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EURONETRES European Network on Education and Training in Renewable Energy Sources United Nations · Educational, Scientific and · SIMMETRIA PUBLICATIONS Cultural Organization . EURONETRES European Network on Education and Training in Renewable Energy Sources SOLAR THERMAL CONVERSION Active solar systems Editor Petros J. Axaopoulos Wi th the support of Venice Office United Nations . SIMMETRIA PUBLICATIONS Educational, Scientific and * Cultural Organization ♦ ATHENS 2011 Central disposal SIMMETRIA PUBLICATIONS Ioannou Theologou 80, 15773 Zografou Athens - Greece Tel. +0030 210.77.10.548 - fax +0030 210.77.10.581 e-mail: [email protected] All Rights Reserved Copyright: © 2011, Petros J. Axaopoulos No part of the material, protected by this copyright notice, may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner. The designations employed and the presentation of material throughout this publication do not imply the expression of any opinion whatsoever on the part of UNESCO concerning the legal status of any country, territory, city or area of its authorities, or concerning the delimitation of its frontiers or boundaries. The authors are responsible for the choice and the presentation of the facts contained in this publication and for the opinions expressed therein, which are not necessarily those of UNESCO and do not commit the Organization. ISBN 978-960-266-328-8 Editing - Printing: S. ATHANASSOPOULOS & CO IOAN. THEOLOGOU 80 ZOGRAFOU, T#l 210.77 10.648 · 210.77.02.033 · 210.77 07.114, FAX. 210.77.10 Ul www.simrnetrla.ar List of contributors Prof. Petros J. Axaopoulos Energy Technology Department Technological Educational Institute of Athens Athens - Greece Email: [email protected] Prof. Viorel Badescu Candida Oancea Institute Polytechnic University of Bucharest Spl. Independentei 313, Bucharest 060042, Romania Email: [email protected] Prof. Milorad Bojlg Faculty of Mechanical Engineering at Kragujevac University of Kragu)evac Sestre Janjic 6 34000 Kragujevac, Serbia Email: [email protected] Asst. Prof. Yannis G. Caourls Dept, of Mechanical Engineering & Aeronautics University of Patras 26500 (Rio) Patras, Greece Email: [email protected] Assoc. Prof. John Gelegenis Energy Technology Department Technological Educational Institute of Athens Athens - Greece Email: [email protected] Dr. Myrsinl Giannouli Senior Researcher Solar Energy Laboratory Physics Department, University of Patras GR-26500 Patras, Greece Email: [email protected] 4 SOLAR THERMAL CONVERSION - ACTIVE SOLAR SYSTEMS Dr. Paul Gravila West University of Timisoara Department of Physics Timisoara - Romania Email: [email protected] Dr. Soteris Kalogirou Cyprus University of Technology Department of Mechanical Engineering and Materials Sciences and Engineering P.O. Box 50329 3603 Limassol - Cyprus Email: [email protected] Dr. Marius Paulescu West University of Timisoara Department of Physics Timisoara - Romania Email: [email protected] Prof. Panayotis T. Tsilingiris Energy Technology Department Technological Educational Institute of Athens Athens - Greece Email: [email protected] Prof. Panayiotis Yianoulis Director of the Solar Energy Laboratory Physics Department University of Patras, GR-26500 Patras, Greece Email: [email protected] CONTENTS Preface ................................................................................................................ 1. Introduction Petros l Axaopoulos Bibliography ............................................................................................ · 18 2. The sun and its radiation Petros l Axaopoulos 2.1 The sun ............................................................................................ 1*> 2.2 Extraterrestrial solar radiation and atmospheric effects ............. 23 2.3 Components of solar radiation ........................................................ 26 2.4 European distribution of solar irradiation ................................... 31 2.5 The measurement of solar radiation ............................................ 33 2.5.1 Pyranometer ....................................................................... 33 2.5.2 Pyrheliometer .................................................................... 36 2.5.3 Sunshine recorder ........................................... 37 References ............................................................................................... 38 Additional reading .......................................................................... 40 3. Basic principles of solar geometry Petros J, Axaopoulos 3.1 The earth’s rotation ....................................................................... 43 3.2 Longitude and latitude ................................................................... 46 3.3 Basic solar geometry angles ......................................................... 47 3.3.1 Determination of solar angles ............................................. 51 3.4 Solar incidence angle ..................................................................... 56 3.5 Solar time ..................................................................................... 61 References ................................................................................................. 63 Additional reading .................................................................... 