markvart, t. practical handbook of photovoltaics - fundamentals and applications

Handbook of Photovoltaics: Section Finder Crystalline silicon solar cells Thin film technologies Space and concentrator cells Organic and dye sensitised cells d l d2 Principles of s

PRACTICAL H A N D B O O K OF

Phot

iarkvart & 1

ELSEVIER

Handbook of Photovoltaics: Section Finder

Crystalline silicon solar cells

Thin film technologies

Space and concentrator cells

Organic and dye sensitised cells

d l d2

Principles of solar cell operation

Low cost industrial manufacture of crystalline silicon solar cells Characterisation and diagnosis of silicon wafers and devices High-efficiency silicon solar cell concepts

Cadmium telluride thin-film PV modules

GaAs and high-efficiency space cells High-efficiency concentrator silicon solar cells

e l e2

Photoelectrochemical solar cells Organic and plastic solar cells

Photovoltaic systems

a 1 a2

a 3

Energy production by a PV array Energy balance in stand-alone PV systems Review of system design and sizing tools

Page reference

c 3 Installation guidelines: electrical

d l Concentrator systems d2

e l Architectural integration e2

e 3 Solar-powered products

Grid-connection of PV generators: technical and regulatory issues

Operation of solar cells in a space environmenl Performance reliability and user experience

Testing, monitoring and calibration

Economics, environment and business strategy

1 Overview of potential hazards

Practical Handbook of

Photovoltaics: Fundamentals and Applications

Practical Handbook of

Photovoltaics: Fundamentals and Applications

Edited by:

Tom Markvart and Luis Castafier

E L S E V I E R

Elsevier Science Inc 3 6 0 Park Avenue South, New York, NY

Elsevier Japan, Tsunashima Building Annex, 3-20-1 2 Yushima,

Bunkyo-ku, Tokyo 11 3, Japan

1OOXO-1710, USA

Copyright 0 2003 Elsevier Ltd

All rights reserved No part of this publication may be reproduced, stored in

a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without prior permission in writing from the publishers

British Library Cataloguing in Publication Data

Practical handbook of photovoltaics: fundamentals and applications

1 Photovoltaic power systems 2 Photovoltaic power generation

Library of Congress Cataloging-in-Publication Data

Practical handbook of photovoltaics: fundamentals and applications / edited by Tom Markvart and Luis Castafier

Includes bibliographical references and index

1 Photovoltaic cells 2 Photovoltaic power generation I Markvart, Tom 11 Luis Castaiier

p cm

ISBN 1-8 5 6 17-3 90-9

TK8322.P73 2003

No responsibility is assumed by the Publisher for any injury and/or damage

to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein

Typeset by Variorum Publishing Ltd, Lancaster and Rugby

Printed and bound in Great Britain by BiddIes Ltd Guildford and King’s Lynn

Preface

Photovoltaics is about to celebrate 50 years of its modern era During this time, the industry has grown from small satellite power supplies to utility-scale systems that are now routinely installed in many countries of the world

Solar cells capable of producing power in excess of 500 MW were manufactured

in 2002, providing electricity to a variety of applications ranging from small consumer products, power systems for isolated dwellings and remote industrial equipment to building-integrated solar arrays and megawatt-size power stations

summarises the current status of know-how in this field It represents a detailed source of information across the breadth of solar photovoltaics and is contributed to by top-level specialists from all over the world Over 1,000 references, bibliographies and web sites guide the reader to further details, be it specific information for industrial production and research or a broad overview for policy makers Thirty-seven chapters in the handbook cover topics from fundamentals of solar cell operation to industrial production processes, from molecular photovoltaics to system modelling, from a detailed overview of solar radiation to guidelines for installers and power engineers, and from architectural

journals and other sources of information which can be found in a printed or electronic form

The main credit for this handbook must go to the 4 7 contributors who have produced a unique compilation of the contemporary knowledge in photovoltaic science and technology

