accomplish end-use effectiveness. Large-scale sensible heat storage systems (Lund, 1986) have been built to supply district heating (Dalenback, 2010). The majority of large-scale interseasonal storage systems serve housing via distinct heating networks. Systems with output of 7.0MW are currently most prevalent in Denmark as can be seen from Table 4.4.1 which summarises such systems with an output >4MWth in operation in 2010 (Dalenback, 2010). The cost of thermal energy storage systems is dominated by the initial cost of the storage medium. The use of water or steam (assuming a low cost pressurized storage tank) as a storage medium reduces storage fluid costs. In addition, the use of water or steam as a storage fluid in a solar thermal electric system using a steam-driven power generation unit obviates the need for an oil/water steam generator. In a latent heat energy storage system, the • latent heat storage materials must be of low cost and available readily • latent heat storage material, if a mixture, must not separate into component materials after repeated phase change cycles. 134 Solar energy sciences and engineering applications Table 4.4.1 Large-scale interseasonal solar heating systems with output >4MWth (adapted from Dalenback, 2010). Collector Nominal Power In operation Area (m2) Output MWth since 1996 Location Country 18,300 12.8 1996 Marstal Denmark 10,700 7.5 2009 Broager Denmark 10,073 7.0 2009 Gram Denmark 10,000 7.0 2000 Kungälv Sweden 8,012 5.6 2007 Braedstrup Denmark 8,012 5.6 2008 Strandby Denmark 7,300 8.1 2003 Grailsheim Germany 7,284 5.1 2009 Torring Denmark 6,000 4.2 2008 Soenderberg Denmark 5,670 4.0 1997 Neckarsulm Germany • latent heat storage material must not corrode or react with heat-transfer surfaces or solar energy collector materials. • supercooling behaviour of the latent heat storage material on solidification should usually be limited and consistent over numerous freeze/thaw cycles. • toxicity and flammability must satisfy regulating requirements In a thermochemical energy storage system thermal energy separates chemical bonds reversibly. The displacement of the chemical bond energy requires absorbs heat energy resulting in thermal energy storage. The chemical products a useful thermochemical heat storage reaction are unreactive at ambient temperatures. As temperatures increase the chemical bonds are reestablished forming the original chemical with the release of stored heat. Highly endothermic chemical reactions can achieve very dense energy storage per unit material mass. The energy remains stored until it is recovered by an exothermic reaction. 4.4.5 Analytical representation of thermosyphon solar energy water heater In a thermosyphon solar water heater a hot water store is located above and connected by dawncomer and upriser pipes to the solar collector. The height difference between the collector and store inhibits nocturnal reverse circulation (Norton and Probert, 1983). A finite difference transient heat transfer and momentum analysis in which the thermal capacitances of both the collector plate and the cover are implicit can be applied to the liquid (i.e., water) circulating through the collector, upriser, hot water store, and downcomer of a thermosyphon loop (Hobson and Norton, 1988). All fluid properties and heat-transfer coefficients can be assumed to be temperature-dependent based on either the ambient or mean component temperatures. These coefficients can be updated at each timestep in a numerical calculation. An appropriate store model includes a simulation of buoyancy-induced mixing between stratified layers that occurs Solar energy collection and storage 135 when warmer fluid is introduced below a cooler layer. Friction factors can be calculated using correlations appropriate to both the nonisothermal thermally-destabilized low Reynolds number flow and the isothermal developing laminar flow present in the lengths of straight pipeline. Empirically-determined laminar heat loss coefficients for the pipe bends are employed usually. Time variations of insolation, ambient temperatures and hot water withdrawal are the inputs to most simulation, models with the transmission of the glass collector cover being a function of the sun-hour angle. A two-dimensional finite difference approach takes account of the thermal capacitance of the collector. In the derivation of the energy equations for the collector model, a glass cover, opaque to long wave radiation, and parallel fin-and-tube collector plate are treated usually as two large, parallel, grey bodies for the analysis of radiative heat exchanges. In addition, the glass cover is assumed to be at a uniform temperature at each moment in time and is therefore represented by a single node. It is also assumed usuallythat the temperature gradient through each thin fin is constant so that twodimensional planar conduction is assumed to prevail. Conduction within the collector fluid in the direction of the main flow is taken to be negligible in most analysis as are the thermal capacities of the thermal insulations applied to the hot-water store, collector, and connecting pipes. An energy balance on an incremental volume of the fin gives, for a two-dimensional plate temperature distribution, .fCf df .Tf .t = kf df  .2Tf .x2 + .2Tf .y2  + hf .g(Tg - Tf ) + Uf ,a(Ta - Tf ) +  s e -1 g + e -1 f - 1  (T4 g - T4 f ) + (Ta)eI. (4.4.4) The boundary conditions are: (i) from symmetry of adjacent nodes .Tf .x  (.2 ,o,t) = .Tf .x  (w,L,t) = 0 (4.4.5) and, (ii) as there is no heat flux through ends of the plate .Tf .y  (x,o,t) = .Tf .y  (x,L,t) = 0 (4.4.6) The boundary condition relating the temperatures of the fluid in the risers and the plate is obtained from a heat balance or an incremental volume of the fluid. The temperature of the pipe wall is assumed to be that of the fin at x=0. Thus: .wCw  pD2r  .Tw .t + Cw .m c N .Tw .y = hrwpDr × (Tf (o,y,t) - Tw). (4.4.7) 136 Solar energy sciences and engineering applications The cover temperature is obtained via .gdgCg .Tg .t = hgf  - Tf - Tg  +  s e -1 g + e -1 f - 1  ×  - T4 f - T4 g  +hwind(Ta - Tg) + seg(T4 sky - T4 g ). (4.4.8) To simulate accurately the transient response time of the collector, it is essential to take into account the thermal capacity of the water in the collector’s header pipes. The transient response of the fluid in this component has been shown to account for the experimentally-observed the time lapse between a change in the temperature of the fluid leaving the end of the riser pipes and when a corresponding perturbation appears at the outlet of the header pipe. Assuming a uniform flow distribution between the collector risers, a heat balance on the nth section of the header pipe gives W.w .m  pD2h 4  .Tn .t = n(Tn-1 - Tn) + (T0 - Tn-1). (4.4.9) for the connecting pipes, a heat balance for an element of fluid within the pipes gives .wCw  pD2h 4  .Tw .t + Cw .m c .Tw .y = Us,apD.(Ta - Tw). (4.4.10) for the hot water store, an energy balance on an incremental section of fluid which is remote from the end sections of the tank gives .wCwAs .Tw .t + Cw .m s .Tw .y = kwAs .2Tw .y2 + Us,aPs(Ta - Tw) (4.4.11) where .m s = .m c - .m L. The boundary conditions for the hot-water store are determined by considering incremental sections of fluid in contact with either the top or the base of the tank as dy - 0, i.e., (i) for the top of the tank, Us,a,T(Ta - Tw) + kw .Tw .y = 0 (4.4.12) (ii) for the base of the tank Us,a,B(Ta - Tw) - kw .Tw .y = 0 (4.4.13) In a simple mixing model, when warm fluid is introduced below cooler water, it is assumed that complete mixing ensues and the two adjacent nodes attained a single temperature. This process is repeated down the tank until a stable themocline is restored. Solar energy collection and storage 137 A transient momentum balance on the four components comprising the thermosyphonic loop, assuming one-dimensional incompressible flow, gives  Lr Ap + L2 Ap + Ls As + Ld Ap  . .m c .t = g . f . .w sin(.)dy - Pm - .m c 2  frLr .wDrA2r N2 + fuLu .wDpA2p + fuLu .wDpA2p  + fdLd .wDpA2p . (4.4.14) Equations expressed in finite difference forms are solved simultaneously using their appropriate boundary conditions. When simultaneous equations are solved using a Gauss-Seidel iterative method in order to find T(x,y,t+t) for each node, the solution is stable unconditionally and the size of the time step is only limited by the accuracy required. 4.4.6 Solar water heater design From a transient heat balance on a generic directly heated thermosyphon solar-energy water heater, the following dimensionless parameters Y, Z and X, designated the Heywood, Yellot and Brooks numbers respectively, have been identified (Hobson and Norton, 1988b): Heywood number: Y = FAVAC(ta)eH MSCW(Ta - Tm) (4.4.15) Yellot number: Z = [FAVACUL + (UA)S]t (MSCW) (4.4.16) Brooks number: X = fQtot MLCW(Ta - Tm) (4.4.17) X 1 - exp ( - Z) = Y Z + 1 (4.4.18) The mean daily circulation number is Np and Specific load: W = ML MS (4.4.19) The Bailey number, K, which represents the system parameters effecting flow within the system is defined as: Bailey number: K = .ßgTref [h3 - h2/2] v .m ref [Lr/N(D4r ) + L./D4. )] (4.4.20) where Tref and mref are given the values 10.C and 10-1 kg -1 respectively. The relationships between the Yellot, Z, Bailey, K, Heywood, Y, Brooks, K, numbers and the specific load,W, may be summarised as a nomogram shown in Figure 4.4.3. The Heywood, Y, and Yellot, Z, numbers and the specific load,W, are functions 0.20 K = 4 W = 2.0 W = 1.5 W = 1.2 W = 1.0 W = 0.8 W = 0.7 W = 0.6 W: Specific load K: Bailey number W = 3.0 W = 5.0 .m3Displacement of gradient from max value K = 5 K = 7 K = 9 K = 12 K = 16 K = 21 K = 27 0.00 Z = 0.2 Z = 0.3 Z = 0.4 Z = 0.5 Z = 0.6 Z = 0.7 Z = 0.8 Z = 0.9 Z = 1.0 Z = 1.1 0.10 Y = –80 Y = –60 Y = –40 Y = –30 Y = –20 Y = –10 Y = –5 Y = 5 Y = 10 Y = 20 Y = 30 Y = 40 Y = 60 Y = 80 0.20 0.30 0.40 0.50 Y: Heywood number –60 –50 –40 –30 –20 –10 X: Brooks number 0 10 20 30 40 50 60 0.60 0.70 Z = Reference Yellot number 0.80 0.90 1.00 1.10 1.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0.00 0.10 0.20 Z: Yellot number 0.30 mJ characteristic gradient 0.40 0.50 0.60 0.70 0.80 0.90 Figure 4.4.3 Nomogram representing the design formulae given by Equations 4.3.8, 4.3.9 and 4.3.5. Solar energy collection and storage 139 Table 4.4.2 Details of the configuration, operating conditions and thermal properties of the thermosyphon solarenergy water-heater used in the sample calculation. System parameters Ac= 2.0 (m2) Fav =0.9 (ta)e =0.72 UL =3.5 (Wm-2 K-1) (UA)s =3 (WK-1) N=8 M2 =297 (kg) h3 =1.8 (m) h2 =1.7 (m) Lr =1 (m) Dr =0.015 (m) Lp =8.72 (m) Dp =0.025 Htd =19.2 (M Jm-2) Weather conditions Ta =16 (.C) Tm =15 (.C) t=59,220 (s) Hot-water demand ML =208 (kg) TL =46 (.C) Fluid properties .w =998 (kgm-3) µw =10-3 (Nsm-2) vw =1.00×10-6 (m2s) Cw =4190 ( J kg-1 K-1) ßw =2.1×10-4 (K-1) of the applied conditions, whereas the Bailey number, K, is a function essentially of the system design. However, all these dimensionless groups include information available readily to a designer who, using the nomgram, can thus determine the Brooks number X, and thus the solar fractions. A worked example of using the correlations to predict the daily solar fraction The component specifications of a thermosyphon solar- energy water heater and climatic conditions (for a typical day in June) used in the following example are given in Table 4.4.2. Evaluating the parameters K, W, I and Z gives K=12, W=0.7, Y=20, and Z=0.3 respectively. Also, since the thermal performance is being determined for the reference month of June, ZJ =Z=0.3. A three-stage algorithm is used to determine the dimensionless Brooks number, X, from which the solar fraction can be calculated. Stage I is to determine the deviation, mJ (due to circulation number effects) of the characteristic gradient from the maximum value, mj,max. m* =0.055, giving mj =0.051. This stage corresponds to the first quadrant of the nomogram shown in Figure 4.3.4. Stage two is to determine the maximum gradient, mJ,max for the system and subtract the gradient displacement, m, to give the actual gradient, mJ of the characteristic curve. mJ,max, =0.673, the actual gradient, mJ, is then given by mJ = mJ,max - (mJ) = 0.622 (4.4.21) 140 Solar energy sciences and engineering applications or using the nomogram, the value of mJ can be read off from the second quadrant (moving anticlockwise). Stage three is using the characteristic curve, X from which the solar=10.71. This calculation corresponds to the third and fourth quadrants of the nomogram. The total thermal load, Qtot , when ML kg of water are heated from the mains cold water supply temperature, Tm to the required temperature of TL is Qtot = MLCw(TL - Tm) = 27.02 × 106 J (4.4.22) From the definition of the Brooks number, X the daily solar fraction can be determined: f = XMLCw(Ta - Tm) Qtot = 0.345 (4.4.23) 4.5 SOLAR ENERGY COLLECTION AND STORAGE FOR DRYING CROPS High crop losses can ensue from inadequate drying, fungial attacks, and rodent and insect encroachment in traditional “open-sun’’ drying. Solar-energy tropical-crop dryers that enclose the crop and enable air to circulate around it compete economically with traditional open-sun drying because they (i) require a smaller area of land in order to dry similar amounts of crop that would have been dried traditionally in the open, (ii) will yield a relatively high quality of dry crop, because insects and rodents are unlikely to infest it during the drying process, (iii) have a shorter drying period, (iv) they afford protection from sudden rain, (v) incur relatively low capital and running costs, and (vi) give improved crop quality achieved after drying. In an integral type natural-circulation solar-energy dryer, the crop to be dried is placed in a drying chamber with transparent walls; heat is supplied to the crop by direct absorption of solar radiation and by convection from the heated internal surfaces of the chamber. The heat abstracts the moisture from the product, while also lowering the relative humidity of the resident air mass thus increasing its moisture carrying capability. The direct absorption of solar radiation makes these dryer particularly appropriate for greenish fruits as during dehydration, the decomposition of residual chlorophyll enhances the proper colour “ripening’’. For certain varieties of grapes and dates this direct exposure to sunlight is considered essential for the development of the required colour in the dried products. Similarly exposure to sunlight of arabica coffee develops full flavour of the bean. Conversely, for some fruits, exposure to sun reduces considerably the vitamin content or blemishes pigments; such produces are thus best dried in enclosed opaque-wlled cabinets or silos to which solar heated air is provided. Solar energy collection and storage 141 Integral type natural-circulation solar-energy dryers are both simple and cheaper to construct than those of the distributed type for the same loading capacity. However the potential drawbacks of the former are (i) a liability to localised over-heating and (ii) relatively slow overall drying rates. To overcome these limitations, a “solar chimney’’ is employed to increase the buoyant force on the air stream and thus provide an increased rate of moist-air removal. Two generic dryer types can thus be identified, namely: the cabinet dryer and the ventilated greenhouse dryer. A natural-circulation solar-energy cabinet dryer is simply a single or double-glazed insulated container referred to as a cabinet at small scales or a silo at large scales. Solar radiation is transmitted through the cover and is absorbed on the blackened interior surfaces as well as on the product itself, thus raising the internal temperature. Vents at both the base and lower parts of the cabinet or silos enable air ventilation, with warm air leaving via upper apertures under the action of buoyant forces, drawing in replenishing fresh air at the base. Shallow layers of the product are placed on perforated trays inside the enclosure. Solar cabinet dryers, constructed from cheap locally-available materials, are usually relatively small units used to preserve “household’’ quantities of fruits, vegetables, fish and meat. The major drawback of small-scale cabinet dryers is the poor air circulation often that reduces the drying rate, and incurs very high internal temperatures that can overheat crops. Drying air temperatures above 70.C are excessive, particularly for perishables fruits and vegetables. Relatively large air inlet ducts with appropriatelydesigned solar chimneys are essential for effective circulation within the cabinet to minimise temperature elevations. Natural-circulation solar cabinet dryers are probably the most widely used type of solar dryer. A cabinet dryer equipped with a solar chimney shows higher efficiency than that of a natural-circulation distributed type which the incoming air was heated as it passed through a solar-energy collector and for more efficient than open sun drying. Natural circulation solar-energy greenhouse dryers are larger than most cabinet dryers and are characterised by extensive glazing on their sides. Usually the glazing is on the front side (i.e. sun facing side) of the dryer while the rear side is insulated. Insulant panels may be drawn over the glazing at night to reduce heat losses and heat storage may also be provided. Designed properly, a solar greenhouse dryer allows a greater degree of control over the drying process than the solar cabinet dryer and are more appropriate for large-scale drying. Typical later designs of natural-circulation solar greenhouse dryers include the widely-reported polyethylene-tent fish dryer built (Doe et al., 1977) that consists of a ridged tent-like bamboo framework clad with clear polyethylene sheet both on the side orientated towards the sun and on the ends. The rear side was clad with black polyethylene sheet, which was also spread on the floor. The cladding at one end was arranged to allow access into the drying chamber for loading and unloading. The clear plastic cladding at the bottom edge of the front side was rolled around a bamboo pole which could be adjusted to control airflow into the chamber, while the vents at the top of the ends served as the exit for the moist exhaust air. The maximum temperature for the drying of fish is 50.C: above which the fish will cook. Dryer temperatures of about 45.C are however desirable as flies and larvae infestations within the fish are boiled. The high 5.C temperature merger is difficult to maintain with thermometry connected to vent aperature controls. 142 Solar energy sciences and engineering applications 4.6 SOLAR ENERGY COLLECTOR AND STORAGE FOR THERMAL POWER GENERATION Parabolic troughs, the most deployed widely solar energy concentrator, consist usually of long curved mirror-surfaced troughs, which concentrate direct insolution on a tube at the focal axis of a parabolic mirror (Fernandez-Garcia et al., 2010). Parabolic trough concentrators are made usually of back-silvered glass for high reflectance and durability. A stainless steel tube receiver is usually coated with a highly solar selective absorbing ceramic and metal blend that is durable at high temperatures material. The absorber is surrounded usually covered by a geometrically concentric and evacuated borosilicate glass tube envelope. In a central receiver system a large array of heliostats mirrors individually tracks the sun to reflect insolation onto a fixed receiver mounted on a tower, that absorbs the heat. The heated fluid (usually molten salt) convectively removes the receiver’s heat energy and is then transported from the receiver to drive a turbine generator or stored. Parabolic dish concentrators reflect solar energy onto a receiver mounted at the focal point. Parabolic dishes typically use multiple curved reflective panel segments made of glass or laminated films. These concentrators are mounted on a two-axis solar tracking system. The concentrated sunlight at the receiver may be utilized directly by a cycle heat engine mounted on the receiver, or simply heats a fluid that is transported for storage. Using multiple smaller engines means: (i) smaller engines can be replaced readily (ii) a plant can deliver close to rated power while engines are being repaired, and (iii) the system can be easily expanded by adding modules to accommodate growth. The thermal efficiency of an engine is proportional to the difference between the maximum collector and heat rejection temperatures. Most real engines operate with efficiencies of just over half of the ideal Carnot efficiency. Whilst engine efficiency is higher at increased operating temperature, the efficiency of a solar collector decreases as its operating temperature increases. The optimal operating temperature thus depends on the particular efficiency trends of the specific engine and collector employed. Typically organic Rankine cycle or Sterling engines are used with evacuated tubes in parabolic troughs or two axis tracking parabolic dishes respectively. Rankine and Brayton cycles both have constant-pressure heat-addition processes readily applicable to solar heating. Stirling engines use a reciprocating piston design can be solar heated directly. A working fluid can pass through the absorber directly, or there can be an intermediate heat-transfer fluid flowing in a closed loop between the absorber and a heat exchanger to heat another fluid via a heat exchanger. Incorporating an intermediate heat-transfer fluid requires another pump, heat exchanger as well as two fluids. Utilizing an intermediate heat-transfer fluid a lower vapor pressure reduces the receiver’s mass and obviates the need for high-pressure piping. 4.7 OVERALL SYSTEM OPTIMIZATION Solar water heating systems can be designed to meet hot water heating (DGS, 2005), industrial process heating (Gordon and Rabl, 1982); Kalogirou, 2003; Kulkarni et al., Solar energy collection and storage 143 2008), domestic space and water heating (Letz et al., 2009; Yazdanshanas and Furbo, 2008; Yazdanshanas et al., 2008). They can be augmented by the inclusion of heat pumps (Troi et al., 2008; Huang and Lee, 2005; Morrison, 1984). A wide variety of methodologies are available for the sizing of system components and determining the optimal operating parameters to satisfy a known set of characteristics of the energy load (Norton et al., 2001). These methodologies include: utilizability (Collares-Pereira et al., 1984) empirical correlations (Gopffarth et al., 1968), simplified analysis, semi-analytical simulation, stochastic simulation, simplified representativeday simulations (Garg et al., 1984) and detailed hour-by-hour simulations (Morrison and Braun, 1985; Hobson and Norton, 1988a). Each of these will be considered individually. There is a minimum threshold insolation at which the solar heat gained by a collector corresponds to its heat losses at a particular ambient temperature. Only above this minimum insolation threshold does the collector supply a useful heat yield. Utilizability is a statistical attribute of the location-specific variation of insolation over a given duration. For example hourly utilizability is the fraction of hourly incident insolation that can be converted to heat by a collector with ideal heat removal and no optical losses. As all solar collectors have heat losses, utilizability always has a value of less than one. Utilizability can be related to other statistical properties of diurnal and annual patterns of insolation (Reddy, 1987) to produce mathematical terms to which specific collector parameters can be attached. Various generalised expressions have thus been be derived, for example, for the yearly total energy delivered by flat plate collectors whose tilt angles equalled the latitude of their notional location (Rabl, 1985). This methodology can be very useful in initial design, but limitations include possible inaccuracy of underlying insolation data correlations particularly when extended to new locations collector inclinations and orientations. The technique has been applied to interseasonal storage (Braun et al., 1981), where due to the very large thermal store mass required, collector inlet temperatures are invariant. Correlation-based system design techniques are predicated on the high probability that for a given solar energy process heat system, in a given period. More insolation will lead to solar energy satisfying a larger share of the heat load. A dimensionless or normalised solar energy input has usually been plotted against a similarly parameterised output for a given system configuration from which correlations were obtained. Simplified analyses consider solely the key driving parameters of system performance assuming all other variables remain constant (Braun et al., 1981). For solar industrial heat loads that over the operating period have largely constant flow rates and temperatures, simplified analysis have been developed that can employed for feasibility and initial design of industrial hot water system with heat storage (Collares-Pereira et al., 1984). Simplified analyses maintain a physical basis for the relationships between parameters that is lost in empirical correlations whose equations are polynominal curve fits. Semi-analytical simulation use detailed numerical models. However rather than undertaking hour-by-hour calculations using insolation, ambient temperature and load data, in this approach sinusoidal and linear functions are used to describe the insolation and load respectively with ambient temperature either varying sinusoidally or remaining constant. This approach has largely been superseded by hour-by-hour analysis as 144 Solar energy sciences and engineering applications the computing resources required to successfully undertake the latter have become widely available. To represent insolation, ambient temperature and load characteristics in stochastic simulations, Markov chain models can be produced from several years hour-by-hour data for a specific location. 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Solar Energy 80, 1021–1030. Chapter 5 Basics of the photovoltaic thermal module Krishnan Sumathy Department of Mechanical Engineering, North Dakota State University, Fargo, USA 5.1 INTRODUCTION Energy plays a crucial role in social and economic development. Remarkable increases in oil prices have stimulated research in the renewable energy field as it contributes to the diversity in energy supply. Compared to other types of energy, renewable energy reduces the dependence on fossil fuel resources and carbon emissions to the atmosphere. Nobuyuki (2006) gives the latest data which show that renewable energy technologies provide 13.3% of the energy need around the world (Baños et al., 2010) and will be greatly dependent on the future, considering their sustainability and wide public acceptance. The total incoming solar radiation is about 3.8 million EJ per year, which can meet the entire energy demand if harnessed properly. Solar energy can be either utilized for producing electricity directly or to produce heat. The combination of photovoltaic and thermal systems (PV/T) is particularly attractive because of its efficiency in directly converting solar energy into electricity and heat, concurrently. A photovoltaic/thermal (PV/T) module mainly comprises a photovoltaic panel and a solar thermal collector, forming the core components of a photovoltaic/thermal (PV/T) system. Solar thermal systems and PV systems are essentially different because solar systems generate heat energy while PV systems generate electricity. The differences between PV system, PV thermal system, and solar collector are demonstrated in Figure 5.1.1. Solar energy can be captured in various ways: in a PV system, only a small fraction of solar radiation absorbed by the panel is converted into electricity. That is to say, a subsection of absorbed solar radiation is used by PV cells to generate electricity; letting most of the radiation turn into waste heat. This results in the decrease of the Figure 5.1.1 Comparison between PV, PV/T and solar thermal systems. 150 Solar energy sciences and engineering applications PV module’s efficiency due to the rise of temperature of PV cells. Natural or forced circulation of air or other fluids can be used to cool PV cells which heats up the photovoltaic thermal systems (PV/T) and can be used as a substitute for the conventional PV modules. A photovoltaic thermal system (PV/T) integrates heat extraction devices to the PV cell to reduce the temperature of the PV module and to increase electricity production as well as useful heat simultaneously, thereby increasing the overall efficiency of PV/T system. For more than thirty years, many researchers have used and discussed the idea of PV/T both experimentally and numerically. The initial focus was on glazed collectors utilizing air and liquid as the heat transport media. Later, unglazed heat pump-based collectors were also introduced. PV/T systems are mainly classified based on the type of fluid being utilized for heat removal. Compared to PV/T air systems, PV/T water systems relatively have a higher efficiency, because water has a higher thermal capacity and conductivity (Prakash, 1994). But considering the leakage and corrosion that can be caused by water, more robust construction must be included in a PV/T water system to make it water-tight and corrosion-free. Therefore, the easiest way of heat extraction from PV modules is by circulating natural or forcing air through an air channel either on the top or rear-side of the PV surface. Other than giving rise to diverse ways of heat extraction, the types of PV modules also influence the operational state of the PV/T system. Crystalline-silicon (c-Si), polycrystalline silicon (pc-Si), and recently exploited thin films of amorphous-silicon (a-Si) cells can be used in PV module construction. The c-Si cells are exorbitantly expensive due to the comprehensive and energy-intensive procedures needed to produce them, even though they are highly efficient. In the past two decades, attempts have been made to lower the price of c-Si cells through various manufacturing methods to further improve module efficiency. The PV/T systems have a wide range of applications. One of the feasible ways to utilize photovoltaic system is to incorporate the PV modules into the building envelope, as they can be incorporated into the façade and roof of buildings easily. Such building integrated PV (BIPV) technology is one of the most widespread applications of photovoltaic systems in urban buildings. Hence these systems have received a great interest from engineers and architects. The PV/T systems have a grander prospect by producing green, safe and tactically significant alternatives to power generation. The superiorities of grid-connected PV electricity to customers are reflected in both economic and environmental concerns. Customers can partially fulfill their electricity needs while using utility-generated power at night and on gloomy days by using a grid-connected PV system where utility power is attainable. The domestic or commercial buildings can also use the extracted heat for heating purpose. Most photovoltaic thermal (PV/T) systems are set up in districts or areas where grid and telephone networks are rarely available, or may be difficult to reach. The PV/T systems have a relatively long service lifetime with nearly no maintenance. Therefore, autonomous or stand-alone PV/T systems are more welcome among remote rural areas. Furthermore, researchers are trying to improve solar output and power production by using solar trackers and concentrator photovoltaic thermal systems, which are most feasible when space is scarce. One of the main advantages of a concentrator integrated PV/T system is to reduce the number of solar cells, as well as increasing power output. A concentrator collects solar radiation that is received on a relatively large surface Basics of the photovoltaic thermal module 151 area and converts it to a concentrated radiation onto a small area with solar cells. The concentrators can be made from simple plastic lenses or conventional mirrors, or polished light water surfaces. Fins are added to the back of the PV panel to further improve the performance of air PV/T systems. Numerous research has been attempted to advance the performance of photovoltaic thermal systems. Generally speaking, the conversion rate of PV modules available in the commercial market is very low. They can only convert about 6–18% of solar radiation incident on the PV panel to electricity. The rest of the solar energy is turned into waste heat affecting the cell competence and some is reflected back to the atmosphere. A PV/T system has the following advantages: (i) Dual function: the integrated system can generate both electricity and heat; (ii) Flexible and efficient: with limited roof surface area, the integrated system has a higher efficiency than two independent systems, especially using the BIPV technology; (iii) Wide application: the heat generated can not only be used for heating but also for cooling, for instance it can be used for desiccant evaporative cooling; (iv) Economical and practical: the PV/T system is simple to be retrofitted or integrated into different types of buildings with insignificant remolding. A PV/T system can be used as roofing material and has a shorter payback period. Compared to parallel connecting of photovoltaic panels to solar thermal collectors, the integrated PV/T modules are not only able to produce more energy per unit surface area, but also have less initial and production cost. The collector area can be reduced up to 40% by using PV/T system producing the same amount of energy according to an ECN report (Energy research Centre of the Netherlands) (IEA, 2007). Furthermore, PV/T modules cater to the same aesthetic need as PV. This chapter will focus on different technologies and module aspects of heat transformation of the PV/T systems. Modifications are suggested with the main objective being to improve heat transfer in the PV/T systems. A detailed description on hybrid PV/T solar systems is included in a recently published Roadmap (Zondag et al., 2005; Affolter et al., 2006), where several aspects regarding technology, present status and future perspectives of these solar energy conversion systems are presented (Arif Hasan and Sumathy, 2010). 5.2 PV/T DEVICES PV/T devices can vary in design to suit various applications, ranging from PV/T domestic hot water systems to ventilated PV facades and actively cooled PV concentrators. The markets for both solar thermal and PV are growing rapidly and have reached a very substantial size. For PV/T, a similar growth can be expected, since the technical feasibility is proven and as such it can be integrated with other domestic applications. PV/T has broad range of applications, that is, it is not only suitable for domestic hot water heating (glazed PV/T collectors), but also for commercial buildings (ventilated PV to preheat ventilation air during winter and to provide the driving force for natural 152 Solar energy sciences and engineering applications Table 5.2.1 Recommendation of the collector type based on the type of demand. Demand Recommendation Water High temperature Use glazed liquid collector. Also, an unglazed collector can be used if PV/T has to be integrated to a heat-pump. Low temperature To meet only summer demand, use unglazed liquid collector. On the other hand, to meet both summer and winter demands, use glazed liquid collector; an unglazed collector can be chosen if PV/T has to be integrated to a heat-pump. Air High temperature Use glazed air collector or unglazed collector.Ventilated PV can be used as a heat source if PV/T has to be integrated to a heat-pump. Low temperature To meet only summer demand or for the place receiving high irradiation in winter,use unglazed air collector or ventilated PV.On the other hand, to meet both summer and winter demands, use glazed air collector; an unglazed collector can be a choice if PV/T has to be integrated to a heat-pump. ventilation during summer). Hence, the market for PV/T might even be larger than the market for thermal collectors. Depending on the application, the required thermal demand can be covered by choosing appropriate PV/T system. There exist various forms of PV/T system which depend on the type of PV module as well as its design, type of heat removal fluid (water/glycol or air) and on the concentration of the incoming radiation. The existing PV/T designs can be classified as: (i) Liquid PV/T collector (ii) Air PV/T collector (iii) Ventilated PV with heat recovery (iv) PV/T concentrator. Irrespective of the type of collector, the absorber of each PV/T collector is provided with a glass cover to reduce the thermal losses. If such a cover is present, the collector is referred to as “glazed’’, otherwise as “unglazed’’. Glazed collectors have less thermal losses, and hence they could higher collector fluid temperatures. For medium to high temperature applications, this significantly improves the annual thermal yield. However, glazed collectors result in high stagnation temperatures that may be critical for certain types of PV encapsulant (risk of yellowing and delamination) resulting in hot spots. In addition, bypass diodes may get overheated due to the additional insulation. Reflection losses at the glazing further reduce electrical performance. Increased temperature levels lower the electrical yield. In summary, whether the collector should be glazed or not, it is important to find a good balance (illustrated in Table 5.2.1) between the increased thermal yield on one hand, and the reduction in electrical yield and the issues related to possible degradation on the other hand. Basics of the photovoltaic thermal module 153 5.2.1 Liquid PV/T collector In order to improve the energy performance of the photovoltaic system, much effort has been spent on research and development of the hybrid PV/T technology. One of the design modifications is to increase the PV module performance by circulating water to extract the heat using water as the coolant. These liquid PV/T collectors are similar to conventional flat-plate liquid collectors; an absorber with a serpentine tube or a series of parallel risers is applied, onto which PV has been laminated or glued as an adhesive epoxy joint. Two common configurations used in PV/T systems are: “Parallel plate configuration’’, and “Tube-in-plate configuration’’. Prakash (1994), Huang et al. (2001), Tiwari and Sodha (2006) and Tiwari et al. (2006) have worked on the parallel plate design, while Zondag et al. (2002), Chow (2003), Chow et al. (2006), Kalogirou (2001), Huang et al. (2001) and Tiwari and Sodha (2006) have carried out an in-depth study on tube-in-plate design. Within the first works on PV/T water system, Bergene and Lovvik (1995) initially conducted a theoretical study on PV/T water system composed of flat-plate solar collector with solar cells. Their proposed system is particularly suitable to preheat the domestic hot water. More recently, Zondag et al. (2003) grouped the design concepts of water-type PV/T collectors into four main types: sheet and-tube collectors, channel collectors, free-flow collectors, and two-absorber collectors. These collector types are designed for pump (forced) circulation (Figure 5.2.1). Based on numerical analysis it has been suggested that a channel should be provided to effect liquid flow below the transparent PV module to effect higher collector efficiency (Chow et al., 2006). Nevertheless, from the viewpoint of good overall performance and structural simplicity, single-glazing sheet and tube hybrid PV/T collector is regarded as the most promising design. Dubey and Tiwari (2008) designed an integrated photovoltaic (glass-to-glass) thermal (PV/T) solar water heater system and tested it in outdoor conditions of India. Similarly, Erdil et al. (2008) constructed and tested a hybrid PV/T system for energy collection at geographical conditions of Cyprus, where they used water as the cooling fluid. It was reported that the payback period for their proposed modification was less than 2 years which made their hybrid system economically attractive. Also, Daghigh et al. (2011) used water as the working fluid and had presented the advances in liquid based photovoltaic/thermal (PV/T) collectors. The liquid-based photovoltaic thermal collector systems are practically more desirable and effective than air-based systems. Temperature fluctuation in liquid based PV/T is much less than the air-based PV/T collector. The future direction of water-cooled and refrigerant hybrid photovoltaic thermal systems was also presented. Their study revealed that the direct expansion solar-assisted heat pump system could achieve a better cooling effect than the PV/T collector. Similar to the above modeling work, Chow et al. (2006) had developed a numerical model of a photovoltaic-thermosyphon collector system using water as a working fluid and verified the model’s accuracy by comparing with measured data. The energy performance of the collector system was examined, through reduced-temperature analysis and the study was further extended to analyze the performance of the system in the “hot summer and cold winter’’ climate zone of China. The numerical results were found to be very encouraging, and according to them the equipment is capable of extending the 154 Solar energy sciences and engineering applications Figure 5.2.1 Various collector concepts: (A) sheet-and-tube PVT, (B) channel PVT, (C) free flow PVT, (D) two-absorber PVT (insulated type) (Zondag et al., 2003). PV application potential in the domestic sector. Apart from the said study, Chow et al. (2009) also carried out analytical simulation to investigate the annual performance of building-integrated photovoltaic/water-heating system for the Hong Kong climate and found that annual thermal and cell conversion efficiencies were about 37.5% and 9.39%, respectively. Based on the results, they confirmed that PV/T systems could be applicable even to hot-humid regions. Though the liquid collectors have proven to be technically feasible, economic feasibility is yet questionable. Compared to the air heating PV/T system, not many developments are seen in the literature on liquid-heating systems due to their inherent limitations such as: additional cost of the thermal unit pipes for the water circulation, and the inherent freezing problem of working fluid when used in low temperature regions, etc. 5.2.2 Air PV/T collector The PV/T air collectors are similar to a conventional air collector with a PV laminate functioning as the top cover of the air channel. PV/T air collectors are cheaper than the PV/T liquid collectors because of the flexibility that conventional PV modules can be easily converted to a PV/T system, with very few modifications. PV/T air collectors can Basics of the photovoltaic thermal module 155 either be glazed or unglazed. In general, air collectors are mostly applied if the end-users have a demand for hot air, space heat, dry agricultural products, or to condition the indoor air (air cooling). At present, air heating systems are mainly designed to directly use the air for space heating. However, the opportunity for this application depends directly on the market share of air heating systems, which is low in most countries. A niche market is given by preheating of ventilation air for large volume buildings (stores, sport halls, schools and other commercial buildings) where temperatures in the range of 15 to 25.Care desirable.With the very same air systems, hot water preparation is often possible through an air/water heat exchanger, which is generally done during the summer season in order to increase the overall performance of the system. The application of air as a heat transport medium compared to liquid, has significant advantages along with few inevitable disadvantages. Using air as a medium of heat transport avoids the damage caused by leakages, freezing or boiling. The disadvantages that brought up are the low heat transfer efficiency and high volume transfer demand due to the lower heat conductivity and density. It also loses more heat energy if leakage happens. Tonui et al. (2007) carried out an experimental study on air-cooled PV/T solar collectors in which a few low cost performance improvements were introduced. Both water and air have been used for PV cooling through a thermal unit attached to the back of the PV module. Compared to water, air is preferred due to minimal use of material and low operating cost despite its poor thermo-physical properties. The study has investigated the performance of two low cost heat extraction improvement modifications in the channel such as the use of a thin flat metal sheet suspended at the middle or finned back wall of an air channel in the PV/T air configuration. A theoretical model was developed and validated against experimental data, where a good agreement between the predicted results and measured data were achieved. The validated model was then used to study the impact of various design parameters such as the channel depth, channel length and mass flow rate on electrical and thermal efficiency, PV cooling and pressure drop. The study had confirmed that the suggested modifications positively improve the performance of the PV/T air system. As the heat transfer in the air cooled PV/T system is much more critical than in the liquid cooled PV/T system, it is important to model the heat transfer properly. Sopian et al. (1996) presented a performance analysis of single-pass and double-pass PV/T air systems. The performance of single-pass and double-pass combined photovoltaicthermal collectors are analyzed with steady-state models, with air as the working fluid. The performances of the two types of combined photovoltaicthermal collectors were compared. The results show that the new design, the double-pass photovoltaicthermal collector, has superior performance over the conventional design. For a flow through a tube or duct, entrance-effect plays an important role in the heat transfer. Eicker (2003) presented an overview of entrance-effect heat transfer relations for air-collectors, showing a variation of about 10% on the average Nusselt number when integrated over the entrance length and reported that for a sufficiently wide channel, the hydraulic diameter should be twice the channel height. Hegazy (2000) analyzed four types of air PV/T model design including single glazed collectors with air flow over (Model I) or below (Model II) the absorber; with air flow on both sides of the absorber in a single pass (Model III); and a double pass model (Model IV) as shown is Figure 5.2.2. The effects of air specific flow rate and the selectivity 156 Solar energy sciences and engineering applications Figure 5.2.2 Schematics of the various PV/T models along with heat transfer coefficients (Hegazy, 2000). Basics of the photovoltaic thermal module 157 of the absorber plate and PV cells on the performances were examined. The results showed that the Model I collector has the lowest performance under similar operational conditions. Other collectors achieved comparable thermal and electrical yields, among which the Model III collector requires the least fan power, followed by Models II and IV. The impact of air flow induced by buoyancy and heat transfer through a vertical channel heated from one side by the PV module on the PV/T performance was investigated numerically and experimentally by Moshfegh and Sanberg (1998) and Sanberg and Moshfegh (2002). The study reports that the induced velocity increases the heat flux non-uniformly inside the duct and its impact depends on the exit size and design. More analysis and modeling on passively cooled PV/T air systems continue to appear (Tiwari et al., 2006; Tiwari and Sodha, 2006; Naphon, 2005; Garg et al., 1994; Tripanagnostopoulos et al., 2006) and a substantial amount of research has been specifically carried out (Brinkworth and Sandberg, 2006; Benemann et al., 2001; Hodge and Gibbons, 2004; Pottler et al., 1999; Tiggelbeck et al., 1993; Tonui et al., 2007; Tripanagnostopoulos, 2007) to improve heat transfer to the air of both buoyancydriven and forced air flow systems. Their studies were focused generally on channel geometry, creation of more turbulence in the flow channel and increasing the convective heat transfer surface area in the channel. Most of these studies used simulation models for their experimental work where the PV module was simulated by a heated foil. Similar to the liquid collectors, various types of solar air systems exist and an overview has been given by Hastings and Morck (2000). The main concepts on aircooled PV/T systems were presented in the works of Kern and Russel (1978), Hendrie (1979), Florschuetz (1979), Raghuraman (1981) and Cox and Raghuraman (1985). The exclusive theoretical aspects of PV/T systems with air as the heat extraction fluid are detailed by Bhargava et al. (1991), Prakash (1994) and Sopian et al. (1996). 5.2.3 Ventilated PV with heat recovery In general, for building integrated photovoltaic panels, to ensure the modules are not overheated, the ambient air is circulated by thermosiphoning beneath or at the rear end of the PV panel, which is commonly referred to as “ventilated PV.’’ If this waste heat can be harvested and be used for secondary purposes, it functions as a PV/T collector, providing additional benefits: (i) A PV-facade may limit the thermal losses in a building by infiltration. Also the PV facade has the advantage of shielding the building from solar irradiance, thereby reducing the cooling load. Hence, such facades are especially useful for retrofitting poorly insulated existing offices. (ii) If there is no demand for the generated heat, then air collectors and PV-facades can use their buoyancy induced pressure difference to assist the ventilation. (iii) Facade integration of PV has additional cost incentives of substituting expensive facade cladding materials. However, the ventilated PV-façade may contribute to the building’s cooling load in hot summer, which is not desirable. To overcome this issue, a desiccant cooling cycle can be employed which can be energized with an additional collector. It is a novel open driven system introduced by Li et al. (2006), in which the required room 158 Solar energy sciences and engineering applications space is affected by evaporative cooling and the PV-driven air heating system provides the necessary regeneration of air. With such systems, it is possible to achieve a solar fraction of 75% with an average COP of 0.52. Such hybrid systems have proven to be most effective to offset the capital costs involved with BIPV. Detailed studies on such systems have been carried out by several researchers. For instance, Ricaud and Capthel (1994) have recommended improved air heat extraction methods and Yang et al. (1994) have exclusively worked on roof integrated air-cooled systems. Infield et al. (2006) presented a methodology to evaluate the thermal impact on building performance of an integrated ventilated PV facade. This was based on an extension of the parameters to take account of the energy transfer to the facade ventilation air. Four terms describing ventilation gains and transmission losses in terms of irradiance and temperature components were defined to characterize the performance of the facade in total. Steady state analysis has been applied in order to express these four parameters in terms of the detailed heat transfer process within the facade. This approach has been applied to the ventilated facade of the public library at Mataró, Spain and was used for validating their developed model. Several researchers (Posnansky et al., 1994; Ossenbrink et al., 1994; Moshfegh et al., 1995) have worked extensively on the building integrated PV/T systems. Later, Brinkworth et al. (1997), Brinkworth (2000), Brinkworth et al. (2000) and Krauter et al. (1999) presented design and performance studies regarding air type building integrated hybrid PV/T systems. In addition, Eicker et al. (2000) have presented on the performance of a BIPV PV/T system which was operated during winter for space heating applications and during summer for active cooling. Yet another comprehensive examination of PV and PV/T in built environments has been presented by Bazilian et al. (2001). The study highlighted the fact that PV/T systems are well suited to low temperature applications. Furthermore, they pointed out that the integration of PV systems into the built environment could achieve “a cohesive design, construction and energy solution’’. However, it should be noted that there exists a need for further research in the said field, before combined PV/T systems become a successful commercial reality. The building integrated photovoltaic is going to be a sector which would serve as a wider PV module application. The works of Hegazy (2000), Lee et al. (2001) and Chow et al. (2003) as well as Ito and Miura (2003) have given interesting modeling results on air-cooled PV modules. Recent work on building integrated air-cooled photovoltaic includes the study on the multi-operational ventilated PVs with solar air collectors (Cartmell et al., 2004), the ventilated building PV facades (Infield et al., 2004; Guiavarch and Peuportier, 2006; Charron and Athienitis, 2006) and the design procedure for air cooling ducts to minimize the loss in PV module efficiency. On the other hand, according to Elazari (1998), smaller size PV and PV/T systems, using an aperture surface area of about 3–5m2 and a water storage tank of 150–200 l, could be installed for small (one family) domestic houses, while large sized systems of about 30–50m2 and 1500–2000 l water storage are more suitable for multi-flat residential buildings, hotels, hospitals and various food processing industries. Further, Charalambous et al. (2007) suggested that the building-integrated PV/T collectors are most suited for climatic regions with low ambient temperatures so that the heat from PV surface can be put into effective use for space heating. Battisti and Corrado (2005) investigated the EPBT (energy payback period) for a conventional multi-crystalline Basics of the photovoltaic thermal module 159 building integrated with a PV system (retrofitted on a tilted roof) in Rome receiving an annual solar insolation of about 1530 kWh/m2/year. The study reported that EPBT was reduced from 3.3 years (standalone system) to 2.8 years by integrating the PV modules to the building. Despite these improvements, commercial application of PV/T air collectors is still marginal, but it is expected to be wider in the near future with many building facades and inclined roofs expected to be covered with photovoltaics. PV facades are already well established and are closely identical to PV/T facades. Hence, by replacing expensive facade cladding materials by PV facades, it is expected that the costs on a module level will be low compared to all other applications. However, on a system level the situation may be different; since PV facades are often unglazed, the temperature levels that can be reached are limited, and the costs of the additional infrastructure required may outweigh the benefits of the use of this heat, so it is essential to come up with alternative low-cost system designs. Yet another issue is that these systems are not yet standardized. However, due to the current strong link between this type PV/T systems and the existing building projects, efforts are made to formulate codes based on an architectural point of view and PV manufacturing constraints (Butera et al., 2005). 5.2.4 PV/T concentrator The combination of solar radiation concentration devices with PV modules appear to now be a viable method to reduce system cost, replacing the expensive cells with a cheaper solar radiation concentrating system. By concentrating, a (large) part of the expensive PV area is replaced by a less expensive mirror area, which is a way to reduce the payback time. This argument serves as the main driving force behind PV concentrators. Concentrating photovoltaics present higher efficiency than the typical ones, but this can be achieved only when the PV module temperature is maintained as low as possible (Othman et al., 2005). The concentrating solar systems use reflective and refractive optical devices and are characterized by their concentration ratio (CR). Concentrating systems with CR >2.5 must use a system to track the sun, while for systems with CR <2.5, stationary concentrating devices can be used (Winston, 1974). The distribution of the solar radiation on the absorber surface (PV module) and the increase in its temperature are two problems that affect the electrical output. The uniform distribution of the concentrated solar radiation on the PV surface and the suitable cooling mode together can contribute to an effective system operation and the achievement of high electrical output. PV/T absorbers can be combined with low, medium or high concentration devices, but so far, only low CR PV/T systems have been mainly developed so far. Reflectors of low concentration, either of flat type (Sharan et al., 1985; Al Baali, 1986; Garg et al., 1991) or of Compound Parabolic Concentrator (CPC) type (Othman et al., 2005; Garg and Adhikari, 1998; Garg and Adhikari, 1999; Garg and Adhikari, 2000) have been suggested. Tripanagnostopoulos et al., (2002) suggested a diffuse reflector to increase both electrical and thermal output of PV/T systems. Garg et al. (1991) presented a simulation study of the single-pass PV/T air heater with plane reflector. They further extended their work on a hybrid PV/T collector with integrated CPC troughs (Garg and Adhikari, 1998; Garg and Adhikari, 2000). Both the studies confirmed that the total efficiency of a PV/T collector with a reflector was marginally 160 Solar energy sciences and engineering applications higher compared to the systems without concentrators. Due to the increase in solar radiation, the average plate as well as solar cell temperatures had shown a sharp rise, as expected. Hence, the system performance in terms of electrical efficiency was low, due to the fact that the cell performance is dependent on its temperature. To overcome such overheating issues, Othman et al., (2005) designed a new double-pass photovoltaicthermal air collector with fins to enhance the heat extraction. It was observed that the cell temperature was reduced by a few degrees which had a positive influence on the cell efficiency. A simple low concentrating water-cooled type PV/T collector of the building integrated type investigated by Brogren and Karlsson (2001). It incorporates PV/T string modules with low cost aluminum foil reflectors with a CR of 4.3 times.With reference to medium concentration devices, PV/T systems based on linear parabolic reflectors (Chemisana et al., 2011) or linear Fresnel reflectors (Rosell et al., 2005) have been investigated. Although concentrators of low or medium CR are interesting devices to be combined with photovoltaics, 3D Fresnel lens or reflector type concentrators have been recently developed, aiming at the market of concentrating photovoltaics. The concept of combined linear Fresnel lenses with PV/T absorbers has also been attempted (Tripanagnostopoulos et al., 2007). Chemisana et al. (2011) carried out a study on a photovoltaic-thermal module for Fresnel linear concentrator. An advanced solar unit was designed to match the needs of building integration and concentrating photovoltaic/thermal generation. The unit contained three basic components: a domed linear Fresnel lens as primary concentrator, a compound parabolic reflector as secondary concentrator and a photovoltaic-thermal module. Models for the electrical and thermal behavior of the system were developed and validated experimentally and were found that the predicted results showed a good agreement with experimental measurements. Even though the PV efficiencies of concentrated PV/T systems have proven to be high, the market share for such systems is very minimal, which is mainly due to the fact that these systems are rather bulky, disqualifying them for many PV applications. Also, since concentrating devices require tracking either one axis or two axes, it makes building integration impossible. Furthermore, not all climates are suitable for high ratio concentration, because it depends on the amount of direct irradiation received. In the aesthetic point of view, the concentrating systems provide different reflections and optical effects, which are unusual to the built environment and also they might prevent such systems from being placed visibly in the facade construction. One of the feasible options may be to install the concentrator on a horizontal roof (e.g. PV/T systems integrated with booster reflector in parallel rows). One more additional point worthy to note is, though the small cell area allows the use of more efficient and expensive PV material, the combination of glazing and reflectors increases the stagnation temperature which may in turn lead to degradation of materials. For electrical performance, the uniformity of the irradiance may be compromised, increasing mismatch losses. However, this drawback might be overcome to certain extent by using diffuse reflectors. 5.3 PV/T MODULE CONCEPTS In a PV cell, part of the solar spectrum does not contribute to the electricity production. Photons with energy lower than the band gap do not have enough energy to create Basics of the photovoltaic thermal module 161 Figure 5.3.1 Cross section of a basic PV/T collector. Figure 5.3.2 PV/T concept with liquid flowing on top of the PV module. Figure 5.3.3 Channel PV/T concept with liquid flow beneath the PV cells. photon-hole pairs and could in principle fully contribute to the generation of heat. This generation can take place either in the cell or outside the cell if the cell material does not absorb light at these wavelengths. The position of the heat generation in the device determines the possible PV/T device geometries. In most concepts the entire heat is generated in a simple device. In a two-absorber PV/T collector, however, part of the heat is generated outside the PV cells (van Helden et al., 2004). 5.3.1 Different types of PV/T modules The design concepts can be categorized into four types based on most other articles about the PV/T systems. The most simple and basic design is a PV module attached on the back of a metallic heat absorber plate as shown in Figure 5.3.1. In this module concept, the distance between the heat generation device and heat collector determines the performance of the PV/T modules. The second type of PV/T modules has water that flows over the photovoltaic panel as shown in Figure 5.3.2. The third type of modules, which is demonstrated in Figure 5.3.3, also uses a liquid, but to improve its performance, the water flows through multiple channels underneath the PV panel to remove generated heat. The fourth type of design makes the PV cells transparent and 162 Solar energy sciences and engineering applications Figure 5.3.4 Two-absorber PV/T model. applies two-absorber geometry. As shown is Figure 5.3.4, this design can lower the temperature of the PV cells on average, but it is relatively complicated to manufacture. 5.4 TECHNIQUES TO INPROVE PV/T PERFORMANCE There are numerous methods to enhance the performance of PV/T air collectors such as the use of fins attached to the PV rear surface, corrugated sheet or wire mesh in the air channel or providing air circulation on both front and rear surfaces of the PV module. Elements of several geometries can be placed between PV module and opposite channel wall, as well as on the back wall, by which air heat extraction can be effected more efficiently (Tripanagnostopoulos, 2007). Roughening the opposite channel wall with ribs or/and using a wall surface of high emissivity, which is a considerably lower cost air heating improvement, has also been adapted (Figure 5.4.1a). In addition, a corrugated sheet inside the air channel along the air flow can be attached on the PV rear surface as well as on the opposite channel wall surface (Figure 5.4.1b). An alternative modification is to insert lightweight pipes along the air flow in the air channel, with slight elasticity to ensure satisfactory thermal contact with the PV rear surface and channel wall (Figure 5.4.1c). These pipes can effectively extract heat from the PV panel by all three modes of heat transfer (conduction, convection, and radiation) thereby restricting the opposite channel wall surface temperature being overheated. Tiwari et al. (2011) presented four types of photovoltaic modules and their applications. They are crystalline PV modules, thin film PV modules, single and multi-junction PV modules. Based on their cost analysis results, the BIPVT systems are reported to be more favorable than the conventional BIPV systems. In extension to the said work, they have also evaluated the overall performance of four types hybrid PV/T model, which are unglazed hybrid PV/T with tedlar, unglazed hybrid PV/T without tedlar, glazed hybrid PV/T with tedlar, and glazed hybrid PV/T without tedlar (Tiwari and Sodha, 2007). Experiments were conducted to validate the thermal model for unglazed PV/T air heating systems for summer conditions and the study concluded that the glazed hybrid PV/T without tedlar had the best performance, especially when in operation Basics of the photovoltaic thermal module 163 Figure 5.3.1 Modified PV/T dual systems provided with twoTMS (Arif Hasan and Sumathy, 2010). during the summer. Also, Tripanagnostopoulos et al. (2001a) carried out a comparison between different types of air PV/T systems. PV module provided with a float glass on either sides of a tedlar integrated to the rear end of the PV module and compared its performance to a pc-Si PV module using transparent tedlar on the front where only float glass is integrated to the rear end of the PV module. The experimental work revealed that the latter system has a higher electrical efficiency by reducing the temperature of the PV significantly. They also presented hybrid PV/T systems with dual heat extraction modes (Tripanagnostopoulos et al., 2001b). Three different design modes of PV/T systems were tested, with (i) a heat exchanger element was provided on the rear surface of the PV module, (ii) heat exchanger element provided in the middle of an air channel, and (iii) heat exchanger element provided on an opposite air channel surface. Results show that for both air and water circulation, PV with a heat exchanger on its rear surface produces the best thermal performance of the system. Joshi et al. (2009) evaluated a hybrid photovoltaic thermal (glass-to-glass) system. They compared the performance of two types of photovoltaic module, which were PV module with glass-to-glass and glass-to-tedlar respectively. In the glass-to-glass case, the insulated base has both a black surface and solar cells to absorb the solar radiation, and then the heat from both the black surface and solar cell is transferred to the flowing air underneath the insulated base. In the glass-to-tedlar case, both the solar cell and the ethylene vinyl acetate absorb the solar radiation, and then transfer the heat to the flowing air underneath the base of the tedlar for thermal heating. The results obtained from both PV modules compared for a composite climate show that 164 Solar energy sciences and engineering applications the hybrid air collector with glass-to-glass PV module has an approximately 2% higher overall thermal efficiency than the PV module with glass-to-tedlar. Dubey et al. (2009) presented an analytical expression for electrical efficiency of PV/T hybrid air collector. They tested four different configurations of photovoltaic modules which are: glass-to-glass PV module with duct, glass-to-glass PV module without, glass-to-tedlar PV module with duct, and glass-to-tedlar PV module without duct. For electrical efficiency, the results show that the differences between PV modules with glass-to-glass and PV modules with glass-to-tedlar with and without duct are 1.24% and 0.086% respectively; the difference between the electrical efficiency of PV modules with glass-to-glass with and without duct is 0.66%. Similar to Dubey et al.’s work, Dupeyrat et al. (2011) had also worked to improve the PV module optical properties specifically to suit for hybrid PV/T collector application. It was shown that “the design of a PV module for a PV-T collector allows the use of alternative materials for the encapsulation process and a new encapsulation setup has been developed. This is a combination of a front layer with a low refractive index instead of glass cover and a low UV absorbing layer instead of the conventional EVA material.’’ The results showed that the said configuration of the PV/T encapsulation module increased the generated current density at least 2 mA/cm2. Compared to equivalent mc-Si cells laminated as a conventional standard glass/EVA/mc-Si/EVA/Tedlar module, the PV/T encapsulation module also has higher solar absorption coefficient. They designed, built and tested a prototype of PV/T collector based on experiments, and the results show 79% thermal efficiency and 8.7% electrical efficiency, which is 87% total efficiency. Low cost performance improvements of PV/T solar collectors for natural air flow operation were introduced by Tonui and Tripanagnostopoulos (2007) and Tonui and Tripanagnostopoulos (2008). Two low-cost modifications of heat extraction were investigated in PV/T air system channel to cool down the PV as well as increase the thermal yield. They suggested using either finned back wall or thin flat metal sheet suspended at the middle of an air channel in the PV/T air configuration. The system consists of the PV module with a simple air channel attached on the back. It is very similar to using the PV module as an absorber plate with conventional air collectors. For the improved systems, the channels are modified by attaching a rectangular profile fins on the other side of the wall to the PV rear surface or by suspending a thin flat aluminum metal sheet in the middle of the air channel (Arif Hasan and Sumathy, 2010). Compared to attaching fins to the PV rear surface, attaching fins at the back wall was relatively easier, because attaching fins at the back of the PV module requires special designated features during the PV modules production. They pointed out that since the fins and metal sheets are easily obtained from the existing material which is not expensive, and the fabrication and modification are not complicated, being incorporated in the middle or on the opposite wall of the air channel, the total cost of the design models for the PV/T sir collectors is low. They also modified the duct geometry by changing the hydraulic diameter of the duct. The test results show that, when the hydraulic diameter of the duct is decreasing, the heat transfer surface area in the channel and convection heat transfer coefficient increased, thus more heat can be transferred from the PV panel to the air stream. Both the PV cooling capability and thermal efficiency of the system is increased. Tripanagnostopoulos et al. (2000) have also presented low cost improvements in integrated air cooled hybrid PV thermal systems specifically for buildings. They noted that the heat exchanging surface area Basics of the photovoltaic thermal module 165 of the air channel is increased by attaching fins of about 15 cm on to the opposite air channel surface. To form fin plate elements, they applied the 1.5 cm aluminum and 4 cm of p profile respectively. 5.5 CONCLUSION The feasibility of the PV/T system will be dependent upon its technical and economic competitiveness with respect to other alternatives. The technical feasibility can be evaluated by comparing the electrical module efficiency and thermodynamic efficiency of such systems with those of the conventional ones, while the economic feasibility (energy metric analysis) can be tested by balancing the capital cost of the solar system against the savings in conventional fuel costs. As the economic feasibility is heavily dependent on the financial parameters based on some assumptions (e.g. the inflation rate of conventional fuel costs), it is certain that the viability of such solar systems will be more pronounced when the environmental costs of conventional electricity production are factored in. As referred to in earlier sections, several studies (both theoretical and experimental) shows that most of the systems could only achieve a maximum thermal efficiency of about 60% for air-cooled and a slightly higher for water-cooled PV/T systems. The reduction in thermal efficiency might be due to reflection losses (since PV surfaces are not spectrally selective), and also due to the fact that the heat resistance between the absorbing surface and the heat transfer medium is increased because of the additional layers of material (e.g. tedlar). Hence, it is necessary to keep all layers between the PV panel and the absorber as thin as possible. It should be pointed out that several researchers (Tiwari and Sodha, 2007; Dubey et al., 2009) have confirmed to use glass instead of tedlar, as tedlar becomes a barrier for extracting thermal energy, in turn reducing both the electrical and overall efficiency of the system. Poor thermal contact was also reported to be a problem by Sudhakar and Sharon (1994) who found a temperature difference of about 15.C between PV laminate and water output temperature. Hence, the objective of future research should aim to optimize the air channel geometry of the PV/T system and to simulate the PV/T collector characteristics, and further investigate the influence of various heat transfer promoters on the cell temperature of the PV module for different operating conditions. It is also essential to establish an analytical expression for the electrical efficiency of the PV module with and without air flow as a function of climatic and design parameters, which can be derived based on a detailed energy balance of each component of the chosen configuration. For the case of PV/T liquid collectors, though the sheet-and-tube design performs efficiently, the channel plate constructions may provide interesting ways of further increasing the heat transport, provided that the channels are made sufficiently thin. For an unglazed PV/T water collector, a heat pump can be integrated to the PV/T system, which may be a promising development for the future. To make solar energy devices more attractive for potential applications, it is essential to develop a thermal model of integrated photovoltaic and thermal solar systems, which could be used to analyze the overall system performance under various climatic as well as design conditions. The possibility of generating electricity and heat energy from PV/T solar collector with either forced or natural flow (using water or 166 Solar energy sciences and engineering applications air) has been demonstrated by various researchers. PV/T systems contribute immensely towards energy savings and mitigation of energy supply of buildings and consequently lower CO2 emission among other social benefits. The choice of technique depends on the location and its application which dictates the usage of appropriate design considerations. Hybrid PV/T systems are especially suitable in regions with a cold climate since PV/T systems integrated to building integrated applications lower the temperature of the PV’s with air and can supply hot air for space heating. 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In: Proceedings of 16th European PV Solar Energy Conference, 1–5 May, Glasgow, UK. pp. 1874–1899. van Helden, W.G.J., van Zolingen, R.J.C. and Zondag, H.A. (2004) PV Thermal Systems: PV Panels Supplying Renewable Electricity and Heat. Progress in Photovoltaics: Research and Application, 12, 415–426. Winston, R. (1974) Principles of solar concentrators of a novel design. Solar Energy, 16, 89–95. Yang, H.X., Marshall, G.H. and Brinkworth, B.J. (1994) An experimental study of the thermal regulation of a PV-clad building roof. In: Proceedings of 12th European Photovoltaic Solar Energy Conference, April 11–15, Amsterdam, The Netherlands. pp. 1115–1118. Zondag, H., Bakker, M., van Helden,W.G.J., Affolter, P., Eisenmann,W. and Fechner, H. (2005) PVT roadmap: A European guide for the development and market introduction of PVT technology. In: Proceedings of (CD) 20th European Photovoltaic Solar Energy Conference, June 6–10, 2004. Barcelona, Spain. Zondag, H.A., de Vries, D.W., van Helden, W.G.J., van Zolengen, R.J.C. and Steenhoven, A.A. (2002) The thermal and electrical yield of a PV thermal collector. Solar Energy, 72, 113–128. Zondag, H.A., de Vries, D.W., van Helden, W.G.J., van Zolengen, R.J.C. and Steenhoven, A.A. (2003) The yield of different combined PV-thermal collector designs. Solar Energy, 74, 253–269. Chapter 6 Thermal modelling of parabolic trough collectors Soteris Kalogirou Department of Mechanical Engineering and Materials Science and Engineering, Cyprus University of Technology, Limassol, Cyprus 6.1 INTRODUCTION As shown in Figure 6.1.1, in a parabolic trough collector (PTC) a sheet of reflective material is formed in parabolic shape. At the focal point of the parabola a metal black pipe, covered with a glass tube to reduce thermal losses, is placed. When the axis formed by the centre of the parabola and the receiver faces the sun, the parallel rays incident on the reflector are reflected and focused onto the receiver tube. In this way the concentrated radiation reaching the receiver tube heats the receiver pipe and thus the fluid circulating through it, transforming the solar radiation into useful heat. As long collector modules are usually produced, a single axis tracking of the sun is used (Kalogirou 2004, 2009). The collector can be orientated in a north-south direction, tracking the sun from east to west, or orientated in an east-west direction, tracking the sun from north to south. The advantages of the latter tracking mode is that very little collector adjustment is required during the day and the full aperture always faces the sun at noon time but the collector performance during the early and late hours of the day is greatly reduced due to large incidence angles (cosine loss). North-south orientated troughs have their highest cosine loss at noon and the lowest in the mornings and evenings when the sun is due east or due west. Over the period of one year, a horizontal north-south trough field usually collects a little more energy than a horizontal east-west one. However the north-south field collects a lot more energy in summer and much less in winter. On the other hand the east-west field collects more energy in winter than a north-south field and less in summer, providing a more constant Figure 6.1.1 Schematic diagram of a parabolic trough collector and receiver detail.The collector comprise a sheet of reflective material in parabolic shape together with a metal black pipe, covered with a glass tube to reduce thermal losses, placed along its focal line. 172 Solar energy sciences and engineering applications Figure 6.1.2 Photographs of parabolic trough collectors (left picture Industrial Solar Technology collector, right picture Eurotrough collector). annual output. Therefore, the choice of orientation should depend on the application and whether more energy is needed during summer or winter (Kalogirou 2004, 2009). Parabolic trough collector technology is the most advanced of the solar thermal technologies because of considerable experience gained so far and the development of a small commercial industry to produce and market these systems. Parabolic trough collectors are built in modules that are supported from the ground by simple pedestals at specific intervals or at either end if the collector is short. Photographs of two parabolic trough collectors are shown in Figure 6.1.2. Parabolic trough collectors are the most mature solar technology to generate heat at temperatures up to 400.C for solar thermal electricity generation or process heat applications. This is due to the application of this type of system in the Southern California power plants, which have a total installed capacity of 354MWe (Kearney and Price 1992) and are known as Solar Electric Generating Systems (SEGS). SEGS I is 14MWe, SEGS II–VII are 30MWe each and SEGS VIII and IX are 80MWe each. Recently, new systems have been installed in Spain and the USA, and new plants are under development in many Middle East countries. New developments in the field of parabolic trough collectors are focussed on cost reduction and improvements of the technology. In one such development, the collector is washed automatically during the night, thus reducing drastically the maintenance cost, as this is the most used maintenance process (Kalogirou 2009). A linear receiver is used in a parabolic trough which is a metallic tube placed along the focal line of the parabola surrounded by a glass cover envelope (see Figure 6.1.1 detail). The size of the tube, and therefore the concentration ratio, is determined by the size of the reflected sun image and the manufacturing tolerances of the trough. The surface of the metal receiver is usually plated with selective coating that has a high absorptance for solar irradiation but a low emittance for thermal radiation. The purpose of the glass cover tube placed around the receiver tube is to reduce the convective heat loss from the receiver, so as to decrease the heat loss coefficient. A disadvantage resulting from the use of the glass cover tube is that the reflected light from the concentrator must first pass through the glass to reach the receiver, and in doing so a transmittance loss is added of about 0.9, when the glass is clean. The glass envelope usually has an anti-reflective coating to improve transmissivity. Particularly Thermal modelling of parabolic trough collectors 173 for high temperature applications, the space between the glass cover tube and the receiver is evacuated to further reduce convective heat loss from the receiver tube and thereby increase the performance of the collector. The total receiver tube length of PTCs is usually from 25m to 150 m. In this chapter a detailed thermal model of the receiver of the collector is presented. Many researchers have published studies of energy models of parabolic trough collectors. The most important ones are the following. Edenburn (1976) predicted the efficiencies for focusing collectors which consist of a cylindrical parabolic reflector and a collector tube surrounded by a transparent envelope and which heat a fluid flowing through the collector tube. These efficiencies have been predicted using analytical heat transfer methods. The analysis considers visible radiation transfer, IR radiation exchange, conductive and convective losses and energy transferred to a fluid flowing through the collector tube. The collector may have a tilted north-south axis, an east-west axis or it may fully track the sun and geometric parameters associated with tracking the sun are considered. Both evacuated and nonevacuated cases are considered and the predicted results are in excellent agreement with collector performances measured using Sandia Laboratories’ collector test facility. Clark (1982) analyzed the effects of design and manufacturing parameters that influence the thermal and economic performance of parabolic trough receivers. This is achieved by an identification of the principal design factors that influence the technical performance of a parabolic trough concentrator and which relate directly to design and manufacturing decisions. These factors include spectral-directional reflectivity of the mirror system, the mirror-receiver tube intercept factor, the incident angle modifier and absorptivity-transmissivity product of the receiver tube and cover tube, the end loss factor and a factor describing the effect of tracking errors and receiver tube misalignment. Each of these factors has been quantified in terms of design and manufacturing tolerances and associated performance degradation. Other design considerations that relate to thermal loss from the receiver tube are low emissivity coatings, evacuation and anti-reflection coating. The analysis of energy costs using the parabolic trough concentrator determines both the break-even, current metered cost of energy and the annual cash flow over periods of investment ranging from 5 to 15 yr. The economic factors include investment tax credit, energy equipment tax credit, income tax bracket, cost of auxiliary system, foundations and controls, cost of collector, costs of maintenance and taxes, costs of fuel, cost of capital, general inflation rate and fuel escalation rate. Karimi et al. (1986) applied a piecewise two-dimensional model of the receiver, in which the receiver of the collector is divided into longitudinal and isothermal nodal sections as shown in Figure 6.1.3, performed by considering the circumferential variation of solar flux and applying the principle of energy balance to the glazing and receiver nodes. Heidemann et al. (1992) studied the temperature field in the absorber tube of a direct steam generating parabolic trough collector. Steady-state and transient operating conditions are considered. They formulated a two dimensional heat transfer model for calculating the absorber wall temperature of a DSG collector under both conditions. A universal program was developed for solving the two-dimensional transient temperature field using a modular nodal point library. The temperature field is extremely asymmetric due to the variation of the heat transfer coefficient at the inner surface and the solar irradiation at the outer surface of the absorber tube. High temperature peaks 174 Solar energy sciences and engineering applications Figure 6.1.3 Piecewise two-dimensional model of the receiver assembly with the receiver divided into longitudinal and isothermal nodal sections (Karini et al., 1986). are found, especially in stratified flow at higher void fractions. The numerical solution showed that a sudden drop of irradiation induces a very high temperature gradient inside the absorber tube in a short period of time. Thomas and Thomas (1994) studied the design data required for the computation of thermal loss in the receiver of a parabolic trough concentrator for specific absorber tube diameters, various ambient temperatures, wind velocity and absorber temperatures from 50 to 350.C in steps of 10.C. Curve-fitting equations based on a numerical heat transfer model for the heat losses for the above parameters are given to enable the designer to generate the required data for any absorber temperature, absorber diameter, ambient temperature, wind velocity and emissivity of the solar selective coating of the absorber. Dudley et al. (1994) developed an analytical model of SEGS LS-2 parabolic solar collector. The thermal loss model for the heat collection element was a one dimensional steady state model based on thermal resistance analysis. This model was validated with experimental data collected by Sandia National Laboratories (SNL) for different receiver annulus conditions: vacuum intact, lost vacuum (air in annulus), and broken annulus cover (bare tube). The results showed a reasonable agreement between the theoretical and experimental heat losses. Odeh et al. (1998) studied the thermal performance of a parabolic trough solar collector used as direct steam generator for different solar radiation levels and geometric configurations. This heat transfer model showed better agreement with the in-focus test results than the polynomial curve fit equation obtained by Dudley et al. (1994). The thermal losses calculated for water were based on the receiver wall temperature and the results showed that thermal losses calculated for steam as heat transfer fluid were lower than those obtained for synthetic oil. Forristall (2003) built and analysed both a 1-D and a 2-D heat transfer models of a PTC receiver implemented in EES software. For this purpose a detailed heat transfer solar receiver model was used. A one-dimensional energy balance for several segments was used for short and long receivers respectively. This model was used Thermal modelling of parabolic trough collectors 175 to determine the thermal performance of parabolic trough collectors under different operating conditions. Garcia-Valladares and Velasquez (2009) developed a detailed numerical model for a single pass and double pass solar receiver and validated it. The governing equations inside the receiver tube, together with the energy equation in the tube walls and cover wall and the thermal analysis in the solar concentrator were solved iteratively in a segregated manner. The single-pass solar device numerical model has been carefully validated with experimental data obtained by Sandia National Laboratories (SNL). The effects of recycling at the ends on the heat transfer are studied numerically and show that the double-pass arrangement can enhance the thermal efficiency compared with the single-pass. Cheng et al. (2010) in their contribution examined the solar energy flux distribution on the outer wall of the inner absorber tube of a parabolic solar collector receiver by adopting the Monte Carlo Ray-Trace Method (MCRT Method). They found that the non-uniformity of the solar energy flux distribution is very large. Three-dimensional numerical simulation of coupled heat transfer characteristics in the receiver tube is calculated and analyzed by combining the MCRT Method and FLUENT software, in which the heat transfer fluid was the Syltherm 800 liquid oil and the physical model was the LS2 parabolic solar collector from the testing experiment of Dudley et al. (1994). Temperature-dependent properties of the oil and thermal radiation between the inner absorber tube and the outer glass cover tube are also taken into account. Compared with test results from three typical testing conditions, the average difference is within 2%. Gong et al. (2010) presented an optimised model and tested China’s first high temperature parabolic trough receiver. The model is written in Matlab and computes the receiver’s major heat loss through the glass envelope, and then systematically analyzes the major influence factors of heat loss in both 1-D and 3-D. Comparison shows the original 1-D model agrees with the ‘ends of the receiver covered test’ while remarkably deviating from the ‘ends exposed’ test. For the purpose of identifying the influence of the receiver end on total heat loss, an additional 3-D model was built using a CFD software to further investigate the different heat transfer processes of receiver’s end components. The 3-D end model is verified by heating power and IR temperature distribution images in the test. Combining the optimized 1-D model with the new 3-D end model, the comparison with test data shows a good agreement. He et al. (2011) used a coupled simulation method based on Monte Carlo Ray Trace (MCRT) and Finite Volume Method (FVM) to solve the complex coupled heat transfer problem of radiation, heat conduction and convection in a parabolic trough solar collector system. A coupled grid checking method is established to guarantee the consistency between the two methods and the validations to the coupled simulation model were performed. The heat flux distribution curve could be divided into 4 parts: shadow effect area, heat flux increasing area, heat flux reducing area and direct radiation area. The heat flux distribution on the outer surface of absorber tube was heterogeneous in the circumferential direction but uniform in the axial direction. Finally, the concentrating characteristics of the parabolic trough collectors (PTCs) were analyzed by the coupled method, the effects of different geometric concentration ratios (GCs) and different rim angles were examined. The results show that both variables affect the heat flux distribution. 176 Solar energy sciences and engineering applications Padilla et al. (2011) in their paper present a detailed one-dimensional numerical heat transfer analysis of a PTC. The receiver and envelope were divided into several segments and mass and energy balances were applied in each segment. The partial differential equations developed were discretized and the nonlinear algebraic equations were solved simultaneously. Finally, to validate the numerical results, the model was compared with experimental data obtained from Sandia National Laboratory (SNL) and other one-dimensional heat transfer models. The model presented in this chapter takes into consideration all modes of heat transfer: forced convection into the receiver pipe and from the glass cover to ambient air (usual case when there is wind); natural convection in the annulus between the receiver and the glass cover; conduction through the metal receiver pipe and glass cover walls; and radiation from the metal receiver pipe to glass cover and from glass cover to the sky. 6.2 THE ENERGY MODEL Although for low-temperature applications a bare tube receiver can be used, as for this kind of applications low technology collectors the flat-plate can be used, in this chapter only a glazed receiver is considered, which is the usual case for PTCs. For the annulus between the receiver and the glass cover two conditions are considered: the vacuum, usually used in high temperature applications, and the air case, which is used for lower temperature applications, and for cases when the vacuum is lost from the former design. The model is written in Engineering Equation Solver (EES) software (Klein 2002). This is done for two reasons; the EES includes routines to estimate the properties of various substances by specifying any two properties, such as temperature and pressure, and EES can be called from TRNSYS which allows the development of a model which can use the capabilities of both programs. The model is validated with known performance of existing collectors, and subsequently is used to perform an analysis of the collector installed at Archimedes Solar Energy Laboratory at Cyprus University of Technology. The collector performance model uses an energy balance between the fluid flowing through the receiver, usually a heat transfer fluid (HTF), and the atmosphere. It includes all equations necessary to predict the various expressions of the energy balance, which depend on the ambient conditions and the collector receiver optical properties and condition. A cross-section of the collector receiver and the subscript definitions are shown in Figure 6.2.1a whereas Figure 6.2.1b shows the energy balance of the receiver and Figure 6.2.1c the steady-state thermal resistance model. The model assumes that all temperatures, heat fluxes, and thermodynamic properties are uniform around the circumference of the receiver. This assumption is not very accurate as it is well known that the radiation profile is not uniform, and the bottom part receives much higher solar flux than the top part because of the radiation reflected by the parabolic mirror. For small solar collectors however, this simplification does not introduce severe inaccuracies. Additionally, all flux directions shown in Figure 6.2.1b are positive. It should be noted that in the resistance model the incoming solar energy and optical Thermal modelling of parabolic trough collectors 177 Figure 6.2.1 Collector receiver model a) nomenclature, b) energy balance and c) thermal resistance network for the cross-section of the receiver. losses have been omitted for clarity. The optical losses are due to imperfections in the collector mirrors, tracking errors, shading and cleanliness of the mirror and receiver glazing. The incoming solar energy, which effectively is equal to the solar energy input minus any optical losses, is absorbed by the glass envelope (qgo,SolAbs) and receiver pipe (qpo,SolAbs). Most of the energy that is absorbed by the receiver is conducted through the receiver pipe material (qpi-po,cond) and eventually transferred to the HTF by convection (qf-pi,conv). The remaining energy is transmitted back to the glass envelope by convection (qpo-gi,conv) and radiation (qpo-gi,rad). The energy reaching the glass cover from radiation and convection then passes through the glass envelope wall by conduction (qgi-go,cond) and along with the energy absorbed by the glass envelope wall (qgo,SolAbs) is lost to the environment by convection to ambient air (qgo-a,conv) and radiation towards the sky (qgo-s,rad). The energy balance equations are determined by considering that the energy is conserved at each surface of the receiver cross-section, shown in Figure 6.2.1. Therefore: qf-pi,conv = qpi-po,cond (6.2.1) qpo,SolAbs = qpo-gi,conv + qpo-gi,rad + qpi-po,cond (6.2.2) qpo-gi,conv + qpo-gi,rad = qgi-go,cond (6.2.3) 178 Solar energy sciences and engineering applications qgi-go,cond + qgo,SolAbs = qgo-a,conv + qgo-s,rad (6.2.4) qHeatLoss = qgo-a,conv + qgo-s,rad (6.2.5) It should be noted that the solar absorption at the outside pipe, qpo,SolAbs and outside glass, qgo,SolAbs surfaces are treated as heat flux expressions, which simplifies the solar absorption expressions as it considers the heat conduction through the receiver pipe and glass envelope wall to be linear. Actually, the solar absorption in the glass envelope wall (semitransparent material) and receiver pipe (opaque metal material) are volumetric phenomena. However, it is well known from heat transfer textbooks (Cengel 2006) that most of the absorption in a metallic surface (receiver pipe) occurs very close to the surface (within a few µm) and although solar absorption occurs throughout the thickness of the glass envelope wall, its absorptance is very small (a=0.02). Thus, the error in treating solar absorption as a surface phenomenon is very small. The various heat transfer interactions are analysed in different sections below, starting from the heat transfer fluid inside towards the ambient air and sky outside the receiver assembly. 6.2.1 Convection heat transfer between the HTF and the receiver pipe Newton’s law of cooling states that the convection heat transfer from the inside surface of the receiver pipe to the HTF is given by hA(Ts–T8). Therefore in the case of the PTC model and using the nomenclature adopted in Figure 6.2.1: qf-pi,conv = hfpDpi  Tpi - Tf  (6.2.6) The convection heat transfer coefficient at the inside pipe diameter, hf is given by: hf = NuDpi kf Dpi (6.2.7) where: hf =HTF convection heat transfer coefficient at Tf (W/m2-.C); Dpi =inside diameter of the receiver pipe (m); Tpi =inside surface temperature of receiver pipe (.C); Tf =mean (bulk) temperature of the HTF (.C); NuDpi =Nusselt number based on Dpi; and kf =thermal conductivity of the HTF at Tf (W/m-.C). In Equation 6.2.6, both Tf and Tpi are independent of angular and longitudinal directions of the receiver. The same applies for all temperatures and properties in the energy model. The Nusselt number depends on the type of flow through the receiver pipe. Although the flow in the receiver pipe is well within the turbulent flow region at typical operating conditions, the model includes conditional statements to determine the type of flow. When the Reynolds number is lower than 2300, laminar flow exists in the receiver pipe and the Nusselt number is constant. For pipe flow, the constant value, assuming constant heat flux, as in the case of a PTC, is equal to 4.36 (Cengel 2006). Turbulent and transitional cases occur at Reynolds number >2300. Therefore, Thermal modelling of parabolic trough collectors 179 the following Nusselt number correlation developed by Gnielinski (1976) is used for the convective heat transfer from the receiver pipe to the HTF: NuDpi = fpi/8  ReDpi - 1000  Prf 1 + 12.7  fpi/8  Pr2/3 f -1   Prf Prpi 0.11 For 0.5 < Prf < 2000 and 2300 < ReDpi < 5 × 106 (6.2.8) with fpi = 1.82 log (ReDpi ) - 1.64 !-2 (6.2.9) where: fpi =friction factor for the inside surface of the receiver pipe, Dpi; Prf =Prandtl number evaluated at the HTF temperature, Tf ; and Prpi =Prandtl number evaluated at the receiver pipe inside surface temperature, Tpi. Except for Prpi, all fluid properties are evaluated at the mean HTF temperature, Tf . The correlation assumes that the receiver pipe has a smooth inside surface and that the heat flux and temperature are uniform. The above equations are valid for both turbulent pipe flow and the transitional flow which occur for Reynolds numbers between 2300 and 4000 (Cengel, 2006). Furthermore, the above correlations are adjusted for fluid property variations between the receiver pipe wall temperature and the bulk fluid temperature. The program will display a warning message if the correlation is used out of the range of validity, shown in Equation 6.2.8. 6.2.2 Conduction heat transfer through the receiver pipe wall Conduction heat transfer through the receiver pipe wall is determined by Fourier’s law of conduction through a hollow cylinder (Cengel, 2006) given by: qpi-po,cond = 2pkpipe  Tpi - Tpo  ln  Dpo Dpi  (6.2.10) where: kpipe =receiver pipe thermal conductivity at the average receiver pipe temperature (Tpi +Tpo)/2 (W/m-.C); Tpi =receiver pipe inside surface temperature (.C); Tpo =receiver pipe outside surface temperature (.C); Dpi =receiver pipe inside diameter (m); and Dpo =receiver pipe outside diameter (m). In this equation the thermal conductivity is considered as constant, and evaluated at the average temperature between the inside and outside receiver pipe surfaces. The thermal conductivity depends on the receiver pipe material type. The receiver performance model includes one copper and three types of stainless steels (304L, 316L, and 321H), which can be chosen by the user at the beginning. If copper is chosen, the thermal conductivity is constant and equal to 385 W/m-.C. If stainless steel 304L or 316L is chosen, the thermal conductivity is calculated from: kpipe = (0.013)Tpi-po + 15.2 (6.2.11) 180 Solar energy sciences and engineering applications and if stainless steel 321H is chosen the thermal conductivity is calculated from: kpipe = (0.0153)Tpi-po + 14.775 (6.2.12) Both equations were determined by linearly fitting data from Davis (2000). 6.2.3 Heat transfer from the receiver pipe to the glass envelope As is mentioned before, between the receiver pipe and the glass envelope heat transfer occurs by convection and radiation. Convection heat transfer depends on the annulus pressure (KJC, 1993). At low pressures (~ <0.013 Pa), heat transfer is by molecular conduction, whereas at higher pressures the heat transfer is by free convection. Radiation heat transfer also occurs because there is a difference in temperature between the outsider receiver pipe surface and the inside glass envelope surface. The radiation heat transfer calculation is simplified by assuming gray surfaces, for which (.=a) and that the glass envelope wall is opaque to infrared radiation. All these are examined separately in the following sections. 6.2.3.1 Convection heat transfer As mentioned above, two heat transfer mechanisms are considered in the determination of the convection heat transfer between the receiver pipe and glass envelope wall (qpo-gi,conv). These are the free-molecular and natural convection (KJC, 1993). The cases of vacuum and pressure in the annulus are examined separately. a) Vacuum in annulus When the annulus is under vacuum (pressure ~ <0.013 Pa), the convection heat transfer between the receiver pipe and glass envelope occurs by free-molecular convection (Ratzel et al., 1979) and is given by: qpo-gi,conv = pDpohpo-gi(Tpo - Tgi) (6.2.13) where hpo-gi = kstd Dpo 2 ln  Dgi Dpo  + b.  