`
`HANWHA 1021
`
`

`

`

`

`SOLAR
`ENERGY
`
`THE STATE OF THE ART
`
`
`
`ISES POSITION PAPERS
`
`FDITED BY JEFFREY GORDON
`
`
`
`

`

`Published by James & James (Science Publishers) Ltd
`35-37 William Road, London, NW1 3ER, UK
`
`© 2001 ISES
`
`All rights reserved. No part of this book may be reproduced in any form
`or by any means electronic or mechanical, including photocopying, recording
`or by any information storage and retrieval system without permission in
`writing from the copyright holder and the publisher.
`
`A catalogue record for this book is available from the British Library.
`
`ISBN 1 902916 23 9
`
`Printed in the UK by The Cromwell Press
`
`Cover photos courtesy of AEE (1 and 2), FLABEG Solar International (3),
`Sandia National Laboratories (4),
`
`

`

`Contents
`
`List of Authors
`Preface
`
`KV
`
`XV1i
`
`Solar and natural resources for a better efficiency in the
`built environment
`M. Santamourts
`Introduction
`Energy consumption of buildings
`The built environment: the urban priority
`1.3.1
`Energy consumption of buildings and urban climate
`US.2
`Passive cooling techniques: the role of materials
`1.3.3
`Passive cooling techniques: the role of green areas
`1.3.4
`Using natural ventilation in urban buildings
`1.3.5
`Using passive solar radiation in urban buildings
`Towards a better environmental quality of buildings:
`the solar challenge
`Looking to the future
`References
`
`ee WN
`
`1.4
`
`1S
`
`Glazings and coatings
`K. G. Terry Hollands, J]. L. Wright and C. G. Granqvist
`Introduction
`Nature of ambient radiation and radiative properties
`2.2.1
`Ambient radiation
`Dee
`Radiative properties
`The conventional double-glazing: a baseline
`2. 3el
`Overview
`Daunte
`The short-wave calculation
`Bite
`The beat flow analysis
`2.3.4
`Indices of merit
`2.3.5
`Values of the indices of merit for the baseline
`2.3.6
`The frame and edge areas
`2.3.7
`Natural convection in the cavity
`
`

`

`VI
`
`Contents
`
`2.4
`
`Baa
`
`2.6
`
`2.7
`
`2.8
`
`2.9
`
`3.4
`
`The advanced multi-pane glazing
`2.4.1
`Mechanisms to alter the solar transmittance
`2.4.2
`Use of low-e coating
`2.4.3
`Substitute fill gases
`2.4.4
`Optimized pane spacing
`2.4.5
`Scope for improvements in low-e coatings
`Physical basis and preparation of low-e coatings
`2.5.2
`Techniques for coating glass: a primer
`2.5.2
`Low-e coatings of the noble metal type
`2 Sk
`Low-e coatings of the doped-oxide
`semiconductor type
`Evacuated and other advanced glazings
`2.6.1
`The evacuated glazing
`2.6.2
`Honeycombs
`2.6.3
`Aerogels
`Solar energy control for energy efficiency
`2.24
`Goals for coatings, and suitable materials
`2.7.2
`Rejecting the near-infrared and the ultraviolet
`2.7.3
`Electrochromic coatings
`2.7.4
`Thermochromic coatings
`2.7.5
`Coatings with angular selectivity in transmittance
`Analytical tools for advanced glazings
`2.8.1
`Code models for radiative transfer in the solar
`range
`Heat transfer model
`2.8.2
`Indices of merit
`end
`Conclusions
`References
`
`Selectively solar-absorbing coatings
`E. Wackelgard, G. A. Niklasson and C. G. Granqvist
`Introduction
`Selectively solar-absorbing surfaces: design and data
`3.201
`Principles
`3.2.2
`Results for some practically useful surfaces
`Models for microstructure and optical properties
`3.3.1
`Microstructure
`3.3.2
`Effective medium theories for the optical properties
`$:3;8
`Computational procedures for multilayered
`coatings
`The use of modelling
`3.4.1
`Theoretical optimization of selectively
`solar-absorbing surfaces
`3.4.2 Micro-structure evaluation from modelling
`3.4.3
`Evaluation of solar absorbers by the use of models
`
`109
`
`109
`112
`112
`114
`122,
`122
`123
`
`127
`128
`
`128
`129
`129
`
`

