Comparative Study of Two Various Methods in Estimation of Clearness Index in Iranian Cities

The solar radiation that reaches to the earth's surface varies in the yearly duration and depends on the weather conditions. To predict the global solar radiation at the earth’s surface' the clearness index should be calculated. In this study, the daily clearness index has been calculated using the daily average measured global solar radiation data, which has been reported by IRIMO. The daily average clearness index results have been compared with two available methods (Collares, Duffie). The results show that if global solar radiation is estimated by Collares method, the calculated clearness index will be more close to the measured data.