Dpo Dgi + 1  For: RaDgi < (Dgi/(Dgi - Dpo))4 (6.2.14) and b = (2 - a)(9. - 5) 2a(. + 1) (6.2.15) . = 2.331 × 10-20(Tpo-gi + 273) (Pad2) (6.2.16) where: Dpo =outside receiver pipe diameter (m); Dgi =inside glass envelope diameter (m); hpo-gi =convection heat transfer coefficient for the annulus gas at Tpo-gi (W/m2-.C); Tpo =outside receiver pipe surface temperature (.C); Tgi =inside glass envelope surface temperature (.C); kstd =thermal conductivity of the annulus gas at Thermal modelling of parabolic trough collectors 181 standard temperature and pressure (W/m-.C); b=interaction coefficient; .=meanfree- path between collisions of a molecule (cm); a=accommodation coefficient; . =ratio of specific heats for the annulus gas (air); Tpo-gi =average temperature (Tpo +Tgi)/2 (.C); Pa =annulus gas pressure (mmHg); and d=molecular diameter of annulus gas (cm). This correlation slightly overestimates the heat transfer for very small pressures (~ <0.013 Pa). The molecular diameter of air, d, is obtained from Marshal (1976) and is equal to 3.55×10-8 cm, the thermal conductivity of air is 0.02551 W/m-.C, the interaction coefficient is 1.571, the mean-free-path between collisions of a molecule is 88.67 cm, and the ratio of specific heats for the annulus air is 1.39. These are for an average fluid temperature of 300.C and pressure equal to 0.013 Pa. Using these values, the convection heat transfer coefficients (hpo-gi) obtained from Equation 6.2.14 is equal to 0.0001115W/m2-.C. b) Pressure in annulus If the receiver is filled or partially filled with ambient air (~ pressure>0.013 Pa) or if the receiver annulus vacuum is lost, the convection heat transfer between the receiver pipe and glass envelope occurs by natural convection. For this purpose the Raithby and Holland’s correlation for natural convection in an annular space (enclosure) between horizontal concentric cylinders is used, given by (Cengel, 2006): qpo- gi,conv = 2pkeff ln (Dgi/Dpo) (Tgi - Tpo) For: 0.7 = Prpo-gi = 6000 and 102 = FcylRapo-gi = 107 (6.2.17) keff kag = 0.386  Prpo-gi 0.861 + Prpo-gi 1/4 (FcylRaDpo)1/4 (6.2.18) Fcyl = [ln(Dgi/Dpo)]4 L3 c (D-3/5 gi - D-3/5 po )5 (6.2.19) In these equations the critical length is given by: Lc = (Dgi -Dpo) 2 where: kag =thermal conductivity of annulus gas at Tpo-gi (W/m-.C); Tpo =outside receiver pipe surface temperature (.C); Tgi =inside glass envelope surface temperature (.C); Dpo =outside receiver pipe diameter (m); Dgi =inside glass envelope diameter (m); Prpo-gi =Prandtl number for gas properties evaluated at Tpo-gi; RaDpo =Rayleigh number evaluated at Dpo; and Tpo-gi =average temperature, (Tpo +Tgi)/2 (.C). This correlation assumes long, horizontal, concentric cylinders at uniform temperatures, which is perfectly applied for a PTC. All physical properties are evaluated at the average temperature (Tpo +Tgi)/2. 6.2.3.2 Radiation heat transfer Several assumptions were made in deriving an equation for the radiation heat transfer, as follows: • The surfaces are gray, • Diffuse reflections and irradiation, 182 Solar energy sciences and engineering applications • Non-participating gas in the annulus, • Long concentric isothermal cylinders, and • The glass envelope is opaque to infrared radiation. These assumptions are not all completely accurate as the glass envelope wall is not completely opaque for the entire thermal radiation spectrum and the glass envelope wall and the selective coatings are not gray (Touloukian and DeWitt, 1972). However, any errors associated with the assumptions are relatively small. The radiation heat transfer between the receiver pipe and glass envelope (qpo-gi,rad) is estimated with the following equation, applied for infinitely long concentric cylinders (Cengel, 2006): qpo-gi,rad = spDpo(T4 po - T4 gi)  1 epo +  (1 - egi)Dpo egiDgi  (6.2.20) where: s =Stefan-Boltzmann constant (=5.67×10-8 W/m2-K4); Dpo =outside receiver pipe diameter (m); Dgi =inside glass envelope diameter (m); Tpo =outside receiver pipe surface temperature (K); Tgi =inside glass envelope surface temperature (K); epo =receiver pipe selective coating emissivity; and egi =glass envelope emissivity. 6.2.4 Conduction heat transfer through the glass envelope The anti-reflective treatment on the inside and outside surfaces of the glass envelope is assumed not to introduce any thermal resistance or to have any effect on the glass emissivity. This is reasonably accurate since the treatment is usually a chemical etching which does not add any additional elements to the glass surface (Forristall, 2003). The conduction heat transfer through the glass envelope uses the same equation as the conduction through the receiver pipe wall described in Section 6.2.2. As in the receiver case, the temperature distribution is assumed to be linear. Furthermore, the thermal conductivity of the glass (kglass) is assumed constant – as explained in Section 6.2.1 – with a value of 1.04, which corresponds to Pyrex glass (Touloukian and DeWitt, 1972). In equation form this is given by: qgi-go,cond = 2pkglass(Tgi - Tgo) ln  Dgo Dgi  (6.2.21) 6.2.5 Heat transfer from the glass envelope to the atmosphere The heat transfer from the glass envelope to the atmosphere occurs by convection and radiation. Depending on whether there is wind the convection will either be forced or natural. Radiation heat loss occurs due to the temperature difference between the glass envelope and sky. All these are examined separately below. 6.2.5.1 Convection heat transfer The convection heat transfer is determined by knowing the Nusselt number, which depends on whether the convection heat transfer is natural (no wind) or forced (wind Thermal modelling of parabolic trough collectors 183 case). When there is wind, the convection heat transfer from the glass envelope to the atmosphere gives a much bigger heat loss. This is estimated from Newton’s law of cooling: qgo-a,conv = hgo-apDgo(Tgo - Ta) (6.2.22) and hgo-a = kair Dgo NuDgo (6.2.23) where: Tgo =glass envelope outside surface temperature (.C); Ta =ambient air temperature (.C); hgo-a =convection heat transfer coefficient for air at (Tgo -Ta)/2 (W/m2-.C); kair =thermal conductivity of air at (Tgo -Ta)/2 (W/m-.C); Dgo =glass envelope outside diameter (m); and NuDgo =average Nusselt number based on the glass envelope outside diameter Dgo. a) No wind When there is no wind, the convection heat transfer from the glass envelope to the environment occurs by natural convection and the correlation developed by Churchill and Chu is used to estimate the Nusselt number (Cengel, 2006): NuDgo = . .. 0.60 + 0387R1/6 Dgo-a " 1 +  0.559/ Prgo-a 9/16 #8/27 . .. 2 105 < RaDgo < 1012 (6.2.24) RaDgo = gß(Tgo - Ta)D3 go .2 go-a Prgo-a (6.2.25) ß = 1 Tgo-a (6.2.26) Prgo-a = .go-a ago-a (6.2.27) where: RaDgo =Rayleigh number for air based on the glass envelope outside diameter, Dgo; g=gravitational constant (=9.81 m/s2); ago-a =thermal diffusivity for air at Tgo-a (m2/s); ß=volumetric thermal expansion coefficient (ideal gas) (1/K); Prgo-a =Prandtl number for air at Tgo-a; .go-a =kinematic viscosity for air at Tgo-a (m2/s); and Tgo-a =film temperature (Tgo +Ta)/2 (K). This correlation assumes a long isothermal horizontal cylinder. Also, all the fluid properties are determined at the mean film temperature, (Tgo +Ta)/2. b) Wind When there is wind, the convection heat transfer from the glass envelope to the environment occurs by forced convection. The Nusselt number in this case is estimated with Zhukauskas’ correlation for external forced convection flow normal to an isothermal cylinder (Incropera et al., 2007): NuDgo = CRemD goPrn a  Pra Prgo 1/4 0.7 < Pra < 500 and 1 < ReDgo < 106 (6.2.28) 184 Solar energy sciences and engineering applications Table 6.2.1 Constants for Equation 6.2.27. ReD C m 1–40 0.75 0.4 40–1,000 0.51 0.5 1,000–200,000 0.26 0.6 200,000–1,000,000 0.076 0.7 The constants C andm are given in Table 6.2.1, obtained from Incropera et al. (2007) whereas the constant n is equal to 0.37 for Pr<=10 and is equal to 0.36 for Pr>10. All fluid properties are evaluated at atmospheric temperature, Ta, except Prgo, which is evaluated at the glass envelope wall outside surface temperature. 6.2.5.2 Radiation heat transfer In this model, only the useful solar irradiation is considered in the solar absorption expressions. Therefore, the radiation transfer between the glass envelope wall and sky is caused by the temperature difference between the glass cover and the sky. This is done by assuming that the cover is a small convex gray object in a large blackbody cavity, i.e., the sky. The net radiation transfer between the glass envelope and sky is given by (Cengel, 2006): qgo-s,rad = segopDgo(T4 go - T4 s ) (6.2.29) where: Dgo =outside glass envelope diameter (m); ego =emissivity of the glass envelope outside surface; Tgo =glass envelope outside surface temperature (K); and Ts =effective sky temperature (K). It should be noted that the sky, especially during non-clear conditions, does not act as a blackbody; however, it is common practice to model it as such and to use an effective sky temperature to compensate for the difference (Kalogirou, 2009). Despite the fact that several relations have been proposed to relate the effective sky temperature for clear skies to measured meteorological data, to simplify the model, an approximate relation is used for the effective sky temperature as Ta -8.C. 6.2.6 Solar irradiation absorption In this model the optical efficiency terms are estimated and combined to form an effective optical efficiency, which is subsequently used to determine the optical loss and solar absorption expressions. The optical properties used in the collector performance model were obtained from a combination of sources, i.e., the parameters used to estimate effective optical efficiencies are generated from the National Renewable Energy Laboratory (NREL) report (Price et al., 2002), which was based on field tests conducted by Dudley et al. (1994), and software performance modelling. These are combined in the intercept factor and the actual values are as follows: esh =Receiver shadowing (bellows, shielding, supports), 0.974 etr =Tracking error, 0.994 Thermal modelling of parabolic trough collectors 185 ege =Geometry error (mirror alignment), 0.98 .cl =Clean mirror reflectance, 0.935 edm =Dirt on mirrors (reflectivity/.cl) [reflectivity is an input parameter, usual value: 0.88–0.93] eda =Dirt on receiver, (1+edm)/2 eun =Unaccounted, 0.96. It should be noted that these parameters are valid only for normal solar incidence irradiation. To account for incident angle losses, the incident angle modifier is used, which accounts for end shading of the trough, reflection and refraction loses, and selective coating incident angle effects. The terms, esh, etr, ege, and eun, shown above are estimates. The clean mirror reflectance .cl is a known value, and the two dirt effects edm and eda are obtained from recommendations by Duffie and Beckman (1991). The above list of parameters account for collector geometric effects (shadowing, tracking, alignment), mirror and glass envelope transmittance effects (mirror reflectance and dirt), and a parameter for unexplained differences between field test data and modelled data. All these values can be altered by the user if in the future better and more accurate values become available. Generally, the incident angle modifier is used to account for cases when the solar irradiation is not normal to the collector aperture (Kalogirou 2004, 2009). This is a function of the solar incidence angle (.) to the normal of the collector aperture. The equation determined from a collector testing carried out at Sandia National Laboratory (SNL) is given by (Dudley et al., 1994): K. = cos (.) + 0.000884. - 0.00005369.2 (6.2.30) Other optical properties required include the selective coating absorptance and emittance, and the glass envelope transmittance, absorptance and emittance. The glass envelope absorptance and emissittance are constant (independent of temperature) and independent of selective coating type. The values used in the model are a=0.02 and e=0.86 and can be changed by the user if it is required. The glass envelope transmittance and the selective coating absorptance and emittance depend on the type of selective coating. Both the envelope transmittance and the coating absorptance are constants; whereas the coating emittance is a function of temperature. The properties of the Luz cermet selective coating type used in the model are as follow (Forristall, 2003): • Envelope transmittance=0.935 • Coating absorptance=0.92 • Coating emittance=0.06 at 100.C and 0.15 at 400.C. The emittance equation used for the selective coating considered, which coincide with the emittance values given above (Forristall, 2003): Coating Emittance, epo =0.000327(T + 273.15) - 0.065971 (6.2.31) 186 Solar energy sciences and engineering applications It should be noted that the temperature in Equation 6.2.31 is in degrees Celsius and that the emittance values between the two reference points, of 100.C and 400.C, are nearly linear. 6.2.6.1 Solar irradiation absorption in the glass envelope As stated in Section 6.2.1, to simplify the model and although physically this is not true, the solar absorption into the glass envelope wall is treated as a heat flux. In fact, the solar absorption in the glass envelope wall is a heat generation phenomenon and as such is a function of the glass wall thickness. However, this assumption introduces an insignificant error since the glass envelope wall is relatively thin and the solar absorptance coefficient for glass is very small, 0.02 (Touloukian and DeWitt, 1972). Additionally, the optical efficiency is used to calculate the solar absorption in the glass envelope given by: qgo,SolAbs = qsol.envaenv (6.2.32) with .env = ..clK. (6.2.33) where: qsol =solar irradiation per receiver length (W/m); .env =effective optical efficiency of the glass envelope; aenv =absorptance of the glass envelope (Pyrex glass); K. =incident angle modifier, as defined by Equation 6.2.30; and .=intercept factor [.=eshetregeedmedaems]. All parameters in Equation 6.2.33, except the incidence angle modifier (K.), are taken from the list presented before. Furthermore, the solar irradiation term (qsol) in Equation 6.2.32 is determined by multiplying the direct normal solar irradiation (DNI) by the projected normal reflective surface area of the collector, i.e., aperture area, and dividing by the receiver length. In both equations, all terms are assumed to be independent of temperature. 6.2.6.2 Solar irradiation absorption in the receiver pipe As stated before, the solar energy absorbed by the receiver pipe occurs essentially at the surface; therefore, it is treated as a heat flux (see Section 6.2.1). Therefore, the equation for the solar absorption in the receiver pipe is given by: qpo,SolAbs = qsol.absaabs (6.2.34) with: .abs =.envtenv (6.2.35) where: .abs =effective optical efficiency at receiver pipe; aabs =absorptance of receiver pipe; and tenv =transmittance of the glass envelope. In Equation 6.2.34, the effective optical efficiency of the glass envelope, .env is obtained by Equation 6.2.33 and as before, all terms are assumed to be independent of temperature. Thermal modelling of parabolic trough collectors 187 6.3 CODE TESTING The code developed is tested using performance measurements for known collectors from test carried out at SNL and presented in Dudley et al. (1994). The information required to input to EES code is the following: 1 Direct normal irradiation (DNI) [W/m2] 2 Wind speed [m/s] 3 Ambient temperature [.C] 4 Solar incidence angle [.] 5 Coating absorptance [-] 6 Coating emittance at 100.C [-] 7 Coating emittance at 400.C [-] 8 Mirror reflectivity [-] 9 Glass envelope transmittance [-] 10 Annulus pressure or vacuum [-] 11 Annulus absolute pressure [kPa] 12 HTF flow rate [m3/s] 13 Type of heat transfer fluid 14 Receiver inside diameter [m] 15 Receiver outside diameter [m] 16 Glass envelope inside diameter [m] 17 Glass envelope outside diameter [m] 18 Collector aperture area [m2] 19 Shadowing [-] 20 Tracking error [-] 21 Dirt factor on glass envelope [-] 22 Dirt factor on mirror [-] 23 Collector inlet temperature [.C]. A comparison of the performance of the code developed and the tests conducted at SNL is shown in the following figures. Figures 6.3.1 and 6.3.2 show a comparison of the actual efficiency and heat loss of the collector with the values determined from the EES code developed, when vacuum exists in the receiver annulus. Similar results for air in the receiver annulus are presented in Figures 6.3.3 and 6.3.4. In all cases the agreement between the experimental results and those obtained by the EES code is acceptable. The agreement is better for the air case whereas in both cases the difference increases with increasing operating temperature. It should be noted that the relatively high percentage differences shown in Figure 6.3.2 are due to the magnitude of the actual numbers considered and generally the model underestimates the heat loss at high receiver temperatures and overestimates it at low receiver temperatures. In general, the reason for the deviation presented in the case of heat loss (Figures 6.3.2 and 6.3.4) is the possible dependence of the optical properties on the temperature. Finally, the code developed is used with the characteristics of the collector erected at the premises of the Cyprus University of Technology and in particular at the Archimedes Solar Energy Laboratory. The collector, shown in Figure 6.3.5, is supplied 188 Solar energy sciences and engineering applications Figure 6.3.1 Comparison of measured against predicted thermal efficiency with vacuum in receiver annulus. Figure 6.3.2 Comparison of measured against predicted heat loss with vacuum in receiver annulus. from the Australian company NEP-SOLAR and has the characteristics presented in Table 6.3.1. The collector is installed at the roof of the laboratory and has a length of 12.2m (one-half of the standard module). It consists of galvanised steel mounts, lightweight, Thermal modelling of parabolic trough collectors 189 Figure 6.3.3 Comparison of measured against predicted thermal efficiency with air in receiver annulus. Figure 6.3.4 Comparison of measured against predicted heat loss with air in receiver annulus. stiff and precise parabolic reflector panels manufactured from reinforced polymeric material, a structurally efficient galvanised steel torque tube, a tubular receiver and an accurate solar tracking system. As the collector is able to operate up to about 200.Cthe selective coating properties are assumed to be constant to all possible temperature range. The results of the program are shown graphically in Figure 6.3.6 and give a thermal efficiency of 58.16% at 200.C, which is very satisfactory. These results were obtained at a solar radiation of 190 Solar energy sciences and engineering applications Figure 6.3.5 Photograph of the collector installed at Archimedes Solar Energy Laboratory. Table 6.3.2 Characteristics of the NEP solar collector installed at Archimedes Solar Energy Laboratory. Parameter Value Collector Length 1993mm Collector width 1208mm Parabola focal distance 647mm Mirror reflectivity 93.5% Receiver material Stainless steel 304 L Receiver external diameter 28mm Receiver internal diameter 25mm Glass tube transmittance 0.89 Selective coating absorptance 0.93 Selective coating emittance 0.18 900W/m2, wind speed of 0.45 m/s, flow rate of 8.8 kg/s, ambient temperature of 25.C and ambient air at atmospheric pressure in receiver annulus. It should be noted that the manufacturer gives for these conditions an efficiency equal to 58%, which is very close to the obtained result. It should be noted that if vacuum exists in the receiver annulus the efficiency is increased to 66.76% and the heat gain is equal to 735.8 W/m. A major objective of the work carried out at Archimedes Solar Energy Laboratory is to develop improved selective coatings for parabolic trough collector receivers. Thermal modelling of parabolic trough collectors 191 Figure 6.3.6 Performance of the collector installed at Archimedes Solar Energy Laboratory. By using conservative properties of these coatings as receiver absorptance=0.93 and emittance=0.18 (both at 100 and 400.C) the performance obtained is; heat gain=341.0 W/m, heat loss=417.2W/m and collector efficiency=31.36%. By using improved properties of receiver absorptance=0.95 and emittance=0.1 (both at 100 and 400.C) the performance obtained is; heat gain=427.1 W/m, heat loss=347.0W/m and collector efficiency=39.29 (7.93% improvement). These were obtained by using in the program the weather properties mentioned above, receiver temperature of 400.C and air in the receiver annulus. This is a significant improvement and the author believes that such values can easily be obtained with coatings based on diamond like carbon (DLC). 6.4 CONCLUSIONS In this chapter a detailed thermal model, which can be used for the analysis of a parabolic trough collector receiver is presented. The model is written in the Engineering Equation Solver (EES) software and takes into consideration all modes of heat transfer; convection into the receiver pipe, in the annulus between the receiver and the glass cover, and from glass cover to ambient air; conduction through the metal receiver pipe and glass cover walls; and radiation from the metal receiver pipe to the glass cover and from glass cover to the sky. The validation of the model is done using the known performance of existing collectors tested at Sandia National Laboratories, and its performance is very satisfactory. Finally, the model is used to perform an analysis of the collector installed at the Archimedes Solar Energy Laboratory of the Cyprus University of Technology with good prediction of the efficiency compared to the manufacturer data. Therefore, the model can be used in the future for system optimisation studies and performance prediction under variable weather conditions. It is also planned to modify the model so as to be called from TRNSYS and thus used for the annual performance prediction of the collector. 192 Solar energy sciences and engineering applications Nomenclatures a accommodation coefficient (-) b interaction coefficient (-) Dpi inside diameter of the receiver pipe (m) Dpo outside receiver pipe diameter (m) Dgi inside glass envelope diameter (m) Dgo outside glass envelope diameter (m) fpi friction factor for the inside surface of the receiver pipe, Dpi (-) h convection heat transfer coefficient (W/m2-.C) k thermal conductivity (W/m-.C) K. incident angle modifier (-) hf HTF convection heat transfer coefficient at Tf (W/m2-.C) g gravitational constant (=9.81 m/s2) Nu Nusselt number (-) Tf mean (bulk) temperature of the HTF (.C) Tpi inside surface temperature of receiver pipe (.C) Pa annulus gas pressure (mmHg) qsol solar irradiation per receiver length (W/m) Pr Prandtl number (-) Ra Rayleigh number (-) Ta ambient air temperature (.C) Tpi receiver pipe inside surface temperature (.C) Tpo receiver pipe outside surface temperature (.C) Tgi glass envelope inside surface temperature (.C) Tgo glass envelope outside surface temperature (.C) Tpo-gi average temperature (Tpo +Tgi)/2 (.C) Ts effective sky temperature (K) Greek symbols aabs absorptance of receiver pipe (-) aair thermal diffusivity for air at Tgo-a (m2/s) aenv absorptance of the glass envelope (-) ß volumetric thermal expansion coefficient (ideal gas) (1/K) . ratio of specific heats for the annulus gas (-) d molecular diameter of annulus gas (cm) epo receiver pipe selective coating emissivity (-) egi glass envelope emissivity (-) ego emissivity of the glass envelope outside surface (-) .abs effective optical efficiency at receiver pipe (-) .env effective optical efficiency of the glass envelope (-) . solar incidence angle (.) .air kinematic viscosity for air at Tgo-a (m2/s) . mean-free-path between collisions of a molecule (cm) s Stefan-Boltzmann constant (=5.67×10-8 W/m2-K4) tenv transmittance of the glass envelope (-) Thermal modelling of parabolic trough collectors 193 REFERENCES Cengel, Y. A. (2006) Heat transfer and mass transfer: A practical approach. McGraw Hill book company. Cheng, Z., He, Y., Xiao, J., Tao, Y. and Xu, R. (2010) Three-dimensional numerical study of heat transfer characteristics in the receiver tube of parabolic trough solar collector. International Communications of Heat and Mass Transfer, 37, 782–787. Clark, J. (1982) An analysis of the technical and economic performance of a parabolic trough concentrator for solar industrial process heat application. International Journal of Heat Mass Transfer , 25, 1427–1438. Davis, J. R. (2000) Editor, Alloy Digest, Sourcebook, Stainless Steels. Materials Park, OH: ASM. Dudley, V. E., Kolb, G. J., Sloan, M. and Kearney, D. (1994) Test Results: SEGS LS-2 Solar Collector, SAND94-1884, Albuquerque, NM. Duffie, J.A. and Beckman, W. A. (1991) Solar Engineering of Thermal Processes. JohnWiley & Sons, New York. Edenburn, M. W. (1976) Performance analysis of a cylindrical parabolic focusing collector and comparison with experimental results. Solar Energy, 18, 437–444. Forristall, R. (2003) Heat transfer analysis and modelling of a parabolic trough solar receiver implemented in Engineering Equation Solver. NREL/TP-550-34169. Garcia-Valladares, O. and Velazquez, N. (2009) Numerical simulation of parabolic trough solar collector: improvement using counter flow concentric circular heat exchangers. International Journal of Heat and Mass Transfer, 52, 597–609. Gnielnski, V. (1976) New equations for heat and mass transfer in turbulent pipe and channel flow. International Chemical Engineering, 562, 359–363. Gong, G., Huang, X., Wang, J. and Hao, M. (2010) An optimized model and test of the China’s first high temperature parabolic trough solar receiver. Solar Energy, 84, 2230–2245. He, Y., Xiao, J., Cheng, Z. and Tao, Y. (2011) A MCRT and FVM coupled simulation method for energy conversion process in parabolic trough solar collector. Renewable Energy, 36, 976–985. Heidemann, W., Spindler, K. and Hahne, E. (1992) Steady-state and transient temperature field in the absorber tube of a direct steam generating solar collector. International Journal of Heat Mass Transfer, 35, 649–657. Incropera, F., DeWitt, D., Bergman, T. L. and Lavine, A. S. (2007) Fundamentals of Heat and Mass Transfer. Sixth Edition, New York, John Wiley and Sons. Kalogirou, S. A. (2009) Solar energy engineering: Processes and systems. Academic Press, Elsevier Science. Kalogirou, S. A. (2004) Solar thermal collectors and applications. Progress in Energy and Combustion Science , 30, 231–295. Karimi, A., Guven, H. M. and Thomas, A. (1986) Thermal analysis of direct steam generation in parabolic trough collectors. In: Proceedings of the ASME Solar Energy Conference, 13–16 May, Anaheim, California, USA. pp. 458–464. Kearney, D. W. and Price, H. W. (1992) Solar thermal plants – LUZ concept (current status of the SEGS plants). In: Proceedings of the 2nd Renewable Energy Congress, 13–18 September, Reading, UK. pp. 582–588. Klein, S. A. (2002) Engineering Equation Solver for Microsoft Windows, Professional Version, Madison WI, F-Chart Software. KJC Operating Company (1993) Final Report on HCE Heat Transfer Analysis Code, SANDIA Contract No. AB-0227. Marshal, N. (1976) Gas Encyclopedia, New York, Elsevier. Odeh, S., Morrison, G. and Behnia, M. (1998) Modeling of parabolic trough direct steam generation solar collectors. Solar Energy, 62, 395–406. 194 Solar energy sciences and engineering applications Padilla, R. V., Demirkaya, G., Goswami, D. Y., Stefanakos, E. and Rahman, M. M. (2011) Heat transfer analysis of parabolic trough solar receiver. Applied Energy, 88, 5097–5110. Price, H., Lupfert, E., Kearney, D., Zarza, E., Cohen, G. and Gee, R. (2002) Advances in parabolic trough solar power technology. Journal of Solar Energy Engineering, 124, 109–125. Ratzel, A., Hickox, C. and Gartling, D. (1979) Techniques for reducing thermal conduction and natural convection heat losses in annular receiver geometries. Journal of Heat Transfer, 101, 108–113. Thomas, A. and Thomas, S. (1994) Design data for the computation of thermal loss in the receiver of a parabolic trough concentrator. Energy Conversion and Management, 35, 555–568. Touloukian, Y. S. and DeWitt, D. P. (1972) Editors, Radiative Properties, Nonmetalic Solids, Thermophysical Properties of Matter, Vol. 8, New York, Plenum Publishing. Chapter 7 Salinity gradient solar ponds Abhijit Date & Aliakbar Akbarzadeh School of Aerospace, Manufacturing and Mechanical Engineering, RMIT University, Melbourne, Australia 7.1 INTRODUCTION The most common form of solar pond is a salt-water solar pond. Salt water ponds exist naturally in a variety of locations, the first ponds being discovered in Eastern Europe at the beginning of the 20th century at a natural salt lake in Transylvania then part of Romania. Most of the salt water ponds operated today, however, are artificial, simulating natural solar ponds but taking advantage of engineering technologies to advance their operation and application for practical purposes. In the case of fresh water ponds all the solar radiation that falls on the surface is absorbed by the top 3 meters of fresh water and this thermal energy is rapidly lost to the atmosphere through natural convection heat transfer. So the temperature of a fresh water pond never rises and is almost constant throughout the fresh water pond depth. A solar pond utilizes a large body of salinity gradient water to absorb the radiation from the sun and store it in form of heat at the bottom. Figure 7.1.1 shows the schematic of a salinity gradient solar pond, which consists of three regions. The cold upper layer or Upper Convective Zone (UCZ) is a homogeneous thin layer of low salinity brine or fresh water. The middle gradient layer or Non-Convective Zone (NCZ) has a salinity gradient with salinity increasing from top of NCZ to the bottom of NCZ, this helps suppress the natural convection heat loss. The bottom layer or Lower Convective Zone (LCZ) has salinity close to saturation (high concentration brine) that absorbs and stores solar radiation that reaches the LCZ in form of thermal energy. Out of the 100% of solar radiation that is incident on the surface of the solar pond, around 5% is reflected back to the atmosphere; around 45% is absorbed by the water in the UCZ and eventually is lost to the atmosphere by convection; around 20% is absorbed by the water in the NCZ and eventually is conducted to the top UCZ and then lost to atmosphere; around 25% is absorbed by the water in LCZ and the remaining 5% is lost to the ground. Heat loss upwards in the pond from the storage zone is prevented since natural convection currents in the gradient zone are suppressed. This suppression and hence insulating effect occurs because of the density gradient present (Weinberger, 1964; Tabor, 1980). Experiments show the formation of separate salinity/density gradient layers in the NCZ (seen in Figure 7.1.2). When a particular layer of solution is heated its density is slightly reduced, but remains higher than that of the layer above. Hence there can be no movement upwards by the ‘buoyancy’ effect that drives natural convection in 196 Solar energy sciences and engineering applications Figure 7.1.1 Schematic diagram of salinity gradient solar pond. Figure 7.1.2 Experimental setup shows formation of separate salinity/density gradient layers in the NCZ. a constant density water body. The only mode of heat transfer from the lower layers to the upper layers is by pure conduction and hence NCZ is sometimes also called as insulation layer. During peak summer the temperatures of the LCZ can reach close to boiling temperatures if no heat is removed. Salinity gradient solar ponds 197 Figure 7.1.3 Typical salinity and density profiles in a solar pond. Figure 7.1.3 shows the typical solar pond profiles of salinity, density and temperature. The UCZ is typically 0.2 m–0.3m thick and is maintained at temperature close to local daily ambient. The UCZ requires continuous washing with fresh or low salinity water to remove the diffused salt from the saline layers below and to compensate for the evaporation water loss. In order to maintain the salinity gradient, salt crystals or saturated brine must be added to the LCZ to compensate for the salt lost by diffusion (Akbarzadeh et al., 2005). 7.2 SOLAR POND – DESIGN PHILOSOPHY 7.2.1 Sustainable use of resources Solar ponds are very sensitive to different applications and hence knowing the application before design starts is very crucial. Knowing the end application of the solar pond would help estimate the energy requirements and hence an optimised solar pond could be designed. For example, the characteristics of a solar pond to supply hot water at say 35.C to an aquaculture facility will be very different from that of a solar pond to be used to generate electricity where sustained performance at higher temperatures of 80.C or above is essential. Obviously the solar pond must be located as close to its application as possible (Akbarzadeh et al., 2005). Solar ponds would be more economical if constructed using local labour, materials and other resources. It is essential to have a local supply of salt or brine and low salinity water. Flat land is better with high solar radiation for easy construction and optimum operation of a solar pond. Since construction of the solar pond basically involves earth moving and plumbing, it makes good sense to use local contractors. 198 Solar energy sciences and engineering applications In order to match the thermal output of a solar pond to the energy and temperature requirements end application, it is very important that the temperature of the lower convective zone is always maintained 3–5.C above the end use temperature. When the LCZ temperature is much higher than the delivery temperature the heat loss will increase and some economic penalties would increase. 7.2.2 Best site characteristics Site selection is very important for easy construction and operation of a solar pond. A potential site for solar pond should have easy access to salt or brine, low salinity water, ample flat land, consistent soil to be used for building pond walls and the most importantly the land should not be cultivable. The site should not be windy for most part of the year, as high wind can disturb the stability of the pond. The sites for small solar ponds should not be surrounded by buildings or tall trees. The site should receive plenty of solar radiation as this will directly affect the performance of the solar pond. However, it is still possible to build ponds that will operate well in high latitudes, with increased area compensating for less available radiation per unit area of surface. The performance of the solar pond would also depend on the local evaporation rates and depth of the natural underground water table. High evaporation and a shallow water table would make the solar pond performance drop as the heat loss to the atmosphere and ground would increase. Heat loss to the ground water can be reduced by insulating the floor of the solar pond, but this would add to the construction cost (Tabor, 1980; Hull, 1989; Akbarzadeh et al., 2005). An ideal site for solar pond would have free draining soil, free salt available nearby to reduce costs, easy access to water, flat land to minimize earthmoving requirements, easily compactable soil for structural stability, low prevailing wind speeds to minimize wave-induced mixing and the depth of the top mixed zone, an environmentally acceptable disposal method or recycling ability for closed-salt inventory balancing, dry soil for good thermal insulation, high incident solar radiation for good thermal performance, low evaporation to minimize the need for make-up water, soil with good cohesion for forming stable walls for above-ground ponds, a low amount of wind-borne debris to easily maintain cleanliness, a stationary or deep groundwater table to minimize heat loss within the ground, most importantly proximity to end use application. 7.2.3 Performance and sizing The thermal performance of a solar pond mainly depends on the absorption of solar radiation in the layers of the ponds. Sun light attenuation as it passes through the top layers of a solar pond puts an upper limit to the amount of solar radiation that can reach the lower convective zone. Further, the amount of sunlight that can reach the lower convective zone would decrease with an increase in turbidity, so it is very important to maintain high water clarity in a solar pond. The more radiation that penetrates, the higher the energy efficiency and operating temperature of the pond will be. In a welldesigned and set up solar pond, upward heat losses from the LCZ are small. Therefore most of the solar radiation that gets through to the LCZ is stored there, apart from the small amount lost by conduction to the ground. A well maintained solar pond with a total depth of 3 metres, with 1m deep LCZ (storage zone) would receive around Salinity gradient solar ponds 199 20–25% of the radiation incident upon the pond’s surface. After accounting for losses to the ground, in practice around 15–20% of the incoming radiation is available for extraction to an application, with the heat delivered 40 to 50 degrees above the local daily average temperature (Weinberger, 1964). Thermal energy output capacity of a solar pond further depends on the surface area and depth of the storage zone. For a given application with a known heat load the size of a solar pond that can meet the load can be approximated through the following steps, for example the heat load is 3000 GJ/year, (for this approximation it is assumed that the solar pond is clear and well maintained): 1 Find the annual solar energy incident per square metre on a horizontal surface for the proposed solar pond site, for example 6 GJ/m2/year. 2 Now estimate the solar energy that will reach the storage zone and be available for extraction, divide the annual solar energy value found in the previous step by 4 and 8, for example 6/4=1.5 GJ/m2/year; 6/8=0.75 GJ/m2/year. 3 Now estimate the solar pond surface area, divide the heat load by the fraction of solar radiation that is available for extraction as estimated from the previous step, for example solar pond surface area=3000/1.5=2000m2; area=3000/0.75=4000m2. The size of a real pond to supply the heat load will depend upon the brine clarity (transparency) and the rate of heat extraction. For continuous heat extraction the pond will be larger, while rapid peak load heat extraction the pond will be smaller. Thus the important design parameters will include: information on ground thermal conductivity, requiring site specific figures for soil thermal conductivity and permeability; pond configuration requirements, requiring knowledge of pond site characteristics; local weather features, including long-term weather and solar data; and pond depth characteristics. The temperature and quantity of the heat storage in a solar pond depends upon and can be estimated by knowing the salinity gradient of the pond, transparency and the depth of the pond. The depth determines such factors as maximum pond temperature, heat losses to the surrounding soil and atmosphere, and temperature decay time for the storage zone. There are lot of assumptions to be made, and the experience of practical solar pond operations provide a very useful guide for design sizing purposes. A solar pond with a deep storage zone (typically of the order of 2 to 5 m) will store a large quantity of heat for a long time. Heat losses will be lower and the collection and storage efficiencies of the pond will be high. In contrast, a shallower storage zone (typically of the order of 1 to 2 m) can readily attain higher temperatures (since there is not so much thermal mass in storage), but will then have higher heat losses to the air and ground and a shorter storage capacity. Hence the design of a solar pond would mainly depend on the application. 7.2.4 Liner, salt and water Liners are a very important component of a solar pond as they prevent saline water from leaking into the soil underneath the pond and endangering the purity of the aquifer. Hot brine leaking out from a pond will carry away with it salt and heat, which 200 Solar energy sciences and engineering applications in most places is not environmentally acceptable. In order to minimise the heat losses to the ground it is desirable that the underground water table is 5 metres or more below the natural ground surface. If the water table is shallower, then insulating the bottom of the pond may be considered using insulation materials such as sheets of polystyrene. The liner material should be able to withstand the anticipated maximum pond temperature, be resistant to ultraviolet radiation, and should not react with salt. Above all it should be mechanically strong. Failure of liners has been one of the main problems encountered with working solar ponds. More environmentally friendly liners for solar ponds can be made from compacted clays. Not all clays are suitable to be used as natural liners for solar ponds. Hot NaCl brine can cause some clay to flocculate, making them more porous. Where unlined ponds can be operated effectively, the cost of solar ponds is lowered significantly since lining is one of the main cost components. However, in many locations pond lining is necessary, for both environmental as well as performance reasons (Almanza and Castaneda, 1993). Different polymeric liner materials can be used for lining the floor of a solar pond. Very often low-density polyethylene (LDPE) and high-density polyethylene (HDPE) are used along with natural clay for lining the floor of a solar pond. For small solar ponds commercially available 10mwide standard liners can be used. But for large solar ponds with a surface area of a few hundred hectares it is advisable to make the polymer liners on site so that they will cost less and also can be made to desired widths, depending on the available liner-making technology. Liners made from LDPE and HDPE should be protected against ultraviolet radiation and hence should be covered with a thin layer of soil or sand. The best liner laying practice is to make sandwich layers of clay and polymer liners; this will help achieve good leakproofing (Almanza and Castaneda 1993; Akbarzadeh et al., 2005). Sodium Chloride (NaCl), often called common salt, is the most commonly used in salinity-gradient solar ponds for construction of a salinity gradient. Magnesium Chloride (MgCl2), also known as bittern, is the second most common salt used in the construction of solar ponds. Bittern is a by-product of a NaCl salt production factory. The density of sodium chloride solution can be increased up to 1300 kg/m3. However, this density can be increased to more than 1500 kg/m3 if the salt used is mainly magnesium chloride. It can be seen from Figure 7.2.1 that the solubility of the sodium chloride is fairly constant with temperature, while that of magnesium chloride increases (IUPAC, 2007). It would be possible to construct and maintain stable gradients with both salts. When setting up a solar pond as an integral part of a commercial salt production facility, it makes good economic and environmental sense to set up the solar pond using bittern, and save the more valuable sodium chloride for salt making. Due to the large salinity difference between the lower convective zone and the upper convective zone, there is upward salt diffusion. To maintain the salinity gradient the top layer of the pond is continuously washed with fresh or low salinity water, while salt is added to the storage zone. It is important to recycle the salt extracted from the pond by surface washing to have minimum economic and environmental impact. An evaporation pond that is at least equal if not twice the total surface area should be constructed next to the solar pond in order to recycle the washed salt. Alternatively the flushed salt solution can enter a sequence of evaporation ponds in a salt production facility. Salinity gradient solar ponds 201 Figure 7.2.1 Solubility of NaCl and MgCl2 in water at different temperatures. One of the most important criteria in determining the viability of a solar pond is the availability of fresh or low-salinity water (less than 50,000 ppm salt concentration or a density of less than 1050 kg/m3). The amount of low-salinity water required to establish a pond is about the volume of the water in the pond measured from the surface to the middle of the gradient layer. The amount of water needed to maintain the gradient depends on evaporative losses, and the flow rate of the overflow system removing surface washing water containing the salt that has diffused upwards. As a rule of thumb, a surface washing water flow rate of two to three times the yearly average rate of evaporation is required. The rate of adding water to the surface zone must exceed the rate of removal through the overflow by the rate of evaporation (Akbarzadeh et al., 2005). For large ponds (greater than about 1000m2), it is best to construct the pond by establishing the walls using the soil excavated from the inner periphery of the pond. The bottom of the pond will thus be below the surrounding ground level. This arrangement provides the head required for gravity feeding of the surface water from the solar pond to adjacent evaporation ponds, as well as siphoning off brine samples from different depths in the pond for the required analysis. 7.2.5 Transient performance prediction To predict the thermal performance of large solar ponds, the thermal process in these solar ponds can be treated as one-dimensional unsteady conduction loss with heat generation from incoming solar radiation and heat addition from any other source of thermal energy; here the finite difference method is used. As shown in Figure 7.2.2 the NCZ of the solar pond is divided into a number of small divisions (divisions can be non-equal) and the location of the node for determination of temperature is assumed 202 Solar energy sciences and engineering applications Figure 7.2.2 Schematic illustrating the vertical divisions of solar pond and external heat extraction and addition. to be at the middle of each division. By applying an energy balance to the divisions in the different layers of the solar pond when extracting heat as shown in Figure 7.2.2, a finite difference model can be developed for the temperatures of three adjacent nodes (Wang and Akbarzadeh, 1982). To estimate the performance of the initial temperature of the solar pond, density, conductivity, specific heat capacity, thickness, depth, incoming solar radiation and the time increment for each node are required as input boundary conditions. The solar radiation that penetrates the solar pond surface is calculated using the formula discussed by Bryant and Colbeck (Bryant and Colbeck, 1977) h = H[0.36 - 0.08 ln(x)]. (7.2.1) Here H is the solar radiation reaching the top surface of the solar pond after deducting the reflective losses; h is the solar radiation that penetrates the solar pond surface and reached the depth of x. 7.3 SOLAR POND – CONSTRUCTION AND OPERATION 7.3.1 Set-up and maintenance Initial steps in setting up a solar pond are very similar to that of constructing an artificial fresh water pond, similar to the rain water collection pond used for irrigation. The land is excavated and the excavated soil is used to build the side walls of the pond with a slope of about 1:2 (Hull et al., 1989). The newly exposed pond floor is compacted with heavy rollers and small sharp stones and dried soil clusters are removed before laying the liners. The liners are then covered by a thin layer of locally available clay. Salinity gradient solar ponds 203 Figure 7.3.1 The process of setting up the salinity gradient is shown schematically. Fresh or low salinity water is injected horizontally into the body of water in the solar pond. Note that the mixing occurs only in the region above the level of injection. There are a number of different methods for setting up a salinity gradient in a solar pond. The method described here is simple and used all over the world. The solar pond is filled with fresh water up to 1.3m from bottom; this is approximately equal to the thickness of the storage zone and half of the gradient layer. Now solid salt crystals are added to the bottom of the pond and allowed to dissolve to create concentrated brine with around 25% salinity; this requires about 325 kg of salt per m2. The first location of fresh water injection determines the thickness of the storage zone. If the fresh water is injected from 1m above the bottom then the storage zone gets a thickness of 1m and so on. The fresh water is injected using a diffuser as shown in Figure 7.3.1 and Figure 7.3.2 while the diffuser is gradually moved upwards. The mixing of rising fresh water with the high concentration brine creates the gradient layer. The diffuser should rise 2 cm for every 1 cm rise in the level of solar pond water (Tabor, 1980; Alagao et al., 1993). The salt steadily diffuses upwards due to salinity differences between the storage zone and top layer. The rate of salt diffusion depends upon the temperature and the salinity gradient present in the solar pond. As a guideline the rate of diffusion in a sodium chloride pond can be up to 20 kg/m2/year. In order to maintain the difference in salinity between the bottom and top of the pond, salt must be continuously removed from the surface zone by surface flushing, and an equal amount added to the storage zone as shown in Figure 7.3.3. A ‘stability margin’ number has been defined to establish an operational safety limit for the salinity gradient within a pond. Basically the density gradient must be kept above a certain minimum level to prevent convection currents starting and the 204 Solar energy sciences and engineering applications Figure 7.3.2 Diffuser used to setup gradient. Figure 7.3.3 Passive salt replenishment and surface washing. salinity profile of the pond becoming unstable (Tabor, 1980). To maintain the stability of the pond one should monitor and limit the stability margin number within the predetermined safety limit. The use of a scanning injection diffuser system allows local instabilities in the gradient layer to be eliminated. 7.3.2 Turbidity control Turbidity is a measure of the clarity of water or in other words the degree of transparency; its unit of measurement is nephelometric turbidity units (NTU). Desired values of turbidity are less than 0.5 NTUs. Dust, live or dead organic material and any other suspended material reduces the clarity of the water and hence reduces the thermal efficiency of a solar pond. Heavy dust particles would sink to the bottom of the pond and will not affect the pond clarity, while the dead organic material like leaves, pollen etc. if not removed on time can reduce the thermal performance of the pond and also help the growth of algae in the pond. All dead organic material acts as food material for the growth of algae. So Salinity gradient solar ponds 205 Figure 7.3.4 In-pond (left) and external (right) heat exchangers at RMIT University, Melbourne, Australia. the best practice will be to prevent and outbreak of algae pond in the first place. Earlier studies have shown that adding copper sulphate to the solar pond water or making the solar pond water more acidic will help reduce the algae growth (Gasulla et al., 2011). Acidification of the pond provides a simple and reliable maintenance method for preventing or inhibiting algal blooms and maintaining high transparency. The pH level of the pond should be monitored and hydrochloric acid added to keep the level below 5.5. Another approach is to add brine shrimp to the pond to feed off and hence control the algae level (Wang and Seyed-Yagoobi, 1995; Malik et al., 2011). 7.3.3 Heat extraction The main purpose of a solar pond is to supply heat and this heat is stored in the storage zone of the pond, and this is in addition to the heat that is stored in the gradient layer. There are different ways in which heat can be extracted from the storage zone of the solar pond. In the case of a large solar pond external heat exchangers are used with hot brine being extracted from the storage zone; after it passes through the external heat exchanger this brine is pumped back. In the case of small solar ponds in-pond heat exchangers are better suited, made from plastic tubes or copper nickel tubes (Jaefarzadeh, 2006). Figure 7.3.4 shows the in-pond and external heat exchangers in the 50m2 solar pond at RMIT University, Melbourne, Australia. The most common method of heat extraction from solar ponds is pumping hot brine from the storage zone through a diffuser located just below its interface with the gradient zone to a heat exchanger located near the pond. After delivering heat to the heat exchanger, the cooler brine is returned to the bottom of the pond. This brine removal and return can be accomplished without causing instability in the gradient layer. The pond must also always be operated with the salt concentration in the storage zone below the saturation level. Otherwise there can be some crystallisation of salt in the pipes, pumps and other equipment used in the heat extraction system, especially when it is not operating. The resulting blockages can be difficult to remove. Alternatively heat can be extracted using in-pond heat exchangers. For instance, use of plastic pipes connected to weights on 206 Solar energy sciences and engineering applications the pond bottom by ropes to overcome buoyancy forces has proven to be a simple and reliable method of heat extraction from the 3000m2 demonstration solar pond in Pyramid Hill, Australia. Recent investigations have shown that heat extraction from the gradient zone in addition to the storage zone helps improve the overall efficiency of the solar ponds. In-pond heat exchangers for heat extraction from the gradient zone are only effective in case of small ponds. To extract heat from the gradient zone of large solar ponds, a selective withdrawal method should be used. Here hot saline water from different depths is extracted and is passed through a respective external heat exchanger where the heat transfer fluid is preheated using the gradient zone (Andrews and Akbarzadeh, 2005; Leblanc et al., 2011; Yaakob et al., 2011). 7.3.4 Performance monitoring For optimum operation and steady performance of a solar pond it is very important to have a reliable monitoring system and procedures. Physical parameters such as temperature, salinity profiles in the solar pond should be routinely monitored. It is also very important to monitor the water clarity and for this measurement of turbidity of water is essential, at the same time monitor the pH of water. Control of algae growth is very critical to the efficient performance of a solar pond as discussed in the earlier section. The temperature, density, turbidity and pH of the solar pond should be monitored once every two weeks. Failure to do this will lead to a decrease in the thermal efficiency of the solar pond. Temperature and salinity profiles help to examine the stability of the gradient zone of the solar pond. The simplest method of monitoring these parameters is by using a sample extraction device as shown in Figure 7.3.5 (top) (Malik et al., 2011). This sample extraction device can be made from a 3.5m long 25mm diameter plastic tube. The sampling tube is used to withdraw brine samples from different levels in the pond (guideline: 5 cm intervals). Thermocouples are connected to the inlet of the tube such that the temperature at the point in the pond from which the sample has been withdrawn can be measured. An automated sampling system as shown with a schematic in Figure 7.3.5 (bottom) would be more practical for large solar ponds. It is also useful to monitor the total global solar radiation incident upon a horizontal surface near the pond surface to keep records of solar energy input for calculation of the solar pond thermal efficiency. Inspection of temperature and salinity profiles is a direct and simple way to locate the depth of the gradient zone with the surface zone and storage zone, as well as the presence of any convective layers. 7.3.5 EEE (Energy, Environmental and Economic) benefit evaluation It is simple to calculate the thermal utilisation coefficient of the solar pond: it is defined as the ratio of the useful heat extracted from the pond to the actual solar radiation received by the pond. The rate of heat extraction from the solar pond and the temperature of the heat delivered should be monitored and recorded. So to accurately calculate the thermal utilisation coefficient of a solar pond in operation, the total thermal energy provided by the pond over a period of several months should be determined. Similarly Salinity gradient solar ponds 207 Figure 7.3.5 Manual sampling device (top) and automated sampling system (bottom). calculate the average collective global horizontal solar radiation incident upon the top surface of the pond. Now the ratio of these two quantities will provide the value of the average thermal utilisation coefficient or thermal efficiency of the pond over the period of measurement. Solar ponds can be environment friendly and economical when constructed at a suitable location and using local resources. For these situations solar ponds are lot cheaper than any other large-scale solar thermal collectors. The main advantage of the solar pond compared with other solar thermal collectors is that they have an integrated thermal energy storage system. So the solar ponds can supply a substantial amount of thermal energy on a continuous basis. The costs of a solar pond vary widely according to location and application, so it is essential to perform an economic analysis of the specific design, site and application 208 Solar energy sciences and engineering applications in mind. Using correct local quotes for the average costs of the construction and operation of a solar pond is very important; this will help to make a sound economic judgement. Irrespective of the location of the proposed solar pond the economics of a solar pond will depend on the costs associated with land purchase or lease, operating and maintenance costs, and capital equipment, the lifetime of the pond and the associated depreciation rate, and the real discount rate or return on investment sought. In addition to the land the other major expensive components of a solar pond are its liners (installed), solid salt or brine, excavation of earthen dam walls, monitoring system, heat exchangers. There are good economic benefits of scale, for example, in excavation and setting up nearby evaporation ponds for salt recycling, so that larger solar ponds are more economically favourable where the land is available. However, the maintenance of pond stability and heat extraction may become more difficult as the area increases. Although solar ponds from hundreds of square meters to thousands of square kilometers are feasible, the sizes likely to find greatest application are in the range of 1 to 10 hectares. Larger facilities than this would probably use unit sizes of 10–20 hectares rather than one extremely large pond, for reasons of operational safety and reliability. Solar ponds can be economically viable for industrial process heating, including manufacturing processes requiring low-temperature heat and aquaculture and drying applications, at sites where land and water (brackish or sea water) are available, and solar radiation is high. In the right situation solar ponds for heating can readily be economical in areas where natural gas is not available and the only alternative fuels are LPG or oil, and may even compete against natural gas where the price of the latter is high. Of course, a key benefit of a solar pond is a zero greenhouse emissions source of heat. Solution mining uses low temperature heat for extracting minerals from the mines. The by-product of such mining operations is plenty of high salinity or brackish water. One of the major requirements for constructing a solar pond is a large and continuous supply of saline water and in return the solar pond can continuously supply a large amount of low temperature heat. So the economics would be more favourable when a solar pond is integrated with a solution mining site. The use of solar ponds as the source of heat for thermal desalination processes such as multiple effect evaporation or multistage flash processes is also potentially one of the most economic zero-emission options for providing fresh water from saline ground water or sea water. With the world running desperately short of fresh water, solar ponds for desalination is a potentially important area of application. Life cycle analysis, covering embodied energy and emissions in the materials used and in construction, indicate that solar ponds have one of the lowest greenhouse gas emissions per unit of thermal energy produced over their lifetime of any of the renewable energy options. This is if a solar pond is constructed and operated using locally available resources. The use of local labour in the construction and operation of a solar pond can create employment opportunities in regional areas. Hence on a ‘triple bottom line’ evaluation, solar ponds in a well-chosen application can rate very well on economic, environmental and social criteria (Esquivel et al., 1993; Akbarzadeh et al., 2005). Salinity gradient solar ponds 209 7.4 SOLAR PONDS – WORLDWIDE Since the 1950s a number of demonstration and a few industrial solar ponds have been constructed and operated around the world. This section presents a few examples of such solar ponds from around the world. 7.4.1 Solar ponds – Israel Figure 7.4.1 shows the solar pond power station at Bet Ha Arava in Israel. Ormat constructed two solar ponds with a combined surface area of 250,000m2 near Bet Ha Arava north of the Dead Sea. These two solar ponds supplied required thermal energy to the Ormat power plant with a power production capacity of 5MWe. Due to geopolitical reasons this solar pond power station was decommissioned in 1990. Solar pond technology is again gaining some interest for industrial process heating rather than power generation application (Tabor and Doron, 1990). 7.4.2 Solar ponds – Australia Figure 7.4.2 shows the 50m2 experimental solar pond that was constructed in 1998 at the Bundoora East Campus of RMIT University located in Melbourne, Australia. The solar pond is circular in shape, 8m in diameter and 2.5m deep; it has an observation window to check water clarity. The overflow level control system maintains the solar pond water depth at 2.05m from the bottom. The solar pond walls and floor is made from 0.2m thick reinforced concrete and to protect the concrete floor and the inner surface of the concrete walls and steel reinforcement from corrosion due to salt, they are coated with a layer of epoxy. This solar pond is a partially in-ground type and has Figure 7.4.1 The Dead Sea solar pond power station near Bet Ha Arava, Israel. 210 Solar energy sciences and engineering applications Figure 7.4.2 50m2 solar pond at RMIT University, Melbourne,Australia. a wall around 1.2m above the ground. As the pond wall is partially above the ground level, it allows using a gravity assisted overflow system to maintain the level of water in the pond. Salt lost by diffusion is replenished by adding solid sodium chloride salt to the bottom of the pond with a cylindrical salt charger (Andrews and Akbarzadeh, 2005; Leblanc et al., 2011; Yaakob et al., 2011). The diffused salt that reaches the top convective zone must be removed to maintain the salinity gradient; at RMIT solar pond the top surface of the pond is continuously flushed with low salinity water, and this helps to remove the diffused salt. The flow rate of flushing should be kept at around twice the local yearly average evaporation rate. To reduce stirring of the top convective zone by natural wind, 0.6m diameter floating rings made from high density polyethylene are spread over the top surface of the pond; these rings help reduce the amplitude of the waves formed due to natural wind. At present, the clarity of the water in this solar pond is maintained by keeping the pH in the range of 5 to 6. Figure 7.4.3 shows the 3000m2 solar pond designed and constructed by RMIT University along with two industrial partners in early 2000 for supplying heat for salt drying process at Pyramid Salt Factory (Akbarzadeh et al., 2005). This pond has a total depth of 2.3 m, with a storage zone that is 0.8m thick and a gradient zone of about 1.2 m. Initially this pond mainly supplied heat for the process of salt drying while later some amount of heat was also supplied to an aquaculture farm on the factory site. Saline ground water is used for surface flushing and the water that overflows is carried to a large evaporation pond. The location of the pond was around 200m from Pyramid Hill’s salt production plant. Unfortunately after the flash flooding in part of Victoria, Australia, the salt production facility was relocated and the solar pond is not in operation anymore. 7.4.3 Solar ponds – USA In 1983 the University of Texas at El Paso along with Bruce Foods, Inc., constructed a 3700m2 solar 20km northwest of El Paso city centre as shown in Figure 7.4.4 (Lu and Sandoval, 1993; Akbarzadeh et al., 2005). Through well-designed procedures for Salinity gradient solar ponds 211 Figure 7.4.3 Solar pond at Pyramid Hill, in northern Victoria, Australia; In operation (Top 2 photos); Photo taken in August 2012 – of out of operation Pyramid Hill solar pond (bottom). Figure 7.4.4 Solar pond at the University of Texas at El Paso. 212 Solar energy sciences and engineering applications maintaining the gradient and clarity of the pond, a very high level of performance has been achieved, with storage zone temperatures remaining above 80.C year-round. This solar pond produced 120kW maximum electrical power with an Ormat ORC engine in the summer of 1992. This solar pond operated for 16 years before it encountered problems with liner failure most likely due to high temperature deterioration of the polymer liners and was decommissioned at the end of 2003. A 2000m2 experimental solar pond was constructed at the University of Illinois, USA to study and develop simple and cost effective construction methods (Newell et al., 1990). At a 2000m2 solar pond in Miamisburg, Ohio, salt-gradient supplies the heat to warm-up a summer outdoor swimming pool and in winter a recreational building and the installation costs were only $35/m2 (Sabetta, Pacetti et al. 1985). 7.4.4 Solar ponds – Tibet, China Figure 7.4.5 shows the lithium carbonate (salt) solar ponds in Tibet, China. These ponds are constructed in a hilly barren land on the southwest of Zabuye Salt Lake. The solar pond has a top surface area of 3588m2 (78m×46 m), the depth is 4m and the pond walls have a slope of 1.5:1. This is an in-ground solar pond, and after excavation Figure 7.4.5 Photos clockwise from top left: (1) Zabuye salt lake; (2) Solar Pond in operation; (3) Lithium carbonate harvesting, and; (4) Lithium carbonate sheets (Nie, Bu et al. 2011). Salinity gradient solar ponds 213 the soil was properly compacted. The pond walls and floor have been covered with 8 cm thick steel reinforced concrete. The smooth concrete walls and the floor of this pond have been insulated with 0.5mm thick sheet of Ethylene-Propylene- Diene-Monomer (S-801EPDM) to prevent brine leakage. This solar pond has an automatic temperature measurement system installed prior to the start of operation of the pond. Thirty ponds of similar size are been used to produce lithium carbonate at the same time in Zabuye (Nie et al., 2011). 7.4.5 Solar ponds – India A 1200m2 pond was constructed at Central Salt and Marine Chemicals Research Institute (CSMCR) in Bhavnagar, Gujarat in 1970. This solar pond used magnesium chloride to create the salinity gradient. Magnesium chloride is a waste product from the process of making edible salt. In 1980 a 100m2 experimental solar pond was constructed and operated for two years in Pondicherry. This pond used sodium chloride to create the salinity gradient and used low density polyethylene liners. In 1980 another solar pond was constructed with a surface area of 1600m2 at CSMCR in Bhavnagar, Gujarat. This pond also used magnesium chloride to create the salinity gradient and had problems with the clarity of bittern. In 1984 a 240m2 solar pond was constructed and operated for a long period at the Indian Institute of Science, Bangalore. Study of this pond has produced very useful performance data for a small solar pond in south India. It has also proven the technical and economic viability of small solar ponds. Since the 1980s several small size solar ponds have been installed and operated for town water heating. A 400m2 solar pond was constructed to supply the hot water needs of a rural community at Masur on the west coast of India. A similar solar pond with 300m2 surface area was constructed to supply hot water to student hostels for an engineering college at Hubli in Karnataka. Figure 7.4.6 shows the solar pond at Bhuj, India. The 6000m2 solar pond that was built at a dairy in Bhuj stood out in many regards. This was the first-ever solar pond in India to have connected itself to an industrial process, supplying heat to the Kutch Dairy. To reduce the construction cost of the solar pond the project developed a cost-effective, indigenous lining; it used locally mined clay and plastics. While the pond attained a record 99.8.C under stagnation, stability of the salinity gradient was maintained even at such elevated temperatures. With only one injection diffuser on one side of the pond, the desired salinity profile was achieved even at the farthest end. Here an external heat exchanger is used to extract heat from the storage zone of this pond. Hot brine is withdrawn from the bottom of the pond and is pumped through a shell-and-tube heat exchanger where it heats the feedwater up to a temperature of 70.C. Further, this hot water was delivered to the Kutch Dairy plant to be used as pre-heated boiler feed water as well as for cleaning and washing. The entire exercise at the Bhuj solar pond successfully demonstrated the expediency of the technology by supplying 80,000 litres of hot water daily to the plant (Kumar and Kishore, 1999). 214 Solar energy sciences and engineering applications Figure 7.4.6 Solar pond in Bhuj, India. 7.5 SOLAR PONDS – APPLICATIONS 7.5.1 Heating In the past solar ponds have been used for water heating and in some cases also for industrial process heating. Solar ponds have the highest potential in suitable applications and locations to supply low-temperature heat at competitive costs. The suitable heating applications of a solar pond are town water heating, salt drying, fruit drying, wood drying, hot water for the food industry, solution mining operations (Hull, 1989). 7.5.2 Aquaculture Pyramid Hill solar pond has been used to supply low temperature heat for aquaculture for growing warm water fish and shrimps. Solar ponds are suitable where the desired supply temperature is low and a large amount of heat is required. Using fossil fuels for supplying the low temperature process heat is not sustainable; hence solar ponds should be used for heating aquaculture ponds to grow fish, shrimps, algae etc. The heat from the solar ponds can be used for controlling the temperature of the environment for growth control well as to supply other thermal energy needs of the plant. Salinity gradient solar ponds 215 This makes a solar pond a very economical and attractive technology (Akbarzadeh et al., 2005). 7.5.3 Desalination Solar ponds can easily supply the large amount of heat that is usually required for thermal desalination processes. The low pressure (below atmosphere) thermal desalination systems like multiple effect evaporation (MEE) and multistage flash (MSF) can be easily coupled with solar ponds to produce fresh water with minimum environmental footprint. The waste brine from these desalination systems can be fed to the bottom of the pond to maintain the salinity gradient (Esquivel et al., 1993; Zhao et al., 2009). 7.5.4 Power production There have been several large demonstration projects in past like the 5MWsolar pond power plant in Israel, a 15 kW power plant in Alice Springs, Australia and the 70kW power plant at El Paso, USA (Akbarzadeh et al., 2005). Organic Rankine Cycle (ORC) engines developed specifically to produce electric power from lower-temperature heat sources (80–90.C) have been used in these applications. However, their thermodynamic performance is well below the Carnot limit, so designs have low net thermal-to-electric energy conversion efficiencies (~7%), which adversely affects their economic viability. With advancement in the low temperature heat engine technology, the electrical power production from low temperature heat stored in solar ponds will get economically competitive with other renewable energy technologies. Consequently to date there has been very little commercial exploitation of solar ponds for generating electricity, despite the reduction in greenhouse gas emissions from burning primary fossil fuels that their utilization would yield. 7.6 FUTURE DIRECTIONS The solar pond technology is now very well matured and has been successfully used on a trial basis around the world in the last half century for power production, industrial process heating, salt drying etc. The applications presently of greatest interest are industrial process heating, thermal desalination, aquaculture, mariculture, greenhouse heating, biogas production, and waste brine or other saline effluent management. Lines of inquiry in solar pond science and technology that still require further investigation include alternative salts and new lining techniques that may increase stability and reduce construction costs; simpler more cost-effective methods to limit wind-driven mixing and thickening of the surface zone; increasing the thermal efficiency of solar ponds and maintaining high-temperature operation; and possibly the development of a new generation of solar ponds based on artificial solar pond liquids. Globally, excellent sites for solar ponds are abundant and many solar pond applications may become economically viable as the prices of fossil fuels rise. Solar ponds as a renewable energy source capable of yielding environmental and social benefits can help make the transition to a truly sustainable energy system. 216 Solar energy sciences and engineering applications GLOSSARY Density gradient: The variation with depth of the density of the saline solution in a salt water solar pond (see ‘salinity gradient’). Lower Convective Zone (LCZ): The bottom zone which is generally about or Storage Zone 1 to 1.5m thick and has near-saturated brine stores the incoming solar radiation in the form of heat. Organic Rankine Cycle engine: A heat engine based on the Rankine Cycle and using a low boiling point organic working fluid that can be used to generate electricity from low temperature heat (less than 100.C) as supplied by a solar pond. Non-Convective Zone (NCZ) The central zone, in a vertical direction, or Gradient zone: in a salt water solar pond in which there is a salinity gradient. Salinity gradient: The variation in salinity with depth used in a salt-water solar pond to create a density gradient that suppresses convection currents. In such a solar pond salinity is near saturation at the bottom of the pond and rises to near fresh water level in the top layer. Salt water solar ponds are also commonly called ‘salinity gradient solar ponds’. Solar pond: A large-area collector of solar energy in the form of a shallow pond that stores heat for use for practical purposes. Designs include salt-water ponds, gel ponds, and ponds with covers. Incoming solar radiation is stored in the lower layer of the pond by suppressing the convection currents in the layer above that would otherwise lead to heat loss to the surroundings. Utilisation coefficient (of a solar pond) on average for a given period or thermal efficiency: is the total thermal output delivered to an application divided by the cumulative solar radiation incident upon the pond’s surface over that period. Normally the period chosen to assess thermal efficiency would be a full year, after the solar pond had warmed up to its operating temperature range. Upper Convective Zone (UCZ) The upper layer of a salt water solar pond or Top layer or Surface zone: (also called the ‘upper convective zone’) comprising low salinity or fresh water that is continually added to the pond and removed at an overflow to take away salt that diffuses to the surface from the layers beneath. The top layer which is about 20 to 30 cm thick has very low and uniform salinity. Salinity gradient solar ponds 217 REFERENCES Akbarzadeh, A., Andrews, J. and Golding, P. (2005) Solar Pond Technologies: A review and Future Directions. Advances in Solar Energy. Earthscan. London, UK, 16: 233–294. Alagao, F.B., Akbarzadeh, A. and Johnson, P. (1993) The Design, Construction and Initial Operation of a closed cycle Salt Gradient Solar Pond. 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R., Nielsen, C.E. and Golding, P. (1989) Salinity-gradient solar ponds. Boca Raton, Fl, USA, CRC Press. IUPAC (2007) NIST Standard Reference Database 106 IUPAC-NIST Solubility Database, NIST. Jaefarzadeh, M.R. (2006) Heat extraction from a salinity-gradient solar pond using in pond heat exchanger. Applied Thermal Engineering, 26, 1858–1865. Kumar, A. and Kishore, V.V.N. (1999) Construction and operational experience of a 6000m2 solar pond at Kutch, India. Solar Energy, 65, 237–249. Leblanc, J., Akbarzadeh, A., Andrews, J., Lu, H. and Golding, P. (2011) Heat extraction methods from salinity-gradient solar ponds and introduction of a novel system of heat extraction for improved efficiency. Solar Energy, 85, 3103–3142. Lu, H. and Sandoval, J. (1993) Experienences of clarity monitoring and maintanance at the El Paso solar pond. In: Proceedings of 3rd International Conference on Progress in Solar Ponds, May 23–27, El Paso Texas, USA. 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Chapter 8 The solar thermal electrochemical production of energetic molecules: Step Stuart Licht Department of Chemistry, George Washington University, Washington, DC, USA 8.1 INTRODUCTION Anthropogenic release of carbon dioxide and atmospheric carbon dioxide have reached record levels. One path towards CO2 reduction is to utilize renewable energy to produce electricity. Another, less explored, path is to utilize renewable energy to directly produce societal staples such as metals, bleach, fuels, including carbonaceous fuels. Whereas solar-driven water splitting to generate hydrogen fuels has been extensively studied (Vayssieres 2009; Rajeshwar et al., 2008), there have been few studies of solar driven carbon dioxide splitting. “CO2 is a highly stable, noncombustible molecule, and its thermodynamic stability makes its activation energy demanding and challenging (Ohla et al., 2009).’’ In search of a solution for climate change associated with increasing levels of atmospheric CO2, the field of carbon dioxide splitting (solar or otherwise), while young, is growing rapidly, and as with water splitting, includes the study of photoelectrochemical, biomimetic, electrolytic, and thermal pathways of carbon dioxide splitting (Graves et al., 2011; Barber 2009). Recently we introduced a global process for the Solar Thermal Electrochemical Production (STEP) of energetic molecules, including CO2 splitting (Licht 2009; Licht et al., 2010; Licht 2011) as well as the solar production of metals, fuels, bleach and other staples (Licht 2009, Licht et al., 2010a; Licht et al., 2010b; Licht and Wang 2010; Licht 2011; Licht et al., 2011b; Licht, and Wu. 2011; Licht et al., 2011a). The direct thermal splitting of CO2 requires excessive temperatures to drive any significant dissociation. As a result, lower temperature thermochemical processes using coupled reactions have recently been studied (Stamatiou et al., 2010; Venstrom and Davidson 2011; Chueh and Haile, 2010; Miller et al., 2008). The coupling of multiple reactions steps decreases the system efficiency. To date, such challenges, and the associated efficiency losses, have been an impediment to the implementation of the related, extensively studied field of thermochemical splitting of water (Rajeshwar et al., 2008). Photoelectrochemistry probes the energetics of illuminated semiconductors in an electrolyte, and provides an alternative path to solar fuel formation. Photoelectrochemical solar cells (PECs) can convert solar energy to electricity, (Licht, 1987; Licht and Peramunage, 1990; Oregan and Gratzel, 1991; Licht, 1998; Licht, 2002) and with inclusion of an electrochemical storage couple, have the capability for internal energy storage, to provide a level output despite variations in sunlight (Licht et al., 1987; Licht et al., 1999). Solar to photoelectrochemical energy can also be stored 220 Solar energy sciences and engineering applications externally in chemical form, when it is used to drive the formation of energetically rich chemicals. Photochemical, and photoelectrochemical, splitting of carbon dioxide (Yan et al., 2011; Zhou et al., 2011; Richardson, Holland, Carpenter 2011; Barton et al., 2008; Kaneco et al., 2009; Pan and Chen, 2007) have demonstrated selective production of specific fuel products. Such systems function at low current density and efficiencies of ~1 percent, and as with photoelectrochemical water splitting face stability and bandgap challenges related to effective operation with visible light (Licht, 2002; Murphy, 2008; Currao, 2007). The electrically driven (nonsolar) electrolysis of dissolved carbon dioxide is under investigation at or near room temperature in aqueous, non-aqueous and PEM media (Narayanan, et al., 2011; Delacourt and Newman 2010; Dufek et al., 2011; Gangeri et al., 2009; Innocent et al., 2008; Wang et al., 2009; Chu et al., 2008; Yano et al., 2007; Hori et al., 2005; Ogura et al., 2004). These are constrained by the thermodynamic and kinetic challenges associated with ambient temperature, endothermic processes, of a high electrolysis potential, large overpotential, low rate and low electrolysis efficiency. High temperature, solid oxide electrolysis of carbon dioxide dates back to suggestions from 1960 to use such cells to renew air for a space habitat, (Martin, 1965; Chandler et al., 1966; Erstfield, 1979; Stancati et al., 1981; Richter, 1981; Mizusaki et al., 1992; Tao et al., 2004; Green et al., 2008) and the sustainable rate of the solid oxide reduction of carbon dioxide is improving rapidly (Meyers et al., 2011; Kim-Lohsoontorn et al., 2011; Ebbesen et al., 2010; Jensen et al., 2010; Fu et al., 2010; Stoots et al., 2010; Fu et al., 2010). Molten carbonate, rather solid oxide, fuel cells running in the reverse mode had also been studied to renew air in 2002 (Lueck et al., 2002). In a manner analogous to our 2002 high temperature solar water splitting studies and described below (Licht, 2002; Licht, 2003; Licht et al., 2003; Licht, 2005), we showed in 2009 that molten carbonate cells are particularly effective for the solar driven electrolysis of carbon dioxide, (Licht, 2009; Licht, Wang et al., 2010a; Licht et al., 2011a; Licht 2011) and also CO2-free iron metal production (Licht and Wang, 2010; Licht et al., 2011b; Licht, 2011). Light-driven water splitting was originally demonstrated with TiO2 (a seminconductor with a bandgap, Eg >3.0 eV) (Fujishima and Honda, 1972). However, only a small fraction of sunlight has sufficient energy to drive TiO2 photoexcitation. Studies had sought to tune (lower) the semiconductor bandgap to provide a better match to the electrolysis potential (Zou et al., 2001). In 2000, we used external multiple bandgap PVs (photovoltaics) to generate H2 by splitting water at 18% solar energy conversion efficiency (Licht et al., 2000; Licht, 2001). However, that room temperature process does not take advantage of additional, available thermal energy. An alternative to tuning a seminconductor bandgap to provide a better match to the solar spectrum, is an approach to tune (lower) the electrolysis potential (Licht et al., 2003; Licht, 2005). In 2002, we introduced a photo-electrochemical thermal water splitting theory, (Licht, 2002) which was verified by experiment in 2003, for H2 generation at over 30% solar energy conversion efficiency, and providing the first experimental demonstration that a semiconductor, such as Si (Eg =1.1 eV), with bandgap lower than the standard water-splitting potential (E. H2O(25.C)=1.23 V), can directly drive hydrogen formation (Licht et al., 2003; Licht, 2005). With increasing temperature, the quantitative decrease in the electrochemical potential to split water to hydrogen and oxygen had been well known by the 1950s (deBethune and Licht, 1959; The solar thermal electrochemical production of energetic molecules: Step 221 Chase, 1998). In 1976Wentworth and Chen wrote about “simple thermal decomposition reactions for storage of solar energy,’’ with the limitation that the products of the reaction must be separated to prevent back reaction (and without any electrochemical component), (Wentworth, 1976) and as early as 1980 it was noted that thermal energy could decrease the necessary energy for the generation of H2 by electrolysis (Bockris, 1980). However, the process combines elements of solid state physics, insolation and electrochemical theory, complicating rigorous theoretical support of the process. Our photo-electrochemical thermal water-splitting model for solar/H2 by this process, was the first derivation of bandgap restricted, thermal enhanced, high solar water-splitting efficiencies. The model, predicting solar energy conversion efficiencies that exceed those of conventional photovoltaics was initially derived for AM(Air Mass)1.5, terrestrial insolation, and later expanded to include sunlight above the atmosphere (AM0 insolation) (Licht, 2002; Licht, 2003). The experimental accomplishment followed, and established that the water-splitting potential can be specifically tuned to match efficient photo-absorbers, (Licht et al., 2003; Licht, 2005) eliminating the challenge of tuning (varying) the semiconductor bandgap, and which can lead to over 30% solar to chemical energy conversion efficiencies. Our early process was specific to H2 and did not incorporate the additional temperature enhancement of excess super-band gap energy and concentration enhancement of excess reactant to further decrease the electrolysis potential, in our contemporary STEP process. 8.2 SOLAR THERMAL ELECTROCHEMICAL PRODUCTION OF ENERGETIC MOLECULES: AN OVERVIEW 8.2.1 STEP theoretical background A single, small band gap junction, such as in a silicon PV, cannot generate the minimum photopotential required to drive many room temperature electrolysis reactions, as shown in the left of Scheme 8.2.1. The advancement of such studies had focused on tuning semiconductor bandgaps (Zou et al., 2001) to provide a better match to the electrochemical potential (specifically, the water-splitting potential), or by utilizing more complex, multiple bandgap structures using multiple photon excitation (Licht et al., 2000; Licht 2001). Either of these structures are not capable of excitation beyond the bandedge and cannot make use of longer wavelength sunlight. Photovoltaics are limited to super-bandgap sunlight, h.>Eg, precluding use of long wavelength radiation, h.1000.C) with concentrations of~2000 suns. Beam splitters can redirect sub-bandgap radiation away from the PV (minimzing PV heating) for a direct heat exchange with the electrolyzer. Solar heating can decrease the energy to drive a range of electrolyses. Such processes can be determined using available entropy, S, and enthalpy, H, and free-energy, G, data, (Chase, 1998) and are identified by their negative isothermal temperature coefficient of the cell potential (deBethune and 1959). This coefficient (dE/dT)isoth is the derivative of the electromotive force of the isothermal cell: (dE/dT)isoth = S/nF = (H - G)/nFT (8.2.1) The starting process of modeling any STEP process is the conventional expression of a generalized electrochemical process, in a cell which drives an n-electron charge transfer electrolysis reaction, comprising “x’’ reactants, Ri, with stoichiometric coefficients ri, and yielding “y’’ products, Ci, with stoichiometric coefficients ci. n-electron refers to the number of electrons gained to form the cathode products and lost to form the anode products in the electrolysis reaction. Electrode 1 | Electrolyte | Electrode 2 Using the convention of E=Ecathode -Eanode to describe the positive potential necessary to drive a non-spontaneous process, by transfer of n electrons in the electrolysis reaction: n-electron transfer electrolysis reaction: x i=1 riRi . y i=1 ciCi (8.2.2) 224 Solar energy sciences and engineering applications At any electrolysis temperature, TSTEP, and at unit activity, the reaction has electrochemical potential, E. T. This may be calculated from consistent, compiled unit activity thermochemical data sets, such as the NIST condensed phase and fluid properties data sets, (Chase 1998) as: E. T = -G.(T = TSTEP)/nF; E. ambient = E. T(Tambient); here Tambient = 298.15K = 25.C, and: G.(T = TSTEP) = y i=1 ci(H.(Ci, T) - TS.(Ci, T)) - x i=1 ri(H.(Ri, T) - TS.(Ri, T)) (8.2.3) Compiled thermochemical data are often based on different reference states, while a consistent reference state is needed to understand electrolysis limiting processes, including water (Light et al., 2005; Licht, 1987). This challenge is overcome by modification of the unit activity (a=1) consistent calculated electrolysis potential to determine the potential at other reagent and product relative activities via the Nernst equation (Licht, 1985; Licht et al., 1991). Electrolysis provides control of the relative amounts of reactant and generated product in a system. A substantial activity differential can also drive STEP improvement at elevated temperature, and will be derived. The potential variation with activity, a, of the reaction: xi =1riRi. y i=1ciCi, is given by: ET,a = E. T -  RT nF  · ln $xi =1 a(Ri)ri $y i=1 a(Ci)ci  (8.2.4) Electrolysis systems with a negative isothermal temperature coefficient tend to cool as the electrolysis products are generated. Specifically in endothermic electrolytic processes, the Equation 8.2.4 free-energy electrolysis potential, ET, is less than the enthalpy based potential. This latter value is the potential at which the system temperature would remain constant during electrolysis. This thermoneutral potential, Etn, is given by: Etn(TSTEP) = -H(T) nF ; (8.2.5) H(TSTEP) = b i=1 ciH(Ci, TSTEP) - a i=1 riH(Ri, TSTEP) Two general STEP implementations are being explored. Both can provide the thermoneutral energy to sustain a variety of electrolyses. The thermoneutral potential, determined from the enthalpy of a reaction, describes the energy required to sustain an electrochemical process without cooling. For example, the thermoneutral potential we have calculated and reported for CO2 splitting to CO and O2 at unit activities, from Equation 8.2.5, is 1.46(±0.01)V over the temperature range of 25–1400.C. As represented in Scheme 8.2.3 on the left, the standard electrolysis potential at room The solar thermal electrochemical production of energetic molecules: Step 225 Scheme 8.2.3 Comparison of solar energy utilization in STEP and Hy-STEP implementations of the solar thermal electrochemical production of energetic molecules. Modified with permission from Licht 2011. temperature, E., can comprise a significant fraction of the thermoneutral potential. The first STEP mode, energetically represented next to the room temperature process in the scheme, separates sunlight into thermal and visible radiation. The solar visible generates electronic charge which drives electrolysis charge transfer. The solar thermal component heats the electrolysis and decreases both the E. at this higher T, and the overpotential. The second mode, termed Hy-STEP (on the right) from “hybrid-STEP’’, does not separate sunlight, and instead directs all sunlight to heating the electrolysis, generating the highest T and smallest E, while the electrical energy for electrolysis is generated by a separate source (such as by photovoltaic, solar thermal electric, wind turbine, hydro, nuclear or fossil fuel generated electronic charge). As shown on the right side, high relative concentrations of the electrolysis reactant (such as CO2 or iron oxide will further decrease the electrolysis potential). 8.2.2 STEP solar to chemical energy conversion efficiency The Hy-STEP mode is being studied outdoors with wind or solar CPV generated electricity to drive Eelectrolysis. The STEP mode is experimentally more complex and is presently studied indoors under solar simulator illumination. Determination of the efficiency of Hy-STEP with solar electric is straightforward in the domain in which Eelectrolysis 0.67 eV) sunlight. A portion of this>~250kWm-2 available energy, is extracted through heat exchange at the backside of the CPV. Another useful source for consideration as supplemental heat is industrial exhaust. The temperature of industrial flue stacks varies widely, with fossil fuel source and application, and ranges up to 650.C for an open circuit gas turbine. The efficiency of thermal energy transfer will limit use of this available heat. A lower limit to the STEP efficiency is determined when no heat is recovered, either from the CPV or remaining solar IR, and when heat is not recovered via heat exchange from the electrolysis products, and when an external heat source is used to maintain a constant electrolysis temperature. In this case, the difference between the electrolysis potential and the thermoneutral potential represents the enthalpy required to keep the system from cooling. In this case, our 0.9V electrolysis occurs at an efficiency of (0.90 V/1.46 V) · 54.7%=34%. While the STEP energy analysis, detailed in Section 8.4.2 for example for CO2 to CO splitting, is more complex than that of the Hy-STEP mode, more solar thermal energy is available including a PV’s unused or waste heat to drive the process and to improve the solar to chemical energy conversion efficiency. We determine the STEP solar efficiency over the range from inclusion of no solar thermal heat (based on the enthalpy, rather than free energy, of reaction) to the case where the solar thermal heat is sufficient to sustain the reaction (based on the free energy of reaction). This determines the efficiency range, as chemical flow out to the solar flow in (as measured by the increase in chemical energy of the products compared to the reactants), from 34% to over 50%. 8.2.3 Identification of STEP consistent endothermic processes The electrochemical driving force for a variety of chemicals of widespread usewill be shown to significantly decrease with increasing temperature. As calculated and summarized in the top left of Figure 8.2.1, the electrochemical driving force for electrolysis of either carbon dioxide or water, significantly decreases with increasing temperature. The ability to remove CO2 from exhaust stacks or atmospheric sources, provides a response to linked environmental impacts, including global warming due to anthropogenic CO2 emission.From the known thermochemical data for CO2, CO and O2, and in accord with Equation 8.2.1, CO2 splitting can be described by: CO2 (g) . CO (g) + 1/2O2 (g); E. CO2-split = (G. CO + 0.5G. O2 - G. CO2 )/2F; E. (25.C) = 1.333V (8.2.9) As an example of the solar energy efficiency gains, this progress report focuses on CO2 splitting potentials, and provides examples of other useful STEP processes. As seen in Figure 8.2.1, CO2 splitting potentials decrease more rapidly with temperature than those for water splitting, signifying that the STEP process may be readily applied to CO2 electrolysis. Efficient, renewable, non-fossil fuel energy-rich carbon sources are The solar thermal electrochemical production of energetic molecules: Step 231 needed, and the product of Equation 8.2.9, carbon monoxide is a significant industrial gas with a myriad of uses, including the bulk manufacturing of hydrocarbon fuels, acetic acid and aldehydes (and detergent precursors), and for use in industrial nickel purification (Elschenbroich and Salzer, 1992). To alleviate challenges of fossil fuel resource depletion, CO is an important syngas component and methanol is formed through the reaction with H2. The ability to remove CO2 from exhaust stacks or atmospheric sources, also limits CO2 emission. Based on our original analogous experimental photo-thermal electrochemical water electrolysis design, (Licht et al., 2003; Licht, 2005) the first CO2 STEP process consists of solar driven and solar thermal assisted CO2 electrolysis. In particular, in a molten carbonate bath electrolysis cell, fed by CO2. cathode: 2CO2 (g) + 2e-.CO= 3 (molten) + CO (g) anode: CO= 3 (molten) . CO2 (g) + 1/2O2 (g) + 2e- cell: CO2 (g) . CO (g) + 1/2O2 (g) (8.2.10) Molten alkali carbonate electrolyte fuel cells typically operate at 650.C. Li, Na or K cation variation can affect charge mobility and operational temperatures. Sintered nickel often serves as the anode, porous lithium doped nickel oxide often as the cathode, while the electrolyte is suspended in a porous, insulating, chemically inert LiAlO2 ceramic matrix (Sunmacher, 2007). Solar thermal energy can be used to favor the formation of products for electrolyses characterized by a negative isothermal temperature coefficient, but will not improve the efficiency of heat neutral or exothermic reactions. An example of this restriction occurs for the electrolysis reaction currently used by industry to generate chlorine. During 2008, the generation of chlorine gas (principally for use as bleach and in the chloralkali industry) consumed approximately 1% of the world’s electricity, (Pellegrino, 2000) prepared in accord with the industrial electrolytic process: 2NaCl + 2H2O . Cl2 + H2 + 2NaOH; E. (25.C) = 2.502V (8.2.11) In the lower left portion of Figure 8.2.1, the calculated electrolysis potential for this industrial chlor-alkali reaction exhibits little variation with temperature, and hence the conventional generation of chlorine by electrolysis would not benefit from the inclusion of solar heating. This potential is relatively invariant, despite a number of phase changes of the components (indicated on the figure and which include the melting of NaOH or NaCl). However, as seen in the figure, the calculated potential for the anhydrous electrolysis of chloride salts is endothermic, including the electrolyses to generate not only chlorine, but also metallic lithium, sodium and magnesium, and can be greatly improved through the STEP process: MCln . n/2Cl2 + M; (8.2.12) E. MCl-split (25.C) = 3.98V-M = Na, 4.24V-K, 3.98V-Li, 3.07V-Mg The calculated decreases for the anhydrous chloride electrolysis potentials are in the order of volts per 1000.C temperature change. For example, from 25.C up to the 232 Solar energy sciences and engineering applications MgCl2 boiling point of 1412.C, the MgCl2 electrolysis potential decreases from 3.07V to 1.86V. This decrease provides a theoretical basis for significant, non-CO2 emitting, non-fossil fuel consuming processes for the generation of chlorine and magnesium, to be delineated in Section 8.3.4, and occurring at high solar efficiency analogous to the similar CO2 STEP process. In Section 8.3.2 the STEP process will be derived for the efficient solar removal/recycling of CO2. In addition, thermodynamic calculation of metal and chloride electrolysis rest potentials identifies electrolytic processes which are consistent with endothermic processes for the formation of iron, chlorine, aluminum, lithium, sodium and magnesium, via CO2–free pathways. As shown, the conversion and replacement of the conventional, aqueous, industrial alkali-chlor process, with an anhydrous electrosynthesis, results in a redox potential with a calculated decrease of 1.1V from 25.C to 1000.C. As seen in the top right of Figure 8.2.1, the calculated electrochemical reduction of metal oxides can exhibit a sharp, smooth decrease in redox potential over a wide range of phase changes. These endothermic process provide an opportunity for the replacement of conventional industrial processes by the STEP formation of these metals. In 2008, industrial electrolytic processes consumed ~5% of the world’s electricity, including for aluminum (3%), chlorine (1%), and lithium, magnesium and sodium production. This 5% of the global 19×1012 kWh of electrical production, is equivalent to the emission of 6×108 metric tons ofCO2 (Pellegrino, 2000). The iron and steel industry accounts for a quarter of industrial direct CO2 emissions. Currently, iron is predominantly formed through the reduction of hematite with carbon, emitting CO2: Fe2O3 + 3C + 3/2O2 . 2Fe + 3CO2 (8.2.13) A non-CO2 emitting alternative is provided by the STEP driven electrolysis of Fe2O3: Fe2O3 . 2Fe + 3/2O2 E. = 1.28V (8.2.14) As seen in the top right of Figure 8.2.1, the calculated iron-generating electrolysis potential drops 0.5V (a 38% drop) from 25.C to 1000.C, and as with the CO2 analogue, will be expected to decrease more rapidly with non-unit activity conditions, as will be delineated in a future study. Conventional industrial processes for these metals and chlorine, along with CO2 emitted from power and transportation, are responsible for the majority of anthropogenic CO2 release. The STEP process, to efficiently recover carbon dioxide and in lieu of these industrial processes, can provide a transition beyond the fossil fuel-electric grid economy. The top left of Figure 8.2.1 includes calculated thermoneutral potentials for CO2 and water-splitting reactions. At ambient temperature, the difference between Eth and ET does not indicate an additional heat requirement for electrolysis, as this heat is available via heat exchange with the ambient environment. At ambient temperature, Etn -ET for CO2 or water is respectively 0.13 and 0.25V, is calculated (not shown) as 0.15±0.1V for Al2O3 and Fe2O3, and 0.28±0.3V for each of the chlorides. We find that molten electrolytes present several fundamental advantages compared to solid oxides for CO2 electrolysis. (i) Molten carbonate electrolyzer provides 103 to 106 times higher concentration of reactant at the cathode surface than a solid The solar thermal electrochemical production of energetic molecules: Step 233 oxide electrolyzer. Solid oxides utilize gas phase reactants, whereas carbonates utilize molten phase reactants. Molten carbonate contains 2×10-2 mol reducible tetravalent carbon/cm3. The density of reducible tetravalent carbon sites in the gas phase is considerably lower. Air contains 0.03% CO2, equivalent to only 1×10-8 mol of tetravalent carbon/cm3, and flue gas (typically) contains 10–15% CO2, equivalent to 2×10-5 mol reducible C(IV)/cm3. Carbonate’s higher concentration of active, reducible tetravalent carbon sites, logarithmically decreases the electrolysis potential, and can facilitate charge transfer at low electrolysis potentials. (ii) Molten carbonates can directly absorb atmospheric CO2, whereas solid oxides require an energy consuming pre-concentration process. (iii) Molten carbonates electrolyses are compatible with both solid and gas phase products. (iv) Molten processes have an intrinsic thermal buffer not found in gas phase systems. Sunlight intensity varies over a 24-hour cycle, and more frequently with variations in cloud cover. This disruption to other solar energy conversion processes is not necessary in molten salt processes. For example as discussed in Section 8.4.3, the thermal buffer capacity of molten salts has been effective for solar to electric power towers to operate 24/7. These towers concentrate solar thermal energy to heat molten salts, which circulate and via heat exchange boil water to drive conventional mechanical turbines. 