`

`Contents
`
`Vil
`
`aos
`
`3.6
`
`4.2
`
`4.3
`
`4.4
`
`4.5
`4.6
`
`4.7
`
`4.8
`
`3.4.4 Modelling used to elucidate some physical
`phenomena
`Degradation and durability
`3.5.1
`Qualification test
`3.5.2
`Lifetime evaluations
`Conclusion and remarks
`References
`
`Solar collectors
`Graham L. Morrison
`Flat-plate solar collectors
`4.1.1
`Absorber plate and fluid passageways
`44.2
`Covers
`4.1.3
`Insulation
`Unglazed collectors
`4.2.1
`Panel collectors
`4.2.2
`Strip collectors
`Flat-plate air heating collectors
`4.3.1
`Glazed air heating collectors
`4.332
`Unglazed transpired air beating collectors
`Evacuated collectors
`4.4.1
`Metal-fin-in-vacuum tubes
`4.4.2
`Dewar tubes
`4.4.3
`Evacuated collector arrays
`4.4.4
`Evacuated flat-plate collector
`Reverse flat-plate collectors
`Collector efficiency
`4.6.1
`Unglazed collectors
`4.6.2
`Glazed flat-plate collectors
`4.6.3
`Evacuated tubular collectors
`Flat-plate collector performance analysis
`4.7.1
`Linear analysis
`4.7.2
`Transmittance—absorptance product (Ta)
`Heat loss from glazed solar collectors
`4.8.1
`Single-glazed collector: heat loss
`4.8.2
`Double-glazed collector: heat loss
`4.8.3
`Heat loss from the cover
`4.8.4
`Linearized top loss coefficient
`4.8.5
`Typical heat transfer coefficients
`4.8.6
`Back surface and edge heat losses
`4.8.7
`Accuracy of linearized top loss heat transfer
`coefficients
`Absorber plate temperature distribution
`4.8.8
`Collector efficiency factor, F’
`4.8.9
`4.8.10 Heat removal factor, Fp
`
`132
`134
`136
`138
`140
`142
`
`145
`
`145
`146
`148
`149
`149
`149
`149
`150
`151
`151
`152
`152
`154
`155
`158
`158
`159
`162
`162
`162
`162
`162
`163
`164
`164
`165
`165
`166
`167
`167
`
`168
`170
`171
`175
`
`

`

`vilt
`
`Contents
`
`4.8.11 Mean temperature efficiency correlation function
`4.8.12 Linear functions for collector efficiency
`4.8.13 Conversion between linear correlations of
`measured collector efficiency
`4.8.14 Effect of working fluid flow rate
`4.8.15
`Series connection of collectors
`4.8.16 Collector loop heat exchangers
`4.8.17 Collector loop pipe losses
`4.8.18
`Prediction of prototype performance
`Heat loss from unglazed collectors
`4.9.1
`Long wave sky radiation
`4.9.2
`Long wave radiation on inclined surfaces
`4.9.3
`Heat loss from unglazed transpired air heating
`collectors
`Stagnation temperature
`Incidence angle modifier
`4.11.1
` Bi-axial incidence angle modifier
`Characterization of solar collector performance
`4.12.1
`Performance characterization functions for
`collectors
`Solar collector performance testing
`4.13.1
`Steady state testing of unglazed collectors
`4.13.2
`Steady state testing of glazed collectors
`4.13.3
`Steady state testing of evacuated tubular
`collectors
`Steady state testing using a solar simulator
`4.13.4
`4.13.5 Dynamic test method
`Quality test procedures for solar collectors
`4.14.1 Material degradation processes in solar collectors
`References
`
`Solar water heating
`Graham L. Morrison
`Solar water heating
`$1.2
`Thermosyphon systems
`Sadak
`Active systems
`5.1.3
`Solar boosted heat pumps
`5.1.4
`Factors governing solar water system performance
`5225
`Thermosyphon SDHW system characteristics
`5.1.6
`Pumped circulation systems
`eg New pumped system configurations
`5.1.8
`Evacuated tubular solar water beaters
`S459
`Seasonally biased collectors
`5.1.10
`PV electric solar water heaters
`Solar water heater performance evaluation
`
`178
`179
`
`180
`181
`182
`183
`185
`186
`188
`189
`190
`
`190
`192
`194
`195
`196
`
`199
`203
`203
`206
`
`208
`209
`211
`212
`213
`219
`
`223
`
`223
`223
`226
`229
`233
`241
`20%
`260
`269
`273
`274
`276
`
`4.9
`
`4.10
`4.11
`
`4.13
`
`4.14
`
`5.1
`
`5.2
`
`

