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Global Solar Radiation

The radiations are received from the sun, without change of direction is called direct or beam radiation, with change of the direction is called diffuse radiation. The diffuse solar radiation is defined as the difference between total solar radiation and direct radiation [1, 2]. From figure-1 [2] the upper graph shows that under most favourable atmospheric conditions, the maximum intensity observed at noon on an oriented surface at sea level is 1 kW/m2. At an altitude of 1000 meters, the value rises to about 1.05 kW/m2. And in higher mountains value slightly above 1.1 kW/m2 are obtained, compared with 1.353 kW/m2 (the solar constant) in outer space. The later value sometimes called air mass O[1,2]. The upper curve applies at the outer limit of the atmosphere (AMO) [2]. The other lower curve applies to the earth’s surface during clear days for a sea level location, for AM1. Dotted curve is for a black body at 5900K. The lower two curves are applying for diffuse components for some haze and clear sky conditions respectively [2]. Although sun light outside the earth’s atmosphere is relatively constant, the situation at the earth’s surface is more complex. Terrestrial sunlight changes dramatically and unpredictably in availability, intensity and spectral composition. On clear days, the length of the sunlight’s path through the atmosphere or the optical air mass is an important parameter. The indirect or diffuse component of sunlight can be particularly important for less ideal conditions. Reasonable estimates of global radiation (direct plus diffuse) received annually on horizontal surfaces are available for most regions of the world. However, there are uncertainties involved in using this for a specific site because of the large deviations that can be caused by local geographical conditions and approximations involved in conversion to radiation on inclined surface [1,2].