MODELLING HOURLY AND DAILY DIFFUSE SOLAR RADIATION USING WORLD-WIDE DATABASE SAIMA MUNA WWAR B. Tech. in Petrochemical Engineering A thesis submitted in partial fulfilment of the requirements of Napier University for the degree of Doctor of Philosophy. April 2006 \ qodis J{e Wlio lias created tlie liea'CJens and tlie eartli and sends down water (rain) from tlie sRy, and tliere6y 6rouglit fortli fruits as pro'CJision foryou; andJ{e lias made tlie sliips to 6e of service to you, tliat tliey may sai[ tlirougli tlie sea 6y J{is Command; andJ{e lias made ri'CJers (afso) to 6e ofservice to you. jlndJ{e lias made tlie sun and tlie moon, 60tli constantfy pursuing tlieir courses, to 6e ofservice to you; andJ{e lias made tlie niglit and tlie day, to 6e of service to you. (Quran: Cliapter 14; Verses 32, 33) (])edicated to my P atlie'0 Prof ({ate) :M.unawwar )l1(Zo6airi Abstract ABSTRACT Solar energy is an alternative to fossil fuels for more sustainable and reliable energy options; with a huge potential to meet many times the present world energy demand. Readily available solar radiation data is a key to design and simulation of all solar energy applications. Whereas, global radiation is a frequently measured parameter, diffuse irradiance is often not measured, and therefore needs to be estimated from robust, reliable models. This research project aims to develop regional diffuse solar radiation models on both hourly and daily levels (using nine world-wide sites) for each of the following countries: India, Japan, Spain and UK. Qualitative analysis is performed to investigate the bearing of sunshine fraction, cloud cover and air mass on the clearness index (kt) -diffuse ratio (k) relationship, fundamental to presently proposed modelling. This is followed by a quantitative assessment by developing a series of hourly models based on these parameters along with global radiation, and comparing the performance results with the conventional k-kt model. Eventually, after thorough statistical evaluation, optimal region-wise hourly models are recommended. Daily radiation models are inherently different from their hourly counterparts and, nevertheless, are required for various other applications. Therefore, models for daily diffuse radiation are developed using daily sunshine fraction and daily-averaged cloud cover along with daily clearness index to estimate daily diffuse ratio. As in hourly modelling, all possible combinations of the parameters under focus are used to develop a set of models for each location. Statistical analysis similar to that in case of hourly radiation models is performed. This is followed by validating the daily models against other sites within a region to be able to safely propose region-based models for each of the four countries. It is revealed that uncertainty in measurements plays an important role in establishing the accuracy with which models can be used. Through this work, important parameters for hourly and daily diffuse radiation models are established. The proposed models are believed to provide reasonable accuracy if used within a given region where diffuse solar radiation is not measured. A case study for the futuristic sustainable production of solar electricity for India is also included to demonstrate the application of solar radiation. Acknowledgments ACKNOWLEDGEMENTS My utmost gratitude is to God Almighty, the Creator and the Sustainer of the heavens and the Earth and all that exists in between. I am highly indebted to Prof Tariq Muneer under whose able supervision this research was conducted. His guidance was a true source of inspiration and motivation for me and I am grateful for his constant support, suggestions and advice throughout the course of my research. I deeply admire his prudence and intriguing intellect and have found working with him an enjoyable challenge. Sincere thanks are due to Prof J Kubie for his invaluable feed back on the earlier drafts of my thesis. I would also like to extend my deepest gratitude to Dr M Asif for his intricate reviewing of my thesis and complementary supervision during the research tenure. I am thankful to my colleagues, Serge and Haroon, and my supervisor's ex-students: Mehreen, Fatema, Naser (in chronological order) who have each extended their help and support, by engaging in intellectual discussions and sharing knowledge. Among the non-teaching staff, special thanks are due to Bill Campbell for being so helpful in providing computer technical support and to Sandra Bell and others in the school office. I am extremely grateful to Dr Christian Gueymard (USA) for providing me useful information and deep in-sight in the technicalities within the subject of solar radiation. His criticism and feedback, though mostly through e-mail, aided me to improve