8.3 DEMONSTRATED STEP PROCESSES 8.3.1 STEP hydrogen STEP occurs at both higher electrolysis and higher solar conversion efficiencies than conventional room temperature photovoltaic (PV) generation of hydrogen. Experimentally, we demonstrated a sharp decrease in the water splitting potential in an unusual molten sodium hydroxide medium, Figure 8.3.1, and as shown in Figure 8.3.2, three series connected Si CPVs efficiently driving two series molten hydroxide water-splitting cells at 500.C to generate hydrogen (Licht et al., 2003; Licht, 2005). Recently we have considered the economic viability of solar hydrogen fuel production. That study provided evidence that the STEP system is an economically viable solution for the production of hydrogen (Licht et al., 2003; Licht, 2005). 8.3.2 STEP carbon capture In this process carbon dioxide is captured directly, without the need to pre-concentrate dilute CO2, using a high temperature electrolysis cell powered by sunlight in a single step. Solar thermal energy decreases the energy required for the endothermic conversion of carbon dioxide and kinetically facilitates electrochemical reduction, while solar visible generates electronic charge to drive the electrolysis. CO2 can be captured as solid carbon and stored, or used as carbon monoxide to feed chemical or synthetic fuel production. Thermodynamic calculations are used to determine, and then demonstrate, a specific low energy, molten carbonate salt pathway for carbon capture. Prior investigations of the electrochemistry of carbonates in molten salts tended to focus on reactions of interest to fuel cells, (Sunmacher, 2007) rather than the (reverse) electrolysis reactions of relevance to the STEP reduction of carbon dioxide, typically in 234 Solar energy sciences and engineering applications Figure 8.3.1 VH2O, measured in aq.saturated or molten NaOH, at 1 atm. Steam is injected in the molten electrolyte. O2 anode is 0.6 cm2 Pt foil. IR and polarization losses are minimized by sandwiching 5mm from each side of the anode, oversized Pt gauze cathode. Inset:At 25.C, 3 electrode values comparing Ni and Pt working electrodes and with a Pt gauze counterelectrode at 5 mV/s. Modified with permission from Licht 2011. alkali carbonate mixtures. Such mixtures substantially lower the melting point compared to the pure salts, and would provide the thermodynamic maximum voltage for fuel cells. However, the electrolysis process is maximized in the opposite temperature domain of fuel cells, that is at elevated temperatures which decrease the energy of electrolysis, as schematically delineated in Scheme 8.2.1. These conditions provide a new opportunity for effective CO2 capture. CO2 electrolysis splitting potentials are calculated from the thermodynamic free energy components of the reactants and products (Licht, 2009; Licht et al., 2010a; Chase, 1998) as E=-G(reaction)/nF, where n=4 or 2 for the respective conversion of CO2 to the solid carbon or carbon monoxide products. As calculated using the available thermochemical enthalpy and entropy of the starting components, and as summarized in the left side of Figure 8.3.3, molten Li2CO3, via a Li2 O intermediate, provides a preferred, low energy route compared to Na2CO3 or K2CO3 (via Na2O or K2O), for the conversion of CO2. High temperature is advantageous as it decreases the free energy energy necessary to drive the STEP enodthermic process. The carbonates, Li2CO3, Na2CO3 and K2CO3, have respective melting points of 723.C, 851.C and 891.C. Molten Li2CO3 not only requires lower thermodynamic electrolysis energy, but in addition has higher conductivity (6 S cm-1) than that of Na2CO3 (3 S cm-1) or K2CO3 (2 S cm-1) near the melting point (Zhang and The solar thermal electrochemical production of energetic molecules: Step 235 Figure 8.3.2 Photovoltaic and electrolysis charge transfer of STEP hydrogen using Si CPV’s driving molten NaOH water electrolysis. Photocurrent is shown for 1, 2 or 3 1.561 cm2 HECO 335 Sunpower Si photovoltaics in series at 50 suns. The CPV’s drive 500.C molten NaOH steam electrolysis using Pt gauze electrodes. Left inset: electrolysis current stability. Modified with permission from Licht 2011. Wang, 2006). Higher conductivity is desired as it leads to lower electrolysis ohmic losses. Low carbonate melting points are achieved by a eutectic mix of alkali carbonates (Tmp Li1.07Na0.93CO3: 499.C; Li0.85Na0.61K0.54CO3: 393.C). Mass transport is also improved at higher temperature; the conductivity increases from 0.9 to 2.1 S cm-1 with temperature increase from 650.C to 875.C for a 1:1:1 by mass mixture of the three alkali carbonates (Kojima et al., 2008). In 2009 we showed that molten carbonate electrolyzers can provide an effective media for solar splitting of CO2 at high conversion efficiency. In 2010 Kaplan, et al., and our group separately reported that molten lithiated carbonates provide a particularly effective medium for the electrolytsis reduction of carbon dioxide (Licht et al., 2010a; Kaplan et al., 2010). As we show in the photograph in Figure 8.3.3, at 750.C, carbon dioxide is captured in molten lithium carbonate electrolyte as solid carbon by reduction at the cathode at low electrolysis potential. It is seen in the cyclic voltammetry, CV, that a solid carbon peak that is observed at 750.C is not evident at 950.C. At temperatures less than ~900.C in the molten electrolyte, solid carbon is the preferred CO2 splitting product, while carbon monoxide is the preferred product at higher temperature. As seen in the main portion of the figure, the electrolysis potential is <1.2V at either 0.1 or 0.5 A/cm2, respectively at 750 or 850.C. Hence, 236 Solar energy sciences and engineering applications Figure 8.3.3 The calculated (left) and measured (right) electrolysis of CO2 in molten carbonate. Left: The calculated thermodynamic electrolysis potential for carbon capture and conversion in Li2CO3 (main figure), or Na2CO3 or K2CO3 (left middle); squares refer to M2CO3 to C+M2O+O2 and circles to a M2CO3 to CO+M2O+1/2O2.To the left of the vertical brown line, solid carbon is the thermodynamically preferred (lower energy) product. To the right of the vertical line, CO is preferred. Carbon dioxide fed into the electrolysis chamber is converted to solid carbon in a single step. Photographs: coiled platinum cathode before (left), and after (right),CO2 splitting to solid carbon at 750.C in molten carbonate with a Ni anode. Right: The electrolysis full cell potential is measured, under anode or cathode limiting conditions, at a platinum electrode for a range of stable anodic and cathodic current densitites in molten Li2CO3. Lower midde: cathode size restricted full cell cyclic voltammetry, CV, of Pt electrodes in molten Li2CO3. Modified with permission from Licht et al. 2010a. the electrolysis energy required at these elevated, molten temperatures is less than the minimum energy required to split CO2 to CO at 25.C: CO2 . CO + 1/2 O2 E.(T = 25.C) = 1.33V (8.3.1) The observed experimental carbon capture correlates with: Li2CO3 (molten) . C (solid) + Li2O (dissolved) + O2 (gas) (8.3.2A) Li2CO3 (molten) . CO (gas) + Li2O (dissolved) + 1/2 O2 (gas) (8.3.2B) When CO2 is bubbled in, a rapid reaction back to the original lithium carbonate is strongly favored: Li2O (dissolved) + CO2 (gas) . Li2CO3 (molten) (8.3.3A) Li2CO3 . Li2O + CO2 (8.3.3B) In the presence of carbon dioxide, reaction (8.3.3A) is strongly favored (exothermic), and the rapid reaction back to the original lithium carbonate occurs while CO2 is bubbled into molten lithium carbonate containing the lithium oxide. The solar thermal electrochemical production of energetic molecules: Step 237 The carbon capture reaction in molten carbonate, combines Equations 8.3.2 and 8.3.3: CO2 (gas) . C (solid) + O2 (gas) T = 900.C (8.3.4A) CO2 (gas) . CO (gas) + 1/2O2 (gas) T = 950.C (8.3.4B) The electrolysis of carbon capture in molten carbonates can occur at lower experimental electrolysis potentials than the unit activity potentials calculated in Figure 8.3.3. A constant influx of carbon dioxide to the cell maintains a low concentration of Li2O, in accord with reaction 23. The activity ratio, , of the carbonate reactant to the oxide product in the electrolysis chamber, when high, decreases the cell potentials with the Nernst concentration variation of the potential in accord with Equation 8.3.2, as: ECO2/X(T) = E. CO2/X(T) - 0.0592V · T(K)/(n · 298K) · log(); n = 4 or 2, for X = Csolid or CO product (8.3.5) For example, from Equation 8.3.5, the expected cell potential at 950.C for the reduction to the CO product is ECO2/CO =1.17V-(0.243 V/2) · 4=0.68V, with a high =10,000 carbonate/oxide ratio in the electrolysis chamber. As seen in the Figure 8.3.3 photograph, CO2 is captured in 750.C Li2CO3 as solid carbon by reduction at the cathode at low electrolysis potential. The carbon formed in the electrolysis in molten Li2CO3 at 750.C is in quantitative accord with the 4 e- reduction of Equation 8.3.2, as determined by (i) mass, at constant 1.25 A for both 0.05 and 0.5 A/cm2 (large and small electrode) electrolyses (the carbon is washed in a sonicator, and dried at 90.C), by (ii) ignition (furnace combustion at 950.C) and by (iii) volumetric analysis in which KIO3 is added to the carbon, converted to CO2 and I2 in hot phosphoric acid (5C+4KIO3 +4H3PO4.5CO2 +2I2 +2H2O+4KH2PO4), the liberated I2 is dissolved in 0.05 M KI and titrated with thiosulfate using a starch indicator. We also observe the transition to the carbon monoxide product with increasing temperature. Specifically, while at 750.C the molar ratio of solid carbon to CO-gas formed is 20:1, at 850. in molten Li2CO3, the product ratio is a 2:1, at 900.C, the ratio is 0.5:1, and at 950.C the gas is the sole product. Hence, in accord with Figure 8.2.2, switching between the C or CO product is temperature programmable. We have replaced Pt, with Ni, nickel alloys (inconel and monel), Ti and carbon, and each are effective carbon capture cathode materials. Solid carbon deposits on each of these cathodes at similar overpotential in 750.C molten Li2CO3. For the anode, both platinum and nickel are effective, while titanium corrodes under anodic bias in molten Li2CO3. As seen in the right side of Figure 8.3.3, electrolysis anodic overpotentials in Li2CO3 electrolysis are comparable, but larger than cathodic overpotentials, and current densities of over 1Acm-2 can be sustained. Unlike other fuel cells, carbonate fuel cells are resistant to poisoning effects, (Sunmacher, 2007) and are effective with a wide range of fuels, and this appears to be the same for the case in the reverse mode (to capture carbon, rather than to generate electricity). Molten Li2CO3 remains transparent and sustains stable electrolysis currents after extended (hours/days) carbon capture over a wide range of electrolysis current densities and temperatures. 238 Solar energy sciences and engineering applications Figure 8.3.4 Left: Species stability in the lithium carbonate, lithium oxide, carbon dioxide system, as calculated from Li2CO3, Li2O, and CO2 thermochemical data. Right:Thermogravimetric analysis of lithium carbonate. The measured mass loss in time of Li2CO3. Not shown: The Li2CO3 mass loss rate also decreases with an increasing ratio of Li2CO3 mass to the surface area of the molten salt exposed to the atmosphere. This increased ratio,may increase the released partial pressure of CO2 above the surface, increase the rate of the back reaction (Li2O+CO2.Li2CO3), and therefore result in the observed decreased mass loss. Hence, under an open atmosphere at 950.C, the mass loss after 5 hours falls from 7% to 4.7%, when the starting mass of pure Li2CO3 in the crucible is increased from 20 to 50 g. Under these latter conditions (open atmosphere, 950.C,50 g total electrolyte), but using a 95% Li2CO3, 5% Li2O mix, the rate of mass loss is only 2.3%. Modified with permission from Licht et al. 2011a. As delineated in Section 8.2.3, in practice, either STEP or Hy-STEP modes are useful for efficient solar carbon capture. CO2 added to the cell is split at 50% solar to chemical energy conversion efficiency by series coupled lithium carbonate electrolysis cells driven at maximum power point by an efficient CPC. Experimentally, we observe the facile reaction of CO2 and Li2O in molten Li2CO3. We can also calculate the thermodynamic equilibrium conditions between the species in the system, Equation 8.2.3B. Using the known thermochemistry of Li2O, CO2 and Li2CO3, (Chase, 1998) we calculate the reaction free-energy of Equation 8.2.1, and from this calculate the thermodynamic equilibrium constant as a function of temperature. From this equilibrium constant, the area above the curve on the left side of Figure 8.3.4 presents the wide domain (above the curve) in which Li2CO3 dominates, that is where excess CO2 reacts with Li2O such that pCO2 · aLi2O 1, would further decrease the thermodynamic potential to produce iron. The measured electrolysis potential is presented on the right of Figure 8.3.5 for dissolved Fe(III) in molten Li2CO3, and is low. For example 0.8V sustains a current density of 500mAcm-2 in 14m Fe(III) in Li2CO3 at 950.C. Higher temperature, and higher concentration, lowers the electrolysis voltage, which can be considerably less than the room potential required to convert Fe2O3 to iron and oxygen. When an external source of heat, such as solar thermal, is available then the energy savings over room temperature iron electrolysis are considerable. Electrolyte stability is regulated through control of the CO2 pressure and/or by dissolution of excess Li2O. Electrolyte mass change was measured in 7m LiFeO2 & 3.5m Li2O in molten Li2CO3 after 5 hours. Under argon there is a 1, 5 or 7wt% loss respectively at 750.C, 850.C or 950.C), through CO2 evolution. Little loss occurs under air (0.03% CO2), while under pure CO2 the electrolyte gains 2–3wt% (external CO2 reacts with dissolved Li2O to form Li2CO3). The endothermic nature of the new synthesis route, that is the decrease in iron electrolysis potential with increasing temperature, provides a low free energy opportunity for the STEP process. In this process, solar thermal provides heat to decrease the iron electrolysis potential, Figure 8.3.5, and solar visible generates electronic charge to drive the electrolysis. A low energy route for the carbon dioxide free formation of iron metal from iron ores is accomplished by the synergistic use of both visible and infrared sunlight. This provides high solar energy conversion efficiencies, Figure 8.2.2, when applied to Equations 8.2.14 and (8.3.6) 20 in a molten carbonate electrolyte.We again use a 37% solar energy conversion efficient concentrator photovoltaic (CPV) as The solar thermal electrochemical production of energetic molecules: Step 243 Figure 8.3.7 STEP and (wind) Hy-STEP iron. Left:STEP iron production in which two molten carbonate electrolysis in series are driven by a concentrator photovoltaic.The 2.7V maximum power of the CPV can drive either two 1.35V iron electrolyses at 800.C (schematically represented), or three 0.9V iron electrolyses at 950.C.At 0.9V,rather than at E.(25.C)=1.28V, there is a considerably energy savings, achieved through the application of external heat, including solar thermal, to the system. Right: The Hy-STEP solar thermal/wind production of CO2 free iron. Concentrated sunlight heats, and wind energy drives electronic transfer into the electrolysis chamber. The required wind powered electrolysis energy is diminished by the high temperature and the high solubility of iron oxide. Bottom: Iron is produced at high current density and low energy at an iron cathode and with a Ni anode in 14m Fe2O3 +14m Li2O dissolved in molten Li2CO3. Modified with permission from Licht et al. 2011b. a convenient power source to drive the low electrolysis energy iron deposition without CO2 formation in Li2CO3, (Licht, 2009) as schematically represented in Figure 8.3.7. A solar/wind Hybrid Solar Thermal Electrochemical Production iron electrolysis process is also demonstrated (Licht et al., 2011b). In lieu of solar electric, electronic energy can be provided by alternative renewables, such as wind. As shown on the right side of Figure 8.3.7, in this Hy-STEP example, the electronic energy is driven by a wind turbine and concentrated sunlight is only used to provide heat to decrease the energy required for iron splitting. In this process, sunlight is concentrated to provide effective heating, but is not split into separate spectral regions as in our alternative implmentation. Hy-STEP iron production is measured with a 31.5 ×44.5 Fresnel lens (Edmund Optics) which concentrates sunlight to provide temperatures of over 950.C, and a Sunforce-44444 400W wind turbine provides electronic charge, charging series nickel metal hydride, MH, cells at 1.5 V). EachMHcell, provides a constant discharge potential of 1.0–1.3V, which are each used to drive one or two series connected iron electrolysis cells as indicated in the right side of Figure 8.3.7, containing 14m Fe(III) molten Li2CO3 electrolysis cells. Electrolysis current is included in the lower right of Figure 8.3.7. Iron metal is produced. Steel (iron containing carbon) may be directly formed via the concurrent reduction ofCO2, as will be delineated in an expanded study. 244 Solar energy sciences and engineering applications 8.3.4 STEP chlorine and magnesium production (chloride electrolysis) The predominant salts in seawater (global average 3.5±0.4% dissolved salt by mass) are NaCl (0.5 M) and MgCl2 (0.05 M). The electrolysis potential for the industrial chlor-alkali reaction exhibits little variation with temperature, and hence the conventional generation of chlorine by electrolysis, Equation 8.2.11, would not benefit from the inclusion of solar heating (Licht, 2009). However, when confined to anhydrous chloride splitting, as exemplified in the lower portion of Figure 8.2.1, the calculated potential for the anhydrous electrolysis of chloride salts is endothermic for the electrolyses, which generate a chlorine and metal product. The application of excess heat, as through the STEP process, decreases the energy of electrolysis and can improve the kinetics of charge tranfer for the Equation 8.2.12 range of chloride splitting processes. The thermodynamic electrolysis potential for the conversion of NaCl to sodium and chlorine decreases, from 3.24V at the 801.C melting point, to 2.99V at 1027.C (Licht, 2009). Experimentally, at 850.C in molten NaCl, we observe the expected, sustained generation of yellow-green chlorine gas at a platinum anode and of liquid sodium (mp 98 .C) at the cathode. Electrolysis of a second chloride salt, MgCl2, is also of particular interest. The magnesium, as well as the chlorine, electrolysis products are significant global commodities. Magnesium metal, the third most commonly used metal, is generally produced by the reduction of calcium magnesium carbonates by ferrosilicons at high temperature, (Li and Xie, 2005) which releases substantial levels of carbon dioxide contributing to the anthropogenic greenhouse effect. However, traditionally, magnesium has also been produced by the electrolysis of magnesium chloride, using steel cathodes and graphite anodes, and alternative materials have been invesitgated (Demirci and Karakaya, 2008). Of significance here to the STEP process is the highly endothermic nature of anhydrous chloride electrolysis, such as for MgCl2 electrolysis, in which solar heat will also decrease the energy (voltage) needed for the electrolysis. The rest potential for electrolysis of magnesium chloride decreases from 3.1V, at room temperature, to 2.5V at the 714.C melting point. As seen in Figure 8.3.8, the calculated thermodynamic potential for the electrolysis of magnesium chloride continues to decrease with increasing temperature, to ~2.3V at 1000.C. The 3.1V energy stored in the magnesium and chlorine room temperature products, when formed at 2.3V, provide an energy savings of 35%, if sufficient heat applied to the process can sustain this lower formation potential. Figure 8.3.8 also includes the experimental decrease in the MgCl2 electrolysis potential with increasing temperature in the lower right portion. In the top portion of the figure, the concurrent shift in the cyclic voltammogram is evident, decreasing the potential peak of magnesium formation, with increasing temperature from 750.C to 950.C. Sustained electrolysis and generation of chlorine at the anode and magnesium at the cathode (Figure 8.3.8, photo inset) is evident at platinum electrodes. The measured potential during constant current electrolysis at 750.C in molten MgCl2 at the electrodes is included in the figure. In the magnesium chloride electrolysis cell, nickel electrodes yield similar results to platinum, and can readily be used to form larger electrodes. The nickel anode sustains extended chlorine evolution without evident deterioration; the nickel cathode may slowly alloy with deposited magnesium. The magnesium product forms both as The solar thermal electrochemical production of energetic molecules: Step 245 Figure 8.3.8 Photograph lower left: coiled platinum before (left), and after (right), MgCl2 electrolysis forming Mg metal on the cathode (shown) and evolving chlorine gas on the anode. Main figure: cathode size restriced cyclic voltammetry of Pt electrodes in molten MgCl2. Inset: The measured full cell potential during constant current electrolysis at 750.C in molten MgCl2. Lower right:Thermodynamic and measured electrolysis potentials in molten MgCl2 as a function of temperature. Electrolysis potentials are calculated from the thermodynamic free energies components of the reactants and products as E=-G(reaction)/2F. Measured electrolysis potentials are stable values on Pt at 0.250A/cm2 cathode (Licht et al., 2011a). Lower right:A schematic representation of a separate (i) solar thermal and (ii) photovoltaic field to drive both water purification, hydrogen generation, and the endothermic electrolysis of the separated salts to useful products. Modified with permission from Licht et al. 2011a. the solid and liquid (Mg mp 649.C). The liquid magnesium is less dense than the electrolyte, floats upwards, and eventually needs to be separated and removed to prevent an inter-electrode short, or to prevent a reaction with chlorine that is evolved at the anode. In a scaled-up cell configuration (not shown in Figure 8.3.8, a larger Ni cathode (200 cm2 cylindrical nickel sheet (McMaster 9707K35) was employed, sandwiched between two coupled cylindrical Ni sheet anodes (total 200 cm2, of area across from the cathode) in a 250 ml alumina (Adavalue) crucible, and sustains multiamp large ampere currents. The potential at constant current is initially stable, but this cell configuration leads to electrical shorts, unless liquid magnesium is removed. 246 Solar energy sciences and engineering applications One salt source for the STEP generation of magnesium and chlorine from MgCl2 are via chlorides extracted from salt water, with the added advantage of the generation of less saline water as a secondary product. In the absence of effective heat exchanger, concentrator photovoltaics heat up to over 100.C, which decreases cell performance. Heat exchange with the (non-illuminated side of) concentrator photovoltaics can vaporize seawater for desalinization and simultaneously prevent overheating of the CPV. The simple concentrator STEP mode (coupling super-bandgap electronic charge with solar thermal heat) is applicable when sunlight is sufficient to both generate electronic current for electrolysis and sustain the electrolysis temperature. In cases requiring both the separation of salts from aqueous solution followed by molten electrolysis of the salts, a single source of concentrated sunlight can be insufficient, to both drive water desalinization and to also heat and drive electrolysis of the molten salts. Figure 8.3.8 includes a schematic representation of a Hybrid-Solar Thermal Electrochemical Production process with separate (i) solar thermal and (ii) photovoltaic field to drive both desalinization and the endothermic carbon dioxide-free electrolysis of the separated salts, or water splitting, to useful products. As illustrated, the separate thermal and electronic sources may each be driven by insolation, or alternatively, can be (i) solar thermal and (ii) (not illustrated) wind, water, nuclear or geothermal driven electronic transfer. 8.4 STEP CONSTRAINTS 8.4.1 STEP limiting equations As illustrated on the left side of Scheme 8.2.2, the ideal STEP electrolysis potential incorporates not only the enthalpy needed to heat the reactants to TSTEP from Tambient, but also the heat recovered via heat exchange of the products with the inflowing reactant. In this derivation it is convenient to describe this combined heat in units of voltage via the conversion factor nF: QT = i Hi(Ri, TSTEP)- i Hi(Ri, Tambient)- i Hi(Ci, TSTEP)+ i Hi(Ci, Tambient); EQ(V) = -QT(J/mol)/nF (8.4.1) The energy for the process, incorporates ET, EQ, and the non-unit activities, via inclusion of Equation 8.4.1 into Equation 8.2.4, and is termed the STEP potential, ESTEP: ESTEP(T, a) = [-G.(T) - QT - RT · ln( %x i=1 a(Ri)ri/ %y i=1 a(Pi)pi)]/nF; E. STEP(a = 1) = E. T + EQ (8.4.2) In a pragmatic electrolysis system, product(s) can be be drawn off at activities that are less than that of the reactant(s). This leads to large activity effects in Equation 8.4.2 at higher temperature, (Licht, 2009; Licht et al., 2010a; Licht and Wang, 2010; Licht et al., 2011b; Licht et al., 2011a; Licht, 2002; Licht, 2003; Licht et al., 2003; Licht, 2005) as the RT/nF potential slope increases with T (e.g. increasing 3-fold from 0.0592 V/n at 25.C to 0.183 V/n at 650.C). The solar thermal electrochemical production of energetic molecules: Step 247 The STEP factor, ASTEP is the extent of improvement in carrying out a solar driven electrolysis process at TSTEP, rather than at Tambient. For example, when applying the same solar energy, to electronically drive the electrochemical splitting of a molecule which requires only two thirds the electrolysis potential at a higher temperature, then ASTEP =(2/3)-1 =1.5. In general, the factor is given by: ASTEP = ESTEP(Tambient, a)/ESTEP(TSTEP, a); e.g.Tambient = 298K (8.4.3) The STEP solar efficiency, .STEP, is constrained by both photovoltaic and electrolysis conversion efficiencies, .PV and .electrolysis, and the STEP factor. In the operational process, passage of electrolysis current requires an additional, combined (anodic and cathodic) overpotential above the thermodynamic potential; that is Vredox =(1+z)Eredox, Mobility and kinetics improve at higher temperature and .(T>Tambient)<.(Tambient,) (Light, 1987; Sunmacher, 2007). Hence, a lower limit of .STEP(VT) is given by .STEP-ideal(ET). At Tambient, ASTEP =1, yielding .STEP(Tambient)=.PV · .electrolysis. .STEP is additionally limited by entropy and black body constraints on maximum solar energy conversion efficiency. Consideration of a black body source emitted at the sun’s surface temperature and collected at ambient earth temperature, limits solar conversion to 0.933 when radiative losses are considered, (Solanki and Beaucarne, 2007) which is further limited to .PV <.limit =0.868 when the entropy limits of perfect energy conversion are included (Luque and Marti 2003). These constraints on .STEP-ideal and the maximum value of solar conversion, are imposed to yield the solar chemical conversion efficiency, .STEP: .STEP-ideal(T, a) = .PV · .electrolysis · ASTEP(T, a) .STEP(T, a) ~= .PV · .electrolysis(Tambient, a) · ASTEP(T, a); (.STEP < 0.868) (8.4.4) As calculated from Equation 8.2.3 and the thermochemical component data (Chase, 1998) and as presented in Figure 8.2.1, the electrochemical driving force for a variety of chemicals of widespread use by society, including aluminium, iron, magnesium and chlorine, significantly decreases with increasing temperature. 8.4.2 Predicted STEP efficiencies for solar splitting of CO2 The global community is increasingly aware of the climate consequences of elevated greenhouse gases. A solution to rising carbon dioxide levels is needed, yet carbon dioxide is a highly stable, noncombustible molecule, and its thermodynamic stability makes its activation energy demanding and challenging. The most challenging stage in converting CO2 to useful products and fuels is the initial activation of CO2,for which energy is required. It is obvious that using traditional fossil fuels as the energy source would completely defeat the goal of mitigating greenhouse gases. A preferred route is to recycle and reuse the CO2 and provide a useful carbon resource. We limit the non-unit activity examples of CO2 mitigation in Equation 8.3.1 to the case when CO and O2 are present as electrolysis products, which yields aO2 =0.5aCO, and upon substitution into Equation 8.4.2: ESTEP(T, a) = E. STEP(T) - (RT/2F) · ln(N); E. (25.C) = 1.333V; N = v 2aCO2a-3/2 CO (8.4.5) 248 Solar energy sciences and engineering applications The example of ESTEP(T,a =1) on the left side of Figure 8.4.1 is derived when N=100, and results in a substantial drop in the energy to split CO2 due to the discussed influence of RT/2F. Note that at high temperature conditions in the figure, ESTEP <0 occurs, denoting the state in which the reactants are spontaneously formed (without an applied potential). This could lead to the direct thermochemical generation of products, but imposes substantial experimental challenges. To date, analogous direct water-splitting attempts are highly inefficient due to the twin challenges of high temperature material constraints and the difficulty in product separation to prevent back reaction upon cooling (Kogan, 1998). The STEP process avoids this back reaction through the separation of products, which spontaneously occurs in the electrochemical, rather than chemical, generation of products at separate anode and cathode electrodes. The differential heat required for CO2 splitting, EQ, and the potential at unit activity, E. STEP, are calculated and presented in the top of Figure 8.4.1. EQ has also been calculated and is included. EQ is small (comprising tens of millivolts or less) over the entire temperature range. Hence from Equation 8.4.2, E. STEP does not differ significantly from the values presented for E. T for CO2 in Figure 8.2.2. ECO2split(25.C) yields ASTEP(T)=1.333 V/E. STEP(T) with unit activity, and ASTEP(T)=1.197 V/ESTEP(T) for theN=100 case. Large resultant STEP factors are evident in the left of Figure 8.4.1. This generates substantial values of solar to chemical energy conversion efficiency for the STEP CO2 splitting to CO and O2. A STEP process operating in the .PV · .electrolysis range of 0.20 to 0.40 includes the range of contemporary 25 to 45% efficient concentrator photovoltaics, (King et al., 2007; Green et al., 2011) and electrolysis efficiency range of 80 to 90%. From these, the CO2 solar splitting efficiencies are derived from Equations 8.4.4 and 8.4.5, and are summarized on the right side of Figure 8.4.1. The small values of ESTEP(T) at Figure 8.4.1 Top: Calculated STEP parameters for the solar conversion of CO2. Bottom: Solar to chemical conversion efficiencies calculated through Equation 8.4.4 for the conversion of CO2 to CO and O2. In the case in which the product of the photovoltaic and electrolysis efficiency is 27.2% (.PV · .electrolysis =0.272), the STEP conversion efficiency at unit activity is 35%, at the 650.C temperature consistent with molten carbonate electrolysis, rising to 40% at the temperature consistent with solid oxide electrolysis (1000.C). Non-unit activity calculations presented are for the case of v 2 aCO2a-3/2 CO =100.A solar conversion efficiency of 50% is seen at 650.C when N=100 (the case of a cell with 1 bar of CO2 and ~58 mbar CO). Modified with permission from Licht 2009. The solar thermal electrochemical production of energetic molecules: Step 249 higher T, generate large STEP factors, and result in high solar to chemical energy conversion efficiencies for the splitting of CO2 to CO and O2. As one intermediate example from Equation 8.4.5, we take the case of an electrolysis efficiency of 80% and a 34% efficient photovoltaic (.PV · .electrolysis =0.272). This will drive STEP solar CO2 splitting at molten carbonate temperatures (650.C) at a solar conversion efficiency of 35% in the unit activity case, and at 50% when N=100 (the case of a cell with 1 bar of CO2 and ~58 mbar CO). 8.4.3 Scaleability of STEP processes STEP can be used to remove and convert carbon dioxide. As with water splitting, the electrolysis potential required for CO2 splitting falls rapidly with increasing temperature (Figure 8.2.1), and we have shown here (Figure 8.2.2) that a photovoltaic, converting solar to electronic energy at 37% efficiency and 2.7V, may be used to drive three CO2 splitting lithium carbonate electrolysis cells, each operating at 0.9V, and each generating a 2 electron CO product. The energy of the CO product is 1.3V (Equation 8.2.1), even though generated by electrolysis at only 0.9V due to synergistic use of solar thermal energy. As seen in Figure 8.2.5, at lower temperature (750.C, rather than 950.C), carbon, rather than CO, is the preferred product, and this four electron reduction approaches 100% Faradaic efficiency. TheCO2 STEP process consists of solar-driven and solar thermal assistedCO2 electrolysis. Industrial environments provide opportunities to further enhance efficiencies; for example fossil-fueled burner exhaust provides a source of relatively concentrated, hot CO2. The product carbon may be stored or used, and the higher temperature product carbon monoxide can be used to form a myriad of industrially relevant products including conversion to hydrocarbon fuels with hydrogen (which is generated by STEP water splitting in Section 8.3.1), such as smaller alkanes, dimethyl ether, or the Fischer Tropsch generated middle-distillate range fuels of C11–C18 hydrocarbons including synthetic jet, kerosene and diesel fuels (Andrews and Logan, 2008). Both STEP and Hy-STEP represent new solar energy conversion processes to produce energetic molecules. Individual components used in the process are rapidly maturing technologies including wind electric, (Barbier, 2010) molten carbonate fuel cells (Sunmacher, 2007), and solar thermal technologies (BrightSource, 2012; AREVA, 2012; Siemens, 2011; Solar Reserve, 2012; Amonix, 2012; Energy Innovations, 2012; Pitz-Paul, 2007). It is of interest whether material resources are sufficient to expand the process to substantially impact (decrease) atmospheric levels of carbon dioxide. The buildup of atmosphericCO2 levels from a 280 to 392 ppm occurring over the industrial revolution comprises an increase of 1.9×1016 mole (8.2×1011 metric tons) of CO2,(Tans, 2009) and will take a comparable effort to remove. It would be preferable if this effort results in useable, rather than sequestered, resources. We calculate below a scaled up STEP capture process can remove and convert all excess atmospheric CO2 to carbon. In STEP, 6kWhm-2 of sunlight per day, at 500 suns on 1m2 of 38% efficient CPV, will generate 420 kAh at 2.7V to drive three series-connected molten carbonate electrolysis cells to CO, or two series-connected molten carbonate electrolysis cells to form solid carbon. This will capture 7.8×103 moles ofCO2 day-1 to form solid carbon (based on 420 kAh · 2 series cells/4 Faraday mol-1 CO2). The CO2 consumed per day is three fold higher to form the carbon monoxide product (based on 3 series cells and 250 Solar energy sciences and engineering applications 2 F mol-1 CO2) in lieu of solid carbon. The material resources to decrease atmospheric carbon dioxide concentrations with STEP carbon capture, appear to be reasonable. From the daily conversion rate of 7.8×103 moles of CO2 per square meter of CPV, the capture process, scaled to 700km2 of CPV operating for 10 years can remove and convert all the increase of 1.9×1016 mole of atmospheric CO2 to solid carbon. A larger current density at the electrolysis electrodes, will increase the required voltage and would increase the required area of CPVs. While the STEP product (chemicals, rather than electricity) is different than contemporary concentrated solar power (CSP) systems, components including a tracker for effective solar concentration are similar (although an electrochemical reactor replaces the mechanical turbine). A variety of CSP installations, which include molten salt heat storage, are being commercialized, and costs are decreasing. STEP provides higher solar energy conversion efficiencies than CSP, and secondary losses can be lower (for example, there are no grid-related transmission losses). Contemporary concentrators, such as based on plastic Fresnel or flat mirror technologies, are relatively inexpensive, but may become a growing fraction of cost as concentration increases (Pitz-Paal et al., 2007). A greater degree of solar concentration, for example 2000 suns, rather than 500 suns, will proportionally decrease the quantity of required CPV to 175km2, while the concentrator area will remain the same at 350,000km2, equivalent to 4% of the area of the Sahara desert (which averages ~6kWhm-2 of sunlight per day), to remove anthropogenic carbon dioxide in ten years. A related resource question is whether there is sufficient lithium carbonate, as an electrolyte of choice for the STEP carbon capture process, to decrease atmospheric levels of carbon dioxide. 700km2 of CPV plant will generate 5×1013 A of electrolysis current, and require ~2 million metric tonnes of lithium carbonate, as calculated from a 2 kg/l density of lithium carbonate, and assuming that improved, rather than flat, morphology electrodes will operate at 5 A/cm2 (1,000km2) in a cell of 1mm thick. Thicker, or lower current density, cells will require proportionally more lithium carbonate. Fifty, rather than ten, years to return the atmosphere to pre-industrial carbon dioxide levels will require proportionally less lithium carbonate. These values are viable within the current production of lithium carbonate. Lithium carbonate availability as a global resource has been under recent scrutiny to meet the growing lithium battery market. It has been estimated that the current global annual production of 0.13 million tonnes of LCE (lithium carbonate equivalents) will increase to 0.24 million tonnes by 2015 (Tahil, 2008). Potassium carbonate is substantially more available, but as noted in the main portion of the paper can require higher carbon capture electrolysis potentials than lithium carbonate. An additional modified barium carbonate STEP electrolyte has been introduced (Licht et al., 2013), and STEP mechanisms continue to be probed (Cui and Licht, 2013), and the portfolio of new STEP processes and products, such as STEP Cement (Licht, 2012) and STEP Water Treatment (Wang et al., 2012; Wang et al., 2103), continues to expand. 8.5 CONCLUSIONS To ameliorate the consequences of rising atmospheric carbon dioxide levels and its effect on global climate change, there is a drive to replace conventional fossil fuel driven electrical production by renewable energy driven electrical production. In addition to the replacement of the fossil fuel economy by a renewable electrical economy, The solar thermal electrochemical production of energetic molecules: Step 251 we suggest that a renewable chemical economy is also warranted. Solar energy can be efficiently used, as demonstrated with the STEP process, to directly and efficiently form the chemicals needed by society without carbon dioxide emissions. Iron, a basic commodity, currently accounts for the release of one quarter of worldwide CO2 emissions by industry, which may be eliminated by replacement with the STEP iron process. The unexpected solubility of iron oxides in lithium carbonate electrolytes, coupled with facile charge transfer and a sharp decrease in iron electrolysis potentials with increasing temperature, provides a new route for iron production. Iron is formed without an extensive release ofCO2 in a process compatible with the predominant naturally occurring iron oxide ores, hematite, Fe2O3, and magnetite, Fe3O4. STEP can also be used in direct carbon capture, and the efficient solar generation of hydrogen and other fuels. 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ChemCatChem, 3, 513–528. Zou, Z., Ye, Y., Sayama, K. and Arakawa, H. (2001) Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst. Nature, 414, 625–627. Chapter 9 Solar hydrogen production and CO2 recycling Zhaolin Wang1 & Greg F. Naterer2 1Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, Ontario, Canada 2Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John’s, Newfoundland and Labrador, Canada 9.1 SUSTAINABLE FUELS WITH SOLAR-BASED HYROGEN PRODUCTION AND CARBON DIOXIDE RECYCLING The need for energy will continue to increase rapidly as the world aims to improve its living quality. As the usable fossil fuel resources are diminishing due to the ongoing depletion, the scenario of using carbon-based fuels is unsustainable. It was estimated by the International Energy Agency (IEA) and the US Energy Information Administration (EIA) that the global primary energy demand will be still met primarily by fossil fuels in the medium term before 2035 (IEA, 2011, 2012; EIA, 2012). In addition, the usage of fossil fuels generates toxic pollutants threatening the ecobalance and our health. The CO2 emissions induce undesirable climate effects. Therefore, clean energy alternatives and carbon recycling are needed for solving the future energy sustainability problems. Among several clean energy substitutes such as nuclear, hydroelectric, geothermal, and solar, nuclear fission energy has been well proven as a technology widely used in the world. However, nuclear waste disposal challenges and unpredictable accidents such as Three Mile Island, Chernobyl, and Fukushima are often cited by the public (Bodansky 2001; Grady, 2011; Makhijani, 2009; Pearce, 2008). As to hydroelectric power, it is limited by the availability of waterways. River dams may also cause unpredictable influences on the aquatic ecosystems, fisheries, and river transport. Regarding deep underground geothermal electricity, the usable energy is often located kilometres below the surface and there are concerns about seismic impacts. In comparison, solar energy is a safe, clean and unlimited resource (Roeb et al., 2010). However, the availability of sunlight on the earth’s surface is intermittent because it is not available at night and on rainy and cloudy days. The energy distributed on various areas may differ significantly. Therefore, if the solar energy captured at daytime or regions rich in sunlight can be stored, then the intermittency issue would be resolved. The usage of solar energy for hydrogen production is a good option for solar energy storage, because hydrogen has a much higher mass energy density than most current fossil fuels (Envestra, 2010; EVWorld, 2010; ForestBioEnergy, 2010). The only product of hydrogen combustion is water vapour, which can be recycled to produce hydrogen with solar energy. It was reported that the efficiency of a hydrogen internal combustion engine could be 10–40% higher than a gasoline engine. The hybrid electric motor and fuel cell vehicle could even be 2 to 3 times more efficient than an internal gasoline combustion engine (Berger, 258 Solar energy sciences and engineering applications 2006). Therefore, solar-based hydrogen production with water is widely viewed as a very promising option for a sustainable future. Even in the conventional fossil fuel industry, hydrogen has a major role in the upgrading of petroleum products. Also, hydrogen is a necessity for the production of fertilizers in the agricultural industry. Currently, oil upgrading and fertilizer production account for about 50% and 40% of the hydrogen consumption, respectively (Dalcor, 2005; Freedonia, 2010; Kramer, 2005). The rising need of hydrogen by modern agriculture and petroleum products will strongly advance the hydrogen economy (Forsberg, 2002; Naterer et al., 2008). However, the major hydrogen production methods of today are generally not “clean’’ because more than 95% of the global hydrogen is produced from fossil fuels, i.e., 48% from steam methane reforming (SMR), 30% from refinery/chemical off-gases, and 18% from coal gasification (IEA, 2010; NYSERDA, 2010). Water electrolysis accounts for less than 4%. Even this 4% is not fully clean because the electricity used for hydrogen production is not fully generated from clean fuels. The usage of fossil fuels to produce hydrogen generates large amounts of greenhouse gases. Therefore, the future hydrogen production pathway from solar-based water splitting is a promising solution. Another option for reducing the depletion rate of our planet’s fossil fuel reserves is to recycle the CO2 emissions with solar energy. This may improve the renewability of limited fossil fuels and at the same time store the solar energy and minimize the impact of intermittency of the sunlight. In addition to the renewable benefit, the CO2 recycling is also a safer measure compared with the geological sequestration of CO2 into deep oceans or geological formations, because the sequestratedCO2 has many unpredictable risks such as leakage of CO2 back to the atmosphere and the change of ocean water properties (Yang, 2011; Little et al., 2010; Spicer, 2007). Even though care is taken to identify the appropriate geological areas for the storage of CO2, there is always a likelihood of leakage due to different reasons such as an earthquake. The liquefied CO2 on the basin of the ocean floor may have a much higher CO2 concentration than normal levels. This makes it difficult for some ocean organisms to survive near the ocean basin and as a result, the whole ecosystem is disturbed. As for deep ground sequestration, the leaked CO2 may mix with groundwater and consequently make the water toxic and unsuitable for human consumption. Although deep saline reservoirs potentially have a large capacity to store CO2, high pressure CO2 can significantly acidify the fluids in the reservoir and dissolve minerals such as calcium carbonate. As a consequence, the permeability is increased which could allow CO2-rich fluids to escape the reservoir along new pathways and contaminate aquifers used for drinking water (Kharaka et al., 2006). Since CO2 alone is not a fuel and most fuels and organic compounds comprise hydrogen, so hydrogen is a necessity in the conversion of CO2 into other useful fuels and organic compounds such as syngas, methanol, and dimethyl ether. However, nowadays more than 96% of the world’s hydrogen is produced from fossil fuels through processes such as steam-methane reforming (SMR) and coal gasification with steam (IEA, 2012). As discussed previously, these hydrogen production processes are depleting the fossil fuel reserves and emitting large amounts of pollutants and greenhouse gases. Hence, to find renewable and low-cost methods of producing hydrogen in large capacities is also critical to the CO2 recycling. Solar hydrogen production and CO2 recycling 259 Engineers and scientists have been developing numerous methods for clean hydrogen production and CO2 recycling. This chapter will examine the energy efficiencies, requirements, and thresholds of different technologies. The major water splitting and CO2 recycling technologies will be compared and categorized according to the reaction mechanisms and engineering approaches. The engineering approaches would be the focus from the perspectives of apparatus, components, materials, equipment and layout of processes. 9.2 SOLAR-BASED HYDROGEN PRODUCTION WITH WATER SPLITTING METHODS This section examines the technologies of utilizing solar energy for hydrogen production with water splitting, which are different from conventional fossil fuel-based methods. The conventional methods such as modified steam methane reforming and coal gasification suitable for the usage of solar energy, are still fossil fuel-based although the green house gas emissions are significantly reduced due to the usage of solar thermal energy to replace the combustion of an extra amount of methane or coal for the supply of reaction heat. Therefore, the solar-assisted fossil fuel-based conventional methods for hydrogen production won’t be discussed in this chapter. Since the only feedstock to a water splitting cycle is water and the products are hydrogen and oxygen with no greenhouse gas emissions, so the hydrogen production with water splitting is the focus of this chapter. 9.2.1 Solar-to-hydrogen efficiency of water splitting processes In water splitting methods, the only feedstock to the hydrogen production cycle is water, and the only products are hydrogen and oxygen. Since the combustion of hydrogen produces water again, the solar-based water splitting methods are fully renewable compared with the prior modified conventional fossil fuel-based methods for the usage of solar energy. This section will focus on these unconventional methods. Engineers and scientists have been developing numerous methods for the clean hydrogen production from water splitting with solar energy, such as thermolysis, thermochemical, water electrolysis, photoeletrolysis, photoeletrochemical, photochemical, photodissociation, photodecomposition, photolysis, photodegradation, photocatalytic, photobiological, and hybrid methods. This section aims to introduce and compare the reaction mechanisms and the major water splitting technologies and engineering approaches. Different water splitting mechanisms and engineering approaches may indicate various hydrogen production efficiencies. Considering the variety of complex efficiency related factors, the following efficiency definition is adopted for a more consistent comparison: . = mP · H0 Liq,298 IS (9.2.1) where . is the solar-to-hydrogen production efficiency, IS is the total incident solar irradiance of the whole solar spectrum on the basis of the sunlight receiving area of the 260 Solar energy sciences and engineering applications device for the reacting system, and its units are J · s-1. Also, mP in Equation (9.2.1) is the hydrogen production rate in units of mole · s-1, and H. Liq is the standard enthalpy change of the following water splitting reaction at 298K (25.C) in units of J · mole-1 with a value of 285,800 J/mol: H2O (liquid) = H2 (gas)+½O2 (gas) H0 298 = 285,800 J/mol (9.2.2) If 25.C is also used as the reference temperature for the higher heating value (HHV), then the HHV is equal to the enthalpy change H. Liq. The product of mP and HLiq indicates the theoretical minimum energy needed to split water into hydrogen and oxygen, or the maximum energy that can be recovered from hydrogen when hydrogen is used as a fuel. The reason to use liquid water at 25.C rather than gaseous water is because the starting state of the water used in industry for hydrogen production is mostly in the form of liquid at an ambient temperature, although in the hydrogen production reactor it could be in other forms and the temperature could be slightly different. Also, since the efficiency is discussed from the perspectives of production rather than usage, the actual energy input to the production may attract more engineering interest. If the reported values in past literature are based on HGas, GGas andGLiq, they will be converted toHLiq in this chapter for a consistent comparison. The reason to use the whole spectrum in Equation (9.2.1) for the efficiency comparison is to make the devices working at different wavelengths to be more comparable, and also more convenient for the evaluation of the energy losses due to the non-use of other wavelengths. The efficiency is also influenced by the working wavelength of the devices and the sunlight absorbing material may work only for a specific range of wavelengths. Consequently, other wavelengths of the solar spectrum will be unused and the efficiency of sunlight usage is reduced. Different devices may work at different wavelengths of the solar spectrum, so the comparison is not made on the same basis when using the working wavelength of the devices to evaluate their performance. Different wavelengths correspond to different portions of solar irradiance. Table 9.2.1 shows the energy distribution of different wavelengths of the solar spectrum from past data (Thuillier et al., 2003). It can be concluded that if the hydrogen production device works only in the ultraviolet region, then it can only use a maximum of 10% of the total incident solar energy. By comparison, if it works in the infrared region, then potentially 50% of the total incident solar energy can be used. In addition, various devices and technologies have different solar irradiance tracking and capturing capabilities. As a result, the requirement of land area and dimensions of auxiliary equipment may be quite different. Table 9.2.1 Energy distribution of the solar spectrum. Irradiance Wavelength % of total Infrared 700–2,400 nm 49.4 91.7 Visible 400–700 nm 42.3 Ultraviolet A 320–400 nm 6.3 8.3 Ultraviolet B 290–320 nm 1.5 Ultraviolet C 200–290 nm 0.5 Solar hydrogen production and CO2 recycling 261 Currently, some other efficiency definitions are also used to evaluate the solar-tohydrogen technologies and equipment. The existence of multiple definitions arises from the difference of reaction mechanisms, which will be discussed in the following sections. A direct comparison for different solar-to-hydrogen technologies is challenging because a wide scope of technologies is involved and many technologies are still at an early stage of development. This section will provide some general comparisons on the basis of similar standards reported in literature. 9.2.2 Matching the temperature requirements of solar-based hydrogen production methods In many unconventional hydrogen production methods, e.g., thermochemical water splitting and high temperature electrolysis, high temperature heat is needed (Hinkley et al., 2011; Monnerie et al., 2011; Corgnale et al., 2011; Summers et al., 2009; Xiao et al., 2012). Therefore, a large amount of solar irradiance must be concentrated to reach the temperature requirements and a large additional land area is needed to concentrate the solar irradiance. The high temperature requirements bring some significant engineering challenges. For example, it is still a challenge to find a coating material for the solar receivers to improve the absorptance and reduce the emittance at 600.C (Sergeant et al., 2010; Barshilia et al., 2006; Kennedy et al., 2005). Furthermore, it is challenging to select an appropriate working fluid and equipment material for a solar irradiance receiver and reactor. The working fluids include water, thermal oils, molten salts, steam, air, and other gases. Due to the good heat transfer performance and low melting points, thermal oils are widely used in the solar concentrating devices such as solar troughs. However, thermal oils are volatile and toxic, and may decompose at a high temperature, so the thermal oils are currently operated below 450.C (Moens et al., 2003, 2004; Eck et al., 2007; Wu et al., 2001), which is not sufficiently high to cover the temperature threshold of some thermochemical hydrogen production cycles (Xiao et al., 2012; Le Gal et al., 2010; Corgnale et al., 2011), which will be discussed in later sections. The solar irradiance concentrating devices include solar troughs, lenses, parabolic dishes, heliostats, and reflection mirrors. Currently, a solar trough can concentrate more power than a lens and a parabolic dish, but it is challenging to reach up to 500.C even if it uses a molten salt as the working fluid (Herrmann et al., 2004), because the receiving area of the tubular irradiance receiver makes the relative number of suns fewer than that otherwise concentrated to a focal spot by a lens and dish. So the temperatures provided by solar troughs are not suitable for some thermochemical cycles with higher temperature requirements (Xiao et al., 2012; Le Gal et al., 2010). A solar tower capable of concentrating thousands of suns as well as tens of megawatts of irradiance with heliostats or reflection mirrors can reach a temperature range of 500–1000.C (Schramek et al., 2009; Spelling 2009; Dersch et al., 2011). When utilizing a molten salt as the working fluid, the operating temperature of large lab-scale equipment can reach up to 900.C (Forsberg et al., 2007; Patel 2011; Dunn et al., 2012; Moore et al., 2010; Matsunami et al., 2000). Some recent small industrial scale construction projects utilizing molten salts operate at up to 650.C (Khan et al., 2004; Martín, 2007; NREL, 2010). A disadvantage of molten salts is their higher melting points than thermal oils and gases, which limits the heat transfer 262 Solar energy sciences and engineering applications and storage performance. Therefore, utilizing gas as the working fluid of the solar tower is another option for obtaining a high operating temperature range of 700– 1000.C (Schwarzbözl et al., 2006; Ahlbrink et al., 2009; Göttsche et al., 2010). Some operational small industrial scale solar thermal plants using air and other gases can operate at a temperature of 1,000.C. Currently, a high temperature up to 3,500.C can be obtained with solar concentrating furnaces of laboratory and pilot scale equipment (Haueter et al., 1999; Riveros-Rosas et al., 2010). This is a promising scenario for the high temperature hydrogen production cycles. 9.2.3 Thermolysis, thermal decomposition and thermochemical methods The water in Equation (9.2.2) may take part in the reaction in the form of either liquid water or steam. Since the generation of steam is also from liquid water, then the liquid form for Equation (9.2.2) is more widely used. The changes of standard enthalpy of the water splitting at 298K are equal to the negative of the higher and lower heating values of hydrogen if the same reference temperature is adopted: H0 Liq,298 = -HHV = -285.8kJ/mol = 2.97 eV/molecule (9.2.3) H0 Gas,298 = -LHV = -241.8kJ/mol = 2.52 eV/molecule (9.2.4) where the superscript “0’’ means the standard state, the subscript 298 means the temperature of 298K (25.C), and the subscripts Liq and Gas mean the liquid and gaseous states, respectively. Figure 9.2.1 shows the influence of the temperature on the standard enthalpy change of water splitting reaction for the gaseous form of water. It can be found that the reaction enthalpy at 5,000.C is only about 7% higher than at room temperature. This means the value of the reaction enthalpy change at room temperature can well represent the values under current engineering temperature ranges. Also, the value adopted in Equation (9.2.1) for the efficiency definition is reasonable. However, the enthalpy change mainly gives the energy balance and requirements. The balance is not sufficient for examining the spontaneity of direct water splitting, which is reflected by the changes of the Gibbs free energy. The values of the standard Gibbs free energy of Reaction (9.2.2) at 298K are (Licht, 2005): G0 Liq = 237.0kJ/mol = 2.47 eV/molecule (9.2.5) G0 Gas = 228.4kJ/mol = 2.38 eV/molecule (9.2.6) It can be found that the values of the Gibbs energy for liquid and gas forms differ by only 3.8%. So either value can be used to examine the spontaneity of water splitting. The large positive values of the Gibbs free energy in Equations (9.2.5) and (9.2.6) indicate that the direct decomposition of water is far from spontaneous except that the temperature is increased or a large amount of non-PV work such as electrical work is injected into the water molecules. To study the spontaneity, Figure 9.2.2 shows the standard Gibbs energy change of the water splitting reaction at different temperatures. It can be found that the transition temperature under standard conditions is about Solar hydrogen production and CO2 recycling 263 Figure 9.2.1 Standard enthalpy change of water splitting reaction vs. temperature. Figure 9.2.2 Standard Gibbs energy change of water splitting reaction vs. temperature. 4,000.C, at which the Gibbs energy change becomes negative and the water splitting becomes spontaneous. From the Gibbs energy change in Figure 9.2.2, the water decomposition equilibrium constant can be calculated. Then according to the equilibrium constant, the water decomposition percentage can be estimated. Figure 9.2.3 shows the direct water decomposition percentage at different temperatures. It can be found that the below 2% of water directly split at 2,000.C, and if 40% of water is split, the temperature must be higher than 4,000.C. This is a very high temperature that would forego engineering 264 Solar energy sciences and engineering applications Figure 9.2.3 Direct water splitting extent at different temperatures. practice in the near future, although currently a high temperature up to 3,500.C can be obtained with solar concentrating furnaces of laboratory and pilot scales (Haueter et al., 1999; Riveros-Rosas et al., 2010). At this high temperature, it is challenging for refractory materials and construction of equipment. Moreover, the direct thermolysis product is a gas mixture of hydrogen and oxygen, which has a considerable explosion risk. To lower the water themolysis temperature, some auxiliary chemicals can be introduced to form at least one intermediate in an association step and then the intermediate releases hydrogen and/or oxygen separately in other dissociation steps. This will form an “indirect’’ water splitting cycle. The integration of the association and dissociation steps forms a closed cycle wherein the net effect of the integration is that only water is decomposed and the auxiliary chemicals are recycled inside the cycle. This type of water decomposition is often termed as a “thermochemical’’ cycle. If a small portion of energy is supplied to the cycle as electricity, it is then a type of hybrid thermochemical cycle rather than a purely thermal cycle. About three hundred thermochemical cycles, either purely thermal or hybrid, have been reported previously (Abanades et al., 2006). For fully thermal hydrogen production, two-step water splitting cycles based on metal redox reactions are leading examples. These cycles usually consist of an endothermic reduction reaction where oxygen is produced from a metal oxide, and a hydrolysis reaction where hydrogen is produced (Xiao et al., 2012; Le Gal et al., 2010): MxOy . MxOy-1 + ½O2 (9.2.7) MxOy-1 + H2O . MxOy + H2 (9.2.8) where M denotes a metal, and the subscripts x and y mean the numbers of the metal and oxygen atoms in a metal oxide molecule. Zinc is a metal example currently under Solar hydrogen production and CO2 recycling 265 active investigation (Haltiwanger et al., 2010; Steinfeld 2002; Melchior, 2009): ZnO = Zn + ½O2 (9.2.9) Zn + H2O = ZnO + H2 (9.2.10) The redox pairs of metal oxides that have been reported include Fe3O4/FeO, TiO2/TiOx, Mn3 O4/MnO, CeO2/Ce2O3, Co3 O4/CoO, Nb2O5/NbO2, In2 O3/In, WO3/W, and CdO/Cd, among others (Xiao et al., 2012; Le Gal et al., 2010; Haltiwanger et al., 2010; Steinfeld, 2002; Charvin et al., 2007). A significant advantage of using these cycles for hydrogen production is that only two chemical reactions are involved, which reduces the challenges of the system integration that otherwise occurs for three or more chemical reactions. However, the temperature for the oxygen production reaction is usually in the range of 1,500–2,500.C. This is a major challenge for equipment materials, which at the same time are expected to have the characteristics of high solar absorptance, low thermal emittance, corrosion resistance, and thermal stability. Another leading example of fully thermal cycles is the sulfur–iodine (S-I) cycle, which was first investigated at General Atomics in 1970s (Schultz, 2003; Riccardi et al., 2011; Khan, 2004). An advantage of the S-I cycle over the metal oxide redox cycles is its lower temperature requirement of 850.C. The S-I cycle has been scaled up from proof-of-principle tests to a larger engineering scale by the Japan Atomic Energy Agency (JAEA) (Kubo et al., 2004; Lewis et al., 2006; Sakurai et al., 2000; Terada et al., 2007). The scale of the S-I cycle at JAEA can reach 0.065 kg/day of hydrogen production at present. Commissariat à l’énergie atomique (CEA, (Anzieu et al., 2006)) and the Sandia National Laboratory (SNL, (Moore et al., 2007)) are also active developers of the S-I cycle. There are several types of S-I cycles. Table 9.2.2 shows a typical three-step purely thermal cycle that is commonly studied (Riccardi et al., 2011; Khan et al., 2004; Kubo et al., 2004; T-Raissi et al., 2003). The temperatures for each step of the S-I cycle adopted by different researchers have some differences, depending on the reactor technology. However, these differences are not significant enough to have major differences in the S-I cycle. The presence of iodine-based chemicals in the S-I cycle brings some significant engineering challenges. For example, great precaution must be taken to process the mixture of combustible H2 and I2 at 450.C. Also, the separation of HI, H2, and I2 is a complex multiple-stage process and the distillation of azeotropic HI would significantly enhance the energy cost of the cycle (Elder et al., 2005; Stewart 2005; Guo et al., 2011). To avoid these challenges, another sulfur-based thermochemical cycle named as a “hybrid sulfur cycle’’ or “Westinghouse cycle’’ has attracted attention (Riccardi et al., 2011; Hinkley et al., 2011; Monnerie et al., 2011; Corgnale et al., 2011; Roeb et al., 2010). As shown in Table 9.2.2, in the hybrid sulfur cycle, hydrogen is produced from the electrolysis of an aqueous solution of SO2 and the operating temperature is about 120.C, which is significantly lower than the HI decomposition temperature of 450.C for the H2 production in the S-I cycle. Both the S-I and the hybrid sulfur cycles require an input temperature of 850.C, which still brings many high temperature-related challenges, although the temperature is significantly lower than redox cycles with metal oxide pairs. In recent years, the 266 Solar energy sciences and engineering applications Table 9.2.2 Major chemical processes of a fully thermal S-I cycle and hybrid sulfur cycle. Step Process and heat flow Major reaction Processes in an S-I thermochemical cycle 1 Hydrolysis step I2(l+g)+SO2(g)+2H2O(g)=2HI(g)+H2SO4(l)+Q, at 120.C (exothermic) 2(a) Oxygen production H2SO4 (g)+Q= SO2(g)+H2O(g)+0.5O2(g), at 800–1000.C step (endothermic) 3 Hydrogen production 2HI(g)+Q=I2(g)+H2(g), at 450.C step (endothermic) Major chemical processes of a hybrid sulfur cycle A Hydrogen production SO2(aq)+2H2O(l)+VE =H2SO4(aq)+H2(g) at 80–120.C step (electrolytic) B(a) Oxygen production H2SO4 (g)+Q=SO2(g)+H2O(g)+0.5O2(g), at 850.C step (endothermic) Symbols: aq – aqueous, g – gas, l – liquid,Q – heat,VE – electricity (a) This step can be divided H2SO4(aq)+Q=SO3(g)+H2O(g), at 300–450.C into two separate steps: SO3(g)+Q=SO2(g)+1/2O2(g), at 800–1000.C copper-chlorine (Cu-Cl) hybrid cycle has gained major attraction due to its lower temperature requirement of 530.C, which can be accommodated by more technologies of solar thermal energy (Khan et al., 2004; Xu et al., 2012; Litwin et al., 2010). The Cu-Cl cycle also has several variations with various numbers of steps from 2 to 5 depending on reaction conditions [Lewis, 2008; Wang et al., 2008, 2009]. The cycle with 4 steps shown in Table 9.2.3 is a typical hybrid Cu-Cl cycle. The energy structure and heat requirements are also shown in Table 9.2.3, so as to provide a basis for the efficiency evaluation of the hybrid cycle. A solarium laboratory apparatus that can absorb a maximum of 50kW solar irradiance and temperature of 800.C is under development for the study of a solar-based Cu-Cl cycle and other photochemical processes at the University of Ontario Institute of Technology (UOIT). The scale-up of the cycle from proof-of-principle to a larger engineering scale of 3 kg/day is also in progress at UOIT (Wang et al., 2008, 2009; Naterer et al., 2008) in collaboration with partners that include Atomic Energy of Canada Limited (AECL). As suggested in Tables 9.2.2 and 9.2.3, more steps are needed by the fully thermal S-I and hybrid Cu-Cl cycles than the hybrid metal redox cycles to complete the water decomposition in a closed loop. Multiple chemical reactors and auxiliary equipment for chemical reactions and heat transfer are needed. This may increase the capital cost of the equipment and operating cost. Therefore, the S-I and Cu-Cl thermochemical cycles are more appropriate for large scale hydrogen production to offset the costs arising from multiple chemical processes. Since heat is the major form of energy input to thermochemical cycles, the energy loss in the conversion of heat to electricity is then avoided. This indicates a great potential to improve the overall thermal efficiency for hydrogen production. For example, it was estimated that the efficiency could reach 40–56% by Zn/ZnO cycles (Xiao et al., 2012; Haltiwanger et al., 2010; Melchior, 2009; Schunk et al., 2009), 39–45% by Fe3O4/FeO cycles (Xiao et al., 2012; Charvin et al., 2008), 35–46% by hybrid sufur Solar hydrogen production and CO2 recycling 267 Table 9.2.3 Major chemical processes of a hybrid copper-chlorine cycle and energy distribution. Step Process and heat flow Major reaction Processes in Cu-Cl thermochemical cycle I Electrolytic hydrogen 2CuCl (s)+2HCl (aq)+VE =H2 (g)+2CuCl2 (aq) in aqueous production solution, at 30~100.C II Drying of cupric chloride CuCl2 (aq)+nfH2O (l)+Q=CuCl2 · nhH2O (s)+(nf -nh) H2O, (endothermic) where nf >7.5, nh =0~4, depending on temperature. Below 80.C, crystallization; at 100~200.C, spray drying. III Hydrolysis of cupric 2CuCl2 · nhH2O (s)+H2O (g)+Q=CuOCuCl2 (s)+2HCl (g)+ chloride (endothermic) nhH2O (g), at 400.C IV Oxygen production CuOCuCl2 (s)+Q=2CuCl (molten)+0.5O2 (g), at 530.C (endothermic) Symbols: aq – aqueous, g – gas, l – liquid, nf – number of free water, nh – number of hydrated water, Q – heat, s – solid,VE – electricity Energy distribution: In the total energy input, thermal energy and electricity occupy 70–90% and 10–30%, respectively. Heat requirements for various hydrogen production scales H2 production rate, tonnes/day 0.001 (1 kg/day) 1 50 100 200 Heat requirement,MWth 0.00263 (2.63 kWth) 2.63 132 263 525 cycles (Hinkley et al., 2011; Monnerie et al., 2011; Corgnale et al., 2011; Summers et al., 2009), and 40–60% by S-I and Cu-Cl cycles. These efficiencies have the potential to compete with current steam methane reforming (Lewis, 2008; Wang et al., 2008, 2009). Another advantage of thermochemical cycles is that water decomposition may utilize separate facilities that are independent of the capturing and processing of solar thermal energy. Therefore, the design and maintenance of the hydrogen production and solar thermal energy facilities can be separately performed. The solar thermal energy plant can be designed in a compact fashion that mainly aims at efficiently capturing and concentrating the solar irradiance, wherein a solar tracking system can be readily utilized. If the captured solar thermal energy needs to be transported by a heat transfer fluid over a distance from a solar tower to the thermochemcial hydrogen production cycle, the heat losses must be controlled to below 30% so as to compete with water electrolysis and steam methane reforming. The pipeline diameter (including the thermal insulation) for the heat transport between solar thermal and thermochemical hydrogen production plants must be large, either utilizing molten salt or pressurized helium as the heat transfer fluid when the heat transport lies in the range of 100–700MWth which corresponds to 40–200 tonnes of hydrogen production per day. A long distance (>10 km), heat transport is not suggested. 9.2.4 Water electrolysis The energy input for Equation (9.2.2) could also be in the form of electricity. This means the solar energy must be converted to electricity and the corresponding facilities should then be designed for the distribution of electrical current. Regarding the usage of electricity as the major energy input, the water molecule is split by an imposed 268 Solar energy sciences and engineering applications electric potential (Bockris et al., 1983): 2H2O(l) = O2 (g) + 4H+ (aq) + 4e-,A = 1.23V on surface of anode (oxidation) (9.2.11) 2H+ (aq) + 2e- = H2 (g),C = 0.00V on surface of cathode (reduction) (9.2.12) where 1.23V is the standard potential of the anode that indicates the theoretical minimum requirement. If viewed from the level of molecules, water electrolysis, photoelectrolysis, and photoelectrochemical methods can be categorized as the same type. However, their engineering approaches may be very different, which will be discussed in detail in the following sections. As to splitting of water molecules with electricity as indicated by Equations (9.2.11) and (9.2.12), an electrode efficiency defined on the basis of the gap between the actual potential bias and the theoretical minimum value of 1.23V is often used to assess the performance of the electrode (Licht, 2005; Bockris et al., 1983). This will not be discussed in detail in this chapter. In water electrolysis, water is split with an electric current to produce hydrogen. Direct current (DC) passes through two electrodes immersed in water, i.e., anode and cathode, and hydrogen is produced on the surface of the cathode when the electric potential is sufficiently high. The electrodes can be shaped to rods or plates, and the reactions taking place on the surface of the electrodes are shown in Equations (9.2.11) and (9.2.12). In order to avoid confusion with other terminology such as photoeletrolysis and photoelctrochemical methods, this paper suggests that the terminology “water electrolysis’’ is only used when the electricity is fully obtained from an external power generated from photovoltaic panels or turbines driven by solar-generated steam or other gases. Therefore, the electrolyzer and water do not receive the sunlight for water splitting. This categorization considered the engineering flexibility and engineering practicality for the integration of independent power sources and various electrolyzers. As shown in Figure 9.2.4, the basic components of the hydrogen production unit include two electrodes (anode and cathode) and one external power source. It can be found that the overall efficiency of the hydrogen production depends on the electricity-to-hydrogen efficiency of the electrolyzer, and the solar-to-electricity conversion efficiency. Table 9.2.4 shows the power consumption and efficiency of different industrial electrolysis systems in the U.S. (Ivy, 2004), Europe (European Commision, 2001) and China (CSPCS, 2009). Since conventional electrolysis is mature technology and the electricity-tohydrogen efficiency of a commercially available electrolyzer lies in the range of 50%–80% either using alkaline or polymer electrolyte membrane electrolyzers, the electricity generation dominates the overall efficiency of the hydrogen production. Currently, the power generation efficiency with photovoltaic panels is about 10–20% (van Helden et al., 2004; Yamada et al., 2011; Hanna et al., 2006). Therefore, the maximum overall efficiency of hydrogen production is below 16% (Khaselev et al., 2001). It is anticipated that the power generation efficiency of photovoltaic panels can be enhanced in the future by use of new materials to accommodate more irradiation of the solar spectrum. Solar hydrogen production and CO2 recycling 269 Figure 9.2.4 Components of a solar-based electrolyzer. Table 9.2.4 Power consumption and efficiency of different industrial electrolyzers. High heating value Electricity required Electricity required efficiency of Electrolyzer model for system, for electrolyzer only, electrolysis system Country or type kWh/m3(H2) kWh/m3(H2) (electricity-to-H2), % US(a) Stuart: IMET 1000 4.8 4.2 73.9 Teledyne: EC-750 5.6 N/A 63.3 Proton:HOGEN 380 6.3 4.3 56.3 Norsk Hydro: 4.8 N/A 73.9 Atmospheric Type No. 5040 (5150 Amp DC) Avalence: 5.4 N/A 65.7 Hydrofiller 175 Europe(b) Membrane 8.8 N/A 40.1 Amalgam 11.3 N/A 31.4 Diaphragm 9.4 N/A 37.7 China(b) Membrane 8.9 N/A 39.8 (a) The electrolyte is an aqueous solution of KOH and the electrolysis system is specifically designed for hydrogen production from water electrolysis. (b) The electrolyte is aqueous solution of NaCl and the electrolysis system also produces Cl2 and NaOH as commercial products. The external electricity can also be generated from a solar thermal plant that uses a sunlight concentrating device to generate high temperature fluid and then use the heat captured by the fluid to generate electricity. The concentrating devices and working fluids have been discussed in the former section regarding thermochemical cycles. Currently, thermal oils are usually used to generate steam to drive a steam turbine 270 Solar energy sciences and engineering applications (Moens et al., 2003, 2004; Eck et al., 2007; Wu et al., 2001), and molten salts are planned to be used for gaining a higher temperature than thermal oils (Forsberg et al., 2007; Patel, 2011; Dunn et al., 2012; Moore et al., 2010), and air and other gases are used to drive a gas turbine (Schwarzbözl et al., 2006; Ahlbrink et al., 2009; Göttsche et al., 2010). The currently operational solar thermal plant can generate the working fluid at more than 500.C and reach 1,000.C, so the conversion efficiency from a working fluid to electricity is in the range of 30–60%. Currently operational solar thermal plants show that the solar radiation capturing efficiency of solar concentrating devices with a tracking system is usually higher than 70% (European Commission, 2010; Schmitz, 2009; Taggart, 2008). Therefore, the integration of an electrolyzer and solar thermal power plant can deliver higher hydrogen production efficiency (15%–56%) than an electrolyzer and photovoltaic panels. In addition to the engineering maturity and flexibility of integration with various solar power generation technologies, another major advantage of water electrolysis is that the hydrogen production can still operate at nights or days by using power when sunlight is not available. The power could be either generated from the stored solar thermal energy for the use at nights and undesirable weather conditions, or directly from the power grid. In a concentrated solar power plant utilizing solar troughs or solar towers, a large amount of solar energy can be stored in thermal oils or molten salts for the times when sunlight is not available (Moens et al., 2003, 2004; Wu et al., 2001; Herrmann et al., 2004; Patel et al., 2011; Dunn et al., 2012). As for electricity from the power grid, even if the energy sources on the power grid may not be “clean’’, the impacts of unpredicted weather conditions can be minimized. Since water electrolysis utilizes an external power source, the design of the power generation plant does not need to consider the location of the solar power plant. It can be designed in a compact way that aims at efficient sunlight capturing. A solar tracking system can be readily utilized for collecting or concentrating the sunlight. In addition, unpredictable mutual safety impacts are minimized and the distance between the solar thermal power plant and the facilities of electrolyzer are flexible. The electrolytic facilities do not need to occupy the space where sunlight is more suitable for the power generation. 9.2.5 Photoelectrolysis and photoelectrochemical water splitting As to the water electrolysis presented in the former section, whether the electricity is fully obtained from an external source does not cause a significant difference from the perspectives of the chemical reaction mechanisms on the electrode surface. For example, the electricity can be generated by an electrode on its own if the electrode is made of materials that can create electric potential due to its exposure to sunlight (Licht, 2005). The materials could be either n-type or p-type semiconductors. However, from the aspect of an engineering and equipment setup, the difference is significant. In this section, it is suggested that “photoelectrolysis’’ and “photoelectrochemical’’ water splitting are not categorized as “water electrolysis’’. Instead, they are adopted when at least one light absorbing electrode is needed, and only a part or no electricity for the reduction or oxidization reaction on the electrode is obtained from external power sources. Solar hydrogen production and CO2 recycling 271 Figure 9.2.5 Components of a photoelectrolysis or photoelectrochemical unit. Figure 9.2.5 shows the basic components of a photoelectrolysis or photoelectrochemical (PEC) hydrogen production unit, including a sunlight absorbing electrode (typically made or coated with semiconductor) and a counter electrode (typically metal) immersed in an electrolyte. The sunlight absorbing electrode must be arranged to face the sunlight window to capture sufficient solar radiation to generate electric potential. The sunlight absorbing electrode should be wired either externally or internally with the other counter electrode so as to form a closed circuit. The reaction mechanism is described in Equations 9.2.11 and 9.2.12. It can be found that the sunlight absorbing material has a dominating role in determining the hydrogen production efficiency. The band gap, i.e., the potential, created by the electrode material must exceed the bottom theoretical limit of 1.23 eV to split water molecules, plus overcoming the electric resistance of the closed circuit. It was also reported that the materials for the sunlight absorbing electrode are very likely subject to electrochemical corrosion. Therefore, many studies are conducted to the development of new anti-corrosion and high efficiency materials (Grätzel, 2003). Another option to improve the performance of photoelectrolysis is to utilize external power to boost the electrode potential. This is a type of hybrid system of water electrolysis and photoelectrolysis. In the hybrid system, the electrode must be a sunlight absorbing material, so this section suggests that the system is still viewed as photoelectrolysis rather than water electrolysis due to the distinct sunlight capturing and electricity generation patterns. Recently, there is an alternative design of a PEC that makes the entire device a microparticle, nanoparticle, or nanofibre (Vayssieres, 2009; Solarska et al., 2012; Yang, 2011; Grimes et al., 2007). The device includes the mini-cathode, mini-anode and photovoltaic components sandwiched together in single particles that are suspended in an aqueous solution. Hydrogen is evolved at the cathode and oxygen at the anode. The reaction byproducts, OH- and H+ recombine in the solution completing 272 Solar energy sciences and engineering applications the cycle. No external wiring is required as all components are internally connected. Short electron pathways and large surface areas result in improved efficiencies. The major drawback of this design is the generation of both H2 and O2 at the same location. Therefore, hydrogen is not produced separately in the reactor, although each hydrogen molecule is formed separately if viewed from the level of a single molecule. The mixture of hydrogen and oxygen is quite sensitive to sparks that may lead to an explosion. The separation of hydrogen and oxygen is a major engineering challenge. It is difficult to directly compare the photoelectrolysis and water electrolysis because the range of technologies is very broad, so only a few general conclusions are made here on the basis of approximate similarity. Currently, the band gaps that can be provided by photoelectrode materials are still large, e.g., greater than 3.2 eV, although there are some materials capable of providing smaller band gaps. This makes the solar irradiance of larger wavelengths such as infrared less available, hence the sunlight usage efficiency is low (Walter et al., 2010; Conibeer et al., 2007; Currao, 2007; Prakasam, 2008; Grimes et al., 2007). Therefore, the overall solar-to-hydrogen efficiency of current photoelectrolysis or photoelectrochemical hydrogen production units rarely reaches higher than 16% (Solarska et al., 2012; Yang et al., 2011; Prakasam 2008; Licht et al., 2000; Mohapatra et al., 2007). Also, in comparison with water electrolysis, it is more challenging for the photoelectrolysis or photoelectrochemical unit to efficiently track the sun because of the structure and operating complexity of the equipment to simultaneously process hydrogen, oxygen, water, and sunlight window. For example, the contact between an electrode and water may be changed when the equipment is tilted for efficient sunlight tracking. In addition, the auxiliary components of the system may occupy a large portion of the sunlight projection area or shade the sunlight in the sunlight tracking operation. The lack of a combination of a stable, efficient light absorption system consisting of suitable photoelectrodes and light windows partly accounts for the low efficiency. No reliance on the external power source may bring some advantages, including simplicity of system design because of the elimination of the auxiliary components required by the electrolyzer, and potentially a large photoanode and photocathode surface with nanosize materials (Yang et al., 2011; Walter et al., 2010; Conibeer et al., 2007; Currao, 2007). Also, photoelectrolysis or photoelectrochemical hydrogen production plants can be more readily distributed in hydrogen fueling stations or remote geographic areas to avoid building an expensive power transmission and distribution grid for otherwise using water electrolysis. 9.2.6 Photochemical, photocatalytic, photodissociation, photodecomposition, and photolysis In addition to using concentrated solar thermal energy or electricity, there is another way to use the irradiance for the water splitting. As shown in Equations 9.2.3 and 9.2.5, for a single water molecule, if the photons are directly trapped by some auxiliary substances (sensitizers and catalysts) to activate the electrons to a higher energy state, then the water molecules can capture the activated electrons from auxiliary substances. As a result, the water molecules are activated to a high energy state, preparing for further formation of hydrogen and oxygen atoms (Hagiwara et al., 2006). This series Solar hydrogen production and CO2 recycling 273 of photochemical reactions are described as follows: 4S + 4h. = 4S* (9.2.13) 4S* + 4CR = 4S+ + 4(C- R)* (9.2.14) 4(C- R)* + 4H+ = 4CR + 4H* (9.2.15) 4S+ + 4COX = 4S + 4C+ OX (9.2.16) 4C+ OX + 2H2O = 4COX + 2O* + 4H+ (9.2.17) 4H* = 2H2 (9.2.18) 2O* = O2 (9.2.19) where h is the Planck constant (6.626×10-34 J · s), . is the photon frequency, and their product means a photon and its energy. Also, S means sensitizer and CR and COX indicate the catalysts for the reduction and oxidization reactions, respectively. The superscript asterisk means the activated state. If viewed from the reaction mechanism shown in Equations 9.2.13–9.2.19, photochemical, photodissociation, photodecomposition, photodegradation, photocatalytic, and photolysis can be categorized as the same type. The reaction mechanism may suggest very different engineering approaches, compared with the heat and electric potential driven water spitting, which will be discussed in later sections. In this type of water splitting processes, the performance of the sensitizers and catalysts is often assessed with photon-use efficiency, which is defined on the basis of the absorbed photons (Melis 2004), which is often adopted for describing the efficiency of the photocatalytic reactions. Another useful efficiency is called “quantum efficiency’’, which is defined as the ratio of the number of charge carriers collected by a solar cell to the number of photons illuminating on the solar cell (Park et al., 2009). This efficiency is often adopted to evaluate the yield of incident photon to charge carriers for photovoltaic panels. Since photon energy varies with wavelength, consequently the quantum efficiency may vary for different wavelengths of light. There are also definitions on the values of enthalpy and Gibbs free energy (Rajeshwar et al., 2008) but in units of ev/molecule from the molecular level. Their values are shown in Equations 9.2.3 and 9.2.5, i.e., 2.97 ev/molecule and 2.47 ev/molecule for energy balance and spontaineity threshhold, respectively. As discussed in the previous sections, an electrode must be utilized to create sufficient potential and sunlight must be converted to electric current in the water electrolysis and photoelectrolysis. If an electrode is not needed, then the terminologies “photochemical’’, “photocatalytic’’, “photodissociation’’, “photodecomposition’’, and “photolysis’’ can be regarded to have a common engineering setup and similar reaction mechanisms that do not utilize the electric potential to break the chemical bond of hydrogen and oxygen. If there is no electrode, the energy of the photons must be absorbed and stored in some intermediate reagents and then delivered by the reagent to water molecules. Water is transparent to a large portion of the photons in the terrestrial solar spectrum and the photons cannot be directly utilized to break the hydrogen and oxygen bond. Firstly, a reagent is needed that must have the ability to serve as a photon sensitizer to absorb photons and use the photons to activate 274 Solar energy sciences and engineering applications Figure 9.2.6 Components of a photochemical unit. the electrons to a higher energy state. Secondly, two electrons are needed to form a hydrogen molecule. However, the energy carried by a single photon in the solar spectrum can activate only one electron for the reduction of a proton. Therefore, another reagent (catalyst) is needed to accumulate the activated electrons for the formation of hydrogen molecules. In order to split water molecules rather than other substances, the catalyst, or again one more catalyst, must also have the ability to capture electrons from the negative valence oxygen atom of the water molecule (Kudo, 2007). As to the formation of an oxygen molecule, four electrons must be extracted by the catalyst to prepare for the oxidization process. The processes and reactions taking place on the sensitizers and catalysts are shown in Equations 9.2.13–9.2.19. Figure 9.2.6 shows the most basic components needed by a photochemical hydrogen production cell, including a sunlight window, sensitizer and at least one catalyst. It can be found that a sensitizer and catalyst are preferably distributed uniformly in the water so as to intercept more sunlight and approach sufficient contact with water. As with particle-based PEC, hydrogen is not produced separately, leading to similar challenges. It can also be found that the preparation and performance of the sensitizer and catalyst may greatly influence the efficiency and economics. For example, if the lifetime of the catalyst or sensitizer is not long, then it must be replenished frequently. This may increase the operating cost and generate a waste stream, which then may not be a strictly clean hydrogen production technology. There are many engineers and scientists focusing on the development of low-cost, highly-efficient, and long-lifetime sensitizers and catalysts (Grätzel, 2003; Hagiwara et al., 2006). By improving the Solar hydrogen production and CO2 recycling 275 technology of manufacturing the sensitizer and catalyst, for example with nanotechnology, then photochemical hydrogen production has the potential to be improved significantly. Recent advancement in the creation of supramolecular catalysts has combined the sensitizer and the catalyst into a single unit. These units are designed to be either to be a Hydrogen Evolving Reaction (HER) (Vayssieres, 2009) or Oxygen Evolving Reaction (OER) (Crabtree, 2010). Each of these reactions operates as half cells. The HER requires an influx of electrons and light, then reduces water to produce hydrogen gas and OH- ions. The OER requires light and oxidizes water to produce oxygen gas, H+ ions, and an excess of electrons. By coupling the two half cells together using electrodes and a proton exchange membrane (PEM), a complete reactor can be built (Zamfirescu et al., 2011). Alternatively, the OER could instead use light to oxidize the OH- produced from the HER to produce oxygen gas, water and an excess of electrons. Overall, this is a lower energy pathway than the OER presented above, but there are larger challenges in finding a suitable membrane. Even though electrodes are present, this is not classified as electrolysis or photoelectrolysis as the reactions do not occur at the electrodes but at the catalysts. The electrodes are only present to complete the electron circuit. Unlike the previously described method, the hydrogen and oxygen are produced separately, creating similar engineering challenges to those discussed in photoelectrolysis. The advantages of photoelectrolysis over conventional electrolysis are also expected for photochemical water splitting, such as the elimination of a power source and auxiliary components of the electrolyzer. In addition, photochemical processes can be implemented in homogeneous catalytic compounds promoting the HER (Wang et al., 2011). Hence, the processing of catalysts can be greatly simplified to the processing of a fluid in engineering. Also, greater tunability is possible with modular architectures and precise details of molecular scale transformations are more accessible for the research (Teets et al., 2011). By comparison, the photonelectrode cannot be homogeneous even if the size is at the nanoscale, otherwise the band gap won’t be satisfied and the photovoltage cannot be created. Similar to photoelectrolysis, it is challenging for the photochemical unit to efficiently make use of the solar irradiance of all wavelengths due to the wavelength selectivity of catalysts (Maeda et al., 2006, 2010; Li et al., 2011). The solarto- hydrogen efficiency of an operational small pilot-scale photocatalytic hydrogen production demonstration of 1.88 liters per hour is even below 1% (Jing et al., 2010). Therefore, the overall efficiency of current photochemical hydrogen production units rarely reaches 10%, although the quantum efficiency at a specific wavelength could reach 56% (Li et al., 2011; Maeda, 2010, 2011; Kudo, 2009). Considering the sunlight tracking challenges due to the structure and operating complexity of the equipment to simultaneously process hydrogen, oxygen, water, and a sunlight window, it can be concluded that much further research and development is needed towards commercialization of the photocatalytic hydrogen production. 9.2.7 Hybrid and other hydrogen production methods Two or more of the technologies presented in the previous sections can be combined together for the production of hydrogen, for a hybrid production technology possessing 276 Solar energy sciences and engineering applications Figure 9.2.7 Hybrid use of concentrator photovoltaic panel and high temperature electrolysis. the potential to deliver a higher energy and hydrogen production efficiency. A reported hybrid technology is a concentrator photovoltaic (CPV) utilizing water-cooled multisun photovoltaic panels to receive tens or hundreds of suns to generate electricity with visible light at a much higher efficiency than one-sun PV panels, and at the same time generate steam with concentrated infrared for high temperature electrolysis (HTE) (Lasich, 1999). As shown in Figure 9.2.7, the sunlight is concentrated by a parabolic dish or trough, and the water is heated by cooling the PV panels and evaporated by receiving the reflected infrared radiation. At the focal spot, a spectral splitter reflects infrared radiation but allows for the transmission of visible light to high-efficiency solar photovoltaic cells behind the splitter. The reflected infrared radiation is conducted to the steam generator for the HTE process. The electricity for the electrolysis is generated from the concentrated visible light. It was reported that this type of system could provide 40% for the solar-to-hydrogen efficiency in the near term (McConnell et al., 2005, 2006; Thompson, 2005). A major challenge of this technology is that the hydrogen and oxygen are produced in a mixture, which may have drawbacks for the separation in the scale-up of the system. Solar hydrogen production and CO2 recycling 277 9.3 SOLAR-BASED CO2 RECYCLING WITH HYDROGEN As presented previously, solar-based hydrogen production is not only a substitute for fossil fuels in the future, it is also a necessity for CO2 recycling and hydrogenation in synfuel production.With the addition of hydrogen toCO2, methanol and its derivatives can be produced. This will significantly increase the sustainability of our limited fossil fuel resources. Currently, there are several methods of adding hydrogen to CO2. A technology is methane-assisted processes utilizing the hydrogen in methane to convert CO2 to carbon-based fuels, for example (Von Zedtwitz-Nikulshyn, 2009), CH4 (g) + CO2 (g) = 2H2 (g) + 2CO (g) (9.3.1) The CO2 present in Equation (9.3.1) can also be captured by a CaO-based cycle and then used in the following reaction to synthesize fuels: CaO (s) + CO2 (g) = CaCO3 (g) (9.3.2) CaCO3 (s) + CH4 (g) = CaO (g) + 2CO (g) + 2H2 (g) (9.3.3) However, this technology is not strictly renewable because methane is used in the processes. So it won’t be further discussed in detail in this section. A renewable option is to use H2 and CO2 to synthesize methanol catalytically (Fornero et al., 2011; Olah et al., 2009): 3H2 (g) + CO2 (g) = CH3OH (g) + H2O (g), 260.C,H. = -49.7kJ/mol (9.3.4) The enthalpy change of the preheating process of the reactants is: [3H2 (g) + CO2 (g)] of 20.C = [3H2 (g) + CO2 (g)] of 260.C,H. = 30.6kJ/mol (9.3.5) The enthalpy change of Reaction 9.3.4 is a negative value, indicating an exothermic reaction. As the preheating of 3 moles of H2 and 1 mole of CO2 from 20.C to 260.C requires 30.6 kJ, which is smaller than the heat released from Reaction 9.3.4, the methanol production process can be assumed as a self-sustained process if the heat losses and the heat recovered from the cooling of the products can offset the electricity requirement. Then it can be approximated that the energy consumption for the CO2 recycling is mainly established by the H2 production and CO2 capture, which will be examined in the following sections. Table 9.3.1 lists the energy requirements of hydrogen production with the solarbased conventional water electrolysis and hybrid Cu-Cl thermochemical cycle. The energy requirement of the hybrid Cu-Cl cycle for hydrogen production is 222 MJ/kg H2 of solar thermal energy and 32 MJ/kg of solar electricity (Wang et al., 2010). The energy requirements of CO2 recycling for synfuel production include the capture and purification of CO2 from industrial emissions and ambient air. A challenge in the industrial design of CO2 recycling is the major energy requirements of CO2 capture, which corresponds to a high energy cost. The energy requirements are influenced 278 Solar energy sciences and engineering applications Table 9.3.1 Energy requirements of electrolysis and hybrid thermochemical Cu-Cl cycle. Energy requirement Solar thermal energy-to thermal H2 production hydrogen efficiency MJth/kg electricity method % (MJth/kmol) MJe/kg Water electrolysis 40% 0 161 (0) Cu-Cl cycle 50% 222 32 (444) by many factors. They have been actively studied by others for the capture of carbon dioxide from flue gases or air (Finkenrath, 2011; Von Zedtwitz-Nikulshyna, 2009; Stolaroff, 2006). The focus of this section is to examine the heat requirements by sorption processes. The energy requirements include thermal energy for CO2 release from sorbents and electricity for capturing and transporting emissions comprising CO2 and other gases to CO2 absorption equipment. As the electricity requirement is influenced by many parameters such as the emission composition, distance between the solar thermal energy capture site andCO2 capturing plant, and the flow type forCO2 absorption and desorption processes, this section will focus on the thermal energy requirement of aCO2 capture method, which is characterized by both quality and quantity influencing the feasibility of linking the CO2 capture process with a nuclear reactor. Table 9.3.2 summarizes the heat requirements of CO2 absorption and desorption processes of some CO2 capture cycles currently under active investigation, including Na2CO3-based (Nikulshina et al., 2008; Liang et al., 2004; Lee et al., 2008), K2CO3- based (Zhao et al., 2010; Lee et al., 2004, 2008), CaO-based (Blamey et al., 2010; MacKenzie et al., 2007; Salvador et al., 2003), CaO-NaOH-based (Mahmoudkhani et al., 2009; Siriwardane, 2007), and MEA-based methods (Han et al., 2011; Yeh et al., 2001). The adsorption heat is evaluated with data of the National Institute of Standards and Technology (NIST, 2012), and other investigators (Han et al., 2011; Yeh et al., 2001). The table shows that all adsorption processes are exothermic but occur below 100.C, so heat is not easily recovered. As for the CO2 desorption processes, they are all endothermic. Also, CaO-based and NaOH-CaO-based cycles may directly provide high purity CO2 because of no need of separation of CO2 and water vapour. However, these two processes have a temperature threshold of 900.C, which can only be satisfied by a large intensity of concentrated solar irradiance. The temperature requirements of CO2 desorption processes of other cycles are below 200.C, which can be satisfied by current industrial solar concentrators. This provides a good flexibility for the linkage of a solar thermal power plant and a CO2 capture plant. The enthalpy changes of different desorption processes may differ significantly in the range of 135–180 kJ/mol CO2, as shown in Table 9.3.2. Except for CaO-based and NaOH-CaO-based cycles, other carbon capture cycles require a thermal energy range of 135–165 kJ/mol CO2 below 200.C. To have an approximation of the CO2 capture capacity with off-peak hours of a nuclear plant, the thermal efficiency of the CO2 Solar hydrogen production and CO2 recycling 279 Table 9.3.2 Heat requirements of some typical CO2 capture cycles. Process enthalpy H(a), Cycle Function Processes T, .C kJ/mol Na2CO3-based Absorption Na2CO3 (s)+H2O (g)+CO2 (g)=2NaHCO3 (s) 20–60 -135.5 Desorption 2NaHCO3 (s)=Na2CO3 (s)+H2O (g)+CO2 (g) 120–180 135.5 K2CO3-based Absorption K2CO3 (s)+H2O (g)+CO2 (g)=2KHCO3 (s) 20–60 -140.9 Desorption 2KHCO3 (s)=K2CO3 (s)+H2O (g)+CO2 (g) 120–180 140.9 CaO-based Regeneration CaO (s)+H2O (l)=Ca (OH)2 (s) 100 -65.3 Absorption Ca (OH)2 (aq)+CO2 (g)=CaCO3 (s)+H2O (g) 100 -69.8 Desorption CaCO3 (s)=CaO (s)+CO2 (g) 900 179.2 CaO-NaOH- Absorption 2NaOH (s)+CO2 (g)=Na2CO3 (s)+H2O (g) 20–60 -127.2 based Precipitation Na2CO3 (s)+Ca (OH)2 (aq)=CaCO3 (s)+ 20–60 -5.3 2NaOH (aq) Desorption CaCO3 (s)=CaO (s)+CO2 (g) 900 179.2 Alkalization CaO (s)+H2O (l)=Ca (OH)2 (s) 100 -65.3 MEA-based(b) Absorption RNH2 +H2O (l)+CO2 (g)=RNH+ 3 +HCO- 3 38 -72.0 Desorption RNH+ 3 +HCO- 3 =RNH2 +H2O (g)+CO2 (g) 120 165.0 (a) H is the process enthalpy change. A positive value for H means endothermic (requiring heat), otherwise exothermic (releasing heat). (b) MEA, also ETA, is monoethanolamine, which is often denoted by RNH2, where R is “OH (CH2)2’’ [Ali 2004]. capture from flue gases rather than air is assumed to be 50%, which is an average value of the 40%–60% range reported by investigators for various CO2 capture methods (Tzimas, 2009; Von Zedtwitz-Nikulshyna, 2009; David et al., 2000). Therefore, the thermal energy forCO2 capture from flue gases is approximately 270–360 kJ/molCO2. Other energy requirements in the CO2 capture process include at least three portions: (i) work to transport the flue gas to the CO2 capture process for the separation of CO2 and other gases; (ii) work to compress the concentrated CO2 to the reservoir pressure, and (iii) work to move the compressed CO2 into a distant storage location including a storage tank or geologic formation. It can be shown that the lower bound of the total work with ideal Second-Law efficiencies for these three portions is about 9, 13 and 2 kJ/mol CO2, respectively, assuming the flue gas comprises 78% N2 from the atmosphere, 15% CO2 from the oxidation of the carbon in the hydrocarbon, 7% steam, reservoir pressure of 70 bars, and the ground water depth is only 2 km. Assuming further the isothermal compression efficiency is 65%, then the total electricity requirement to complete the above three steps is approximately 37 kJ/mol CO2 (House et al., 2009). Taking the value of 45% as the conversion efficiency for the solar thermal energy conversion to electricity, then the primary solar thermal energy is about 82 kJ/mol CO2. Consequently, the total thermal energy requirement for CO2 capture and storage lies in the range of 352–442 kJ/mol CO2. Table 9.3.3 summarizes the energy requirements of the H2 production, CO2 capture and compression for methanol synthesis for the production of 1 mole of methanol with Reaction 9.3.4. Figure 9.3.1 shows the percentage of CO2 capture in the synthesis of methanol production based on the data of Table 9.3.3. It can be concluded that a key to CO2 recycling is an economic hydrogen source, because the energy required 280 Solar energy sciences and engineering applications Table 9.3.3 Distribution of energy requirements of methanol production. H2 production: equivalent thermal CCS: equivalent thermal energy requirement energy requirement for 3 moles of H2 for 1 mole of CO2 Method kJ method kJ Cu-Cl cycle 1759 Na2CO3-based 353.0 1759 K2CO3-based 363.8 1759 CaO-based 440.4 1759 MEA-based 412.0 Water electrolysis 2147 Na2CO3-based 353.0 2147 K2CO3-based 363.8 2147 CaO-based 440.4 2147 MEA-based 412.0 Figure 9.3.1 Percentage of CO2 capture energy in the synthesis of methanol production. by hydrogen preparation accounts for the majority of energy cost in the methanol production process. As Reaction 9.3.4 can be self-sustained and the solar reactors for hydrogen production have been presented in the hydrogen production section of the chapter, this section will focus on the reactors for CO2 capture, particularly the extraction of solid carbonates from the aqueous solution and the release of CO2 from the carbonates or sorbents. As the carbonation and calcination operations are very mature in industry, Solar hydrogen production and CO2 recycling 281 the major modifications are conducted for the usage of solar thermal energy. A solarbased spray carbonator and fluidized bed calcinator were tested at a large laboratory scale (Nikulshina et al., 2006, 2009). Both reactors have a transparent section allowing for the direct heating of the solid reactants. As indicated in Table 9.3.2, solid and gas must be processed in the same reactor. Sometimes the aqueous solution of the solid is used for the CO2 capture, so the heat transfer is a multiphase process and preferable that the solid can be directly heated by the solar irradiation due to the poor heat transfer performance of solid particles. This explains why the reported reactors are transparent, which are different from solar hydrogen production reactors, where only gases are processed. 