`

`Contents
`
`ix
`
`5.3
`
`Solar water heater markets
`References
`
`Photovoltaic physics and devices
`Martin A. Green
`Introduction
`The photovoltaic process
`6.2.1
`Background and essentials
`6.2.2
`Semiconductors, Fermi-levels and light absorption
`6.2.3
`Doped semiconductors
`p-n junction theory
`6.3.1
`Characteristics in the dark
`6.3.2
`p-n junction solar cell
`Energy conversion efficiency limits
`6.4.1
`Detailed balance: light absorption and emission
`6.4.2
`Approaches for exceeding previous limits
`Heterojunctions, metal—-semiconductor contacts,
`tunnelling junctions
`6.5.1
`Heterojunctions
`6.5.2 Metal-semiconductor contacts
`6.5.3
`Tunnelling junctions
`Silicon solar cells
`6.6.1
`Advantages ofsilicon
`6.6.2
`Crystalline silicon cells
`6.6.3
`Multicrystalline and ribbon wafers
`6.6.4
`Polycrystalline silicon thin film cells
`6.6.5
`Amorphous silicon solar cells
`Compound semiconductor cells
`6.7.1
`Il-V crystalline cells
`6.7.2
`Polycrystalline thin-film compound
`semiconductor cells
`Other photovoltaic approaches
`6.8.1
`Basic requirements for photovoltaics
`6.8.2
`Photoelectrochemicalcells
`6.8.3
`Nanocrystalline dye cell
`Conclusion
`References
`
`6.3
`
`6.4
`
`6.5
`
`6.6
`
`6.7
`
`6.8
`
`6.9
`
`Solar Concentrators
`R. Winston
`Limits to concentration
`Imaging devices and their limitations
`Non-imaging optics
`The edge-ray or ‘string’ method
`The flow-line or phase space method
`
`284
`286
`
`291
`
`307
`311
`
`313
`313
`315
`315
`316
`316
`317
`327
`330
`331
`337
`aoe
`
`341
`347
`347
`348
`349
`351
`350
`
`357
`
`357
`358
`359
`362
`363
`
`

`

`Contents
`
`7.6
`
`Put
`7.8
`
`79
`
`7.10
`
`7.11
`
`FL
`7S
`7.14
`7.15
`7.16
`fh?
`7.18
`7.19
`7.20
`
`Tras
`Fete
`
`7.23
`7.24
`7.25
`7.26
`Puet
`7.28
`
`Designs which are functionals of the acceptance angle: ‘tailored
`edge-ray design’ method for non-imaging concentrators
`Advantages and features of two-stage concentrating systems
`Design considerations for non-imaging secondary
`concentrators
`Compound parabolic concentrators and compound
`elliptical concentrators (CECs)
`Refracting high index non-imaging concentrators, DCPCs
`and DTIRCs
`Flowline or trumpet concentrators
`New secondary concentrator designs
`Other secondary design considerations
`Solar concentrators: overview
`Stationary concentrators (CR < 2)
`Adjustable concentrators (CR = 2-10)
`Tracking concentrators, one axis (CR = 15-35)
`Tracking receiver, stationary reflector
`Tracking concentrators, two-axes (CR = 50-1000)
`Dish-thermal applications (reflecting trumpet secondary:
`n = 1.0)
`Central receiver (CR = 500-1000)
`Demonstration and measurement of ultra high solar
`fluxes (CR up to 100 000)
`7.22.1 Measurement techniques
`7.22.2
`Exceeding the n = 1 limit: small-scale experiments
`7.22.3 A new record for the concentration of sunlight
`7.22.4
`‘Brighter than the Sun’
`7.22.5
`Large-scale experiments at the NREL high flux
`solar furnace
`7.22.6 Measurement of high flux at kilowatt levels in
`air (n = 1.0)
`7.22.7 Measurement of high flux at high power levels
`in a refractive medium (n> 1.0)
`Applications using highly concentrated sunlight
`Solar-pumpedlasers
`Solar processing of materials
`Solar thermal applications of high-index secondaries
`Solar thermal! propulsion in space
`Discussion
`Acknowledgements
`References
`Bibliography
`
`367
`370
`
`372
`
`373
`
`374
`374
`375
`375
`376
`376
`378
`381
`388
`389
`
`391
`395
`
`398
`401
`404
`405
`406
`
`407
`
`408
`
`415
`421
`421
`426
`429
`430
`432
`434
`434
`436
`
`

`

`Contents
`
`xt
`
`Ce es
`
`8.3
`
`8.4
`
`8.5
`
`GoCOCO CON]A
`
`8.9
`
`The cost of pollution and the benefit of solar energy
`A. Rabl and J. V. Spadaro
`Introduction
`Economic valuation
`8.2.1
`Ground rules
`8.2.2
`Valuation of mortality
`8.2.3
`Discounting
`Atmospheric dispersion
`8.3.1
`Dispersion models
`8.3.2
`Site dependence of impacts
`8.3.3
`Secondary pollutants
`8.3.4
`Typical damage estimates
`Health impacts and costs
`8.4.1
`General remarks
`8.4.2
`Particles
`8.4.3
`Sulphur dioxide
`8.4.4
`Oxides of nitrogen
`8.4.5
`Carbon monoxide
`8.4.6
`Ozone
`8.4.7
`Sulphate and nitrate aerosols
`8.4.8
`Other pollutants
`8.4.9
`Health impacts of radiation
`Other impacts
`§.5.1
`Impacts on crops
`8.5.2
`Impacts on ecosystems
`8.5.3
`Impacts on buildings and materials
`8.5.4
`Upstream impacts
`8.5.5
`Wastes
`Global warming
`Results for damage costs per kilogram of pollutant
`Results for damage costs per kilowatt-hour
`8.8.1
`Fossil fuels
`8.8.2
`The nuclear fuel chain
`8.8.3
`Renewable energy technologies
`Conclusions
`Acknowledgements
`Glossary
`References
`
`Solar process heat: distillation, drying, agricultural
`and industrial uses
`B. Norton
`Introduction
`Industrial process hot water provision
`Materials processing
`
`437
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`437
`439
`439
`439
`440
`441
`441
`441
`445
`445
`446
`446
`449
`449
`449
`451
`451
`452
`453
`453
`454
`454
`454
`455
`455
`456
`457
`460
`464
`464
`466
`468
`470
`470
`471
`472
`
`477
`
`477
`480
`482
`
`

