A new analytical-empirical model for the instantaneous diffuse radiation and experimental investigation of its validity

Abstract

Most of the diffuse fraction correlations are based on hourly or daily data. However, the performance of solar collectors depends on instantaneous solar radiation. Moreover, in most cases the diffuse fraction is defined as I d I g , or G d G g , where I g and G g are hourly, and daily global radiation. But in many cases it is more suitable to define the diffuse fraction as I d I bn or G d G bn , as was done in the ASHRAE model. At present there are some fairly accurate methods, e.g. the parameterization method, to predict direct beam radiation, I bn. In combination with an accurate model for diffuse fraction defined as I d I bn , these methods would allow the prediction of instantaneous, hourly and daily beam, diffuse, ground reflected, and total solar radiation. The model developed in this paper allows the calculation of instantaneous diffuse radiation directly from the extraterrestrial radiation. According to this model, the instantaneous diffuse radiation and I d I bn fraction can be simulated with the help of the following formulae: I d =I on · K t · e −(m−1)K t e K 1 −1 (0.5K s +0.7K st ƒ) I d I bn = K t e K t e K t −1 (0.5K s +0.7K st ƒ) where I on is the direct normal extraterrestrial solar radiation; K t is the total attenuation coefficient; K s is the Rayleigh scattering coefficient; K st is the turbidity scattering coefficient; and f is an empirical function of zenith angle and air mass. If meteorological parameters are known, values of K t, K s and K st are found with the help of the parameterization method.