this book would have never seen the light of day

Luis Castafier

Barcelona

Tom Markvart

Southampton

Guide to Usage of the Handbook

I Svstem analvsis and evaluation

Solar energy resource

PART II SOLAR CELLS

/la Introduction

Ilb Crystalline silicon solar cells

2 Low cost industrial manufacture of crystalline silicon solar cells

J Szlufcik, S Sivoththaman, 1 Nijs, R.P Mertensand

4

2 2 7

Characterisation and diagnosis of silicon wafers and devices

A Cuevas and R Sinton

Ilc Thin film technologies

/Id Space and concentrator cells

viii Practical Handbook of Photovoltaics: Fundamentals and Applications

Ile

Organic and dye sensitised cells

Ilia Introduction

1

2

3 Review of system design and sizing tools S Silvestre

Energy balance in stand-alone PV systems

and K Fragaki

T Markvart andK Fragaki

L Castaner, S Bermejo,

lllb Balance-of-system components

2 Batteries in PV systems D Spiers

lllc Grid-connected systems

1

3 Installation guidelines: electrical M Cotterell

Grid-connection of PV generators: technical and regulatory

issues J Thornycroft and T Markvart

llld Space and concentrator systems

2 Operation of solar cells in a space environment S Bailey and R.Rafaelle

llle Case studies

2 Performance, reliability and user experience U Jahn

3 Solar-powered products P Wove

PART V ECONOMICS, ENVIRONMENT AND BUSINESS STRATEGY

APPENDICES

List of Contributors

Institute for Sustainable Development and Innovation, Utrecht University, Padualaan 14, NL-3584 CH Utrecht, The Netherlands

email: e.a.alsema@chem.uu.nl

Polytekhnicheskaya str., St Petersburg 19402 1, Russia

email: vmandreev@mail.ioffe.ru

Sheila Bailey, Photovoltaic and Space Environments Branch/5410, NASA Glenn

Emilio Fernandez Lisbona, Solar Array Section,ESA-Estec, Keplerlaan 1, 2200

AG Noordwijk, The Netherlands

email: Emilio.Fernandez.Lisbona@esa.int

Francesca Ferrazza, Eurosolare S.p.A Via August0 D’Andrea 6 , 0 0 0 4 8 Nettuno, Italy

email: francesca.ferrazza@ eurosolare.agip.it

Katerina Fragaki, School of Engineering Sciences, University of Southampton,

email: A.FRAGAKI@soton.ac.uk

USA

email: fthenakis@bnl.gov

Photonics, University of New South Wales, Sydney NSW 2 0 5 2 , Australia

email: m.green@ unsw.edu.au

Ulrike Jahn, Institut fur Solarenergieforschung GmbH Hameln/Emmerthal

email: ujahn@easynet.de

email: ptl(dmaths.soton.ac.uk

Seccio Arquitectura Sostenible, c/Baldiri Reixac 4 - 6 , 0 8 0 2 8 Barcelona, Spain email: rleal@pcb.ub.es

email: t.markvart(@Jsoton.ac.uk

xii Practical Handbook of Photovoltaics: Fundamentals and Applications

Augustin McEvoy, Institute for Molecular and Biological Chemistry, Faculty of

email: Carl-osterwald @nrel.gov

Roger van Overstraeten (deceased), formerly of Interuniversity Microelectronic Center (IMEC), Kapeldreef 75, Leuven B-3001, Belgium

John Page 1 5 Brincliffe Gardens, Sheffield, S119BG, UK

List of Contributors xiii

xiv Practical Handbook of Photovoltaics: Fundamentals and Applications

Philip Wolfe, WolfeWare, Rose Cottage, Dunsomer Hill, North Moreton, O X 1 1

Introduction

This Practical Handbook of Photovoltaics aims to give a detailed overview of all aspects of solar photovoltaics in a way that can be easily accessed by the expert and non-specialist alike It reflects the current status of this modern power- generating technology which, despite its mature status, continues to explore new directions to improve performance and reduce costs A focus on practical aspects, however, does not imply the neglect of fundamental research and theory which are both covered in depth, as are the environmental impacts, commercial aspects and policy views

Solar cell manufacturing technologies are discussed in Part 11 An overview of the principal issues, including the physics of solar cell operation, materials and modelling and the fundamental theoretical framework form a n introduction

to the device aspects of photovoltaics given in Part IIa Part IIb gives a detailed account of crystalline silicon technology, from the manufacture and properties

of silicon (Chapter IIb-1) to industrial and high-efficiency solar cells based on wafer silicon (Chapters IIb-2 and -5), and thin silicon cells (Chapter IIb-3) Part IIc examines all aspects of thin-film solar cells, including amorphous silicon, cadmium

which is rapidly gaining ground Part IId focuses on high-efficiency cells for space and concentrator use Part IIe deals with devices based on molecular structures The testing and calibration of both terrestrial and space solar cells is discussed

in Part IV An overview of material characterisation methods for silicon wafers and devices will be found in Chapter IIb-4

Solar radiation has been called the fuel of photovoltaics, and its characteristics form the basis of system design, from array construction to the reliability of electricity supply by stand-alone photovoltaic systems The understanding of solar radiation forms arguably the most ancient part of physical science but it is only recently that the statistical nature of solar energy has been understood in some detail A number of sophisticated computer models are now available and are described in detail in Part I, which also summarises the relevant aspects of solar radiation as a n energy source and examines the principal attributes and

