Experimental and theoretical investigation of diffuse solar radiation: Data and models quality tested for Egyptian sites

In order to obtain reliable irradiation data for the design, operation and economic assessment of solar power stations, eight widely used standard models predicting diffuse irradiation values are tested. A statistical analysis of hourly, daily and monthly correlations is carried out, using a 5yr data archive (1999–2003) of hourly global and diffuse solar irradiances obtained at five selected meteorological stations over greater Egypt area. The accuracy of the candidate correlations are performed in terms of the two widely used statistical indicators, mean bias and root mean square errors; standard deviation, standard error and F-test, were also introduced. The results indicate that the correlations relating the diffuse fraction (KD) with both clearness index (KT) and the sunshine fraction (S/SO) are more reliable for diffuse irradiation predictions in the Egyptian environment than using each variable separately.