A model for diffuse and global irradiation on horizontal surfaces

Abstract

The intensity of the direct radiation and the diffuse radiation at any time on a horizontal surface are each expressed as fractions of the intensity of the extraterrestrial radiation. Using these and assuming a random distribution of the bright sunshine hours and not too wide variations in the values of the transmission coefficients, a number of relations for estimating the global and the diffuse irradiation are derived. Two of the relations derived, including the Angström correlation for estimating the global irradiation, are already known empirically while several new correlations have been derived. The relations derived in this paper are: ▪The formulation lends more confidence in the use of the already empirically known relations (i) and (ii) by providing them a theoretical basis and affords mare flexibility to the estimation techniques by supplying new equations (iii) to (viii). The study also enables to throw light into the nature of the constants appearing in the various equations. It identifies three independent basic parameters and the constants appearing in the various equations as simple functions of these three basic parameters. This provides unification and interrelationships between the various constants. Experimental data for the diffuse irradiation, the global irradiation and the bright sunshine duration for Macerata (Italy), Salisbury and Bulawayo (Zimbabwe) is found to show good correlation for the linear eqns (i) to (vii), and the nature and the interrelationships of the constants is found to be as predicted by the theory.