Fundamentals Of Solar Radiation

Description

The sun radiates a tremendous amount of energy, called solar energy or solar radiation, which is the main natural source of energy on the Earth, by far. Because solar radiation is the almost unique supplier of energy to the Earth, it has a primary influence on life and activities on the Earth. The climate is a first example, but there are many others, such as plant growth or human health, or even the design of buildings, the production of energy, notably electrical and thermal, or even aging materials. This book aims to provide simple answers to anyone who has questions about solar radiation. Its ambition is to help by presenting the fundamental elements of the solar radiation received on the ground. The book includes many examples and numerous illustrations, as well as some simple but fairly precise equations to calculate the various elements covered and to reproduce the figures and graphs. The first of the three parts of this book is devoted to the relative geometry between the direction of the sun and an observer on the ground as well as to the solar radiation emitted by the sun and received at the top of the atmosphere. The orbit of the Earth around the sun and the solar declination are described. The concept of time is introduced which is closely linked to the solar cycle and the rotation of the Earth on itself. Equations are given to calculate the solar radiation received on a horizontal or inclined surface located at the top of the atmosphere. The spectral distribution of the extraterrestrial solar radiation is described. The second part of this book addresses how the solar radiation incident at the top of the atmosphere is attenuated and modified in its downward path to the ground. The reflection of the radiation by the ground is presented. The solar radiation received on the ground by a horizontal or inclined collector plane, such as a natural slope or a rooftop, is discussed, as well as its spectral distribution. The variability of the radiation is addressed in relation to the properties of solar radiation estimated from the measurements. The third part deals with direct or indirect measurements of the solar radiation received on the ground over a given integration time (minute, hour, day, or month), whether for total radiation or radiation in a spectral range such as ultraviolet (UV), or daylight, or photosynthetically active radiation (PAR). It also explains how to check the plausibility of the measurements. Fundamentals of Solar Radiation will be a valuable resource to all professionals, engineers, researchers, students, and other practitioners that seek an understanding of solar radiation. Cover......Page 1 Half Title......Page 2 Title Page......Page 4 Copyright Page......Page 5 Table of Contents......Page 6 List of Figures......Page 10 List of Tables......Page 20 Preface......Page 22 Acknowledgments......Page 30 1 The definition of time and different time systems......Page 32 1.1 Reminder – geocentric and geographic coordinates – angle of incidence......Page 33 1.2 Orbit of the Earth around the sun – distance between the sun and the Earth......Page 36 1.3 Solar declination......Page 38 1.4 Definitions of time: from year to second......Page 40 1.4.1 Year – astronomical season – month – week......Page 41 1.4.2 Day – hour – minute – second......Page 43 1.5 Mean solar time – true solar time......Page 44 1.6.1 Coordinated universal time (UTC)......Page 46 1.6.2 Legal time – civil time – time zone......Page 47 1.6.3 To sum up: converting the legal time to the true solar time......Page 49 2 The course of the sun over an observer on the ground......Page 50 2.1 Position of the sun seen by an observer on the ground......Page 52 2.1.1 Solar zenithal angle – solar elevation – azimuth......Page 53 2.1.2 Direction of the sun in the case of an inclined plane......Page 56 2.2 Solar zenithal angles at solar noon during a year......Page 57 2.3 Times of sunrise and sunset......Page 58 2.4 Daytime – daylength......Page 61 2.5 Solar angles during equinoxes and solstices......Page 62 2.6 Effective solar angles......Page 67 2.6.1 Methods for computing effective angles......Page 68 2.6.2 How to practically use one of these methods......Page 69 3 Solar radiation incident at the top of the atmosphere......Page 71 3.1.1 Radiance......Page 72 3.1.2 Irradiance......Page 73 3.1.3 Irradiation – radiant exposure......Page 74 3.1.4 Converting irradiance to irradiation and vice versa......Page 75 3.2 Solar activity – extraterrestrial radiation......Page 76 3.3 Extraterrestrial radiation received on a horizontal plane......Page 77 3.3.1 Typical values of hourly extraterrestrial total radiation......Page 78 3.3.2 Daily extraterrestrial radiation......Page 82 3.4 Extraterrestrial radiation received on an inclined plane......Page 84 3.5 Spectral distribution of the extraterrestrial radiation......Page 87 4 Radiative transfer in the atmosphere......Page 91 4.1 Absorption by gases in the atmosphere......Page 92 4.2 Scattering – overview – case of the air molecules......Page 95 4.4.1 Air mass......Page 98 4.4.2 Optical