Iqbal - An introduction to solar radiation

This book is written for those who must determine quantitatively the amount of solar radiation incident on a surface on the earth. The material has been derived from meteorological and engineering literature. The emphasis is on an understanding of the subject as well as on the methods of calculation.

The audience of this book can be divided into six groups: (1) energy analysts who are responsible for developing national programs on solar energy utilization and who are required to assess the potential of solar energy in their region; (2) designers of thermal devices, such as flat-plate collectors or concentrating devices, who need to calculate hourly and daily values of the incident radiation; (3) photovoltaic engineers who require an estimation of the spectral values of the direct plus diffuse solar radiation under cloudless skies; (4) architects and engineers who need to calculate thermal loads and natural illumination of buildings; (5) agronomists who need to estimate the radiative environment of plants; and (6) hydrologists who need to compute the amount of snowmelt and evaporation from reservoirs.

The required background for this book is some college-level elementary physics, chemistry, and calculus, and the book can form part of a course on solar energy utilization at senior-graduate levels or be used in an introductory course on solar radiation meteorology.

deals with extraterrestrial radiation. Detailed formulations treating instantaneous, hourly, and daily quantities of radiation incident on inclined planes are presented.

. The first part of this chapter describes earths cloudless-sky atmosphere (U.S.S.A. 1976), concentration of the various molecules, vertical distribution of ozone and its seasonal variation, vertical distribution of water vapor, evaluation of the precipitable water through its partial pressure or dew-point temperature, and description of the aerosols. The second part deals with the calculation of optical masses for air, ozone, and water vapor.

. This chapter essentially has four parts. In the first part, the basic Bouguer equation is laid down; Rayleigh and aerosol scattering are also described, and aerosol attenuation is described through ngstrms turbidity formula. In the second part, molecular absorption is described, and tables of the monochromatic attenuation coefficients are given for ozone, uniformly mixed gases, and water vapor. In the third part, the formulation to compute direct spectral irradiance is presented. In the fourth part, a simple empirical approach to calculate the diffuse spectral irradiance is demonstrated; however, in this approach multiple scattering is ignored. The diffuse irradiance is considered to have three components: (1) Rayleigh-scattered diffuse radiation, (2) aerosol-scattered diffuse radiation, and (3) multiply reflected diffuse radiation.

, in which three parameterization models of calculating direct and diffuse irradiance are described and compared with each other. Finally, a comparison is made between one of the three models and the ASHRAE algorithm for clear-sky irradiance. A revised ASHRAE algorithm is recommended.

treats the estimation of solar radiation arriving at horizontal surfaces on the earth through cloudy skies. Here, it is assumed that mean daily global (direct plus diffuse) radiation either is known from meteorological data or can be easily estimated through simple procedures such as the use of empirical correlations between global radiation and the number of bright sunshine hours or cloud cover. A number of such correlations are described, site dependent as well as site independent. The rest of the chapter discusses methods of predicting diffuse radiation when the global value is known. A critical examination of the predictive correlations for global as well as diffuse radiation is presented.

the ground albedo and its spectral and angular variation are described, and tables of the albedo values for natural ground covers and man-made surfaces are presented.

deals with calculation of angle factors. The angle factors between the ground and inclined surfaces and between the sky and inclined surfaces are developed.

. Direct, ground-reflected, and sky diffuse irradiance are taken up separately. Angular distribution of sky diffuse intensity is studied, and models of the anisotropic sky diffuse irradiance are presented.

. Development of the pyrheliometers beginning from Abbot water-flow and ngstrm electrical compensation to the modern cavity-type radiometers is presented. The different types of pyranometers and sunshine recorders are discussed. Sources of errors and methods of calibration are presented.

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