64 6 SOLAR THERMAL CONVERSION - ACTIVE SOLAR SYSTEMS 4. Heat transfer topics Milorad Bojic 4.1 Conduction .......................................................................................... 65 4.2 Convection .......................................................................................... 66 4.3 Radiation ............................................................................................ 66 4.4 Combined heat-transfer .................................................................... 68 4.5 Heat exchangers ................................................................................ 73 4.6 Conclusion .......................................................................................... 79 References .................................................................................................... 79 5. Solar radiation incident on horizontal and tilted surfaces at ground level Viorel Badescu 5.1 Model accuracy ................................................................................ 82 5.2 Solar radiation on horizontal surfaces at ground level .............. 83 5.2.1 Models to evaluate solar irradiance values on clear sky days 85 5.2.1.1 Very simple clear sky solar irradiance models 85 5.2.1.2 Simple clear sky solar irradiance models ......... 87 5.2.2 Computation of hourly average solar irradiance ............ 92 5.2.2.1 Very simple cloudy sky models to compute hourly average solar irradiance ...................................... 92 5.2.2.2 Simple cloudy sky models to compute hourly ave rage solar irradiance ....................... 93 5.2.3 Computation of daily average solar irradiation .............. 97 5.2.3.1 Computation by integration from solar irradiance data ........................................................................ 97 5.2.3.2 Relationships to evaluate daily diffuse and beam irradiation .............................................................. 99 5.2.4 Models to evaluate monthly averaged irradiation values 101 5.2.4.1 Relationships for global irradiation based on sun shine data .............................................................. 101 5.2.4.2 Relationships for global irradiation based on point cloudiness data .................................................... 103 5.2.4.3 Relationships for diffuse and beam irradiation 104 5.3 Solar radiation on arbitrary tilted and oriented surfaces ............ 105 5.3.1 Solar irradiances on tilted surfaces .................................... 105 Contents 7 5.3.2 Daily solar irradiation on tilted surfaces .................. 109 5.3.2.1 Daily optimum tilt and orientation angles ___ 112 5.3.3 Monthly averages of daily solar irradiation on tilted surfaces 113 5.3.3.1 Monthly and seasonal optimum tilt and orientain angles ................................................................. 116 5.4 Recommendations about computing solar radiation on horizontal and tilted surfaces ........................................................................ 118 References .............................................................................................. 119 Glossary .................................................................................................. 121 6. Optical characteristics of materials Marius Pauiescu and Paul Gravila Introduction .................................................... 123 6.1 Optics summary ............................................................................ 124 6.1.1 Reflectance - transmittance - absorptance ..................... 124 6.1.2 Transmittance - absorptance product ............................. 138 6.1.3 Total emitance, absorptance and transmittance .............. 141 6.2 Transparent cover platem aterials ................................................. 144 6.3 Opaque collector plate surfaces ................................................ 146 Summary ............................................................... 149 References ..................................................................... 149 7. Solar collectors Soteris A. Kaloglrou 7.1 Flat plate collectors (FPC) ............................................................ 152 7.1.1 Flat - plate collector components ................................... 154 7.1.1.1 Cover plate ....................................................... 154 7.1.1.2 Collector absorbing plates ................................... 155 7.1.2 Liquid heating flat-plate collectors ................................ 158 7.1.3 Air heating flat-plate collectors — ............................. 160 7.1.4 Thermal analysis for a Hat-plate collector ............... |61 7.1.4.1 Collector efficiency factor .................................. 164 7.1.5 Collector efficiency ........................................................... 168 7.1.5.1 Collector incidence angle modifier ................... 170 7.1.5.2 Collector time constant ............ 171 8 SOLAR THERMAL CONVERSION - ACTIVE SOLAR SYSTEMS 7.1.6 Collector performance and quality testing ...................... 172 7.1.7 Factors affecting the solar collector efficiency .............. 174 7.2 Evacuated tube collectors (ETC) .................................................... 181 7.3 Concentrating collectors ................................................................. 184 7.3.1 Compound parabolic collectors (CPC) ........................... 186 7.3.2 High concentration collectors ............................................ 190 7.3.2.1 Parabolic trough collectors ................................ 