the quality of my work at various stages. My many friends and flat-mates, Reema, Arsala and Sahdia, deserve due credit for their moral support and for standing by my side in all odds. I thank my brother and sister and their respective families for simply being there. Finally, I would like to thank my Mum from the deepest corners of my heart; for it is due to her that I am here, pursuing my PhD. She is the greatest source of my strength and encouragement behind this task. ii Declaration DECLARATION I hereby declare that the work presented in this thesis was solely carried out by myself at Napier University, Edinburgh, except where due acknowledgement is made, and that it has not been submitted for any other degree. SAIMA MUNAWWAR (CANDIDATE) Date iii Table a/Contents TABLE OF CONTENTS Page Abstract Acknowledgements 11 Declaration III Table of contents IV List of Figures IX List of Tables XV Nomenclature XV11 CHAPTER 1 INTRODUCTION 1 1.1 Energy 1 1.1.1 Energy and environment 1 1.1.2 Energy and civilization 1 1.2 Sources of Energy and their current status 2 1.2.1 Conventional, non-renewable energy resources 2 1.2.2 Non-conventional, renewable energy resources 4 1.3 Energy challenges 8 1.3.1 Limited resources and their vulnerability 8 1.3.2 Environmental concerns 10 1.3.3 Socio-economic and political impacts 12 1.4 The Global Energy Future trends 13 1.4.1 Implications set by the ongoing conventional energy use 13 1.4.2 Role and prospects of renewable energy 15 1.5 Solar Energy: The Fuel of the Future 17 1.5.1 Solar thermal systems 18 1.5.2 Solar thermal power generation 19 1.5.3 Solar Photovoltaic 20 1.5.4 Solar cooking, detoxification~ desalination, and miscellaneous Applications 21 1.6 The Solar Radiation Data 25 1.6.1 Solar radiation basics 25 iv Table a/Contents 1.6.2 Solar data: its use and significance 27 1.6.3 Need for solar radiation data modelling, in particular diffuse radiation 29 1.7 The Present Research Project 30 1.7.1 Problem statement 30 1.7.2 Aims and objectives 30 1.7.3 Outline of the thesis 31 1.8 Summary 32 CHAPTER 2 REVIEW OF THE RELEVANT LITERATURE 38 2.1 Measurement and Processing of Solar Data and other Meteorological Parameters 38 2.1.1 Brief history of solar radiation measurement 38 2.1.2 Solar radiation instruments (radiometers) 39 2.1.3 Measurement of bright hours of sunshine 42 2.1.4 Cloud cover measurements 44 2.1.5 Uncertainties and errors associated with solar radiation measurements 45 2.1.5.1 Calibration uncertainties 45 2.1.5.2 Sources of error 46 2.1.5.3 Measurement uncertainties 47 2.1.6 Proposed methods to improve the quality of data 48 2.1.6.1 Quality assessment of the data 48 2.1.6.2 Shadow band correction 52 2.2 Radiation Modelling 54 2.2.1 Estimation of solar radiation components from global radiation 55 2.2.1.1 Diffuse ratio- clearness index regressions 55 2.2.1.2 Diffuse transmittance index-clearness index regreSSIOn 59 2.2.1.3 Direct transmittance index-clearness index regreSSIOns 59 2.2.2 Estimation of diffuse radiation from beam/direct radiation 59 2.2.2.1 ASHRAE model 59 v Table a/Contents 2.2.2.2 Direct transmittance index - diffuse transmittance index regression 60 2.2.3 Estimation of solar radiation from meteorological parameters 60 2.2.3.1 Estimation of solar radiation from sunshine fraction 60 2.2.3.2 Additional descriptors used with clearness index for radiation models 62 2.2.3.3 Estimation of diffuse radiation using various other parameters 64 2.2.3.4 Cloud cover models 65 2.2.3.5 Atmospheric transmittance models 67 2.2.4 Satellite based models 69 Statistical Measures used for Evaluation of Models 70 2.4 Summary 74 CHAPTER 3 THE PRESENT DATABASE 82 3.1 Geographical and Database Information for the Sites under Study 82 3.2 Hourly Data 83 3.2.1 Processing 83 3.2.2 Quality control 84 Daily Data 87 3.3.1 Assimilation 87 3.4 Summary 89 CHAPTER 4 PRELIMINARY QUALITATIVE ANALYSIS FOR DIFFUSE IRRADIANCE MODELLING 97 4.1 Significant Factors in Characterization of Diffuse Irradiance: Need for Improvement over the Conventional k-kt Model 98 4.2 Inter-comparison of Sunshine Fraction, Cloud Cover and Air Mass Influence on Diffuse Irradiance 101 4.2.1 The effect of sunshine fraction (SF) 101 4.2.2 The effect of cloud cover (CC) 103 4.2.3 The effect of air mass (m) 103 4.3 Evaluation of Air Mass Influence on Diffuse Fraction using Cloud Cover and Sunshine Fraction Filters for Clear Skies 104 vi Table a/Contents 4.3.1 Basic clear sky filter using cloud cover 104 4.3.2 Basic clear sky filter using sunshine fraction 105 4.4 Summary 106 CHAPTERS HOURLY DIFFUSE IRRADIANCE MODELLING 131 5.1 Development of Models 132 5.1.1 Clearness index for diffuse irradiance: monovariate model 132 5.1.2 Clearness index and sunshine fraction/cloud cover/air mass: bivariate model 133 5.1.3 Clearness