Correlation between global solar radiation and its direct and diffuse components

Work on typical correlations between global solar radiation and its direct and diffuse components is reviewed. Data for diffuse, global horizontal radiation measured at the City University of Hong Kong for the period 1991–1994 are analysed. Three correlations are considered — the diffuse coefficient (Kd) and the clearness index (Kt), the diffuse fraction (K) and Kt, and the direct normal (Idn) and Kt. A hybrid correlation model based on hourly measured data is proposed for the prediction of hourly direct and diffuse components from the global solar radiation for Hong Kong.