The eqn (1) can be used for both concentrated and unconcentrated solar radiation. When unconcentrated radiation is considered, the solid angle subtended by the Sun (say Qs) can be computed by eqn (9), taking into account the simple geometrical relationship 8 = tg’^Rsmj/d) where Rg^ (= 6.9599 x lO^m) is Sun's radius while d is the distance to the Sun. Then the energy flux is computed by using the approximate procedure (eqns 1-4) or the more elaborate treatment (equations (1) and (5)-(8)). When concentrated radiation is considered, first we observe that in the presence of the concentrator the Sun is viewed from the receiving surface under an enlarged solid angle say Qc ) The concentration ratio C is naturally defined (see equation (1) as in [13]): It is a simple question to verify that in case of both isotropic and non-isotropic radiation eqn (10) reduces to:
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