2 Practical Handbook of Photovoltaics: Fundamentals and Applications

System engineering is discussed in Part 111 Part IIIa provides an introduction to

including a review of the relevant modelling and simulation tools The balance of system components is discussed in Part IIIb providing an in-depth analysis

of battery operation in photovoltaic systems (Chapter IIIb-2) and an overview of

The rapidly growing area of grid-connected systems is considered from several viewpoints in Part IIIc which examines the technical and regulatory issues of the grid connection, and the building and electrical installation of domestic systems The International Energy Agency data about user experience and performance indicators are analysed in Chapter IIIe-2; the allied subject of system monitoring

is reviewed in Chapter IV-2

Solar cells as a source of power for consumer products are reviewed in Chapter IIIe-3 Arguably the most exciting aspect of photovoltaics -the visual impact of

numerous breathtaking examples of the new trends

An in-depth analysis of the world photovoltaic markets is given in Part V alongside an overview of support mechanisms Part V also reviews the potential hazards in solar cell manufacture, and examines broader environmental issues, including C 0 2 emissions and the energy payback times

The space application of solar cells has always been considered a unique and a special area of photovoltaics The device aspects, including radiation damage, are discussed in Chapters IId-1 and IIId-2 with a brief introduction to the material aspects in Chapter IIa-2 Chapter IIId-2 gives a thorough review of the operation of solar cells in the space environment together with its history and space mission requirements

The use of high-efficiency cells is not confined to space but is finding an increasing application in concentrator systems which are examined Chapter IIId-1 The corresponding solar cells are discussed in Part IId

The handbook contains a number of chapters that contain a strong research element The ultimate efficiencies that can be reached by a solar cell are discussed

in Chapter IIa-3 High-efficiency concepts in crystalline silicon photovoltaics, which have driven the progress in this field over several decades, are reviewed in Chapter IIb-5 The dye-sensitised and organic/plastic solar cells that are attracting a large research investment are examined in Part IIe Part IId and Chapter IIc-2 cover wide areas at the boundary between industrial production and research, as do many other chapters on thin-film solar cells in Part IIc Each chapter gives a self-contained overview of the relevant aspect of photovoltaic science and technology They can be read on their own although ample cross-referencing provides links that can be followed to build a knowledge base for any particular purpose at hand For the non-specialist, the introductory chapters of Parts I1 and I11 can serve as a starting point before proceeding to explore other parts of the handbook The supplementary chapters and appendices, including bibliography, solar radiation data for selected sites and the lists of standards, web sites and news sheets, act as pointers to more specific

Part I

I

Climatology in the Design of

2.6.2 Examples of Simplified Radiation Climatologies for Europe

2.6.3 Basic Solar Radiation Climatology of the Humid Tropics

2.6.4 Solar Radiation in Desert-Type Climates 2.6.5 Special Issues in Mountainous Areas Radiation Climatology of Inclined Planes for Photovoltaics Applications

6 Practical Handbook of Photovoltaics: Fundamentals and Applications

2.9 Conclusions Concerning Basic Radiation Climatology

Quantitative Processing of Solar Radiation for Photovoltaic

The Stochastic Generation of Solar Radiation Data

4.1 The Basic Approach and the Implicit Risks

4.1.1 General Principles 4.2 Estimating Time Series ofDaily Global Radiation from

Monthly Means using KT-Based Methods 4.3 Improvements in the Stochastic Estimation of Daily Solar

Radiation in the SoDa Project 4.3.1 Validation of Daily Irradiation Generation Models 4.4 Generating Hourly Values of Global Radiation from Daily

Values 4.4.1 Generating Mean Daily Profiles of Irradiance from Specific Values of Daily Radiation

4.4.2 Profiling of the Global Irradiance 4.4.3 Stochastic Generation of Hourly Values of Global Irradiation

4.5 Splitting the Global Radiation to Diffuse and Beam

4.6 Assessment of Progress

5.1 Angular Movements of the Sun Across the Seasons

5.2 Time Systems Used in Conjunction with Solar Geometry

and Solar Radiation Predictions 5.3 Conversion ofLocal Mean Time (LMT) to Local Apparent

Time (Solar Time) 5.4 Trigonometric Determination of the Solar Geometry

5.4.1 5.4.2 Climatological Algorithms for Estimating 5.4.3

5.5 The Calculation of the Angle of Incidence and Vertical and Horizontal Shadow Angles

5.6 Establishing the Accurate Noon Declination and the

Accurate Solar Geometry The Estimation of Hourly Global and Diffuse Horizontal

5 Computing the Solar Geometry

Key Angles Describing the Solar Geometry Declination

The Calculation of Solar Altitude and Azimuth Angles, also Astronomical Day Length