`

`X11
`
`ONOSESOSNSmoONAWSD
`
`10.1
`10.2
`
`10.3
`
`10.4
`
`10.5
`
`10.6
`
`11
`
`ti
`
`Contents
`
`Solar detoxification
`Distillation
`Solar drying of crops
`Cold storage
`Livestock building heating
`Solar cookers
`Conclusion
`Acknowledgements
`References
`
`Solar resource assessment: A review
`R. Perez, R. Aguiar, M. Collares-Pereira, D. Dumortier,
`V. Estrada-Cajigal, C. Gueymard, P. Ineichen, P. Littlefair,
`H. Lund, J. Michalsky, J. A. Olseth, D. Renné, M. Rymes,
`A. Skartveit, F. Vignola and A. Zelenka
`Introduction
`Background and definitions
`10.2.1
`Basic remarks and radiative transfer
`10.2.2.
`Solar radiation components
`10.2.3 Radiation-related meteorological parameters
`Measurement of solar radiation components
`10.3.1
`Instrumentation
`10.3.2 Measurement context
`Models
`Input—output and parameterization
`10.4.1
`10.4.2. Component-to-component models
`10.4.3. Meteorological models
`10.4.4
`Satellite models
`10.4.5
`Stochastic models: background theory, multi
`correlation issue
`User products
`10.5.1
`Time-site specific data
`10.5.2.
`Typical data
`10.5.3 Averaged/condensed data sets
`10.5.4 Data forecasts
`Concluding remarks
`10.6.1
`Research and development issues
`10.6.2
`Selling resource assessment
`References
`
`Solar thermal electricity
`D. R. Mills
`Introduction
`11.1.1
`Conversion of solar beat to electricity
`11.1.2 Basic solar collector types
`11.1.3
`Solar thermal conversion process
`
`485
`486
`488
`491
`492
`493
`493
`494
`494
`
`497
`
`497
`498
`498
`499
`502
`S02
`502
`509
`$13
`516
`519
`531
`533
`
`536
`540
`541
`547
`552
`558
`539
`560
`561
`562
`
`577
`
`577
`578
`580
`580
`
`

`

`Contents
`
`Xi
`
`11.2
`
`11.3
`
`11.4
`
`11.9
`
`Solar energy
`11.2.1
`Solar energy resource characteristics
`11.2.2
`Solar collector orientation: position of solar
`beam relative to collector aperture
`Limits of optical concentration in solar collectors
`11.2.3
`11.2.4 Appendix A: More about solar radiation
`11.2.5 Appendix B: Radiation loss minimization using
`selective absorbers
`|
`11.2.6 Appendix C: Thermodynamic limits to optical
`concentration
`Line focus systems
`11.3.1
`Parabolic trough technology
`11.3.2.
`Linear Fresnel technology
`Point focus STE systems
`11.4.1
`Introduction
`11.4.2 Distributed paraboloidal dish/Stirling technology
`11.4.3. Central collection paraboloidal dishes
`11.4.4 Central receiver
`Non-imaging systems
`11.5.1
`Practical non-imaging concentrators
`11.5.2. Non-imaging secondary concentrators
`11.5.3. Appendix A: Design of basic CPC for a
`tubular absorber
`Ponds and chimneys
`11.6.1
`Introduction
`11.6.2
`Solar pond technology
`11.6.3.
`Solar chimney technology
`11.6.4 Appendix A: Basic mathematical analysis of
`the chimney
`Storage and hybridization
`11.7.1
`Introduction
`11.7.2
`Plant capacity factor
`11.7.3
`Storage
`11.7.4 Hybridization
`Market issues
`11.8.1
`Introduction
`11.8.2
`Economic and environmental benefits of STE
`systems
`Subsidy or regulation?
`
`11.8.3
`Summary
`References
`
`12
`
`12.1
`
`Wind energy review
`D. Milborrow
`Introduction
`
`S82
`582
`
`585
`590
`592
`
`593
`
`S95
`597
`597
`605
`610
`610
`612
`615
`618
`624
`624
`626
`
`627
`628
`628
`629
`631
`
`634
`635
`635
`636
`636
`639
`640
`640
`
`642
`646
`647
`648
`
`653
`
`653
`
`

