Establishing new empirical models for predicting monthly mean horizontal diffuse solar radiation in city of Isfahan, Iran

Diffuse solar radiation data are very required for many solar energy applications. The main objective of the present study is to establish new empirical models for predicting the monthly mean diffuse solar radiation on a horizontal surface for city of Isfahan situated in central part of Iran. For this aim, 16 empirical models from previous studies were nominated and categorized as: (1) diffuse fraction is function of relative sunshine duration; (2) diffuse fraction is function of clearness index. Also, using long-term sunshine duration and horizontal global solar radiation data as well as applying SRT (statistical regression technique), six new diffuse solar radiation models were established. The accuracy of the new models and those models selected from literature were evaluated in terms of different statistical indicators. It was found that all of the new models perform better than those nominated from literature. However, the best performance was achieved for the third degree model from category (1) in which sunshine duration plays role as a variable.