I The Role of Solar Radiation Climatology in the Dcsign of Phntovnltair Systrnrs 7

6.3 The Estimation of Mean Daily Profiles of Global Solar

Irradiation from Monthly Means

6.3.1 Method 1 : Estimating the Monthly Mean Daily

Irradiance Profile of G h ( t ) from the Monthly Mean

Extraterrestrial Irradiance Profile 6.3.2 Method 2: Estimating theMonthly Mean Daily

Profile of G h ( t) from the Clear Sky Irradiance Profile 6.4 The Estimation of Hourly Diffuse Radiation on Horizontal Surfaces

6.4.1 Introduction

6.4.2 Estimating the Monthly Average Daily Diffuse

Horizontal Irradiation from the Monthly Average

from Hourly Time Series of Horizontal Irradiation

Principles

7.2 Direct Beam Irradiation on Inclined Planes

7.3 The Estimation of the Hourly Diffuse Irradiation on

Inclined Surfaces from the Hourly Horizontal Diffuse

Irradiation

7.4 The Estimation Process for Sunlit Sun-Facing Surfaces

for Solar Altitudes below 5.7"

8 Practical Handbook Photovoltairs: Fundamentals and Applications

1 Introduction

Climate and solar radiation impact both on system supply side issues and on system demand side issues Designers need both solar data and temperature data Temperature affects the performance of photovoltaic devices per se It also has a strong bearing on the demands of the energy required for heating and cooling Relating supply and demand within any renewable energy structure requires study of the inter-relationships between supply and demand One has to establish the resources that need to be devoted to energy storage to achieve a n acceptably reliable energy supply from a n intermittent supply resource Ideally one needs long-term time series of solar radiation data and temperature data for each specific site at the hourly level Such data are relatively rare, so in recent years statistical approaches have been developed to help fill the gap Long series of daily data are needed for sizing and modelling of stand-alone systems (see Part

IIIa, Chapters 2 and 3) Effective statistical approaches have to recognise the

links between solar radiation data and temperature data

features of solar radiation climatology that are important for photovoltaics

the currently available quantitative techniques available for compiling solar radiation design data for specific sites The goal is to help optimise future photovoltaic design through the application of improved climate knowledge Sections 4-7 explain in detail the quantitative procedures that are suitable for developing quantitative solar radiation data in the forms needed for the study of the performance of photovoltaic systems

Attention is &awn throughout to important sources of systematic climate data that are already readily accessible to designers These modern approaches, aiming to provide the user with specialised meteorological data for specific sites, usually try to deliver information to users through user-friendly PC-based methodologies The most successful now use advanced CD-ROM-based computational toolboxes as a matter of routine For example, the digitally based

alone CD-ROM toolbox The ESRA toolbox includes coupled applications modules that enable users to address in a user-friendly way the design of stand- alone and grid-connected PV systems for any site in the mapped area The

needs with a wide range of supporting algorithms Global coverage based on CD-

ROM approaches is available in the Meteonorm system marketed by Meteotest of

the US NASA enhanced with support from UNEP provides another example of a CD-ROM-based design tool This contains photovoltaic applications tools A recent UK CD-ROM-based achievement is the Chartered Institution of Building Services Engineers (CIBSE) Guide J Weather, solar and illuminance data [7] It is

hour by hour observed climate databases may be purchased for the UK The pdf

I - The Role of Solar Radiation Clinmtology in the Design of Photovoltaic Systems 9

tabulated data into spreadsheets to enhance their practical utility in design, for example for facilitating inter-site interpolation and for creating site specific design graphics

More recently developed systems provide user friendly live interactive IT methodologies, operating through the World Wide Web After development which is costly, such systems allow users to specify their own climate data requirements directly through the Internet and then receive back nearly immediate application oriented answers to their questions for any part of the globe Designated server sites interacting within organised computational networks offer users intelligently programmed access to the use of various distributed and interlinked data bank systems to provide advanced solar radiation data and other climate design data The user-requested results are delivered back through the internet The recently completed EU-funded

EU-based Sate1 Light program provides another web-based example, covering

the area 34"N to 68"N 20"W to S0"E with data for five years (1996 to

design [9, 101

Extensive use is now made of digital mapping techniques in such programmes Such maps have the advantage that they can be accessed in very user-friendly ways using computer mouse clicks to extract climate data associated with any specific pixel from the database resource The data associated with any pixel are arranged in arrays spatially structured

systems enable users to work directly from the maps themselves The mouse is used to gain computational access to detailed design databanks associated with any specific pixels held in the database This input data is then used in structured algorithmic systems to supply outputs that match declared user

orientation

This chapter does not attempt to reproduce the advanced technological studies that underlie these new approaches Readers should consult the above listed publications to find out more about them These new approaches include the use of satellite technology to estimate radiation data for regions of the world

with no ground observational resources for measuring solar radiation [ 1, 81

This processed satellite data then becomes part of the organised database resource