`

`xiv
`
`Contents
`
`12.5
`
`12.9
`
`12.2 Historical summary
`12.3 Wind energy resources
`12.4 Types of wind turbine
`12.4.1 Horizontal axis turbines
`12.4.2. Vertical axis wind turbines
`Principles of operation
`12.5.1 Aerodynamics
`12.5.2.
`Electrical systems
`12.5.3
`Power limitation
`12.6 Types of modern wind turbine
`12.6.1
`Present-day practice
`12.6.2.
`Large machines
`12.6.3.
`Small wind turbines
`12.6.4 Vertical axis wind turbines
`12.7 Blade materials
`12.8 Exploitation of wind energy
`12.8.1 Off-grid applications
`12.8.2 Offshore wind
`Integration of wind in electricity systems
`12.9.1
`Electricity supply system operations
`12.9.2 Capacity credit
`12.10 Economics
`12.10.1 Wind energy plant costs and energy prices
`12.10.2 Energy price calculation methods
`12.10.3 Historical summary
`12.10.4 Current plant costs and energy prices
`12.10.5 Reference energy prices
`12.10.6 Offshore wind energy prices
`12.10.7 Generation costs of competing fuels
`12.10.8 Small wind turbines
`12.10.9 Other price and cost issues
`12.11 Environmental aspects
`12.11.1 Noise
`12.11.2 Television and radio interference
`12.11.3 Birds
`12.11.4 Visual effects
`12.12 Future developments
`12.12.1 Markets
`12.12.2 Technology
`12.12.3 Future price trends
`12.12.4 Offshore
`References
`
`Index
`
`654
`656
`657
`657
`657
`658
`658
`659
`660
`661
`662
`664
`666
`668
`669
`669
`670
`672
`674
`675
`677
`678
`679
`679
`681
`682
`684
`686
`687
`689
`689
`691
`691
`693
`693
`694
`694
`694
`695
`695
`696
`696
`
`699
`
`

`

`List of Authors
`
`R. Aguiar
`INGTI, Portugal
`
`M. Collares-Pereira
`INGTI, Portugal
`
`D. Dumortier
`ENTPE, France
`
`V. Estrada-Cajigal
`SOLAR-TRONICS, Mexico
`
`C. G. Grangqvist
`Department of Materials Science,
`The Angstrém Laboratory,
`Uppsala University, Uppsala,
`Sweden
`
`Martin A. Green
`Centre for Third Generation
`Photovoltaics, University of New
`South Wales,
`Sydney,
`Australia
`
`C. Gueymard
`2959 Ragis Rd.,
`Edgewater,
`FL 32132,
`USA.
`
`K. G. Terry Hollands,
`Department of Mechanical
`Engineering, University of Waterloo,
`Waterloo,
`Ontario,
`Canada
`
`P. Ineichen
`University of Geneva,
`Switzerland
`
`P. Littlefair
`Building Research Establishment,
`UK
`
`H. Lund
`Thermal Insulation Laboratory,
`Denmark
`
`J. Michalsky
`The University of Albany,
`USA
`
`D. Milborrow
`Consultant,
`Lewes,
`East Sussex,
`UK
`
`D. R. Mills
`Department of Applied Physics,
`School of Physics,
`Building A28,
`New South Wales, 2006,
`Australia
`
`Graham L. Morrison
`School of Mechanical and
`Manufacturing Engineering,
`University of New South Wales,
`Sydney,
`Australia
`
`

`

`xvi
`
`List of Authors
`
`G. A. Niklasson
`Department of Materials Science,
`The Angstrém Laboratory,
`Uppsala University,
`Uppsala,
`Sweden
`
`Brian Norton
`Centre for Sustainable
`Technologies,
`University of Ulster,
`Newtownabbey,
`BT37 OQB,
`WN. Ireland
`
`J. A. Olseth
`University of Bergen,
`Norway
`
`R. Perez
`The University of Albany,
`USA
`
`A. Rabl
`Ecole des Mines,
`60 boul. St.-Michel,
`F-75272 Paris Cedex 06,
`France
`
`D. Renné
`NREL,
`USA
`
`M. Rymes
`NREL,
`USA
`
`M. Santamouris
`Group of Building Environmental
`Studies,
`Department of Physics,
`University of Athens,
`Panepistimioupolis, 15784,
`Athens,
`Greece
`
`J. V. Spadaro
`International Atomic Energy Agency,
`Wagramerstrasse 5, A-1400
`Vienna,
`Austria
`
`A. Skartveit
`University of Bergen,
`Norway
`
`F. Vignola
`University of Oregon,
`USA
`
`E, Wackelgard
`Department of Materials Science,
`The Angstrém Laboratory,
`Uppsala University,
`Uppsala,
`Sweden
`
`R. Winston
`The Enrico Fermi Institute and the
`Department of Physics,
`University of Chicago,
`Chicago,
`Illinois,
`USA
`
`J. L. Wright
`Department of Mechanical
`Engineering, University of Waterloo,
`Waterloo,
`Ontario,
`Canada
`
`A. Zelenka
`MeteoSuisse,
`Switzerland
`
`

