Evaluation of different models to estimate the global solar radiation on inclined surfaces

Global and diffuse solar radiation intensities are, in general, measured on horizontal surfaces, whereas stationary solar conversion systems (both flat plate solar collector and solar photovoltaic) are mounted on inclined surface to maximize the amount of solar radiation incident on the collector surface. Consequently, the solar radiation incident on a tilted surface has to be determined by converting solar radiation from horizontal surface to the tilted surface of interest. This study evaluates the performance of 14 models transposing 10 min diffuse solar irradiation from horizontal to inclined surface. Solar radiation data from 8 months (April to November 2011) which include diverse atmospheric conditions and solar altitudes, measured on the roof of the radiation tower of the Royal Meteorological Institute of Belgium in Uccle (Longitude 4.35° E, Latitude 50.79° N) were used for validation purposes. Individual model performance is assessed by an inter-comparison between the calculated and measured solar global radiation on a south-oriented surface tilted at 50.79° using statistical methods. The relative performance of the different models under different sky conditions has been studied. Because statistical validation procedures revealed that none of the considered model performs well under all types of sky conditions a new model resulting from the coupling of three models acting under different sky conditions, is developed for Belgium. The sensitivity of the proposed model to the ground reflection formulation is assessed. Finally, the ability of the coupled model to handle hourly and daily data is discussed.