The world is a big place No one can be familiar with more than a small part of

it There are great dangers in photovoltaics design, if designers attempt to guess

in the assessment of design risks the properties of the radiation climate a t

unfamiliar places As globally based climatological tools are becoming more

widely available, designers should make themselves aware of the powers of the new tools for design assessment in renewable energy design Such tools should not be looked on as luxuries They deliver quality controlled programmed results very efficiently to users This chapter draws heavily on the resources available in the above publications

10 Practical Handbook of Photovoltaics: Fundamentals and Applications

2 Key Features of the Radiation Climatology in

2.1 Some Definitions and Associated Units

The solar short-wave radiation falling on a horizontal surface from sun and sky combined is known as the global short-wave radiation The global short-wave

symbol G The most commonly used unit is watts per square metre (W mP2) The integral of irradiance flux over any period is called the irradiation Typical

the integration period The global radiation may be split into its two components, beam and diffuse The method of indicating the integration period for these

irradiation data corresponds to that used for global irradiation, yielding Bh, B d ,

B,,, and ( B d ) , as the symbols for the horizontal beam irradiation and Dh, Dd, D ,

and (Dd), as the symbols for the horizontal diffuse irradiation over the indicated periods, h hour, d day, and m month and monthly mean The symbols for

irradiance, G, E and D carry no time subscripts, so making a clear distinction

between irradiance and irradiation

Solar radiation data are often presented in a dimensionless form called the

dividing the global irradiation at the surface on the horizontal plane by the corresponding extraterrestrial irradiation on a horizontal plane for the same time period Three integration time periods are in common use The hourly clearness index KTh is the ratio G h / G o h The daily clearness index KTd is the ratio

Gd/God The monthly mean clearness index K T , is (Gd),/(God), Goh, God and

(Go& are the corresponding extraterrestrial global irradiation quantities for the time integration periods, as indicated by the subscript

The solar constant 1, is the extraterrestrial irradiance of the solar beam at

earth from the sun varies slightly according time of year The symbol used for the correction to mean solar distance is E It is dimensionless The extraterrestrial irradiance normal to beam is therefore &Io W mP2

2.2 The Variability of Solar Radiation and the implications for Design

Solar photovoltaic designers are essentially facing the design problem of achieving optimal performance from a n input of a natural energy resource of great intrinsic variability Three basic sky conditions can be identified, cloudless skies, partially clouded skies and overcast skies Cloud cover is the primary cause

of the variability in solar radiation energy supply from one minute to another

I - The Role of Solar Radiation Climatology in the Design of Photovoltaic Systems 11

to geographic locations In desert climates, there may be no cloud cover day after day In high latitude maritime climates, overcast skies may persist day after day

in winter with only rare breaks of sunshine Statistics of cloud cover and daily sunshine, which are widely available, yield qualitative information that can help inform design Such statistics are available in a systematic mapped form for

solar radiation energy fluxes The starting point is to understand the basic radiation climatology for the three basic conditions: clear sky, partially cloudy skies and overcast skies

2.3 Cloudless Sky Global Solar Radiation at Different latitudes

The global irradiation is made up of two components, the horizontal beam irradiation and the horizontal diffuse irradiation coming from the hemispherical sky dome The cloudless sky global irradiation may be calculated using the

of the site, the date in the year, which determines the solar geometry and the clarity of the atmosphere The clarity of the sky is described by an index known as the Linke turbidity factor Dust, man-made pollution and water vapour in the cloudless atmosphere deplete the clear sky beam irradiation and increase the

clear sky diffuse irradiation Figure 1 plots the calculated clear sky irradiance

normal to the solar beam as a function of the sea level solar altitude for different values of the Linke turbidity factor Figure 2 plots the corresponding values of the diffuse irradiance on horizontal surfaces Representative spectra of the clear sky

global and diffuse irradiance are shown in Figure 3

By combining the information in Figures 1 and 2 with the geometric

information about the solar altitude, a clear day irradiance plot can be produced

Sdar altitude, degrees

Figure 1 European Solar Radiation Atlas clear sky beam irradiance model [ l , 111 estimates of the irradiance normal to the beam in Wh m-2 at sea level at mean solar distance for a range of Linke turbidity factors of 1.5-8.5 In practice the sea level Linke turbidity factor is seldom below about 3.5 Values above 6 arecommonindesert areasdue todust in the atmosphere