9.4 SUMMARY This chapter presented scenarios of using solar-based hydrogen and CO2 recycling to provide a sustainable solution to the increasing demand of clean energy and ongoing depletion of conventional fossil fuels. The intermittency issue of solar energy can also be significantly addressed with the usage of solar-based hydrogen as well as synfuels produced from recycled CO2 and H2. Then this chapter examined the solarto- hydrogen reaction mechanisms and technologies including thermochemical cycles utilizing solar thermal energy to split water molecules, conventional electrolysis utilizing solar-generated electricity to split water molecules, and photochemical processes utilizing photon-activated electrons of auxiliary reagents (sensitizer and catalyst) to activate and split water molecules. This chapter also examined and suggested some categorization criteria for technologies from the reaction mechanisms and engineering approaches, particularly the latter. The basic components for the hydrogen production apparatus and major advantages and challenges of the technologies were also examined. It was concluded that conventional water electrolysis powered by solar generated electricity is more mature than other technologies, but the solar-to-hydrogen efficiency is currently below 16% due to the energy loss in the conversion of solar irradiation to electricity. Thus, its efficiency improvement is mainly determined by the increase of solar-to-electricity conversion efficiency, which is a maximum of about 20% for currently operating PV panels and solar thermal plants. A hybrid method involving a high temperature electrolysis and spectrum splitter may utilize more heat than conventional electrolysis. Consequently, the hydrogen production efficiency is greatly increased to 40–50%, but it is challenging to find appropriate materials for the electrodes and electrolyte that must withstand high temperature steam, hydrogen and oxygen. Thermochemical cycles benefit from large production scales in order to minimize the energy losses arising from high temperature requirements and multiple auxiliary processes for an integrated operation of the thermochemical cycle. A high temperature of 1,500–2,500.C required by the metal oxide redox pair cycles may keep the cycles from being utilized in the short term, although the cycles usually have only two chemical reactions. The solar-to-hydrogen efficiency of thermochemical cycles was estimated to be in the range of 40–60%, which is higher than conventional electrolysis because the cycles use thermal energy as the major energy input with no energy loss due to the conversion of thermal energy to electricity. 282 Solar energy sciences and engineering applications The solar-to-hydrogen efficiencies of both photoelectrochemical and photochemical technologies are significantly limited by the activity and wavelength range of the photoelectrodes and photocatalysts. Even if the quantum efficiency reaches as high as 56%, the solar-to-hydrogen efficiency is still below 16% and 10% for the existing photoelectrochemical and photochemical cells, respectively. Both thermochemical cycles and water electrolysis may need additional hydrogen distribution systems. By comparison, photoelectrochemical and photochemical technologies are more suitable at hydrogen fueling stations because fewer processes are needed. Therefore, they are more suitable for serving as hydrogen fueling stations with no need of extra hydrogen distribution systems. The extension of the working wavelength of the materials for a solar PV panel, photoelecltrodes, and photo catalysts, to visible light (400–700 nm) and the infrared (700–2400 nm) range is a useful future research direction to improve the solar-to-hydrogen efficiency, as these two spectral ranges occupy more than 90% of the total solar irradiance. This chapter also examined the energy requirements of synfuel production from captured CO2 and hydrogen. It was found that the synfuel production reaction is selfsustainable, so it was concluded that the energy requirements of synfuel production are mainly determined by the hydrogen production and CO2 capture. The hydrogen production energy consumption is about 5–7 times the level for CO2 capture. The CO2 capture methods such as Na2CO3-based, K2CO3-based, CaO-based, and MEA-based processes were examined. 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(2006) Hydrogen production via the solar thermal decarbonization of fossil fuels. Solar Energy, 80, 1333–1337. Zedtwitz, P. v. and Steinfeld, A. (2003) The solar thermal gasification of coal – energy conversion efficiency and CO2 mitigation potential. Energy, 28, 441–456. Zhao, C., Chen, X. and Zhao, C. (2010) Multiple-cycles behavior of K2CO3/Al2O3 for CO2 Capture in a fluidized-bed reactor. Energy and Fuels, 24, 1009–1012. This page intentionally left blank Chapter 10 Photoelectrochemical cells for hydrogen production from solar energy Tania Lopes, Luisa Andrade & Adelio Mendes Laboratório de Engenharia de Processos, Ambiente e Energia (LEPAE), Faculdade de Engenharia da Universidade do Porto, Rua Roberto Frias, Porto, Portugal 10.1 INTRODUCTION The awareness concerning carbon dioxide emissions and the depletion of fossil fuel reserves motivates the development of innovative processes to take advantage from renewable energy sources (Grätzel, 2005). The world power consumption is currently about 13TW and it is expected to increase up to 23TW by 2050 (Dobran, 2010). With approximately 120PW of solar energy continuously striking the earth at any given moment, the challenge in converting sunlight into electricity via photovoltaic (PV) cells is to reduce the cost per watt of delivered solar electricity (Krol et al., 2008 and Nathan, 2005), which is already approximately 0.65 a/Wp for crystalline silicon modules. The solar PV technology has greatly evolved in the last decade and it is now a well-established way to convert solar energy into electric energy, which accounts presently more than 21GW installed worldwide (The Energy Report, 2011). Nevertheless, this technology only works on a daily basis and it largely depends on the amount of solar radiation available. Thus, an effective method to store energy for later dispatch is still needed (Trieb, 2005). A practical way to convert sunlight into a storable energy form is using a photoelectrochemical (PEC) cell that splits water into hydrogen and oxygen by light-induced electrochemical processes (Grimes et al., 2008). Hydrogen production via photoelectrochemical water-splitting is a thriving alternative that combines photovoltaic cells with an electrolysis system (Bard and Fox, 1995; Khaselev and Turner, 1998). The major advantage is that solar harvesting, conversion and storage are combined in a single integrated system (Nathan, 2005). The hydrogen generated by this process has the potential to be a sustainable carbon-neutral fuel since it is produced from a renewable source and it can be stored or transformed into other chemicals such as methanol or methane (Grimes, 2008; Zerta, 2008). 10.2 PHOTOELECTROCHEMICAL CELLS SYSTEMS OVERVIEW 10.2.1 Solar water-splitting arrangements Converting sunlight into hydrogen and oxygen through water-splitting can be accomplished via different technologies, as sketched in Figure 10.2.1. More specifically, via three general types of devices: i) composed devices – photovoltaic (PV) cell associated with an electrolyzer or photovoltaic (PV) cell associated with a PEC cell; ii) stand 294 Solar energy sciences and engineering applications Figure 10.2.1 Solar water splitting based on PEC cells, PV cells or combined arrangements systems. The red line represents the “holy grail’’ of the PEC system. alone devices – semiconductor-liquid junction (SCLJ) photoelectrochemical cell (Krol et al., 2008; Grimes et al., 2008; Aruchamy et al., 1982; Minggu et al., 2010); and iii) by thermochemical cycles (Coelho et al, 2010). The third technology will not be considered in this chapter. 10.2.1.1 Composed devices Up to today, no semiconductor photoelectrode is able to efficiently perform alone water-splitting and thus an extra bias must be supplied. The most developed technology is the PV device/electrolyzer arrangement where the photovoltaic cells are silicon based, achieving maximum efficiencies of 15%, and efficiencies of electrolyzers is often around 75%1. For instance, combining commercial 12% efficiency PV modules with a water electrolysis unit operating with an energy conversion efficiency of approximately 65% (output voltage of 1.9 V) results in a solar-to-hydrogen efficiency of about 7.8% (Kruse et al., 2002; Bansal et al., 1999; Bilgen, 2001). Only by combining optimized PV technologies it is possible to achieve higher solar-to-hydrogen conversion efficiencies (Green et al., 2008; Conibeer and Richards, 2007). Even if biasing an electrolyzer with a separate set of solar cells is very attractive from an efficiency point of view, the fact of involving two separate devices complicates the system and increases the cost (Krol et al., 2008), besides being more energy dissipative. Furthermore, in a system PV+electrolyzer at least four silicon PV cells connected in series are required to generate the desired voltage for water-splitting, which under 1The efficiency of an electrolyzer is defined as .Z =E./V, where E. is the thermodynamic cell potential (1.23V for water electrolysis) and V is the voltage applied to the cell under operating conditions. Photoelectrochemical cells for hydrogen production from solar energy 295 unfavorable climatic conditions such as partial shading, haze or cloudiness may ultimately interrupt the photoelectrolysis. Similar problems are observed when a PV cell is used as external bias for a PEC cell to promote water-splitting. In this case, the electric current generated by the PV cell goes directly to the PEC cell instead of feeding an electrolyzer, resulting in a cheaper but not necessarily more efficient embodiment (Grimes et al., 2008; Minggu et al., 2010). Still, two separated parts must be considered when estimating the initial and operating costs. Moreover, the available area for solar exposure must be substantially increased since both PV and PEC cells have to be illuminated (Minggu et al., 2010; Conibeer and Richards, 2007). In this sense, a more effective approach would be to “merge’’ the PV cell with an electrolyzer to make photoelectrochemical devices with a semiconductor-liquid junction. Thus, several efforts have been made to design a monolithic system to partially avoid the previously mentioned technological and economic drawbacks (Minggu et al., 2010). 10.2.1.2 Single devices Single water-splitting devices (Figure 10.2.1) can be divided into biased and zero biased systems. Concerning biased systems, there are chemically biased photo-assisted photoelectrolysis cells and tandem devices (Grimes et al., 2008; Minggu et al., 2010). In the first case, the bias is achieved using two different electrolytes (e.g. acid and basic electrolytes) placed in two separated half-cells. However, this configuration is not selfsufficient, relying not only on sunlight but also on additional input of chemicals to stabilize the electrolyte solutions (Minggu et al., 2010). In the tandem approach, the cell is normally characterized by layered stacked or hybrid structures involving several different semiconductor films placed on top of each other. In this configuration, at least one of the substructures must work as a bias source. These internal biased photoelectrode tandem structures can be subdivided into: i) PV/PEC (Miller et al., 2005); ii) PV/PV (Khaselev et al., 2001) and; iii) PEC/PEC (Grätzel and Augustynski, 2001). The use of PV/PEC systems has an advantage over PV/PV systems because the PEC face (layer) can replace the face conductor grids that partially obscure the PV layer. Consequently, PEC panels are able to reduce some cost components and improve photon capture of PV layer (James et al., 2009). Recently a new multiphoton combination of a PEC cell and two dye cells (tandem arrangement) was proposed (Brillet, 2010). Three different architectures were suggested. The authors found that the “trilevel’’ tandem architecture (hematite/squaraine dye/black dye) produces the highest operating current density. The expected highest solar-to-hydrogen efficiency was about 1.36%. However, this value is far below the expected 3.3% that should be possible with the nanostructured hematite photoanodes used (Brillet, 2010). Photoelectrochemical devices with no additional bias represent a prospective pathway to overcome the complexity of biased systems. No-bias photoelectrochemical devices comprise single and multiple photo-system arrangements. The possible arrangements of single photo-system are: i) n-type semiconductor photoanode and a metal counter-electrode (Figure 10.2.2a) (Kay et al., 2006); ii) p-type semiconductor photocathode and a metal counter-electrode (Figure 10.2.2b) (Chandra et al., 1985); 296 Solar energy sciences and engineering applications Figure 10.2.2 No-biased single photo-system configurations for solar water splitting (SC – semiconductor; M – metal;WE – working electrode; CE – counter-electrode) – (adapted from Minggu et al., 2010): a) n-type semiconductor photoanode and a metal counter-electrode; b) p-type semiconductor photocathode and a metal counter-electrode; and c) monolithic configuration. Figure 10.2.3 Different no-biased multiple photo-system configurations for solar water splitting (SC – semiconductor; M – metal;WE – working electrode; CE – counter-electrode) – (adapted fromMinggu et al., 2010): a) n- and p-type semiconductors wired; b) n- and p-type semiconductors linked by an ohmic contact; and c) hybrid systems. iii) monolithic configuration – bipolar system and a layered metal counter-electrode (Figure 10.2.2c). Concerning multiple photo-system arrangements, the following configurations can be identified: (Minggu et al., 2010) i) n- and p-type semiconductors (acting as photoanode and photocathode, respectively) (Nozik, 1976), wired (Figure 10.2.3a) or linked by an ohmic contact (Figure 10.2.3b); Photoelectrochemical cells for hydrogen production from solar energy 297 Figure 10.2.4 Schematic representation of the relevant processes involved in the photo-hydrolysis of water. ii) hybrid systems with layered structures involving several different semiconductor films stacked on top of each other (Figure 10.2.3c). All the presented routes to convert solar energy into hydrogen show advantages and disadvantages. Nevertheless, there is a consensus that the single photo-system is the holy grail of PEC technology in terms of simplicity, packaging and overall system costs (Krol et al., 2008). 10.2.2 Working principles of photoelectrochemical cells for water-splitting The principle of converting sunlight into hydrogen by water photoelectrolysis using a single photon-system, taking as an example a n-type semiconductor in an alkaline media, is illustrated in Figure 10.2.4. The single-photon PEC system for water-splitting is composed of a semiconductor photoelectrode that absorbs photons with sufficient energy to inject electrons from the valence to the conduction band, creating electron-hole pairs – Equation 10.2.1. 2hv . 2e- + 2h+ Photon-induced electron-hole pair generation (10.2.1) As sketched in Figure 10.2.4, the excited electrons percolate through the semiconductor layer reaching the counter-electrode, via the external circuit, to promote water reduction at its surface – Equation 10.2.2 – while holes oxidize water in the semiconductor surface – Equation 10.2.3 (Krol et al., 2008; Archer and Nozik, 2008). The 298 Solar energy sciences and engineering applications cycle is closed when the electrolyte anions generated at the counter-electrode diffuse back to the surface of the semiconductor to recombine with holes. Cathode: 2H2O + 2e- . 2H2 + 2OH- E. H2O/H2 = -0.828V (10.2.2) Anode: 2OH- + 2h+ . H2O + 1 2 O2 E. O2/OH- = 0.401V (10.2.3) If an acid media is considered, instead of having hydroxyl anions traveling from the counter electrode to the surface of the semiconductor we have hydrogen ions, as described by the following equations: Cathode: 2H+ + 2e- . H2 E. H+ /H2 = 0.0 V (10.2.4) Anode: H2O + 2h+ . 2H+ + 12 O2 E. H2O/O2 = 1.23V (10.2.5) In both cases, i.e. for alkaline or for acid media, the overall PEC water-splitting reaction can be written as follows: H2O + 2 hv . H2 + 1 2 O2 (10.2.6) A similar phenomenon occurs when a p-type semiconductor is used. Nevertheless, for this case the dominant (or the majority) charge carrier is holes, which will travel through the external circuit towards the metal counter-electrode, working now as the anode. On the other hand, electrons travel to the surface of the semiconductor in contact with the electrolyte to reduce water (Nozik, 1978). The minimum potential of -1.23V at 25.C is needed to electrolyze water. The negative sign identifies the process as not being spontaneous and so the reaction cannot occur without additional energy from an external electrical power source. This value is obtained from the following relation: Eo = -Go neF (10.2.7) G. is the standard Gibbs free energy change (+237 kJ · mol-1), representing a thermodynamic minimum for splitting water into the gaseous hydrogen and oxygen at 25.C and 1 bar; Eo is the electric standard potential of the reaction. For a direct photoelectrochemical water-split using a single-photon system, several key criteria must be simultaneously fulfilled: i) The semiconductor system must generate sufficient voltage upon irradiation to split water; ii) The bulk bandgap must make efficient use of the solar spectrum; iii) The band-edge potentials at the surface must straddle the hydrogen and oxygen redox potentials according to the half-reactions described in Equations 10.2.2– 10.2.5; iv) Low overpotentials; v) The system must exhibit long-term stability in aqueous electrolytes; Photoelectrochemical cells for hydrogen production from solar energy 299 Figure 10.2.5 Schematic illustration of a semiconductor with a hypothetically ideal bandgap of 1.9 eV. Right: Intensity of sunlight vs.wavelength for AM1.5 conditions. The grey area represents the part of the spectrum that can be absorbed by a semiconductor with a bandgap of 1.9 eV. vi) The charge transfer from the surface of the semiconductor to the solution must be selective for water-splitting and exhibit low kinetic overpotentials (Chen et al., 2010); vii) The material must be sufficiently abundant, harmless and cost-effective. Since overpotentials are required at various points in the system to ensure sufficiently fast reaction kinetics, i.e. related to the electrochemical reaction kinetics at anode and cathode and charge transfer (inside the electrodes and in the electrolyte), the minimum bandgap required to split water is at least 1.9 eV. This value also imposes that the semiconductor is able to absorb light for wavelengths lower than 650 nm, as show in Figure 10.2.5 (Krol et al., 2008). Despite the research efforts to date no single semiconducting material has been found that will fulfill all the requirements needed to generate standalone devices for solar hydrogen production from water-splitting (Krol et al., 2008; Sivula et al., 2011). 10.2.3 Materials overview The keystone in water photoelectrolysis is the development of an efficient, robust, reliable, cost-effective, and stable photoelectrode system (Grimes et al., 2008). The first material recognized to split water under UV light was TiO2, reported by Fujishima and Honda in 1971 (Fujishima and Honda, 1972). Thenceforward, extensive efforts have been made to find a suitable material for efficient photoelectrodes. Thus, during the last three decades, different types of semiconductors were studied such as metal 300 Solar energy sciences and engineering applications Figure 10.2.6 Energy band positions for various semiconductors at pH 14 (Krol et al., 2008). oxide (e.g. Fe2O3, SrTiO3, TiO2 , WO3 , BiVO4, Cu2 O, etc.) and non metal oxide semiconductors (e.g. GaAs, CdS, InP, etc.) (Grimes et al., 2008). The photocorrosion stability of photoanode or photocathode, its wavelength response (bandgap) and current-voltage characteristic are important factors that determine the semiconductor performance in water-splitting – Figure 10.2.6. 10.2.3.1 Metal oxide semiconductor Concerning the metal-oxide semiconductors, only a few are able to fulfill the bandgap and band edge requirements for operating at zero bias voltage; SrTiO3, KTaO3 and ZrO2 are among them but only SrTiO3 has been in fact studied (Memming, 2001). SrTiO3 generates hydrogen without any additional bias even if with a barely small efficiency, less than 1%, which was ascribed to its large bandgap energy (3.4–3.5 eV) (Mavroides et al., 1976). Preparing a semiconductor oxide that meets all criteria needed to achieve efficient water-splitting is a great challenge and the most frequently studied photoelectrode materials are TiO2, WO3, Fe2O3, BiVO4 SnO2 and Cu2O and their modifications. The well known titanium dioxide (TiO2), with different crystalline structures (anatase mostly but also rutile) and arrangements as single crystal, polycrystalline or thin films, have been largely investigated (Nowotny et al., 2007a; Nowotny et al., 2007b; Nowotny et al., 2007c; Nowotny et al., 2007d; Grimes et al., 2008) mainly due to its excellent stability over a wide range of pH and applied potentials, low cost and abundance (Nowotny et al., 2007a). As a major drawback, TiO2 only absorbs in the UV light spectrum due to its large bandgap of approximately 3.2 eV. Several attempts have been made to extend TiO2 spectral response into the visible light by doping it with aliovalent ions, such as W, Ta, Nb, Zn, In, Li and Ge (Karakitsou et al., 1993), Pb (Rahman et al., 1999), Mo and Cr (Wilke and Breuer, 1999), Cr (Bak et al., 2002), C (Khan et al., 2002) and N (Nakamura et al., 2004; Babu et al., 2012). Wilke and co-workers showed the important effect of doping TiO2 with Mo and Cr ions on the Photoelectrochemical cells for hydrogen production from solar energy 301 decrease of TiO2 bandgap (Wilke and Breuer, 1999). An impressive reduction on the bandgap of TiO2 photoanodes was achieved also by Khan et al. (2002) with carbon incorporation into the TiO2-x lattice during heating in a natural gas flame (Khan et al., 2002). Nevertheless, the studies concerning the doping effect of TiO2 semiconducting material do not provide clear conclusions, since the dopant that may have a positive effect on the bandgap (Eg) reduction, and thereby increasing the light absorption, has a negative effect in the energy conversion efficiency (ECE). Moreover, the reduction of Eg should be followed by changing other relevant functional properties. Finally, the procedures used to incorporate the dopants are often arbitrarily selected. Thus, without a solid knowledge of the time and the temperature required to incorporate the dopants, it is truly difficult to replicate the TiO2 material and to obtain a homogeneous distribution in the semiconductor (Bak et al., 2002). The same holds for tin dioxide (SnO2) semiconductor; it has also a large bandgap in the range of 3.1–3.3 eV that makes this material able to absorb only the UV solar spectrum (Grimes et al., 2008). Nevertheless, an n-type single crystal of SnO2 doped with Sb was investigated byWrighton and co-workers for H2 and O2 production with an applied bias of 0.5V under UV light illumination (Wrighton et al., 1976). However, comparing TiO2 with SnO2, the latter requires a slightly higher potential to achieve the photocurrent onset (Grimes et al., 2008). Tungsten trioxide (WO3) is an interesting semiconductor since it has an attractive bandgap of 2.5–2.7 eV (Grimes et al., 2008; Santato et al., 2001; Butler, 1977). Theoretically, a bandgap of 2.7 eV allows use of 12% of the AM 1.0 solar spectrum, a very high value compared to the barely 4% achieved with TiO2 (Grimes et al., 2008; Butler, 1977). Although this material had been widely studied by Deb in 1972, it was Hodes in 1976 that first recognized it as an active visible-light driven photoanode for watersplitting (Hodes et al., 1976). This material shows good stability in water for pH<4 and a favorable energy band edge for oxygen evolution (Butler, 1977). Nevertheless, the minority carrier (hole) diffusion length plays a limiting role in the photoresponse of tungsten trioxide photoanodes due to the indirect bandgap transition (Solarska et al., 2012). Usually, WO3 photoanode is used as thin films and can be found either in the crystalline or in the amorphous forms. To obtain efficient WO3 photoanodes, a highly crystalline structure is desirable since it minimizes the imperfections and the surface contaminations which may lead to a charge trapping and carrier recombination (Meda et al., 2010). Recently, a nanostructured WO3 photoanode has been described capable of producing a photocurrent of about 3mAcm-2 in 3M CH3HSO3 (AM 1.5 G) (Solarska et al., 2012). Alternatively, small bandgap materials can be considered as a starting point in the research into single-photon systems, such as Fe2O3 or BiVO4 (Krol et al., 2008; Kay et al., 2006; Luo et al., 2008; Long and Cai, 2008; Liang et al., 2008; Khan and Akikusa, 1999). Particular attention has been given to hematite (a-Fe2O3) and it has actually been considered a material with great potential for PEC applications. Hematite is one of the most abundant and inexpensive oxide semiconductors with an interesting bandgap of 1.9–2.3 eV, is a non-toxic material and is stable in water (Satsangi et al., 2010). As a drawback, pure-phase a-Fe2O3 has intrinsically poor charge carrier transportation, which limits its quantum efficiency. Moreover, it has poor oxygen evolution reaction (OER) kinetics and the band edges are not well positioned to directly carry out the reduction of water. Intensive research efforts have been conducted to improve 302 Solar energy sciences and engineering applications hematite’s intrinsic electronic properties by varying the deposition method or by doping the photoelectrode with Si, Ti, Pt, Mo and Cr, among other atoms (Kay et al., 2006; Satsangi et al., 2010; Glasscock et al., 2007; Hu et al., 2008; Kleiman-Shwarsctein et al., 2008; Brillet et al., 2010). In an ideal hematite photoanode, the onset is just anodic of the flat band potential with a photocurrent plateau of 12.6mAcm-2 (Tilley et al., 2010). However, until the present, no research group has been successful in splitting water by means an a-Fe2O3 photoanode without assistance of an external bias voltage (Krol et al., 2008). Another interesting material is BiVO4 semiconductor, with a bandgap of 2.4– 2.5 eV and a reasonable band edge alignment with respect to water redox potentials; in fact it has the ability to carry out the water photosplitting reaction (Sayama et al., 2006). Moreover, it has been reported that BiVO4 is able to show both semiconducting properties, n- and p-types, (Vinke et al., 1992) as well as a high photon-to-current conversion efficiencies (>40%) at 420nm (Memming, 2001; Karakitsou and Verykios, 1993). Nevertheless, further improvements on its fundamental electronic structure and stability are still needed (Sayama et al., 2006). Over the past few years several efforts have been made in order to find an efficient harvesting semiconductor under visible light. Cuprous oxide (Cu2O), which works as a photocathode, has an interesting bandgap of 2.0–2.1 eV (Hara et al., 1998). Theoretical calculations indicate that Cu2O can produce up to 14.7mAcm-2, corresponding to a light-to-hydrogen conversion efficiency of 18% based on the AM 1.5 spectrum (Paracchino et al., 2011). For solar water-splitting purposes, Cu2O has favorable energy band positions; the conduction band is located 0.7V negative of the hydrogen evolution potential with the valence band lying just positive of the oxygen evolution potential (Paracchino et al., 2011). Since no overpotential is available for oxygen evolution, the reduction band edge is close to the water reduction potential, the p-type Cu2O can drive half of the water-splitting reaction but an external bias must be applied to conduct the other half reaction (water oxidation) (Paracchino et al., 2011). However, the limiting factor of this material is the poor stability in aqueous solutions, since the redox potentials for the reduction and oxidation of monovalent copper oxide lie within the bandgap (Hara et al., 1998; Paracchino et al., 2011). The corrosion sensitivity issue of cuprous oxide under illumination can be addressed by depositing very thin protective layers by e.g. atomic layer deposition (ALD). In fact, using this methodology, Grätzel and co-workers have designed, up to now, the best performing oxide photoelectrode, 7.6mAcm-2 at 0VRHE, using Cu2O electrodes protected with nanolayers of Al-doped zinc oxide and titanium dioxide activated for hydrogen evolution with electrodeposited platinum nanoparticles, i.e. Cu2O was coated with layers of n-type oxides with structure 5×(4nm ZnO/0:17nm Al2O3)/11nm TiO2 (Paracchino et al., 2011). 10.2.3.2 Non-oxide semiconductor Non-oxide semiconductors (p-type and/or n-type photoelectrodes) are known to efficiently harvest sunlight, converting it into electricity: amorphous, polycrystalline and crystalline silicon (a-Si, p-Si and c-Si), gallium arsenide (GaAs), cadmium telluride (CdTe), gallium phosphide (GaP), indium phosphide (InP), copper indium diselenide (CIS), copper indium gallium diselenide (CIGS) and gallium indium phosphide Photoelectrochemical cells for hydrogen production from solar energy 303 Table 10.2.1 Non-oxide n-type semiconducting materials with small bandgap (Grimes et al., 2008). Semiconductor Bandgap CdSe 1.7 eV CdTe 1.4 eV GaP 2.24 eV GaAs 1.35 eV InP 1.35 eV MoS2 1.75 eV MoSe2 1.5 eV (GaInP2) (Grimes et al. 2008). As mentioned, most of the oxide semiconductors studied for water-splitting show bandgap and stability issues, thus, particular attention has been given to non-oxide materials since they have smaller bandgaps enabling the capture of a larger portion of the solar spectrum energy. Cadmium sulfide (CdS) has well positioned band edges to efficiently reduce and oxidize water with an optical absorption of 520nm (corresponding to a bandgap of 2.4 eV). However, it suffers from anodic photodecomposition by the photogenerated holes (Grimes et al., 2008). Similar problems of photocorrosion can be found with other n-type non-oxide semiconductors as the ones presented in Table 10.2.1. Considering now p-type non-oxide materials, they usually show stable behavior against cathodic photodecomposition since the photoelectrons in excess migrate towards the semiconductor/electrolyte interface such as the p-Si and p-GaP (Grimes et al., 2008). However, the flat band position of these materials is unfavorable for the H2O/O2 redox and a large bias voltage must be applied (Nozik, 1978). One approach to overcome stability issues is covering the unstable photoelectrodes with thin films of stable wide bandgap semiconductors with suitable band edges or with thin metal films (e.g. by chemical vapor deposition (CVD), sputtering or atomic layer deposition) (Nozik, 1978). Although single-photon systems seem to be the preferable route to produce hydrogen from solar energy, either with oxide or non-oxide materials, significant improvements should be accomplished on electronic structure and stability in order to be used alone in PEC systems – Figure 10.2.2a and 2b. Clearly, there are three routes that may result in a high efficient system for watersplitting and without need of an additional bias. These are illustrated in Figures 10.2.2c and 10.2.3a and 10.2.3c. All strategies share the feature of having two semiconductors with different bandgaps. This provides a mechanism by which a single electron is photoexcited twice and, correspondingly generates a larger bias from light. It has been calculated that this type of system could realistically achieve a solar-to-hydrogen conversion efficiency of 21.6% (Bolton et al., 1985). The most compelling approach is however illustrated in Figure 10.2.2c and Figure 10.2.3a. Here, various combinations of n-type and p-type semiconductors, oxide and non-oxide, such as n-TiO2/p-GaP, n-SrTiO3/p-GaP, n-Fe2O3/p -Fe2O3 have been used to eliminate the bias needed for water-splitting – Table 10.2.2. Because of the low performance of the individual electrodes in these dual-photoelectrode devices, the resulting overall efficiency is low. 304 Solar energy sciences and engineering applications Table 10.2.2 Examples of n-p photoelectrochemical cells for water splitting (FIGURE 10.2.3a). Energy conversion n-SC/p-SC Electrolyte Efficiency, . Reference n-TiO2/p-GaP 0.2M H2SO4 0.25% Nozik, 1976 n-SrTiO3/p-GaP 1M NaOH 0.67% Ohashi et al., 1977 n-Fe2O3/p-Fe2O3 0.1 M H2SO4 0.10% Ingler et al., 2006 10.2.4 Stability issues – photocorrosion In photoelectrochemical cells, stability issues are one of the major problems to be solved (Krol et al., 2008). Usually, when a semiconductor electrode is placed in contact with an electrolyte solution some reactions may occur, for instance ionic oxidation or reduction of the semiconductor with simultaneous reduction or oxidation of a component (Gadgil, 1990). The electrolytic reduction of a semiconductor is often associated with the electrons in the valence band, while the electrolytic oxidation reaction is related to holes in the conduction band as electronic reactants (Gadgil, 1990). Following the Gerischer’s derivations, it is possible to formulate the simplest type of decomposition reaction involving a binary semiconductor MX and the solvation (complexing) of the elements (labeled hereafter as “solv’’) as stated next (Memming, 2001): MX + ze- + solv . M+ Xz- solv (10.2.8) for a cathodic reaction, and MX + zh+ + solv . Mz+ solv + X (10.2.9) for an anodic reaction. Using H+/H2 standard potentials as reference, the corresponding reaction for hydrogen may be written as: 1 2 zH2 + solv . zH+ solv + ze- (10.2.10) The addition of Equation 10.2.10 to Equation 10.2.8 or to Equation 10.2.9 yields the corresponding equations for the free energy values, nGsH and pGsH, respectively. The decomposition potentials equations are: pEdecomp = pGsH/z (10.2.11) for the oxidation, and nEdecomp = -nGsH/z (10.2.12) for the reduction of the semiconductor (Memming, 2001). Photoelectrochemical cells for hydrogen production from solar energy 305 Figure 10.2.7 Relative positions of decomposition Fermi Levels of a semiconductor with respect to its band edges: a) cathodically and anodically stable, b) cathodically and anodically unstable, c) cathodically stable but anodically unstable and d) anodically stable but catodically unstable. Adapted from ref (Gerischer, 1977). Figure 10.2.8 Positions of band edges and decomposition Fermi levels for different oxide and non-oxide semiconductors at pH 7. Adapted from (Memming, 2001). The energy positions of the electron-induced potential nEd and the hole-induced corrosion value pEd can be plotted with respect to the band edges Ec and Ev, as shown in Figure 10.2.7. In fact, the criterion for thermodynamic stability of the semiconductor is: pEd > Eredox >n Ed (10.2.13) Figure 10.2.8 shows some decomposition potentials for various semiconductors used to carry out the water-splitting reaction under solar radiation. However, these diagrams show some practical limitations since, besides the thermodynamic stability, the reaction kinetics may also play an important role in the stability definition of a given 306 Solar energy sciences and engineering applications semiconductor material. Actually, none of the semiconductors presented in Figure 10.2.8 have their Fermi level edges positioned as shown in Figure 10.2.7a, meaning that they are cathodically and/or anodically unstable. However, some of these semiconductors show cathodic and/or anodic stability because the reaction kinetics can help in preventing crystal decompositions where the charge transfer of photogenerated carriers in the interface is faster compared to the crystal decomposition (Gadgil, 1970). The stability of a semiconductor in contact with an electrolyte solution strongly depends on the competition between anodic dissolution and redox reaction, which are controlled by thermodynamic and kinetic parameters, respectively (Memming, 2001). Thus, even if the semiconductor oxides are not thermodynamically stable, following Gerischer’s approach, their stabilities can only be achieved in the presence of a suitable redox system for kinetic reasons (Sinn et al., 1990). For instance, even if the metal oxides are thermodynamically stable, based on the nEd and pEd positions, towards cathodic photocorrosion, most of them are unstable towards anodic photocorrosion. Nevertheless, there are some n-type semiconductors, such as a-Fe2O3 and TiO2, which are sufficiently stable in aqueous electrolytes because their decomposition is controlled by very slow corrosion reaction kinetics (Krol and Schoonman, 2008). As suggested by Krol, if the material has a tendency for photodecomposition, this may be prevented by adding a suitable co-catalyst to favor the water oxidation route (Krol and Schoonman, 2008). 10.2.5 PEC reactors A photoelectrochemical cell combines the harvesting of solar energy and the electrolysis of water process in a single device. Thus, when a semiconductor with the ideal set of properties is immersed in an aqueous electrolyte and illuminated, the corresponding photon energy is directly used to split water into hydrogen and oxygen (i.e. in a chemical energy). The basic setup for water-splitting comprises two electrodes immersed in an aqueous electrolyte solution, where one or both electrodes are photoactive. The electrolyte container must be transparent, or have at least a transparent window for allowing light to strike the photoelectrode; then, water-splitting occurs when the energetic requirements are met (Minggu et al., 2010). In a laboratory setup, to measure the PEC cell efficiency, a three-electrode configuration is normally used, the third electrode being a reference one. However, to simulate a real PEC cell application the two-electrode configuration is preferable. The more common PEC cell design is the conventional electrochemical cell used for corrosion studies, with an optically transparent window, as reported by Chen et al. in 2010 (Chen et al., 2010). The optically transparent window is very important for PEC cells to work properly, for instance, a normal soda lime glass cuts off the transmission for wavelengths lower than 350 nm, while a quartz window will normally have a transmittance higher than 90% from 250 nm. Nevertheless, a cheaper material can be used with similar performance; fused silica (amorphous silica) allows transmission values higher than 90% and shows an excellent stability in both acid and alkaline aqueous solutions (except for fluoridric acid) (Krol and Schoonman, 2008). Normally, the research laboratories in water photosplitting usually manufacture their own PEC cells, e.g. simple cubic or cylindrical open vessels, closed vessels equipped with an ion exchange membrane separating hydrogen from oxygen evolutions, H-type PEC cells, sandwich assembly, Photoelectrochemical cells for hydrogen production from solar energy 307 Figure 10.2.9 Example of PEC cells for water splitting with different designs, in a three-electrode configuration. among other more complex cells, such as the ones that allow tandem configurations (PV+PEC system in a single embodiment) – Figure 10.2.9 (Minggu et al., 2010). Presently there are some PEC cells available commercially, e.g. Pine Research Instruments Company (Pine Instrument Company); however, these cells are normally limited in what concerns innovative configurations and characterization methods (Krol and Grätzel, 2012). The best option is still the PEC cells in-house designed and built. Figure 10.2.10 shows an example of a versatile PEC cell designed by the authors; for small (Figure 10.2.10a) and for large photoanodes (Figure 10.2.10b), up to 10×10 cm2. The developed cells, named Portocell, have two removable windows (front and back) screwed to a transparent acrylic part. The small cell has a mask that allows an illumination area of 4 cm2, crossing a synthetic quartz window (Robson Scientific, England), which is pressed against an o-ring by means of five screws. After assembling the black and the transparent acrylic part, the cell is then filled with the appropriate electrolyte solution where both electrodes are immersed. An acrylic cap can then be screwed on the top of the cell, allowing a reference electrode to be connected, if a three electrode configuration is desired, or just permitting the evolution of the electrolysis decomposition gases. Portocell permits back and front illumination and allows one to place a separator between the electrodes to avoid gas (hydrogen and oxygen) mixture. This separator can be a Nafion® membrane that allows just protons to permeate or a porous hydrophobic Teflon® membrane, which exhibits a very small ionic transfer resistance and prevents hydrogen and oxygen gas bubbles to mix (Mendes, et al., under protection). 308 Solar energy sciences and engineering applications Figure 10.2.10 Photoelectrochemical cells designed by the authors for small scale photoelectrodes (a) and for large scale photoelectrodes (b) – (Lopes et al., 2012). In the upper part of the larger Portocell, a membrane separates the electrolyte vessel and the gas collecting chamber. This membrane, of porous Teflon®, allows the gases to permeate but prevents the electrolyte to cross and consequently, in outdoor applications, the cell can be tilted to maximize the sunlight harvesting without fearing electrolyte leaks. Moreover, it avoids the use of a complex electrolyte feeding control system in continuous operating cells since it is just necessary to feed anode and cathode sides and the exit stream leaves from the other side without leaking to the gas collecting chamber (Mendes, et al., under protection). 10.2.5.1 The electrolyte The electrolyte chemistry is a key factor that can dramatically influence the photoresponse of a PEC cell (Archer and Nozik, 2008). In electrochemical cells the electrolyte Photoelectrochemical cells for hydrogen production from solar energy 309 consists of a solvent with active species to be reduced or oxidized, depending if it is an alkaline or an acid media. Nevertheless, pure water is not conductive and thus supporting ions must be added to ensure the desired charge transfer (Krol et al., 2008). The photoactive semiconductor immersed in a redox electrolyte is greatly affected by the solution properties, redox level and stability, interfacial kinetics (adsorption), viscosity, conductivity, ionic activity and transparency within the crucial wavelength region (Archer and Nozik, 2008). As mentioned, the choice of a suitable electrolyte solution is very important, mainly in what concerns the redox couple selection. It should improve the charge-transfer kinetics, the photoelectrode stability, and also should help in preventing undesirable phenomena such as surface recombination and trapping (Archer and Nozik, 2008). Also, the electrolyte concentration should be sufficiently high to avoid large ohmic voltage losses. As reported by Roel and co-authors, the voltage drop is given by Vloss =I ×RE, where I is the total current flowing between the working electrode and the counter-electrode, and RE is the electrolyte resistance (Krol and Grätzel, 2012). The electrolyte conductivity strongly depends on the type of ions and the corresponding concentration value. It is important to add that there is not a linear relation between conductivity and ion concentration due to incomplete dissociation of anions and cations and/or ion-solvent interactions. Moreover, deviations from linearity can occur for concentrations above 1 mmol L-1; at high concentrations, i.e. >1M, the formation of ion-pairs can result in a decrease of the conductivity with the concentration increase. This behavior explains why the conductivity starts to decrease for concentrations higher than ~6M of KOH aqueous solutions. To avoid large ohmic losses it is then important to guarantee concentrations of at least 0.5M (Krol and Grätzel, 2012). Usually, acid electrolytes such as aqueous H2SO4 or HCl solution (0.5–1 M) are often used with WO3 and TiO2 photoelectrodes (Krol and Grätzel, 2012). Aqueous NaCl solutions are also used with WO3 photoanodes, simulating sea water conditions (Alexander and Augustynski, 2010). For electrodes such as a-Fe2O3, where alkaline or neutral electrolyte solutions are preferable, concentrations of 0.5–1M NaOH or KOH should be used. Metal oxide semiconductors that are only stable in fairly neutral environments such as BiVO4, 0.5M Na2SO4 or K2SO4 solutions should be used and the electrolyte should be buffered (KH2PO4/K2HPO4) to prevent local pH fluctuations (Krol and Grätzel, 2012). In photoelectrochemical cells bubbles usually get stacked in the electrode surface which can generate excessive noise on the photocurrent signal and thus it is necessary to remove them. This can be done at lab level flashing using a nitrogen or argon stream or simply by using a magnetic stir bar for stirring the electrolyte (Krol and Grätzel, 2012). Moreover, by using these procedures, the back-reaction of dissolved hydrogen and oxygen again to water can also be prevented ensuring that the redox potentials do not change over time. The Portocell with the integrated electrolyte recirculation systems helps to remove the stacked bubbles, being a solution applicable for both laboratory and industrial contexts. 10.2.5.2 The counter-electrode As stated before, the preferable configuration is the single-photon system, in which the photoactive semiconductor works as working-electrode and a metallic material as 310 Solar energy sciences and engineering applications Table 10.2.3 Common reference electrodes for PEC research overview (Peterson, 2012). Reference Electrode Filling Solution Potential (vs. SHE) Reversible Hydrogen The electrode is in the actual E0 =0.0+0.059×pH Electrode (RHE) electrolyte solution and not separated by a salt bridge Standard Hydrogen Acid solution with activity E0 =0.0 Electrode (SHE) equal to 1 (=normal hydrogen) [H+]=1.18M Calomel (Hg/Hg2Cl2) 0.1M KCl 0.334 1M KCl (NCE) 0.281 3.5M KCl 0.250 Saturated KCl (SCE) 0.242 Saturated NaCl (SSCE) 0.236 Silver/Silver chloride (Ag/AgCl) 0.1M KCl 0.288 1M KCl 0.237 3M KCl 0.210 3.5M KCl 0.205 Saturated KCl 0.198 3M NaCl 0.209 Saturated NaCl 0.197 SeaWater 0.25 counter-electrode. The reaction at the counter-electrode should be as fast as possible and should have a high catalytic activity in order to prevent performance limitations (Krol and Grätzel, 2012). Usually, platinum is used as counter-electrode; this material presents good stability over a wide range of electrolytes and pH, as well as showing low overpotentials for hydrogen evolution (~0.1 V). To avoid inhomogeneous current densities at the working electrode, the counter-electrode should face it symmetrically; this is critical for electrolytes concentration lower than 0.5M(Krol and Grätzel, 2012). Moreover, the counter-electrode area should be twice as large than the photoelectrode area (Krol and Grätzel, 2012). In PEC systems, a compromise must be maintained between the working electrode, the counter electrode and the electrolyte solution in order to ensure low overpotentials, fast charge transport and efficient light absorption. 10.2.5.3 The reference-electrode In single photon-system PEC cells, the applied potential is an important parameter when studying the properties of the photoelectrode (photoanode or photocathode). The three-electrode configuration allows one to measure the applied potential with respect to a fixed reference electrode, allowing to turn visible the independent response of the working electrode to any change in the applied potential. The same cannot be hold for the metal counter-electrode since its overpotential at the interface with the electrolyte is usually unknown and varies with the amount of current flowing through the cell according to the Butler Volmer relation. Table 10.2.3 shows an overview considering the reference electrodes most used in PEC research applications. As exemplified in Table 10.2.3 there are several choices for the reference electrode. However, the most commonly used reference electrode is the silver/silver chloride. Photoelectrochemical cells for hydrogen production from solar energy 311 In water-splitting studies the applied potential is reported against RHE, thus the potential measured with the Ag/AgCl electrode must be converted into RHE scale using the following expression: ERHE = EAg/AgCl + E0 Ag/AgCl vs. SHE + 0.059 × pH (10.2.14) where the E0 Ag/AgCl vs. SHE is the potential of the Ag/AgCl reference electrode with respect to the SHE, see Table 10.2.3 (Peterson, 2012). There are experimental reasons for the choice of a reference electrode; one important selection parameter is its stability on the electrolyte solution where it is immersed as well as the operating temperature (Peterson, 2012). All the reference electrodes are very sensitive and so it is crucial their good maintenance; the lifetime of a reference electrode is 2–3 years with a daily basis usage. The feasibility of a reference electrode can be checked using three identical reference electrodes and by observing potential differences at every two weeks and confirming if the deviation between any two individual electrodes is less than ±3mV (Krol and Grätzel, 2012). 10.3 ELECTROCHEMICAL IMPENDANCE SPECTROSCOPY Several photoelectrochemical techniques have been used to characterize photoelectrodes with the goal of understanding its performance and limitations. This kind of measurement is usually performed under steady-state and includes simulated sunlight measurements, as photocurrent-voltage, wavelength-dependent measurements, photocurrent action spectra and quantum efficiencies. Nevertheless, more detailed properties cannot be extracted from steady-state measurements and so dynamic techniques should be considered to identify performance-limiting steps or to determine certain materials properties. These techniques allow the interpretation of the charge transfer kinetics, mainly characterized by diffusion coefficients and lifetime of the different charge carriers. One of the most powerful characterization techniques of photoelectrochemical cells involving transient probing is Electrochemical Impedance Spectroscopy (EIS). EIS is a dynamic technique that has many advantages, not only because it is userfriendly, but also because of to its sensitivity and ability to separate different complex processes, such as those occurring in a photoelectrochemical system (Bisquert, 2002; Bisquert, 2003). The foundations of EIS began in the 19th century with the controversial but extraordinary work of Oliver Heaviside (Heaviside, 2012), where he defined the terms “impedance’’, “reactance’’ and “admittance’’. At the end of the 19th century,Warburg derived the impedance function for a diffusional process in a remarkable work where he extended the concept of impedance to electrochemical systems (Macdonald, 2006). In the early 20th century, EIS experiments were performed mainly for capacitance measurements of ideally polarizable electrodes, e.g. mercury, using reactive bridges at relatively high frequencies. Even though these studies yielded important information about the double layer behavior, complete impedance studies including the low frequency range were only possible at the 1940s with the invention of electronic potentiostats. Three decades later, the frequency response analyzer (FRA) was developed, allowing one to probe electrochemical interfaces at sub-millihertz range 312 Solar energy sciences and engineering applications (Chang and Park, 2010). Since then, EIS has been widely used in electrochemistry, photoelectrochemistry and corrosion studies. However, this method is viable only for a stable and reversible system in equilibrium, as the system’s linearity, stability and causality must be ensured (Macdonald, 2006). 10.3.1 Fundamentals EIS is a technique widely used for characterizing the electrical behavior of systems in which the overall performance is determined by a number of strongly coupled processes, each proceeding at a different rate. The most common and standard procedure in impedance measurements consists of applying a small voltage sinusoidal perturbation and monitoring the resulting current response of the system at the corresponding frequency. An EIS measurement can be performed under any bias illumination and at any working condition of the solar cell. Nevertheless, the single-frequency voltage perturbation is usually done in open-circuit conditions with a modulation signal of magnitude V0: V(t) = VOC + V0cos(.t) (10.3.1) The response in current has the same period as the voltage perturbation but will be phase-shifted by f: I(t) = IOC + I0cos(.t - f) (10.3.2) V0 and I0 are the amplitudes of the voltage and current signals, respectively, and .(=2pf ) is the radial frequency in radians per second; the open-circuit current IOC is zero – Figure 10.3.1. Similar to resistance, impedance is a measure of the ability of a system to impede the flow of electrical current. Thus, impedance is the ratio of a time-dependent voltage and a time-dependent current as defined by Equations 10.3.1 and 10.3.2: Z = V0 cos(.t) I0 cos(.t - f) = Z0 cos(.t) cos(.t - f) (10.3.3) The impedance is therefore expressed in terms of a magnitude, Z0, and a phase shift f. Applying complex notation, the impedance response of a system can be described in terms of real and imaginary components, as follows (Barsoukov and Macdonald, 2005): Z = Z0 exp(j.t) exp(j.t - jf) = Z0(cos f + j sin f) (10.3.4) Knowing the values of ZReal =Z0cos f, ZImag =Z0sin f and the phase angle f, Bode and Nyquist diagrams can be plotted. The Bode diagram is the representation of the symmetric of the phase angle f vs frequency. This is a semi-logarithmic plot because both the impedance and the frequency often span orders of magnitude. Bode plots explicitly show the frequency-dependence of the impedance of the device under test. A Nyquist plot is the representation of the imaginary impedance, -ZImag, which is Photoelectrochemical cells for hydrogen production from solar energy 313 Figure 10.3.1 Sinusoidal voltage perturbation and resulting sinusoidal current response, phase-shifted by f. V0 – amplitude of the voltage signal; I0 – amplitude of the current signal; Voc – open-circuit voltage; Ioc – open-circuit current (Andrade et al., 2010). indicative of the capacitive and inductive character of the cell, vs the real impedance of the cell, ZReal. Nyquist plots have the advantage that activation-controlled processes with distinct time-constants show up as unique impedance arcs and the shape of the curve provides insight into possible mechanisms or governing phenomena. However, this format of representing impedance data has the disadvantage that the frequencydependence is implicit; therefore, the AC frequency of selected data points should be indicated. Because both data formats have their advantages, it is usually best to present both Bode and Nyquist plots – Figure 10.3.2. It is important to note that impedance analysis is based on the assumption that the system under study behaves linearly. Since linear systems typically exhibit features and properties that are much simpler than the general nonlinear cases, the analysis becomes less complex. A system is linear if it complies with both homogeneity and additivity principles, which state that: i) when a perturbation is imposed to a system, the response will be proportional and of the same type as the input signal (for instance, if a tensile strength applied to a sample increases twofold, the corresponding strain will double); ii) if the perturbation imposed on a system consists of the weighted sum of several signals, then, the output is simply the weighted sum of the system’s responses to each input signal. Mathematically, let us consider y1(t) the response of a continuous time system x1(t) and y2(t) the output corresponding to the input x2(t). Then the system is linear if: i) Principle of homogeneity: the response to a · x1(t) is a · y1(t) ii) Principle of additivity: the response to x1(t) + x2(t) is y1(t) + y2(t) 314 Solar energy sciences and engineering applications Figure 10.3.2 Graphical representation of theAC impedance of a PEC cell in 3-electrodesconfiguration: (a) Nyquistdiagram; (b) impedance Bode diagram; (c) phase Bode diagram. Figure 10.3.3 Current versus voltage curve showing pseudo-linearity. (Current-voltage characteristic is a steady-state technique that determines the performance response of a photoelectrode in the dark and under different light conditions. The I-V characteristic applied for water splitting is usually performed in a three electrodes configuration (being the third one is the reference electrode, usually Ag/AgCl)). Attempting for the system under study, i.e. a photoelectrochemical cell for watersplitting and its I-V characteristic shown in Figure 10.3.3, it is clear that the response to a voltage input signal is not linear. The way to circumvent this situation is to consider only a small portion of the cell’s current versus voltage curve, which appears to be linear – Figure 10.3.3. In practice, for EIS measurements a small voltage perturbation (1–20 mV) is applied to the cell, ensuring that the response is in the pseudo-linear range (Conway et al., 2002). Photoelectrochemical cells for hydrogen production from solar energy 315 Figure 10.3.4 Graphical representation of the AC impedance of a resistor (R=5 k): (a) Nyquist diagram; (b) impedance Bode diagram; (c) phase Bode diagram. 10.3.2 Electrical analogues EIS data usually represents the electrochemical systems as an electronic circuit, which may consist of resistors, capacitors, inductors and more complex elements, assembled in series or in parallel. Equivalent electrical analogues are a useful tool for the interpretation of experimental results, by fitting the experimental data to specific arrangements of electrical elements. This can provide relevant information concerning reaction kinetics, ohmic conduction processes and even mass transfer phenomena occurring in electrochemical systems. The different circuit elements in alternating current (AC) are briefly described hereafter. However, complementary knowledge about standard circuit elements is strongly encouraged (Barsoukov and Macdonald, 2005). 10.3.2.1 Ohmic resistance The equivalent analogue for an ohmic conduction process is a simple resistor, which according to the Ohm’s law represents the resistance to electric charge transfer. For a sinusoidal perturbation the impedance of a resistor ZR in the complex plane is simply defined as: ZR = R (10.3.5) For the case of a simple resistor, the correspondent Nyquist plot is just a point in the real axis with value R with no imaginary component and independent of frequency – Figure 10.3.4. 10.3.2.2 Double layer capacitance An electrical double layer exists at the interface between an electrode and its surrounding electrolyte as shown in Figure 10.3.5. This double layer is formed due to the charge 316 Solar energy sciences and engineering applications Figure 10.3.5 Schematic representation for an n-type semiconductor of: depletion layer zone and the electrical double layer (Helmholtz layer). separation that occurs across the interface: an excess of ions of opposite charge to that on the electrode will be found at the electrolyte phase boundary. A simple way of understanding the double layer behavior is to imagine that ions at each side of the interface approach the electrode surface as closely as possible, originating two parallel layers of equal and opposite charge, one on the electrode side and the other on the electrolyte side – Figure 10.3.5. This double-layer will act as a charge storage (Bard and Faulkner, 2001), i.e. a capacitor with an impedance response defined as follows: ZC = 1 j.C (10.3.6) In real cells, formed by nanoporous semiconductors, the double layer capacitor does not behave ideally. Instead it acts like a constant phase element (CPE), a non-ideal capacitance with a non-uniform distribution of current in the heterogeneous material. In this case, the impedance of the double layer capacitance is defined as: ZC = 1 j.Cnz (10.3.7) where nz (0300 nm). Biological degradation of a chemical refers to its elimination by the metabolic activity of living organisms, usually microorganisms, particularly the bacteria and fungi living in natural water and soil. In this context, conventional biological processes do not always provide satisfactory results, especially for industrial wastewater treatment, since many of the organic substances produced by the chemical industry are toxic or resistant to biological treatment (Muñoz and Guieysee, 2006; Lapertot and Pulgarin, 2006). Conventional methods of water decontamination that can address many of these problems are often chemically, energetically and operationally intensive, and when used in large systems require a considerable infusion of capital, engineering expertise and infrastructure. This practically precludes their use in much of the world. Furthermore, intensive chemical treatments (such as those involving ammonia, chlorine compounds, hydrochloric acid, sodium hydroxide, permanganate, alum and ferric salts, coagulation and filtration aids, anti-scalants, corrosion control chemicals, and ion exchange resins and regenerants) and residuals resulting from treatment (sludge, brines, toxic waste) can add to the problems of contamination and salting of freshwater sources. Air stripping and adsorption, which merely transfer toxic materials from one medium to another, are not long-term solutions. Incineration converts toxics into carbon dioxide, water and inorganic acids, but negative public perception has very often prevented its implementation. In view of all of the above, a feasible 376 Solar energy sciences and engineering applications option for such biologically persistent wastewater is the use of the advanced technologies based on chemical oxidation called advanced oxidation processes (AOPs), which are widely recognized as highly efficient treatments for recalcitrant wastewater (Pera- Titus et al., 2004). These methods rely on the formation of highly reactive chemical species which degrade even the most recalcitrant molecules into biodegradable compounds. Although reacting systems vary (Comninellis et al., 2008), all of them are characterized by the production of hydroxyl radicals (OH), which are able to oxidize and mineralize almost any organic molecule, yielding CO2 and inorganic ions. They are also non-selective, which is a useful attribute for wastewater treatment and solution of pollution problems. The versatility of the AOPs is also enhanced by the fact that hydroxyl radicals may be produced in different ways, facilitating compliance with the specific treatment requirements. Methods based on UV, H2O2/UV, O3/UV and other combinations use photolysis of H2O2 and ozone to produce the hydroxyl radicals, but generation of UV radiation by lamps and ozone production are expensive. So future applications of these processes could be improved through the use of catalysis and solar energy. Therefore, research is focusing more and more on those AOPs which can be driven by solar irradiation. Of special interest is photo-Fenton, which is based on addition of H2O2 to dissolved iron salts and irradiation with UV-VIS light, because sunlight can be used for it (Pignatello et al., 2006). 12.2 SOLAR PHOTO-FENTON Fenton and Fenton-like processes are probably among the advanced oxidation processes most applied in the treatment of industrial wastewater (Suty et al., 2004). The first proposals for wastewater treatment applications were reported in the 1960s. Yet it was not until the early 1990s that the first studies on the application of the photo-Fenton process for the treatment of wastewater were published by the groups of Pignatello, Lipcznska-Kochany, Kiwi, Pulgarín and Bauer (Pignatello et al., 2006). Much of the literature on photo-Fenton includes the possibility of driving the process with solar radiation because it seems to be the most suitable of all AOPs for being driven by sunlight, because soluble iron-hydroxyl and especially iron-organic acid complexes even absorb part of the visible light spectrum (Figure 12.2.1), not only ultraviolet radiation (Malato et al., 2009). Hydrogen peroxide is decomposed to water and oxygen in the presence of iron ions in the Fenton reaction in aqueous solutions, Equation 12.2.1, as first reported by H.J.H. Fenton (Fenton, 1894). Mixtures of ferrous iron and hydrogen peroxide are called Fenton reagents. Equations 12.2.1–12.2.3 show the basic reactions in the absence of other interfering ions and organic substances. Regeneration of ferrous iron from ferric iron by Equations 12.2.2 and 12.2.3 is the rate limiting step in the catalytic iron cycle, if iron is added in small amounts. Fe2+ + H2O2 . Fe3+ + OH- + OH• (12.2.1) Fe3+ + HO• 2 . Fe2+ + O2 + H+ (12.2.2) Fe3+ + O•- 2 . Fe2+ + O2 (12.2.3) Decontamination of water 377 Figure 12.2.1 Normal solar irradiance (I) on the Earth’s surface (ASTM E891-87, air mass 1.5), main light absorbing gases and light absorption of Fe3+ species. If organic substances (quenchers, scavengers, or in the case of wastewater treatment, pollutants) are present in the system Fe2+/Fe3+/H2O2, they react in many ways with the hydroxyl radicals generated. The organic radicals generated continue reacting, prolonging the chain reaction and thereby contributing to reducing the consumption of oxidants in wastewater treatment by Fenton and photo-Fenton. In aromatic pollutants, the ring system is usually hydroxylated before it is broken up during oxidation, typically into intermediate degradation products containing quinone and hydroquinone structures. In any case, sooner or later, ring opening reactions further mineralize the molecule. One important drawback of the Fenton method, especially for total mineralisation of organic pollutants, is that carboxylic intermediates cannot be further degraded. Carboxylic and dicarboxylic acids (L: Mono- and Dicarboxylic acids) are known to form stable iron complexes, which inhibit the reaction with peroxide (Kavitha and Palanivelu, 2004). Hence, the catalytic iron cycle reaches a standstill before total mineralisation is accomplished (Equation 12.2.4). Fe3+ + nL . [FeLn]x+ H2O-2., dark no further reaction (12.2.4) The primary step in the solar photoreduction of dissolved ferric iron is a ligand-tometal charge-transfer reaction in which intermediate complexes dissociate as shown in the reaction in Equation 12.2.5. The ligand may be any Lewis base able to form a complex with ferric iron (OH-, H2O, HO- 2, Cl-, R-COO-, R-OH, R-NH2 etc.). Depending on the reacting ligand, the product may be a hydroxyl radical such as in Equation 12.2.6 or other radical derived from the ligand. The direct oxidation of an organic ligand is possible as well, as shown for carboxylic acids in Equation 12.2.7. [Fe3+L] + h. -. [Fe3+L]* -. Fe2+ + L• (12.2.5) [Fe(OH)]2+ + h. -. Fe2+ + OH• (12.2.6) [Fe(OOC-R)]2+ + h. . Fe2+ + CO2 + R• (12.2.7) 378 Solar energy sciences and engineering applications The ferric iron complex has different light absorption properties depending on the ligand, so Equation 12.2.5 takes place with different quantum yields and also at different wavelengths. Consequently, pH plays a crucial role in the efficiency of the photo-Fenton reaction, because it strongly influences which complexes are formed. Thus, pH 2.8 has frequently been postulated as optimum for photo-Fenton treatment, because there is no precipitation yet and the predominant iron species in solution is [Fe(OH)]2+, the most photoactive ferric iron-water complex. In fact, as shown in its general form in Equation 12.2.5, ferric iron can form complexes with many substances and undergo photoreduction. Carboxylic acids are of special importance because they are frequent oxidation intermediate products, and ferric iron-carboxylate complexes may have much higher quantum yields than ferric iron-water complexes. Fe3+ complexes present in mildly acidic solutions absorb an appreciable amount of light in the UV and into the visible region, and may complex with certain target compounds or their by-products. These complexes typically have higher molar absorption coefficients in the near-UV and visible regions than aquo complexes. Polychromatic quantum efficiencies from 0.05 to 0.95 are common in the UV/visible range (Pignatello et al., 2006), making the photo-Fenton process suitable for being driven by sunlight. 12.2.1 Solar photo-Fenton hardware Much solar detoxification system component equipment (Blanco and Malato, 2003) is identical to what is used for other types of water treatment, and construction materials are available on the market. Most piping may be made of polyethylene or polypropylene, but not metal or composite materials that could be degraded by the oxidizing conditions of the process. Neither may reactive materials that would interfere with the photocatalytic process be used. All materials used must be inert to degradation by solar UV light so they last the minimum required system lifetime. Photocatalytic reactors must transmit UV-Vis light efficiently because of the process requirements. The best reflecting/concentrating material is aluminium, because, while aluminium coated mirrors are low-cost and highly reflective in the UV-Vis band of the terrestrial solar spectrum, the reflectivity (reflected radiation/incident radiation) from 300 to 400 nm of traditional silver-coated mirrors is very low. Aluminium, which is the only metal surface that is highly reflective throughout the ultraviolet spectrum, has a reflectivity range from 92.3% at 280nm to 92.5% at 385 nm. Comparable values for silver are 25.2% and 92.8%, respectively. Aluminium also reflects perfectly in the visible range. The photocatalytic reactor must be transparent to UV-Vis radiation. Visible range transmissivity of different materials is usually high, and it is in the UV range where restrictions appear. The choice of materials that are both transmissive to UV light and resistant to its destructive effects is limited. Common materials that meet these requirements are fluoropolymers, acrylic polymers and several types of glass. Quartz has excellent UV transmission as well as good temperature and chemical resistance, but its high cost makes it completely unfeasible for photocatalytic applications. Fluoropolymers are a good choice of plastics for photoreactors due to their good UV transmittance, excellent ultraviolet stability and chemical inertness. However, in order to achieve minimum pressure resistance, the wall thickness of the fluoropolymer tube has to be increased, which in turn lowers its UV transmittance. Other low cost polymeric materials are Decontamination of water 379 significantly more susceptible attack by hydroxyl radicals. Standard glass is not satisfactory because it absorbs part of the UV radiation that reaches it, due to its iron content. Low-iron borosilicate glass, which has good transmissive properties in the solar range with a cut-off at about 285nm (Blanco et al., 2000), would seem to be the most adequate. Therefore, although both fluoropolymers and glass are valid photoreactor materials, if a large field with a considerable number of photoreactors is being designed, there will be a high system pressure drop. So in such cases, fluoropolymer tubes are not the best choice of material, and borosilicate glass is a better solution. The original solar photoreactor designs (Dillert et al., 1999) for photochemical applications were based on line-focus parabolic-trough concentrators (PTCs). In part, this was a logical extension of the historical emphasis on trough units for solar thermal applications. Furthermore, PTC technology was relatively mature and existing hardware could be easily modified for photochemical processes. The main disadvantages are that these collectors (i) use only direct radiation (ii) are expensive (iii) have low efficiencies as they concentrate sunlight therefore increasing temperature and promoting iron precipitation, and (iv) a high iron concentration is needed to absorb concentrated sunlight. On the other hand, one-sun (non-concentrating) collectors have no moving parts or solar tracking devices. They do not concentrate radiation, so efficiency is not reduced by factors associated with concentration and solar tracking. As there is no concentrating system (with its inherent reflectivity), the efficiency is higher than for PTCs, and they are able to utilize the diffuse as well as the direct portion of the solar radiation. An extensive effort in the design of small non-tracking collectors has resulted in the testing of several different non-concentrating solar reactors (Blanco et al., 2007). Although one-sun collector designs possess important advantages, the design of a robust one-sun photoreactor is no simple matter, due to the need for weather-resistant and chemically inert ultraviolet-transmitting reactors. In addition, non-concentrating systems require significantly more photoreactor area than concentrating photoreactors and, as a consequence, full-scale systems must be designed to withstand the operating pressures for fluid circulation. Design of a solar collector for a photo-Fenton reactor is subject to some major optimization constraints: (1) collection of maximum solar UV-Vis radiation, (2) working temperatures below 50oC, (3) efficiency at low iron concentrations, (4) its construction must be economical, and finally (5) the system pressure drop must be low. Tubular photoreactors therefore have a decisive advantage in the inherent structural efficiency of tubing for flowing water. Tubing is also available in a large variety of materials and sizes and is a natural choice for a pressurized fluid system. A particular type of low concentration collector called the Compound Parabolic Concentrator (CPC) is used in thermal applications. This combination of parabolic concentrators and static flat systems is also an attractive option for solar photochemical applications (Ajona and Vidal, 2000). CPCs are static collectors with an ideal reflective surface according to non-imaging optics that can be designed for any given reactor shape. The entire circumference of the receiver is illuminated, rather than just the front, as in conventional flat plates. The ideal optics of these concentrating devices thus combines both the advantages of the PTC and static systems (Colina-Márquez et al., 2010). The concentration factor (RC) of a two dimensional CPC collector is given by Equation 12.2.8. RC,CPC = 1 sin .a = A 2p r (12.2.8) 380 Solar energy sciences and engineering applications The normal values for the semi-angle of acceptance (.a) for photochemical applications is 90 degrees, whereby RC =1 (non-concentrating solar system). If the CPC is designed for an acceptance angle of +90. to -90. (Figure 12.2.2), all incident solar direct and diffuse radiation can be collected. The light reflected by the CPC is distributed all around the tubular receiver so that almost the entire circumference of the receiver tube is illuminated. CPCs have the advantages of both the PTC and non-concentrating collector technologies and none of the disadvantages, so they seem to be the best option for solar photocatalysis. They can make highly efficient use of both direct and diffuse solar radiation, without the need for solar tracking. An important factor in photoreactor design is its diameter. As mentioned above, the Fenton reagent consists of an aqueous solution of hydrogen peroxide and ferrous Figure 12.2.2 Schematic drawing and photo of CPC with a semi-angle of acceptance of 90.. Decontamination of water 381 ions producing hydroxyl radicals. When UV/visible radiation is added, it is called photo-Fenton, which is a catalytic process. Fe3+ (and related species and organic complexes) absorbs solar photons as a function of its absorptivity. This must be taken into consideration when the optimal photoreactor load is calculated as a function of lightpath length. After long experimentation with different photoreactors under sunlight at the Plataforma Solar de Almería (PSA) in Spain, the optimal concentration proposed is 0.2–0.5mM of iron depending on photoreactor diameter (Malato et al., 2009). Under the “SOLARDETOX’’ project (Solar Detoxification Technology for the Treatment of Industrial Non-Biodegradable Persistent Chlorinated Water Contaminants, Brite Euram III Program, 1997–2000, BRPR-CT97-0424), a European consortium coordinated by the PSA was formed for the development and marketing of solar detoxification of recalcitrant water contaminants. The main goal of the project was to develop a commercial non-concentrating solar detoxification system using the compound parabolic collector technology (CPC). A full-size demonstration plant for field demonstration was constructed at Hidrocen facilities (Madrid, Spain). The same collectors have also been used to treat paper mill effluents in Brazil and Germany, and paper mill effluents, surfactants, and textile dyes in Spain (Malato et al., 2007). In 2004, a new CPC plant was installed in the context of a project for the collection and recycling of plastic pesticide bottles using advanced oxidation process (AOP) driven by solar energy funded by the European LIFE-ENVIRONMENT programme. This plant, which is now in routine operation, has a total collector surface of 150m2 and photo-reactor volume of 1.06m3. More recently, in a new step forward, solar photo-Fenton and aerobic biological processes have been combined, in a 100m2 CPC solar photo-Fenton reactor and an aerobic biological treatment plant based on a 1m3 immobilized-biomass activated sludge reactor (Figure 12.2.3). The overall efficiency Figure 12.2.3 Solar photo-Fenton plant with 100m2 of CPC. 382 Solar energy sciences and engineering applications in the combined system was about 95% mineralization. 50% of the initial TOC was degraded in the photo-Fenton pre-treatment, while 45% was removed in the aerobic biological treatment (Blanco et al., 2009). The following sections of this chapter focus mainly on how to enhance the solar photo-Fenton efficiency by integration with biotreatment. 12.3 STRATEGY FOR COMBINING SOLAR ADVANCED OXIDATION PROCESSES AND BIOTREATMENT Chemical oxidation for complete mineralization is usually expensive, because the oxidation intermediates formed during treatment tend to be more and more resistant to their complete chemical degradation, and furthermore, they all consume energy (radiation, ozone, etc.) and chemical reagents (catalysts and oxidizers) which increase with treatment time (Oller et al., 2011). One attractive potential alternative is to apply these chemical oxidation processes in a pre-treatment to convert the initially persistent organic compounds into more biodegradable intermediates, which would then be treated in a biological oxidation process at a considerably lower cost. Therefore, the main role of the chemical pre-treatment is partial oxidation of the biologically persistent part and produce biodegradable reaction intermediates. The percentage of mineralization should be minimized during pre-treatment to avoid unnecessary expenditure of chemicals and energy, thereby lowering the operating cost. The choice depends on the quality standards to be met and the most effective treatment at the lowest reasonable cost. Therefore, the main factors in the decision on which wastewater treatment technologies to be applied are: (i) the quality of the original wastewater, (ii) the quality of the final effluent, (iii) economic studies and (iv) Life Cycle Assessment of the treatment technology. As information on AOPs efficiency in eliminating certain specific pollutants in wastewater compared to conventional options is necessary, bench-scale and pilot plant studies must be done to develop the technologies and generate information on new industrial wastewater treatments. Such scaled studies are even more decisive for combining decontamination technologies for a specific industrial wastewater, as described in this chapter. When preliminary chemical oxidation is applied in a combination treatment line, its effect may be insignificant or even harmful to the properties of the original effluent, even though it is conceptually advantageous. This underlines the need to establish a step-by-step research methodology which takes these effects into account, because the effect of the operating conditions on the pre-treatment stream (contact time, oxidant and/or catalyst type, dose and toxicity, temperature, etc.) must be known. Such studies must employ analytical tools to infer the reaction mechanisms, pathway and kinetics, evaluate the effect of the chemical pre-treatment on toxicity and biodegradability, the effect of cations and anions in the wastewater matrix, and the application of various techniques for determining biodegradability and toxicity (Rizzo, 2011). Appropriate techniques must be combined to provide technically and economically feasible options. The performance of an AOP treatment may be enhanced in several ways. The first is placing the AOP in a physical, chemical and biological treatment Decontamination of water 383 sequence. Such an approach often involves at least one AOP step and one biological treatment step (Mandal et al., 2010). Whether the AOP or the biological process comes first in the treatment line, the overall purpose of reducing costs is nearly the same as minimizing AOP treatment and maximizing the biological stage. The individual biological and chemical oxidation efficiencies must be calculated to find the optimal operating conditions for the combined process. This involves detailed knowledge of both biological and chemical processes. Therefore, several analytical parameters must be monitored during each step of the treatment line. The usual chemical parameters measured are total organic carbon (and/or chemical oxygen demand), the concentration of specific pollutants in the target wastewater, and heteroatoms from contaminants completely degraded during the AOP treatment released (Cl, N, P,…) as inorganic species (Cl-, NO- 3, PO3- 4 ,…) into the medium. Toxicity analyses (with organisms like Vibrio fischeri, Daphnia magna, activated sludge, etc.) and biodegradability tests (using activated sludge) are very important to ensure that AOP effluent conditions are suitable for treatment by conventional biodegradation. The following sections highlight the main parameters necessary for proper evaluation of an AOP to determine the best way of combining it with a biotreatment. 12.3.1 Average oxidation state One of the most widely used parameters in wastewater biodegradability assessment is the Average Oxidation State (AOS), which can be calculated with Equation 12.3.1 (Scott and Ollis, 1995), where TOC (total organic carbon) and COD (chemical oxygen demand) are expressed in moles of C/L and moles of O2/L, respectively: AOS = 4 ×  TOC - COD TOC  (12.3.1) The AOS is from +4 (for the most oxidized state of carbon, CO2) to -4 (for the most reduced state of carbon, CH4). The AOS, which varies with treatment time and indicates the oxidation state of the organic compounds in the wastewater, can be used to determine how the AOP is modifying them. Figure 12.3.1 shows an example of how the AOS commonly evolves during AOP wastewater treatment. Although the AOS rises rapidly at first, this increase later slows down, suggesting that the chemical nature of the reaction intermediates generated did not vary significantly after certain stage. Furthermore, when contaminants are oxidised before mineralization, it usually means that biodegradability is increasing, as in the transformation of chlorophenol into phenol, and phenol into oxalic acid, for example. This is because when the AOS stabilizes, oxidation is producing mineralization (the last step of the process), and no further substantial change in wastewater biodegradability is expected. So if high biodegradability is to be achieved, it must be before or right at the moment the AOS stabilizes. From that point on, the photocatalytic treatment only mineralizes organic carbon, even though the chemical nature of the organic compounds does not change substantially. It may therefore be concluded that the AOS provides indirect information about wastewater biodegradability. The example in Figure 12.3.1 shows the TOC, COD and AOS during solar photo-Fenton treatment of an industrial waste water. As observed, the AOS rises to a maximum of around 2 and remains there 384 Solar energy sciences and engineering applications Figure 12.3.1 Evolution of TOC, COD and AOS during the solar photo-Fenton treatment of an industrial wastewater. almost until the end of the experiment (circled in blue). This means that from this point on, the chemical nature (oxidation state) of the mixture is not going to change significantly, because oxidation is causing mineralization. Parallel behavior of theCOD and TOC also makes this very clear. Before this point (treatment time less than 200 minutes), the AOS rose sharply (sharp drop in COD along with slow fall in TOC). It can therefore be assumed that the biodegradability of the effluent should change from 0 to 200 minutes, but not after, and therefore, any bioassay for determining biodegradability enhancement should be applied while AOS is still changing and only until it stabilizes but not afterwards. As bioassays are usually difficult, expensive and slow, reliable and rapid chemical analyses, such as COD and TOC are useful aids in deciding the best time for their application. 12.3.2 Activated sludge respirometry Respirometry assays measure the Oxygen Uptake Rate (OUR) in live biomass and are an indicator of the microbiological activity present in it. The oxygen demand determined in respirometric assays has recently been found to be an excellent control parameter, as it represents a direct measure of the proper activity and viability of microorganisms in aerobic activated sludge. Furthermore, as this test directly assesses the primary function of an activated sludge process, it can be used for efficient measurement of any acute toxicity in industrial wastewater that could affect the activated sludge in a MunicipalWastewater Treatment Plant (MWWTP) (Gutiérrez et al., 2002). Aerobic biomass activity at different stages of AOP treatment of industrial wastewater Decontamination of water 385 Figure 12.3.2 Measurement of readily biodegradable COD in a respirometric assay. could be affected by the amount of biodegradable or hardly biodegradable substances present in the medium and the presence of any compounds which are toxic or inhibit cell activity. The active organisms in activated sludge biomass require molecular oxygen to oxidize the organic load in the wastewater and provide organic carbon to synthesize the compounds necessary for their continuous growth. The two processes are concurrent and together determine the treatment’s wastewater organic load elimination rate. The toxicity analysis consists of loading the respirometer with the desired amount of activated sludge from the WWTP and continuously aerating it until air saturation. Then, the sample is added and oxygen consumption measured. The percentage of inhibition can be calculated from the oxygen measurement according to the equipment protocol. Apart from toxicity assays, activated sludge respirometry analyses are also employed for assessing the biodegradability of industrial wastewater partially treated by an AOP. It is used as a short-term biodegradability assay, to evaluate the oxygen uptake rate during the time the sample and the activated sludge are in contact. Biodegradation parameters such as the maximum oxygen uptake rate and dissolved oxygen consumption found in the respirometric tests are realistic analyses for evaluating the efficiency of partial wastewater oxidation (textile wastewater, landfill leachate wastewater, phenol wastewater, etc.) by an AOP (Goi et al., 2009). In this assay, the oxygen uptake rate from a mixture of a certain amount of the pretreated wastewater and activated sludge (in the endogenous phase and with inhibited autotrophic bacterial activity) is measured during a contact period of around 20 minutes. In the end, the readily biodegradable fraction of the COD (CODrb) is found (as a function of the total oxygen consumption and the biomass growth rate). The CODrb/COD ratio shows the sample’s biodegradability. Over 0.1 means that it is biodegradable and below 0.05 is not. The different wastewater COD fractions (biodegradable, non-biodegradable, non-soluble, etc.) can also be determined by respirometric assay (Lagarde et al., 2003). Figure 12.3.2 is a typical oxygen uptake rate graphic. As observed, the area 386 Solar energy sciences and engineering applications under the curve shows the readily biodegradable fraction of COD in a wastewater sample containing non-biodegradable contaminants partially treated by solar photo- Fenton. In this particular case, COD was 197 mg/L and the CODrb found in the respirometric analysis was 37.4 mg/L. Thus CODrb/COD is 0.29, so the sample is considered biodegradable and completely biocompatible for discharge into a conventional municipal WWTP. 12.3.3 Zahn-Wellens test Study of the inherent biodegradability of a chemical compound enables its potential for biodegradation under optimal aerobic conditions, such as in a conventional WWTP, to be determined. In these assays, the chemicals are exposed to microorganisms, sometimes previously adapted to the substance for a long period of time to increase compound degradation. The Zahn-Wellens method (Z-W) is standardized by a European Union protocol (Directive 88/302/EEC), and is recommended much more for biodegradability testing than BOD for several reasons. First, the procedure is similar to a real activated sludge biological reactor, and the biomass can even adapt to the compounds in the sample, since the Z-W test lasts 28 days, and second, biodegradation efficiency can also be evaluated by TOC and HPLC-UV to ensure the reliability of the results. The main drawbacks of this biological assay are that it is not applicable to volatile or semi-volatile compounds, or to compounds with water solubility under 50 mg of carbon per litre (application range 50 to 400 mg/L organic carbon). The Z-W procedure consists of placing activated sludge (preferably from the WWTP which is going to receive the industrial wastewater) and nutrients in contact with the target compound as the only carbon source in the medium. This mixture is kept under proper aeration (and agitation) and in the dark or under diffuse light for 28 days (ambient temperature around 20–25.C). The percentage of biodegradability can be determined at any given time by monitoring total organic carbon and using Equation 12.3.2: Dt =  1 -  TOCt - TOCb TOCa - TOCba  × 100 (12.3.2) where Dt is the percentage of biodegradability after time t, Ca is the TOC (mg/L) in the sample measured three hours after the beginning of the experiment (to take the effect of adsorption of the compound on the biomass into account), Ct is the TOC measured at time t (usually measured daily), Cb is the blank TOC (containing only the same amount of activated sludge as the samples, distilled water and nutrients in order to evaluate the TOC produced only by the biomass metabolism) measured at time t and Cba is the blank TOC measured three hours after the beginning of the experiment. The biodegradability threshold is 70%. The biodegradability of a reference compound such as diethylene glycol should be evaluated as a control to ensure the method is working properly and the correct activity of the activated sludge. 70% elimination of the TOC in this substance in less than 14 days demonstrates proper activity of the activated sludge. As a practical example of the use of biodegradability analyses, Figure 12.3.3 shows photo-Fenton degradation of an industrial wastewater. As photo-Fenton is an Decontamination of water 387 Figure 12.3.3 Photo-Fenton degradation of an industrial wastewater: (a) AOS during treatment, and; (b) Z-W biodegradability analyses of selected samples. oxidation process, more oxidized organic intermediates are formed at the beginning (Note how fast COD drops until H2O2 is 10mM) without any substantial mineralization (very low, as measured by TOC, until H2O2 is 10 mM). After a certain amount of H2O2 has been consumed, COD and TOC behave similarly, stabilizing the AOS. After 25mM of H2O2, another increase in AOS, though not as sharp, and another steady state after 35mM of H2O2 were observed, indicating that the very different intermediates formed go through different oxidation-mineralization steps. Formation of more oxidized intermediates is usually an indirect demonstration of 388 Solar energy sciences and engineering applications improved wastewater biodegradability. Biodegradability of the mixture during phototreatment was evaluated in samples taken over 28 days by the Zahn-Wellens (Z-W) test. S1 was non-biodegradable (around 50% biodegradability in 28 days), whereas the rest of the samples were biodegradable according to the Z–W test. S2, S3 and S4 were biodegradable after a long period of 8 days or more. However, after S5 (345 mg/L TOC), all the samples became biodegradable in a short time (5 days or less). S6 had the highest biodegradation percentages. Therefore, the most suitable point for combining photo-Fenton with the biological treatment is somewhere between S5 and S6. 12.3.4 Factors to be considered in designing a combined system Design of a combined chemical and biological wastewater treatment consider how the characteristics of each individual treatment can improve the destruction of a persistent contaminant. The chemical oxidant to be used (photo-Fenton or Fenton reagent, O3/H2O2, O3/UV, H2O2/UV, TiO2/UV, etc.) must be decided based on tests to determine which has the highest rate in the key parameter selected (TOC, COD, biodegradability, toxicity or a combination thereof) with the lowest chemical consumption. The rest of the characteristics to be taken into consideration are widely known: the ability of the chemical oxidation process, its potential for forming toxic intermediates or not, change in pollutant behaviour, choice of biological agent, comparison of different cultures, comparison of acclimated and non-acclimated cultures, use of monospecific cultures, anaerobic cultures, etc. Measurement of the combined process efficiency depends on the purpose of the treatment, but usually requires independent optimization of each chemical and biological step. For example, the extent of mineralization of the organic compounds may be a measure of efficiency if highly pure water is used or the effluent has a specific dissolved organic carbon limit. The main purpose of other treatments may be total elimination of toxicity or of a specific pollutant. Determining the target is an essential step in combination studies since it helps define process efficiency and provides a basis for comparing operating conditions and optimizing the process. Nevertheless, if the influent concentration is expected to change, correct scaling and design of the photoreactor may be complicated, because the correlation of the required treatment time and substrate concentration cannot be estimated directly, but must be determined experimentally. Therefore, several analytical parameters must be monitored during each step of the treatment train, as commented before. Chemical parameters, biological assays for toxicity (with various organisms like Vibrio fischeri, Daphnia magna, but recommended with activated sludge, etc.) and biodegradability (always using activated sludge) are very important to ensure the optimal conditions for complete effluent treatment. In the biological system itself, and in addition to daily control analyses such as total suspended and volatile solids, total organic carbon or chemical oxygen demand, pH and dissolved oxygen in the system, etc., it is also essential to measure anions and cations in the biological medium, since nutrients are vital to the microorganisms that make up the activated sludge populations and monitoring of the nitrogen species provides much information on nitrification and denitrification that take place during biotreatment. This whole series of analytical parameters is necessary for engineering the design of the combined strategy. For further understanding of the underlying processes, Decontamination of water 389 additional analytical methods may be necessary for the identification of unknown intermediate degradation products (chromatography coupled with mass spectrometry). Considerable effort and sophisticated analytical equipment may be necessary to explain (Gómez-Ramos et al., 2011) why, for example, acute toxicity rises during treatment by pinpointing a single intermediate product much more toxic than the original pollutant, or when the purpose is to degrade certain contaminants to below a limit (usually µg/L) in complicated water containing other organics, and therefore COD, TOC or HPLC/UV cannot be used. 12.4 COMBINING SOLAR ADVANCED OXIDATION PROCESSES AND BIOTREATMENT: CASE STUDIES 12.4.1 Case study A:An unsuccessful AOP/biological process In the first example the targets are two biorecalcitrant substances used as synthesis intermediates in the pharmaceutical industry, 2-(2, 4-dichlorophenyl)-2-(1H-imidazol- 1-dylmethyl)-1,3-dioxolan-4-ylmethanol (CAS 84682-23-5) (DIM) and 2-(2,4- dichlorophenyl)-2-(1H-1,2,4-triazol-1-ylmethyl)-1,3-dioxolan-4-ylmethanol (CAS 67914-85-6) (DTIM). These two non-biodegradable compounds mixed in a distilled water matrix were degraded by solar Photo-Fenton. Each contaminant was dissolved at a concentration of 200 mg/L (COD of 700 mg/L) because they are usually found at this concentration in industrial wastewater. Figure 12.4.1 shows wastewater degradation by solar photo-Fenton, toxicity results and Zahn-Wellens biodegradability analyses. It may be observed that the target compounds were susceptible to complete degradation and mineralization by photo-Fenton. Both substances were completely eliminated after 25 minutes of illumination (DIM and DTIM=0) with 27.5mM of hydrogen peroxide. The AOS was also calculated from TOC and COD results, but did not increase until around 80% ofTOChad been mineralized. Toxicity analyses showed that inhibition remained the same until practically the end of the treatment. Furthermore, biodegradability monitored by Zahn-Wellens showed that DIM and DTIM biodegradability were only slightly enhanced when photo-Fenton pretreatment was extended until TOC was below 98 mg/L (at this point, biodegradability after 11 days was 60%). From these results, it can be concluded that the best treatment option for wastewater containing DIM and DTIM is to apply solar photo-Fenton (or other AOP) only until almost complete mineralization. In this case a combined AOP/biological process strategy is not feasible. 