190 7.3.2.2 Linear fresnel reflector (LFR) .......................... 192 7.3.2.3 Parabolic dish reflector (PDR) .......................... 193 7.3.2.4 Heliostat field collector (HFC) .......................... 194 References ................................................................................................... 196 Further reading ........................................................................................... 197 Glossary ...................................................................................................... 197 8. Thermal energy storage Panayotis T. Tsilingiris 8.1 Introduction ....................................................................................... 203 8.2 The specific heat capacity of substances ........................................ 206 8.3 The technology of sensible heat storage ........................................ 209 8.4 The analysis of the water heat storage .......................................... 211 8.4.1 The well mixed storage ..................................................... 213 8.4.2 The influence of thermal stratification ........................... 216 8.5 The analysis of heat storage in packed bed solid materials — 220 8.6 The heat storage in building elements and structures .................. 224 8.6.1 Human comfort and thermal storage ................................ 224 8.6.2 Heat storage in passive building design ........................... 225 8.6.3 The heat strorage in structural elements and the building envelope ............................................................................... 228 8.6.4 The combined influence of thermal resistance and heat ca pacity in building elements ................................................ 233 8.7 Long term seasonal storage ........................................................... 238 8.7.1 Limiting dimensions of seasonal heat storage systems and examples ............................................................................... 240 8.8 Solar ponds ...................................................................................... 249 8.8.1 The fundamental system description and operation ____ 250 8.8.2 Modeling of the physical processes ....................................‘252 8.8.3 Stability and zone boundary migration considerations .. 257 8.8.4 The potential of solar ponds as large low-grade heat sto rage systems ......................................................................... 259 9 Contents 8.9 Latent heat storage ....................... 261 8.9.1 The characteristics of candidate phase-change materials 261 References ............................................................................................... 269 9. Solar thermal systems Soteris A. Kalogirou 9.1 Domestic hot water ....................................................................... 274 9.1.1 Thermosiphon systems ...................................................... 274 9.1.2 Direct circulation systems (active) .................................. 277 9.1.3 Indirect water heating systems (active) ......................... 279 9.1.4 Air systems for hot water production (active) ................. 280 9.2 Solar space heating and cooling .................................................... 281 9.2.1 Air systems ......................................................................... 283 9.2.2 Water systems ................................................................... 284 9.2.3 Heat pump systems ............................................................ 285 9.2.4 Energy saving and environmental considerations .......... 286 9.2.4.1 Application ........................................................ 289 9.3 Guaranteed solar results ............................................................... 290 9.3.1 The framework of guarantee ............................................ 293 References ................................................................................................ 294 Further reading ........................................................................................ 294 Glossary ................................................................................................... 295 Abbreviations ............................................................................................ 296 10. Active solar heating systems design Yannis G. Caouris Introduction .............................................................................................. 297 10.1 Preliminary analysis ...................................................................... 298 10.2 Feasibility study ....................................................... 307 10.3 Detailed design ............................................................................. 308 10.3.1 System schematic ....... 309 10.3.2 Collector subsystem .......................................................... 311 10.3.2.1 Array design ...................................................... 311 10.3.2.2 Piping design ...................................................... 317 10.3.2.3 Component requirements ................................. 319 10.3.3 Storage subsystem ............................................................. 324 10.3.4 Control subsystem ..................................................... 329 References ................................................................................................ 332