`

`Preface
`
`Periodically, it is beneficial to pause and commit to the printed page a retro-
`spective summary of the progress, status and leading challengesin a scientific
`discipline. This bookis an initiative of the International Solar Energy Society
`(ISES) for the broad and rich interdisciplinary field of solar energy. The
`approach of the year 2000 in part provided the motivation and timing for
`this undertaking.
`The book comprises 12 chapters each of which covers a major solar
`energy sub-discipline and is authored by a leading scientist in the partic-
`ular field. These background papers are intended to survey the history of
`the particular technology and field, to review the state-of-the-art, to discuss
`the rudiments, to present major applications, and to identify the principal
`remaining challenges. The authors were asked to tailor their chapters to an
`audience of university students, fellow researchers, engineers in the field,
`and solar energy practitioners.
`This book took form in early 1997 while Terry Hollands was spending
`his sabbatical with me. Terry, in his capacity then as editor-in-chief of Solar
`Energy, and David Mills in his capacity as president of ISES at the time,
`recommended that a book of this nature be published under the auspices
`of ISES during the year 2000. Terry and David asked if I would accept the
`challenge of editing the book, namely, of identifying the principal categories
`for the chapters, of securing the agreement of the foremostscientists in the
`respective areas to write the chapters, to oversee the assembly of the book
`including adherence to deadlines, and to coordinate matters among the
`authors, ISES and the publisher. With Terry’s valuable input, the major
`categories were selected, a list of prospective authors was drafted, and the
`project was underway.
`What motivated me was the prospect of emerging with an invaluable
`reference book for any serious student or scientist entering one of the trib-
`utaries of solar energy research or development - the type of book that
`would concentrate a wealth of accumulated wisdom and experience between
`
`

`

`xvii
`
`Preface
`
`two covers, and would save considerable efforts in literature searches and
`attempts to gain a clear understanding of the basic concepts required to
`move forward in solar energy studies. With nearly twenty years having
`elapsed since the publication of a book of comparable scope and high scien-
`tific level, I concurred it would be an honor,as well as a gratifying experience,
`to be part of this endeavor.
`If you, the reader, find the list of topics incomplete, it is not necessarily
`because we overlooked or belittled other disciplines. Rather it stems from
`human frailty. There were other chapters to which prominent scientists
`committed themselves. But, after more than one year into the project, they
`reneged and left us with insufficient time to find a replacement andstill
`meet our deadline.
`Those colleagues who did accept the chalice imparted the unique perspec-
`tive of the intellectual leaders who have pioneered progress in each field,
`have gained extensive experience teaching it in university settings, and can
`succinctly condense that wisdom into one chapter. To them, I, and I trust
`all readers of this book, extend profound gratitude.
`As editor, I also wish to express my appreciation to the reviewers of
`these chapters. Whereas with authors of this caliber an accept-or-reject
`review was uncalled for, I did feel that a constructively critical review by
`a colleague of comparable scientific standing would help each author to
`improve and sharpen his chapter.
`In order to preserve the promised
`anonymity of the reviewers, I am not at liberty to thank them by name
`here. Nonetheless, I wanted to explain that each chapter has undergone
`peer review at the highest level and was indeed revised in accordance with
`the referees’ comments.
`Recognition is also in order to ISES publication officers Chris Findlay
`and Burkhard Holder of ISES headquarters in Germany. It is thanks to
`their dedication to this project that a respectable publishing house was
`found, legal and financial arrangements were reached, and the book was
`published.
`For all the years I’ve spent as a researcher and instructor in solar energy
`studies, I was excited and humbled at the amountI learned from reading
`these chapters and supervising their review. I can only hope that you, the
`reader, will share that intellectual exhilaration along with a renewed enthu-
`siasm to tackle the challenges that remain in solar energy research in the
`years ahead.
`
`Jeffrey M. Gordon
`Department of Solar Energy and Environmental Physics
`Jacob Blaustein Institute for Desert Research
`Ben-Gurion University of the Negev
`Sede Boger Campus
`Israel
`jeff@menix.bgu.ac.il
`
`