12.4.2 Case study B:A successful AOP/biological process Industrial wastewater with a low organic load (TOC under 500 mg/L) can usually be treated in a combined AOP/biological process. This is the case in the following example, the successful treatment of saline industrial wastewater containing around 600 mg/L of a non-biodegradable compound (R-methylphenylglycine, MPG, also from the pharmaceutical industry) and 400 to 600 mg/L dissolved organic carbon (TOC). Tests performed in solar photo-Fenton pilot plants [Gernjak et al., 2006] were used to design a large-scale hybrid solar photocatalytic-biological plant with a 4m3 daily treatment 390 Solar energy sciences and engineering applications Figure 12.4.1 Degradation of wastewater containing two biorecalcitrant compounds by solar photo- Fenton: (a) TOC elimination and toxicity, and; (b) Biodegradability of several samples from intermediate stages of the treatment DIM (I) and DTIM (II) Structures are also shown. capacity. This demonstration plant was erected in the grounds of a pharmaceutical company in the south of Spain (see Figure 12.2.3). Using the same protocol described in Section 12.4.1, and after performing photo-Fenton experiments, biodegradability was monitored by Zahn-Wellens tests with the results presented in Figure 12.4.2. Biodegradability was enhanced and the threshold was reached at TOC<150 mg/L (70% biodegradability in 7 days). A sample with an initial TOC of around 180 mg/L was biodegradable, but after a longer period of around 15 days. This means that with Decontamination of water 391 Figure 12.4.2 Biodegradability of several samples from intermediate stages of the photo-Fenton treatment of biorecalcitrant wastewater containing 550 mg/L of TOC. Samples at TOC>250 mg/L are not shown as biodegradability belowTOC=254 mg/L. a period of adaptation, higher organic loads could be fed to the bioreactor. This is discussed later, under full-sized plant results. From these results, it can be concluded that the best treatment option for this wastewater, containing around 550 mg/L of TOC was solar photo-Fenton until around 60% mineralisation followed by biotreatment. The full-sized solar photo-Fenton reactor consists of a 3000-L buffer or recirculation tank and 100m2 solar collector field made up of three rows of compound parabolic collectors (CPCs) specially developed for photo-Fenton applications. The total system volume is 4000 L (1260 L of illuminated volume). The aerobic biological treatment part of the plant consists of three modules: a 5000 L neutralization tank, a 2000 L conditioner tank, and a 1000 L fixed-bed biofilm reactor colonized by activated sludge from the wastewater treatment plant installed at the pharmaceutical company itself. This biological system was operated directly in continuous mode. It was operated in batch mode only during the start-up phase (inoculation, bacteria fixation, and growing, etc.) and for the first experiments, such as the one shown in Figure 12.4.3. A detailed description can be found elsewhere (Oller et al., 2007). From the point of view of operation, the industrial saline wastewater partially oxidized by photo-Fenton is discharged into the neutralization tank where water is roughly neutralized with concentrated NaOH and iron is settled and removed when necessary. Then the photo pre-treated effluent is transferred to the conditioner tank, where the pH is automatically adjusted to between 6.8 and 7.5. Then the effluent is pumped to the bioreactor. According to previous laboratory and pilot plant-scale biological tests performed (Figure 12.4.2), biodegradability enhancement of industrial wastewater by photo- Fenton is accomplished when TOC is reduced to approximately 40%. At that moment partially treated wastewater is transferred to the aerobic biological reactor. No mineral medium is usually added to the photo-Fenton pretreated effluent as the seawater 392 Solar energy sciences and engineering applications Figure 12.4.3 Degradation of wastewater containing biorecalcitrant compounds in a combined solar photo-Fenton and biotreatment. matrix and the ammonium generated by the degradation of wastewater (approximately 4 mM), fulfill the C and N, P requirements (8–10 mg/L depending on wastewater composition). Figure 12.4.3 shows TOC and nitrogen concentrations in the photo-Fenton and IBR. It should be observed that the water was transferred from photo-Fenton to biotreatment at around 200 mg/L and that it fell to around 30 mg/L very quickly (48 hours) compared to the Z-W test, demonstrating that an adapted bioreactor produces more efficient results than the Z-W test carried out with fresh biomass. Nitrification was also complete. In this case the combined AOP/biological process strategy can be applied. ACKNOWLEDGEMENTS The authors wish to thank the Spanish Ministry of Science and Innovation for funding under the EDARSOL Project (Reference: CTQ2009-13459-C05-01). REFERENCES Ajona, J. A. and Vidal, A. (2000) The use of CPC collectors for detoxification of contaminated water: design, construction and preliminary results. Solar Energy, 68, 109–120. Blanco, J., Malato, S., Fernández, P., Vidal, A., Morales, A., Trincado, P., de Oliveira, J.C., Minero, C., Musci, M., Casalle, C., Brunotte, M., Tratzky, S., Dischinger, N., Funken, K.-H., Decontamination of water 393 Sattler, C., Vincent, M., Collares-Pereira, M., Mendes, J.F. and Rangel, C.M. (2000) Compound parabolic concentrator technology development to commercial solar detoxification applications. Solar Energy, 67, 317–330. Blanco, J. and Malato, S. (2003) Solar Detoxification, UNESCO Publishing, France. Blanco-Galvez, J., Fernández-Ibáñez, P. and Malato-Rodríguez S. (2007) Solar photocatalytic detoxification and disinfection of water: Recent overview. Journal of Solar Energy Engineering, 129, 4–15. Blanco J., Malato S., Fernández-Ibañez P., Alarcón D., GernjakW. and Maldonado M.I. (2009) Review of feasible solar energy applications to water processes. Renewable and Sustainable Energy Reviews, 13, 1437–1445. Colina-Márquez J., Machuca-Martínez F. and Li Puma G. (2010) Radiation adsorption and optimisation of solar photocatalytic reactors for environmental applications. Environmental Science and Technology, 44, 5112–5120. Comninellis, C., Kapalka, A., Malato, S., Parsons, S.A., Poulios, I. and Mantzavinos, D. (2008) Advanced oxidation processes for water treatment: advances and trends for R&D. Journal of Chemical Technology & Biotechnology, 83, 769–776. Dillert R., Cassano A. E., Goslich R. and Bahnemann D. (1999) Large scale studies in solar catalytic wastewater treatment. Catalysis Today, 54, 267–282. Fenton, H.J.H. (1894) Oxidation of tartaric acid in presence of iron. Journal of the Chemical Society, 65, 899–910. Gernjak W., Fuerhacker M., Fernández-Ibáñez P., Blanco J. and Malato S. (2006) Solar photo-Fenton treatment—Process parameters and process control. Applied Catalysis B: Environmental, 64, 121–130. Goi D., Di Girogio G., Cimarosti I., Lesa B., Rossi G. and Dolcetti G. (2009) Treatment of landfill leachate by H2O2 promoted wet air oxidation: COD-AOX reduction, biodegradability enhancement and comparison with a fenton-type oxidation. Chemical and Biochemical Engineering, 2, 343–349. Gómez-Ramos, M.D.M., Pérez-Parada, A., García-Reyes, J.F., Fernández-Alba, A.R. and Agüera, A. (2011) Use of an accurate-mass database for the systematic identification of transformation products of organic contaminants in wastewater effluents. Journal of Chromatography A, 1218, 8002–8012. Gutiérrez, M., Etxebarria, J. and De Las Fuentes, L. (2002) Evaluation of wastewater toxicity: Comparative study between Microtox® and activated sludge oxygen uptake inhibition. Water Research, 36, 919–924. Kavitha, V. and Palanivelu, K. (2004) The role of ferrous ion in Fenton and photo-Fenton processes for the degradation of phenol. Chemosphere, 55, 1235–1243. Lagarde, F., Tusseau-Vuillemin, M-H., Lessard, P., Hèduit, A., Dutrop, F. and Mouchel, J.-M. (2003) Variability estimation of urban wastewater biodegradable fractions by respirometry. Water Research, 39, 4768–4778. Lapertot, M. and Pulgarin, C. (2006) Biodegradability assessment of several priority hazardous substances: Choice, application and relevance regarding toxicity and bacterial activity. Chemosphere, 65, 682–690. Malato S., Blanco J., Alarcón D.C., Maldonado M.I., Fernández-Ibáñez P. and Gernjak W. (2007) Photocatalytic decontamination and disinfection of water with solar collectors. Catalysis Today, 122, 137–149. Malato, S., Fernández-Ibañez, P., Maldonado, M.I., Blanco, J. and Gernjak, W. (2009) Decontamination and disinfection of water by solar photocatalysis: Recent overview and trends. Catalysis Today, 147, 1–59. Mandal, T., Maity, S., Dasgupta, D. and Datta, S. (2010) Advanced oxidation process and biotreatment: Their roles in combined industrial wastewater treatment. Desalination 250, 87–94. 394 Solar energy sciences and engineering applications Muñoz, R. and Guieysee, B. (2006) Algal-bacterial processes for the treatment of hazardous contaminants: A review. Water Research, 40, 2799–2815. Oller I., Malato S., Sánchez-Pérez J.A., Maldonado M.I., Gernjak W., Pérez-Estrada L.A., Muñoz J.A., Ramos C. and Pulgarín C. (2007) Pre-industrial-scale combined solar photofenton and immobilised biomass activated-sludge bio-treatment. Industrial & Engineering Chemistry Research, 46, 7467–7475. Oller I., Malato S. and Sánchez-Pérez J.A. (2011) Combination of advanced oxidation processes and biological treatments for wastewater decontamination-A review. Science of the Total Environment, 409, 4141–4166. Pera-Titus, M., García-Molina, V., Baños, M.A., Giménez, J. and Esplugas, S. (2004) Degradation of chlorophenols by means of advanced oxidation processes: A general review. Applied Catalysis B: Environmental, 47, 219–256. Pignatello, J.J., Oliveros, E. and MacKay, A. (2006) Advanced oxidation processes for organic contaminant destruction based on the fenton reaction and related chemistry. Critical Reviews in Environmental Science and Technology, 36, 1–84. Rizzo, L. (2011) Bioassays as a tool for evaluating advanced oxidation processes in water and wastewater treatment. Water Research, 45, 4311–4340. Scott J.P. and Ollis D.F. (1995) Integration of chemical and biological oxidation processes for water treatment: review and recommendations. Environmental Progress, 142, 88–103. Suty, H., De Traversay, C. and Cost, M. (2004) Applications of advanced oxidation processes: present and future. Water Science and Technology, 49, 227–233. Chapter 13 Solar driven advanced oxidation processes for water decontamination and disinfection Erick R. Bandala1 & Brian W. Raichle2 1Energy and Environmental Research Group., Universidad de las Américas, Puebla. Sta., Catarina Mártir, Cholula 72820 Puebla, Mexico 2Department of Technology and Environmental Design, Appalachian State University, Katherine, Harper Hall, Boone, NC, USA 13.1 INTRODUCTION The industrial revolution of the late 18th century brought about new paradigms, including unprecedented and sustained population growth and a shift from a manual labor economy towards machine-based manufacturing. As a result, generations of industrial and domestic waste started to accumulate, resulting in new problems related to waste management and site contamination. As industrialization increases and populations rise, the amount of waste inevitably will surpass the natural capacity of ecosystems to self-purifiy. The continued production of waste during the last few decades has exceeded this capacity and has caused disorder, instability, harm, or discomfort to these ecosystems. Efforts to mitigate the negative effects of waste from anthropogenic activities fall into two main categories. One category strives to decrease or eliminate waste generation through design and implementation of cleaner industrial processes. The second involves site restoration using novel state-of-art technologies able to remove waste with less impact on the surroundings. Many technological approaches for improving water, air and soil quality have been developed over the last few decades. With an increasing emphasis placed on sustainability, technological solutions are evaluated not only by their cost-effectiveness but also by their ability to withdraw pollutants from the environment without generating by-products and, preferably, by their use of renewable sources of energy. Among the different technological approaches developed, Advanced Oxidation Processes (AOPs) have recently emerged as very interesting alternatives for water treatment. AOPs were initially defined as processes involving the generation of highly reactive oxidizing species able to attack and degrade organic substances (Bolton, 2001). Nowadays, AOPs are considered physical-chemical processes with high thermodynamic viability and the ability to produce deep changes in the chemical structure of contaminants as a result of the participation of free radicals in Redox reactions (Domenech et al., 2004). Free radicals, mainly hydroxyl radicals (HO), are of particular interest for environmental restoration because of their high oxidation capability. However, other studies have suggested that, besides hydroxyl radicals, AOPs can also generate other oxidizing species (Anipsitakis and Dionysiou, 2004). Generated radicals 396 Solar energy sciences and engineering applications are able to oxidize organic pollutants mainly by hydrogen abstraction or by electrophylic addition to double bonds that generates organic free radicals (R•). These free radicals can react with oxygen molecules forming peroxy-radicals and initiate oxidative degradation chain reactions that may lead to the complete mineralization of the organics. AOP-generated free radicals involved in the degradation process may be produced by photochemical and non-photochemical procedures as has been widely reported previously (Quiroz et al., 2011). In particular, photochemical AOPs have generated great interest in the last decade since these procedures have led to the use of renewable sources of energy to promote the chemical procedures involved. Solar radiation has been identified as a potential source for driving photochemical AOPs with interesting potential for real applications, specifically for water detoxification and disinfection (Orozco et al., 2008; Bandala et al., 2008a,b). The most studied technological approaches to water disinfection using solar radiation are homogeneous and heterogeneous photocatalysis. Both processes have been widely tested at the laboratory, bench and pilot-plant scale for water detoxification and disinfection with interesting results that will be discussed in later sections of this chapter. 13.2 SOLAR RADIATION COLLECTION FOR AOPs APPLICATIONS Solar driven AOPs possess interesting advantages when compared with artificial light promoted AOPs reactions. Solar radiation availability, reduced cost, and simplicity are the most commonly cited. Nevertheless, use of solar radiation also has some challenges that must be faced being solar radiation collection probably the most significant. Despite the availability of free solar radiation almost everywhere around the world, its use requires an efficient and cost effective optical system that focuses and uniformly distributes the radiation of a surface. This optical system is known as a solar radiation collector. The first reported attempts to collect solar radiation for the promotion of AOPs were in the early 90’s at Sandia National Laboratories (USA). These early efforts used parabolic trough collectors, usually employed for solar thermal applications. The initial project objectives were not achieved at that time since the high concentration optical system required an expensive sun tracking system for optimal operation. After initial use of high concentration solar systems for driving AOPs, interest shifted toward the use of low or non-concentrating, i.e. non imaging systems, since it was identified that non-tracking systems may be able to promote AOPs without the disadvantages of high concentrating, i.e. tracking, collectors. Since the first use of the non-tracking, low concentration solar collectors for AOP applications, a wide variety of different solar collection geometries have been tested. Some of the important geometries are shown in cross section in Figure 13.2.1. The main differences, advantages and disadvantages of these different geometries have been discussed in the past and the discussion continues today. Some of the main findings have been summarized by Bandala et al. (2004). These authors reported that reactors based on non-imaging collectors have attracted interest (Blanco et al., 1994) since these reactors share some of the advantages of tracking parabolic troughs and non-concentrating reactors (Malato et al., 1997), which has been confirmed by several studies comparing Solar driven advanced oxidation processes 397 Figure 13.2.1 Schematic representation of different concentrating and non-concentrating solar collection geometries usually reported for photocatalytic applications. Figure 13.2.2 Ray tracing approach showing the capability of CPC geometry to concentrate diffuse solar radiation. non-concentrating reactors with parabolic troughs (Curco et al., 1996; Malato et al., 1997; Gimenez et al., 1999). Probably, the most studied non-imaging reactor for AOP applications is the compound parabolic collector (CPC). The most frequently quoted advantages of nonimaging CPC reactors are their ability to use global solar radiation (radiation coming from all directions in the sky, as shown in Figure 13.2.2), simplicity of operation, and ability to operate in the turbulent flow regime (which improves mass transfer). However, other non-imaging reactors offer these advantages in principle, to different degrees. There are many possibilities for the geometry of the reflectors that illuminate the tubes. Reflector geometries other than the CPC that have received attention in the 398 Solar energy sciences and engineering applications Table 13.3.1 Main chemical reactions involved in the dark Fenton and photo-Fenton processes; k values in every case represents rate constant. (1) Fe2+ +O2.O•- 2 +Fe3+ k=1.15M-1 s-1 (2) H2O2.HO- 2 +H+ k=1.26×10-2 M-1 s-1 (3) Fe2+ + H2O2.Fe3+ +HO• +HO- k=55M-1 s-1 (4) Fe(OH)2+ +hv.Fe2+ +HO• k=2×10-3 M-1 s-1 (5) H2O2 +hv.2HO• fHO· =0, 98, 254 nm (6) HO• +H2O2.HO• 2 +H2O k=2.7×107 M-1 s-1 (7) Fe2+ +HO•.Fe3+ +HO- k=2.7×107 M-1 s-1 past include V-trough and L-shaped collectors, as shown in Figure 13.2.1. The performance of other non-imaging geometries have been found quite close to those found for CPCs, due mainly to a more uniform photon distribution in the photorreactor rather than radiation intensity reaching the receiver (Brito et al., 2012). 13.3 SOLAR HOMOGENOUS PHOTOCATALYSIS The Fenton process is one of the most widely used AOPs for water and wastewater treatment. Fenton’s reaction uses hydrogen peroxide and ferrous salt to generate hydroxyl radicals (HO), chemical species possessing inherent properties that enable them to mineralize dissolved organic pollutants into CO2, water, and mineral acids (Bandala et al., 2007). When this process is driven by ultraviolet (UV) radiation, visible light, or both it is known as the photo-Fenton process. The photo-Fenton process possesses several advantages over the dark Fenton reaction, mainly an increased reaction rate and the possibility of using a cheap, clean, and widely distributed energy source: solar radiation. Table 13.3.1 depicts the main chemical reactions involved in both dark Fenton and photo-Fenton processes along with reaction rate constants for each reaction. In agreement with the information in Table 13.3.1, it is worthy to note Equations 13.3.3 and 13.3.4 since they are the main processes involved in the solar photo-Fenton reaction. Equation 13.3.3 shows the actual decomposition of hydrogen peroxide catalyzed by ferrous salt. Hydrogen peroxide decomposition catalyzed by Fe2+ generates one hydroxyl ion and one hydroxyl radical. Hydroxyl radicals are the most important specie generated during the reaction since it may react with organic matter to oxidize it. In the case of dark Fenton reactions, ferrous salt may be considered the limiting reagent anytime the reaction described in Equation 13.3.3 stops once all the available ferrous iron has been oxidized to ferric iron independent of the amount of hydrogen peroxide added to the reaction. The advantage of including radiative energy in the process is shown in Equation 13.3.4. In the case of photo-assisted Fenton reactions, radiation of a certain wavelength (usually UV and part of the visible radiation) may generate the so-called photo-reduction of Fe3+ to Fe2+ depicted in Equation 13.3.4 plus one mole of hydroxyl radicals. Photo-reduced iron can participate again in the decomposition of hydrogen peroxide as in Equation 13.3.3, and the process may keep occurring if enough hydrogen peroxide is provided to the reaction mixture. As stated earlier, the useful wavelength range for carrying out the catalytic Solar driven advanced oxidation processes 399 Figure 13.3.1 The solar spectrum. process (300–500 nm) may be provided by solar radiation since it has been widely reported that the sun is an important light source for driving photo-Fenton process in the wavelength above 300 nm as shown in Figure 13.3.1. 13.3.1 Degradation of organic pollutants by solar driven photo-Fenton processes Several different types of organic pollutants have been tested for the application of solar driven photo-Fenton processes including dyes and textile wastewater effluents, surfactants and algal toxins, among many others. The following are some of the most widely reported organic pollutants tested for solar driven photo assisted Fenton and Fenton-like processes. 13.3.1.1 Pesticide degradation Solar driven photo assisted Fenton (SDPAF) and Fenton-like processes have been tested for pesticide degradation. Recently, Quiroz et al., (2011) published a complete review on the application of advanced oxidation processes for pesticide removal in aqueous media, including interesting tabulated data on references related with Fenton and Fenton-like processes. Specifically, triazinic pesticides have been successfully removed from water by using solar driven Fenton-like processes (Bandala et al., 2007; Perez et al., 2006) as well as dimethylurea pesticides (Farre et al., 2007; Perez et al., 2006); organophosphorous (Farre et al., 2007; Hincapié et al., 2005); chloracetic acid (Bandala et al., 2007a); and phenol and phenolic derivatives (Farre et al., 2007). An interesting effect that should be taken into account when applying homogeneous photocatalysis for pesticide degradation is the presence of salt counterions. Inorganic anions (Cl-, SO2- 4 , HPO2- 4 ) present in the water or added as reagents have a significant effect on the reaction rates of Fenton processes such as complexation with Fe2+ 400 Solar energy sciences and engineering applications or Fe3+, affecting iron species reactivity and distribution; precipitation reactions leading to a decrease of the active dissolved Fe3+; or scavenging of hydroxyl radicals and oxidation reactions involving these inorganic radicals. It has been well documented that, for example, chloride ions show an inhibitory effect for oxidation reactions of phenols (Tang and Huang, 1996), dichlorvos (Lu et al., 1997), and atrazine (De Laat et al., 2004). 13.3.1.2 Dye degradation Textile wastewater is considered highly polluting because of its high organic loads and the presence of color. Colored wastewater is not usually considered as toxic, however it has been well documented that it may cause serious impact once release to the environment (Orozco et al., 2008). Removal of waste products resulting from the textile industry, specifically dyes, is probably one of the most successful applications of solar driven Fenton-like processes. A wide variety of dyes and pigments have been remediated with good results such as azo-dyes (Chacón et al., 2006; Orozco et al., 2008) and benzidine-based dyes (Bandala et al., 2008c), as well as dye mixtures in real textile wastewater (Bandala et al., 2008c). As with pesticide degradation, several variables affecting dye degradation have being identified related with iron salt counterions, reaction pH, and reagent concentration, among many others (Orozco et al., 2008). 13.3.1.3 Surfactants degradation Surfactants (surface active agents) are molecules which include in their chemical structure a hydrophilic head and a hydrophobic tail. This structure allows surfactants to increase the aqueous solubility of hydrophobic compounds by solubilization. Surfactants are frequently used in soap and as active ingredients in detergent formulations like shampoos and dishwashing liquids. They play an important role in the paper, food, polymers, cosmetics, food, pharmaceuticals, and oil recovery industries (Bandala et al., 2008a). Besides their environmentally undesirable characteristics, surfactants produce aesthetic effects after being released into the natural water courses, inhibit gas transference between the water and the atmosphere, and consume dissolved oxygen. Removal of up to 99% of surfactants from wastewater has been also successfully carried out using SDPAF and Fenton-like processes in short reaction time (Bandala et al., 2008b; Lin et al., 1999; Amat et al., 2004). This process has been applied to the treatment of real wastewater from the surfactant enhanced soil washing (SESW) process commonly applied to the restoration of oil polluted sites (Bandala et al., 2008a). Results obtained in this case were also very interesting since the SDPAF and Fenton-like processes tested were capable not only to completely remove the surfactant but also eliminate 69% of the TPHs and other pollutants in the influent, measured as chemical oxygen demand (COD). The resulting effluent has a high potential to be further treated with conventional, i.e. biological, wastewater treatments. 13.3.2 Microorganisms inactivation by solar driven photo-Fenton processes Despite several different examples of the application of SDPAF for organic pollutant remediation, relatively little work have been done on its application for inactivation of Solar driven advanced oxidation processes 401 microorganism in water. The removal of pathogenic species from water is important due to their ability to generate immediate adverse health effects on the population forced to consume non-safe water. Just to have an idea of the problem’s magnitude, it has been estimated that in Africa, Latin America and the Caribbean alone, nearly one billion people no not have access to safe water supplies (WHO/UNICEF, 2000). As a result of this situation, waterborne diseases result in the death of 1.5 million children every year (Montgomery and Elimelech, 2007). Besides health concerns, lack of access to safe drinking water is also associated with poverty and limits sustainable development (Bandala et al., 2011a). 13.3.2.1 Bacteria Solar water disinfection (SODIS) has been identified in the past decade as a simple, environmentally friendly, and low cost point-of-use treatment technology for drinking water purification capable to use the bacteriostatic effect of the UV-A part of the solar spectrum (320–400 nm) and dissolved oxygen in water to inactivate pathogenic species through the production of reactive forms of oxygen (ROS). According to previous systematic studies, the best bacteria inactivation effect is reached on sunny days when heat and UV radiation combine synergistically (EWAG, 2002; Schmid et al., 2008). Application of advanced oxidation processes to water disinfection using solar radiation, coined as enhanced photocatalytic solar disinfection (ENPHOSODIS) by Bandala et al. (2011a), has improved SODIS disinfection efficiency, solved some disadvantages identified previously with SODIS, and allows the efficient inactivation of not only common waterborne bacteria but other highly resistant microorganisms (Guisar et al., 2007). In this regard, SDPAF processes have been tested for the inactivation of bacterial strains commonly infecting water such as Escherichia coli and Pseudomonas aeruginosa with very satisfactory results (Bandala et al., 2011a,c). 13.3.2.2 Helminth egg Species with the ability to resist adverse conditions may survive conventional disinfection processes even after long periods of treatment. One example of these kinds of undesirable species is helminth eggs. The WHO has estimated that about 1 billion people in developing countries are infected by Ascaris and that 25–33% of this population is affected by helminthiasis alone. These diseases are importantly related to poor physical growth and development, as well as retardation of intellectual and cognitive development in children less than 15 years of age (Bandala et al., 2012). Inactivation of helminth eggs in water is not an easy task mainly because their basic structure makes them resistant to external agents. Conventional sanitary engineering processes have been applied to the removal of helminth eggs from wastewater. For these cases, 80–100% helminth egg removal within 20–35 h of treatment has been achieved. Presence of these microorganisms, however, is not exclusive to wastewater. Several studies have reported the presence of helminth eggs in surface and even ground water and it is well documented that they possess high resistance to disinfection, resisting conventional drinking water treatment and emerging live from domestic taps (Bandala et al., 2012). 402 Solar energy sciences and engineering applications 13.3.2.3 Spores as indicators Helminth eggs are not the only microorganisms with the capability to resist conventional water disinfection processes. Other well-known pathogenic species with such high resistance are Cryptosporidium parvum oocyst and Giardia lamblia cysts, among many others. Resistance of C. parvum oocysts to chlorine disinfection has been extensively documented. Chlorine concentrations of 4 mg/L, the highest residual allowed by U.S. regulations, require more than 15 hours of contact time to inactivate 99% of C. parvum oocysts at pH 6.0 and 20.C. The chlorine dosage necessary for treatment increases dramatically with increasing pH and decreasing temperature (Guisar et al., 2007). Having in mind all these considerations, it is clear that using E. coli as pathogenic indicator of drinking water disinfection is, by far, not enough. In the search for a more reliable indicator, Bacillus spores have emerged as an interesting alternative. It is known that spores of Bacillus spp., are highly resistant to inactivation by different physical stress conditions such as toxic chemicals or biocidal agents, desiccation, extreme pressure and temperature, as well as exposure to high doses of UV or ionizing radiation. Spores of Bacillus subtilis are commonly used test organisms for inactivation studies due to their high degree of resistance to various sporicidal treatments, reproducible inactivation response, safety (B. subtilis is not pathogenic to humans) and resistance to UV radiation, and have become a useful conservative index for water disinfection anytime it is considered that, once B. subtilis spores has been inactivated, anything else with less resistance will surely be inactivated (Bandala et al., 2011b). SDPAF processes have been demonstrated as highly efficient for Bacillus subtilis spore inactivation in water as a surrogate microorganism for C. parvum oocysts. Using relatively low Fenton reagent concentrations and low solar radiation intensity (about 1 sun), Guisar et al., (2007) obtained up to 96% spore viability reduction in only 1.5 hours of exposure to solar radiation. Bandala et al. (2011a) used heminth eggs to demonstrate that the use of highly resistant microorganisms as a conservative index for water disinfection is desirable and that an adequate solar radiation dose is required to ensure the final required water quality. In their work, these authors used Ascaris suum eggs, very similar to A. lumbricoides, the actual specie infecting humans, to assess the amount of radiation necessary to inactivate>5-log (99.999% removal) of helminth eggs. They found that approximately 140 kJ L-1 was required to achieve this task. When they tested the same experimental conditions (Fe2+ and H2O2 concentration) for the inactivation of E. coli and P. aeruginosa, they found that less than 10 kJ L-1 were required to reach up to>6-log inactivation (99.9999% removal) of both bacteria. Finally, they found no significant increase in the inactivation dose required when up to 5mgL-1 natural organic matter (NOM) was added to the bacterial suspension. In relation to Bacillus subtilis spore inactivation, several different Fenton reagent concentrations were tested in combination with solar UV-A radiation (.max =365 nm) for spore inactivation. The best spore inactivation conditions were found to be [Fe2+]=2.5mM and [H2O2]=100mM and radiation. Under these conditions, over 9-log inactivation was reached after only 20 min of reaction. The effect of ionic strength and natural organic matter (NOM) on spore inactivation kinetics was also tested. In both cases, an important decrease of the inactivation rate, fitted to the delayed Chick-Watson inactivation kinetics, was observed (Bandala et al., 2011b). Solar driven advanced oxidation processes 403 13.3.2.4 Sequential processes In other recent work (Bandala et al., 2012), the same research group reported the use of sequential coupled disinfection processes, in this case SDPAF processes followed by chlorine. In their work they suggested that the high efficiency of ozone–chlorine sequential disinfection previously reported could be generalized to different reactive oxygen species (i.e. hydroxyl radicals) which could synergistically enhance the oxidative properties of chlorine, thus improving the inactivation process. If true, it means that other methods to produce hydroxyl radicals might produce a similar synergistic effect in sequential processes. They assessed the photo-assisted Fenton process alone under different H2O2 and Fe2+ concentrations to test its ability to inactivate Ascaris suum eggs. The effect of free chlorine alone was also tested. Using the best reaction conditions, free chlorine only treatment achieved 83% egg inactivation after 120 min of reaction time, while the sequential photo-assisted Fenton process plus chlorine treatment achieved over 99% of egg inactivation after 120 kJ L-1 (about one hour of solar radiation). No effect on helminth eggs inactivation was observed with free chlorine alone after 550 mg min L-1, whereas egg inactivation in the range of 25– 30% was obtained for sequential processes (ENPHOSODIS then chlorine) using only 150 mg min L-1. 13.4 SOLAR HETEROGENOUS PHOTOCATALYSIS Heterogeneous photocatalytic degradation is usually related with the use of a stable, solid semiconductor capable of stimulating, under the effect of irradiation, a redox reaction at the solid/solution interface, while remaining unchanged after many turnovers of the redox system. When the semiconductor is in contact with a liquid electrolyte solution containing a redox couple, charge transfer occurs across the interface to balance the potentials of the two phases. An electric field is formed at the surface of the semiconductor, bending its energetic bands from the bulk of the semiconductor toward the interface. During the absorption of a photon with appropriate energy (photo excitation) by the semiconductor, band bending provides the conditions for carrier separation. The two generated charge carriers should react at the semiconductor/ electrolyte interface with the species in solution and, under steady state conditions; the amount of charge transferred to the electrolyte must be equal and opposite for the two types of carriers. When the charge carriers, usually denominated electron/hole pairs, are generated in the semiconductor the electron moves away from the surface to the bulk of the semiconductor as the hole migrates towards the surface. Metal oxides are common semiconductor materials suitable for photocatalytic purposes. Table 13.4.1 lists some selected semiconductor materials, which have been used for photocatalytic reactions in the past, along with their band gap energy and the wavelength range required to activate the catalysts. Among all these possibilities, TiO2 is a widely analyzed, low-cost, nontoxic, stable, highly photoreactive, and chemically and biologically inert photocatalyst. Solar driven heterogeneous photocatalysis (SDHP) using titanium dioxide is theAOPmost widely used as an alternative to conventional water decontamination and disinfection as well as air remediation technologies (Castillo et al., 2011). 404 Solar energy sciences and engineering applications Table 13.4.1 Optical properties for some photocatalytic semiconducting materials. Band gap equivalent Semiconductor Band gap (eV) wavelength (nm) BaTiO3 3.3 375 CdO 2.1 590 CdS 2.5 497 CdSe 1.7 730 Fe2O3 2.2 565 GaS 1.4 887 GaP 2.3 540 SnO2 3.9 318 SrTiO3 3.4 365 TiO2 3.0 390 WO3 2.8 443 ZnO 3.2 390 ZnS 3.7 336 TiO2 has proven to be highly effective in the nonselective decomposition of organic molecules and inactivation of microorganisms due to high decomposition and mineralization rates when used in combination with specific radiation sources (see Table 13.4.1). Furthermore, water treatment with SDHP neither requires the addition of other chemicals reactants nor generates hazardous waste by-products. The basic reactions occurring within the photocatalyst particles after absorption of the proper wavelength radiation are as follows: TiO2 .hv e- + h+ + TiO2 (13.4.1) e- + h+ + TiO2 . TiO2 + hv (13.4.2) (TiOIV - O2- - TiIV) - OH + h+ BV . (TiOIV - O2- - TiIV) - OH + H+ O2(ads) + e- BC . O·- 2(ads) (13.4.3) As shown in Equations 13.4.1 to 13.4.4, several oxidizing species may be generated as a result of the interaction between the photocatalyst and the radiation, although evidence supports the idea that the hydroxyl radical (·OH) is the main oxidizing specie responsible for the photo-oxidation of most organic compounds studied. Once the absorption of one photon with the required energy (ultraviolet radiation, .<390 nm) occurs, the first step is the generation of electron/hole pairs, which are separated between the conduction and valence bands (Equation 13.4.1). Recombination of the generated charge carriers (Equation 13.4.2) may occur. However, if the dissolvent is redox active (i.e. water) it may acts as a donor and acceptor of electrons avoiding recombination and improving the photonic efficiency, producing that on a hydrated and hydroxylated TiO2 surface, the holes trap ·OH radicals linked to the surface and avoid recombination reactions (Equation 13.4.3). It should be emphasized Solar driven advanced oxidation processes 405 that even trapped electrons and holes can rapidly recombine on the surface of a particle (Equation 13.4.2). The recombination process can be partially avoided through the capture of the electron by pre-adsorbed molecular oxygen, forming a superoxide radical (Equation 13.4.4). 13.4.1 Degradation of organic pollutants by solar driven heterogeneous photocatalysis Degradation of organic pollutants using photocatalytic processes is probably the most investigated AOP over the last three decades. It has been widely tested for the degradation of mono aromatics (i.e. benzene, dimethoxybenzenes, halobenzenes, nitrobenzene, chlorophenols, nitrophenols, benzamide, and aniline) and, consequently, these pollutants appear as model compounds in a wide variety of scientific papers. In addition to these, several other types of molecules have been investigated as substrates for photocatalytic degradation. Some of the most frequently reported are water-miscible solvents (i.e. ethanol, alkoxyethanol), haloaliphatics (i.e. trichloroethylene, tetrachloromethane), pesticides and surfactants, among many others. Despite the wide variety of reports on the application of heterogeneous photocatalytic processes for water and air treatment, relatively few reports are available dealing with the application of solar radiation to drive these processes. Solar driven heterogeneous photocatalytic processes (SDHPC) have been, however, applied to the treatment of organic contaminants. Some of the most important applications are described below. 13.4.1.1 Pesticide degradation Strongly colored compounds can be removed from surface or wastewater by conventional water treatment processes (i.e. adsorption), however these phase change processes have to be discouraged anytime they result in solid matrix waste. It is highly desirable to degrade dyes by oxidation to avoid the risk of contamination. Solar driven heterogeneous photocatalytic processes have been extensively applied for pesticide degradation. Quiroz et al. (2011) have recently published an interesting review about the different applications of SDHPC for the removal of these compounds. To mention some examples, chlorinated insecticides (Bandala et al., 2002; Hincapié et al., 2005); triazine derivatives (Parra et al., 2004); carbamate derivatives (Arancibia et al., 2002); haloacetic acid derivatives (Terashima et al., 2006) and organophosphorus (Pichat et al., 2007), among many others, have been successfully removed from water (Bandala and Torres, 2008d). 13.4.1.2 Dye degradation As mentioned early for homogeneous photocatalytic processes, SDHPC degradation of dyes and pigments has been extensively tested. The most recent published works dealing with SDHPC dye degradation focus mainly on azo dyes (Sajjad et al., 2010; Chung and Chen, 2009; Malato et al., 2009). However, a few reports dealing with SDHPC removal of other dye types have been published. 406 Solar energy sciences and engineering applications 13.4.1.3 Surfactant degradation Use of SDHPC for surfactant removal in aqueous media has been also widely reported for many different surfactant structures. Among the most recently reported are nonionic (Du et al., 2008; Eng et al., 2010) and cationic (Han et al., 2009; Naldoni et al., 2009; Natoli et al., 2012) surfactants; relatively few reports are available dealing with anionic surfactants. 13.4.2 Microorganisms inactivation by solar driven heterogeneous photocatalysis TiO2 semiconductor photocatalyst is widely reported, as suspended powder or thin film, to inactivate different organisms such as viruses, vegetative cells, and spores of organisms with a high resistance to desiccation and radiation (i.e. Escherichia coli, Lactobacillus acidophilus, Saccharomyces cerevisiae, Bacillus atrophaeus, Aspergillus niger, and Kocuria rhizophila) with very interesting results (Castillo et al., 2011; Muranyi et al., 2009). Some other specific strains have been tested for SDHPC disinfection, for example Fernandez-Ibañez et al. (2008) tested the inactivation of Fusarium solani and Fusarium sp spores, a pathogen infecting food crops using slurry TiO2 and a solar CPC photorreactor. They found that these pathogens are susceptible to solar photocatalytic disinfection with TiO2 in distilled water not only at the laboratory scale but also at the pilot plant scale (Fernandez-Ibañez et al., 2008; Polo-Lopez et al., 2010). The effect of some water quality parameters on disinfection processes has been estimated at pilot plant scale and some authors have proposed that natural organic matter (NOM) and hardness may inhibit the SDHPC process due to the hydroxyl radical scavenging effect of NOM (Bandala et al., 2011b), the formation of calcium carbonate film adhering to the internal glass wall of the photoreactor which is in contact with the liquid being treated, and to the presence of calcium carbonate precipitates on catalyst surface (Acevedo et al., 2012). In a very recent work, Byrne et al. (2011) reviewed the available literature of SDHPC enhancement for solar disinfection of water, including an analysis of parameters affecting the process and a comparison with the widely known solar disinfection (SODIS) technology. 13.5 CHALLENGES AND PERSPECTIVES Solar driven water disinfection technologies hold great promise as low cost, effective replacements for conventional water treatments, potentially bringing clean drinking water to a large number of people. However, before widespread adoption is realized, several significant technological hurdles must be overcome. 13.5.1 Photorreactor design UV at ground level includes both direct beam and diffuse radiation at almost similar portions. During cloudy days, this proportion in solar UVA spectrum may change to 60% diffuse and 40% direct. Since direct beam radiation may represent as little as 40% of the total radiation available, non-imaging solar collecting systems capable to Solar driven advanced oxidation processes 407 use global radiation arriving from any direction have a clear advantage, as stated earlier, compared to expensive imaging/concentrating optic-based systems which cannot harvest diffuse radiation. The major advantage with non-imaging collectors is that the collection factor remains constant for all values of sun zenith angle within the acceptance angle limit. Therefore, many different non-imaging geometries have been used in the design of larger-scale solar disinfection systems, including up to small community scale (Bandala and Estrada, 2007). A solar collecting system for use in, for example, rural, isolated communities in developing countries should have many specific attributes: high illuminated volume/total volume ratio; low flow operation to maximize residence time; serve as a UVA dosimetric indicator (considering that both, photocatalytic detox and disinfection are dose dependent); high UVA reflectivity; high (90%) UVA transmission in the receiver; robust to harsh environmental conditions; minimal lifecycle cost; low environmental impact; low maintenance requirements and ease of access to replacement parts; and minimal external power requirement. In addition, the photoreactor design must also provide electron acceptors, typically dissolved oxygen, since the concentration of dissolved oxygen will be rapidly consumed in the initial stages of the reaction as water temperature increases in static batch systems. As said earlier, several technological approaches have been reported that strive to offer the characteristics described previously. However, more research is required to demonstrate satisfactory full scale performance before widespread deployment of this technology for point-of-use water or industrial wastewater treatment. 13.5.2 Suspended vs. immobilized photocatalyst One of the main disadvantages for heterogeneous photocatalytic processes in water treatment is the generation of catalyst slurries. In agreement with previous studies of pilot plant scale systems, suspended TiO2 is more effective than immobilized TiO2, probably due to limitations imposed by mass transfer processes on the latter’s reaction rate (Bandala and Torres, 2008). Immobilized TiO2, however, present some important advantages when compared with suspensions, such as reduced material loss, cost reductions, and the possibility to escape further recovery steps after the water treatment process. Many materials have been tested for the immobilization of TiO2 as well as a wide variety of immobilization methodologies (Gelover et al., 2006). However, the controversy remains over the performance of immobilized TiO2 photocatalyst in comparison with suspended TiO2, since some authors have found no advantages at all for the use of immobilized systems (Sordo et al., 2010) whereas others claims important improvements when using immobilized applications (Gelover et al., 2006; Acevedo et al., 2012). It is probable that this controversy is related to the lack of a proper comparison methodology rather than a real difference between the tested materials. Several chemical techniques for TiO2 film deposition on solid surfaces have been described in the past: chemical vapor deposition (CVD), serigraphy, galvanoplasty, anodization, electrophoresis, electroless deposition, spray pyrolysis, controlled precipitation or chemical deposition, sol-gel chemical deposition, and magnetron sputtering. In the same way, a wide variety of solid matrix materials for TiO2 immobilization are reported. It is reasonable to speculate that the variation in results is at least in part due 408 Solar energy sciences and engineering applications to the range of experimental conditions. Development of a standardized testing protocol must be considered another interesting research challenge to the field deployment of solar driven photocatalytic processes. 13.5.3 Visible light active photocatalyst materials As a result of its large band gap (3.2 eV), TiO2 semiconductors exhibits photocatalytic activity only within UV radiation wavelengths (=400 nm). This specific characteristic limits the photosensitivity to theUVpart of the solar spectrum and is an important technological limitation. Since sunlight consists of only about 5% UV radiation, efficiency enhancements are needed to enhance the viability of SDHPC processes. A recent emerging field of research is the development of photocatalytic materials excitable by visible solar radiation, which account for 45% of the solar spectrum. Several modifications to TiO2have produced visible radiation active materials with improved photosensitivity and quantum yield. Several ways to get this so-called daylight photocatalysis have been reported recently including dye sensitization, coupling TiO2 with other semiconductors possessing favorable band gaps and potentials, surface deposition of metal clusters, and doping the crystal lattice with metals (Fe, Co, Ag) and/or nonmetal foreign atoms (N, C, F, S). According to the literature, one of the more promising approaches to achieve visible light active TiO2 is by doping with nonmetal elements including N and S. After initial reports of visible-light photo-active nitrogen-doped TiO2, many groups have demonstrated that anion-doped TiO2 has extended optical absorbance into the visible region (Asahi et al., 2001). However, the number of publications concerning the photocatalytic activity of these materials for SDPC processes is still limited. In many cases, the UV activity of undoped TiO2 has been reported much greater than the visible light activity of the doped material (Reginfo-Herrera and Pulgarin, 2010). Therefore, photocatalysts developed for SDHOC applications should be tested under simulated solar irradiation or, preferably, under real sun conditions. However, in agreement with these authors, N-doped TiO2 materials did not exhibit enhanced photocatalytic degradation of phenol or the photocatalytic inactivation of E. coli under simulated solar light, as compared to Degussa P25. They suggest that although N, or N-S co-doped TiO2 may show visible light response, the localized states responsible for visible light absorption are not important in the photocatalytic activity. Other studies, however, report that although solar visible radiation displays a lower activity than solar UV radiation it is possible to observe interesting photocatalytic activity for N-doped TiO2 under these conditions and the overall effect of using complete (UV+visible) radiation is higher than that observed for regular TiO2 using only UV solar radiation (Castillo et al., 2011). More research is required to determine if visible light active materials can deliver an increased efficiency of photocatalytic processes under solar radiation. 13.6 CONCLUSIONS Solar driven AOPs were demonstrated as cost-effective emerging methodologies for water decontamination and disinfection with very interesting advantages compared with conventional water and wastewater treatment processes such as higher efficiency, Solar driven advanced oxidation processes 409 lower cost, easy operation and maintenance but mainly the opportunity of using a wide available, cheap and interesting alternative source of energy: the Sun. Several different applications for both, homogeneous and heterogeneous photocatalytic processes, solar driven AOPs have been reported in the past. Some of them are included in this work as a representative example for applications at laboratory, bench and full scale of these technologies in the removal of organic pollutants or pathogenic microorganisms. 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(1996) 2,4-dichlorophenol oxidation kinetics by Fenton’s reagent. Environmental Technology, 17, 1371–1378. Terashima, Y., Ozaki, H., Giri, R.R., Tano, T., Nakatsuji, S., Takami, R. and Taniquechi, S. (2006) Photocatalytic oxidation of low concentration 2,4-dichlorophenoxyacetic acid solutions with new TiO2 fibber catalyst in a continuous flow reactor. Water Science and Technology, 54, 55–63. WHO/UNICEF. (2000) Global water supply and sanitation assessment report, NewYork/Geneva. Chapter 14 Solar energy conversion with thermal cycles Giampaolo Manzolini & Paolo Silva Dipartimentio di Energia, Politecnico di Milano, Milano, Italy 14.1 INTRODUCTION The increasing concentrations of gases such as carbon dioxide (CO2) and methane (CH4) – producing the so-called “greenhouse effect’’ – in the atmosphere are regarded by many members of the scientific community as a consequence of human activities. The use of energy represents the largest source of emissions, accounting for over 90% of anthropogenic greenhouse gases, while electricity production produces about 35% of the total CO2 emissions (Key World Energy, 2012). Renewable energies (i.e. hydropower, biomass, solar, wind) for electricity production are seen as one of the best options for reducing the impact of human activities on the environment. Electricity production from renewable energy is by definition CO2 neutral1, with no resulting impacts on global CO2 emissions and concentration. Among renewable energies, solar energy could play a fundamental role in satisfying energy demand in countries with high solar radiation. In particular, solar thermal power could easily cover the commercial demand for bulk electricity in the range of tens to hundreds of MW. Focusing on power production, photovoltaic and thermal systems are the available technologies. Photovoltaics consist of the direct conversion of solar radiation into electricity by means of photovoltaic effect. In solar thermal power plants solar radiation is first converted into thermal energy through a concentrator, then into electricity through a thermodynamic cycle, as in fossil fuel-based plants. These kinds of plants are usually called Concentrated Solar Power plants (CSP). Whilst photovoltaics seems to be a promising and suitable technology for distributed generation (i.e. small-size plants in the range of 1–100 kW), thermodynamic plants might be an attractive solution for centralized large-scale electricity production in the range of tens to hundreds of MW, with predictable low costs and relatively low land demand. The most significant advantage of solar thermal plants over photovoltaics is the adoption of thermal energy storage (TES) which can decouple the electricity production from the energy source. Moreover, TES increases the operating hours of the plant, with economic benefits. 1While solar, wind and hydro systems do not emit CO2 during the power production process, biomass combustion processes, the emissions from which were previously subtracted from the atmosphere, do emit CO2. The overall balance depends on the non-renewable resources used. 414 Solar energy sciences and engineering applications Depending on the type of concentrator, the CSP can be based on linear receivers (i.e. parabolic trough or Fresnel collectors), or point focus receivers, such as solar towers or parabolic dish systems. Linear concentrators, mainly parabolic trough collectors, are currently the most proven solar thermal electric technology and are becoming the reference technology for commercial applications. A summary of the CSP plants operating globally is given in the Appendix. Today, several companies are active in the field of solar thermal technologies. These include Schott (Schott Solar), Acciona (Acciona Energy), Abengoa (Abengoa Solar), Areva (Areva CSP), Siemens (Siemens Concentrated Solar Power) and Archimede Solar Energy (Archimedes Solar Energy). This chapter introduces the principles of solar thermal energy and discusses the advancements, as well as the potentiality, in power production. The last section makes a comparison between the actual costs of CSP versus competitive technologies. 14.2 SOLAR CONCENTRATION CONCEPT IN THERMAL SYSTEMS The sun has been worshipped as a life-giver to our planet since ancient times. Every hour the amount of energy that the Earth intercepts is about 500 million TWh (values at the top of the atmosphere) which corresponds to the total energy consumption of the entire world over a thousand years. However, this energy is diluted in space: the total integrated spectral irradiance has been made to conform to the value of the solar constant accepted by the space community, i.e. 1366.1W/m2. The atmosphere along with atmospheric gases such as ozone (O3), oxygen (O2), water (H2O) and carbon dioxide (CO2) strongly absorb light for some wavelengths. Other causes of energy reduction occur once the irradiance has passed through the atmosphere include scattering by aerosols and dust particles. By the time light reaches the earth, total energy density is about 1000W/m2 for Air Mass 1.5. As anticipated in the introductory paragraph, of the two technologies which convert solar radiation into electricity, this chapter will focus on Concentrated Solar Power plants. The power production process in CSP is based on two steps: the conversion of solar energy into heat and then into power via a conventional thermodynamic cycle. The overall conversion efficiency can be seen as the product of each step: .ideal = .th · .carn (14.2.1) where .th is the efficiency conversion of solar radiation in thermal power and .carn stands for power cycle efficiency. In order to fully understand the solar concentration concept and the advantages of concentrating solar radiation, the two terms will now be discussed. Starting with power cycle efficiency (.carn), the second law of thermodynamics states that the ideal conversion from thermal to mechanical power depends only on the temperature of the heat source. The real conversion cycle will never achieve the efficiency of the ideal cycle. However, the higher the efficiency of the thermodynamic limit, the higher can be the real cycle efficiency. Using Carnot’s cycle, which Solar energy conversion with thermal cycles 415 can be representative of a Rankine cycle2, the conversion efficiency (.carn) is as follows: .Carn = 1 - Tamb Tmax (14.2.2) where Tamb is the ambient temperature and Tmax is the maximum temperature in the cycle. From this formula, it can be deduced that the higher the maximum temperature of the cycle, the higher the ideal conversion efficiency from thermal power to electricity. For example, moving from a maximum temperature of 600K to 1000 K, the Carnot efficiency increases from 50% to 70%. Moving on to the second term of ideal conversion efficiency, .th represents the ratio between the net heat absorbed by the collector, .Q ABS, and the heat concentrated on the collector itself .Q CONCENTRATED: .th = ·Q ABS ·Q CONCENTRATED (14.2.3) In a CSP plant, solar energy is concentrated on an absorber by a collecting structure; the entire system is designed to capture most of the solar energy and transfer it to a fluid inside the absorber. In order to produce useful heat for the power cycle, the absorber must be at a high temperature and consequently it will emit energy to the environment as infrared emissions3. Therefore the net heat absorbed by the collector ( .Q ABS) can be written as the difference between the energy received from the collecting system and the energy emitted to the environment, as expressed by the following equation: ·Q ABS = ·Q CONCENTRATED - ·Q EMITTED = a · C · G - s · e · (T4 abs - T4 amb) (14.2.4) where a is the hemispherical absorptivity of the absorber, C is the geometrical concentration ratio of the collector, G is the direct normal irradiance [W/m2], epsilon is the hemispherical emissivity of the absorber, sigma is the Stefan-Boltzmann constant [5.67e–08W/m2K4], Tabs is the average temperature of the absorber and Tamb is the sky temperature or the temperature viewed by the absorber [K]. With regard to the non-dimensional coefficient, the thermal efficiency of a collector (.th) can be expressed as the ratio between the net heat absorbed by the collector and the heat concentrated on the collector itself (all these formulae assume a unitary surface): .th = ·Q ABS ·Q CONCENTRATED = ·Q ABS C · G - ·Q EMITTED C · G = a - s · e · (T4 abs - T4 amb) CG (14.2.5) 2See section 14.5 for a detailed discussion of a Rankine cycle. 3Any body at a temperature above 0 K emits radiation energy expressed by the Stefan-Boltzmann Law (Incropera & DeWitt, 2007). 416 Solar energy sciences and engineering applications Figure 14.2.1 Ideal conversion efficiency from solar energy to mechanical work assuming different concentration ratios (C) and absorber temperatures. From this equation it can be noted that the thermal efficiency and, consequently, the heat recovered from the collecting system increases with the concentration ratio; .Q EMITTED is constant since it depends only on absorber temperature, but, being divided by the concentration factor, its relative contribution decreases. In order to better explain these concepts, the resulting ideal conversion efficiency (see Equation 14.2.1) as a function of maximum temperature and concentration ratio is summarized in Figure 14.2.1 (for this example a and e are assumed equal to 0.94 and G to 800W/m2). In addition to the thermodynamic advantages previously discussed and shown in Figure 14.2.1, the adoption of a concentration system substitutes expensive components working at high temperature, like the absorber, with cheaper mirrors at ambient temperature. Moreover, since the performance of the absorber is fundamental and the amount required is reduced, research activity can focus on its improvement, pushing its performances to higher values. An example of the material absorptivity/emissivity impact on system efficiency is shown in Figure 14.2.2. For simplicity a constant absorptivity is assumed, while three different values of emissivity are considered (0.94, 0.5 and 0.1). Reducing the emissivity while keeping a high absorptivity reproduces the properties of an advanced material with high performance values, though probably at higher cost. However, a very high concentration ratio can significantly reduce the economic impact because of the overall limited influence on total plant costs. Moreover it can be noted that low emissivity is fundamental for a low-medium concentration ratio (in the range of 100) and high absorber temperature (>800 K). Solar energy conversion with thermal cycles 417 Figure 14.2.2 Ideal conversion efficiency from solar energy to mechanical work for different concentration ratios (C), absorber temperatures and at different epsilon. As a final remark on the concentration concept, we should underline that concentrating technologies require tracking systems in order to maximize collected solar energy; however, tracking systems increase plant complexity as well as costs, with a potential negative impact on the overall availability of the plant. However, as will be clearly described in the following sections, the advantages of solar concentration overcome the few above-mentioned drawbacks. 14.3 CONCENTRATING SOLAR TECHNOLOGIES The previous section described the basics of solar power concentration. This section will deal with existing solar collectors used with the two different types of concentration. Solar energy can be concentrated on a line, called linear focus collectors, or on a point, called point focus collectors. Another characteristic which distinguishes the two concentrations is that linear collectors are single-axis tracking while point focus collectors are two-axis tracking. Solar tracking is fundamental in maximizing the capture of solar energy. Without it, solar radiation would be collected on the absorber only when the beam incidence angle is equal to the design conditions of the collector, i.e. for just a few minutes per day. Another aspect of solar collectors is the system of concentration, which can be continuous or discrete. Continuous concentrators are mirrors in the form of a parabola 418 Solar energy sciences and engineering applications which reflect the solar energy at the focus of the parabola; the parabolic shape is made by a rigid metallic structure. Discrete concentration is achieved by several mirrors which move independently in order to collect the solar energy at the same focus point. Most of the existing solar thermal plants (2050MWvs. 2120MWdata from 2012 (“NREL Database,’’ n.d.)) are based on a linear focus technology, but the point focus is seen as the most attractive because of its potential for cost reduction. Before discussing the technology in detail, we need to define solar multiple (SM) and introduce the general approach to CSP design. SM is the ratio between the power delivered by the solar field at design conditions ( .Q SF,design) and the nominal thermal input of the power cycle .Q PB,design. SM = ·Q SF,design ·Q PB,design (14.3.1) A solar multiple equal to one corresponds to a solar field aperture area which delivers at nominal conditions the design thermal energy input of the power cycle. The SM coefficient is one of the optimization parameters of a CSP plant. It is usually above one in order to: (i) mitigate solar transient and fluctuation in irradiance; and, if available, (ii) store part of the collected thermal energy in a Thermal Energy Storage. The advantage of a SM above one, even without thermal storage, is to increase plant operating hours. One drawback, during high radiation days, is that part of the solar field might be defocused in order to respect the thermal input to the power block, thus wasting potential solar radiation. Existing plants are usually designed with SM higher than one and with a TES for the above-mentioned advantages. Moving to the energy conversion process, all concentrating solar technologies convert solar energy into thermal energy supplied to a fluid (i.e. the fluid increases its temperature in the solar field). Thus, solar field efficiency can be introduced as the ratio between the thermal power absorbed by the fluid and the direct normal solar radiation available on the collector area. Solar field efficiency (.SF) is defined as: .SF = ·Q FLUID ·Q SUN = ·Q FLUID G · A (14.3.2) where ·Q FLUID [kW] is the thermal power transferred to the fluid and ·Q SUN [kW] is the solar energy coming from the sun. .Q SUN can be expressed as a product of the direct beam radiation, G (W/m2) and the collector surface area A [m2]. Solar field efficiency can be further divided into two different efficiencies: optical and thermal. In order to better explain this concept, a schematic of a concentrating system with main conversion steps4 is represented in Figure 14.3.1. 4A more rigorous representation would also consider other phenomena such as absorbance of the receiver. Solar energy conversion with thermal cycles 419 Figure 14.3.1 Schematic of conversions and losses in a generic concentrating system. Optical efficiency accounts for the difference between solar radiation available at the reflector and the amount of radiation effectively transferred to the receiver, while thermal efficiency accounts for the thermal losses to the ambient mainly as radiative losses. The resulting solar field efficiency can be written as: .SF = ·Q FLUID G · A = ·Q FLUID ·Q REC × ·Q REC G · A = .opt × .th (14.3.3) where ·Q REC is the heat concentrated on the receiver, .opt is the optical efficiency and .th is the thermal efficiency. Solar field efficiency strongly depends on the operating conditions: ambient temperature, wind speed, sun position, solar radiation and material properties, etc. In particular, optical efficiency varies significantly with the position of the sun, while thermal efficiency is dependent on ambient temperature and solar radiation. Optical efficiency is usually defined at the design conditions (nominal optical efficiency) and then corrected during different operating conditions to take account of the variation in material properties according to the incidence angle of radiation. The overall optical efficiency depends on several contributions, which are usually split to outline their differences: nominal optical efficiency, geometrical losses and the K(.) as expressed in equation 3.4. .opt = .opt-peak · .geom · K(.) (14.3.4) where .opt-peak is the nominal optical efficiency, .geom is the geometrical efficiency, which includes shadowing, tail-end losses and blocking, and K(.) is the correction of the efficiency for the incidence angle. K(.) takes into account the variation in 420 Solar energy sciences and engineering applications material optical properties with the incidence angle, together with the cosine effect. The cosine effect consists of the reduction of radiation by the cosine of the angle between solar radiation and a surface normal. The cosine effect occurs for both single-axis and double-axis tracking systems being more important for the single-axis technology (about 60% on a yearly base) than in the double-axis (about 80%). This is because the former can reduce to zero only the azimuth angle. Variation in material properties is usually expressed in the incidence angle modifier (IAM), which includes t and a variation with .. The best way to predict optical efficiency at different solar positions is by means of laboratory experiments, owing to the difficulty of splitting the contribution of t and a. The contribution of the K(.) is usually predominant over other terms, making the average yearly optical efficiency of single axis-tracking systems much lower than double-axis tracking ones. Nominal optical efficiency is defined at peak conditions with an incidence angle (.) equal to zero. It can be seen as representative of the property of the collector materials (i.e. reflector, absorber, etc.) under perpendicular solar radiation and is defined as: .opt-peak = . · . · t · a|.=0 (14.3.5) where . is the mirror reflectivity, . the intercept factor, t the vacuum glass transmissivity and a the receiver absorptivity. After this short general introduction on concentration systems, the description will be divided between adopted tracking systems. 14.3.1 Linear focus Linear focus concentration is based on parabolically curved mirrors or segmented mirrors, according to the Fresnel principle, which concentrates solar energy onto a receiver pipe. This configuration allows single-axis tracking. Acollector field comprises many collectors (usually named trough) in parallel rows aligned with the tracking axis orientation. The fluid is heated in the receiver and then sent to the power block to convert the thermal energy into electricity. The distribution of the fluid along the field is made through pipes called headers. The header that brings the fluid from the power block to the trough is named “cold header’’, since it is at lower temperature, while the header that collects the fluid is named “hot header’’ because of the higher temperature. The particularity of these pipes is in the variation in diameter along with the variation in fluid flow: the target is to keep an almost constant fluid velocity. The connection between the header and the power cycle is made by connecting pipes which transfer the fluid from the solar field to the power cycle. An example of a solar field layout is given in Figure 14.3.2. In particular, the two typical configurations are shown: on the left side the “H’’ configuration is reported. On the same side of the power block there are two different sections of the solar field, with two cold and two hot headers. This configuration is adopted in Andasol project (Herrmann and Geyer, 2002), SEGS VIII and IX. In the “I’’ configuration (used in SEGS VI, (Patnode, 2006)), reported on the right side, there is just one section. In both the “H’’ and “I’’ configurations one loop is composed of two solar collector rows, connected by Solar energy conversion with thermal cycles 421 Figure 14.3.2 Schematic of two possible configurations of solar fields (the central line must be seen as symmetric axis). a “U shape’’ pipe: the cold and hot headers can be on the same side leading to the socalled “central-feed configuration’’. This layout minimizes the piping and allows direct access to each collector row without buried pipes. One drawback is that this configuration is not balanced from a pressure drop point of view; therefore pressure balancing valves are required at each row, adding significant pressure drops. An alternative layout can be represented by the “direct return’’ and “reverse-return’’ configurations (Kreith and Goswami, 2007), where the cold and hot headers are on different sides of the rows. In particular, the inverse return configuration balances the pressure drop but requires longer connecting pipes. To summarize, the typical layout configuration is the central feed since it reduces piping length and thermal losses, keeping a good access to the collectors. The tracking orientation of the trough can be either in a N-S direction, an E-W direction or anywhere in between; the direction which guarantees the highest collection of solar energy is N-S. As an example, the electricity produced in a year from a parabolic trough-based solar plant located in the United States at a latitude of 34. is shown in Figure 14.3.3. The only difference between the two cases is in the axis orientation. 422 Solar energy sciences and engineering applications Figure 14.3.3 Monthly electricity output for N-S and E-W orientations. North-south tracking produces 15% more electricity yearly than an E-W orientation: production is higher during the summer, while the situation changes in winter. The qualitative result can be extended at different latitudes, with changes from quantitative standpoint: at lower latitudes the difference between tracking orientation increases, while at higher latitudes it reduces. Since in the two cases (N-S tracking and E-W tracking) the ambient conditions, power cycle configuration and performance, and the collecting system are the same, the only difference can be due to optical efficiency. As discussed before, optical efficiency depends on the solar incidence angle: N-S tracking follows the solar azimuth angle, hence reducing its impact, while solar altitude remains. Consequently, in winter at moderate latitude, the solar incidence angle can be significant also at midday, where the solar altitude is lower than 40. (during the winter solstice it is equal to 90. – latitude – ecliptic (23.27)). E-W tracking does the opposite, following solar altitude, with advantages during the middle hours of the day in winter. This effect explains the variation in the electricity production of different tracking axes as a function of the season. 14.3.2 Parabolic trough Commercial applications of solar plants based on parabolic trough (PT) technology began in the mid-1980s with the construction of Solar Energy Generating Systems (usually known by the acronym SEGS) by LUZ in California. Nine different SEGS plant were built, named SEGS I to SEGS IX, for a total installed capacity of 354 MW (Cohen and Kearney, 1999). Afterwards, this type of technology was forgotten until Solar energy conversion with thermal cycles 423 Figure 14.3.4 Schematic of a parabolic trough. Main components are outlined. 1998 when the Eurotrough project started. This project was cost-shared by a group of European companies with the aim of developing an innovative parabolic trough collector with high performances and reduced costs. Then, after 2000, the construction of solar thermal plants started again with Nevada Solar One and the installation of “Plataforma Solar de Almeria’’ in Spain. In fact, the “Plataforma Solar de Almeria’’ is not a power plant but a test facility where different solar technologies can be tested and compared. Nowadays, several companies, such as Siemens (Siemens Concentrated Solar Power), Archimede Solar Energy (Archimede Solar Energy), Schott (Schott Solar), Abengoa (Abengoa Solar) to mention just a few, develop parabolic trough systems or manufacture components. Between 2009 and 2011 total installed capacity increased significantly, reaching almost 2.1GW. Most of this capacity was installed in only two nations: the United States and Spain. An example of recently built solar power plants is the Andasol project which has three plants of 50MW(The parabolic trough power plants Andasol 1 to 3, 2008). The basic component of the solar field is the Solar Collector Assembly (SCA), a schematic of which is shown in Figure 14.3.4. Each SCA consists of trough-shaped mirrors, also named parabolic trough reflectors, supported by a structure and the tracking systems. The parabolic trough collectors have a characteristic cylindrical shape with a parabolic curvature. Solar radiation is reflected and concentrated on an absorber tube/pipe (also called the Heat Collection Element) which transfers the thermal power to a fluid flowing inside. The parabolic trough collectors are designed to achieve high performance at low cost with high 424 Solar energy sciences and engineering applications reliability and durability. These general targets can be translated into the following sub-tasks: • High optical and tracking accuracy; • Low heat losses; • Manufacturing simplicity; • Reduced number of parts and field erection costs; • Increased aperture area. Hence, R&D activities aim at fulfilling these targets in order to make the parabolic trough technology competitive to commercial power generation technologies from an economic point of view. At present, the solar collector assemblies have an aperture area ranging from 5.77m for Siemens (Siemens Sunfield LP) to 6m for Skyfuel (Skytrough Brochure)5. The length for elements is up to 12m while the total trough length, which is the union of elements, reaches a total length of about 100–120m (“Skytrough brochure,’’ n.d.). The absorber tube diameter is usually 70mm (SCHOTT PTR® 70 Brochure; Archimede Solar Energy), making the concentration ratio (CR) of parabolic trough collectors in the range of 80. The concentration ratio is defined as follows6: CR = aperture width adsorber diameter = W Dabs (14.3.6) After this short and general introduction, a detailed description of parabolic trough technology is now presented, in which the three main components – reflectors, heat collection element and structure – are discussed. 14.3.3 Reflectors Reflectors must reflect and concentrate solar direct beam radiation onto the linear receiver located at the focus of the parabola, called the heat collection element (HCE). Aside from pure performances whilst new and in clean conditions, the reflectors must maintain constant reflectivity over the years. Since solar plants are usually installed in places with high solar radiation, often in desert locations, the ambient is abrasive because of sand and dust transported by the wind. Receivers must be resistant to these conditions, making the endurance test really challenging. For this reason, protective coatings are deposited on the reflector surface to keep high reflectivity and reduce wear effects. In addition, the reflectors must guarantee high reflectivity at all solar incidence angles: reflectivity t is usually defined at zero incidence angle where it has the maximum values. Usually, it is known that reflectivity varies as a function of solar incidence angle. For higher incidence angles, mirror reflectivity lowers, affecting optical and overall plant efficiency. Variation in performance with the incidence angle is usually 5It should be noted that Skyfuel is testing a solar collector with an aperture area of about 8m. 6There is discussion on how to define the concentration ratio. Some authors prefer to adopt the receiver diameter instead of the circumference as in this work. Solar energy conversion with thermal cycles 425 Figure 14.3.5 Heat collection element (Flabeg Solar; Price et al., 2002). defined by the incidence angle modifier (IAM). However, the IAM not only takes into account reflectivity variation, but also glass and absorber tube property variations with the angle. Examples of IAM trends will be presented later, in the parabolic trough performance section. Trough-shaped mirrors can be made by a glass layer with low iron concentration and a reflective silvered film. For example, glass-based technology is developed by Flabeg (Flabeg). Another option is multiple layers of polymer film with a layer of pure silver to provide for high specular reflectance (Skytrough Brochure). Other companies which are active in reflector development are Alanod (Alanod), 3M (3M). Alanod products, for example, are based on polished aluminium which has a slightly lower reflectivity but a higher resistance to wear and lower weight, with advantages for structure and plant reliability. Typical reflectivity values are 94.4% for glass-based reflectors and slightly lower for polymer-based ones (93%) and aluminium (92%). The higher investment cost and weight of glass-based reflectors is nowadays balanced from an economic point of view by a higher efficiency compared to polymer and aluminium-based reflectors. 14.3.4 Heat collection element The absorber tube, also called heat collection element (HCE), consists of a steel tube with a selective coating layer. The coating is added in order to increase the absorption properties within the solar spectrum while keeping emissivity low. The development of new coatings has increased the performance of absorber tubes thanks to higher absorptivity and lower emissivity. An example of the improvement in this technology is given in Figure 14.3.6 which shows the efficiencies of collectors with different selective coatings. The coatings adopted for use in SEGS plants were LUZ Black Chrome and Luz Cermet with efficiency in the range of 60%. R&D activity has increased absorber performance, reaching an overall collector efficiency of more 426 Solar energy sciences and engineering applications Figure 14.3.6 Commercial collectors efficiency as function of fluid temperature (DNI=800W/m2) (Manzolini et al., 2011a). In this case, Schott PTR 70 performances do not correspond to the latest version. Table 14.3.1 Features of commercial absorbers (Price et al., 2002; Forristall, 2003). Luz Black Luz Solel Solel Schott Archimede Solar Chrome Cermet UVAC UVAC 2008 PTR 70* Energy Absorption 0.94 0.92 0.955 0.97 0.955 0.945 Transmittance 0.935 0.935 0.965 0.97 0.965 0.97 Absorber’s Length [m] 4.06 4.06 4.06 4.06 4.06 4.06 Din absorber [mm] 64 64 64 64 64 64 Dout absorber [mm] 70 70 70 70 70 70 Din glass envelope [mm] 109 109 109 109 119 109 Dout glass envelope [mm] 115 115 115 115 125 115 than 70% at temperatures below 400.C. Moreover, recent coatings can withstand tube temperatures of about 550.C, with significant thermodynamic advantages. The main properties of HCE are summarized in Table 14.3.1. Because the absorber tube works at high temperature, a glass tube surrounding the absorber tube is also adopted. A schematic of a heat collection element with all the fundamental components is shown in Figure 14.3.4. The glass tube, usually made by Pyrex®, makes a vacuum annulus between the glass and the tube which prevents coating oxidation and reduces heat loss; pressure within the annulus is typically about 1×10-2 Pa (Price et al., 2002). The glass tube has an antireflective coating on both surfaces to maximize solar transmittance while Solar energy conversion with thermal cycles 427 limiting reflective losses. Along with the antireflective coating, significant developments have been made in improving absorber performance. Most of heat collection elements are applied to stand-alone concentrated solar power plants with synthetic oil as the heat transfer fluid. The synthetic oil decomposes at high temperature, producing hydrogen which permeates across the tubes and ends up in the vacuum annulus. The presence of hydrogen in the vacuum annulus is detrimental for the thermal performance of the collector. For this reason the vacuum is usually filled with getters which absorb hydrogen and other gases that eventually permeate during operation. The getters are barium-based and, as additional components required for the HCE, they affect the overall costs. The adoption of vacuum glass is beneficial for HCE efficiency, but this requires a dedicated glass-to-metal sealing. In addition, the adoption of bellows is necessary to equalize the thermal expansion of metal and glass in order to guarantee the vacuum conditions in the annulus. A drawback of the bellows is the shadow thrown onto the absorber tubes, with consequent efficiency penalties. However, recently SCHOTT PTR 70® have reduced the impact of bellows, leading to an active area above 96.7% (SCHOTT PTR® 70 Brochure). Active length is defined as the active aperture area of the receiver on the total receiver area. Commercial HCE efficiency as a function of heat transfer fluid (HTF) temperature is shown in Figure 14.3.6. Finally, in order to limit bending of the tube, HCE length is usually equal to 4m. For this reason, several HCEs are placed in series to make a trough. 14.3.5 Structure The structure, usually of metal, serves to hold the heat collection element and the reflectors at the correct position. The parabolic shape of the reflector is usually formed by the structure. The structure has to fulfil the following requirements: efficient use of material, ease of transportation onto the site, easy to assemble, and able to withstand atmospheric conditions for at least 30 years. Moreover, it must have a high torsional and bending stiffness under wind loads (the structure must be designed to work under wind conditions, typical of desert locations). For example, Andasol plant can continue operating in winds up to 13.6 m/s (about 50 km/h); above this wind speed, the collectors are put in a wind-protected position. It must be remembered that the aperture length of the parabola is in the range of 6m and wind drag and lift can be really significant. For these reasons, research activity on structure development is carried out using computational fluid dynamics (CFD) analysis and wind tunnel testing to determine the required torsional and bending stiffness in order to achieve a desired interception factor at defined conditions. An example of CFD analysis on parabolic troughs performed at Politecnico di Milano is shown in Figure 14.3.7. Research activity has led to different types of structure becoming available on the market, each based on a different concept. Flagsol’s Eurotrough and Ultimate Trough are based on a torque box design and made from galvanized steel (Graf and Nava, 2011). The torque box guarantees savings on materials, uses thick and reliable mirrors, and reduces wind loads compared to other configurations. SENER proposes a torque tube plus stamped steel cantilever, with the mirrors supported by arms. The design of Acciona solar power is based on recycled aluminium or steel struts and geo 428 Solar energy sciences and engineering applications Figure 14.3.7 Wind velocity field around five different parabolic troughs. hubs. The advantages of this configuration are higher rigidity via interlinking and no cutting or welding during construction. Lastly, the ENEA design is based on a torque tube with precise reflector supported by arms. The structure has to be fixed to the ground with foundations which add significantly to the overall cost of solar fields. To reduce these associated costs, research activity is also being done on advanced foundations, an example of which is steel-reinforced, drilled pier foundations. 14.3.6 Parabolic trough performance This section tries to give an overview of parabolic trough performances during a typical year. As mentioned above, the parabolic trough transfers solar radiation to a fluid flowing in the HCE: this conversion is affected by optical and thermal losses. Optical losses depend mainly on K(.) contribution, and secondarily on end-losses and shadowing. In order to give an idea of the impact of K(.) on overall performance, the optical efficiency ratio (OR) is introduced. This is calculated as the ratio between offdesign optical efficiency and nominal optical efficiency. The hourly OR calculated for a parabolic trough-based solar plant located in the United States at a latitude of 34. with N-S axis tracking is shown in Figure 14.3.8. It can be noted that during the winter, even at midday, optical efficiency is about half the nominal optical efficiency. This is due to the above-mentioned high incidence angle consequence of low solar altitude. For example, the cosine effect, which is only one part of the optical penalty, is equal to 0.6 (during winter, solar altitude at midday is about 40.–50.). On the contrary, during summer, optical efficiency at noon is close to nominal conditions because the incidence angle is about 10.. Solar energy conversion with thermal cycles 429 Figure 14.3.8 Annual maps of optical efficiency ratio (OR) for PT technology (Giostri et al., 2013). For the United States location at latitude north 34., the yearly optical efficiency is 53% (meaning that optical losses account for almost half of the solar radiation that is lost), while the nominal optical efficiency was about 75%. In addition to optical losses, parabolic troughs are affected by thermal losses. As a general consideration, the solar field is operated such that inlet and outlet temperatures are constant; hence, absolute heat losses are constant. Focusing on thermal efficiency, which was defined in Equation 14.3.3 as the heat transferred to the fluid divided by the solar radiation impinging on the receiver, it can be seen also as: .th = ·Q FLUID ·Q REC = ·Q REC - ·Q HT LOS ·Q REC = 1 - ·Q HT LOS ·Q REC = 1 - ·Q HT LOS G · A · .opt (14.3.7) where QHT LOS is the heat losses [W], G is the solar normal radiation [W/m2], A is the collector area [m2] and .opt is the optical efficiency as defined in Equation 14.3.5. Keeping in mind that absolute heat losses are constant, depending only on absorber size and temperature, it can be noted that thermal efficiency drops when G·A· .opt is lower than nominal conditions. Hence, during spring, fall and mostly in winter, thermal efficiency is significantly lower than in summer. Thermal losses depend also on ambient temperature since they are proportional to the temperature difference between the receiver and the ambient (Equation 14.2.3). However, this is a second order effect, because the temperature variation in typical solar plant sites is about 30–40.C, while the average temperature difference between the receiver and the ambient is in the range of 300.C. 430 Solar energy sciences and engineering applications Figure 14.3.9 Linear Fresnel Reflector concept. The yearly thermal efficiency of commercial collectors is in the range of 90% (10% of radiation concentrated on the receiver is lost) while, at nominal conditions, thermal efficiency is about 95%. This section has briefly described parabolic trough technology and has given an idea of the expected efficiency of this kind of technology. Taking into account only the two main losses (optical and thermal) related to the solar field, yearly efficiency is below 50%. Piping losses and consumption by fluid recirculating pumps further penalize efficiency, even if they account for less than 6%. 14.3.7 Linear Fresnel The second type of linear focus technology is the linear Fresnel reflector (LFR). This concept is based on several mirrors which can be moved independently and which collect radiation on the HCE: the parabolic shape is substituted by several ground-based mirrors which have different curvatures and which move independently. A schematic of the LFR technology is shown in Figure 14.3.9. Compared to the parabolic trough, this is a rather new technology. Actually, there are only two commercial manufacturers for LFR: Novatech Biosolar (Novatech Biosol, 2012) and, more recently, Areva (Areva Solar, 2012). However, several companies (e.g. Skyfuel) and research centres (e.g. NREL and Fraunhofer) are investigating Solar energy conversion with thermal cycles 431 the application of LFR technology since it is considered promising, with potential economic advantages for the following reasons: (i) ground-based mirrors allow the adoption of lighter structures thanks also to reduced wind drag effect; (ii) land area is minimized due to reduced shading between collector rows; (iii) the receiver is fixed and tracking energy consumption is decreased; (iv) the absence of ball joints lowers pumping losses; and (v) ground-based mirrors are easier to clean. Moreover, the concentration ratio (CR) of LFR can be higher than with parabolic troughs since it is not limited by parabola aperture width. The adoption of more mirrors would not change the wind drag effect, hence the same structure can be kept. The CR for LFR is defined in Equation 14.3.87. CR = aperture width adsorber diameter = n ·W Dabs (14.3.8) whereDabs [m] is the absorber diameter, n is the number of primary mirrors andW [m] is the aperture of each mirror. The aperture width in this case is the aperture width of each mirror times the number of mirrors. Commercial LFRs have a concentration ratio of about 160 (which is about twice that of parabolic trough systems). LFRs can have a secondary reflector which collects solar radiation from the primary mirrors to the absorber tube. The concentration ratio definition for this case is given in Equation 14.3.8; the only difference is that the solar radiation can be subjected to multiple reflections. The HCE can be the same as for parabolic trough, while the glass tube can also be absent where there is a secondary receiver. The first application of LFR was in direct steam generation plants (DSG, see Section 14.5 for a detailed discussion of this technology) with saturated steam. In order to increase the conversion efficiency of collected heat in the power section, the production of superheated steam in the solar field was recently investigated (Novatech biosol, 2012), as was the adoption of molten salts as HTF (Areva CSP). At the time this chapter was written (end of 2012), there are just a few solar plants in operation based on LFR technology. The most important is Puerto Errado 2, built by Novatec Solar with a 30MWelectric power output (Puerto Errado 2). In this plant, saturated steam at 55 bar and 270.C is produced in the solar field. Other operating plants, though with significantly lower power output, are the Kimberlina solar thermal power plant (Kimberlina Solar Plant) and the Liddel plant by Areva, formerly Ausra, (Liddel Solar plant). In terms of size, a commercial Fresnel linear collector has a width of about 16.6m and a height of 7–8 m. In general, the width affects the concentration ratio, while the height affects optical efficiency. This part will be discussed in detail later. The next section describes the main components of Fresnel collectors. The LFR structure is not treated in detail for the sake of brevity, and because Fresnel mirrors are ground-based with consequent less complexity than parabolic trough reflectors. 7As for parabolic troughs, there is a debate on how to define the concentration ratio. Some authors prefer to adopt the receiver diameter instead of the circumference, as in this work. 432 Solar energy sciences and engineering applications Figure 14.3.10 Schematic of heat collection element adopted in LFR. 14.3.8 Heat collection element The heat collection element transfers concentrated solar energy to the fluid. Compared to trough technology, there are several heat collection element concepts available, all of which can be merged into three different categories: – Single absorber tube; – Secondary reflector/concentrator; – Cavity receiver. A schematic of each category is shown in Figure 14.3.10. The single absorber tube concept is similar to that of parabolic trough. Solar radiation is concentrated by the primary mirrors on the heat collection element, which has a glass tube to limit thermal losses. Compared to PT, the HCE is much simpler since it does not move with the structure; connections and differential thermal expansions are much simpler. In this case, the size of each mirror is similar to the heat collection element. Mirrors are usually flat or have a small curvature. The second category is based on the adoption of a secondary reflector or concentrator which further concentrates solar radiation coming from the primary mirrors on the absorber tube. Considering that the mirror’s width is about the same as the secondary reflector’s (i.e. larger than HCE), this configuration can increase the concentration ratio of the single absorber tube configuration. Novatec (Novatec Biosol, 2012) and ENEA concepts are based on the secondary receiver (Grena & Tarquini, 2011). Besides the higher concentration ratio, the advantages of the secondary reflector concepts are lower thermal losses: the upper part of the absorber tube does not see the sky temperature, but the temperature of the reflectors which is higher. Moreover, the secondary reflector can be insulated and a glass plate can be used to close the secondary reflector cavity, reducing convective heat loss. A disadvantage of this configuration is the additional reflection of solar radiation, with optical penalties. Optimization of the secondary reflector shape has been rigorously investigated and optimized in the literature, since the optical analysis has much higher degrees of freedom than the single absorber tube. A picture showing the optical and thermal Solar energy conversion with thermal cycles 433 Figure 14.3.11 Optical assessment (left side), thermal analysis (center and right) for the Novatech secondary reflector configuration (Binotti et al., 2011). assessment for Novatec’s secondary receiver is shown in Figure 14.3.11. Similar assessments were performed by Barale et al. (2010), Veynandt et al. (2010) and Morin & Dersch (2009). The third category involves a cavity receiver, which places this concept in between the single absorber and the secondary concentrator. The configuration is based on multiple absorber tubes being placed in a cavity, which provides thermal insulation, thus reducing heat loss. In general, the single absorber tube has higher heat losses but is much simpler and cheaper than the secondary reflector and cavity concepts. Adoption of a cavity or secondary reflector system increases complexity, but also cost. Moreover, both cavity receiver and secondary reflector systems shadow part of the primary reflector, reducing optical efficiencies. The absorber tubes in LFR must have the same physical properties as PT applications. High absorptivity and low emissivity are required in order to reduce as much as possible the heat losses. For this reason the same technology adopted in parabolic trough systems can be used in Fresnel (i.e. Schott PTR 70® (SCHOTT PTR® 70 Brochure). 14.3.9 Reflectors Reflectors in LFR are also called primary to distinguish them from the secondary reflector. Their function is to reflect and concentrate solar direct beam radiation onto the HCE. Compared to PT, where just one parabolic reflector is assumed, LFR is based on several reflectors, which can be flat or have a very small curvature. The reflector width is generally close to the absorber diameter for the single tube configuration; the width of secondary reflectors is equal to the aperture width of the secondary receiver. Reflectors must have a high reflectivity when new and operating in clean conditions, as well as over their entire lifetime. Since the reflectors are ground-based, i.e. below 1–2m altitude, wind speed over the system is lower than in parabolic trough installations, thus reducing wear, especially by sand in desert locations. Since the function of mirrors is the same in both LFR and PT applications, their reflectors share the same manufacturers. Therefore mirrors can be made from: (i) a glass layer with low iron concentration and a reflective silvered film (Flabeg); (ii) 434 Solar energy sciences and engineering applications multiple layers of polymer film with a layer of pure silver to provide for high specular reflectance (Skytrough Brochure); and (iii) polished aluminium (Alanod). 14.3.10 Linear Fresnel performance This section describes the performance of a solar field using LFR technology and compares it to PT systems. As before, performance will be described from both optical and thermal perspectives. With regard to thermal analysis, it is much more difficult to provide a general indication since LFR technology involves more than one receiver design. Moreover, the receivers are strongly affected by the concentration ratio as well as by the absorber tube operating temperature, which also varies with manufacturer. As a general consideration, Fresnel collectors usually have a higher concentration ratio than PT, hence the potential for thermal loss reduction (see Equation 14.2.3). As regards receiver configuration, the single absorber tube case is similar to PT technology and will have the highest thermal losses due to the radiative contribution. The secondary receiver and cavity configuration limit radiative losses thanks to the insulation layer. Since further general considerations are not possible, thermal efficiency of the Novatech collector will be presented. Experimental results (Novatech Biosol. Technical data – NOVA 1) show that heat losses for the Fresnel collector at design conditions are in the range of 15–30W/m2, with a thermal efficiency of 95% (whereas parabolic trough efficiency was in the range of 90%). However, these losses were calculated at lower temperature than the corresponding PT case. Moving to optical efficiency, this is defined as the collected radiation divided by the DNI multiplied by the total mirror surface8 (see Equation 14.3.9). .SF = ·Q REC G · Atotal = ·Q REC G · n · Amir (14.3.9) where the total reflecting area (Atotal) is equal to the number of mirrors (n) times the single mirror reflecting area (Amir). Assuming this efficiency (in the literature another definition of optical efficiency is proposed), LFR has a lower optical efficiency than PT even at design conditions or where the incidence angle is equal to 0.. A schematic explaining the above-mentioned concept is shown in Figure 14.3.12. This is because even with solar incidence angle equal to 0., the primary mirror must be inclined to centre the solar radiation on the absorber tube (the angle is equal to . which corresponds to the angle between the solar and the reflected radiation). Hence, optical efficiency is penalized because of the cosine effect on the mirror’s area (A· cos(.i)). This effect is more significant for mirrors further from the receiver. This effect was not present in PT because the parabolic shape makes the mirrors parallel to the solar radiation at any given point. In order to increase the optical efficiency in LFR, the average incidence angle should be minimized; minimization can be performed in two different ways: (i) by increasing the height of the receiver; 8There is discussion on this definition, since in parabolic trough systems the aperture area is considered and not the reflective area as in Fresnel. A recent study showed that a different definition has limited impact on final results (Giostri et al., 2013). Solar energy conversion with thermal cycles 435 Figure 14.3.12 Characteristic angles of Fresnel collectors at 0. incidence angle. or (ii) by reducing the concentration ratio. As a drawback, both solutions increase the cost of the solar field. For this reason, when developing a Fresnel technology, it is not correct to focus on only one parameter, but the minimization of the cost of electricity must be the final target. Moving to yearly optical efficiency, the IAM for the linear Fresnel concentrators will now be discussed. The starting point is that LFR requires two projections of the incidence angle: one on the longitudinal plane and the other on the transversal plane (see Figure 14.3.13): .|| is defined as the angle between the vertical axis and the beam vector projection on the longitudinal plane, and .. is defined as the angle between the vertical axis and the beam projection on the transversal plane. In addition, another characteristic angle, .i, can be defined as the angle between the sunray vector and its projection on the transversal plane. This angle corresponds to the above-described incidence angle of PT technology. Relations between angles are summarized in Equation 14.3.10 to 14.3.12: .. = arctan(|sin(.)|tan(.z)) (14.3.10) .