Solar Radiation, its Measurement and Application in Solar Energy Utilization Programme

Abstract

The electromagnetic radiation emitted by the sun, covers a very large range of wavelengths, from radiowaves through the visible to X-rays and gamma rays. But 99 percent of this energy is contained in the region 0.2 to 4μ, in the near ultraviolet, visible and near infra red regions of the solar spectrum, with a maximum about 0.5μ. Roughly one half of this radiation lies in the visible region between 0.38 and 0.77μ and the remainder in the ultraviolet and infrared regions. The radiation received from the sun, on an average, on a surface of unit area exposed normal to the sun’s rays outside the atmosphere, at the mean distance of the earth from the sun, is called the solar constant. It has a value of roughly 2cal/cm/min or 1.36 KW/m². In its passage through the atmosphere, solar radiation is attenuated by scattering and absorption by air molecules, watervapour, dust and aerosols and clouds, and reduced by geometric factors, so that the maximum intensity received at the earth’s surface’ is only about 1.0 KW/m², even with very clear, dry air and a cloudless sky.