`

`Ss
`
`Selectively solar-absorbing coatings
`
`E. Wackelgard, G. A. Niklasson and C. G. Granqvist
`Department of Materials Science, The Angstrom Laboratory,
`Uppsala University, Uppsala, Sweden
`
`3.1
`
`INTRODUCTION
`
`An energy-efficient solar collector should absorb incident solar radiation,
`convert
`it to thermal energy, and deliver the thermal energy to a heat-
`transfer medium with minimum loss at each step. Figure 3.1 serves as a
`convenient introduction to the design of a flat-plate solar collector and
`outlines the most salient components. The collector comprises a thermally
`well-insulated arrangement whose upward-facing side is transparent so that
`solar radiation can penetrate to an absorbing surface, with carefully tailored
`properties,
`in contact with a heat-transfer medium such as water or air.
`Thermal losses are diminished by placing the absorber surface below a cover
`glass. Even smaller losses can be obtained by the use of transparent insu-
`lation in the air gap between the absorber and the cover glass. In principle,
`the energy efficiency of the collector can be boosted by surface coated glass:
`antireflection coatings as well as infrared-reflecting coatings are of interest.
`It should be stressed that Figure 3.1 refers to the commonly used fixed
`flat-plate collector. Most of the treatment below will be done with such
`collectors in mind, but some discussion of absorber surfaces designed for
`high-temperature applications primarily in evacuated tubular collectors is
`also included. Some solar collector constructions have solar reflecting and
`tracking facilities.
`The requirements for energy efficiency can be introduced with reference
`to the ‘natural’ radiation in our surroundings. The pertinent radiative prop-
`erties are shownin Figure 3.2. The solid curve reproduces a typical spectrum
`for solar irradiance at the ground. Specifically, the curve gives an air mass
`(AM) 1.5 spectrum according to the ISO standard 9845-1 (1992). It is seen
`that almost all of the solar radiation comes at a wavelength \ below 3 wm.
`The absorber surface of the solar collection device should absorb this energy.
`The surface then heats up and emits thermal radiation. The dashed curves
`
`

`

`110
`
`E. Wackelgard, G. A. Niklasson and C. G. Granqvist
`
`
`
`Figure 3.1 Principle design ofa flat-plate solar collector. The numbers
`denote glazing (1),
`transparent insulation (2), absorber (3), insulation
`(4), collector box (5) and pipes (6). The different components are not
`to scale.
`
`in Figure 3.2 indicate black-body spectra at three temperatures; the emitted
`energy is negligible at X < 3 wm for temperatures below 100°C. The losses
`associated with thermal emission should be avoided in order to gain energy
`efficiency. This can be accomplished in two different ways. The first one
`relies on a cover glass with a coating that reflects at \ > 3 4m so that the
`radiation emitted from the absorber surface is brought back to this same
`
`(GW/m’)
`
`
`
`Powerdensity
`
`Blackbody
`
`f™
`
`Wavelength (um)
`
`Figure 3.2 Spectra for characteristic solar irradiance and for black-
`body exitance pertaining to three temperatures.
`
`

`

`Selectively solar-absorbing coatings
`
`1i1
`
`surface. Another, and more commonly used, way to diminish the heatlosses
`is to have an absorber surface whose thermal emittance is low. The second
`condition leads to the selectively solar-absorbing surfaces to be discussed
`in this chapter. They are characterized by low reflectance at A < dX, and
`high reflectance at \ > X\, where A. = 3 ~m for temperatures below 100°C.
`At increased operating temperatures, A, should be displaced towards a lower
`value, and at 300°C it is adequate to put A, = 2 pm.
`Qualitative performance criteria can be formulated by use of the normal
`solar absorptance A,,, and the normal or hemispherical thermal emittance
`(E°..-m and E,,.... respectively). These parameters are defined by
`
`_ |daddys; (A)(1 — R(A, 0))
`Heo
`J besos ON)
`
`= J Adher (A, T)(1 — R(A, 0))
`n
`
`Stheemn (7) =
`| drda.... (7)
`
`/2
`
`| dr [ d(sin® @)drnerm(Ast)(1 — R(A,8))
`[ ANdgherm(A5T)
`
`Perm\T) at
`
`2ee
`
`deherm(Ast) = €, X~ [exp (ep /(AT)]
`
`en
`
`(3.2a)
`
`is20)
`
`(3.3)
`
`irradiance (for example the AM1.5 spectrum),
`where ,,, is the solar
`R(A, 0)
`is the reflectance as a function of wavelength and incidence angle,
`t is the absorber surface temperature, c, = 3.7814 * 10°'° Wm and c,
`= 1.4388 =< 10°?mK. Clearly, the desired spectral selectivity implies that
`A,,, should be close to unity and that E,,.,,,, should be minimized. The normal
`thermal emittance is often used for characterization, although the hemi-
`spherical emittance is more adequate. Thermal emittance for other specific
`angles than normal can also be calculated with Equation 3.2a by replacing
`R(A, 0) with the reflectance R(X, 0) for the actual angle of incidence.
`Coatings and surface treatments with high A,,, and low E,,.,,, were subject
`to intense research and development in many laboratories around the world
`during the 1970s and early 1980s. This work has been reviewed in consid-
`erable detail
`(Meine! and Meinel, 1976, Seraphin and Meinel, 1976,
`Seraphin, 1979, Lampert, 1979a, b, Agnihotri and Gupta, 1981, Niklasson
`and Grangqvist, 1991). An annotated bibliography (Niklasson and Granqvist,
`1983), covering the period 1955-1981, was published in 1983. It lists 565
`scientific papers, including studies of almost 280 different coatings or surface
`treatments. Much laboratory research was done during this period and it
`resulted in a number of coatings for commercial production of solar
`absorbers. Rather than reviewing this vast, and somewhat stale, field once
`
`

