A comparison of several diffuse solar radiation models for Greece

Summary

This paper attempts to test the applicability of existing correlation models to the estimation of diffuse radiation with respect to measured values at a station. There are two types of model: The first type depends on the fraction of monthly average daily diffuse radiation to total solar radiation,$K = {{\bar H_d } \mathord{\left/ {\vphantom {{\bar H_d } {\bar H}}} \right. \kern-\nulldelimiterspace} {\bar H}}$, as a function of the “clearness index”,$K_T = {{\bar H} \mathord{\left/ {\vphantom {{\bar H} {\bar H}}} \right. \kern-\nulldelimiterspace} {\bar H}}_O$. The second type expresses the fraction${{\bar H_d } \mathord{\left/ {\vphantom {{\bar H_d } {\bar H}}} \right. \kern-\nulldelimiterspace} {\bar H}}$ or${{\bar H_d } \mathord{\left/ {\vphantom {{\bar H_d } {\bar H}}} \right. \kern-\nulldelimiterspace} {\bar H}}_O$ as a function of the sunshine fraction${{\bar n} \mathord{\left/ {\vphantom {{\bar n} {\bar N}}} \right. \kern-\nulldelimiterspace} {\bar N}}$ Therefore, it presents statistically based correlations between global radiation and its diffuse component on a horizontal surface and suggests two equations to determine the ratio of diffuse radiation to total radiation received on a horizontal surface. The results of these correlation equations are compared with other accepted equations.