|| = arctan(cos(.) tan(.Z)) (14.3.11) .i = arcsin(cos(.) sin(.z)) (14.3.12) where . is the azimuth angle and .Z is the zenith angle. The incidence angle modifier which is a function of the zenith and azimuth angle includes the cosine effect, the primary mirrors mutually blocking and shading, the secondary reflector and support shading, variation in optical properties and modification of the interception factor. 436 Solar energy sciences and engineering applications Figure 14.3.13 Angles definition of a linear Fresnel reflector with horizontal N-S orientation tracking axis (Mertins, 2009). For simplicity, the overall IAM of the collector is calculated as the product of two different IAMs related to .|| and .. characteristic incidence angles, as follows: IAM(.z; .) = IAM(..) · IAM(.i) (14.3.13) This methodology was introduced by McIntire (1982) and Ronnelid et al. (1997) and recently confirmed by Mertins (2009). As an example, the IAM (.i) and IAM (..) of a commercial collector are shown in Figure 14.3.14. These data are taken from a commercial simulation tool (Thermoflex® database). From the result, it can be noted that IAM (.i), which is not the tracking axis, gives the larger contribution to the IAM, while IAM (..) exhibits an irregular trend for incidence angles between 0. and 45. because of the secondary reflector shading over primary mirrors, reducing effective mirror aperture area. To summarize, the optical efficiencies of LFR are defined as: .optical_LFR = .optical_LFR|0. IAM(..)IAM(.i).end_loss (14.3.14) The LFR optical efficiency ratio shown in Figure 14.3.15 summarizes the optical efficiency of the collector for every month of the year. IAM (.i) presents a maximum during the day at 10 h and 16 h, while the presence of IAM (..) in LFR leads to a smoother shape, with lower efficiency, in particular for high incidence angles. Compared to a parabolic trough collector (see Figure 14.3.8), which is affected only by .i, linear Fresnel has a lower optical efficiency. Solar energy conversion with thermal cycles 437 Figure 14.3.14 Longitudinal and transversal incidence angle modifier (IAM) trend as a function of incidence angle (Giostri et al. 2013). Figure 14.3.15 Annual maps of optical efficiency ratio (OR) for LFR technology (Giostri et al. 2013). The yearly optical efficiency of the commercial collector with the performances taken from the Thermoflex® database is in the range of 38%; this is 15% points lower than PT. With regard to thermal losses, thermal efficiency is about 90% like PT. To summarize, linear Fresnel collectors have good potentiality to reduce costs, but they are affected by lower efficiencies than PT. In particular, the concept of several flat mirrors introduces significant optical penalties compared to the parabola system. 438 Solar energy sciences and engineering applications Table 14.3.2 Costs for main components of a solar field based on parabolic trough technology (Manzolini et al., 2011a). Support Structure a/m2 64 Reflecting mirror a/m2 54 HCE a/m 200 Driver/controls a/m2 15.7 Foundations a/m2 19.2 Assembling a/m2 22.8 Contingencies a/m2 8 Solar field BOP % 30 14.3.11 Cost comparison of linear focus technologies The last section, dedicated to the linear focus technology, tries to give an idea about the costs of parabolic trough and linear Fresnel. Before going into details, it must be admitted that the two technologies have a different level of development: 30 years have passed since the first solar plant based on PT was built, while the older Fresnel plant is less than five years old. Obviously, this level of development affects the cost of the technology. Besides this, the cost of a parabolic trough field is in the range of 220 to 300 a/m2 (Giostri et al., 2013; Graf and Nava, 2011; Morin et al., 2012). The cost of the solar field can be split into the component parts shown in Table 14.3.2. For every linear metre of trough, where the aperture width is about 6m, the cost of the support structure and reflecting mirrors accounts for more than 50% of the overall cost of the solar field. The remainder is due to the HCE (15%) and to the civil works and drivers. Current research activity focuses on increasing the aperture area of the parabola in order to reduce the ancillary costs of drive units, sensors, control systems and pylon foundations (Graf and Nava, 2011). In addition, cost reductions can be made in solar field assembly: fewer collectors and smaller labour costs. Finally, another topic is the improvement of mirrors in terms of performance and weight; lighter mirrors can reduce the cost of the structure and foundations. Where LFR is concerned, it is much more difficult to find reliable cost information in the literature. Few studies discuss the maximum costs that the technology can support to be competitive with PT systems. Results show that Fresnel must be at least 50% cheaper than PT (Giostri et al., 2013; Morin et al., 2012). The target is feasible since the support structure would be cheaper, along with the foundations, drivers and controls. Moreover, the higher concentration ratio can reduce the HCE share of total costs. 14.3.12 Point focus The second type of solar field is based on point-focusing systems. This technology is based on two-axis tracking systems which have a higher ratio of concentration than single-axis tracking (hundreds of sun vs. tens to sun) with potentially higher working Solar energy conversion with thermal cycles 439 temperatures. In particular, the yearly optical efficiency of two-axis tracking is more constant than single-axis collectors. On the other hand, the very same aspects penalize this configuration from a cost point of view and land occupation of the solar field. The concept of point focus is to transfer the collected solar radiation to a fluid; the thermal power produced is then converted into electricity by a thermodynamic cycle. Considering the very high concentration ratio, the thermal losses of central receiver systems are usually less important than optical losses when compared to linear focus systems. On the contrary, since heat fluxes are significant (500–1000 kW/m2), heat transfer phenomena on the receiver, together with transient conditions, become more important and must be carefully investigated. Point focus technologies can be divided into two main categories featuring different power outputs: (i) the solar tower (also named power tower) concept for net power output above 10MW; and (ii) the solar dish or dish Stirling system which has a power output of up to 200kW. Since all of the solar radiation is collected at the same point, there is no necessity for a piping system as in the trough configuration. In particular, in the solar dish configuration the conversion from heat to electricity is performed directly at the focus of the collecting systems, while with power towers, thermal power is transferred to a fluid and then converted to electricity at the bottom of the tower. By the end of 2012, very few solar plants based on point focus technology were operating worldwide, producing a total installed capacity close to 40MW, with another 17MW under construction in Spain and 110MW in Nevada, US (NREL). Over the next few years the construction of 12 new heavy-duty plants in the United States and Spain will yield a total installed capacity of more than 1.5GW. The most important power tower plants are the PS-10 and PS-20 as well as Gemasolar plants in Siviglia (E), though there are several plants under construction, in particular in the United States. With regard to dish Stirling technology, there is just one plant running in the US, producing a total output of 1.5MW. It must be noted that dish Stirling holds the record for solar-to-electricity efficiency, with a value of 31.25%. From these numbers, it can be noted how this technology is still under development and far behind linear focus technology. 14.3.13 Central receiver systems Central receiver systems, sometimes named power tower systems, are based on several sun-tracking mirrors (usually called heliostats) which reflect incidence radiation onto a receiver. The concentration ratio of this type of technology, defined as receiver area divided by mirrors area, is usually in the range of 500–1000 suns. The CR in this particular case is defined as: CR = total mirror aperture area receiver area = n · Amirr Arec (14.3.15) where n is the number of heliostats and A the area of each heliostat [m2]. Since there are few operating plants based on central receiver technology, most of the example described will refer to the PS-10 plant in Spain. 440 Solar energy sciences and engineering applications 14.3.14 Collector field The collector field is based on a large number of heliostats with a tracking control system to continuously focus direct solar radiation onto the receiver aperture area. Heliostats are usually supported by a metallic structure with the tracking systems. They can be flat or also a parabolic shape with small curvature (Buck and Teufel, 2009). The optical efficiency of the solar field, which depends on the heliostat field performances, is equal to the ratio of the net power intercepted by the receiver and the product of the direct insolation and the total mirror area. This parameter includes the cosine effect, mirror properties as reflectivity, shadowing, blocking, aberration and atmospheric attenuation, and receiver spillage. Since optical efficiency is fundamental for achieving a high solar-to-electricity conversion, and the solar field complexity is significant (the solar field counts hundreds of heliostats), several modelling codes have been developed in order to determine plant performances and to find the optimal configuration (Belhomme et al., 2009; Delsol Modelling Tool; Noone et al., 2012; Pitz-Paal et al., 2011; Wei et al., 2010). These models can predict optical efficiency as well as the thermal flux density with a good degree of accuracy. For example, in the following Figure 14.3.16 there is a comparison of experimental measurement and the simulation results for the PS-10 power plant at noon on 21 March. The simulation required detailed information about the heliostat number, positions and optical properties, as well as receiver characteristics. All these simulation tools are based on optical and geometrical correlations which aim at maximizing the yearly optical efficiency. In the literature, there are several solar field configurations which belong to three different groups: surround fields (the heliostats surround the receiver), north fields (the heliostats are just on the north side of the receiver) and a configuration in between these two. In general, north fields have a higher efficiency for high latitude or high incidence angles, while surround fields are typical of locations close to the equator. Examples of two configurations are shown in Figure 14.3.17. As anticipated, the optical efficiency of central tower systems is higher than single-axis tracking systems since the solar incidence angle on the heliostats is limited. An example of the incidence angle modifier for a central tower system is shown Figure 14.3.16 Measured flux on PS-10 tower (top) and simulation results with Delsol for the same plant on the 21st of March at midday (Colzi et al., 2010). Figure 14.3.17 Representation of optimized field with surround field (a) and north field (b) configurations (Kreith and Goswami, 2007). 442 Solar energy sciences and engineering applications Figure 14.3.18 IAM for PS-10 type central receiver plant. in Figure 14.3.18: this IAM is taken from the Thermoflex™ database. It can be noted that the resulting optical efficiency is affected by high zenith angles, while it is almost constant with the azimuth angle. Single-axis tracking systems are affected by both angles. Yearly optical efficiency can be in the range of 61% for a location in France (Garcia et al., 2008), to 64% for Spain; for comparison, the Spanish case has an optical efficiency at a nominal operational rate of 77%. As expected, these efficiencies are higher than linear focus by 10–17%. 14.3.15 Central receiver Solar radiation is concentrated from heliostats to the central receiver, where it is transferred to a fluid. There are different types of central receiver: external tubular, cavity tubular, billboard tubular and volumetric. The central receiver design depends on the fluid heated and the type of application. For example, volumetric receiver has been suggested for thermochemical applications at high temperature (Pitz-Paal et al., 2011), while tubular design is usually applied to water boilers and heat transfer fluid such as molten salts. This is because in tubular design the high heat transfer coefficient of the fluid (either water or molten salts) can restrain the temperature of the tube. Conversely, when a gas fluid is used, the tubular configuration cannot be adopted because of the Solar energy conversion with thermal cycles 443 Tube receiver Concentrated solar radiation ~ 200 kW/m2 Concentrated solar radiation ~ 1000 kW/m2 Gas Exit Exit Absorber Gas Inlet Inlet T T Wall Volumetric receiver Gas Figure 14.3.19 Tubular receiver (left side) vs. volumetric receiver (right side) (Kreith and Goswami, 2007). poor heat transfer coefficient. For this reason, a volumetric receiver, which is characterized by a porous structure, increases the transfer surface, reducing the temperature on the receiver even with gas fluids. The two options are shown in Figure 14.3.19. The height of the receiver is usually in the range of 100–120 m. For example, the centre of the receiver of the PS-10 tower is 100.5 m. In general, the taller the tower, the higher the optical efficiency of the solar field since the cosine effect, shading and blocking among heliostats are minimized. On the other hand, the cost of the tower increases. It should be said that the tower is usually made of concrete. In addition to these configurations, there is also a more exotic design where the tower acts as secondary receiver and solar radiation is transferred to the fluid on the ground (see Figure 14.3.20). This configuration penalizes the optical efficiency of the solar field but reduces the cost of the tower since it has to support the secondary mirror instead of the boiler, with advantages in terms of weight. An example of a beam-down concept is the Tokyo Tech CSP project in Abu Dhabi (Beam-Down Solar Concentration). The maximum temperature on the receiver depends on the particular heat transfer fluid, the receiver technology and the power cycle adopted to convert the thermal power into electricity. For example, the PS-10 plant is based on a saturated steam boiler working at 250.C and 40 bar. Different fluids have been investigated in order to increase the temperature: molten salts have a maximum temperature of 565.C as in Gemasolar plant (ref), while air can go up to 800.C. Suggested operating conditions for tubular receivers are summarized in Table 14.3.3. 444 Solar energy sciences and engineering applications Figure 14.3.20 Beam-Down solar concentration concept. Table 14.3.3 Main operating conditions for solar tower receivers for two different fluids. Water steam Molten Salt Receiver Outlet Temperature [.C] 250/525 566 Incident Flux [kW/m2] 350 550 Peak flux [kW/m2] 700 800 Maximum pressure [bar] 100–135 – Thermal efficiency [%] 80–93 85–90 The application of central receiver technology to thermochemical processes requires even higher temperature: zinc oxide thermal reduction (which is required in water-splitting processes for hydrogen production) works at 1700.C (Schunk et al., 2009), while coal gasification for syngas production works at 1100.C (Z’Graggen et al., 2006). It is difficult to provide figures for the thermal efficiency of central receivers since there are several different design configurations of the receiver and working Solar energy conversion with thermal cycles 445 Figure 14.3.21 Principal components and tracking mechanisms adopted in a solar dish system: (a) turntable ring – Eurodish (Schlaich, 2001) and (b) pedestal based mechanism (Stirling Energy Systems, 2013). temperatures (ranging from 300.C to 800.C) and the type of receiver. As a general consideration, cavity receivers will probably have a higher efficiency than external tubular receivers, but they will probably be penalized from an optical point of view. To give an idea of a real application, the thermal efficiency of the PS-10 is 92% at nominal rated operation, while the yearly average reduces to 90.2%. 14.3.16 Solar dish A solar dish system is composed of a reflector obtained by a solid of revolution surface, namely a paraboloid, and a receiver positioned at the focal point of the reflector. The solar collector is oriented towards the sun by a rotational movement along two orthogonal axes by a two-axis tracking system. A thermal engine is placed at the top of the receiver with working fluid heated by the concentrated radiation. Parabolic dish systems are generally characterized by high efficiency, modularity and flexibility. Another advantage is that the conversion process does not consume water, as with most thermal-powered generating systems. Actually, the highest solar-toelectricity conversion efficiency among solar technologies (31.25%) has been achieved by a solar dish Stirling collector (Stirling Energy Systems) tested at Sandia National Laboratories in the United States. However, these systems are still experiencing relatively high specific costs and some reliability issues, related to the receiver/engine block working at high temperature. It should be noted that, in relation to costs, there is the potential for economies of scale with mass production in the future, especially where large industrial production volumes are concerned. The parabolic dish can be manufactured by discrete elements (facets) that approximate the geometry of a paraboloid or by a continuous reflector made with metal membranes or glass mirrors approaching the ideal geometry. Figure 14.3.21 shows the two typical structures of solar dish with tracking mechanisms based on a turntable or 446 Solar energy sciences and engineering applications pedestal support, respectively. Figure 14.3.21a shows the components and working principle of a 10 kWe prototype, Eurodish, developed in Europe9. Figure 14.3.21b, shows a more diffused configuration based on pedestal support (Stirling Energy Systems, USA); in this case the solar concentrator dish structure supports an array of curved glass mirror facets. With a solar dish collector, concentration can be as high as 3000, with potential high temperatures on the absorber and, consequently, on the thermal engine. The optical design of components and the accuracy of their manufacture determine solar radiation interception and limit the concentration factor. Cost optimization indicates 10–12m as a maximum diameter for the reflector, therefore limiting the single solar dish to a net power output of about 25–30kW with a solar radiation of 1000W/m2. Nevertheless, solar dish technology can be applied even to multi-MW solar power plants thanks to its modularity: two examples consist of Maricopa Plant in Arizona and a Power Purchase Agreement (2010) for the project of a 664MW plant in Calico (USA) involving about 26,000 dish Stirling “Sun Catcher’’ (25kWe) made by Stirling Energy Systems. Recent market evolution seemed to convince the project’s owner to convert the Calico project entirely to PV technology. 14.3.17 Receiver The receiver of a dish Stirling has two functions: absorbing solar radiation reflected by the concentrator and transferring this energy to the working fluid of the thermal engine. Usually, the receivers used in parabolic dishes are of the cavity type to reduce radiative and convective losses (Mancini et al., 2003). In commercial solar dishes, two kinds of receiver have been used: • Tube receiver: the absorber consists of several tubes which transfer the heat directly to the working fluid of the thermal cycle. The high temperatures of these absorbers (up to 800.C) make it difficult to use selective coatings due to the great overlap between absorbed and emitted radiation. • Reflux receiver (heat pipes): these receivers use a primary loop with liquid metal (usually Sodium) that evaporates on the absorbing surface. The liquid metal then condenses, transferring the heat to the working fluid of the thermal cycle. This solution implies two heat transfer loops, but takes advantage of the high heat transfer coefficients (800 W/cm2) and the metal condensation with a more uniform heating of the working fluid. The absorbing surface of the receiver is generally positioned behind the concentrator focus in order to limit the intensity of the incident thermal solar flux to values of approximately 750 kW/m2. Recently, some models have been proposed in the power range of a fewkWin order to obtain cost reductions through lighter structures and standardized low-maintenance engines. Infinia is a Stirling manufacturer that proposes a 3.2kWe solar dish Stirling (Infinia Company), based on a Beta-free piston configuration (see Section 14.5 for details on the Stirling cycle) with helium as the working fluid. The diameter of the 9Project co-financed by the European Community (under Contract N. Jor3-CT98-0242). Solar energy conversion with thermal cycles 447 Figure 14.3.22 Heat pipes heat exchanger with Sodium for a Stirling engine. solar dish is 4.4 m, the concentration factor 800 and the receiver metal temperature about 650.C, resulting in an overall electrical efficiency of 21%. 14.3.18 Power system The heat-to-electricity conversion in solar dish technology is usually based on a Stirling reciprocating engine, while a Brayton cycle gas turbine is rarely adopted. In Stirling engines for solar applications (Kongtragool and Wongwises, 2003), the working fluid is usually hydrogen or helium. The principle of Stirling engines consists of fluid compressed to about 20 MPa, heated to temperatures generally higher than 700.C owing to the high concentration ratio and then expanded to produce power. The cycle concludes with the cooling of the fluid (further details about the Stirling engine can be found in Section 14.5). Heat pipes with sodium as the intermediate fluid are usually adopted for these engines so as to achieve uniform and controlled temperatures of the fluid (Figure 14.3.22). Brayton engines, on the other hand, use air as the working fluid, with typical maximum pressures of 0.25MPa and turbine inlet temperatures of 850.C. In this kind of application conversion efficiencies are limited to 25–27%. In Brayton cycle systems the receiver is a volumetric absorber where solar concentrated radiation passes through a quartz window and is absorbed by a honeycomb-like matrix which provides a high exchange surface (Figure 14.3.23). An example that represents an attempt to apply a gas turbine cycle to a solar dish system is the adaptation of the Garrett Turbine Engine Company’s automotive gas turbine (Stine and Diver, 1994). The gas turbine is based on an open cycle with a centrifugal compressor and a radial turbine operating at 87,000 rpm. Because of the target operating temperatures of this engine, 1371.C (2500.F), a ceramic turbine and ceramic hot-section components are currently under development. At such operating 448 Solar energy sciences and engineering applications Figure 14.3.23 Receiver cavity for air heating in a gas turbine engine. conditions the net power output of the engine is expected to be equal to 75kW with a cycle efficiency of 47%. 14.4 HEAT TRANSFER FLUIDS AND STORAGE This section focuses on the heat transfer fluids used in concentrated solar power plants and on the thermal energy storage systems. The HTFs collect solar energy in the solar field and transfer it to the power block where it is converted into electricity. Part of this energy can be stored in the TES, hence decoupling the solar radiation from the power production with advantages for the dispatchability of the electricity. The HTFs from the solar field can be directly used in a turbine to produce power, in this case also becoming the working fluid of the thermodynamic cycle (direct configuration). Alternatively, the fluid can be used to transfer heat to the working fluid through heat exchangers (indirect configuration). In this second case the fluid circulating in the solar receiver can be properly referred to as heat transfer fluid. Direct configuration, for example, occurs when water is evaporated and superheated inside the absorber tubes of linear collectors or within the receiver of solar tower systems. The resulting system is defined as a direct steam generation (DSG) plant. Because of the high pressure required, as well as related control issues, this technology has so far been applied in only a few commercial plants. DSG plants have occasionally been applied to central receiver towers or to Fresnel reflector systems producing saturated steam. Parabolic trough DSG plants have recently been studied in the DISS project at the “Plataforma Solar de Almeria’’ producing superheated steam at 400.C/100 bar. Other direct configuration plants are based on compressed gas, heated inside the receiver and then expanded in a gas turbine (central receiver) or in a Stirling engine (solar dish). Indirect configuration with HTF is the commonly used configuration in most commercial solar plants. Heat transfer fluids are employed in parabolic troughs, in linear Fresnel reflectors or in central receiver plants as well. In all these systems HTF circulates Solar energy conversion with thermal cycles 449 Table 14.4.1 Characteristics of the nitrate salts and most used synthetic oil (Kearney et al., 2003). Hitec XL Solar (Calcium LiNO3 Therminol Property Salt Hitec Nitrate Salt) mixture VP-1 Composition, % – – – – biphenyl/diphenyl oxide NaNO3 60 7 7 – – KNO3 40 53 45 – – NaNO2 – 40 – – – Ca(NO3)2 – – 48 – – Freezing point, .C 220 142 120 120 13 Upper temperature, .C 600 535 500 550 400 Density @300.C, kg/m3 1899 1640 1992 – 815 Viscosity @300.C, cp 3.26 3.16 6.37 – 0.2 Heat capacity@300.C, J/kgK 1495 1560 1447 – 2319 through the receiver tubes, increasing its temperature. The HTF is then used to generate high-pressure superheated steam in a steam generator coupled with a conventional reheated steam turbine to produce electricity. Moving to thermal energy storage, in general, solar power plants can operate as stand-alone with TES in order to increase their operating hours, or alternatively they can be coupled with a back-up fossil-fuelled boiler or integrated in conventional power stations. Besides operating hours perspective, thermal energy storage is an inherent capability of indirect configuration plants with HTF, allowing increased electricity production and usually a decrease of its costs. 14.4.1 Heat transfer fluids HTFs in a solar plant should satisfy many technological requirements: stability at high temperature, low vapour pressure at high temperature, low freezing point, high boiling point, low flammability, low corrosivity and relatively low cost. Firstly, the selection of HTF is crucial for increasing the operating temperature of a solar thermal plant, and hence the efficiency conversion from heat to power (see Section 14.2). Conventional HTFs in commercial solar power plants are synthetic oil and molten salts; they allow good cycle performance and assure trade-off between the above-mentioned issues. Table 14.4.1 lists the operating temperatures and the main characteristics of some HTFs used in commercial parabolic troughs and power towers. In existing parabolic trough plants, the most common HTF is synthetic oil (Therminol VP-1), in spite of its characteristics of flammability and toxicity, and temperature limitations (up to 400.C). The use of molten salts would primarily increase the maximum temperature of the solar field to 500–600.C, thereby increasing the cycle efficiency of the power plant. Moreover, molten salts are non-flammable, environmentally friendly and cheaper than other HTFs. The main drawback of molten salts, which definitely limit their application, is the requirement for expensive anti-freezing systems because of salt’s high solidification temperature (about 120–220.C depending 450 Solar energy sciences and engineering applications on the fluid). Advanced HTFs as ionic liquids have been already studied and proposed (Moens et al., 2003), with the aim of reducing the freezing point below even room temperature. Alternatively, liquid metals like sodium can be used in CSP plants as HTF. Sodium’s low melting point (97.7.C) and high boiling point (873.C) allows a much larger range of operational temperatures and the use of advanced cycles such as combined Brayton/ Rankine cycles in central receiver systems. Nevertheless, the use of sodium poses many technological issues due to its high flammability. Another option is the adoption of gases as HTFs, however their application involves high pressure drops and low heat transfer coefficients. In the following the commonly used and most promising HTFs are described. 14.4.1.1 Synthetic oil Synthetic oil is by far the most common solution adopted in solar plants. Commercially adopted synthetic oil is typically a diphenyl/biphenyl oxide. Dowtherm A and Solutia Therminol VP-1 are industrial products that have been used in SEGS plants in the United States or in Andasol plants in Spain. On the other hand, mineral oils are not used in CSP because of their temperature limitations to 300.C, in spite of their low cost (San Diego Regional Renewable Energy Study Group, 2005). The field of exploitable temperature with synthetic oil (see Table 14.4.1) varies between 13.C, where solidification takes place, and a maximum of 400.C, beyond which the phenomenon of thermal cracking occurs. The maximum working temperature in the solar field is therefore typically limited to about 390.C, while the operating pressure of oil is about 12–15 bar in order to keep it in a liquid state and avoid evaporation at normal working temperatures (Giostri et al., 2012). The oil is generally expensive (about 5 a/kg (Manzolini et al., 2011b)), flammable and highly toxic for life and the environment, so that spills and leakages should be avoided. Although collector design has advanced to excellent levels of performance and reliability, occasional spills of HTF may occur, primarily because of piping or equipment failure. Existing plants have reduced HTF spills to very low levels (Cohen and Kearney, 1999); good maintenance practices and the use of ball-joint assemblies rather than flexible hoses between trough collectors are the major contributors to this improvement. In case of any spill or release, the affected collector loop is immediately separated from the rest of the circuit and shut down. An appropriately equipped crew will repair the damage and remove any hazardous wastes, moving it to an on-site bioremediation facility which employs indigenous bacteria to digest the hydrocarbon contamination, and in two to three months restore the soil to a normal condition. Following these operations and maintenance (O&M) best practices, the average fluid losses for SEGS plants at Kramer Junction (USA) between 1996 and 2002 was 2–3% of the site inventory per year (Cohen and Kearney, 1999). 14.4.1.2 Molten salts Molten nitrate salts are, typically, a mixture of NaNO3 and KNO3 of variable composition, even if the most commonly used is a mixture known as “solar salt’’ (respectively 60% NaNO3 and 40% KNO3). The use of molten salt HTF in a trough plant has several advantages, as has already been pointed out. Depending on the fluid, the solar field output temperature can be raised up to 550.C (see Table 14.4.1), thereby increasing Solar energy conversion with thermal cycles 451 the Rankine cycle efficiency with respect to synthetic oil plants. In addition to this, molten salts have excellent heat transfer properties, are non-flammable, non-toxic, environmentally friendly and cheaper than other HTFs. Furthermore, using molten salt in both the solar field and thermal energy storage system eliminates the need for expensive heat exchangers and also allows for a substantial reduction in the cost of the storage system. In fact, with respect to synthetic oil plants, the HTF temperature variation in the solar field can increase up to a factor of 2.5, reducing the physical size of the storage system for a given capacity. The first example of a parabolic trough plant using a molten salt mixture as HTF and thermal storage fluid is “Progetto Archimede’’ in Italy, based on ENEA technology collectors reaching 550.C (Manzolini et al., 2011a). Regarding other CSP technologies, molten salts have also been employed in central receiver plants with storage systems (Gil et al., 2010). The main drawbacks of molten salts are corrosivity at high temperatures and, more importantly, high freezing points (120.C–220.C depending on salt composition). Corrosion issues can be easily solved by adopting stainless steel (AISI 316 or 321) for tubes, piping and storage systems, together with other devices such as a layer of nitrogen at atmospheric pressure in the upper part of storage tanks. As far as high solidification is concerned, this should be avoided at all costs because it can disrupt circuits and cause mechanical failures in the reverse process of liquefaction. In fact, the specific volume of the mixture increases by about 5% when changing from solid to liquid state. Therefore freeze protection methods should be adopted along with specific operational and maintenance requirements. Molten salt technology was first developed in the United States for central receiver systems, thanks to the operation of the 10MWe “Solar Two’’ plant in Barstow, California. In that plant, before filling the boiler with salt each morning, the receiver was heated to approximately 290.C to reduce thermal stresses and to ensure that solidification of salts did not take place inside the tubes. This pre-heating was achieved by focusing a selected subset of the heliostat field onto the receiver so as to reach a uniform temperature distribution both vertically and circumferentially. For parabolic trough plants the technical issues are more challenging. During the first start-up of the power plant a boiler or heater is necessary to obtain melting of the HTF; at the same time pre-heating of the tubes and piping in the whole solar field should prevent thermal stresses during plant filling. The solution is to employ extensive heat-tracing equipment on piping and collector receivers (heating is obtained through resistive Joule effect). The same operation can be used in instances of a restart of one loop following a failure or for routine loop maintenance that requires HTF removal. On the other hand, during operation the solar field cannot be drained and solidification must therefore be avoided. Freeze protection during night-time is achieved by means of a low-flow circulation of hot salt in the solar field: a fossil-fuelled boiler can be used for heating the salt or, alternatively, molten salts can be taken from thermal storage tanks (from a cold tank in case of a two-tank storage system – see the following paragraph). In this way, critical thermal gradients during start-up are prevented. Assuming overnight heat losses of approximately 25W/m2, a storage capacity of 1 hour is suitable for freeze protection operation, according to an annual performance calculation (Kearney et al., 2003). A direct comparison among the various types of molten salts suggests that solar salt has the highest operating temperature limit: it can be used for temperatures up 452 Solar energy sciences and engineering applications to 600.C. Beyond this temperature solar salt degrades and nitrite formation occurs, creating solid precipitates. In addition, it is one of the lowest-cost nitrate salts (about 0.6 a/kg (Manzolini et al., 2011b)). Mechanical integrity studies for parabolic troughs (Hasuike et al., 2006; Yang et al., 2010) have shown that, with salt temperatures of 600.C and heat flux on the receiver up to 800 kW/m2, a maximum surface temperature of 700.Cis ensured for every geometry of the receiver tube, demonstrating the technical feasibility of high-temperature collectors. However, a major disadvantage of solar salt is its relatively high freezing point of 220.C. Hitec salt offers a lower freezing point (about 140.C) but at higher cost. Finally, HitecXL is a calcium nitrate salt mixture with a lower freezing point of about 120.C. The density, viscosity and heat capacity properties are comparable for all nitrate salts, as shown in Table 14.4.1. Research into HTFs has led to experiments with a new category of fluids that exhibit very low freezing points: the so-called room temperature ionic liquids (RTIL) (Moens et al., 2003). They are essentially salt-like materials, usually in the classes of quaternary ammonium compounds, that are composed of organic cations combined with organic or inorganic anions, and which are liquid at or near ambient temperature. 14.4.2 Storage Thermal energy storage systems allow efficient storing of solar energy as heat. Thermal energy storage can avoid the effects of variation of the solar source, which by its nature is highly variable, thereby making the system more flexible and meeting the needs of productive processes. In this way, in CSP plants, the thermal input to the power section can be more constant and in general electricity production can be independent from the collection of solar energy. Alternatively, it is possible to integrate it with fossil fuels or renewable fuels, such as oil, natural gas or biomass, obtaining a so called “hybrid’’ plant. With regard to CSP plants, the sizing of thermal storage can be carried out according to different design philosophies: • Buffering. Here, thermal storage is designed to cancel out the effect of clouds transiting over the power block. The quick variation in steam flow and quality could cause severe dynamic variations of the turbine’s working conditions. In particular, load variations faster than a few MW/minute can be unacceptable for the engine, bringing about a degradation or affecting the lifetime of the turbine itself. The required storage capacity for “buffering’’ purposes is relatively small (typically delivering up to 1 hour of power at full load). • Displacement of the production period. In this case, storage allows a decoupling of the electricity production from sunny periods, when energy demand or prices on the grid can be higher. The displacement of production generally involves the use of a medium-high storage capacity (typically delivering between 3 and 6 hours of power at full load), and does not necessarily require an increase in the surface of the solar field. • Extension of the production period. This type of storage is aimed at extending the operating hours of the plant beyond the insolation period. This solution requires a proper sizing of the storage together with an increase in the solar field surface Solar energy conversion with thermal cycles 453 (in principle the storage can be designed to deliver up 12 hours of power at full load). When TES is employed to increase the number of operating hours of the plant, the optimization parameter “solar multiple’’ (SM) is introduced (see Equation 14.2.1). Without TES, the SM is close to 1 (usually between 1 and 1.25) and all the collected thermal power is immediately used. Values higher than 1 mean that the plant can store the excess thermal energy (SM higher than 2.5 normally allows continuous operation throughout the day). From an economic perspective, the advantage of increasing SM is to extend the working hours of the plant, which also operates at higher efficiency thanks to higher load fraction values. On the other hand, the construction cost of the plant increases proportionally to the capacity of the thermal storage system and SM. For these opposing tendencies there is an optimal size for both TES and SM that maximizes the revenues from sales of electricity or equivalently minimizes the levelized cost of energy (LCOE). An optimum has to be found on a case-by-case basis by means of economic analysis (for further details see Section 14.6). 14.4.2.1 Thermal energy storage materials There are different TES technologies which are characterized by the type of storage medium and its integration into the power plant. The TES medium can be different from the solar field fluid, requiring a heat exchanger to transfer the stored heat to the power plant and a separated loop for the solar field. Commonly used TES exploit “sensible heat’’ variations of a substance, which can be measured by the change in its internal energy/temperature. This type of TES consists of a storage medium, a container (usually a tank) and inlet/outlet devices. Tanks must both contain the storage material and prevent losses of thermal energy. The amount of energy input to TES by a sensible heat device is proportional to the difference between the final and the initial storage temperature, the mass of the storage medium and its heat capacity. The amount of stored heat can be expressed as: Q = mcpT = .VcpT (14.4.1) where cp [kJ/kg K] is the specific heat at constant pressure of the storage material, T [.C] is the temperature variation in the storage, V [m3] is the material volume and . [kg/m3] is the density of the material. However, besides the density and the specific heat of the storage material, other properties are significant for sensible heat storage: namely, allowable operational temperatures; thermal conductivity and diffusivity; vapour pressure; compatibility among materials; thermal stability; heat transfer coefficients; and cost. Sensible heat storage media can be classified as solid or liquid materials: • Solid media (mainly high-temperature concrete and castable ceramics) are usually used in packed beds, requiring a fluid to exchange heat with the solar field or the power block. When the fluid is a liquid, heat capacity of the liquid in the packed bed is not negligible, and the system is called dual-storage. Packed beds also favour thermal stratification, which can be exploited in a profitable way. Another advantage of the dual system is the potential use of inexpensive solids such as rock, sand 454 Solar energy sciences and engineering applications or concrete as storage materials. Concrete, for example, is chosen because of its low cost, availability throughout the life of the plant and easy processing. Moreover, concrete is a material with high specific heat, good mechanical properties (especially when subjected to compression strain), thermal expansion with a coefficient close to one of steel, and high mechanical resistance to cyclic thermal loading. When concrete is heated, a number of reactions and transformations take place which influence its strength and other physical properties: resistance to a compression strain at 400.C is about 20% lower than its value at ambient temperature; the specific heat decreases in the range of temperature between 20.C and 120.C; and the thermal conductivity decreases between 20.C and 280.C (Gil et al., 2010). Resistance to thermal cycling depends on the thermal expansion coefficients of the materials used in the concrete. To minimize such problems, a basalt concrete is sometimes used. Steel needles and reinforcement are sometimes added to the concrete to prevent cracking. At the same time, by doing so, thermal conductivity is increased by about 15% at 100.C and 10% at 250.C. Another material that can be employed is rock, which is a more inexpensive TES material costwise, even if its physical and mechanical properties are not as good as concrete (see Table 14.4.2). • Liquid media: liquids (mainly molten salts, mineral oils and synthetic oils) are more commonly used for sensible thermal storage than solids. One important design consideration of a liquid storage system is the need to maintain a separation between the colder fluid and the warmer fluid. There are mainly two ways of separating temperatures: two-tank systems and stratified thermal storage tanks (thermocline tanks). Two-tank systems separately store the hot and cold fluid in different tanks; as the hot tank is being filled, the cold is being emptied and vice versa. In this way mixing is avoided but a higher storage volume is required compared to the stratified thermal storage tanks. In fact, in a thermocline tank liquid medium maintains natural thermal stratification because of density differences between hot and cold fluid. Thermal stratification ensures the existence of separate volumes of liquid at different temperatures inside the tank and the temperature gradient occurs in a small portion of the tank height, the so-called thermocline. Significant volume and cost reductions are generally achieved with respect to the two-tank configuration. The requirements of this type of TES are that the hot and cold fluids have to be supplied in different parts of the tanks in order to limit fluids mixing: the fluid enters from the upper part of storage during charging, and the cold fluid has to be extracted from the bottom part during discharging. In any case a minimum mixing of hot and cold fluids takes place. For this reason, an accurate design of the geometry of inlet and outlet ducts is necessary to limit fluid velocity. As regards the shape of the tank, a slim storage container is desirable to improve thermal stratification (Dincer and Rosen, 2002). However, the optimum value of the ratio between the tank height and diameter cannot be determined whatever the techno-economic optimization of the power plant. In addition to storage tanks based on sensible heat storage media, there are other TES solutions employing latent heat storage media. In fact, thermal energy can be stored almost isothermally in some substances as latent heat of phase change, as heat of fusion, exploiting solid-to-liquid transition, heat of vaporization, exploiting liquid-to-vapour transition, or even solid structure change (transition from amorphous Solar energy conversion with thermal cycles 455 Table 14.4.2 Materials and fluids applicable for thermal storage (NREL, 2000). Temperature Average Average Volume Media Media Average heat heat specific cost cost per Cold Hot Density conductivity capacity capacity per kg kWhth Storage Medium [.C] [.C] [kg/m3] [W/mK] [kJ/kgK] [kWhth/m3] [$/kg] [$/kWhth] Solid Media Sand-rock-mineral oil 200 300 1.7 1.0 1.30 60 0.15 4.2 Reinforced concrete 200 400 2.2 1.5 0.85 100 0.05 1.0 NaCl 200 500 2.16 7.0 0.85 150 0.15 1.5 Cast iron 200 400 7.2 37.0 0.56 160 1.0 32.0 Cast steel 200 700 7.8 40.0 0.60 450 5.0 60.0 Silifica fire bricks 200 700 1.82 1.5 1.00 150 1.0 7.0 Magnesia fire bricks 200 1200 3 5.0 1.15 600 2.0 6.0 Liquid media Mineral oil 200 300 770 0.12 2.6 55 0.3 4.2 Synthetic oil 250 350 900 0.11 2.3 57 3.0 43.0 Silicone oil 300 400 900 0.10 2.1 52 5.0 80.0 Nitrite salts 250 450 1.825 0.57 1.5 152 1.0 12.0 Nitrate salts 265 565 1.87 0.52 1.6 250 0.7 5.2 Carbonate salts 450 850 2.1 2.0 1.8 430 2.4 11.0 Liquid Sodium 270 530 850 71.0 1.3 80 2.0 21.0 Phase change media NaNO3 308 2.257 0.5 200 125 0.2 3.6 KNO3 333 2.11 0.5 267 156 0.3 4.1 KOH 380 2.044 0.5 150 85 1.00 24.0 Salts-ceramics NaCO3-BaCO3/MgO 500–850 2.6 5.0 420 300 2.00 17.0 NaCl 802 2.16 5.0 520 280 0.15 1.2 Na2CO3 854 2.533 2.0 276 194 0.20 2.6 K2CO3 897 2.29 2.0 236 150 0.60 9.1 to crystalline structure). Nowadays, mainly the solid-to-liquid transition has been applied, and substances used under this technology are called phase change materials (PCM). Storage systems utilizing PCM can be reduced in size compared to single-phase sensible heating systems. However, heat transfer design and media selection are more difficult, and experience with low-temperature salts has shown that the performance of the materials degrades after a moderate number of freeze–melt cycles. Phase change materials allow large amounts of energy to be stored in relatively small volumes as a result of a higher energy density, theoretically resulting in cost reduction in comparison with the other storage concepts. For these reasons their application in CSP plants seems to be attractive, even if the potential reduction of costs and technical feasibility are still to be demonstrated. A further possibility is to exploit the chemical heat of reaction of specific media, obtaining a chemical storage. This type of thermal storage exploits appropriate chemical reactions that are fully reversible. The heat produced is used to promote an endothermic chemical reaction and, if this reaction is completely reversible, the stored 456 Solar energy sciences and engineering applications heat can be completely recovered by the reverse reaction. Usually, catalysts are required to control the reverse reaction. The main advantages of thermal storage with thermochemical reversible reactions are the high density of storage and the possibility of maintaining the energy stored at a temperature close to that of the environment. Major issues are the complexity and the costs of the system itself and of materials, the effective control of the kinetics of reactions with different operating conditions and aspects related to the properties of the components involved in the reactions (thermodynamic properties, toxicity, flammability, etc.). This type of storage will be the object of study and experimentation in the near future but at present it can be considered far from commercialization. 14.4.2.2 Integration schemes between storage and power plant There are different ways in which TES systems can be integrated into a solar plant. These depend on the storage medium, heat transfer fluid in the solar field and the type of process coupled to the solar plant. For simplicity, in the following we refer to the case in which the process is a power plant producing electricity. The integration of TES can use a direct or indirect system. In a direct system, the heat transfer fluid is also used as the storage medium, while in an indirect system a second medium is used for storing the heat. A direct storage system is composed of a plant configuration with two tanks where the HTF produced by the solar field is directly stored in a hot tank. The cooled HTF is pumped to the other, cold tank where it remains until the solar field starts to operate. The adoption of the very same fluid in the solar field and in storage tanks implies that it must have, at the same time, the characteristics of a good HTF and good storage medium. The use of molten salts as an HTF and storage medium allows the solar field to be operated at temperatures higher than current heat transfer fluids such as synthetic oil. This configuration also allows a substantial reduction in the costs of TES systems, owing to the elimination of expensive heat exchangers, improving the performance of the plant (no temperature difference due to the heat exchange) and reducing the LCOE. Moreover it is worth noting that the maximum temperature of HTF can be increased with respect to synthetic oil thanks to the higher thermal stability of salts. Some complications, in the case of molten salts, are due to the high freezing point (from 120.C to 240.C depending on the type of salt used, see also Table 14.4.1) and this means that special care has to be taken to avoid the salt freezing in the solar field. Hence, freeze protection operations must be undertaken, increasingO&Mcosts. Figure 14.4.1 describes the layout of the Solar Tres plant, a solar tower plant which uses molten salts (NaNO3 and KNO3) as HTF (Gil et al., 2010). In an indirect storage system, a second fluid is used for storing the thermal power produced by the solar field. Within this configuration, both the two-tank and singletank systems (the thermocline system, as stated earlier) are the adoptable solutions. The main disadvantages of this configuration relate to the presence of heat exchangers, meaning additional cost, heat transfer irreversibility and the temperature limitation of one of the fluids, usually the one circulating in the solar field. Figure 14.4.2 depicts a power plant based on the indirect two-tank storage system, in which the heat transfer fluid which circulates in the solar field is different from the storage medium. Here, the energy is stored by another medium in the TES (generally Solar energy conversion with thermal cycles 457 RECEIVER SALT 565°C 790°C COLD SALT STORAGE TANK HOT SALT STORAGE TANK SALT TURBINE GENERATOR CONDENSER STEAM HELIOSTAT FIELD GENERATOR SUBSTATION Figure 14.4.1 Example of power plant layout with the two tanks direct storage system (Gil et al., 2010). molten salts), heated by the HTF circulating in the solar field and pumped through heat exchangers (generally synthetic oil). During a thermal storage charge cycle, a portion of the oil from the solar field is directed to the oil-to-salt heat exchanger: molten salts are taken from the cold storage tank and flow in a counter current arrangement through the heat exchanger. During the discharge cycle, the oil and salt flow paths are reversed in the oil-to-salt heat exchanger; heat is then transferred from the salt to the oil to provide the thermal energy for the steam generator. Another active indirect storage system is the single-tank system, in which hot and cold fluids are stored in the same tank (see Figure 14.4.3). Equipping the system with a thermally stratified tank (thermocline) is a potential way of reducing the cost of the plant. The HTF from the solar field passes through a heat exchanger, heating the thermal storage fluid media. Usually, a filler material can be used to aid stratification and reduce the quantity of heat transfer fluid. Experimental studies performed up to now have found that this filler material, depending on the type, can also act as a thermal energy storage medium. Recent research projects conducted by Sandia National Laboratories have identified quartzite rock and silica sands as potential filler materials (Brosseau et al., 2005). Depending on the cost of the storage fluid, the thermocline can result in a substantially lower cost storage system (Gil et al., 2010). The advantages of the single-tank thermocline system are: reduced storage tank costs and lower cost of the filler materials (rocks and sand). A thermocline system has been estimated to be about 35% cheaper than the two-tank storage system (Brosseau et al., 2005). 458 Solar energy sciences and engineering applications Solar Field Solar Superheater Steam Turbine Condenser Solar Preheater Low Pressure Preheater Solar Reheater Expansion Vissel Seam Generator Hot Salt Tank Cold Salt Tank Deaerator 2-Tank salt Storage Figure 14.4.2 Example of power plant layout with the two tanks indirect storage system (Gil et al., 2010). Solar Field 1-Tank Thermooline Storage Solar Superheater Boiler (optional) Steam Turbine Condenser Solar Preheater Low Pressure Deaerator Preheater Solar Reheater Expansion Vessel Seam Generator Fuel Figure 14.4.3 Example of power plant layout with the one tank indirect storage system (thermocline) (Gil et al., 2010). Solar energy conversion with thermal cycles 459 However, the design of the storage system is more complex because of the aforementioned mixing issues. Lastly, an indirect configuration can adopt a solid as storage medium. When using concrete, for example, the solar energy of the solar field is transferred through the HTF to the solid storage material system. The storage material comprises a tube heat exchanger which transfers the thermal energy from the HTF to the storage and vice versa. This heat exchanger has a significant impact on overall investment costs and, moreover, the design of geometry parameters such as tube diameter; the number of pipes is also a key issue, increasing engineering costs. While experimental tests have, however, shown long-term instability of the media (Gil et al., 2010), this technology is potentially advantageous for the very low cost of thermal energy storage media. Another possibility is phase-change materials as storage media. The overall concept of this type of storage system is the same as in solid systems, but the storing material has a melting temperature within the range of the charging and discharging temperatures of the HTF (Bauer, Laing, & Tamme, 2011; Steinmann, Laing, & Tamme, 2010). For the sake of completeness, there is also the possibility of obtaining an indirect storage system in which the secondary fluid is steam (Beckam and Gilli, 1984; Slocum et al., 2011). In principle the advantage of steam storage lies in its direct use in the power block Rankine cycle. The disadvantage is related to the high storage pressures, thus limiting its application to small-size storage tanks, exploiting the buffering of the system. Examples of steam storage are the central-tower commercial plants PS10 (10MWel) and PS20 (20MWel) built in Spain and which started operating in 2007 and 2009 respectively; both of them feature sliding pressure steam accumulators which are able to provide approximately 1 hour of operation at 50% load (Gil et al., 2010). 14.5 FROM HEAT TO POWER This section discusses the part of the plant dedicated to the conversion into electricity of the thermal power collected in the solar field. Operating plants are based on two different technologies: Rankine cycle and Stirling cycle. In large-scale CSP, which can be either linear focus or power tower, technologies, the power cycle is based on the water Rankine cycle, which is used also in other solid fuel-based technologies such as coal plants and waste-to-energy plants. In solar dish, thermal conversion is based on the Stirling cycle, which, though it has a fairly high efficiency even at small power output, is difficult to scale up. One of the main characteristics required to power cycles in solar plant is a high conversion efficiency even at partial load; solar-based plant are characterized by daily start-up and shut-down procedures, as well as by partial load operating conditions as a consequence of the variation in solar radiation during the day. In reality, the adoption of large storage systems (i.e. 7.7 hours as in Andasol) reduces the operating time at partial load, something which cannot be avoided. An example of thermal input for a power cycle with different storage sizes and solar multiple, and in different seasons, is shown in Figure 14.5.1. At large storage size, power production can begin before sunrise because the thermal input stored during the previous day can be used. 460 Solar energy sciences and engineering applications Figure 14.5.1 Power production (solid lines) vs. solar radiation (dashed lines) for two different solar field and storage sizes during a summer day (left side) and mid-season day (right side). Solar dish, which does not have a storage system, is more affected by the variation in solar radiation during the day. The efficiency of power section is defined as follows: .PS = Net Power Output Thermal input = Wnet ·m HTF · (hHTF,in - hHTF,out) (14.5.1) where Wnet is the net power output generated in the thermodynamic cycle [W], ·m HTF is the HTF mass flow rate [kg/s] and hHTF are the enthalpies of the heat transfer fluid at the inlet and outlet of the power section. Wnet depends on the thermodynamic cycle adopted for thermal conversion. Another classification of solar plants is between stand-alone plant and hybrid plant. In stand-alone plant the entire power output of the plant is related to the input of solar power. Another configuration consists of the integration of the solar field in a solid fuel-based power plant. Solid fuel can be either fossil (coal, natural gas) or renewable (e.g. biomass). Integrated solar combined cycle (ISCC) plants, which integrate the solar field in a combined cycle, are already commercially available, as in Indiantown (Florida, USA). Usually, hybrid power plants have a limited solar energy share of total electricity production. The plant mentioned above, for example, has a net power output from the solar plant of 75MW out of 3.7GW of the entire plant (Martin Next Generation Solar Snergy Center). Hybrid plants have a higher flexibility than stand-alone, since fossil fuel can be regulated to maintain a constant thermal input to the power section by balancing solar input fluctuation. Moreover, this configuration exploits an economic advantage as a consequence of the power section having a higher number of operating hours, hence reducing its cost per kWh produced. Since the power section is fundamental for the success of CSP technology and, with actual technologies, has a significant impact on overall plant costs (about 40–45%), research activity has been focusing on other options with the aim of cost reduction and faster start-up, as well as higher conversion efficiency. One of the most investigated Solar energy conversion with thermal cycles 461 candidates is the Brayton cycle, which is cheaper than Rankine and it has faster startup; however, this technology to be competitive requires temperatures in the solar field above 700.C and/or the adoption of fluid in the solar field in gas phase. These two requirements put Brayton cycle still far from the commercialization phase. 14.5.1 Rankine cycle The power section (which converts thermal power into electricity) in all operating CSP plants with a net power output above 1 MW is based on water Rankine cycle. Water Rankine cycles are based on a closed loop where heat is supplied externally (i.e. by the solar field). The power output is obtained by the expansion of water in the gaseous phase in a steam turbine. Compression is performed in the water liquid phase, hence requiring less power. The same cycle is applied in coal-based plant (which accounts for half of the world’s power production (Key World Energy, 2012)) and nuclear plant. Even if the concept is the same, thermodynamic conditions and performance of the Rankine cycle in CSP plant are significantly different than other applications. For example, modern coalbased plant – usually called advanced supercritical or ultra supercritical pulverized coal plant – works with live steam pressure at turbine inlet of between 250 and 300 bar, and temperatures of 600–620.C (Manzolini et al., 2011). This is because the temperature of coal combustion is about 1500–2000.C. The temperature in the solar field ranges between 400.C (linear focus technologies) and 800.C (point focus technologies), hence limiting the maximum temperature of the steam. It must be noted that the only operating solar tower plant works with saturated steam at a temperature of 250.C (ref. PS-10). In general, the higher is the evaporation pressure and the steam temperature, the higher is the conversion efficiency of the Rankine cycle. For this reason, current research activity is focusing on the development of innovative HCE (see Section 14.3) and the adoption of innovative heat transfer fluids (see Chapter 14.4) which can withstand temperatures of up to 550–600.C for linear collectors and 1000.C for point focus concentrators. Two different types of power cycle configurations can be adopted: the indirect cycle and the direct cycle. A schematic of the two configurations is shown in Figure 14.5.2. The indirect cycle configuration is characterized by the adoption of different working fluids between the solar field and the power cycle. In detail, the heat collected in the solar field is transferred to a heat transfer fluid which is sent to the power section. In the power section, the HTF is cooled in heat exchangers by evaporating and superheating steam. This configuration allows a degree of freedom since the mass flow rate and the pressure in the solar field differ from the power cycle; however, it does require the adoption of expensive heat exchangers. The direct cycle configuration is based on direct steam generation inside the solar field. In this configuration the adoption of the HTF and all its associated components, in particular the boiler, can be avoided. The maximum temperature in the solar field coincides with the steam temperature at the turbine inlet, with advantages from an efficiency point of view. Moreover, lower circulating pump consumption occurs in the solar field. Obviously there also some drawbacks, namely: (i) higher fluid pressure inside the solar field (about 100 bar vs. about 35 bar of synthetic oil), with penalties 462 Solar energy sciences and engineering applications Figure 14.5.2 Schematic of the two configurations for the power section: indirect cycle (left side) and direct cycle (right side). Table 14.5.1 Stream properties for two indirect layouts (Giostri et al., 2012). Synthetic oil (Andasol type) Molten Salts (Archimede type) Stream Fluid type M [kg/s] p [bar] T [.C] M [kg/s] p [bar] T [.C] 1 HTF 725.8 25.0* 308.0 355.9 15.0* 312.7 2 HTF 618.7 17.6 390.0* 296.6 3.7 550.0* 3 Steam 63.5 95* 370.0* 47.1 115.0* 540.0* 4 Steam 51.6 14.5* 370.0* 37.9 14.5* 540.0* 5 Steam 49.1 0.096 45.0* 30.8 0.096 45.0* 6 Steam 2.6 9.2 312.8 1.6 9.2 471.4 7 Water 49.1 8.7* 143.9 36.3 8.7* 143.9 8 Water 63.5 117.6 260.4* 47.1 141.2 278.9* 9 Water 63.5 100 299.5 47.1 120.0 313.4 (*) Assumptions from piping and absorber thickness; (ii) potential hot spots on the selective coating, as a consequence of difficult superheater (SH, see below) steam temperature control; and (iii) the absence of reheating (RH, see below) because of the large volumetric flow, which would require a great number of parallel absorber tubes. The only operating plant based on direct cycle configuration is the linear Fresnel collector developed by Novatec solar; this has a maximum steam temperature of 550.C and no RH (Novatec Biosol, 2012). A schematic of the power section for a conventional indirect cycle is given in Figure 14.5.3. This shows a state-of-the-art power section, where the steam turbine has both a super-heater and a re-heater and seven regenerative bleedings (including one for the deareator). This configuration is adopted in most operating plants, including those of Andasol (The parabolic trough power plants Andasol 1 to 3, 2008) and SEGS. Solar energy conversion with thermal cycles 463 Figure 14.5.3 Power block scheme for an indirect cycle. In general, a simpler layout can also be adopted, with fewer regenerative bleedings or without RH. However, the higher the number of regenerators, the higher the power cycle efficiency since all the heat collected in the solar field is used for steam evaporation and superheating, rather than economization. Reheating is fundamental to achieving high conversion efficiencies, as well as a high vapour fraction at the turbine outlet. HTF thermal power is transferred to the power cycle in four different heatexchangers: an evaporator, an economizer, and a superheater and a reheater. Different heat exchanger configurations can be adopted: the RH section can be in parallel with the SH only, or with the whole boiler, as in SEGS VI plant (Patnode, 2006). Optimal configuration depends on heat transfer and steam temperatures, and is usually selected to minimize HTF boiler outlet temperature and heat transfer irreversibilities inside the boiler. The steam turbine is usually divided into a high pressure (HP) section and a low pressure (LP) section; between the two sections the steam is sent to the boiler for reheating. The maximum steam temperature that occurs at the turbine inlet, and after reheating, is in the range of 371.C when synthetic oil is used as the heat transfer fluid; this temperature occurs in all existing plant with indirect cycle configuration because of the maximum working temperature of the synthetic oil (400.C) – a conservative maximum temperature in the solar field of 391.C is usually taken. If molten salts are adopted as the heat transfer fluid, steam temperature can be increased up to 500–540.C. For example, the Archimede plant in Sicily (Italy) works with a steam temperature of 525.C at the turbine inlet. The drawbacks of this HTF have already been discussed in the previous section. Because of the moderate maximum temperature 464 Solar energy sciences and engineering applications Table 14.5.2 Comparison between air cooled and evaporative tower condenser. Air cooled condenser Evaporative tower condenser Ambient temperature (.C) 23 Humidity (%) 60 Air dry bulb temperature (.C) 23 Air wet bulb temperature (.C) 17.74 Heat rejected (MW) 86.2 Air flow (kg/s) 5880.5 1386.9 Air outlet temperature (.C) 37.41 32.74 Condensing pressure (bar) 0.096 0.0689 Water make-up (kg/s) – 36.84 Fan consumption (kW) 842.2 346.6 Pump consumption (kW) – 298.3 of the steam cycle, the evaporating pressure is in the range of 100 bar and RH around 25 bar. The pressures are set in order to limit moisture content in the last stage of of the steam turbine because water droplets during expansion can be detrimental for turbine blades. The steam cycle, as a closed loop, requires a heat exchanger to discharge the heat: in Rankine cycle this heat exchanger is called a condenser because the steam condenses. Condensers can be based on evaporative cooling towers as well as dry coolers. Considering that solar thermal plants are usually placed in arid places with high solar radiation and low precipitation, dry cooling condensers are more typical. Air condensers are more expensive than evaporative cooling systems and are more sensitive to the ambient conditions: the air temperature at the condenser inlet is the dry bulb temperature, and the entire heat rejected is transferred as sensible heat to the air. Conversely, evaporative towers transfer the heat rejected from the condenser also as latent heat by evaporating water in the air, requiring lower amounts of air. In addition, the air temperature at evaporative tower inlet is the wet bulb, which is lower than dry bulb particularly in dry conditions. Comparisons between evaporative tower and air cooled condenser conditions are summarized in Table 14.5.2. The evaporative tower has lower fan consumptions (346.6kW vs. 842.2 kW) because of the lower air mass flow. This advantage is partially balanced by the consumption of the recirculating pump, which is not present in air condensers, and the required water make-up of 36.8 kg/s; the cost of water replacement can be significant because of solar field location. Both condensate extraction and feedwater compression are performed by dedicated pumps which have a motor driver. The number of regenerative bleedings can be variable (between four and seven) with the pressure selected in order to optimize heat exchange in the regenerators. The target is to minimize the temperature difference between the hot stream and the cold stream in order to limit entropy production, hence maximizing efficiency. When direct steam generation configuration is adopted, the power section has a much simpler layout since the entire heat exchanger section can be avoided. The layout of a DSG configuration and the thermodynamic properties of main streams are shown Solar energy conversion with thermal cycles 465 Figure 14.5.4 Power block scheme for a direct cycle. Table 14.5.3 Stream properties for the direct layout (DSG) (Giostri et al., 2012). Stream Fluid type M [kg/s] p [bar] T [.C] 1 Water 73.7 100* 279.3* 2 Steam 73.7 91.3 304.7 3 Steam 56.8 89.3 302.8 4 Steam 56.8 81.0 540.0* 5 Steam 36.7 0.096 45.0* 6 Steam 2.4 9.2 256.1 7 Water 44.1 8.7* 143.9 8 Water 56.8 100* 260.4* (*) Assumptions in Figure 14.5.5 and Table 14.5.3. The DSG plant considered has steam SH, like the more recent Novatec technology, but no RH. The feedwater line is composed of seven regenerators, one being the deareator. The steam turbine, with no RH, has a lower vapour fraction at the outlet compared to the indirect cycle. Evaporation in the solar field is based on a reboiler concept: steam quality at the evaporation section outlet is kept below 1 (i.e. 0.77 (Forristall, 2003)) to guarantee good wettability of the collector, thus preventing the formation of hot 466 Solar energy sciences and engineering applications spots. The separated liquid fraction is sent back at the inlet of the solar field by a recirculation pump and is mixed with feedwater at 260.C. The main drawbacks of the DSG configuration are the absence of available storage suitable for the steam, and temperature control in the SH section. 14.5.2 Rankine cycle performance When a Rankine cycle is adopted for power conversion, the power section efficiency is defined as: .PS = Wnet ·m HTF · (hHTF,in - hHTF,out) = Wgross,tur -  Wpumps -Waux ·m HTF · (hHTF,in - hHTF,out) (14.5.2) where Wgross,tur is the power output generated in the steam turbine [W], Wpumps accounts for pump (feedwater and condensate) consumptions, Waux considers auxiliaries consumption as condenser fan, ·m HTF is the HTF mass flow rate [kg/s] and hHTF are the enthalpies of the heat transfer fluid at the inlet and outlet of the boiler. In case of direct steam generation, the denominator will be ·m steam · (hsteam,TV - hwater,in), where msteam is the steam generated in the solar field [kg/s], hsteam,TV is the enthalpy at the outlet of the solar field (i.e. turbine inlet) and hwater,in is the enthalpy of the water after the feedwater section (i.e. solar field inlet). Thermodynamic conditions and the performance of different power cycles at design conditions are summarized in Table 14.5.3 and Table 14.5.4. The examples given reproduce existing plant beside indirect molten salts; this is because there is no stand-alone plant running with this HTF. Molten salts can work at higher temperature, increasing the power cycle efficiency by 12% compared to conventional synthetic oil configurations. Direct cycle configuration with superheating can achieve almost the same efficiency owing to higher temperature at the turbine inlet, whilst it is penalized for the absence of RH. Lastly, saturated steam cycle in direct configuration is penalized from an efficiency point of view by about 3% points versus commercial synthetic oil. A comparison of the yearly power block efficiency is more difficult since, besides cycle configuration and HTF, this depends also on condensing technology and on thermal storage size: the bigger the thermal storage, the closer power block efficiency is to the nominal working conditions at constant thermal input. Condensing technology strongly affects power block efficiency as, during the summer when solar radiation is higher, ambient temperature also increases, together with the condensing temperature. An evaporative tower is less penalized than an air-cooled condenser, since the wet bulb temperature is more constant than that of the dry bulb, in particular in sites with low relative humidity, such as deserts. 14.5.3 Stirling cycle The Stirling engine is a closed cycle that has been designed for small solar power applications (from a few kW up to 100 kW). It has a potential for high cycle efficiency, the ideal Stirling cycle equalling the efficiency of the Carnot cycle. Efficiency is one Solar energy conversion with thermal cycles 467 Table 14.5.4 Performances at on-design conditions for five selected technologies Indirect Direct Molten Salts Superheating Direct Indirect (Andrea (Andrea saturated Direct Andasol Giostri Giostri (A. Giostri saturated (A. Giostri et al., 2012) et al., 2012) et al., 2013) (Colzi et al., et al., 2013) Archimedes Supernova Novatech 2010) HTF working 297.3–391.0 300.0–550.0 – – – temperature (.C) T steam SH (.C) 371 525 540.0 270.0* 260.0* Pressure solar field (bar) 25.0 15.0 100.0 66.0 45.0 Pressure at turbine 95.0 115.0 81.0 55.0 40.0 inlet (bar) Steam mass flow 63.5 47.1 56.8 77.2 18.0 @turbine inlet (kg/s) RH temperature (.C) 371 525 – – – RH pressure (bar) 14.5 14.5 – – – Condensing pressure (bar) 0.096 0.096 0.096 0.096 NA N. of regenerators 6 7 7 3 4 Temperature at boiler 234.8 278.9 – – – inlet (.C) Temperature at solar – – 260.4 195.9 NA field inlet (.C) Net power output (MW) 50.0 50.0 50.0 50.0 11.0 Thermal input (MW) 144.5 128.9 130.2 156.7 35.8 Power block efficiency (%) 34.6 38.8 38.4 31.9 30.7 *Saturated steam. of the main targets for solar power cycle design because of the reduced size of the collector area, and thus lower costs, for a given power output. Most proposed Stirling applications (Stine & Diver, 1994) are for small (10 to 100 kW) engines placed at the focus of a parabolic dish concentrator. In point of fact, for small power output, the net efficiency of Rankine or Brayton cycle-based engines is seriously degraded, favouring the high efficiency potential of the Stirling engine. On the other hand, component size and costs increase significantly for large-scale Stirling engines. The ideal Stirling cycle results from two constant-temperature and two constantvolume processes with working fluid in the gas phase. Figure 14.5.5 shows the four processes in pressure-volume and temperature-entropy diagrams. Energy is exchanged (either produced or absorbed) by the cycle only during the constant-temperature processes; however, heat must be transferred during all four processes. Because the processes at constant volume (2-3 and 4-1) involve an equal amount of heating and cooling of the working fluid, in the hypothesis of ideal gas behaviour, a regenerator may be used which transfers the heat internally to the cycle, between expanded and compressed gas. As has already been pointed out, with an ideal heat exchanger, heat would only be introduced and discharged from the cycle at constant temperatures, ideally obtaining the same efficiency as the Carnot cycle. However, the 468 Solar energy sciences and engineering applications Figure 14.5.5 Ideal Stirling cycle representations in p-V (a) and T-s (b) diagrams. processes occurring in the engines are not ideal, leading to lower efficiency than Carnot. The main penalties of a real engine that cause performance decay (Walker, 1980) are: • Heat exchange losses: (i) non-isothermal processes (engines are adiabatic in nature; isothermal processes are unfeasible in practice); (ii) T in heat exchangers due to finite surfaces; • Fluid dynamic losses: (i) pressure losses (especially in the regenerator); (ii) losses due to the dead volumes (volumetric efficiency); (iii) leakages of fluid; • Losses due to kinematics: isochores are not perfectly executed in normally adopted mechanisms; • Electrical and mechanical losses. As regards the mechanical device, a reciprocating piston-cylinder arrangement is normally used. The kinematic transmission is based on a rod-crank mechanism in which the pistons move according to a sinusoidal law. Another kinematic transmission involves a Wobble-Joke or a Swash-plate mechanism, with similar effects. As a result the four transformations that compose the cycle are partially overlapped in a real cycle, with consequent reduction of overall efficiency (Figure 14.5.6). 14.5.4 Stirling configurations Depending on the mechanical scheme adopted, existing Stirling engines can be classified as one of three basic arrangements (Martini, 1983; Stine, 2007), as represented in Figure 14.5.7. The Alpha configuration is characterized by the presence of two distinct cylinders with two corresponding pistons. Both pistons have the same pressure at any given time. The crank mechanism is generally kinematic, usually of a rod-crank type. In this way, a sinusoidal motion is induced on the pistons, with a phase shift close to 90.. Solar energy conversion with thermal cycles 469 Figure 14.5.6 Representation of a real Stirling cycle in a p-V diagram. Regenerator Alpha Beta Gamma Cool Cool Cool Heat Crankshaft Cold Cylinder Hot Cylinder Piston Piston Piston Displacer Displacer Heat Figure 14.5.7 Main types of Stirling engine component arrangements. The Alpha configuration characterizes most of the Stirling engines made in the last century: simplicity of construction and the possibility of using technologies derived from the automotive industry. In spite of this simple design, the Alpha configuration has several drawbacks that currently limit its spread. These are mainly related to: 470 Solar energy sciences and engineering applications (i) sealing problems on the hot cylinder; (ii) lubrication issues due to the need to lubricate both the pistons and the mechanism; and (iii) high specific volumes derived from the particular geometry of the engine and by the presence of two pistons. The latter is a factor that increases the specific costs of the system and precludes its use for applications requiring compact solutions. The Eurodish collector discussed in Section 14.3 exploits the Alpha scheme. The Beta configuration is characterized by the presence of a single cylinder and two components moving inside the same cylinder. The first is the piston, responsible for the phases of compression and expansion, and the second is the displacer accomplishing the processes at constant volumes. The displacer is a mechanical device similar to the piston but, unlike the latter, it is not equipped with seals: its surfaces must support only the pressure difference related to the fluid dynamic losses in the regenerator. Piston and displacer may have a different rotating mechanism. In general, they have a sinusoidal motion, associated with a phase shift of 90. between piston and displacer. Recently, a novel configuration called “Beta free piston’’ has been developed by some manufacturers and is characterized by the absence of kinematic connection between piston and displacer (Thombare&Verma, 2008). The motion of piston and displacer is entrusted to resonance phenomena and regulated by the natural frequency of a linear generator, connected to the grid directly or through inverters. This technology is employed, for example, by Infinia in its 3.2kWe solar dish collector described in Section 14.3. In general, the main advantages of the Beta configuration are: (i) its compactness due to the presence of a single cylinder for both the compression and expansion phase; (ii) high power density; (iii) reduced dead volumes; and (iv) the possibility of developing solutions intrinsically sealed with the adoption of a free-piston mechanism (i.e. it is possible to pressurize both the working fluid and the chamber containing the generator, avoiding leakages). Figure 14.5.8 shows a schematic view of a Beta free-piston Stirling generator. It can be noted that the whole engine is encapsulated to avoid leakages of fluid. Another option is the Gamma configuration. This differs from the Beta configuration by having two separate cylinders which contain, respectively, the displacer and the power piston. The Alpha configuration also has two cylinders but both feature a power piston to implement the compression and expansion phases. Like the Beta configuration, the displacer has to overcome only the pressure losses generated during the displacement of the fluid from the hot side to the cold side of the engine, and vice versa. The power piston generates the phases of compression and expansion. The Gamma configuration is rarely used because of its modest efficiency, low specific power and high vibrations generated by the kinematics required to operate the system. In addition to these three classic configurations there is one further scheme that is a variant of Alpha, namely the double-effect Alpha configuration. This configuration derives from the Alpha with three or four interconnected pistons: the lower part of each piston acts as volume of compression and is interconnected to the expansion chamber (upper volume) of the next piston. Figure 14.5.9 shows the working principle of an engine based on a double-effect Alpha configuration. This configuration retains the simple design feature of the Alpha engine, bringing about some major improvements, namely: (i) an increase in the specific power (W/cm3) owing to the double action of the pistons, which allows the displacement of the motor to be halved: (ii) a reduction in leakages (the only seal towards the outside is the ring which allows sliding of the connecting rods of each piston); (iii) high continuity in the movements; and (iv) reduced Solar energy conversion with thermal cycles 471 Linear alternator Stirling engine Regenerator Piston Displacer Front flexure stack Mover Rear flexure stack Figure 14.5.8 Schematic representation of a Beta free-piston Stirling generator. Figure 14.5.9 Double-effect Alpha configuration. inertial effects due to the presence of three or four pistons, respectively phase-shifted by 120. or 90., which improve mechanical efficiency since the dual action works the crank in one direction while compression is directly subtracted from expansion. 14.5.5 Stirling working fluids The working fluid in a Stirling engine is employed in a closed cycle and has to satisfy the following requirements: (i) good heat transfer coefficients; (ii) low viscosity; (iii) high thermal stability; and possibly (iv) low cost (Urieli and Berchowitz, 1984). Light molecule fluids like H2 or He are considered suitable fluids for solar dish Stirling applications because they meet the majority of these requirements; however, a small caveat concerns leakages. In fact, due to the small size of their molecules, these fluids tend to escape through seals. The problem is more serious for H2 because of 472 Solar energy sciences and engineering applications its highly flammable nature. Two of the examples presented in Section 14.3 employ helium as a working fluid, namely Eurodish and Infinia engines, while Stirling Energy Systems “Sun catcher’’ uses hydrogen. 14.6 ECONOMICS AND FUTURE PERSPECTIVES This last chapter tries to give an indication of the economics of solar thermal plants and compares it with commercial technologies for power production. The selected competitive technologies are natural gas combined cycles (NGCC) and advanced supercritical pulverized (ASC) coal plants: the adoption of fossil fuel-based plants occurs because of the more certain costs and, besides renewables, they are the only kind installed. In addition to the above-mentioned reference cases, two technologies with CO2 capture will also be considered. Fossil fuel-based plants with CO2 capture are the competitive technologies of renewable for power production. This is because CO2 is seen as one of the main issues of fossil-fuelled power plants: CO2 concentration in the atmosphere amplifies global warming (some believe that it is also the main reason behind global warming) and power production accounts for 35% of world CO2 emissions. The comparison will be performed using as reference parameters the cost of electricity (COE) and the cost of CO2 avoided. The COE is calculated using International Energy Agency (IEA) models by setting the net present value (NPV) of the power plant to zero (PH3/14; PH4/33). This can be achieved by varying the plant COE until the revenues balance the cost over the whole life time of the power plant. This methodology can be applied both to fossil fuel-based plants and renewable ones. The second parameter, the cost of CO2 avoided, is defined as: Cost of CO2 avoided = (COE)inn - (COE)ref (CO2kWh-1)ref - (CO2kWh-1)inn (14.6.1) where ref is the reference technology for power production and inn is the innovative plant which can be either renewable-based or fossil fuel-based with CO2 capture. The cost of CO2 avoided represents the additional cost of electricity consumed to avoid the emission of 1 kg of CO2 into the atmosphere. Another interpretation of the cost of CO2 avoided is the value of carbon tax that makes the COE for innovative plants equal to the reference plant. The COE and cost of CO2 avoided assumed for the reference cases are summarized in Table 14.6.1 (Franco et al., 2010; Gazzani et al., 2012a, 2012b; Manzolini et al., 2012). The calculated cost of electricity was determined assuming 7500 hrs/y, which is typical of base load plants such as NGCC and ASC. Considering recent renewable energy diffusion (in Europe at least), it is difficult to predict what the operating hours of a power plant are going to be, and whether power plants with CO2 capture will be assimilated into green-energy sources or not. Results show that the COE for conventional NGCC and ASC plants is similar; moreover, the cost of CO2 avoided for the two reference cases is also pretty close and in the range of 50 a/tCO2. To give an idea about the current price of CO2 emissions Solar energy conversion with thermal cycles 473 Table 14.6.1 Summary of reference cases performances and economics (Franco et al., 2010; Gazzani et al., 2012a, 2012b; Manzolini et al., 2012). NGCC with ASC with NGCC CO2 capture ASC CO2 capture Net electric efficiency [%] 58.34 49.9 45.25 33.55 CO2 emissions [kgCO2/MWhel] 352 41 772 104 Specific investment costs [a/kWnet] 630 970 166 2556 COE [a/MWh] 54.1 69.1 54.8 85.5 Investment costs [a/MWh] 9.6 15.6 23.1 35.7 Fixed costs [a/MWh] 3.9 5.2 4.9 10.7 Consumables costs [a/MWh] 0.6 1.4 2.8 7.6 Fuel costs [a/MWh] 40.1 46.9 23.9 32.2 Cost of CO2 avoided [a/tCO2] – 48.5 – 46 trading on the market, the average value in 2012 was between 6 and 9 a/tCO2: today, CO2 capture technologies are not economically sustainable. The COE for fossil fuel-based plants can be split into four different cost centres: (i) investment costs (i.e. equipment and installation costs); (ii) fixed costs (i.e. labour and maintenance costs); (iii) consumables (i.e. water make-up and chemicals); and (iv) fuel costs. Coal-based plants are characterized by high investment costs and low fuel costs, which is the opposite for NGCC. In solar plants, almost the total cost of electricity arises from investment costs since the fuel is “free’’ and labour and maintenance have a limited impact. At current technology development rates, CSP plants have higher investment costs than fossil fuel-based plants. Moreover, the yearly operating hours of CSP are in the range of 2000-4000 hrs rather than 7500 hrs for fossil fuel-based plants. This is because the energy source is not always available (there is no solar radiation during the night) and it varies during the day even in clear sky conditions (solar radiation is much lower in the morning than at noon). Because of the limited amount of CSP installed worldwide, there is little information about the actual plant costs; even less information is available for solar tower and linear Fresnel technologies which are still under development. Another parameter affecting the cost of electricity is plant location: electricity production is almost proportional to the yearly available solar energy, which can vary from 2100 kWh/m2 in Seville (Spain) to 2600 kWh/m2 in the Mohave Desert in California (USA). Finally, plant design and storage optimal size depend on the solar energy available throughout the year. Examples of relative COE variation as a function of the site – Las Vegas (USA), direct normal irradiation (DNI) equal to 2600 kWh/m2; Seville (Spain) and Darwin (Australia), DNI equal to 2100 kWh/m2 – storage size and solar multiple are shown in Figure 14.6.1 (Astolfi et al., 2011). Observing the LCOE curves displayed in Figure 14.6.1, the best results are obtained by the Las Vegas site, which has high storage capacity: the adoption of large storage is necessary to limit defocusing during summer days. LCOE in Las Vegas is equal to 139 a/MWh, which is about 20% lower than the Seville plant where LCOE is equal to 168 a/MWh); this value is very close to the difference in terms of available 474 Solar energy sciences and engineering applications Figure 14.6.1 LCOE variation with storage size for three different solar plant location designed for SM=2 (a) and different SM and storage size in Seville site (b). solar radiation. Seville is the less attractive site because of its lower solar radiation. Finally, Darwin falls in between the other two sites (LCOE equals 155 a/MWh). It requires a smaller storage (5 heq compared to 7.7 heq for Seville and 8 heq for Las Vegas) because of its lower latitude and a more homogeneous solar distribution through the year. A second study was carried out for three different solar multiple sites (1.5, 2, 2.5) in Seville – see the right side of Figure 14.6.1. LCOE curves have a minimum for all three sizes. The optimal heat storage capacity is a compromise between higher plant cost and lower defocusing. As expected, Andasol I storage size (7.7 equivalent hours) proved to be the best of the considered SM. Solar energy conversion with thermal cycles 475 Table 14.6.2 Summary of three selected CSP plants. performances and economics for Las Vegas site: Morin (Morin et al., 2012) and Politecnico (Giostri et al., 2013) Nevada Solar OneType Novatec type Andasol Type Solar field Technology PT LFR PT Plant configuration Indirect Cycle Direct cycle Indirect Cycle Storage No No Yes (7,7 hours) Nominal net power MW 50 50 44.5 Turbine gross power MW 54.6 52.5 50.2 Thermal storage hours – – 8 Total electricity MWh/y 97818 76136 206484 production Overall efficiency % 16.05 10.2 15.6 Total mirror surface m2 235,527 289,101 509,204 Total land surface m2 704,252 593,205 1,522,577 Equivalent hours kWh/kWnom 1,956 1,522 4,666 Assumptions Politecnico Morin Politecnico Morin Politecnico Morin Land specific cost a/m2 0 7 0 7 0 7 Solar field a/m2 220 275 125.73* 150.1* 220 275 Solar field overall Ma 51.8 64.7 36.3 43.4 112.2 150.5 costs Power block specific a/kWgross 667.5 882.64a 605 800.00 667.5 882.64a cost Power block overall Ma 36.5 44.1 30.21 40.0 33.5 44.3 costs Storage costs Ma – – – – 42.8 42.8 Indirect cost & % plant 31 20 31 20 31 20 Contingencies cost Total plant costs Ma 115.7 137.5 89.3 113.9 247.0 272.5 O&M Ma 2.5 3.6 2.1 3.1 4.1 4.6 Consumables a/MWh 2.9 – 2.9 – 2.9 – Reduction % – – 42.85 45.42 – – COE a/kWh 156.3 176.1 156.3 176.1 139.0 161.4 *Calculated to achieve the same COE of the reference parabolic trough case Nevada Solar One type. A tentative economic comparison of existing linear CSP systems is summarized in Table 14.6.2 using Las Vegas as the reference site (2600 kWh/m2). An economic assessment is made for three different cases: Nevada Solar One type plant (synthetic oil as HTF, no storage), Andasol type plant (synthetic oil as HTF, thermal storage) and linear Fresnel technology developed by Novatech (DSG, no storage). For linear Fresnel, with no information available about the collector costs, the costs are calculated based on the LCOE of the Nevada Solar One type plant (neither has any storage). For all of these cases, two different sets of assumptions were made in order to determine their impacts on the overall results. The calculated COE ranges between 139.0 a/MWh and 176.1 a/MWh; as expected, these figures are higher than reference cases based on fossil fuels both with and without capture. Focusing on capital costs, solar field has the larger contribution, in particular when TES is considered. The specific investment costs for the Nevada 476 Solar energy sciences and engineering applications Figure 14.6.2 Cost of Electricity [a/MWh] for different solar field costs; reference cases with COE as function of the carbon tax is also shown. Solar One case are about 2300 a/kWnet higher than ASC and NGCC cases. This is a consequence of solar field costs as well as limited power output (50 MW): CSP suffers from a small-scale power section compared to conventional fossil-fuel plants, with drawbacks from an economic point of view. As a term of comparison, fossil fuel-based plants have a net power output between 750 and 850MW, which is more than 10 times larger than the typical CSP size, with scale-up cost advantages. With regard to Fresnel technology, the equivalent cost should be about 45–50% lower than parabolic trough. This level of cost saving might be achieved as a consequence of improved receiver and tracking systems, cheaper mirrors and structure. Moreover, the investigated LFR has a concentration ratio twice that of PT, reducing absorber-specific costs. The same analysis could also be carried out on the power tower system, however the overall results would show no change since the system has yet to show economic benefits to match PT, and it shares some negative aspects with the linear focus technology in having a small-scale power section. Turning to CSP plants, assuming technological improvements of the solar field and thermal storage system, this could bring cost reductions of between 25% and 50% of current prices. No cost variation is assumed for the power section as this is a mature technology and the only significant breakthrough can come from scaling up. In addition, the cost of electricity is reported as a function of carbon tax price (a/tCO2). The results of this analysis are summarized in Figure 14.6.2. The COE bandwidth considers the resulting cost of electricity set against the two different sets of assumptions (Morin et al., 2012; A. Giostri et al., 2013). This analysis Solar energy conversion with thermal cycles 477 assumed the same power plant efficiency and electricity production shown in Table 14.6.1. Obviously, solar field improvement can also come from a performance standpoint, not only from an economic perspective. Figure 14.6.2 shows that assuming a cost reduction of 50% moves CSP technology closer to the reference fossil fuel-based plants. As regards competitive renewable technologies, photovoltaics have recently become cheaper than CSP, thanks to a higher installed capacity globally (in the range of 100GW, which is 50 times more than CSP) and economies-of-scale effects owing to larger production volumes by manufacturers. For example, PV costs reduced from about 3000 a/kW in 2010, when they were similar to CSP plants, to 1500 a/kW in 2012. Admittedly, however, the operating hours of PV systems are also shorter since they have no storage capability. However, unlike photovoltaic and wind energy, concentrated solar power systems have a great advantage in terms of “dispatchability’’, namely the capability of decoupling electricity production from the availability of the source through thermal energy storage systems. Dispatchability will be the key factor for CSP plants in future scenarios in which renewable energy sources will gain in importance in electricity grids. 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Database Net Electricity existing Turbine production Storage plants Location Start capacity (MWh/yr) Technology HTF (h) Type Andasol -1 Spain 2008 49.9 158000 PT Diphenyl/ 7.5 2 tank Biphenyl oxide indirect Andasol -2 Spain 2009 49.9 158000 PT Diphenyl/ 7.5 2 tank Biphenyl oxide indirect Andasol -3 Spain 2010 49.9 175000 PT Diphenyl/ 7.5 2 tank Biphenyl oxide indirect Archimede Italy 2010 4.72 9200 PT Molten salts 8 2 tank direct Aste 1A Spain 2012 50 170000 PT Thermal oil 8 2 tank indirect Aste 1B Spain 2012 50 170000 PT Thermal oil 8 2 tank indirect Astexol II Spain 2012 50 170000 PT Thermal oil 8 2 tank indirect Augustin France 2012 0.25 – LFR Thermal oil 8 2 tank Fresnel 1 indirect Colorado USA 2010 2 49 PT Integrated Solar Project Extresol-1 Spain 2010 50 158000 PT Diphenyl/ 7.5 2 tank Biphenyl oxide indirect Extresol-2 Spain 2010 49.9 158000 PT Diphenyl/ 8.5 2 tank Biphenyl oxide indirect Gemasolar Spain 2011 19.9 110000 ST Molten salts 15 2 tank Thermosolar direct plant Helioenergy 1 Spain 2011 50 95000 PT Thermal oil none Helioenergy 2 Spain 2012 50 95000 PT Thermal oil none Helios I Spain 2012 50 97000 PT Thermal oil none (Helios I) (Continued) 482 Solar energy sciences and engineering applications Appendix 14.1 (Continued) Database Net Electricity existing Turbine production Storage plants Location Start capacity (MWh/yr) Technology HTF (h) Type Holaniku at USA, 2009 2 4030 PT Xceltherm-600 2 non Keahole Haweii fornito Point Ibersol Spain 2009 50 103000 PT Diphenyl/ NA Ciudad Real Biphenyl oxide (Puertollano) ISCC Hassi Algeria 2011 25 NA PT Thermal oil none R’mel (ISCC Hassi R’mel) ISCC Kuraymat Egypt 2011 20 3400 PT Therminol none (ISCC Kuraymat) VP-1 ISCC Morocco Morocco 2010 20 NA PT Thermal oil none (ISCC Morocco) Jülich Germany 2008 2.5 NA ST Air 1.5 Ceramic Solar Tower heat sink Kimberlina USA, 2008 5 NA LFR Water none Solar Thermal California Power Plant La Dehesa Spain 2011 49.9 175000 PT Diphenyl/ 7.5 2-tank Biphenyl oxide indirect La Florida Spain 2010 50 175000 PT Diphenyl/ 7.5 2-tank Biphenyl oxide indirect La Risca Spain 2009 50 105200 PT Diphenyl/ none (Alvarado I) Biphenyl oxide Lebrija 1 (LE-1) Spain 2011 50 120000 PT TherminolVP1 none Majadas I Spain 2010 50 104500 PT Diphenyl/ NA Biphenyl oxide Manchasol-1 Spain 2011 49.9 158000 PT Diphenyl/ 7.5 2-tank (MS-1) Biphenyl oxide indirect Manchasol-2 Spain 2011 50 158000 PT Diphenyl/ 7.5 2-tank (MS-2) Biphenyl oxide indirect Maricopa USA, 2010 1.5 NA Dish/ non fornito none Solar Project Arizona Engine (Maricopa) Martin Next USA, 2010 75 155000 PT Thermal oil none Generation Florida Solar Energy Center (MNGSEC) Morón Spain 2012 50 100000 PT Thermal oil none Nevada USA, 2007 72 134000 PT Dowtherm A 0.5 0.5 hours Solar One Nevada full-load (NSO) storage Palma del Spain 2011 50 114500 PT Diphenyl/ none Río I Biphenyl oxide Palma del Spain 2010 50 114500 PT Diphenyl/ NA Río II Biphenyl oxide Planta Solar Spain 2011 11 23400 ST Water 1 other 10 (PS10) Planta Solar Spain 2009 20 48000 ST Water 1 other 20 (PS20) (Continued) Solar energy conversion with thermal cycles 483 Appendix 14.1 (Continued) Database Net Electricity existing Turbine production Storage plants Location Start capacity (MWh/yr) Technology HTF (h) Type Puerto Errado Spain 2009 1,4 2000 LFR Water NA Single-tank 1 Thermosolar gross thermocline Power Plant (PE1) Puerto Errado Spain 2012 30 49000 LFR Water 0.5 Single-tank 2 Thermosolar thermocline Power Plant (PE2) Saguaro USA, 2006 1 2000 PT Xceltherm 600 NA Power Arizona (solar field); Plant n-pentane (ORC working fluid) Sierra USA, 2009 5 NA ST Water none SunTower California (Sierra) Solaben 3 Spain 2012 50 100000 PT Thermal oil none Solacor 1 Spain 2012 50 100000 PT Thermal oil none Solacor 2 Spain 2012 50 100000 PT Thermal oil none Solar Electric USA, 1984 13.8 NA PT Therminol 3 2-tank Generating California direct Station I (SEGS I) Solar Electric USA, 1985 30 NA PT Therminol NA Generating California Station II (SEGS II) Solar Electric USA, 1985 30 NA PT Therminol NA Generating California Station III (SEGS III) Solar Electric USA, 1989 30 NA PT Therminol NA Generating California Station IV (SEGS IV) Solar Electric USA, 1989 30 NA PT Therminol NA Generating California StationV (SEGS V) Solar Electric USA, 1989 30 NA PT Therminol NA Generating California Station VI (SEGS VI) Solar Electric USA, 1989 30 NA PT Therminol NA Generating California Station VII (SEGS VII) Solar Electric USA, 1989 80 NA PT Therminol NA Generating California Station VIII (SEGS VIII) (Continued) 484 Solar energy sciences and engineering applications Appendix 14.1 (Continued) Database Net Electricity existing Turbine production Storage plants Location Start capacity (MWh/yr) Technology HTF (h) Type Solar Electric USA, California 1990 80 NA PT Therminol NA Generating Station IX (SEGS IX) Solnova 1 Spain 2009 50 113520 PT Thermal oil none Solnova 3 Spain 2009 50 113520 PT Thermal oil none Solnova 4 Spain 2009 50 113520 PT Thermal oil none Termesol 50 Spain 2011 49.9 175000 PT Diphenyl/ 7.5 2-tank (Valle 2) Biphenyl oxide indirect Thai Solar Thailand 2012 5 8000 PT Water/Steam none Energy 1 (TSE1) http://www.nrel.gov/csp/solarpaces/operational.cfm PT=Parabolic Trough, LFR=Linear Fresnel, ST=Solar tower, DS=Dish Stirling Chapter 15 Solar hybrid air-conditioning design for buildings in hot and humid climates Kwong-Fai Fong Division of Building Science and Technology, College of Science and Engineering, City University of Hong Kong, Hong Kong, China 15.1 INTRODUCTION In hot and humid cities, space conditioning and refrigeration generally consume close to half of energy use in office and residential buildings. In conventional air-conditioning provision, electricity-driven equipment is commonly applied. The projection of energy supply finds that fossil fuels may still play a very significant role in 2035 if there is no abrupt change of energy use (IEA, 2010). The Intergovernmental Panel on Climate Change (IPCC) has recently issued a special report about the role of renewable energy sources in climate change mitigation (IPCC, 2011). It indicates renewable energy, including solar energy, has a large potential to mitigate greenhouse gas emissions. But how can renewable energy be widely used in the building sector? In 1977, the International Energy Agency (IEA) set up the Solar Heating and Cooling Programme to promote the technology development and standardization of solar heating and cooling since (IEA, 2012). In recent decades, solar air-conditioning has been broadly advocated (Altener, 2002; Eicker, 2009), aiming mainly at small and medium applications in buildings. Although solar air-conditioning has been promoted, most of the demonstration projects are found in temperate and cold climate regions where the emphasis is placed on heating rather than cooling. The demand for cooling systems is not a priority and the holistic system design for both cooling and dehumidification is seldom touched. In fact, the market for solar-thermal collectors is growing rapidly around the world. Mainland China has the largest total capacity of evacuated tube and flat-plate collectors in operation, accounting for almost 60% of the world market (IEA, 2011). The capital cost of solar-thermal collectors has been reduced by about 20% for each doubling of installed capacity (IEA, 2007). Therefore, the application potential of solar-thermal energy becomes economically and technically viable from such blooming production. With continuous population and economic growth, the strategic design of solar air-conditioning should secure the increasing energy demand and attain low carbon urbanization in the long run. General design guidelines and demonstration projects have been established to promote wider use of solar air-conditioning (Delorme et al., 2004; Wang et al., 2009). In addition, basic design calculations and discussions have been provided for various common solar refrigeration and air-conditioning systems (Eicker, 2003; Henning, 2004). In recent years, solar energy systems have been designed in building systems through solar heating and cooling, as well as different integrated methods (Wang and Zhai, 486 Solar energy sciences and engineering applications 2010). Solar-thermal desiccant cooling systems with appropriate auxiliary heating can be operated during both daytime and night-time (Enteria et al., 2009). New system design and material for solar desiccant cooling was proposed to enhance its energy performance (Ge et al., 2010). The design and operation of a solar air-conditioning system using parabolic solar collectors and double-effect absorption chiller was evaluated for the application in a conditioned space (Qu et al., 2010). By combining an ejector cooling system and an inverter-type heat pump, a prototype of a hybrid solar cooling and heating system was designed and constructed (Huang et al., 2010). The energy and economic feasibility of a solar air-conditioning system for buildings in temperate and Mediterranean climates has also been evaluated (Calise, 2010). In this chapter, Section 15.2 discusses the possible design approaches and features of solar air-conditioning. Section 15.3 presents the cooling and energy performances of the various solar air-conditioning systems, including the principal systems and different hybrid designs. Section 15.4 demonstrates the application potential of solar hybrid air-conditioning (SHAC) in the various hot and humid cities in Southeast Asia. In conclusion, Section 15.5 looks at future development. 