12 Practical Handbook of Photovoltaics: Fundamentals and Applications

Figure 4 shows the calculated distribution of the hourly global, beam and diffuse

direct beam dominates Under cloudless sky conditions, the diffuse irradiation forms typically about 10-20% of the global radiation If one performs the same calculation, day by day throughout the year and then integrates the hourly values, one can estimate the annual pattern of the clear day daily irradiation at this latitude at any reference level of atmospheric clarity Figure 5 shows the result The inter-seasonal variations due to the changing day length and the

shows the substantial impact of the Linke turbidity factor on the clear sky daily global radiation at latitude 45"N It is very rare to encounter a turbidity below

summer Extremes of dust in the atmosphere may raise the Linke turbidity factor

to 6.0 or more The diffuse proportion of the daily irradiation, which is not plotted, increases as the sky clarity decreases The hour by hour diffuse proportion also becomes greater as the solar altitude angle gets lower The available clear sky global radiation becomes extremely low in December at high Northerly latitudes as one is close to the Arctic Circle and the noon sun is low in

irradiation as a function of the day number As the knowledge of the Linke turbidity factor is so important for making solar energy estimates, considerable

1 The Role of Solar Radiation Climatology in the Design of Photovoltaic Systems 1 3

0 0.5 t 1.5 2 2.5 3 3.5 4

Wavelength I pm Figure 3 Calculated clear sky global and difluse spectral irradiances on a horizontal surface for a solar

altitude of30" The integrated global irradiance was 476 W m-2 and the integrated difluse irradiance was

11 4 Wm-2 Note thedominanceofdifluse radiation in the ultraviolet and the contrasting low clear sky difluse spectral irradiance in the infrared region Water vapour in the atmosphere is an important absorbing agent in the infrared region Source: reference [7]

reliable global database of Linke turbidity factor values

solstice in the northern hemisphere The Equator values show the maximum

effectively defines the near maximum global radiation likely to be observed on

irradiation received per unit area can be achieved by tilting the collector surface

radiation availability in winter The quantitative estimation of the cloudless sky irradiation falling on inclined planes is discussed later

2.3.7 Partially Clouded Conditions

variable in partially clouded climates Dramatic changes in beam irradiance can occur within a few seconds Skies with a lot of scattered cloud tend to be relatively bright Consequently the associated diffuse radiation received during such periods can be quite large However, the short-term variations in the diffuse irradiance are normally not as great as in the case of the beam The beam irradiance may fall from 750 W m-2 to zero within seconds and then, within less

observed radiation records for four European sites measuring both global and diffuse radiation on a continuous basis from [ 151 The four days were selected as being representative of different types of intermittent cloudiness The top curve

Practical Handbook of Photovoltaics: Fundamentals and Applications

solar time (hours)

Figure 4 Estimated hourly horizontal irradiation under cloudless skies a t sea level a t latitude 45"N

showing the split into hourly direct and diffuse irradiation with afixed L i n k turbidityfactor of 3 Source: reference [I 41

Global

Jan Feb Mar Apr May J u n Jul Aug Sep Oct Nov Dec

Figure 5 Estimated daily horizontal irradiation under cloudless skies a t sea level in diflerent seasons, latitude 4 5 " N , showing the split into daily direct anddifluse irradiation with afixed Linke turbidityfactor of

3 Source: reference [ l 4 ]

I - The Role Solar Radiation Climatology in the Design of Photovoltaic Systems 1 5

calculated usinga Linke turbidity factorof3.5 Derivedfrom reference[7]

Cloudless day daily global irradiation falling on horizontal surfaces a t four different latitudes

This is characteristic with broken cloudy under windy conditions At Nantes on

2 2 July 1994, the morning was overcast and the afternoon sunny In Athens

on 2 5 October 1 9 9 4 there was some thin cloud throughout most of the day

Figure 8 Variation of theglobal anddiffuse horizontal irradiance with time of day forfour European sites under conditions of intermediatecloudiness The bottom line

is the diffuse component the top line is the global component When the two lines merge into one line i t is overcast Vaulx and Nantes reveal a cloudy morning followed

byasunnier afternoon Theirradianceat Nanteson 9 May 1994 is very variable Source: referencell51

