Study of Solar Radiation on Inclined Surfaces in Japan

Evaluation of models for predicting monthly mean insolation on inclined surfaces was carried out using the data for 16 months from the experimental measurements in Tsukuba Science City. Three direct solar radiation models, eight sky-diffuse radiation models, one ground-reflected radiation model and twenty four total slope insolation models by combination of them were evaluated. Solar radiation measured on an inclined surface with seven orientations, the measured direct-normal and the global solar radiation on a horizontal surface as well as the ground albedo were used for the evaluation. MBE (mean bias error) and RMSE (root mean square error) were used for ranking of the models as well as the graphical comparison between the predicted and the measured values. It was found that , model for direct radiation, Hay (1)model for sky-diffuse radiation and combined (1) model for total insolation on inclined surfaces were the best. By means of this combined model the monthly mean insolation on inclined surfaces was predicted with MBE of ∼0.2% and ∼0.2% for the south-faced inclination smaller than 60° and greater than 60°, respectively, and with RMSE of and ∼1.4% and ∼5 for the south-faced slope of inclination smaller than 45° and greater than 45°, respectively.