15.2 DESIGN APPROACHES OF SOLAR AIR-CONDITIONING In the context of solar air-conditioning, various types of solar collectors, refrigeration and air-conditioning cycles are interlinked or hybridized so that the indoor design conditions of the buildings can be fulfilled throughout the year. Solar air-conditioning can be developed in the following four approaches according to system complexity, as consolidated in Figure 15.2.1. In any one of the design approaches, electricity is still required to drive the parasitic equipment – such as pumps, fans and cooling towers – of the various solar air-conditioning systems. In the study of solar air-conditioning systems, it is necessary to take a holistic view of the energy consumption of all the equipment involved. 15.2.1 The solar-electric approach Photovoltaic (PV) panels are used to generate electricity, which can be in turn be used to drive conventional vapour-compression refrigeration. The compression chiller driven by direct current can also be considered in order to prevent losses from current conversion. Both roof-mounted and building-integrated strategies can be applied for the installation of PV panels. In this approach, an auxiliary electricity supply, generally from the power grid, is required in case of electricity deficit to the solar-electric airconditioning system. Although it is relatively straightforward to apply the conventional monocrystalline and polycrystalline PV panels for driving the compression chiller, their environmental impacts from production and disposal have been the subject of study in recent years (Gottessfeld and Cherry, 2011; EC, 2011), apart from the new thin film PV which is technologically mature enough to supersede the crystalline types. 15.2.2 The solar-thermal approach Solar-thermal collectors, like flat-plate collectors, evacuated tubes or parabolic concentrators, are applied to generate heat for the thermally driven refrigeration or Solar hybrid air-conditioning design 487 Figure 15.2.1 Feasible design approaches of solar air-conditioning. air-conditioning cycle. Similar to the solar-electric approach, both roof-mounted and the building-integrated strategies can be considered for the collector installation. In this approach, auxiliary heating, typically using fuel gas, is needed in case of thermal deficit in driving the solar-thermal air-conditioning system. The basic system designs include solar absorption refrigeration, solar adsorption refrigeration and desiccant cooling, each of which is described below. 15.2.2.1 Solar absorption refrigeration Absorption chillers have been developed now for almost a century with various working pairs of absorbent and refrigerant, as well as different system configurations, including single-effect, double-effect and even triple-effect. For solar energy application, single-effect absorption refrigeration is the most popular due to its relatively low driving temperature. Figure 15.2.2 illustrates the solar absorption refrigeration system. Solar-thermal gain is firstly collected in the hot water storage tank by the hot water pump. To drive the absorption chiller, the regenerative water pump feeds in the hot water from the storage tank. If the driving temperature is not enough, an auxiliary heater can be used. Chilled water from the chiller is delivered to the air handling unit so that the conditioned supply of air can be provided to the building zone accordingly. As the required outdoor air flow rate is less than that of the supply air, return air from the building zone is drawn back to the air handling unit for the sake of air balancing. 488 Solar energy sciences and engineering applications Figure 15.2.2 Solar absorption refrigeration system for space conditioning. (Abbreviation:A: absorber; C: condenser; E: evaporator; EA: exhaust air;G: generator;HW:hot water;OA: outdoor air; RA: room air; and SA: supply air). When the absorption chiller is operating, the cooling tower removes heat from both the absorber and the condenser in series. The common working pairs are lithium bromide/ water and water/ammonia. For a typical single-effect LiBr-H2Oabsorption chiller the driving temperature is generally between 70.C and 90.C, which can be primarily achieved by solar-thermal gain, together with the assistance of auxiliary heating. 15.2.2.2 Solar adsorption refrigeration Adsorption chillers have a relatively low driving temperature by using an appropriate working pair of adsorbent and refrigerant. The schematic diagram of a solar adsorption refrigeration system is presented in Figure 15.2.3. Generally it is similar to that of solar absorption refrigeration, except that an adsorption chiller is used. The economical adsorption pair is silica gel and water, where silica gel is the adsorbent and water the refrigerant. Other effective adsorption pairs include zeolite/water and activated carbon/ammonia. Compared to the absorption cycle, its driving temperature can be down to about 60.C. Typically, there are two chambers containing adsorbent in the adsorption chiller. While one chamber is used for adsorption, the other is used for desorption. Their roles are interchanged according to the period of adsorption/desorption process (usually 6 minutes for the silica gel/water pair). A pseudo-continuous operation is therefore formed in the refrigeration cycle. Cooling water and regenerative water are fed into the adsorption chamber and the desorption chamber respectively, and these two water circuits are alternatively changed according to the role of the chamber. 15.2.2.3 Solar desiccant cooling A solar desiccant cooling system can directly provide conditioned air to the building zone, as shown in Figure 15.2.4. The core part of this system is the desiccant component, and both solid and liquid sorbents, such as silica gel and lithium chloride respectively, can be applied. Although desiccant cooling using liquid sorbent has the Solar hybrid air-conditioning design 489 Figure 15.2.3 Solar adsorption refrigeration system for space conditioning (New abbreviation: A: adsorber; and D: desorber). Figure 15.2.4 Solar desiccant cooling system for space conditioning. merit of thermal storage, there are health and safety concerns to the building occupants. Since the supply air would be in direct contact with the slightly corrosive liquid sorbent, there is a risk of carry-over to the conditioned space. Desiccant cooling using solid sorbent, however, is stable in the processes of adsorption and desorption, so it is more suitable for direct application in the supply air stream. In this regard, a solid desiccant wheel is the major component of desiccant cooling. The other components include a thermal wheel, direct evaporative coolers, solar collectors, a hot water storage tank and an auxiliary heater. The thermal wheel is used to remove the sensible heat of the process air after passing the desiccant wheel. The evaporative cooler at the supply air stream is used to cool down the process air to become supply air, which still has a sufficiently low humidity ratio for handling the space latent load. The evaporative cooler at the exhaust air stream is used to cool down the room air for better sensible heat recovery at the thermal wheel. 490 Solar energy sciences and engineering applications Solar desiccant cooling systems can enhance indoor air quality owing to the provision of full outdoor air. However, primary energy consumption is higher than in other solar air-conditioning systems, such as solar absorption or adsorption refrigeration systems and conventional vapour compression refrigeration systems, in which the return air scheme can be used at the air handling unit. Adoption of the return air design for solar desiccant cooling depends heavily on the climatic conditions, particularly solar irradiation, air temperature and humidity during summer. In hot and humid climates the cooling performance of solar desiccant cooling systems using outdoor air is better than that using return air, since a larger amount of conditioned air is involved (Fong and Chow, 2007). Auxiliary heating is therefore involved to achieve a satisfactory cooling performance. 15.2.3 A hybrid approach to system design In general, the building cooling load can be divided into zone cooling load (mainly sensible load) and ventilation load (mainly latent load). If the refrigeration cycle is used to handle the former, and the hygroscopic nature of desiccant cooling used to tackle the latter, the cooling load can be effectively handled in such a load-sharing approach. Due to the separate handling of the cooling load, individual controls for zone temperature and zone relatively humidity become more practical. The hybrid approach of system design can be the basic SHAC or SHAC enhanced by high temperature cooling. The details of these two alternatives are described as follows. 15.2.3.1 Principal SHAC Figure 15.2.5 presents the schematic diagram of the principal SHAC system fully driven by solar-thermal energy. In the configuration of the desiccant cooling unit the two evaporative coolers can be omitted and a cooling coil adopted at the supply air stream instead. In this case the absorption/adsorption chiller generates chilled water to the cooling coil located at the downstream of the thermal wheel. As both the heat-driven chiller and desiccant cooling are involved, there would be two sets of regenerative Figure 15.2.5 SHAC system using absorption chiller (New abbreviation: DW: desiccant wheel; EAF: exhaust air fan;OAF: outdoor air fan; SAF: supply air fan; andTW: thermal wheel). Solar hybrid air-conditioning design 491 water pumps and auxiliary heaters, one for the heat-driven chiller and the other for the desiccant cooling cycle. 15.2.3.2 SHAC enhanced by high temperature cooling In this approach the energy performance of SHAC is further facilitated by using the appropriate strategies of high temperature cooling, such as radiant ceiling cooling or a specific indoor ventilation method. Since this allows a higher chilled water supply temperature from the heat-driven refrigeration system, a better solar fraction of the solar energy system and a higher coefficient of performance of the refrigeration cycle is achieved. 15.2.4 A hybrid approach to energy sources and system design 15.2.4.1 SHAC with dual solar energy sources In this design, photovoltaic/thermal (PV/T) panels are utilized, and cogeneration of electricity and heat can happen. The solar electric gain can be used to drive the compression chiller for the zone cooling load, while the solar-thermal gain can regenerate the desiccant cooling for the ventilation load (Fong et al., 2010b), as depicted in Figure 15.2.6. A power regulator is used to allow the power to come from either the PV panels or the regional grid. In this case, both auxiliary electricity supply and auxiliary heating are involved. 15.2.4.2 SHAC system with separate thermal and electrical energy sources An alternative of this hybrid approach is to make use of the solar-thermal gain for fully regenerating the desiccant cooling, using electricity from the grid for the conventional compression chiller. Figure 15.2.7 illustrates the SHAC system energized by the separate thermal and electrical energy sources. Figure 15.2.6 SHAC system with both electricity and heat generation. 492 Solar energy sciences and engineering applications Figure 15.2.7 SHAC system with separate thermal and electrical energy sources. 15.3 PERFORMANCE EVALUATION OF VARIOUS SOLAR AIR-CONDITIONING SYSTEMS With the knowledge of different system configurations of solar air-conditioning approaches, their cooling and energy performances for buildings in hot and humid climates are of great interest. The operating energy would be determined for the various solar air-conditioning systems against conventional types, since energy saving is the primary concern of any newly proposed air-conditioning (AC) system. As a start to investigating solar air-conditioning systems in hot and humid climates, the study area is subtropical Hong Kong (22.32.N, 114.17.E), using its weather data of the typical meteorological year (Chan et al., 2006). In this section, we look at dynamic simulation conducted for a typical office building zone (except Section 15.3.5 which considers premises with high latent load), in which the ratio of the installed collector area and the conditioned space is 1:2. A typical office has an area of 196m2 and an occupant density of 8m2/person. Daily occupancy is 10 hours between 8:00 a.m. to 6:00 p.m. The outdoor air amount is based on 0.01m3/s/person. The lighting heat gain is 17W/m2 (with 70% radiative) and the heat gain of office equipment is 230 W/person. The fenestration to wall ratio is 0.5. Based on indoor design conditions of 25.5.C and 60% in relative humidity, the estimated design zone cooling load and ventilation load are 20kW and 9 kW respectively. The total net area of the solar collectors is 100m2 and the capacity of the hot water storage tank is 5m3. The system simulation model includes the appropriate control components, so as to realize the dynamic interaction between the AC system and the building zone under the changing loading and climatic conditions throughout a year. The solar airconditioning and the conventional AC systems are built using the validated component models of the plant simulation program TRNSYS (SEL, 2006) and its associated component library TESS (TESS, 2004). Meanwhile the dynamic component models of the absorption chiller, the adsorption chiller and the desiccant wheel are specifically constructed according to those validated by Kim and Infante Ferreira (2008), Cho and Kim (1992) and Zhang et al. (2003) respectively. Dynamic simulation is carried out to Solar hybrid air-conditioning design 493 determine the annual total energy consumption of system operation, as well as other kinds of performance indicators, such as solar fraction (SF) and coefficient of performance (COP). In order to have more accurate evaluation of responses and operation of the solar air-conditioning system arising from changing boundary conditions, the simulation time step is set at 6 minutes, so that a total of 87,600 simulations are run for a year-round study. As electrical energy and thermal energy are involved in the various solar air-conditioning systems, they would be converted to primary energy consumption (Ep) for comparison purposes. The conversion of electrical energy to primary energy takes into account the local fuel mix. 15.3.1 Principal solar-thermal air-conditioning systems The basic design of solar air-conditioning is in accordance with the solar-thermal approach, including the solar absorption refrigeration system, the solar adsorption refrigeration system and the solar desiccant cooling system, as described in Section 15.2.2. From the previous study (Fong et al., 2010a), the cooling and energy performances of these solar-thermal air-conditioning systems for office building application are consolidated in Table 15.3.1. Their results are also contrasted with those of the conventional AC systems using air-cooled vapour compression chillers (ACVCC) and water-cooled vapour compression chillers (WCVCC). Although the common solarthermal collectors include the flat-plate collectors, evacuated tubes and parabolic concentrators, the last type is the least effective, as found in the aforementioned study. In principle, parabolic concentrators can harness thermal energy at higher temperature; however, in a fixed space for collector accommodation and the changing incidence of solar irradiation, they have the lowest overall solar-thermal gain in a year. As such, only the flat-plate collectors and the evacuated tubes are involved in the study. Compared to the conventional air-conditioning system in the return air scheme, the solar absorption refrigeration system with evacuated tubes has the best energy performance, with 35.1% and 33.6% less year-round primary energy consumption than the ACVCC and the WCVCC respectively. The solar adsorption refrigeration system, which has about 20% more energy consumption, does not have better energy performance than the conventional system here. Nevertheless, its application potential still exists in the hybrid design of solar air-conditioning systems which are discussed in Section 15.3.3. For the same kind of solar air-conditioning system, evacuated tubes can have better energy performance than the flat-plate collectors in the hot and humid climate from a year-round perspective. Solar desiccant cooling has relatively high year-round primary energy consumption, since it has to handle the extra ventilation load due to the inherent nature of full outdoor air design. Accordingly, the total cooling capacity of desiccant cooling system is much higher than that of the other systems. In addition, the parasitic energy consumption is comparatively high, particularly the supply air fan and exhaust air fan. Compared to conventional AC systems in the outdoor air scheme, solar desiccant cooling systems with evacuated tubes can have 4.9% and 1.5% less primary energy consumption than ACVCC and WCVCC respectively. The feature of solar desiccant cooling in effect more than satisfies the required cooling load. It is able to supply the outdoor air amount far above the minimum requirement of the functional area, 494 Solar energy sciences and engineering applications Table 15.3.1 Summary of cooling and energy performances of principal solar-thermal air-conditioning systems. Year-round Energy saving vs. total Ep per corresponding Type of solar Year-round Year-round AC area ACVCC/ Type of system collectors averaged SF averaged COP (kWh/m2) WCVCC Solar absorption refrigeration (RA) Flat-plate collectors 0.497 0.769 371 4.4%/2.2% Evacuated tubes 0.818 0.763 252 35.1%/33.6% Solar adsorption refrigeration (RA) Flat-plate collectors 0.313 0.435 657 -69.0%/-72.9% Evacuated tubes 0.577 0.437 478 -23.0%/-25.9% Solar desiccant cooling (OA) Flat-plate collectors 0.336 1.066 762 -11.0%/-14.8% Evacuated tubes 0.552 1.059 653 4.9%/1.5% ACVCC (RA) NA NA 2.859 389 NA WCVCC (RA) NA NA 3.195 380 NA ACVCC (OA) NA NA 2.802 687 NA WCVCC (OA) NA NA 3.177 664 NA Remarks: 1. RA refers to the return air scheme, in which a conventional air handling unit is applied with return air and outdoor air mixed together to form the supply air for the building zone. 2. OA refers to the outdoor air scheme, in which the air handling unit takes the full outdoor air to form the supply air for the building zone. 3. NA means not applicable. and this merit can guarantee good indoor air quality and ventilation effectiveness. Therefore the application potential of solar desiccant cooling remains valid. 15.3.2 SHAC with load sharing The SHAC system (i.e. the principal SHAC) with load sharing has been introduced in Section 15.2.3.1. In the study by Fong et al. (2010b) the absorption chiller is designed to handle the zone cooling load of the office building, while desiccant cooling tackles the ventilation load. Solar-thermal gain is used to drive both the related refrigeration cycle and desiccant unit. The performance results also cover the year-round averaged zone temperature (Tz) and zone relative humidity (RHz), the coefficient of performance of chiller (COPch) and the coefficient of performance of desiccant cooling (COPdc), as summarized in Table 15.3.2. As described in Section 15.3.1, evacuated tubes are more effective than flat-plate collectors in hot and humid climates, so only the former type is involved here. Compared to the year-round primary energy consumption of conventional ACVCCs and WCVCCs shown in Table 15.3.1, the SHAC-Ab system with load sharing clearly gives substantial savings of 36.8% and 35.3% respectively. Although the primary energy consumption of the SHAC-Ab system is only 2.4% lower than that of solar absorption refrigeration shown in Table 15.3.1, the SHAC-Ab system has tight control of the design zone temperature and relative humidity. While the SHAC-Ad Solar hybrid air-conditioning design 495 Table 15.3.2 Summary of cooling and energy performances of SHAC with load sharing. Year-round Year-round Year-round Energy saving vs. averaged averaged total Ep per ACVCC/ Type of solar Tz(.C)/ Year-round COPch AC area WCVCC Type of system collectors RHz (%) averaged SF COPdc (kWh/m2) in Table 1 SHAC with absorption chiller (SHAC-Ab) Evacuated tubes 24.9/58.3 0.804 0.779/0.904 246 36.8%/35.3% SHAC with adsorption chiller (SHAC-Ad) Evacuated tubes 24.9/58.4 0.590 0.460/0.919 445 -14.5%/-17.2% system does not have primary energy-saving potential, it does have slightly better energy performance than solar adsorption refrigeration, as shown in Table 15.3.1. 15.3.3 SHAc with radiant cooling The SHAC with radiant cooling is one of the hybrid systems enhanced by high temperature cooling, as discussed in Section 15.2.3.2. Although solar energy is able to power up heat-driven refrigeration, its contribution is quite limited because of the conventionally low chilled water supply temperature at around 6.C. If this temperature could be raised, it would enhance the solar fraction and the solar refrigeration would rely less on auxiliary heating. In the SHAC with radiant cooling, 15.C to 18.C of chilled water can be supplied from the absorption/adsorption chiller to the indoor radiant ceilings and the cooling coil of the desiccant cooling unit. The radiant ceilings would handle the zone sensible load, while the desiccant cooling tackles the zone latent load and the ventilation load. Radiant ceilings apply both the means of radiation and convection for cooling purposes, and their types include chilled panels, passive chilled beams and active chilled beams. The schematic diagram of the SHAC with the chilled panels and the passive chilled beams is shown in Figure 15.3.1, while that for the active chilled beams appears in Figure 15.3.2. The latter SHAC demands additional fans to maintain the effective induction of room air, so as to facilitate indoor convective heat transfer. As the cooling coil is used at the supply air stream, the two evaporative coolers of the original desiccant cooling unit become obsolete. In previous studies (Fong et al., 2010c; 2011a), the three kinds of radiant ceilings are associated with the SHAC-Ab/SHAC-Ad using evacuated tubes, the results for which are presented in Table 15.3.3. As can be seen from Table 15.3.3, whether for the SHAC-Ab or the SHAC-Ad system, both passive and active chilled beams provide satisfactory zone temperatures and relative humidity. However, the chilled panels cannot be adopted, since its average Tz is up to 29.0.C, thus depriving thermal comfort for both types of SHAC systems. SHAC systems with passive or active chilled beams have definite energy saving potential compared to conventional AC systems. The yearly Ep of the SHAC-Ab system using passive 496 Solar energy sciences and engineering applications Figure 15.3.1 SHAC using adsorption chiller for radiant cooling with chilled panels or passive chilled beams. Figure 15.3.2 SHAC using adsorption chiller for radiant cooling with active chilled beams (New abbreviation: RAF: return air fan). Table 15.3.3 Summary of cooling and energy performances of SHAC systems with radiant cooling. Year-round Year-round Energy saving averaged Year-round total Ep per vs. ACVCC/ Type of chilled Tz(.C)/ Year-round averaged AC area WCVCC Type of system collectors RHz (%) averaged SF COPch (kWh/m2) in Table 1 SHAC-Ab Chilled panels 29.0/42.8 0.782 0.840 209 46.2%/45.0% Passive chilled beams 25.4/52.2 0.882 0.830 137 64.7%/63.9% Active chilled beams 25.1/52.2 0.828 0.823 191 50.8%/49.6% SHAC-Ad Chilled panels 29.0/43 0.614 0.564 365 6.1%/3.9% Passive chilled beams 25.4/53 0.754 0.558 228 41.2%/39.9% Active chilled beams 25.0/53 0.689 0.548 311 20.0%/18.1% Solar hybrid air-conditioning design 497 chilled beams are 64.7% and 63.9% less than that of ACVCC and WCVCC respectively, while those of the SHAC-Ad system are 41.2% and 39.9% less respectively. On the other hand, the yearly Ep of the SHAC-Ab system using active chilled beams are 50.8% and 49.6% less than the two conventional systems, while those of the SHACAd system are 20.0% and 18.1% less. It has been found that passive chilled beams have higher SF and lower yearly Ep compared to active ones. This is because the better COPch of absorption/adsorption chillers results in less frequent heat demand for generation/ desorption from the solar-thermal gain. On the other hand, active chilled beams require substantial energy demand from the additional supply air and return air fans. In this hybrid design it is clear that the adsorption chiller also has an essential role in overall energy merit. By adopting an appropriate high temperature cooling approach, the feature of low driving temperature of adsorption chillers can be effective. From this study it can be seen that using passive chilled beams is the best choice for SHAC systems working in hot and humid regions. Passive chilled beams also have other merits, such as silent operation and free from drafts, making this option more attractive. Of course, prevention of condensation is of primary importance during humid weather. The involvement of desiccant cooling in SHAC can control indoor humidity effectively. The problem of infiltration of the building envelope can be avoided by use of good-quality building materials and workmanship, together with positive air pressurization in the building zone. 15.3.4 SHAC coordinated with new indoor ventilation strategies In the conventional design of indoor air distribution of supply air, mixing ventilation (MV) is used in order to have homogenous air conditions within the entire building zone. The supply air temperature of MV is generally around 15.C. If the supply air temperature is raised without sacrificing thermal comfort, this can help to enhance overall energy performance. Therefore displacement ventilation (DV), which allows a supply air temperature of 19.C for office use (Lin et al., 2005), has been promoted. This provides a useful strategy for high temperature cooling for solar air-conditioning. In this sense, the supply air flow rate of DV can be maintained or even reduced compared to that of MV, but a higher return/exhaust air temperature results. This can also reduce ventilation load, and hence the cooling capacity of the entire air-conditioning system. As such, SHAC combined with appropriate indoor ventilation strategies gives another alternative, as described in Section 15.2.3.2. In this hybrid design, either the absorption or adsorption chiller can be paired with the desiccant cooling unit for the return air scheme. As the latent cooling load of the building zone can be handled by the desiccant cycle, the problem of insufficient latent capacity due to the higher supply air temperature of DV can be solved. Figure 15.3.3 depicts the configuration of SHAC coordinated with DV. In DV, air is supplied to the building zone at floor level and exhausted at ceiling level. A temperature gradient, and subsequently a humidity gradient, is developed along the zone height, maintaining thermal comfort mainly within the occupied zone, regardless of the level above it. A common hot water storage tank is used to provide the driving heat for both the desiccant cycle and the heat-driven chiller, but separate auxiliary heaters can be added. The results of cooling and energy performances of SHAC 498 Solar energy sciences and engineering applications Figure 15.3.3 SHAC using absorption chiller for displacement ventilation. Table 15.3.4 Summary of cooling and energy performances of SHAC and conventional systems for DV and MV. Year-round Year-round Type of averaged Year-round total Ep per Energy saving ventilation Tz(.C)/ Year-round averaged AC area vs.WCVCC Type of system collectors RHz (%) averaged SF COPch/COPdc (kWh/m2) using MV SHAC-Ab DV 25.1/59.1 0.905 0.809/0.845 192 49.5% MV 24.9/58.3 0.804 0.779/0.904 246 35.3% SHAC-Ad DV 25.0/59.3 0.741 0.475/0.880 310 18.3% MV 24.9/58.4 0.590 0.460/0.919 445 -17.2% WCVCC DV 24.4/70.3 NA 3.556/NA 273 NA WCVCC MV 24.8/58.9 NA 3.195/NA 380 NA Remarks:NA means not applicable. and conventional AC systems, as shown by Fong et al. (2011b), are consolidated in Table 15.3.4. Table 15.3.4 summarizes the year-round performances of the different systems. All of them can maintain satisfactory indoor conditions of averaged Tz and RHz, except the WCVCC for DV, which has a RHz of 70.3% due to the relatively high supply air humidity ratio. When compared to the conventional WCVCC for MV, the SHAC systems are technically feasible, with a primary energy saving of 49.5% for SHAC-Ab and 18.3% for SHAC-Ad. In the same ventilation strategy the SHAC-Ab has a primary energy saving of 29.7% against the conventional AC system for DV, and 35.3% against that for MV. Even the SHAC-Ab for MV has an energy saving of 9.9% against the conventional system for DV. This really demonstrates the effectiveness of the hybrid design of solar air-conditioning systems using absorption chillers. When compared toMVcounterparts, the adoption of DV can reduce total primary energy consumption from 246 kW/m2 to 192 kW/m2 (a 21.9% drop) in SHAC-Ab; and Solar hybrid air-conditioning design 499 from 445 kW/m2 to 310 kW/m2 (a 30.3% drop) in SHAC-Ad. A higher percentage reduction in primary energy consumption can be achieved by the hybrid system with the adsorption chiller; this is because there is a more significant percentage rise of SF in the adsorption chiller, and hence the decrease in the driving energy is more substantial. Recently, a novel indoor ventilation strategy, called stratum ventilation (SV), has been advocated (Lin et al., 2009). Stratum ventilation uses an even higher supply temperature than displacement ventilation, and the energy saving potential of SHAC can be further advanced. This will undoubtedly enhance solar energy deployment in air-conditioning for buildings. 15.3.5 SHAC for premises with high latent load In conventional AC design, a cooling coil is used to conduct both the cooling and dehumidification processes for the supply air. This is suitable for typical offices and residential units, where the zone sensible load is more significant in the total cooling load. However, some commercial premises have a high zone latent load with which conventional AC provision cannot cope effectively. Such commercial premises are common and include restaurants, indoor food markets and entrance lobbies. The latent heat gain of these building zones comes from humid fresh air, the building’s occupants and indoor services such as hot food, spas and water ponds. Additional latent heat gain can be caused by excessive infiltration and frequent opening to the outdoors. Because of the high latent load for this kind of building zone, substantial subcooling followed by reheating of the supply air is needed in conventional AC design, causing high energy requirements. In addition, it is common for these AC systems to suffer over-cooling problems in these premises. As AC equipment is designed to suit the latent load, so its sensible cooling capacity naturally becomes over-provided. If there is no reheat provision, and no simultaneous temperature and humidity control, this can cause thermal comfort problems. As a result of this over-cooling potential, it is common for people to wear jackets or additional clothing, thus defeating the primary objective of air-conditioning: i.e. thermal comfort. In addition, there is risk of condensation at the supply air grilles because of low supply air temperature. To alleviate the high energy demand and thermal discomfort problem associated with conventional AC design, SHAC with desiccant cooling can fit the purpose; the appropriate system for premises with high latent load is illustrated in Figure 15.2.7 of Section 15.2.4.2. Solar-thermal energy is primarily used for desiccant cooling, and electricity from the power grid for the VCC, so this solar hybrid design is represented by SHAC-VCC. In a previous study carried out by Fong et al. (2011c), dynamic simulation was used for energy evaluation of a Chinese restaurant in which the ratio of the installed collector area and the conditioned space was again 1:2. The restaurant used in the stuady – typical of premises with a high latent load in subtropical Hong Kong – had an area of 196m2 and an occupant density of 1m2/person. The daily occupancy schedule covered 17 hours between 6:00 a.m. and 11:00 p.m. The design outdoor air amount was determined at 0.01m3/s/person. The lighting heat gain was 20W/m2 (with 70% radiative). The other sensible and latent heat gains were 1.23kW (with 50% radiative) and 1.77kW respectively. The total net area of the evacuated tubes was 100m2 and 500 Solar energy sciences and engineering applications the size of the hot water storage tank was 5m3. Based on the design indoor conditions of 22.C and 60%RH, the estimated design zone sensible and latent loads came out at 19kW and 13kW respectively, reflecting the relatively high latent component. The performance results of the SHAC-VCC and conventional systems for the restaurant are presented in Table 15.3.5. For the design RH of 60% in Table 15.3.1, the total primary energy consumption of the SHAC is lower than that of the conventional system by 49.5%. The huge difference is due to the substantial sub-cooling and reheating of the supply air, as well as the large supply air flow rate. It is also apparent that the SHAC-VCC can offer Tz without overcooling and close to the design value of 22.C. In the SHAC-VCC, the zone thermostat and the zone humidistat can control the supply air cooling coil (mainly for handling the sensible cooling load) and the heating coil (mainly for the latent load) separately, so the zone temperature and humidity can be maintained more steadily throughout different loading and climatic conditions in the course of a year. However, this is not the case in the conventional AC system. Although zone thermostat and zone humidistat are provided, they are used to control the one, and only one, cooling coil, which handles both the sensible and latent load simultaneously. As such, the supply air temperature, and hence the zone temperature, tends to be sub-cooled very low in order to fulfil the design humidity. Thus, the independent humidity and temperature controls are not as effective as that of the SHAC. Table 15.3.5 also shows that the annually averaged COPch of the SHAC is better than that of the conventional system by 5%. The solar fraction of the SHAC is about 0.3, indicating that auxiliary heating has a role in supporting the year-round operation. In order to prove better performance by the SHAC system, a scenario of a higher indoor RH of 70% is involved, since this may be favourable to the conventional AC system. To evaluate this, the minimum dewpoint temperature is decreased to 10.C and the supply air temperature is lowered to about 12.C. Since the reheat demand is cut, the capacity of the chiller is therefore reduced and the supply air flow rate drops accordingly. The lower part of Table 15.3.5 presents the performance results of this scenario. It can be seen that the total primary energy consumption of the conventional system is still higher, but much closer to that of the SHAC. Now the SHAC can have 22.7% less Ep at 70%RH, instead of 49.5% less at 60%RH. Table 15.3.5 Summary of cooling and energy performances of SHAC and conventional systems for Chinese restaurant at 60%RH and 70%RH. Year-round Year-round Design averaged Year-round total Ep per Energy relative Tz(.C)/ Year-round averaged AC area saving vs. Type of system humidity RHz (%) averaged SF COPch/COPdc (kWh/m2) WCVCC SHAC-VCC 60% 22.2/56.2 0.295 3.39/0.63 1,699 49.5% WCVCC 60% 21.4/63.0 NA 3.23/NA 3,362 NA SHAC-VCC 70% 22.4/59.8 0.441 3.41/0.80 1,348 22.7% WCVCC 70% 20.2/72.4 NA 3.23/NA 1,743 NA Remarks:NA means not applicable. Solar hybrid air-conditioning design 501 15.4 APPLICATION POTENTIAL OF SHAC IN VARIOUS HOT AND HUMID CITIES IN SOUTHEAST ASIA With the assurance of the energy and cooling merits of the SHAC system in subtropical Hong Kong, it is interesting to consider its application potential in the different hot and humid cities in Southeast Asia and South China, where solar irradiation is abundant. Air-conditioning is essential in maintaining the economic and commercial activities of these urban areas. In this study, six additional cities are included: Bangkok, Guangzhou, Kuala Lumpur, Manila, Singapore and Taipei – see Figure 15.4.1. Particularly in the cities of the Southeast Asia, there is rapid economic growth and increasing energy demand, and with it comes rising fossil fuel consumption which is increasing environmental pressures. Resource availability varies greatly from place to place in this region, which offers large potential for developing renewable energy initiatives. Many Southeast Asian countries have already adopted medium- and long-term targets for renewable energy. For instance, Indonesia, Singapore and Thailand have recently announced reduction targets for carbon dioxide emissions in support of the Copenhagen Accord (Ölz and Beerepoot, 2010). Year-round dynamic simulations of both SHAC and conventional AC systems are being carried out for buildings in these cities, including the same office building zone mentioned in Section 15.3. In these simulations, only the absorption chiller is involved in the SHAC since it is more effective than the adsorption chiller in earlier hybrid designs. The two types of air-conditioning system are specifically designed according to the climatic conditions of these cities, for each of which weather data for a typical meteorological year (DOE, 2011a; DOE, 2011b) are being used. As all the cities are located in the northern hemisphere, the installation of solar collectors faces south, with the tilt angle the same as the latitude of the respective cities in order to harness a maximum of solar irradiation throughout a year. Table 15.4.1 summarizes the results of cooling and energy performances of the SHAC and the conventional AC systems. Figure 15.4.1 Hot and humid cities of Southeast Asia and South China in this study. 502 Solar energy sciences and engineering applications Table 15.4.1 Summary of cooling and energy performances of SHAC and conventional systems in the cities in Southeast Asia and South China. City (latitude, longitude) Annual global horizontal irradiation (kWh/m2) Type of system Year-round averaged SF Year-round averaged COPch/ COPdc Year-round total Ep per AC area (kWh/m2) Energy saving vs. VCC Bangkok 1756.2 WCVCC NA 3.117 548 NA (13.92.N, 100.60.E) SHAC-Ab 0.857 0.773/0.968 273 50.3% Guangzhou 1073.4 WCVCC NA 3.175 366 NA (23.17.N, 113.33.E) SHAC-Ab 0.714 0.782/0.958 271 25.8% Hong Kong 1268.8 WCVCC NA 3.195 380 NA (22.32.N, 114.17.E) SHAC-Ab 0.804 0.779/0.904 246 35.3% Kuala Lumpur 1466.4 WCVCC NA 3.092 536 NA (3.12.N, 101.55.E) SHAC-Ab 0.777 0.777/0.915 313 41.7% Manila 1537.9 WCVCC NA 3.090 550 NA (14.52.N, 121.00.E) SHAC-Ab 0.753 0.774/0.993 330 39.9% Singapore 1587.0 WCVCC NA 3.067 555 NA (1.37.N, 103.98.E) SHAC-Ab 0.809 0.772/0.977 302 45.7% Taipei 1388.1 WCVCC NA 3.171 395 NA (25.07.N, 121.55.E) SHAC-Ab 0.862 0.778/0.859 222 43.9% Remarks:NA means not applicable. From the table it is clear that there is substantial primary energy saving of the SHAC against the conventional system, ranging from 25.8% to 50.3%. Particularly in tropical cities, like Bangkok, Kuala Lumpur and Singapore, the energy saving is well above 40%. This indicates the direct contribution that annual global horizontal irradiation to the SHAC can make, leading to relatively high SF in these places. As the SHAC system has independent temperature and humidity controls, it is suitable for those premises even with high space sensible and latent loads. Along with the various renewable energy incentives and policy implementation advocated in the related cities, solar air-conditioning systems should become technologically attractive and economically competitive. 15.5 CONCLUSION AND FUTURE DEVELOPMENT The cooling and energy performances of solar hybrid air-conditioning systems are assured in hot-humid climates. With its hybrid design and independent temperature and humidity controls, the SHAC system can be designed for sharing zone load and ventilation load; for radiant cooling with passive and active chilled beams; for new indoor ventilation strategies (both DV and SV); and for premises with high latent load. In all these SHAC systems, solar-thermal gain can be the primary energy source. Due to the intermittent nature of solar irradiation, it is inevitable that SHAC will require auxiliary heating, which may demand fossil fuel consumption. Despite this, the primary energy saving of the SHAC is still guaranteed compared to the conventional compression refrigeration system. For buildings with high sensible and latent loads, Solar hybrid air-conditioning design 503 this merit becomes more prominent. As a result, solar air-conditioning is an effective means to reduce the carbon footprint of buildings. In fact, SHAC systems can be further facilitated by building integrated installations of solar-thermal collectors, incorporating technological advancements of sorption refrigeration cycles and component integration in hybrid design. There are now absorption and adsorption chillers with a cooling capacity of below 10kW, which can compete with conventional AC systems in small- and medium-scale applications. For solid desiccant wheels, new sorbent materials, such as inorganic zeolitic adsorbent, are appearing, which would be more effective in regeneration by low temperature heat source. More research work is being carried out on liquid desiccant cooling with the inherent feature of thermal storage. Although adsorption chillers do not currently perform as well as absorption chillers, continual improvement is being made in the working pairs, the method of desorption and the configuration of multiple chambers. This would help in raising the COP of the adsorption refrigeration while keeping the advantage of low driving temperature. In point of fact, the advancement of sorption refrigeration and air-conditioning can be beneficial beyond solar air-conditioning: in combined cooling, heating and power, heat-driven refrigeration cycles are also essential. There are still a number of barriers to wider application of solar air-conditioning systems, among them spatial requirement and economic consideration. It will take time for building practitioners to feel comfortable with this new air-conditioning technology. More demonstration projects and operational experience of solar airconditioning would certainly help in convincing policy makers, building developers, engineering practitioners and market players. Were the life cycle assessments of solar air-conditioning better known, their environmental merit would also become known more widely. In the trend towards sustainable urbanization, solar cooling can play an essential role in realizing zero carbon deployment in regions with hot and humid climates. With appropriate system integration of cooling and heating, together with a strategy of hybrid use of renewable energy sources, more and more opportunities for solar-thermal technologies for building use should emerge in the near future. ACKNOWLEDGEMENT The work described in this paper was fully supported by a grant from the City University of Hong Kong (Strategic Research Grant, Project No. 7002765). Nomenclature A Absorber of absorption chiller or adsorber of adsorption chiller AC Air-conditioning ACVCC Air-cooled vapour compression chiller C Condenser COP Coefficient of performance COPch Coefficient of performance of chiller COPdc Coefficient of performance of desiccant cooling D Desorber DV Displacement ventilation DW Desiccant wheel 504 Solar energy sciences and engineering applications E Evaporator EA Exhaust air EAF Exhaust air fan EC Evaporative cooler Ep Primary energy consumption (kWh) G Generator HW Hot water MV Mixing ventilation NA Not applicable OA Outdoor air or outdoor air scheme OAF Outdoor air fan PV Photovoltaic PV/T Photovoltaic/thermal RA Room air or return air scheme RAF Return air fan RH Relative humidity (%) RHz Zone relative humidity (%) SA Supply air SAF Supply air fan SF Solar fraction SHAC Solar hybrid air-conditioning SHAC-Ab Solar hybrid air-conditioning with absorption chiller SHAC-Ad Solar hybrid air-conditioning with adsorption chiller SHAC-VCC Solar hybrid air-conditioning with vapour compression chiller SV Stratum ventilation TW Thermal wheel Tz Zone temperature (.C) VCC Vapour compression chiller WCVCC Water-cooled vapour compression chiller REFERENCES Altener (2002) Promoting Solar Air Conditioning – Technical Overview of Active Techniques, ALTENER Project Number 4.1030/Z/02-121/2002. 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The Solar Energy Laboratory, University of Wisconsin-Madison, WI, USA. TESS (2004) TESS Component Libraries – Version 2.0. The Thermal Energy System Specialists. Wang, R.Z., Ge, T.S., Chen, C.J., Ma, Q. and Xiong, Z.Q. (2009) Solar sorption cooling systems for residential applications: Options and guidelines. International Journal of Refrigeration, 32, 638–660. Wang, R.Z. and Zhai, X.Q. (2010) Development of solar-thermal technologies in China. Energy, 35, 4407–4416. Zhang, X.J., Dai, Y.J. and Wang, R.Z. (2003) A simulation study of heat and mass transfer in a honeycombed rotary desiccant dehumidifier, Applied Thermal Engineering, 23, 989–1003. Chapter 16 Solar-desiccant air-conditioning systems Napoleon Enteria Enteria Grün Energietechnik, Davao, Philippines 16.1 INTRODUCTION 16.1.1 Energy and environment One of the alarming situations regarding the current energy supply and demand scenario is the peaking of supply due to rapid utilization (Greene et al., 2006). The scenario also causes instabilities and unpredictability for the long-term energy supply situation owing to the domination of energy suppliers from specific regions and groupings. Hence, the main source of global conventional energy sources comes from the Middle East and Russia. A decade ago, the Middle East accounted for 33% of the world’s conventional energy supply (IEA, 2004). In addition, the Russian Federation is becoming the global supplier, particularly of natural gas (Gelb, 2006). Due to the above situation, energy and economic politics have collided, examples being the 1973 oil crisis, the Gulf War and the Russia-Ukraine crisis (Gelb, 2006). Since the start of the industrial revolution, large amounts of greenhouse gases (GHG) have been deposited in the atmosphere (Brown et al., 2009), the increase in global pollutants coming from human activities of urbanization, industrialization and so on (Lin et al., 2008). Greenhouse gases are the primary cause of global warming (IPPC). The ozone layer depleting substance such as CFCs, halons, and other ozonedepleting chemicals are the cause of ozone layer thinning (Calm and Didion, 1998). All of these have consequences for the increase in global temperature, which has serious effects on climate patterns – flooding, cyclones and other weather disturbances (Barrios et al., 2006). The above situation has serious consequences for global sustainability, as concluded by numerous studies (Bard and Frank, 2006). The building sector is one of the primary energy consumers, accounting for 50% inclusive of commercial and industrial buildings (Zimmermann et al., 2005). Energy sector output is used to support electrical appliances, thermal comfort and other requirements such as lighting. As the global population grows, urbanization spreads and standards of living increase, it is expected that energy consumption by the building sector will increase (IEA, 2004; York, 2007; Solecki and Leichenko, 2006). Studies show that population size and age structure have effects on energy consumption (York, 2007). Power sector energy consumption will increase by 119% between 2002 and 2030 (IEA, 2004), while energy consumption in the residential sector, including agriculture, was 56.7% in 2006 compared to 44.2% in 1973 (IEA, 2008). 508 Solar energy sciences and engineering applications The provision of optimum human living conditions results in large amounts of energy consumption. The energy required to maintain human thermal comfort approaches 50% of total building energy consumption. In most cases, the energy needed is high-grade electric power (IEA, 2008). As most power plants consume large amounts of fossil fuel in generating power, so provision for optimal human living standards has contributed to high carbon-based energy consumption and greenhouse gas emissions (IEA, 2004). Alternative methods are therefore needed when talking about reducing conventional energy consumption and cutting greenhouse gases emissions (Coiante and Barra, 1996). However, it is imperative not to sacrifice healthy indoor and thermal comfort conditions for the sake of energy consumption reduction (Costa and Costa, 2006; Day et al., 2009). 16.1.2 The building environment At present, maintaining clean indoor thermal comfort conditions is done using refrigerant-based air-conditioning systems (AC). The operation of refrigerant-based or vapour compression systems is by means of altering the pressure of the working fluid to change the boiling point and thus release or absorb latent heat. However, the operation of changing the pressure can only be done by mechanical means, for which pumps or compressors are used; thus, the so-called mechanical vapour compression system (ASHRAE, 1989). In addition, at present, most of the working substances are made from halocarbon compounds such as CFC and HCFC which affect the ozone layer through emissions of greenhouse gas (GHG) (Calm and Didion, 1998). These materials have a long-term effect on the general environment. Hence, the systems maintaining indoor thermal comfort conditions are gradually harming the natural environmental, causing ozone-layer depletion and greenhouse gases emissions. The provision of indoor thermal comfort conditions for buildings, either through heating or cooling, is done by heat pump systems. These devices are widely called mechanical vapour compression systems. Several studies have been conducted to improve system performance through efficiency and reduce environmental damage. However, these systems still consume large amounts of energy in the form of highgrade electricity. The main energy source of mechanical vapour compression systems comes from electricity power grids. These air-conditioning systems play a major role in the energy consumption of buildings, most particularly in hot and humid climates. In the Middle East, more than 70% of building energy consumption is to support cooling (El-Dessouky et al., 2004). In Europe, 10% of building sector energy consumption is likewise to support cooling demand (Kolokotroni and Aronis, 1999). In Hong Kong, 45% of commercial building energy consumption is also for cooling (Zain et al., 2007). In Japan, 3% of building sector energy consumption is for cooling applications (Murakami et al., 2009). It is expected that in tropical countries which are hot and humid, energy demand for cooling and dehumidification will be very high (Wong and Li, 2007). Commercial, office and industrial buildings commonly use centralized airconditioning systems with heat pumps or refrigerant chillers. However, split-type air-conditioning systems provide an alternative. The application of centralized airconditioning systems will introduce fresh air inside the buildings. However, it will also increase energy consumption as treatment of outdoor air latent and sensible energy Solar-desiccant air-conditioning systems 509 100% rh 70% rh 60% rh 50% rh 30% rh 26ET* 0 25 30 OPERATIVE TEMPERATURE °C 20 –10 –5 0 5 10 winter summer 15 20 5 10 HUMIDITY RATIO g/kg DEW POINT TEMPERATURE °C 15 20ET* EFFECTIVE TEMPERATURE ET* A B C Figure 16.1.1 Singapore buildings indoor temperature and humidity (Sekhar, 1995). is very high. Hence, recirculation methods are commonly applied to AC systems. Refrigerant-based chillers are commonly used in buildings, as evidenced by the many cooling towers that are installed on rooftops. The chillers are used to cool the water used in the cooling of air in the air cooling unit (ACU). Chillers are common in buildings to cool water distributed in buildings in which cool air is produced through a fan coil unit (FCU). In addition, centralized air-conditioning is commonly applied in large building spaces through a network of air ducts. As open spaces such as commercial establishments have a large volumetric supply of air, the treatment of outdoor air sensible and latent load has an impact on air-conditioning system performance. In addition, as in Singapore, buildings operate at lower temperature and higher relative humidity (approximately 23.C and 70% – see Figure 16.1.1). This is due to the non-reheating of cold air to reduce the energy consumption of the air handling unit (AHU). Residential buildings, both public and privately owned, use window and split types of air-conditioning system. However, split-type air conditioning systems are the most common due to the flexibility, unlike window types, of locating the compressor/condenser unit and the evaporator/expander unit. Residential buildings use air-conditioning systems during both day and night. However, natural ventilation and air fans can be used simultaneously during daytime, when air-conditioning systems are not commonly used. As shown in Figure 16.1.2, in the case of Singapore, 510 Solar energy sciences and engineering applications Figure 16.1.2 Singapore pattern of the residential air-conditioning operation (Chua and Chou, 2010). residential buildings commonly operate air-conditioning systems at night, during the hours of sleep. As most residential buildings operate air-conditioning systems at this time, in terms of electric energy consumption, this is the off-peak period. However, in terms of indoor air quality, problems arise due to the recirculation of indoor air, resulting in a poor quality indoor environment. 16.2 THE BASIC CONCEPT 16.2.1 Thermodynamic processes Thermally operated air-conditioning systems run by means of applying heat energy for the production of cooling effect (Grossman, 2002). However, as technologies are varied in operation principles and heat requirements, some thermally operated air-conditioning technologies have limited applications (Henning, 2007; Fan et al., 2007). The main advantage of these systems is the direct application of thermal energy for system operation. Hence, low-grade thermal energy can be used to operate the system. In addition, several thermal energy sources can be utilized for the system operation, such as waste heat (Henning et al., 2007). The concept of a thermally operated air-conditioning system is the utilization of a higher thermal energy source to drive the air-conditioning system and provide cooling effect. Figure 16.2.1 shows the general thermodynamic principle of the thermally operated cooling system for airconditioning applications. The concept is based on four temperatures – the Carnot heat engine (Abrahamsson and Jernqvist, 1993; Hellman, 2002). However, the system still utilizes electric energy for the operation of fans, pumps and the control system. Combined thermal energy and electric energy can be generated from solar energy through a Solar-desiccant air-conditioning systems 511 REFRIGERATOR W HEAT ENGINE TA TH QH TL TA QL QA2 QA1 Figure 16.2.1 Thermodynamic principle of the thermally-operated air-conditioning system (Grossman and Johannsen, 1981). thermo-electric collector (Thermal/Photovoltaic System) (Charalambous et al., 2007). This concept had been described by Mittelman et al. (2007) and Kribus et al. (2006). From a thermodynamic point of view, the system performance is dependent on the thermal energy source temperature and on the cooling effect temperature (Boehm, 1986). The system relies on ambient temperature conditions (Grossman and Johannsen, 1981). Hence, system thermodynamic performance can be obtained based on the system’s operational temperature conditions (Abrahamsson and Jernqvist, 1993; Hellman, 2002). Thermally activated/operated systems have wide potential for application, not only for buildings but also for other systems which produce thermal energy (such as waste heat). Transportation and industrial sectors are sectors with potential for thermally operated air-conditioning systems. Hence, Mazzei et al. (2005) discuss the provision of thermal comfort of desiccant-based thermally activated air-conditioning systems, which can save up to 50% conventional energy (Henning et al., 2001). Thus, the system has potential for further development and application through utilization of thermal energy resources and application of desiccant materials. Figure 16.2.2 shows the operational concept of the desiccant-based air-conditioning system. The processed air from the desiccant dehumidifier becomes hot due to the release of heat through condensation and sorption. Heat recovery devices are used to recover this energy for application again in the desiccant dehumidifier in conjunction with other sources of thermal energy (renewable energy, non-conventional energy and conventional energy). The condition of the air after the heat recovery becomes warm and dry. In hot and humid climates, the air condition is still above the thermal comfort temperature, so that an evaporative cooling process is applied by either direct addition of air moisture (direct evaporative cooling) or indirect addition of air moisture in a secondary air stream (indirect evaporative cooling). Furthermore, application of other air cooling techniques, both conventional and non-conventional, can also be used as an additional air cooler prior to introduction of the air to the indoor environment. Conventional air coolers such as sorption chillers and vapour compression systems are used with an increase in their performance. Non-conventional coolers, such as ground source heat pumps and water source heat pumps, can also be used. These auxiliary 512 Solar energy sciences and engineering applications Figure 16.2.2 General concept of the thermally activated desiccant cooling technologies (Enteria and Mizutani, 2011). coolers are applied when the required temperature of the air after the evaporative cooler is still insufficient to support indoor thermal comfort conditions. 16.2.2 Advantages of the open systems The chemical contents of the air, such as indoor volatile organic compounds (VOCs), are readily absorbed by desiccants (Wolfrum et al., 2008), thus solving the problem of the recirculation of indoor air pollutants in the recirculation method of air-conditioning systems. In addition, desiccant-based air-conditioning systems have controlled indoor air quality, as reported by Zhang et al. (2007). Based on studies conducted by Fang et al. (2008), desiccant-coated wheels have removed VOCs from air, in particular the most common VOCs, toluene and n-hexane (Wolfrum et al. 2008). Therefore, desiccant-based air-conditioning systems have not only solved health problems related to moisture in buildings (Ahman et al. 2000), but have also improved the quality of indoor air. This applies also to the management of the indoor environment for office buildings, as reported by Shaw et al. (2005) of the National Research Council of Canada. Most importantly, desiccant-based air-conditioning systems can be applied to solve the problem of energy, comfort, environment and indoor air for those of sensitive dispositions, such as people in nursing care (Theodosiou and Ordoumpozanis, 2008). The open-cycle desiccant-based air-conditioning system utilizes desiccants which are mostly salt-based. Development of other aqueous absorbent desiccants is Solar-desiccant air-conditioning systems 513 progressing well. As sorption processes are natural and occur on the surface of the desiccant, the desiccant has another advantage: it can treat the biological and chemical contents of air owing to direct contact of the air with the desiccant. Airborne microorganisms can be treated by desiccants (Goswami et al., 1997). Wang et al. (2011) show that desiccants can make airborne fungi inactivate. Thus, serious problems in air quality can also be resolved by the open-cycle desiccant-based air-conditioning system. Nevertheless, it was shown by Goswami et al. (1997) that titanium dioxide (TiO2) desiccant material can be used to control air micro-organisms through a photocatalytic process. 16.2.3 Desiccant materials The adsorption process is a surface phenomenon occurring at the interface of two phases in which cohesive forces, including Van der Waals forces and hydrogen bonding, act between the molecules of all substances irrespective of their state of aggregation (Srivastava and Eames, 1998). This process is called physisorption. Absorption is a chemical process caused by valency forces called chemisorption (Low, 1960). The process of attracting moisture from the air is done either by adsorption or by absorption: the adsorption process is a physical process in which the property of the desiccant material remains the same; while in the absorption process, upon attracting moisture, the physical characteristic of the material changes. The desiccant materials can be either solid or liquid: the solid desiccant and hydrophilic desiccants are silica gel, activated alumina, and zeolites, while calcium chloride is an absorbent desiccant. Commercial hydrophobic solid desiccants are activated carbons, metal oxides, specially developed porous metal hydrides and composite adsorbents (Srivastava and Eames, 1998). Some desiccant materials combine absorbent and adsorbent desiccants to form composites which enhance their physical properties and sorption capacity (Tokarev et al., 2002). The basic mechanism in the sorption of moisture between air moisture and the desiccant material is the difference in the water vapour pressure on the surface of the desiccant and of the material. The uptake of moisture from the air to the desiccant occurs when vapour pressure in the air is high; the removal of vapour from the desiccant material is done when the vapour pressure in the air is lower than on the desiccant material. When the vapour pressure is the same both in the air and on the desiccant material, an equilibrium is reached and the sorption process stops. The only means to make the sorption process proceed is to use outside forces such as increasing the air pressure, decreasing the temperature or by artificial electromotive force (Low, 1960). The same procedure, but in reverse, is applied for the removal of moisture from the desiccant material. The most common absorbents are lithium bromide, lithium chloride, calcium chloride and triethelene glycol. Other possible candidates as absorbents are salt-based solutions or related materials which attract water molecules. Examples of alternative absorbents are potassium chloride and sodium chloride. Other candidates are a mixture of the commonly used absorbents mentioned above. Table 16.2.1 summarizes the common absorbents and their properties and compares them for their thermochemical, environmental, human toxicity and cost properties. Lithium chloride (LiCl) has a low vapour pressure at a given temperature but the material cost is high (Mei and Dai, 2008); Figure 16.2.3 shows LiCl in the psychometric chart. According to 514 Solar energy sciences and engineering applications Table 16.2.1 Comparison of the properties of six liquid desiccants (absorbents) at 25.C, to allow a fair comparison, a concentration giving an equilibrium relative humidity of ERH (Equilibrium Relative Humidity)=50% has been chosen in each case, with the exception of sodium chloride where ERH=75%, this being the minimum achievable (Davies and Knowles, 2006). Aqueous solution Property Unit CaCl2 LiBr LiCl MgCl ZnCl NaCl Cocentration 0.36 0.39 0.26 0.31 0.52 0.26 (mass solute/ mass solution) Hygroscopicity % 50 50 50 50 50 75 (equilibrium RH) Cost US$/m3 560 7300 4600 450 1400 180 Abundance in m3/m3 2.3×10-3 4.0×10-6 3.0×10-6 1.3×10-2 1.0×10-9 9.0×10-2 seawatera Density kg/m3 1.35 1.38 1.4 1.29 1.58 1.2 Viscosity mPa-s 4.6 1.8 2.5 6 4.7 1.8 Specific heat kJ/kg-.C 2.6 2.6 3 2.1 2.3 3.4 capacity Thermal W/m-.C 0.56 0.48 0.56 0.52 0.46 0.58 conductivity Diffusivity of 10-9m2/s 0.54 1.17 0.9 0.91 0.8 1.86 water in the solution Differential heat kJ/kg 80 no data 65 65d no data no data of dilutionb Water absorption kg/m3 85 84 91 76 120 n.a. capacity Human toxicityc L 0.14 0.23 0.10 0.49 0.03 0.66 Ecotoxicity ml/L 4.9(2) no data 0.06 (2) 4.3 (1) 0.001 (6) 20 (5) (Daphnia magna) aVolume of absorbent that could theoretically be extracted from unit volume of seawater, assuming 100% recovery speed. bMass of water that, on absorption in the absorbent, will cause a 10% relative increase in equilibrium relative humidity. cEstimated lethal dose in humans scaled from LD50 values for rats. dAt 50.C. Ameel et al. (1995), the absorber utilizing LiCl solution at 35.C requires about five times the area of an absorber utilizing lithium bromide to achieve the same sorption performance. Also, lithium chloride solutions are not practical at an absorber temperature of 45.C or higher due to solubility limitations (Ameel et al., 1995). At the same absorbent mass flow rate, the dehumidification performance of lithium chloride is better. Where absorbent volumetric flow rate is the same, the dehumidification performance of lithium chloride is almost the same as lithium bromide. Using structured packing for dehumidification and regeneration studies of three absorbents (calcium chloride, lithium chloride and a mixture of 50% calcium chloride and 50% lithium chloride). Al-Farayedhi et al. (2002) show that lithium chloride has a higher rate of liquid-phase mass transfer coefficient than the other two absorbents, owing to its molecular weight. Solar-desiccant air-conditioning systems 515 55% 50% Crystal line Crystal line (a) 55 (b) 50 45 40 35 30 25 20 55 50 45 40 35 30 25 20 45% 60% 80% 40% 30% 60% 80% 20% 40% 40% 0 0.01 0.02 Humidity ratio/(kg/kg) Temperature/°C Temperature/°C 0.03 0.04 Isoconcebtration line Iso-. line Isoconcebtration line Iso-. line 0.05 0 0.01 0.02 Humidity ratio/(kg/kg) 0.03 0.04 0.05 20% . = 40% . = 60% . = 50% . = 20% . = 100% . = 100% Figure 16.2.3 Status of the commonly used liquid desiccants (absorbents) in the psychrometric chart: a) Lithium Bromide (LiBr); b) Lithium Chloride (LiCl) (Liu et al., 2010). 16.3 SOLID-BASED SYSTEM 16.3.1 Basic concept Solid desiccant air-conditioning systems are primarily based on the application of solidbased desiccant materials in controlling air moisture content. The sorption mechanism in the solid material is either through absorption or adsorption. Cooling by means of heat recovery, evaporative cooling or other means is applied to the system. The design of the system is based on the fixed-bed type in alternative operation of moisture sorption and desorption. When the encapsulated phase change materials (EPCM) in the desiccant bed are applied, this absorbs the heat of sorption released during the dehumidification process and lowers the air temperature. However, its humidity is higher compared to the pure desiccant (Rady et al., 2009). For building 516 Solar energy sciences and engineering applications cooling application, this is done through temperature, compared to industrial applications of pressure. The processed (dehumidified) air is pre-cooled through, in most cases, the rotating heat wheel, either through utilization of the cool return air or by means of outside air. As the air in most cases is still warm for application indoors, final air cooling is done by means of evaporative cooling, or chill cooling. The solid desiccant air-conditioning system is the most widely used desiccant airconditioning system. This is due to the simple handling of desiccant materials. The desiccant material is typically impregnated to the honeycomb designed wheels or to the cross-flow heat exchangers. Although typical solid desiccant materials have higher regeneration temperatures than liquid desiccants, new materials have been developed with lower regeneration temperature requirements. 16.3.2 Typical systems Solid desiccant air-conditioning systems are simpler to use and apply due to the easy handling of the desiccant material. Hence, the system is not complicated, unlike liquid desiccant air-conditioning systems in both design and operation. Farooq and Ruthven (1991) investigated the desiccant bed for solar air-conditioning application. They showed that the optimal choice of desiccant can be compensated by the appropriate adjustment of the cyclic time. In addition, the cost of making the desiccant wheel and moisture diffusivity should be given consideration. Jurinak et al. (1984) presented the open-cycle desiccant air-conditioning system both for ventilation cycle and recirculation cycle. They showed that unbalancing the air flow through the dehumidifier improved the desiccant system coefficient of performance (COP) by 10–15% for the ventilation cycle and up to 50% for the recirculation cycle. To make the desiccant airconditioning system competitive, the thermal COP of the high-performance desiccant systems must be improved to compete with conventional vapour compression systems, such as very high heat and mass transfer unit dehumidifiers with large thermal capacitance matrix. The most common solid desiccant air-conditioning system is composed of the two-wheel type, the so-called Munter Cycle shown in Figure 16.3.1. This is the basic design of the solid desiccant air-conditioning system, in which application of the air cooling in both the supply air and in the return air is implemented. Hence, several modifications to this cycle, implementing different operating strategies, are presented by Henning et al., 2007, Jain et al., 1995 and Henning et al., 1995. Figure 16.3.1 The double wheels type solid desiccant air-conditioning system (Enteria et al., 2010a). Solar-desiccant air-conditioning systems 517 Application of the desiccant wheel as the air dehumidifier has factors to be considered. Kang and Maclaine-cross (1989) show that the performance of the desiccant-based air-conditioning system relies much on the desiccant material’s moisture sorption capacity. Kodama et al. (2001) show that there is an optimal speed at which a high sorption rate occurs in the rotating desiccant wheel. Optimal speed increases with increasing regeneration air flow rate, decreasing desiccant wheel depth, and decreasing bulk density of the rotor. Optimal wheel speed decreases with higher humidity and lower regeneration temperature. They also show that the sorption rate is relative to the relative humidity of the air. Zhang and Niu (2002) show that the sorption performance of the desiccant wheel depends on the wheel rotational speed and the number of transfer units; they suggest that the desiccant wheel should have 2.5 transfer units. Subramanyan et al. (2004a) show that increasing the air flow rate reduces the specific cooling load (difference in the enthalpy of the outdoor air and of the processed air). The cooling load increases due to the amount of air mass flow rate. In addition, Subramanyan et al. (2004b) show that increasing the air flow rate increases the supply air moisture content. Harse et al. (2005) show that for higher humidity air the optimal speed of the wheel is greater than for the air with lower humidity content. They show that at higher regeneration temperature, the performance of the desiccant wheel improves. The depth of the wheel affects the dehumidification rate and as the depth increases the dehumidification rate also increases, resulting in a lowering of the optimum wheel speed. Gao et al. (2005) show that the thickness of the desiccant material affects sorption capacity. At higher desiccant material thickness in the channel, higher sorption rate is attained owing to more time to reach the steady state. In addition, a lower desiccant rotor speed makes for optimum wheel speed. Their study shows that channel shapes affect rotor sorption capacity. Hence, for the same cross-sectional area, sinusoidal channel is the best performer due to its lower hydraulic diameter, resulting in higher air velocity and heat transfer coefficient. Furthermore, the study shows that increasing the outdoor air relative humidity increases the processed air temperature. However, the humidity content of both the processed and the exit air increases as relative humidity of the outdoor air increases. Enteria et al. (2010a) presented parameters affecting the performance of the desiccant wheel and performance evaluation for the desiccant wheel dehumidification capability. La et al. (2010) reviewed the development of the rotary desiccant wheel-based system. 16.3.3 Modified systems In most designs, operation of the solid desiccant air-conditioning system is through a dehumidification-humidification process. In this process, air dehumidification is done at very low humidity content to achieve evaporative cooling. For this, the required regeneration temperature is increased. Accordingly, application of constant dehumidification will help to prevent the deep dehumidification in regions with hot and humid climates. Enteria et al. (2010b, 2010c) looked at the constant humidity air cooling cycle of the desiccant, as presented in Figure 16.3.2. Ando et al. (2005) show the double stage dehumidification process, in which two desiccant wheels are employed (Figure 16.3.3). The main purpose of this process is to reduce the air moisture content in the case of humid air with lower regeneration temperature requirements. Ge et al. (2009) investigated the two-stage desiccant air-conditioning system. They show 518 Solar energy sciences and engineering applications WARM WATER HOT WATER WATER OUT 5’ 5 4’ 10 9 A H C D E C HEAT EXCHANGER [HEX-1] DESICCANT WHEEL HEX[2] 1 2 3 12 F N F N F N F N EXIT 11 AIR (a) (b) OUTDOOR AIR 8 4 6 7 SUPPLY AIR RETURN AIR 0 0.022 60°C RT 70 kJ/kg 30 kJ/kg 5 6 1 2 11 12 11 12 11 12 11 12 8 55 50 55 3 10 10 10 10 3 3 4 3 4 4 4 4 4 4 4 9 9 9 9 50 kJ/kg 65°C RT 70°C RT 75°C RT 0.020 0.018 0.016 0.014 Absolute Humidity (kg/kg) 0.012 0.010 0.008 0.006 5 10 15 20 25 30 35 Dry Bulb Temperature (C) 40 45 50 55 60 65 70 75 80 WATER IN Figure 16.3.2 Constant humidity ratio supply air solid desiccant air-conditioning system: a) system diagram; b) psychrometric chart (Enteria et al., 2010b; Enteria et al., 2010c). it has lower regeneration temperature requirements with higher COP. Kodama et al. (2005) show the multi-pass desiccant wheel presented in Figure 16.3.4. This shows that a 50.C regeneration temperature is enough for the desiccant wheel. Furthermore, Kodama et al. (2003) presented several designs of the desiccant air-conditioning system for humid climates, conditions in which the 4-wheel cycle (two desiccant wheels and two heat wheels) can be used. However, the 3-wheel cycle (1 desiccant wheel, 1 heat wheel and 1 total heat exchanger) proved better than the 4-wheel cycle. The fixed-bed solid desiccant air-conditioning system is another type of modified system, on which several design studies have been conducted. The advantage of the fixed-bed system is the sorption process, which can be done in an isothermal way. Solar-desiccant air-conditioning systems 519 OA 1 Dehumidifier Heat Exhanger Heater EA Z Y X Heater Evaporative Cooler 5 RA 4 SA Dehumidifier Heat Exhanger A B 1 9 8 7 6 2 3 Figure 16.3.3 Double stage dehumidification solid desiccant air-conditioning system (Ando et al., 2005). Figure 16.3.4 Principle and diagram of the multi-pass desiccant wheel: a) desiccant wheel design; b) installation diagram (Kodama et al., 2005). 520 Solar energy sciences and engineering applications Z process alror regeneration stream desiccant particles Y X cross cooling or heating stream £ £ Figure 16.3.5 Desiccant coated cross-cooled compact dehumidifier (Yuan et al., 2008). This means that the temperature of the air after passing the desiccant material is not increased. Hence, the sorption process is increased due to the removal of sorption heat. Yuan et al. (2008) proposed a cross-cooled compact solid desiccant dehumidifier. The aim of this design is to make the dehumidification process cooler due to the heat exchange between the dehumidified air and the secondary air. The performance of the design was shown to be better without secondary cooling. Majumdar and Worek (1989) investigated the open-cycle cooled-bed desiccant air-conditioning system using a cooled-bed dehumidifier. Here, system performance is more sensitive to regeneration and indoor temperature and to the outdoor humidity ratio than to the indoor humidity ratio and outdoor temperature. The study showed that the cooledbed system can be regenerated at a low temperature of 50.C. Figure 16.3.5 shows the cross-flow fixed-bed type air-dehumidifier with cooler. Henning et al. (2007) show the application of fixed-bed through a cyclic operation called evaporative cooled sportive heat exchanger (ECOS). 16.3.4 Hybrid systems The typical solid-based desiccant hybrid air-conditioning system comprises the rotating desiccant wheel and the vapour compression system, in which the evaporator serves as the air cooler while the condenser serves as air heater, with a back-up thermal energy source. Dhar and Singh (2001) studied several designs of the solid-based hybrid desiccant air-conditioning systems. The study shows that solid-based hybrid desiccant air-conditioning system gives substantial energy saving as compared to the conventional vapour compression refrigeration system. Jia et al. (2006) show that this system can reduce electricity consumption by 37.5% compared to the ordinary vapour compression system operating in the same air conditions of 30.C and 55% humidity (Figure 16.3.6). Sheridan and Mitchell (1985) investigated the use of solid desiccant hybrid airconditioning systems in Australian climatic conditions. With high sensible load air, Solar-desiccant air-conditioning systems 521 Figure 16.3.6 Schematic diagram of the hybrid solid desiccant air-conditioning system using desiccant wheel ( Jia et al., 2006). Figure 16.3.7 Schematic diagram of the hybrid solid desiccant air-conditioning system using desiccantcoated heat exchanger (Enteria et al., 2011). the hybrid cycle saved 24% to 40% electric energy compared to the conventional cycle. Combining the hybrid system with an indirect evaporative cooler can bring significant savings in energy consumption. The hybrid system saves more energy in hot and dry climates than in hot and humid climates where more energy is consumed than for the conventional cycle. The system can save energy when the air to be treated has a higher sensible load fraction than latent load fraction. However, solar energy can be combined to meet the higher latent load of the air. Enteria et al. (2011) presented the combined desiccant-refrigerant air-conditioning system using the cyclic processes of air cooling+dehumidification; this can be used as an air heater+humidifier during the winter (see Figure 16.3.7). The result shows that the system can support a higher 522 Solar energy sciences and engineering applications COP of above 5. It also shows that the coefficient of performance increases as the air humidity increases. This means that the system is suitable in humid climates where airconditioning is important. The system is also more compact than other solid desiccant air-conditioning systems. 16.4 LIQUID-BASED SYSTEM 16.4.1 Basic concept The design of the liquid desiccant air-conditioning system uses the falling film type in the membrane, with air passing to its surface (Ren et al., 2007). Some designs apply the spray type to increase the surface area of air-desiccant contact. The design of the air dehumidifier uses an isothermal process which passes cool air/water at the back of the falling desiccant film (Yin et al., 2008). Since the regeneration of the desiccant material is by means of heat, many designs of liquid desiccant regenerators are done with solar energy. The cooling of air after the desiccant material is performed in the same way as in solid-desiccant air-conditioning systems. The liquid-based system utilizes liquid desiccant materials in removing air moisture content. The most widely used liquid desiccant materials are lithium chloride, lithium bromide, calcium chloride and glycol-based substances (Yin et al., 2009). The application of these materials depends on cost, type of operations and the source of thermal energy. In addition, some liquid desiccants are corrosive and require proper handling in their application. However, the main advantage of liquid desiccant is its high moisture removal capacity with lower regeneration temperature requirement. Liquid desiccant air-conditioning systems rely on the liquid desiccant to control air moisture content, which is achieved by means of an absorption process. One of the main advantages of liquid desiccant air-conditioning systems is the lower regeneration temperature requirement along with higher thermal and chemical storage. The advantage of the hybrid liquid desiccant air-conditioning system is the complete operation of the system using electric energy at higher performance. This means, for small applications, that the hybrid liquid desiccant air-conditioning system will prevail over the pure liquid desiccant air-conditioning system. 16.4.2 Typical systems Dehumidifiers or regenerators based on spray, wetted wall (falling film) and packed bed tower are the typical arrangements (Jain and Bansal, 2007). The wetted wall system uses a falling film absorbent in the plate while the air is in contact with the absorbent. This concept is practical for applications which do not need complex air dehumidifiers, such as for low thermal capacity buildings. Mesquita et al. (2006) developed the numerical models for simultaneous heat and mass transfer in parallel-plate dehumidifiers. Here, two polypropylene twin-wall plates form the channel, inside which cooling water flows in a cross-flow configuration with respect to the absorbent and air streams. Water mass flow rate is maintained high enough to keep the plates’ walls essentially isothermal, with water temperature gains throughout the plate of less than 0.4.C. The constant thickness and simplified model under-predict the dehumidification, especially for low absorbent flow rates. The numerical model can be adapted Solar-desiccant air-conditioning systems 523 Figure 16.4.1 Schematic diagram of the open-cycle liquid desiccant air-conditioning system either with packed bed tower, spray tower or wetted wall column ( Jain and Bansal, 2007). for non-isothermal conditions, with the introduction of cooling water flow equations. Figure 16.4.1 shows the schematic design of the absorbent dehumidifier/regenerator. 16.4.3 Modified systems Thermo-chemical storage is an option when the available source of thermal energy is not in phase with cooling/dehumidification demand. With this scenario, conventional energy usage can be reduced. Kessling et al. (1998) shows that the absorbency of hygroscopic salts such as LiCl and CalCl2 is up to more than three times greater in energy storage than other adsorbents such as zeolite and silica gel. The schematic diagram in Figure 16.4.2 shows that the high storage capacity is based on the concentration between the strong and diluted salt solutions. Furthermore, Miller (1983) reported that energy storage via absorbents was competitive with phase-change materials, rock-bed storage and water systems. 16.4.4 Hybrid systems Hybrid installation of open-cycle liquid desiccant air-conditioning systems is used to increase system performance. A two-stage open-cycle liquid desiccant air-conditioning system with CaCl2 and LiCl has a COP and exergy efficiency of 0.73% and 23.0% compared to the basic open-cycle liquid desiccant air-conditioning system (Xiong et al., 2010). Figure 16.4.3 shows a multi-stage open-cycle liquid desiccant air-conditioning 524 Solar energy sciences and engineering applications Figure 16.4.2 Schematic diagram of the open-cycle liquid desiccant air-conditioning system with thermo-chemical storage capacity (Kessling et al., 1998). Figure 16.4.3 Schematic diagram of the two-stage open-cycle liquid desiccant air-conditioning system (Xiong et al., 2010). system. Dai et al. (2001) studied the multi-cycle open-cycle liquid desiccant airconditioning system, in particular the refrigerant, liquid desiccant and cooling water cycles. Lazzarin and Castellotti (2007) looked at the combination of an open-cycle liquid desiccant air-conditioning system and vapour compression system. The system Solar-desiccant air-conditioning systems 525 Expansion valve Evaporator Solution pump Total heat Return air exchanger Regeneration Exhaust air unit Condenser Solution groove Four-way valve Total heat exchanger Solution groove Solution groove Send air DehumidificFresh air action unit Compressor Plate heat exhanger Figure 16.4.4 Schematic diagram of the hybrid open-cycle liquid desiccant air-conditioning system with vapour compression system (Zhu et al., 2010). produced 20% to 30% more cooling than the vapour compression system alone. Xiong et al. (2010) also presented a novel open-cycle liquid desiccant air-conditioning system with CaCl2. Compared to the basic system, the thermal coefficient of performance and exergy efficiency of the system increased from 0.24% to 0.73% and from 6.8% to 23.0% respectively. Khalid Ahmed et al. (1997), in a simulation model of the hybrid open-cycle liquid desiccant air-conditioning system, showed that the system provides an excellent alternative to conventional vapour absorption systems, particularly in hot and humid climates. The COP obtained was about 50% higher than that of a conventional vapour absorption system. A maximum COP of 1.25 was obtained during the study and the unit was found to be best suited for hot and humid areas. Figure 16.4.4 shows the hybrid open-cycle liquid desiccant air-conditioning system with vapour compression system. 16.5 SYSTEM APPLICATION 16.5.1 Countries Thermally operated desiccant air-conditioning technologies are a promising alternative to the vapour compression system in handling air sensible and latent energy contents. This is due to the possibility of operating the system by energy sources other than fossil fuel-based electricity – solar energy, waste heat, etc. Research into the solar desiccant ventilation and air-conditioning system is very important owing to the fact that the amount of air thermal energy content is almost in phase with the amount of solar radiation (Tabor, 1962). In hot and humid climates, such as in East Asia 526 Solar energy sciences and engineering applications during the summer and South East Asia year round, air temperature and humidity are high. In addition, as day-long dehumidification is needed compared to other climatic conditions, the more cheaply available night-time (off-peak) electric energy can be stored for daytime operation of the system (Hammou and Lacroix, 2006). Enteria et al. (2010b) show the applicability of night-time electric energy storage for daytime utilization. Combined solar energy for air dehumidification with ground water source for air cooling makes the system utilize natural energy sources, such as done in London (Ampofo et al., 2006). The vapour compression system operates to remove air moisture content by cooling the air below its dew-point temperature. However, as the air after cooling to its dewpoint temperate is very cold, reheating of the air is needed before it can be introduced to the indoor environment. As the Asia-Pacific region is very hot and humid all year round, in South East Asia and during summer in East Asia especially, the vapour compression system operates thoroughly to reduce the very high moisture content of outdoor air. Application of the desiccant material coupled with the vapour compression system minimizes the operating condition of the latter since the desiccant material handles the air latent energy content while the vapour compression system handles the air sensible load (hybrid desiccant). Liquid system applications can have a higher performance (44.5%) in green building (Ma et al., 2006). The advantage of the hybrid desiccant air-conditioning system is its operation in part loading (Jia et al., 2006). Table 16.5.1 shows the development and application of desiccant-based airconditioning systems, which are expanding globally. However, in the hot and humid climate of the Asia-Pacific Region, South America and Africa the system is still not fully utilized. Therefore, investigations of the system for applications in these regions should expand the potential of the system for more wide-scale use. The system has the potential to be a leading air-conditioning technology for energy-efficient, healthy buildings in hot and humid climates (Sekhar, 2007). As such, it should contribute significantly to reducing conventional energy consumption and GHG emissions by the building sector while also providing human thermal comfort conditions. 16.5.2 Temperate regions Solid-based desiccant air-conditioning systems have actually been applied in many different climatic conditions. In addition, feasibility studies through numerical studies on the applicability of the system have also been carried out. White et al. (2009) conducted numerical investigation of the solar-powered solid-based desiccant air-conditioning system in different Australian climatic conditions. The investigation centred on direct application of solar energy for the regeneration of the desiccant wheel. The study showed that the system is applicable in the warm temperate climate of Melbourne and Sydney, but not in the tropical climate of Darwin due to the hot and humid outdoor air. The research also showed that solar energy can support building comfort condition by means of a high ventilation rate. Bourdoukan et al. (2008) conducted numerical investigation of the solar-powered desiccant air-conditioning system using an evacuated tube collector. The researchers showed that collector areas vary with different location due to the required cooling load. For higher outdoor air humidity content, a higher solar collector area was needed in each of three geographical locations: La Rochelle (France), 13.4 g/kg; Bolzano Solar-desiccant air-conditioning systems 527 Table 16.5.1 Global development and application of the desiccant-based air-conditioning technologies (Enteria and Mizutani, 2011). Solid Desiccant Liquid Desiccant Hybrid Desiccant Continent Country System System System Africa Egypt o Kenya o Asia China o o o India o o o Iran o Iraq o Israel o o o Japan o o o Kuwait o Lebanon o Pakistan o Quatar o Saudi Arabia o o Singapore o South Korea o Thailand o o Turkey o Europe France o Germany o o o Italy o o o Poland o Spain o Sweden o o Switzerland o United Kingdom o North America Canada o o Mexico o USA o o o Oceania Australia o o o New Zealand o South America Cuba o Brazil o *Other countries may have research, development and application but no published literatures. (Italy), 11.4 g/kg; and Berlin (Germany), 9.5 g/kg. Furthermore, the study showed that evacuated tube is better than the flat-plate collector owing to the small size of the required back-up thermal energy. Sand and Fischer (2005) investigated the application of the solid desiccant air-conditioning system with a package of HVAC equipment. This showed that the active desiccant module delivers the required air condition with lower cost. Henning et al. (2001) considered the application of solar desiccant air-conditioning in Europe. They showed the possibility for a simple design of desiccant air-conditioning that is totally dependent on solar energy. However, the condition of the indoor air sometimes exceeded the required level. The system is best suited to a temperate 528 Solar energy sciences and engineering applications climate. Further solar desiccant with chiller is feasible in terms of economic and energy point of view in the warm-humid climate in which energy saving of 50% is possible. Mavroudaki et al. (2002) presented a numerical investigation with regard to the application of single-stage desiccant air-conditioning system in European cities. The system is applicable in some parts of southern Europe as long as latent load is not high; this is due to the high regeneration temperature requirement for high relative humidity air. The system is feasible in most of central Europe. Atlantic and inland regions of southern Europe appear to be much more suitable to this technology than Mediterranean costal regions. Smith et al. (1994) investigated the application of solar-powered solid desiccant air-conditioning system in residential buildings in the United States through transient system simulation (TRNSYS) simulation. The study focused on the Pittsburg (Massachusetts), Macon (Georgia) and Albuquere (New Mexico). It showed that building cooling demand was met, and that solar energy is suited to the operation of desiccant AC in the southwest of the US, with 72.7% of energy from solar. In the southeast of the county, 18.0% of desiccant air-conditioning was provided by solar energy. Casa and Schmitz (2005) investigated the application of borehole heat exchanger in desiccant air-conditioning with a gas engine (Figure 16.5.1). The system, installed in a demonstration building, saved 70% of energy for desiccant with a borehole heat exchanger. In the case of desiccant with chiller, it can save up to 30%. Cler (1992), investigating the possibility of applying desiccant dehumidification in military facilities, showed that it is recommended when additional cooling capacity is needed in existing HVAC systems. Also, for higher quantities of outdoor air make-up, a desiccant-based system is ideal for this type of application. In new construction, desiccant dehumidification equipment should be considered. This would reduce the size of chiller and electric energy demand. In addition, when designing new desiccant air-conditioning systems, desiccant regeneration from vapour compression, solar energy, cogeneration and others should be considered in the early phase of design. Halliday et al. (2003) looked at the feasibility of applying solar desiccant airconditioning systems in the UK. This study showed that the solid desiccant with solar power is feasible for application in buildings as long as the system is applied in a proper manner. Henning et al. (2007) investigated the application of desiccant air-conditioning system in a tri-generation system (power + heating and cooling). This used a vapour compression chiller and silica gel desiccant with the electricity to drive the chiller coming from combined cooling, heating and power CCHP, while the regeneration of the desiccant wheel was powered by waste heat from CCHP. An electric saving of more than 30% was made compared to the conventional air handling system. Enteria et al. (2012) conducted a numerical investigation of the solar-powered desiccant air-conditioning system in East Asia (Northeast Asia and Southeast Asia). The system ventilation rate increased from the temperate Northeast Asia to tropical Southeast Asia. The solar desiccanr air-conditioning system is applicable under East Asian climatic conditions as long as the proper specifications are applied, such as the size of the flat-plate collector, inclination of the collector plate, thermal storage tank volume and the required air flow rates going to the building. In addition, an alternative desiccant air-conditioning system in which air cooling can be done independently can reduce the air flow rate requirement, as a pure desiccant air-conditioning system cannot support a lower supply air temperature. Solar-desiccant air-conditioning systems 529 reject air ambient air desiccant wheel CHP plant condesing boiler borehole heat exchanger rot. heat exchanger 1 2 3 4 8 7 6 heater heater cooler room air supply air radiant floor heating/cooling desiccant assisted A/C system 5 Figure 16.5.1 Desiccant assisted HVAC system with borehole heat exchanger (Casa and Schmitz, 2005). Liu et al. (2004) investigated an office building in Beijing (China) with a total floor area of 20,000m2 over 10 stories and an installed open-cycle absorbent airconditioning system. During the summer, the system is used to control the air latent load while the absorption chiller and compression chiller are used to control the air sensible load. With this combined usage of open-cycle absorbent air-conditioning and chillers, the chilled water temperature is increased from 15.C to 18.C. In this situation, the COP of the chiller is increased due to the increased evaporative temperature. The schematic design of this system is shown in Figure 16.5.2. It has an efficiency of over 80% compared to the conventional HVAC system. The thermal storage tank and the absorbent storage tank provide long operating hours, due to the thermo-chemical storage capability of the absorbent. 16.5.3 Sub-temperate regions Fong et al. (2010) conducted investigations with regard to the application of solar desiccant air-conditioning system in the subtropical climate of Hong Kong. Although a typical desiccant air-conditioning system is not energy efficient, it can supply the required fresh air to a building, resulting in good indoor air quality and ventilation 530 Solar energy sciences and engineering applications Outdoor air Outdoor air Hot water Chilled water Bring air Return air Diluted solution Concentrated solution A A A Heat Recovery Exhaust air Exhaust air (a) (b) A A A A A A Diluted Concenerated Heat EX Heat EX Plate HX Plate HX Figure 16.5.2 Schematic of open-cycle liquid desiccant air-conditioning system with multiple airabsorbent contact exchanger (a) air dehumidifier and (b) absorbent regenerator (Liu et al., 2004). effectiveness. Hao et al. (2007) investigated the application of desiccant dehumidification with chilled ceiling and displacement ventilation. This showed it to be feasible in hot and humid climates owing to its capability of responding consistently to cooling demand. In addition, the system reduces building energy consumption by 8.2% compared to conventional air-conditioning. Niu et al. (2002) investigated application of the desiccant dehumidification by desiccant wheel with chilled-ceiling (Figure 16.5.3). The aim of the installation is for the desiccant to reduce air moisture content and thus avoid the condensation of moisture in the ceiling panel, and at the same time cool the air by means of the chilled ceiling. The results showed the combined system can save up to 44% of primary energy consumption in which 70% of the operating hour of the desiccant dehumidification can be provided with less than 80.C regeneration temperature. Solar-desiccant air-conditioning systems 531 Exhaust 9 1 6 7 8 3 10 11 2 11 Sensible Heat Wheel Evaporative Cooler Gas Heater 5 4 12 Ambient Desiccant Wheel Cooling Coil Chilled Ceiling Room Figure 16.5.3 Chilled-ceiling with solid desiccant air-conditioning system (Niu et al., 2002). Heater core Mixed air (a) (b) Mixed air Humid air Evaporator Fan Evaporator Dry air from Desiccant Air Fan Air Figure 16.5.4 Automobile air conditioning system: a) conventional and b) with desiccant (Nagaya et al., 2006). 16.5.4 Hot and humid regions Khalid et al. (2008) conducted numerical investigations with regard to the application of solar desiccant air-conditioning in Pakistan. The system had a higher COP in Lahore’s climatic conditions, without auxiliary cooling. Hirunlabh et al. (2007) considered the applicability of the solid fixed-bed desiccant air-conditioning in Thailand, showing that it can save 24% of electric energy. Furthermore, it is practical for application in large buildings as a centralized air-conditioning system. Nagaya et al. (2006) investigated the application of the solid desiccant wheel in an automobile airconditioning system (Figure 16.5.4). This showed that the system is energy efficient 532 Solar energy sciences and engineering applications ambient air pyranometer manometer orifice plate solar air heater auxiliary heater desiccant heat wheel exchanger evap. cooler venturi meter thermocouple amb. air sol. integ. 