I The Role of the Design of 1 7

followed by a brief overcast period The day and night periods are indicated using night as black This presentation makes the changes in day length evident The numbers at the top right-hand side give the temperatures The daily global radiation is obtained by integrating the area under the curve This yields a single

question is ‘Is the detail important for design?’ The second question, if it is important, is ‘Can the detail be statistically regenerated for design studies?’ The answer to the first question is yes, because one is dealing with non-linear systems The answer to the second question is there are stochastic data generation methods available but they are complex Stochastic methods are discussed in more detail

in Section 4

2.4 Overcast Sky Conditions

There is no direct beam during overcast periods The radiation on overcast days

is not steady The height, type and depth of clouds influence the atmospheric transmission Changes in the cloud cover make the irradiation on overcast days variable but the minute to minute variations in global irradiance tend to be far less than with partially cloudy skies

irradiation at Hamburg They found that the overcast hour diffuse radiation from

There were strong differences between the different cloud types They expressed the overcast hour horizontal radiation data with different cloud types as a fraction of the corresponding clear sky global horizontal radiation data found at

typical overcast day values to the clear day values discussed in Section 2 3 The

impact of rain clouds is especially evident Such days prove serious battery down draw days The practical risk is such days often occur in prolonged runs of adverse weather rather than single days in isolation

2.5 Sequences of Global Radiation Data

great importance in having daily time series of global radiation for making sensibly reliable design decisions Otherwise, the day to day variability patterns

Table 1 Overcast cloud transmittance relative to clear sky values Source: reference 1161

~ _

_ _

Cirrus cirrostratus and

0 2 5 0.18 0.16

18 Practical Handbook of Photovoltaics: Fundamentals and Applications

in solar radiation due to weather change will lost The daily global radiation can

be profiled to match the clear day profile to estimate hourly irradiation values with a n acceptable loss in accuracy The monthly mean values cannot be used with accuracy to simulate daily profiles Figure 9 shows the advantages of using

a profiled daily time series model to simulate performance compared with Figure

differences between the methodologies become even greater when one moves on

to estimating slope irradiation

Another issue is that there is a n auto-correlation between the global radiation

on successive days because of the common persistence of cyclones and anti- cyclones over several days Strings of dull days may therefore occur, which may put a serious strain on battery storage resources These statistical issues are discussed in Section 4

_ _ - - - Algonthmic Cham

I

;

B I I I I I

2.6 Simplified Radiation Climatologies

radiation climatology found in different parts of the world As climate data are compiled on a month by month basis, it is sensible to adopt the calendar month

as basic time scale division in climatological summaries The most important solar radiation climatological value is the long-term monthly mean daily global

few sites, it usually has to be estimated using the methods described later in

representative monthly maximum values of daily global irradiation, Gdrnax The

maximum measured values in each 10-year data series for each site to obtain

Figure 9

daily global irradiation values Source: referenre [ I 41

Using an averageprofile model to generate hourly data, starting with the respective time series of

I - The Role of Solar Radiation Climatology in the Design of Photovoltaic Systems

Observed Time Series

_ _ _ _ _ Repeated Monthly Average Profile

O h

Figure IO

with monthly meandaily global radiation (G& Source: reference [14]

Using a monthly average hourly global radiation profile model to generate hourly data starting

global database of monthly mean Linke turbidity factors However, there is a risk

climates where cloudless days have a near zero probability of occurrence, for example in the rainy season months in Equatorial Africa and in the Monsoon months in countries like India It is also useful to tabulate representative

minimum global values Gdmin The European Solar Radiation Atlas [ 2 3 ] used the

data series Sunshine observation data can be usefully added As the day

length varies so much with season, especially at high latitudes, it is useful to express the monthly mean daily bright sunshine in hours as a fraction of the astronomical day length This ratio is called the relative sunshine duration Experience with the various European Atlases has also indicated the value of

climatological summaries Many of the algorithms used in data preparation,

a n input

2.6.2 Examples of Simplified Radiation Climatologies for Europe

Figure 11, based on observed data for the period 1981-90 for eight European cities located at different latitudes, presents the latitudinal relationships between daily global radiation, daily diffuse radiation, the ratio diffuse/global radiation

month the monthly mean global and diffuse radiation for the period 1981-90

Sde Boker 12 (h) and one humid tropical site, Ilorin in Nigeria Figure 1 l ( a ) immediately makes evident the large latitudinal gradient in monthly mean daily global radiation in winter In mid-summer this strong latitudinal gradient is absent This winter gradient reflects the extraterrestrial values shown in Figure

11 (a) by the dashed line A better understanding of exactly what is happening is

20 Practical Handbook of Photovoltaics: Fundamentals and Applications

Figure 1 1 Monthly mean climatology of selected European sites 1981-90 in June and December Prepared

from reference 111 ( a ) Monthly m m n daily global radiation (points andfull lines) and the radiation outside the atmosphere (dashed lines): ( b ) monthly mean difuse radiation on horizontal surfaces (all in kWh m-2);

