Estimation of the diffuse solar irradiation from global solar irradiation. Daily and monthly average daily values

Abstract

Data sets of total and direct solar irradiation at Valencia (Spain) measured during the years 1990–1991 have been used to analyze diverse methods of estimating the daily values and monthly average daily values of the diffuse fraction of solar radiation from the ratio of the total to the extraterrestrial radiation (clearness index k t). In the case of daily values, all the methods described in the bibliography have led to significant deviations concerning the experimental values, with a MAD (mean absolute deviation) close to 20%. For the monthly average daily values, the methods based on the PDF (probability density function) of daily values of k t provide more accurate results than the deterministic methods based on the correlation of monthly average daily values of k t.