`

`112
`
`E. Wackelgard, G. A. Niklasson and C. G. Granqvist
`
`again, here the focus is on certain key issues and on the developments
`during the 1990s. This chapter first considers design principles for obtaining
`spectral selectivity and data for several solar collector surfaces used in prac-
`tice. Then follows a discussion of theoretical models for the optical properties
`and examples where modelling has been applied in the research on solar
`absorbers. A brief review is given of the work by the International Energy
`Agency (IEA) on accelerated degradation tests for solar absorbers.
`
`3.2
`
`SELECTIVELY SOLAR-ABSORBING SURFACES: DESIGN
`AND DATA
`
`3.2.1
`
`Principles
`
`It is possible to exploit several different design principles and physical mech-
`anisms in order to create a selectively solar-absorbing surface. Six of these
`principles are shown schematically in Figure 3.3. The most straightforward
`one is to use a material whose intrinsic radiative properties have the desired
`kind of spectral selectvity. Generally speaking, this approach has not been
`very fruitful, but work on ZrB, (Randich and Allred, 1981, Randich and
`Pettit, 1981) and on some other compoundsindicates thatintrinsically selec-
`tive materials do exist. In practice, useful solar absorbers are based on two
`layers with different optical properties;
`they are referred to as tandem
`absorbers. A semiconducting or dielectric coating with high solar absorp-
`tance and high infrared transmittance on top of a non-selective highly
`
`a
`
`Intrinsic selective
`material
`
`2
`
`Metal
`Dielectric
`
`
`SeaeT
`Antireflection coating 4
`SSSsgq—3cssssgq
`Silicon
`Yj), x06
`
`
`
`Mera
`
`\\ YYI),rv WII), sre
`
`
`
`Figure 3.3 Cross-sectional schematic designs of six types of different
`coatings and surface treatments for selective absorption of solar energy.
`
`

`

`Selectively solar-absorbing coatings
`
`113
`
`reflecting material (i.e. a metal) constitutes one type of tandem absorber.
`Anotheralternative is to coat a non-selective highly absorbing material with
`a heat-mirror (having high solar transmittance and high infrared reflectance).
`The different choices of material combinations for tandem absorbers are
`described in detail below.
`
`Semiconductor-metal tandems
`These can give the desired spectral selectivity by absorbing short-wavelength
`radiation in a semiconductor whose bandgap is ~0.6 eV and having low
`thermal emittance as a result of the underlying metal. The useful semicon-
`ductors have undesirably large refractive indices, which tends to yield high
`reflection losses. Hence it is necessary to antireflect the surfaces in the range
`of solar radiation. Early work in this category (Seraphin, 1976, 1979, Janai
`et al., 1979) was centred on Si based designs and coatings prepared by
`chemical vapour deposition.
`
`Multilayer-metal tandems
`These can be tailored so that the multilayer stack becomesanefficient selec-
`tive absorber of solar radiation. It is comparatively easy to compute the
`optical performance, which facilitates design optimization. One interesting
`example is an Al,O,/Mo/AI,O,triple layer, which was originally developed
`for the US space programme (Schmidt and Park, 1965). This type of surface
`has been produced by large-area sputtering technology for high tempera-
`ture applications (Thornton and Lamb, 1982).
`
`Metal—dielectric composite-metal tandems
`These have a coating consisting of very fine metal particles in a dielectric
`host. This coating is also labelled ‘cermet’ from the abbreviation of ceramic—
`metal. The ensuing optical properties can be intermediate between those of
`the metal and of the dielectric. The metal—dielectric concept offers a high
`degree of flexibility, and the optimization of the solar selectivity can be
`made with regard to the choice of constituents, coating thickness, particle
`concentration, shape and orientation of the particles. The size of the particles
`is restricted so as to be much smaller than the wavelengths in the solar
`spectral range in order to absorb instead of scatter the solar light. It has
`been shownthat particle sizes should be less than 0.1 to 0.2 wm (Arancibia-
`Bulnes and Ruiz-Suarez, 1998). Effective medium theories, which will be
`treated in a later section, are of great value for modelling the optical perfor-
`mance. The composite coatings can be produced by a variety of techniques
`such as chemical conversion, electroplating, anodization, inorganic