(e) ratio ofmeandaily diffuse radiation on horizontal surfaces to the corresponding global values; ( d ) monthly mean KTvaIues The graph corresponds to solar radiation data shown in the Appendix

relatively sunny winter climate, due to the protection offered by the mountains

immediately to the north In summer London (latitude 5 1.52"N) has a cloudier

I The Role of Solar Radiation Climatology in the of Photovoltaic Systems 2

less radiation than more northerly Stockholm, which has a more continental type of summer

daily diffuse irradiation falling on horizontal surfaces The diffuse radiation is relatively constant across Europe in the period May to August and can be less at southerly latitudes than at northerly latitudes There are large variations however in the relative proportion of beam irradiation Figure l l ( c ) sets down the month by month values of the ratio (D&/(G& Of course (B&J(Gc~),,l = 1- (D&,,/(G& While Figures l l ( b ) and (c) reveal big variations in the diffuse radiation, it shows that, even in relatively sunny climates, the diffuse radiation is

radiant energy, so proper attention must be attached to diffuse irradiation analysis Only the beam radiation can be focused successfully

found in the ESRA User Guidebook of reference [ 11 which identifies three roughly

defined climatic categories, Maritime climates, Continental climates and Mediterranean climates, adding a separate category of high mountainous climates Photovoltaic designers are strongly encouraged to strengthen their understanding of the basic dynamics of climate in the regions in which they plan

to work, before embarking on detailed quantitative studies Such knowledge strengthens understanding of the basic strategic issues that should underlie the detailed engineering design decisions

2.6.3 Basic Solar Radiation Climatology of the Humid Tropics

The humid tropics are located close to the Equator The Tropic of Cancer at latitude 22.5"N and the Tropic of Capricorn at latitude 22.5"s give a broad

definition to the zone The equatorial radiation climate is quite different in nature

to the climate produced by the varying patterns of cyclones and anti-cyclones,

For a start, the month to month variations in day length are very small close to the Equator There are no long summer evenings There are no long

winter nights So the hours during which indoor lighting is needed do not

energy demand

The noon solar elevation of the sun at low latitudes remains fairly high

the global solar energy varies very little across the year Figure 7 in Section 2.3

shows clearly the contrast between the annual pattern of clear sky daily

latitudes However, cloudless conditions are rare in equatorial areas

The wet and dry seasons are usually well-defined periods in tropical regions There is always a considerable amount of cloud during the wet seasons There is

also often quite a lot of cloud in the drier seasons too The daily pattern of irradiation is often dominated by convective clouds The clouds build up as the

surface temperatures increase These clouds frequently obstruct the sun and some produce large amounts of rainfall This rainfall is often associated with

22 Practical Handbook Photovoltaics: Fundamentals and Applications

violent thunderstorms The normal pattern of insolation is therefore a n oscillating mix of periods of bright sunshine intermingled with periods of heavy obstruction of the solar beam The diffuse radiation frequently dominates throughout the year

The variations in the amount and type of cloud cover associated with the different seasons is the main factor influencing the monthly mean global solar radiation available at different times of the year The patterns of radiation experienced reflect the impacts of the general circulation of the earth’s atmosphere in tropical regions Surface warming by the sun induces a global atmospheric circulation pattern of ascending air above the equatorial zone Air from both the northern hemisphere and the southern hemisphere flows horizontally in at the bottom at low levels to drive this vast vertical circulation system in the equatorial zone This air ascends in massive convection cells that move upwards, cooling and shedding moisture as they ascend Vertical clouds that may grow to considerable heights in the atmosphere often form producing heavy rain The air that has ascended, loosing much of its initial moisture on the way up through rainfall, then flows away from the equatorial region towards higher latitudes, moving horizontally a t relatively high levels close to the stratosphere This circulating air eventually descends heated adiabatically by recompression to reach the surface of the land, warm and dry Such descending dry air overlies the surface of the great deserts of the globe and desiccates them These regions of descending dry air are essentially regions of high sunshine and small or virtually non-existent rainfall The rotation of the earth causes a distortion of the convection cells, so, for example, the returning air from the Sahara moving into the west of Africa arrives predominately from a north-east direction

The zone where the air masses from the south and north merge to form the equatorial convection cells is known as the inter-tropical convergence zone The position of the inter-tropical convergence zone oscillates north and south as the solar declination changes with season This movement produces the typical patterns of wet and dry seasons found in tropical climates There are northern and southern limits to the movement of the inter-tropical convergence zone If

season will be experienced Closer to the Equator a pattern of two wet seasons is usually experienced as the rains come up from the Equator and produce the first rainy season The ITCZ then passes on to a region further from the Equator, the rains yield, only to return later with a second season of rain as the inter-tropical convergence zone falls back later in the year towards the other hemisphere The northern hemisphere and southern hemisphere tropical rainy seasons typically

are about 6 months out of phase from each other, reflecting the annual

movements of the inter-tropical convergence zone back and forwards across the Equator

diffuse radiation values tend to occur during hours of broken thin cloud The