depth – optical thickness......Page 99 4.4.3 Transmittance......Page 100 4.4.4 Linke turbidity factor......Page 101 4.4.5 Visibility......Page 102 4.5 Effects of aerosols on radiation......Page 103 4.6 Effects of clouds on radiation......Page 107 4.7 The paths of the sun rays in the atmosphere......Page 109 4.8 Summary of contributions of atmospheric constituents to attenuation of radiation......Page 111 5 Ground reflection......Page 113 5.1.1 Reflection factor......Page 114 5.1.2 Examples of spectra of reflection factor......Page 116 5.2 Bidirectional reflectance distribution function (BRDF)......Page 117 5.3 Albedo......Page 119 6 Solar radiation received at ground level......Page 126 6.1.1 Direct, diffuse, and reflected components – global radiation......Page 127 6.1.2 Several definitions of the direct component......Page 129 6.2.1 Definition......Page 131 6.2.2 An example of 15-min clearness indices......Page 133 6.2.3 An example of monthly clearness indices......Page 135 6.3 Clear-sky index......Page 137 6.4.1 An example of hourly radiation over the North Atlantic Ocean......Page 139 6.4.2 Map of a multi-year average of the total solar irradiance at ground......Page 140 6.6 The most important variables for the solar radiation at ground......Page 143 6.7 Decoupling the effects of clear atmosphere and clouds......Page 145 7 Spectral distribution of the solar radiation at ground......Page 148 7.1.1 Influence of the turbidity......Page 150 7.1.2 Spectral distribution of the direct and diffuse components......Page 152 7.1.3 Influence of the solar zenithal angle......Page 154 7.1.4 Typical irradiances integrated over some spectral intervals in a clear atmosphere......Page 155 7.2.1 Influence of the cloud optical depth......Page 157 7.2.2 Influence of the solar zenithal angle......Page 159 7.2.3 Typical irradiances integrated over spectral intervals in cloudy conditions......Page 160 8 Variability – implications for estimating radiation......Page 163 8.1 Time and space scales......Page 164 8.2 Random nature of meteorological variables – ergodicity......Page 167 8.3 Fluctuations around the average – small-scale variability......Page 170 8.4 Measurement – sampling......Page 175 8.5.1 Sampling theorem......Page 178 8.5.2 What are the smallest observable details in a series of measurements?......Page 179 9 Ground-based instruments for measuring solar radiation at ground......Page 181 9.1.1 Thermopiles and photodiodes – calibration and classes of instruments......Page 183 9.1.2 Pyranometer – pyrheliometer – pyranometer with a shadow ring or ball......Page 184 9.1.3 Other instruments measuring global radiation and its diffuse component simultaneously......Page 186 9.2 Typical uncertainties of pyranometers and pyrheliometers......Page 188 9.3 Incomplete coverage in both space and time by stations......Page 190 9.4.1 Ideal case of a complete time series of measurements......Page 193 9.4.2 Calculation of climate normals in meteorology......Page 195 9.4.3 Case of incomplete time series......Page 196 9.4.4 Overview of the completion of a time series......Page 198 10 Other means for estimating solar radiation at surface......Page 200 10.1 Meteorological analyses and reanalyses......Page 201 10.2 Images acquired by Earth observing satellites......Page 203 10.2.1 Multispectral imagery......Page 204 10.2.2 Orbits of satellites......Page 205 10.2.3 Overview of the methods for estimating the solar radiation......Page 207 10.3 Estimate using nearby ground stations......Page 210 10.4 Usual approach to estimate components from the only knowledge of global radiation......Page 212 10.5.1 Ångström’s relationship between irradiation and sunshine duration......Page 214 10.5.2 Relationships between the total radiation and that in a given spectral range......Page 216 10.5.3 Estimate using other meteorological variables......Page 217 11 Control of the plausibility of measurements......Page 220 11.1 Objective of the control of plausibility......Page 221 11.2 How to control the plausibility of measurements......Page 224 11.3 Checking metadata: an overview......Page 227 11.4 Checking metadata: geographic location of the station......Page 228 11.5.1 Overview of the most common problems......Page 229 11.5.2 Some graphs for the visual inspection of the time-related problems......Page 232 11.5.3 Benefit in using the hourly extraterrestrial irradiation......Page 233 11.6 Checking metadata: checking the measured quantity and its unit......Page 234 12 Visual and automated procedures......Page 238 12.2 Visual inspection of daily measurements......Page 239 12.2.1 Daily and monthly total irradiation......Page 240 12.2.2 An example of visual control of daily spectral values......Page 244 12.3 Visual inspection of hourly and intra-hourly values......Page 245 12.4 Two-dimensional representation of hourly and intra-hourly values......Page 248 12.5 Automatic procedures......Page 253 12.5.1 Hourly and intra-hourly measurements......Page 255 12.5.2 Daily measurements......Page 256 12.5.3 Monthly measurements......Page 257 12.5.4 Spectral radiation measurements......Page 259 Index......Page 262