20° d.b. thermometer w.b. thermometer Figure 16.5.5 Solar-desiccant air-conditioning system ( Joudi and Madhi, 1987). compared to the conventional system. One of the problems encountered is the difficulty of controlling air humidity and temperature due to the heat exchange and coolant flow to the evaporator. Camargo et al. (2005) looked at the application of solid desiccant air-conditioning systems in Latin America and in tropical and equatorial cities. This system comprises a desiccant wheel and evaporative cooler and was shown to be applicable as an alternative to the vapour compression system since it can provide human thermal comfort conditions. Dupont et al. (1994) showed that a silica compact-bed desiccant air-conditioning system powered by solar energy in the tropical climate of Guadeloupe can produce cooling power. However, the system is not efficient due to losses. Hamed (2003) investigated the packed porous bed with burned clay as desiccant carrier, and desiccant impregnated with liquid calcium chloride. He showed that the mass transfer rate had a significant effect on the concentration gradient in the bed. Jain et al. (1995) investigated the solid desiccant air-conditioning system in 16 Indian cities, showing that a cycle with a wet surface heat exchanger gives a higher COP than other cycles. The Dunkle cycle has been found to be better in all climatic conditions. Heat exchanger effectiveness of above 0.8 is desirable for better performance of the cycles not using a wet surface heat exchanger. The effect on COP of the evaporative cooler is insignificant but it can control the room sensible load factor. Joudi and Madhi (1987) investigated the applicability of the solar desiccant air-conditioning system in Basrah, Iraq (Figure 16.5.5). For the local weather conditions, they showed that a regeneration temperature of 70.C can be provided using solar energy in clear skies. Kabeel (2007) investigated the application of the calcium chloride desiccant wheel constructed from iron wire and a cloth layer. The system uses sole solar energy for the regeneration of the desiccant. Tested in the climate of Egypt, the system had a high performance after the solar noon, with the wheel’s effectiveness dependent on solar radiation and air flow rate. The tri-generation plant installed in Politecnico di Torino (Italy) was developed to support the air-conditioning system shown in Figure 16.5.6 (Badami and Portoraro, 2009). The layout of the system, presented in Figure 16.5.7, shows the internal combustion co-generator, open-cycle absorbent air-conditioning system, cooling tower, two Solar-desiccant air-conditioning systems 533 Figure 16.5.6 Building heated and air-conditioned by the tri-generation plant in Italy (Badami and Portoraro, 2009). Building gas/water heat exchanger exhaust air cold water exhaust air moist air outdoor air outdoor air 20.000 m3/h cooling tower outdoor air 20.000 m3/h Exhaust gases to chimney conditioned supply air Hum Hum Rotary Heat Exchanger return air B O N R S M L NG Congenerator electrical power to user water/water heat exchanger Desiccant regenerator Desiccant conditioner Natural gas Figure 16.5.7 Layout of the tri-generation plant in Italy (Badami and Portoraro, 2009). heat exchangers and the connecting pipes. Based on the economic analysis, the system payback time was assessed as 6.8–7.7 years. The system is an interesting alternative to common AC technologies and further investigation and analysis will be done on the system’s actual operation. 534 Solar energy sciences and engineering applications Figure 16.5.8 Actual view of the open-cycle liquid desiccant air-conditioning system showing the absorber/dehumidifier (1), desorber/regenerator (2); air ducts (3), fan (4), rotary air/air heat exchanger (5), control cabinet (6), solar collector field (7) and hot water storage tank (8) (Gommed and Grossman, 2007). An open-cycle liquid desiccant air-conditioning system was installed at the Energy Engineering Centre at the Technion, in the Mediterranean city of Haifa, as depicted in Figure 16.5.8 with schematic diagram presented in Figure 16.5.9 (Gommed and Grossman, 2007). The system uses solar energy for regeneration, with thermal and chemical storage tanks (hot water and absorbent). The analysis of the system revealed a thermal COP of approximately 0.8 with a parasitic loss of around 10%. Based on the analysis, parasitic losses could be minimized with an improvement of the overall COP. A liquid desiccant system was installed at the Energy Park of the Asian Institute of Technology (AIT) in Pathumthani, Thailand (Katejanekarn et al., 2009). Figure 16.5.10 shows the actual installation of the solar-regenerated absorbent ventilation preconditioning system, while Figure 16.5.11 shows the schematic of the installed system. Results from the study showed that the solar open-cycle absorbent air-conditioning system can work in tropical climates like that of Thailand. The humidity of outdoor air can be reduced by 11%, while the temperature of the supply air is almost the same as the outdoor air. This means that an auxiliary air-cooling system is still needed to control the air temperature. Zhao et al. (2012) reviewed the open-cycle absorbent air-conditioning system in a 21,960m2 building in Shenzhen, China. The system can provide the required indoor thermal comfort and air quality based on temperature, humidity and CO2 concentration. With a COP of 4.0, the system is much better than an ordinary AC system. In Singapore, modern applications of liquid-based and solid-based desiccants are used in AC systems in commercial buildings. In one example where 540m2 solar hot water collector panels are used to support a 3000m2 factory hall using liquid-desiccant air-conditioning, the annual energy saving amounted to S$ 50,000 (Singapore Liquid Desiccant). Solar-desiccant air-conditioning systems 535 Supply Air 11 From Solution Storage Tank Absorber 13 TC2 14 Blower Ambient Air TC1 Blower Exhaust Air 16 15 Air/Air H.X. Desorber Ambient Air Splitter SV to Solution Storage Tank 12 10 Drain Solution Pump Water/Solution H.X 4 3 1 2 Hot-Water Pump Hot Water From Storage Tank Cold Water From Cooling Tower C.M2. Cold-Water Pump 5 8 Drain C.M1. 6 7 Solution Pump Water/Solution H.X. to Cooling Tower to Storage Tank to Absorber to Solution Storage Tank 9 Figure 16.5.9 Schematic description of the open-cycle liquid desiccant air-conditioning system: the solid thick lines indicate air flow; the solid thin lines indicate solution flow; the dotted thin lines indicate water flow (Gommed and Grossman, 2007). Cooling Tower Dehumidifier Solar C/Rs Figure 16.5.10 The solar-regenerated open-cycle liquid desiccant air-conditioning system installed at Asian Institute of Technology (AIT) (Katejanekarn et al., 2009). 536 Solar energy sciences and engineering applications Figure 16.5.11 Schematic of the solar-regenerated open-cycle liquid desiccant air-conditioning system installed at Asian Institute of Technology (AIT) (Katejanekarn et al., 2009). 16.6 FUTURE AND PERSPECTIVES Thermally operated desiccant-based air-conditioning technologies are being conceptualized as alternative devices for the control of air temperature and moisture content in artificially controlled indoor environments. The aim of these technologies is leave the general environmental conditions unaffected while controlling the indoor environment. Within this set-up there is potential for the utilization of clean energies converted to thermal energy, as well as the use of clean and environmentally friendly substances. The technology has potential for industrial, commercial and residential sector applications so as to provide comfortable artificially controlled indoor environmental conditions. To achieve this potential the technology must be properly developed and redesigned for practical application. Several studies have considered how the technology may be modified, for example in size reduction, and practically applied. Internal scrutiny of is important to evaluate the behaviour of the technology. Aside for the internal study of the system, the external factors affecting the processes, such as thermal energy sources and how it will be properly coupled to the technology to make it economical and easily maintained, are important when applied in real situations. The desiccant-based air-conditioning technology operated thermally utilizes the capability of desiccant material in controlling air moisture content. As the process is a physical or chemical phenomenon, the uptake of moisture does not require energy and, thus, the process is a natural one. The material needs a continuous re-sorption of moisture, either by pressure or temperature variation, to remove the uptake moisture in the material. The process is already utilized in chemical processes and industrial Solar-desiccant air-conditioning systems 537 controls. The most practical application is the desiccant material used in the control of moisture in sensitive products such as foods. The uptake of moisture is natural as can be observed. Since the process is exothermic, removal of moisture is done mostly using thermal energy, while some is removed through reduction of pressure. For the control of large spaces, such as industrial areas, assembly of complicated and sensitive electronic, electrical, chemical and mechanical systems, and application of the desiccant-based air-conditioning system is done using thermal energy. It should be noted that application of the technology in the commercial sector is undertaken in the same way as for industry, but with some variations. For residential buildings the technology is still at the infant stage, but as the development of larger, ideal systems reaches the middle stages of development, smaller-scale systems will surely follow on in due course, along with the incorporation of other processes for optimization. Several combined systems have been developed, such as Combined Heat and Power (CHP) and Combined Cooling, Heating and Power (CCHP) along with the Distributed Energy System (DES), the purpose being cost-effective implementation and optimization of the technology. In the combined systems the electricity generated is used for different applications, while thermal energy is used for the operation of sorption air-conditioning. This means that the systems provide heating, cooling and power production, and thus have higher overall system efficiency. In other systems the technology is coupled with other devices such as waste-thermal energy from from industrial, commercial or natural processes. This is dependent on the availability of thermal energy sources, and thus design is based upon it. Some are coupled with thermal energy produced from the utilization of clean energy sources such as solar energy, biogas or biomass, or other forms. In addition, utilization of cheaper night-time electricity is used through thermal storage done in sensible or latent storage systems. Storage of unutilized thermal energy from different thermal energy sources, particularly given its variable availability, is indeed an approach that should be optimized from both economic and practical standpoints. Other designs are based on the combined operation and utilization of available clean energy sources, waste-thermal energy sources or conversion of night-time electricity for thermal storage. 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Mallick2 1Polytechnic School, University of Lleida, 25001, Spain 2College of Engineering, Mathematics and Physical Sciences, University of Exeter, TR10 9EZ, United Kingdom 17.1 INTRODUCTION TO BUILDING INTEGRATION OF SOLAR ENERGY SYSTEMS Building integration of solar systems can refer to the roof or the façade of a building. Building Integrated (BI) solutions are of great interest since they have several advantages, such as aesthetically pleasing roof or facade integration, on-site energy generation, higher electrical and/or thermal conversion efficiencies, and better use of space. The term “building integration’’ is generally classified in two ways: (i) Building Integrated Solar Energy (BISE); and (ii) Building Applied Solar Energy (BASE). These are defined as: • BISE – when a solar energy system such as photovoltaics or solar thermal system is directly integrated within the building as a replacement of building fenestrations; for example, in a building integrated photovoltaics (BIPV) system where PV is the replacement for an existing building component or where no added building components are required to integrate such PV systems within the new buildings. • BASE – when a solar energy system such as PV and solar thermal collectors are installed within the existing building or new building without any replacement of building fenestrations; a good example of a building applied photovoltaics (BAPV) system is PV integrated into a roof or a solar thermal collector installed into a roof without any replacement of building materials. In the literature and in the commercial sector there are several BI systems; however, their architectural quality can be further improved. In this way, the use of solar technologies can be increased. Towards this direction, solar concentrating systems could offer several advantages in comparison with non-concentrating ones. Nevertheless, at present the use of concentrating technologies is limited, while most of the existing installations have devices of considerable size (for example, solar power towers, parabolic-trough concentrators, parabolic-dish concentrators, large Fresnel concentrators with 2-axis tracking). With regard to Concentrating Photovoltaics (CPV), more than 30 companies are developing these systems; many are start-ups, while there is a tendency for rapidly increasing production (Kurtz, 2009). The use of concentrating systems requires the development of reliable systems from the producers. On the other hand, utilization of a solar concentrator usually means the necessity of tracking. An important issue is the tracker to be simple in order to 546 Solar energy sciences and engineering applications reduce the complexity and the cost of the system. When comparing such a system with a flat-panel PV device built for the same application, the additional cost of the tracker and its maintenance must be compensated by the advantages that are provided by the use of a concentrating technology. On the other hand, flat-panel systems can be used to replace structural elements of a building and this (in most cases) is not possible by means of concentrating technologies (Swanson, 2000). In terms of the concentrators (reflectives or refractives), their integrability depends on its concentration factor, C (defined as the ratio between the aperture area of the primary concentrator and the active cell area). Concentrating systems with C>2.5X generally use tracking, whereas systems with C<2.5X can be static. However, in the long term static concentrators with higher ratios which make use of luminescence and photonic crystals may appear (Luque-Heredia et al., 2007). Low concentrating ratio systems (C<10X) are of great interest as they are mostly of linear geometry and thus one tracking axis is sufficient for efficient operation (Tripanagnostopoulos, 2008). The combination of improved sheet metal capability with the high capacity of the PV industry can lead to a large deployment of low concentration PVs (Kurtz, 2008). CPV is a feasible method to reduce the high initial cost of PV solar energy since concentrating solar radiation onto solar cells means that the area of semiconductor devices is diminished. Considering that a higher concentration factor has higher cost reduction, within the concentration range where single-axis tracking may be used, the most desirable concentration factor is that which approaches the upper limit of single-axis concentrators. On the other hand, solar thermal systems can be considered as an alternative solution for instances where the priority is the production of heat. Several promising technologies are included in this category, including solar collectors with vacuum tubes, reflectors combined with simple concentrating thermal (CT) systems. Moreover for solar thermal, solutions with low cost and low complexity should be preferred. For all applications of BI systems, certain requirements must be fulfilled. These requirements are associated with factors such as the design of the building, the conformity to the context of the building, an architecturally pleasing design, etc. The first part of this chapter describes the characteristics of BI systems without concentration, and later focuses on concentrating systems specifically. We begin by considering general concepts relating to building integration of solar thermal and photovoltaic systems. In the second part, all the terms and characteristics are adapted and applied to the concentring systems, thermal and photovoltaic. 17.1.1 Solar thermal systems and building integration requirements Building Integration of Solar Thermal (BIST) systems involve incorporating a solar heat generator while preserving and considering the other functions of the building envelope. Some of these functions are, for example, to protect the building interior space from weather conditions, to prevent noise, control daylight, regulate air renovation, and to achieve good insulation conditions leading to energy efficiency. The characteristics of solar thermal collectors present difficulties when being integrated into buildings, due to their size, materials, rigidity, colour and auxiliary installations. Accordingly, thermal collectors are mostly added to fulfil only a technical Building integrated concentrating solar systems 547 function (BASE) in a manner such that their visual impact is kept low and poor integration is minimized. This constrains the placement to the roof top (flat or inclined). Within the roof integration, the tilted configuration maximizes annual efficiency; however, overheating problems may occur in the warmest seasons. In these cases proper system dimensioning must be performed taking into account both the heat demand and overheating possibilities. BIST systems on the façade, at present, are practically non-existent because the main use of thermal collectors to date has been to provide heat, with no or minimum involvement in the building environment and envelope. In this way, installation of the solar panel in the most visible part of the building requires more attention than simply to cover a technical function. At this point a balance becomes necessary, sometimes pitting architecture and engineering against each other – in other words, a conflict between efficiency and aesthetics. To state how to increase the architectural quality of BIST, a clear definition of a successful integration must be established. In this sense, Munari Probst and Roecker (2007) conducted a survey of how architects and engineers perceive integration quality. The authors inferred a set of guidelines synthesizing the criteria highlighted in the survey: 1 The use of the solar energy system as a construction element (facade cladding, roof covering, etc.) facilitates the work of integration design. Certainly, the “logic’’ of the building design is easier to follow when the architect has to balance fewer elements which fulfil more functions. 2 The position as well as the dimensions of the collector field should be evaluated by considering the building as a whole (important issues are: energy production goals, formal integration needs, etc). Another option is the use of dummy elements (non-active elements with a similar appearance). 3 The choice of colours and materials for the system should match with colours and materials characterizing building and context. The initial choice of technology is fundamental because it imposes the material of the external-visible-system layer (glass, metal, plastic, etc). In the frame of the chosen technology, material treatments (surface colour, texture) offered by the various available products can be considered. 4 Module size and shape should be chosen by taking into consideration building and facade/roof composition grids (or vice versa). The proposed jointing types should also be considered while choosing the product (different jointing types differently underline the modular grid of the system in relation to the building). The criteria and guidelines imply that the designer should have knowledge of the existing technologies and available products in order to make the right choice (of products and technologies) to realize a successful building integration. Nevertheless, the above-mentioned integration requirements are difficult to achieve because the currently available collectors on the market have been developed with insufficient awareness of building integration aspects (Munari Probst et al., 2005). In order to properly design collectors oriented to their integration in buildings, a methodology has been defined by Munari Probst and Roecker (2007). Figure 17.1.1 shows the different steps involved in designing the collector: integration, efficiency, users’ preferences and feasibility – all need to be taken into account to cover all points of view. 548 Solar energy sciences and engineering applications Glazed flat plate hydraulic collectors 0. TEAM BUILDING ENSURE FROM START ALL NEEDED COMPETENCES IN ENERGY, CONSTRUCTION, ARCHITECTURE. 0. START FROM SOLAR THERMAL TECHNOLOGY TYPE 2. WITHIN THE CHOSEN TECHNOLOGY, SPECIFY THE COLLECTOR CHARACTERISTICS AFFECTING THE BUILDING APPEARANCE 3. FOR EACH CHARACTERSTIC LISTED ABOVE FIND THE BEST ACCEPTABLE/SUITABLE OPTIONS IN TERMS OF 3. CONSIDER AND BALANCE ALL THESE NEEDS FOR AN OPTIMIZED COLLECTOR DESIGN Unglazed flat plate hydraulic collectors Unglazed plastic hydraulic collectors Unglazed flat plate air collectors Vacuum tube hydraulic collectors Concentrating hydraulic collectors BASIC FORM of collector module characterized by MODULE SHAPE MODULE SIZE MODULE JOINTING BUILDING INTEGRATION: COLLECTOR AS PART OF A CONSTRUCTION SYSTEM ENERGY PERFORMANCES (COLLECTOR EFFICIENCY) PRODUCTION FEASABILITY, SUITABLE STANDARDISATION, AND COSTS FINALIZE COLLECTOR DESIGN USERS’ AESTHETIC PREFERENCES AND REQUIRED FREEDOM SURFACE TEXTURE SURFACE FINISH SURFACE COLOUR MATERIAL of system external layer characterized by Figure 17.1.1 Design methodology (Munari Probst and Roecker, 2007). Towards BIST systems, the Solar Heating and Cooling program (SHC) of the International Energy Agency (IEA) created a Task called the Successful Architectural Implementation of Solar Thermal Systems Project Database. The project database was initiated by Task 39 with support from Task 41 (Solar Energy and Architecture) and Task 37 (Advanced Housing Renovation with Solar and Conservation) and is now online. Some representative cases of BIST systems are included in Figure 17.1.2. From the four examples in the figure, some characteristics are apparent regarding the grade of integration, and pose two questions: (i) has the collector participating in the building envelope any additional function other than to produce heat?; and (ii) how does the collector fit in with the building’s aesthetics? Building integrated concentrating solar systems 549 Figure 17.1.2 Four different configurations of BIST systems. a) Multifamily house and commercial building with roof integrated solar collectors in Graz,Austria (Picture source:D. Chemisana); b) Roof integrated solar collectors ina a single family house in Pietarsaari, Finland (Picture source:A.G.. Hestnes, 1999); c) Transparent façade collectors in Ljubljana, Slovenia (Picture source: C. Maurer et al., 2012), and; d) Student residence with solar collectors integrated in the façade in Dornbirn,Austria (Picture source: Munari Probst and Roecker, 2007). 550 Solar energy sciences and engineering applications In order to answer the first question, it is necessary to see how multifunctional the solar collector is. In Figures 17.1.2a and 17.1.2c the collector is acting as a shading element, whilst in the other two cases (Figures 17.1.2b and 17.1.2d) it works as a double skin roof or façade respectively. As can be seen, the solar system provides additional positive functions to the building envelope. It is impossible from the pictures to see the effects of the insulation or the structure on the building, both of which aspects could also be satisfied by the solar technologies depending on the configuration used. In this sense the cost-effectiveness ratio becomes better when compared to a scenario in which a heat only benefit is gained. The second question concerns the formal integration of the collector, where the module’s characteristics are essential. In the first configuration (Figure 17.1.2a) the solar collectors are totally standard, placed on a specific structure to hold the systems at the required angle for both illumination control and electrical production. In Figures 17.1.2b and c the collectors also have quite standard characteristics. By contrast, in the another case the module’s active area is divided into strips to allow partial lightening (Figure 17.1.2c). IEA SHCP Task 41, concerning the influence and characteristics of the collectors in the appearance of the building, defines a set of key criteria for all types of solar collectors: module shape and size, jointing, visible materials, surface texture, colours, field size and position. When the collector’s response to these aspects is positive, the solar system’s flexibility ensures a proper building integration from a formal point of view. Next, Figure 17.1.3 synthesizes the two questions posed above. As can be seen, inside the house schematic a second column entitled “Constructive’’ is defined. This refers to the constructive properties of the solar collector: insulating properties, waterproofing, resistance to impacts and wind/snow loads, etc. This point has not been considered in the previous explanations due to as a building element, it is clear that must fulfil all constraints as a constructive element in order to be architecturally incorporated. A summary of the requirements for building integration of solar thermal systems are presented in Table 17.1.1. 17.1.2 Solar photovoltaic systems and building integration requirements In Table 17.1.2, several requirements for the building integration of non-concentrating PVs are presented along with their advantages. For grid-connected PV systems, some of their advantages are: they do not require additional land; the cost of the PV wall or roof can be offset against the cost of the building element it replaces; power is generated on site and thus replaces electricity that would otherwise be purchased at commercial rates; and by connecting to the grid, the high cost of storage is avoided and security of supply is ensured. The way people deal with PVs in architecture differs from country to country depending on factors such as the influence of the government on house building. However, in all cases of PV system integration into buildings there are some important issues which should be taken into consideration (Table 17.1.2). At this point it should be mentioned that the integration of PV systems in architecture can be divided into five categories, based on the increasing extent of architectural Building integrated concentrating solar systems 551 Figure 17.1.3 Development methodology for new systems building integration (Munari Probst and Roecker, 2012). integration (Reijenga, 2003): (ii) Applied invisibly; (ii) Added to the design; (iii) Adding to the architectural image; (iv) Determining architectural image; and (v) Leading to new architectural concepts. Several available urban-scale BIPV products and projects, along with details about their applications and characteristics, can be found at: http://www.pvdatabase.org/ products_viewall.php, from IEA PVPS Task 10. Some representative examples of BIPV are included in the following (Figure 17.1.4). From the pictures shown in Figure 17.1.4, the different grades of building integration can be distinguished, according to the criteria defined by IEA PVPS task 7 (see Table 17.1.2) and by Reijenga (2003). In the case of the first photograph (Figure 17.1.4a), the PV modules are placed on the flat roof in such a manner that they cannot be seen from the street. This example reflects the lowest grade of building integration, where the PV modules are applied to the building without replacement of any building material and without any extra functionality other than to produce electricity (BAPV). However, in Figure 17.1.4b the photovoltaic modules can be observed to be part of the façade and to participate directly in the architectural design. The modules are working as a building element at the same time than as an energy source. The PVs are manufactured using transparent encapsulation in order to allow partial illumination in the interior space. In Figure 17.1.4c, the inclined roof PV system can be seen 552 Solar energy sciences and engineering applications Table 17.1.1 Requirements for building integration of solar thermal systems (Munari Probst and Roecker 2007; 2012). Type of building integration Description of the system Requirements Facade and roof use of the solar energy system as integrations a construction element; evaluation of position and dimensions of the collector field by considering the building as a whole; choice of colours and materials based on building and context; module size/shape based on building and facade/roof composition grids (or vice versa) For all the types of ensuring/preserving the functions envelope integration of the envelope: protection (e.g. from rain), insulation, comfort etc. new designs based on: type of building; energy performance; users’ required freedom and aesthetic preferences; production feasibility/standardisation Functional Integration multifunctional collectors envelop functions compatible with integration into building envelope solar heat collection Constructive Integration integrating solar collectors in a to take into consideration the façade constructive characteristics of the specific technology to be integrated along with the specificities of the constructive system hosting the collectors Formal Integration flexibility in terms of all collector characteristics affecting building appearance: module shape/size/ jointing, collector colour, visible surfaces textures etc. to work with the transparency of an atrium and to provide architectural continuity as a roof. The PV panels are performing wholly and perfectly a building function, defining the image of the house and leading to a new architectural concept. The “Schott Ibérica’’ building (Figure 17.1.4d) in Barcelona combines a semi-transparent photovoltaic façade in which the coloured windows are transparent and the cells are opaque, resulting in a singular shading system. The façade constitutes an important element of the building’s aesthetics, both for its interior space and outwardly. As described previously, PV modules can be in the form of: (i) roofing materials; (ii) wall materials; and (iii) photovoltaic flexible modules applicable for construction materials (Shinjo, 1994; Toyokawa and Uehara, 1997). PV building roof integration includes exchangeable PV shingles, prefabricated PV roof panels and insulated PV roof panels (Shinjo, 1994). PV glass curtain walls and PV metal curtain walls are used for integration of PV modules with wall materials (Shinjo 1994; Toyokawa and Uehara, Table 17.1.2 Requirements for building integration of non-concentrating photovoltaics (Reijenga, 2000a; 2000b; 2003; Reijenga and Kaan, 2011). Type of building Advantages (along with the integration Description of the system Requirements production of electricity) Into the roof the system is part the system should be part of an impermeable of the external skin layer in the construction the system is above the the impermeable layer has to be pierced in impermeable layer order to mount the system on the roof if the PVs are transparent . sun protection (avoid overheating in serve as water and sun summer) in glass-covered areas, such as barriers and also transmit sunrooms and atriums daylight Into the façade glass or frameless orientation is important: facade systems might be shade, protection from rain PVs for sunshades, suitable in certain countries, especially at a northern louvers and canopies (above 50.N) or a southern (below 50.S) latitude PV systems as part of PVs replace building elements; PVs are a passive cooling very well ventilated at the back; a separate strategy (specific case) mounting construction is not necessary; the air-conditioning system is eliminated All the types of technical aspects of PV, cables and inverters building integration a structure strong enough to withstand wind or snow loads aesthetic quality . criteria formulated by the IEA PVPS Task 7 workgroup for the evaluation of the aesthetic quality: natural integration, architecturally pleasing designs, good composition of colours and materials, dimensions that fit the gridula, harmony, composition, PV systems that match the context of the building,well-engineered design, use of innovative design ventilation at the back of the modules (not important for thin film a-Si) shadow is not allowed on the modules; the mounting and removing of the modules should be easy; the modules should stay clean (or can be cleaned); the electrical connections should be easy; the wiring should be sun-proof and weather-proof 554 Solar energy sciences and engineering applications Figure 17.1.4 Four different configurations of BIPV systems. a) Flat roof installation in Lleida, Spain (Picture source: D. Chemisana); b) Solar façade with a detail of its view from inside the building in Lleida, Spain. (Picture source:D. Chemisana); c) Glass ceiling with transparent BIPV modules (Picture source:Petter Jelle et al, 2012); d) Solar transparent façade“Schott Ibérica’’ in Barcelona, Spain (Picture source: Munari Probst and Roecker, 2012). 1997). Most roof-mounted systems are retrofitted and hence are not fully integrated into the roof structure but are mounted onto existing roofs (Watt et al., 1999). Fully integrated BIPV roofing systems must perform the function of a standard roof and provide water tightness, drainage and insulation. These characteristics could also be extended to BIST systems. Building integrated concentrating solar systems 555 In terms of the aesthetics of the modules, frameless modules look very similar to window glass and the individual module is hard to recognize. Its smooth surface has a high aesthetic value. On the other hand, framed modules give a totally different effect. Firstly, the frames can be heavy and thus determine the total impression of the surface. Moreover, the highly visible frames divide the surface into modules and every individual module is very recognizable. As a solution, smaller frames in the same colour as the cells can be used and are less visible. Also, the soldering between the cells is a small detail but is important for the image of very visible PV systems. Older techniques had very visible and not very smooth soldering. However, new techniques have hidden the soldering better, e.g. by moving it towards the back. In addition, modules vary significantly in size, while the glazing is available as single and double (insulating) glass. In general, thin-film modules allow greater freedom to select size and colour than c-Si modules (Reijenga, 2003). Regarding the temperature of the PV panels, module efficiency and thus the amount of electricity produced, decreases as the temperature increases for mono and polycrystalline silicon cells (amorphous silicon cells present little temperature dependence). In many non-BIPV applications, modules are mounted on free-standing frames with ambient air on both sides (for cooling of both sides). In contrast, some BIPV applications install the modules in close contact to building material, such as roofs or wall insulation, and the lack of circulating air increases the module’s temperature. Hence, a good design criterion for mono or poly silicon applications is to allow as much cooling as possible by providing air flow behind the module and minimizing the effect of insulation. This is not an important issue for amorphous silicon modules (Fanney et al., 2001; Reijenga, 2003). In terms of cooling of PVs and thus increasing their efficiency, hybrid PVT (Photovoltaic Thermal) panels have been developed. These systems combine a PV panel with a thermal collector and in this way they produce electricity and heat simultaneously. In parallel, they offer other advantages, such as generation of higher electricity output than a standard PV panel, maximization of the available roof space, etc. PVTs are classified according to the kind of heat removal fluid: PVT/water or PVT/air and according to the type of fluid circulation: passive or active. In the literature there are several studies about these types of systems (Tripanagnostopoulos et al., 2002; Tonui and Tripanagnostopoulos, 2007; Ibrahim, 2011). Finally, it should be mentioned that high-efficiency operation requires substantial changes to the traditional inverter technologies. For example, the use of micro-inverters can reduce the overall cost of BIPV systems (Ericsson and Rogers, 2009). From this brief introduction to building integration of solar energy, it can be noted that research concerning BIPV started earlier than BIST. In fact, as an indicator, all the IEA Tasks focused on BIPV have now been concluded, while new Tasks regarding BIST are ongoing. Globally, the characteristics of how to integrate a photovoltaic or a solar thermal system reflect the same principles. The main difference is centred on the materials and the auxiliary systems needed, which with regard to building integration in the case of PV were more beneficial at the outset. At present, research conducted into new materials and configurations for solar thermal systems seeks to overcome difficulties for their building integration and make strong advances in this field. In the next section, the ideas described above are discussed in depth for building integration of concentrating systems. 556 Solar energy sciences and engineering applications 17.2 BUILDING INTEGRATED CONCENTRATING SYSTEMS In the field of Building Integrated Concentrating Solar systems (BICS), solar thermal as well as Photovoltaics (PVs) are included. First, some previous concepts regarding solar concentration are introduced ahead of the presentation of BICPV systems. Some of the configurations presented (e.g. Fresnel concentrators) can also refer to a Building Integrated Concentrating Solar Thermal (BICST) system, since the receiver CPV (or CPVT) can be replaced by a Concentrating Thermal (CT) unit. Later, some representative BICST technologies are included. Lastly, building integration requirements for concentrating systems are summarized. 17.2.1 Physics of concentrating solar system 17.2.1.1 Why solar concentration? The general concept of the PV solar concentrator is to reduce the amount of expensive solar cell by low-cost optical material. The sunlight either focused to a point or to a line is reflected by or refracted through an optical element to increase the solar flux at the solar cell, thus the electrical power of the system. The solar flux at the solar cell can be increased by light trapping using the total internal reflection using polymer material which has properties like glass. A PVsolar concentrator increases insolation intensity at the PV surface, reducing the area of photovoltaic material required per unit of power output. A cost reduction can be achieved for the overall photovoltaic/concentrator system when the concentrator cost is lower than the displaced PV material cost. In the case of thermal receivers, the use of solar concentration enables the attainment of higher working fluid temperatures. This leads to the more effective use of thermal concentrating systems for some applications, such as solar cooling with double-effect absorption chillers or concentrating solar power with higher-efficiency cycles. Optical concentrators can be reflective, refractive, diffractive or a combination of these. 17.2.1.2 The concentration ratio of a CPV system The concentration ratio determines the increase in relative radiation at the surface of the exit aperture/absorber. The concentration ratio can be defined in several ways, as described below. 17.2.1.2.1 Geometric concentration ratio The geometric concentration ratio is defined as the ratio of the area of aperture to the area of the receiver (Duffie and Beckmann, 1991), i.e. C =Aa/Ar. This ratio has an upper limit that depends on whether the concentrator is three-dimensional, such as a paraboloid, or two-dimensional, such as a compound parabolic concentrator. In terms of the half acceptance angle, the concentration ratio is defined as (Rabl, 1976b): C = 1 sin .s for a two-dimensional system C =  1 sin .s 2 for a three-dimensional system ..... .... (17.2.1) Building integrated concentrating solar systems 557 Concentrator R Sun r .s Aa Ar Figure 17.2.1 The half-angle subtended by the sun at a distance R from a concentrator with aperture area Aa and receiver area Ar . 17.2.1.2.2 Optical concentration ratio The optical concentration ratio for an actual system is the proportion of incident rays within the collecting angle that emerge from the exit aperture. This yields an optical concentration ratio defined as (Winston, 1980): Cop = Gr Ga (17.2.2) In other words, it can be defined as the average irradiance (G) over the receiver area, divided by the insolation incident on the collector aperture. 17.2.1.2.3 Limits to concentration An attainable concentration limit follows from physical optics (Rabl, 1976b). The disk of the Sun subtends at the surface of the Earth an angle of 2.s, as shown in Figure 17.2.1. Concentration is achieved by making a small image of the Sun with a given diameter optical device. Rays forming the smallest image make a cone with the largest semi-angle .. When the semi-angle of the image-forming cone is .=p/2, the maximum theoretical limit for the concentration ratio is achieved (Winston and Welford, 1979); i.e. the area of the input aperture of the device divided by the area of the Sun’s image. In three dimensions the maximum concentration ratio is Cmax =1/sin2 .s, for a value of .s =0.27., Cmax is 45,031. 17.2.2 Types of concentrators Imaging types of solar concentrator largely depend on image formation of the Sun to the receiver at the exit area. Alternatively, in some cases the Sun’s image does not form at the exit area, hence the term non-imaging solar concentrator. In Table 17.2.1, an indication of the different types of solar concentrator based on geometrical concentration ratios and their application types is given. This clearly shows that higher-concentration ratio systems are primarily used for large-scale power generation, which requires precise single- or two-axis tracking mechanisms. Low-concentration systems, such as compound parabolic concentrators (CPC) and quasi-stationary devices, are used for building integration and in domestic heating/cooling. 558 Solar energy sciences and engineering applications Table 17.2.1 Classification of solar concentrators based on the concentration ratios and their applications. Concentration ratio (X) Traking requirements, type of system Application 1–2 Stationary, CPC Heating, cooling, building integration 2–10 Quasi-stationary, CPC and Parabolic trough Power generation, heating and cooling 10–100 1-axis tracking, Parabolic trough Power generation 100–10000 2-axis tracking, Parabolic dish, Fresnel lens Power generation, CPV 10000–100000 2-axis, solar tower, solar furnace Power generation, materials assessment, laser 17.2.2.1 Non-imaging optics Non-imaging optics provide effective and efficient collection, concentration, transport and distribution of energy in applications where image forming is unnecessary (Welford and Winston, 1982). In imaging optics an image is formed at the exit aperture or on a screen, whereas for non-imaging optics no image of the object is formed (Winston 1980). In an “ideal’’ non-imaging concentrator the first concentrator aperture is radiated uniformly from a Lambertian source. The absorber then receives a uniform flux (Leutz, 1999b). The Sun approximates to a Lambertian source, although its brightness is not uniform and its wavelength-dependent brightness changes significantly across its disc. Practical non-imaging concentrators are designed with one or two pairs of acceptance half-angles that accept light (for example, diffuse insolation) incident at angles other than the almost paraxial rays of the Sun. Concentrated solar fluxes are thus non-uniform (Winston and Hinterberger, 1995) and flux densities at the absorber in a non-imaging solar concentrator are influenced by solar disc size and solar spectral irradiance (i.e. colour dispersion) (Leutz et al., 2000) and by the proportion of diffuse insolation, particularly at low concentrations (Rabl, 1985). For non-imaging optical systems, the edge-ray principle (Winston, 1974) states that extreme rays entering a concentrating system through an entrance aperture must be extreme rays when leaving this system through another aperture (i.e. receiver or absorber) for maximal optical concentration. Non-imaging systems can be made either by using a refracting lens or by using reflective mirrors (Boes and Luque, 1992). Fresnel lenses may offer flexibility in nonimaging optical design. For photovoltaics, uniformity of solar flux maintains electrical efficiencies by minimizing electrical energy losses (Leutz, 1999b). Non-imaging Fresnel lenses allow uniformity of flux at the photovoltaic material to be achieved as manufacturing errors at the back and front faces of Fresnel lenses are partially self-correcting. In contrast, an angular error in the plane of a mirror leads to twice this error in the reflected beam. 17.2.2.2 Non-imaging optics: examples a) Compound Parabolic Concentrator (CPC) Developed originally for the detection of Cherenkov radiation in particle physics experiments (Hinterberger andWinston, 1966), a CPC for solar energy applications consists Building integrated concentrating solar systems 559 Parabola B Absorber Focus of parabola A Q 2a P Focus of parabola B Axis of parabola Parabola A Aperture Figure 17.2.2 Schematic diagram of a compound parabolic concentrator. of two different parabolic reflectors that can reflect both direct and a fraction of the diffuse incident radiation at the entrance aperture onto the absorber, in addition to the direct solar radiation absorbed directly by the absorber. The axis of the parabola makes an angle .a or -.a with the collector mid-plane and its focus at P (or Q), as shown in Figure 17.2.2 (Rabl, 1976b). The slope of the end point of the parabola is parallel to the collector mid-plane. A CPC reflector shape can be designed in different ways according to the absorber shape. A basic form for a flat one-sided absorber is shown in Figure 17.2.2. The equation of a CPC with a flat absorber For the coordinates in Figure 17.2.3, by rotation of the axis and translation of the origin, in terms of the diameter (2a) and the acceptance angle (.max), the equation for a meridian section CPC reflector is (Welford, 1978) is: (r cos .max + y sin .max)2 + 2a(1 + sin .max)2r - 2a cos .max(2 + sin .max)z -a2(1 + sin .max)(3 + sin .max) = 0 (17.2.3) In polar coordinates, the complete parametric equation becomes (Welford, 1978) r = 2f sin(. - .max) 1 - cos . - a/; z = 2f cos(. - .max) 1 - cos . where f = a/(1 + sin .max) (17.2.4) 560 Solar energy sciences and engineering applications 2a z r CPC axis CPC Parabola axis . Figure 17.2.3 The angle . used in the parametric equations of the CPC. left parabola L right parabola R absorber FL FR 2.a l h fr fl extreme ray extreme ray axis of L axis of R (x,y) vertical line aperture (A) truncation line (xL,yL) Figure 17.2.4 Asymmetric CPC with half acceptance angle 2.a =fl +fr . b) Asymmetric compound parabolic concentrator (ACPC) The foci and end points of the two parabolas of an ACPC make different angles with the absorber surface, as shown in Figure 17.2.4. A is the aperture of the concentrator, R is the right parabola, L is the left parabola, FR is the focus of R and FL is the focus of L. For the ACPC, effective concentration ratio varies with the angle of incidence (Rabl, 1976b). Building integrated concentrating solar systems 561 Rabl (1976b) and Smith (1976) state that this type of concentrator has a maximum concentration ratio of [sin(.max/2)]-1, where .max =fl +fr. However, (Mills and Giutronich, 1978) have shown that the maximum concentration ratio for a parabolic asymmetric concentrator is CPAmax =  1 + sin fr tan (.max/2) - cos fr  [ cos(fr - .)-1] (17.2.5) and the minimum concentration ratio is CPAmin =  1 + sin fl tan(.max/2) - cos fl  (17.2.6) where cos . = sin .max - cos fr + cos fl 2 v (1 - cos fl cos fr) (17.2.7) Truncation of the reflectors of an ACPC reduces the size and cost of a system but results in a loss of concentration. The degree of truncation for a given ACPC can be determined in terms of the coordinates of a full ACPC. As Figure 17.2.4 illustrates, the left half of the ACPC is terminated at the point ( ¯x, ¯y), instead of the end point (xL, yL) of the full ACPC. The right half of the ACPC, is of course, truncated in an analogous manner. Truncation does not change the absorber area. The width (¯l ), height (¯h ), and the position coordinates ( ¯x) of the truncated ACPC are (Rabl, 1976b) ¯l = 2¯x cos . - ¯x2 s(1 + sin .) sin . + s(sin . - cos2 .) (17.2.8) ¯h = ¯x sin . + ¯x2 cos . 2s(1 + sin .) - s 2 cos .(1 + sin .) (17.2.9) and ¯x = s  (1 + sin .) cos . . .-sin . +  1 + ¯h h cot2 . 12 . . (17.2.10) 17.2.3 Building integrated concentrating photovoltaics The following is an analysis of the suitability for architectural integration of the principle types of existing concentrators, categorized by concentration factor. 17.2.3.1 High concentration systems (C >100X) High concentration systems require two-axis tracking with high precision (tolerances below 0.2.). The integrability of such a system will be highly compromised by the fact that it is mobile and by its size and dimensions which, even when minimized, are considerable. Incorporation is best achieved on the roof of the building (particularly 562 Solar energy sciences and engineering applications Figure 17.2.5 Schematic of the Cassegrain concentrator. In the left,2D ray tracing view and in the right, 3D model of the system (Gordon and Feuermann, 2005). (Picture source: Chemisana, 2011). for flat roofs) where the system is invisible from the exterior. This group is currently dominated by point focus Fresnel systems. There are a number of companies producing high concentration systems, some of whom are mentioned below. The Spanish company Sol3g, now absorbed by Abengoa Solar, produced a modular system with a row of 10 Fresnel lenses per module (solar aperture 1200×120mm2) which may be custom-designed according to space and consumption requirements (Chellini et al., 2007). The array of modules is positioned on a high precision tracker fabricated by Feina Ltd. Green and Gold Energy offers a system called SunCubeTM which consists of a device of approximately 1m2 (1064×1064mm2) aperture formed of 9 Fresnel lenses divided into 3 rows. Each system is coupled to a small two-axis tracker (Green and Goldenergy, 2011). Emcore commercializes a module formed by 8 Fresnel lenses in two lines of 4 (Emcore Soliant 1000). The dimensions of each module are 733×378mm2 (Emcore, 2012). Using similar technologies to those previously mentioned and employing Fresnel lenses, Whitfield Solar designed the WS-Si24 system. This achieves a concentration factor of 70X and is therefore a medium concentration system (Anstey et al., 2007). However, the optical and tracking technology used imply characteristics of integration that place it within this section. Using point concentration reflectors, Menova developed PowerSpar in two confingurations: the RFP 20 and the RFP 40 which consist of four RFP 20 units (Menova, 2008), and SVV Technology Innovations a Ring Array Concentrator (RAC), which emulates a point focus lens using reflectors (Vasylyev and Vasylyev, 2002; 2003). In addition to the above systems, it is worth mentioning the concentrator based on Cassegrain Optics (Figure 17.2.5) which has been commercialized by SolFocus (Gordon and Feuermann, 2005; Winston and Gordon, 2005; Mcdonald and Barnes, 2008) and the Light-guide Solar Optics (LSO) system presented by Morgan Solar Inc. (Morgansolar, 2012), which is based on optical light guides (Figure 17.2.6). As with the systems described in the previous paragraph, these can be installed on flat roofs. Both have a minimal receiver size, encapsulating the PV cells within the concentrator Building integrated concentrating solar systems 563 Solar rays LSO concentrator PV cell Figure 17.2.6 Light-guide Solar Optics (LSO) system presented by Morgan Solar Inc. (Morgansolar, 2012). (Picture source: Chemisana, 2011). itself. In 2011, Morgan Solar Inc. developed the Sun Simba, which is based on their standard, mass-produced LSO. Sun Simba is a fully engineered, modular concentrating solar panel, optimally designed to perform in high heat, high wind-load and extreme moisture conditions, leading to lower maintenance costs. Experimental performance results of the Sun Simba were presented by Myrskog et al. (2012). Given that the practical degree of integration of high concentration systems is limited by the need to incorporate them onto a high precision two-axis tracker, their means of integration are analogous between each case. It is therefore considered that the architectural issues are already well explained. Finally, in the field of high concentration systems it is worth mentioning the BICPV for box-window curtain wall assemblies, a day-lighting system within ‘double-skin’ and with a reduction in unwanted solar gain (the DOE Solar Energy Technologies Multi-Year Program Plan). The goals of this façade were: maximization of solar energy utilization, reduction of the overall energy consumption profile of the building (by means of the synergistic combination of power generation (using PV cells) and highquality heat capture with a simultaneous reduction in building cooling and lighting loads). The system consists of multiple concentrator modules which are situated within a glass façade or glass atrium roof of a building and are mounted on a highly accurate, inexpensive tracking mechanism (Dyson et al., 2007). 17.2.3.2 Medium concentration systems (10X< C <100X) Medium concentration systems can generally be divided into two groups: parabolic troughs and those using Fresnel optics in the form of lenses or mirrors. Concentrators which achieve the higher end of this concentration range (60–85X) and which, due to their size, make integration in buildings impossible are the Concentrating Solar Power (CSP) devices. In these type of systems, when decreasing the concentration ratio, building integration is facilitated. 564 Solar energy sciences and engineering applications An important problem associated with linear CPVs is overheating produced by the high density of light flux received by the cells, the majority of which is transformed into heat. The high concentration systems mentioned above use a passive cooling system, facilitated by the reduced dimensions of the cell which allow the use of a fin-based heat sink. Contrarily, in the case of linear concentration systems, passive cooling is complicated due to the larger surface area of the solar cells. This results in less cost-effective dissipators than in the case of insulated cells (Edenburn, 1980). For solar receivers which receive linearly concentrated light, the most adapted means of cooling is by active dissipation using liquids such as heat conducting fluids (Florschuetz, 1975). A new group of solar generators has appeared which take advantage of the evacuated heat stored in the thermal fluid as a bi-product. These are known as hybrid or co-generation Photovoltaic Thermal Concentrators (CPVT). Medium concentration systems present a wide range of possible building integration configurations. The principle designs, grouped by their integration characteristics, are described below. i) Parabolic trough concentrators The installation of parabolic trough concentrators in buildings is similar to that of high concentration systems; they are generally placed on flat roofs and are ideally hidden from view. Solar tracking is achieved by rotation of the entire concentrator/receiver ensemble about a single axis. The majority of devices which use parabolic concentrators are thermal generators (Weiss and Rommel, 2008). Exponents of parabolic CPV systems are: the Combined Heat and Power Solar System (CHAPS) developed at the Australian National University, with a concentration factor of 37X, which employs a photovoltaic/thermal (PVT) module (Coventry, 2005) and the Euclides system designed at the Polytechnic University of Madrid, with a concentration factor of 38.2X, in which the cells are passively refrigerated using fins (Luque et al., 1997; Antón and Sala, 2007). In 2009, Niedermever patented a new concentrating system for PVT generation (Niedermeyer, 2008). ii) Linear Fresnel reflectors With parabolic troughs, daily solar tracking is achieved by moving the entire concentrator/ receiver ensemble. However, within this range of concentrations good versatility is offered by systems which work using Fresnel reflection, some of which are worthy of note (some of the systems described below are included owing to their importance as concentrating technologies, despite being thermal collectors): 1 Concentrators with 2-axis trackers in which tracking is achieved by movement of the entire system, such as the BiFres system developed at the University of Lleida (equipped with a PVT receiver), whose integration in buildings would be restricted to flat (horizontal) roofs (Rosell et al., 2005) (Figure 17.2.7). 2 Static concentrators in which solar tracking is achieved by movement of the receiver. This option offers greater scope for integration in buildings as it can easily be installed on either flat or inclined roofs. Installation on façades, however, presents certain problems: the mirrors prevent light from passing into the building, and the mobile receiver, which protrudes outward from the building, creates strain Building integrated concentrating solar systems 565 Figure 17.2.7 Two-axis Bifres Fresnel reflector (Rosell et al., 2005). The PV receiver is water cooled, getting benefit of the thermal energy (PVT module). (Picture source: Chemisana, 2011). on the building structure and presents an unaesthetic appearance. The main exponent of this technology is the CCStaR system (equipped with a thermal receiver) developed at the University of the Balearic Islands. It should be mentioned that in the most recently presented CCStaR prototypes the Fresnel reflectors are replaced by parabolic reflectors (Pujol et al., 2006). 3 Concentrators in which the tracking is achieved by the movement of the individual mirrors. The possibilities for integrating such systems are similar to those for the previous case of a stationary concentrator. The most important design within this group is the Compact Linear Fresnel Reflector (CLFR) presented in 1997 by Mills and Morrison (Mills and Morrison, 1997) and commercialized by Ausra. The CLFR system is used for direct steam generation. Similar systems to the CLFR have been developed, these being the solar collector Solarmundo presented by (Häberle et al., 2002) and commercialized by Power Group GMBH, and the Mirroxx Fresnel collector commercialized by Mirroxx GMBH, a spin-off of PSE-AG (Berger et al., 2009). Using the same concentration principle, the company Helio- Dynamics have presented a collector, HD211, for integration in buildings with a receiver which can be thermal or PVT (currently the HD211, renamed HD10, only incorporates a thermal receiver) (HelioDynamics, 2004). In 2009, the University of Lleida constructed another such system with a PVT receiver, in collaboration with NUFRI Corporation and Trigen Solar S.L. More recently, research was conducted at the Australian National University on a PVT receptor incorporated into the concentrating system developed by Chromasun (Vivar et al., 2012); more details concerning this device are described below in section 17.2.4. 566 Solar energy sciences and engineering applications Figure 17.2.8 a) Ring Array Concentrator (RAC), and; b) Slat Array Concentrator (SAC) (Vasylyev, 2002; 2005). (Picture source: Chemisana, 2011). A new concept in the field of Fresnel reflection systems is the so called Non-imaging Reflective Lens (NIRL) concentrator, of which there are two types: the axially symmetric Ring Array Concentrator (RAC) and the linearly symmetric Slat Array Concentrator (SAC) (Vasylyev, 2005). These operate by using mirrors to direct and concentrate light onto a receiver behind the optical element, thus emulating a lens (Figure 17.2.8). The high concentration RAC requires two-axis tracking, whereas the medium concentration SAC can be employed with either one- or two-axis tracking (Vasylyev, 2004). This type of concentrator combines the high optical efficiency achievable by mirrors with the flexibility of design which is characteristic of lenses. The principle drawback of these systems is that solar tracking is achieved by movement of the whole system, incurring the aforementioned restrictions with regard to architectural integration. The University of Lleida is currently developing concentration technology which uses reflection, in a similar way to the systems developed by Chemisana and Rosell (2009), but with a design which prioritizes architectural integrability. The system consists of a linear Fresnel reflector which focuses radiation in a manner analogous to a lens. The receiver remains static and solar tracking is achieved by a simple and effective way by rotation of the individual mirrors. Thus, overall movement is minimized, facilitating incorporation into buildings and offering different possibilities for suiting the varied requirements of specific installations (Figure 17.2.9). High and medium concentration reflective systems are summarized in Table 17.2.2. iii) Linear Fresnel lenses Firstly, before commenting on the different properties and characteristics of Fresnel lenses when applied to BICPV, two systems must be mentioned. Although of low architectural integrability, as per the systems described previously, these are the first references of this kind of linear concentrator. These products are both formed by arched Fresnel lenses situated on a solar tracker. The first, designed by Entech Solar (USA) (O’Neill et al., 1990), uses a two-axis tracker and a PV or PVT receiver. The second, designed by SEA Corp. (later Photovoltaics Building integrated concentrating solar systems 567 Figure 17.2.9 Building integrated system presented by Chemisana and Rosell (Chemisana and Rosell, 2009), a) Curtain wall architectural design, and; b) Parasol architectural design. (Picture source: Chemisana, 2011) . 568 Solar energy sciences and engineering applications Table 17.2.2 Concentrating systems which use aspheric/Puntual Fresnel Lenses (PFL) or Linear Fresnel Lenses (LFL) as a primary concentrator (Chemisana, 2011) Company or reference Actual status of the system PFL/LFL C1 Cell type Abengoa Solar (Chellini, 2007) Commercially available PFL 476X 3J2 Green and Gold Energy Commercially available PFL 1370X 3J (Green & Goldenergy, 2011) Emcore (Emcore, 2012) Commercially available PFL 500X 3J Whitfield Solar (Anstey, 2007) Commercially available PFL 70X c-Si3 Photovoltaics International Stopped production in 2000 LFL 10X c-Si (Kaminar, 1991; Bottenberg, 2000) Entech Solar (O’Neill, 1990) Commercially available LFL 20X c-Si Chemisana et al. Demonstration and LFL 30X c-Si (Chemisana, 2011a,b) test installations 1C: Geometric concentration ratio. 23J: triple-junction solar cell. 3c-Si: monocristaline silicon solar cell. Internacional) (Kaminar et al., 1991; Bottenberg et al., 2000) uses a one-axis tracker and a PVT receiver. Recently, Entech Solar announced two new systems: TermaVoltTM II (PVT) and SolarVoltTM II (PV). Both systems are based on the same technology but use different receivers. Entech has resized the initial prototypes designed in the 1980s into these two smaller, low-cost devices applicable for both ground and roof-mount applications. Linear Fresnel lenses have a number of attractive features when used for solar concentration applications: they may be produced in large sizes; their aspect ratio can be designed to be small, leading to a compact concentrating system; they may be very thin, minimizing the cost of optical material and reducing the mechanical load on the supporting structure; and they may be made of reliable and durable material (Chemisana et al., 2009; Chemisana and Ibañez, 2010; Chemisana et al., 2011a,b). The ability of linear Fresnel lenses to separate the beam from the diffuse solar radiation makes them useful for illumination control in a building’s interior space. The Fresnel lenses are advantageous because they can combine within them both the concentrating element and the optically transparent window. The use of Fresnel lenses as a transparent covering material for lighting and energy control of internal spaces has attracted special attention (Tripanagnostopoulos et al., 2007). In addition to mentioning the general benefits of Fresnel lenses, some comparison should be made between those which are image forming and those which are anidolic. Image-forming Fresnel lenses for solar applications require high precision tracking. Non-imaging lenses, often convex and arched in shape and designed for medium concentration using one-axis tracking, have been devised as highly competitive solar collectors. If the tracking requirements are minimized, the cost reduction achieved by reduction of the PV cells’ surface area outweighs the cost of the optical elements (Leutz et al., 1999a; Leutz and Suzuki, 2001). The concept of using a fixed concentrator with a tracking absorber has been mentioned in the past (Kritchman et al., 1979; 1981a; 1981b). It is based on a stationary wide angle optical concentrator that, whatever the location of the sun, transmits the Building integrated concentrating solar systems 569 Figure 17.2.10 Linear Fresnel lens concentrating system developed at the University of Lleida. Right: detail of the concentrated spot on the PVT receiver. input radiation onto a small moving focal area which, in turn, is tracked by the PV receiver. Following this approach, the University of Lleida has developed a prototype (Figure 17.2.10) based on a stationary Fresnel lens which focuses solar radiation onto a PVT receiver which tracks the moving focal area (Chemisana and Ibañez, 2010). The advantages of this type of CPV make it architecturally versatile, allowing integration onto flat or inclined roofs or as lightweight façades, windows etc. Thus, their characteristics correspond perfectly to the requirements of well-integrated systems described by the IEA PVPS Task 7 workgroup (Luque-Heredia et al., 2007; Tripanagnostopoulos, 2008). Refractive systems under high- and medium-concentration ratios are summarized in Table 17.2.3. 17.2.3.3 Low concentration systems (C<10X) Within this group fall an extremely large number of systems and variations based on very distinct technologies. From an intuitive point of view, the simplest system is the V-trough reflector which directs light onto the receiver using flat mirrors (Tabor, 1958; Hollands, 1971; Fraidenraich, 1998a; Rabl, 1976a; Fraidenraich and Almeida, 1991). The V-trough can achieve at most 3X concentration. To ensure uniform illumination of the PV cells, planar reflectors require solar tracking (Freilich and Gordon, 1991; Gordon et al., 1991; Klotz et al., 1995; Fraidenraich, 1998b; Klotz, 2000; Poulec and Libra, 2000; Dobon et al., 2001). If solar tracking is not continuous, the V-trough behaves as an anidolic (non-imaging) optical system. Use of such devices, as with all lowconcentration systems, is beneficial as commercial cells may be used and as cell heating is reduced (King et al., 2000; Klotz et al., 2001). However, despite the low light flux, 570 Solar energy sciences and engineering applications Table 17.2.3 Concentrating systems which use Puntual Reflectors (PR), Parabolic Trough Reflectors (PTR), Linear Fresnel Reflectors (LFR) as a primary concentrator. (Chemisana, 2011) PR/PTR/ Cell Company or reference Actual status of the system LFR C1 Type Menova Energy (Menova, 2008) Commercially available PR 1450 3J2 SVV Technology Innovations (RAC) Commercially available PR 2500X 3J (Vasylyev S. andVasylyevV. 2002; (only concentrator provided) 2003) Solfocus (Gordon and Feuermann, Commercially available PR 500X 3J 2005;Winston and Gordon, 2005; Mcdonald and Barnes, 2008) Aronstis Solar (Bernardo, 2008) Commercially available PTC 10X c-Si3 Euclides system (Luque, 1997; Demonstration and test PTC 37X c-Si Anton and Sala, 2007) installations CHAPS system (Coventry, 2005) Demonstration and test PTC 38.2X c-SI installations BiFres system (Rosell, 2005) Demonstration and test LFR 11X c-Si installations HelioDynamics (Vasylyev, 2004) Commercially available LFR 10X p-Si4 (currently only thermal module) Trigen Solar Demonstration and test LFR 20X c-Si installations SVV Technology Innovations (RAC) Commercially available LFR 40X c-Si (Vasylyev andVasylyev, 2002; 2003) (only concentrator provided) Chemisana and Rosell Demonstration and test LFR 18X c-Si (Chemisana and Rosell, 2009) installations 1C: Geometric concentration ratio. 23J: triple-junction solar cell. 3c-Si: crystalline silicon solar cell. 4p-Si: polycrystalline silicon solar cell. cells may overheat to temperatures above 80.C. Operation is considerably improved through use of a thermal dissipator (Solanki et al., 2008). By taking advantage of the extracted heat, such a system can be converted into a PVT generator. In this range of concentrations, as in medium concentration devices, there are parabolic trough systems. An example is the PVT concentrating system (Bernardo et al., 2011) commercialized by companies Arontis Solar and Absolicon Solar, which concentrates with a ratio of 7.6X onto a Photovoltaic/Thermal (PVT) receiver. Figure 17.2.11 shows an experimental installation and a schematic where the module triangular cross section, which is actively refrigerated using a fluid, is illustrated. Compound Parabolic Concentrators (CPC) form a category of reflectors largely used for static systems. When used to illuminate PV cells, high losses are suffered due to the non-uniform illumination pattern produced on the cell surface. V-trough systems are less prone to producing detrimental hot-spots than are CPC systems (Swanson, 2003). Many works can be mentioned within this category (Almonacid et al., 1987; Goetzberger, 1988; Zanesco and Lorenzo, 2002; Mohedano et al., 1998; Uematsu et al., 1998; Garg and Adhikari, 1999; Brogren et al., 2000; Brogren and Karlsson, 2002; Helgesson, 2004; Nilsson, 2005; Mallick et al., 2004). With the objective of Building integrated concentrating solar systems 571 Figure 17.2.11 Parabolic trough PVT concentrator. Experimental installation in Sweden and schematic of the PVT module (Bernardo et al., 2011). Figure 17.2.12 Section of the stand-alone MaReCo for Stockholm conditions (aperture tilt 30.; optical axes 20. and 65. defined from the horizon) (Adsten et al., 2005). improving the system, many authors have opted for incorporation of bi-facial cells (Bowden et al., 1993; Mayregger et al., 1995; Ortabasi, 1997; Hernandez et al., 2000; Libra and Poulek, 2004; Weber et al., 2006; Parada et al., 1991). These double the amount of radiation or concentration that can be realized at the PV receiver. However, their use is not possible in high concentration systems as they have no un-illuminated surface onto which the essential heat dissipator may be attached. Designs of static concentration systems (with a typical acceptance angle of 30.) are normally intended for use with bifacial cells. Their concentration factor may be increased by use of a dielectric (Winston et al., 2005). In terms of the asymmetric compound parabolic concentrators, Figure 17.2.12 illustrates an asymmetric CPC known as “Maximum Reflector Collector’’ (MaReCo). This system was characterized experimentally for high-latitude bi-facial cell BIPV applications (Adsten, 2002) and different MaReCo configurations were developed 572 Solar energy sciences and engineering applications (stand-alone, roof integrated, wall integration etc). The figure shows the cross-section of roof integrated MaReCo designed for Stockholm conditions (Adsten et al., 2005). The highest optical efficiency which was reported for a bi-facial based MaReCo was 56%. Other examples of optical efficiencies of similar systems are: 91% for dielectricfilled BIPV covers (Zacharopoulos et al., 2000) and 85% for an air-filled asymmetric CPC BIPV system (Mallick et al., 2002a). Some other examples of static concentrators which use dielectrics are presented by Edmonds et al. (1987). The cells are positioned in a V-trough concentrator filled with oil or water (the dielectric) which also serves as a cooling function. The design presented by Shaw and Wenham (2000) uses an anidolic lens to reach a concentration factor of 2X and optical efficiency of 94%. The flat static concentrator described in (Uematsu et al., 2001a; 2001b; 2001c; 2003) has been used to analyse various possible configurations, including use of monofacial cells (1.5X) or bifacial cells (2X) and different types of illumination of the rear face. However, Uematsu et al. did not take into account thermal effects in the PV cells. Two linear dielectric non-imaging concentrating designs (symmetric and asymmetric) for PV integrated building façades were analysed using 3D ray-tracing analysis (Zacharopoulos et al., 2000). A “slim line’’ design was reported to achieve a concentration ratio of 4X (Wenham et al., 1995). Thermal analysis indicated that performance loss through additional heating of the PV cells was more than offset by the gains achieved through concentration. The efficiency of the module was reported to be 15% greater than that of the flat-plate module. Static concentrators offer a compromise between high concentration systems that require tracking and one-sun flat-plate modules (Wenham et al., 1995). Some additional studies in the field of dielectric non-imaging concentrating covers for PV integrated building facades are those of Zacharopoulos (2001) and Korech et al. (2007), where the total internal reflection (within the dielectric material) is used to provide optimal optical efficiency. An image of the second generation of the Photovoltaic Facades of Reduced Costs Incorporating Devices with Optically Concentrating Elements (PRIDE) dielectric-filled system, based on the first studies conducted by (Zacharopoulos et al., 2000) is shown in Figure 17.2.13. This system was studied and did show excellent power output compared to a similar non-concentrating system (they were characterized indoors by using both a flash and continuous solar simulator). Nevertheless, durability and instability (of the dielectric material) occurred under long-term outdoor characterization when the concentrator was made by means of casting technology. With regard to large-scale manufacturing, durability and reduction of the weight and cost of the concentrator, second generation PRIDE designs use 6mm wide solar cells at the absorber of dielectric concentrators. PV concentrator modules achieved a power ratio of 2.01 when compared to a similar non-concentrating system. The solar to electrical conversion efficiency for the PV panel was 10.2% when characterized outdoors. It should be mentioned that in large-scale manufacturing, a module cost reduction of over 40% is potentially achievable by using this concentrator technology (Mallick and Eames, 2007). Systems which concentrate radiation using elements opaque to visible light (CPC, V-trough) cannot be installed on areas of a building through which light is supposed to enter without reducing natural lighting in the interior. To reach a certain concentration Building integrated concentrating solar systems 573 Figure 17.2.13 Second Generation dielectric PRIDE Photovoltaic Concentrator Module (Mallick and Eames, 2007). factor, the reflecting surface area used by these systems is elevated compared to the surface area of PV cells. Given the detrimental effect this has on illumination of interior spaces, low-concentration static concentrators are preferable for architectural integration. These form the vast majority of CPC systems. They may be designed to be installed at any inclination or position which receives solar radiation (flat roofs, inclined roofs, façades, etc.). What’s more, although the reflector area is high with respect to the PV cell area, the volume of the entire system is relatively reduced, the geometry approximately tending to a parallelepiped. The aforementioned linear Fresnel systems with one-axis tracking (Kaminar et al., 1991; Bottenberg et al., 2000) can also be included within the low-concentration group. Leutz et al. (1999a) designed a convex-shaped non-imaging stationary Fresnel lens (1.5–2X) intended for warming up evacuated tubes, but able to be used with PV including secondary optics. Other low-concentration systems which are currently less used but are the subject of some study are: fluorescent/luminescent concentrators, quantum dot concentrators and holographic concentrators. The idea of using fluorescent concentrators (Figure 17.2.14) to concentrate both direct and diffuse radiation without tracking systems first appeared in the late 1970s (Weber and Lambe, 1976; Goetzberger and Greubel, 1977). In a fluorescent concentrator, a matrix of dye molecules absorbs radiation and emits light with a longer wavelength. Most of the emitted light is internally totally reflected and therefore 574 Solar energy sciences and engineering applications Figure 17.2.14 Fluorescent technology. The main part of reemitted photons are trapped in the layers and guided by total internal reflection to the PV cells placed on the edges. Photon loss occurs because of nontrapped emission or absorption by other dyes. Radiation frequencies noncaptured by the dyes are transmitted through the layers and can be captured and collected by a second fluorescent device or other system which take benefit of them. trapped and guided to the edges of the concentrator, where solar cells convert it into electricity. This concept was investigated intensively in the early 1980s (Wittwer et al., 1981; Seybold and Wagenblast, 1989). After 20 years of progress in the development of solar cells, fluorescent dyes and new concepts, several groups (Luque et al., 2005; Van Roosmalen, 2004; Goldschmidt et al., 2006; Richards and Shalav, 2005; Rau et al., 2005; Goldschmidt et al., 2007; Danos et al., 2006; Debije et al., 2007; Slooff et al., 2007) are currently reinvestigating the potential of fluorescent concentrators. In the quantum dot concentrator, the luminescent dye is replaced by quantum dots. Quantum dots are crystalline semiconductors which degrade less than organic dyes. Quantum dots can be tuned to the absorption threshold by the choice of dot diameter. Red shift between absorption and luminescence is primarily determined by the variance of dot sizes, which in turn can be optimized by choice of growth conditions. Reabsorption can therefore be minimized and high efficiencies and high concentration ratios achieved (Barnham et al., 2000). Of the systems which use this kind of technology, the organic dye-based Organic Solar Concentrator (OSC), designed in the Massachusetts Institute of Technology (MIT) and commercialized by Covalent Solar, has had the most impact, particularly within the field of building integration. The primary advantages of this system are that it does not require tracking and that its geometry is completely planar. It is formed of a stack, the principle layers being the OSC and the PV cells. This system is aesthetically superior to conventional PV systems; the colour is tuneable, better views through, transparent metal oxide contacts are not required and they may be formed with flexible plastics. Owing to their versatility, their position in buildings varies from atriums and roofs to windows. The concentration for which the system works with best efficiency, when combined with a variety of PV cell types, is 3 suns (Currie et al., 2008). Building integrated concentrating solar systems 575 The idea of holographic solar concentrators was first proposed in the early 1980s (Horner and Ludman, 1981; Ludman, 1982a,b; Bloss et al., 1982). Holographic elements have a number of advantages over conventional optical elements: they are lightweight, easy to reproduce and one holographic element can be used to perform several different functions. For example, there is a demonstration project which utilizes light-directing holograms for both daylighting and PV power generation (Müller, 1994). Holograms can be fabricated, which concentrate the spectrally disperse solar radiation (Ludman et al., 1992). There has been a surge of interest in this kind of technology thanks to the recent appearance of the Holographic Planar Concentrator™ (HPC) designed by the American company Prism Solar Technologies, Inc. This is the key technology in Prism Solar products. The HPC acts as an extremely low-cost concentrator (3 suns) without mechanical tracking or cooling systems. The bi-facial HPC configuration uses 72% less silicon than a standard module, leading to a more cost-effectiveness product. Furthermore, this new type of concentrator can be installed on rooftops or even incorporated into windows and glass doors (Castro et al., 2010; PrismSolar, 2012). Finally, it should be mentioned that for concentrating systems an important factor is their economic viability. In this direction there are some companies which aim at the development of cheap devices. An example is the “Cool Earth Solar’’ company with its innovative design of an inflated, balloon-shaped concentrator. Each 8-footdiameter concentrator is made of plastic film with a transparent upper hemisphere and a reflective lower hemisphere. When the concentrator is inflated with air, it forms a shape which focuses or concentrates sunlight onto a PV cell placed at the focal point (Coolearthsolar, 2012). Another example is the “Pacific Solar Tech’’ company with its MicroPV TM Concentrator Photovoltaic Modules and a “silicone-short environment’’ (Pacificsolartech, 2006). 17.2.4 Building integrated solar thermal (concentrating) As mentioned previously, the category of BICST may include configurations similar to the aforementioned BICPV, provided that there is a CT (instead of a PV) receiver. Of the examples of BICST systems which follow, some of the technologies have the potential for BI applications, while other devices have low-level potential. 1) Parabolic troughs (on a small-scale) The POWER ROOF™ is a new concept in solar energy because it is a high-temperature solar collector which at the same time also serves as an insulating system. The system integrated building approach is an important characteristic of this technology, along with energy cost savings. POWERROOF™is an attractive solution for large industrial and commercial energy users and is designed for new as well as for existing facilities. A basic advantage is the fact that its temperature range can fulfil an important array of issues. Collection temperatures above 398.C can be achieved, thus the system can provide energy in the form of steam for uses such as space heating and domestic hot water (as well as for industrial applications: desalination, absorption cooling, water purification, etc). In a commercial-building level, Solargenix Power Roof was installed in 2002 on a 930m2 office building in Raleigh, North Carolina. This system utilizes a fixed parabolic reflector and tracking receiver and provides 50 tons (176 kW) of 576 Solar energy sciences and engineering applications cooling as well as heat. It produces 170–175.C water which powers a double-effect absorption chiller. Where needed, the thermal energy can be converted into electricity (Gee et al., 2003). The companies Absolicon Solar and Arontis Solar offer two parabolic trough systems (T10 and MT10) very similar to the one described in 17.2.3.3, but with thermal absorbers. The T10 system is prepared for domestic solar heating and the MT10 for steam and process heat industrial applications (Absolicon Solar Concentrator AB, 2012). 2) Solar collectors with Micro-Concentrators (MCT) A representative example of this type of technology is the system commercialized by Chromasun Solar company. Chromasun and its partners in 2011 activated a 60-collector SCT system at Santa Clara University (SCU) to heat water (the largest rooftop concentrating solar thermal installation in California). The installation is shown in Figure 17.2.15a. From Figure 17.2.15b it can be seen the Fresnel reflector shape which focuses irradiance onto the absorber tube under a concentration ratio of 20–30X. From outside, the MCT has a flat-panel format. Like flat-panel collectors, MCT is easy to install, weighs about the same and is easy to look after. MCTs can produce much higher temperatures and efficiencies than a flat-plate collector and has a much smaller package. The MCT has been designed purposely for rooftop integration (Sultana et al., 2012). In relation to “micro’’ systems, it should be mentioned that in the field of CSP there is a tendency for development of “compact’’ devices (Micro-scaled Concentrated Solar Power (MicroCSP)) in order to facilitate their building integration. An example is a Micro CSP system for air-conditioning from the company Sopogy® (Sopogy, 2012). Figure 17.2.15 (a) Exploded view of a solar micro-concentrator system and (b) Cross-section of the MCT collector (Sultana et al., 2012). Building integrated concentrating solar systems 577 3) Integrated collector storage solar water heaters (ICSSWH) with asymmetric CPC reflector ICSSWHs are simple, low-cost solar devices. However, their disadvantage is the significant ambient heat losses, especially during night-time and non-collection periods (Tripanagnostopoulos and Yianoulis, 1992). Thereby, several studies have been carried out focusing on the improvement of thermal performance of these systems, especially during night-time operation. Horizontal water storage tanks are less effective in terms of water temperature stratification than vertically orientated ones (Smyth et al., 2003); however, they can achieve Concentration Ratios (CR) >1 when combined with CPC reflector troughs. In this way, water storage thermal losses are reduced, due to smaller absorber than aperture surface area (Tripanagnostopoulos and Souliotis, 2004). An example of a recently developed, novel ICSSWH system can be found in the study by Souliotis et al. (2011), who investigated an ICSSWH experimentally and theoretically. The goal of the study was the achievement of low thermal losses during night-time. The unit was based on a heat-retaining ICS vessel design consisting of two concentric cylinders mounted horizontally inside a stationary truncated asymmetric CPC reflector trough. The annulus between the cylinders was partially evacuated and contained a small amount of water, which changed phase at low temperature, producing a vapour and thus creating a thermal diode transfer mechanism from the outer absorbing surface to the inner storage vessel surface (Figure 17.2.16). The optical study, in conjunction with the experimental study of the ICS system, revealed that even if the absorbed solar radiation is distributed non-uniformly on the outer vessel throughout the year, thermal stratification within the inner vessel during daily operation is not significant. The thermal diode mechanism in the annulus plays an important role for the effective operation the device, with the pressure of the water vapour in annulus being an important parameter. Comparison of experimental results between the ICS system and a commercial Flat-Plate Thermosiphonic Unit (FPTU) device did show that the studied ICS system has an effective thermal performance and Figure 17.2.16 a) ICS experimental model mounted at the test field, and; b) Cross-sectional front view of the vessel (Souliotis et al., 2011). 578 Solar energy sciences and engineering applications thermal losses close to the FPTU, making it a promising solar system for domestic water-heating applications. Another possibility is the combination of stationary CPC collectors with multiple absorber segments and several configurations such as tubular and flat-fin type absorbers with pipe (Tripanagnostopoulos and Yianoulis, 1996). These types of solar devices are appropriate for applications in the temperature range of 100–200.C. At this point it should be mentioned that among the systems presented above, only system 1, parabolic troughs (on a small-scale), has the potential to be actually integrated into buildings (maximum level of BI). The other systems show a low level of BI potential since they are merely placed or installed on the roof without replacing elements of the roof. 17.2.5 Concentrating systems and building integration requirements Building integration is directly related with the concentration ratio C of a system. Thereby, the evaluation of the BI potential of a device is based on its concentration. In Table 17.2.4 the requirements for building integration of several concentrating systems are given. Table 17.2.4 Requirements for building integration of several concentrating solar systems (Chemisana, 2011). Description of the Type of building Tracking system integration requirements Restrictions/ High concentration: Flat roofs mainly Two-axis tracking Whole system movement. C>100x (mainly with high precision Control and management point focus (tracking, cleaning, etc). High temperatures Fresnel systems) management (case, surrounding air, etc.) Possible failures require immediate actions due to the very high fluxes effects Medium Flat or inclined Two-axis tracking in Whole system movement concentration: roofs some of the systems (if applicable) 10x5.0 kWh/m2. Legislation to foment the use of renewable energies and solar thermal energy 659 speaking, should not exceed 15%. In building projects, it is obligatory to include a report that states the calculation method. It should specify, at least on a monthly basis, the daily mean values of the energy demand and the solar contribution. The calculation method should include the annual overall capabilities defined by the thermal energy demand, the thermal solar energy contribution, the annual and monthly solar fractions, and the annual mean yield. It is necessary to ascertain whether there is any month of the year in which the energy theoretically produced by the solar installation exceeds the demand corresponding to the actual building occupancy or to any other period of time in which overheating conditions might occur. In such cases, measures should be taken to protect the installation. The performance of the solar collector, independently of the application and technology used, should always be equal to or greater than 40%. In addition, the mean performance of the installation during its period of operation should be greater than 20%. In order to assure system operation and prolong the useful life of the installation, a monitoring plan and a programme of preventive maintenance are implemented. The monitoring plan involves actions to guarantee that the operational values of the installation remain within normal range and thus guarantee its smooth operation. This simple plan includes, among other things, the visual inspection of installation components and maintaining the collectors in a clean condition. The maintenance programme involves a series of actions to ensure optimal operating conditions, capabilities, protection and the general durability of the installation. Such maintenance should be performed by trained personnel, and includes all repairs and replacement of consumable units as well as parts that have suffered deterioration during the useful life of the system. 20.6 MEASURES TO FOMENT THE USE OF RENEWABLE ENERGIES: GOVERNMENT INCENTIVES The EU (e.g. Regulation (EC) No 397/2009), (EP&C, 2009b) as well as the governments of EU member states have provided incentives for initiatives that improve energy efficiency and/or the use of renewable energies in building construction. These incentives are often tax-related (i.e. tax exemptions, deductions or refunds) or are in the form of investment aids and project loans at low interest rates. The beneficiaries of this financial aid can be either the users of the installations, who can use it to purchase solar equipment, or the companies that develop and manufacture renewable energy systems27. For example, for the production of electrical power with photovoltaic solar installations, the Spanish government has included these installations in a special regime. This regime, as specified in the Ley del Sector Eléctrico (1997) [Electrical Sector Regime Law], was conceived to foment power generation by energy sources whose special characteristics deserve higher financial returns than they would ordinarily obtain. Special regime energy sources receive higher remuneration than those belonging to the ordinary regime through a system of feed-in tariffs and premium payments. 27Businesses that market, install, service and sell this equipment or the energy produced by it. 660 Solar energy sciences and engineering applications In the first case, the owners of renewable energy installations receive a feed-in tariff for the energy produced. The amount of this payment is set by the government and depends on the technology. In the case of premium payments, the electricity produced is freely sold on the electrical energy market. It is bought at the market price (or at a freely negotiated price), supplemented by a premium payment from the government. In the same way as the tariff, the amount of the premium depends on the type of renewable energy. Regarding the promotion of renewable energies for heating and cooling in EU countries, there are two types of financial incentives: (i) direct investment aids; and (ii) specific funding programmes for solar thermal installations (M&I, 2010). For example, in 2003–2006, subsidies were given in the following conditions: • Direct aid. 40% of the investment cost of the thermal installation and an added 10% if the applicant was a small- or medium-sized company. The investment was calculated by fixing a maximum of 500 euros/m2 in the case of thermal solar installations for the production of hot water, whatever its use (CDET, 2003). • Financing of installations. Other programmes offered financial aid in the form of low-interest loans that covered up to 100% of the investment costs (thermal solar collectors, storage tanks, heat exchangers, circulation pumps, pipes, valves and connections, expansion tanks, insulation, construction work, etc.), the engineering costs associated with the project, as well as the funds necessary to transact the permits and aids (up to 675 a/m2), (IDAE, 2004). 20.7 ECONOMIC IMPACT OF SOLAR THERMAL ENERGY The residential building sector in Spain experienced spectacular growth in 1996-2007 (see Graph 20.7.1). This was accompanied by a significant increase in the investment in housing. In fact, from a little more than 5% of the gross domestic product (GDP) in the 1990s, it rose to 7.4% in 2007. Not surprisingly, construction was also an Graph 20.7.1 Evolution of the number of residential buildings (Ministry of Development. Spain). Legislation to foment the use of renewable energies and solar thermal energy 661 Figure 20.7.1 Solar collectors on a building rooftop (courtesy of Solaris S.L.). Graph 20.7.2 Percentage of buildings with thermal solar energy installed (Ministry of Development. Spain). important source of employment during this period. Of the more than six million jobs created from 1996 to 2007, 23% were in this sector (Domenech, 2011). After 2007, there was a sharp drop in the number of houses built, and even today the residential sector is still in the process of adjusting to this drastic change. In 2011, the investment in housing construction was less than 4% of GDP. Various studies coincide in affirming that the housing demand will eventually begin to increase again and finally stabilize at around 300,000 houses in future years. The weight of the renewable energy sector in the economy has steadily increased. From 0.47% of GDP in 2005, the percentage rose to 0.70% in 2009. Wind and hydroelectric energy are the most important renewable technologies reflected in the GDP, and together they make up 44.84% of the total contribution of renewables. 662 Solar energy sciences and engineering applications In contrast, photovoltaic solar energy accounts for 38.02% and thermal solar energy for 0.78% (Deloitte, 2011). In 2010, the area in Spain covered by thermal solar panels was approximately 2,400,000 m2(see Figure 20.7.1). If Spain meets the targets in the PANER, the energy generated in the solar thermal sector for heating and cooling will predictably rise from 61 ktoe in 2005 to 644 ktoe in 2020 (M&I, 2010). The development of legislation and especially the requirement in the Spanish Technical Building Code28, enacted in 2006, has made thermal solar technology an integral part of the majority of buildings currently under construction. Although in 2005, less than 10% of the buildings had this technology, in 2010, the percentage of buildings with thermal solar systems soared to almost 80% (see Graph 20.7.2). In 2007 and 2008, the expectations of future growth in this sector were extremely positive despite the drop in the construction of new residential buildings (see Graph 20.7.1). Thermal solar energy has thus become a viable energy alternative in the residential building sector. 20.8 CONCLUSIONS In recent years, the European Union has made a significant effort to develop a regulatory framework to enhance the energy performance of buildings and to foment the use of renewable energies to increase energy efficiency. The goal is to reduce both energy consumption and energy dependence in the EU by regulating a sector responsible for 40% of greenhouse gas emissions. Since energy consumption related to thermal processes in buildings accounts for 69% of the total energy consumption, this means that the use of thermal solar energy in building construction has become a priority for the achievement of energy performance objectives. To implement this technology in building construction and/or renovation in the EU, the following measures have been approved. a) Development of a legislative framework in the EU: – Directive 2002/91/EC of the European Parliament and of the Council of 16 December 2002 on the energy performance of buildings. This directive states that buyers or tenants of buildings should be provided with an energy performance certificate. The rationale behind these certificates is to thus obtain buildings with a high energy performance by improving their thermal properties and fomenting the investment in energy-saving systems. – Directive 2009/28/EC of the European Parliament and of the Council of 23 April 2009 on the promotion of the use of energy from renewable sources. This directive specifies a common framework for all EU countries with a view to fostering the use of energy from renewable sources. – Directive 2010/31/EU on the energy performance of buildings. The objectives of this directive include increasing “the number of buildings which not only 28According to Article 1.5 of this building code, part of the energy should come from thermal solar energy in all new and renovated buildings in which there is a supply of domestic hot water. Legislation to foment the use of renewable energies and solar thermal energy 663 fulfil current minimum energy performance requirements, but are also more energy efficient, thereby reducing both energy consumption and carbon dioxide emissions’’. For this purpose, the directive requires member states to draw up national plans for increasing the number of nearly zero-energy buildings. b) Development of legislation in the member states of the European Union. As an example, the Spanish Technical Building Code requires all new and renovated buildings in which domestic hot water must be installed to use thermal solar energy for part of the energy demand. This legislation is supplemented by a series of programmes for the funding of installations that run on renewable energy. The financial incentives offered can be tax-related (i.e. tax exemptions, deductions or refunds) or in the form of investment aids and low-interest project loans. Thanks to these legislative measures„ as well as to government subsidies, in 2010, 80% of the new and renovated buildings in Spain had thermal solar energy systems installed. REFERENCES Brounen, D. and Kok, N. (2011) On the economics of energy labels in the housing market. Journal of Environmental Economics and Management, 62, 166–179. Commission of the European Communities (2007) Communication from the Commission to the Council and the European Parliament: Renewable Energy Road Map. Renewable energies in the 21st century: building a more sustainable future, COM (2006) 848 final, Brussels; 2007. 3–18. CDET (2003) Council Directive 93/76/EEC of 13 September 1993 to limit carbon dioxide emissions by improving energy efficiency (SAVE) European Official Journal, 22 September 1993. Council of the European Union (2007) Brussels European Council, 8/9 march 2007. Presidency Conclusion. Deloitte (2011) Impacto económico de las energías renovables en el sistema productivo Español. 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REAL DECRETO [Royal Decree] 314/2006 de 17 de marzo, por el que se aprueba el Código Técnico de la Edificación. (In Spanish). 28.03.2006 BOE.; num. 74 Real Decreto [Royal Decree] 47/2007 (2007) de 19 de enero, por el que se aprueba el procedimiento básico para la certificación de eficiencia energética de edificios de nueva construcción. (In Spanish). 31.01.2007 BOE.; num. 27 REAL DECRETO [Royal Decree] 1027/2007 de 20 de julio, por el que se aprueba el Reglamento de Instalaciones Térmicas en los Edificios. (In Spanish). 28.08.2007 BOE.; num. 207 REE (2011) Informe del sistema eléctrico en 2010. (In Spanish.) http://www.ree.es/sistema_ electrico/informeSEE.asp This page intentionally left blank an informa business Solar energy is available all over the world in different intensities. Theoretically, the solar energy available on the surface of the earth is enough to support the energy requirements of the entire planet. However, in reality, progress and development of solar science and technology depends to a large extent on human desires and needs. This is due to the various barriers to overcome and to deal with the economics of practical utilization of solar energy. This book will introduce the rapid development and progress in the field of solar energy applications for science and technology: the advancement in the field of biological processes & chemical processes; electricity production; mechanical operations & building operations enhanced by solar energy. The volume covers bio-hydrogen production and other biological processes related to solar energy; chemical processes for the production of hydrogen from water and other endothermic processes using solar energy; the development of thermo-electric production through solar energy; the development of solar ponds for electric energy production; the mechanical operation with solar energy; the building operation with solar energy optimization and urban planning. This book is an invaluable resource for scientists who need the scientific and technological knowledge of the wide coverage of solar energy sciences and engineering applications. This will further encourage researchers, scientists, engineers and students to stimulate the use of solar energy as an alternative energy source.