Performance Evaluation of Hourly Solar Radiation Models on Inclined Surfaces in Longdong Area, China

Hourly global irradiations on tilted planes are required in various engineering calculations for solar systems which must be determined by converting the solar radiation intensities measured on a horizontal surface to the incident on the tilted surface of interest. There exist a large number of models designed to perform such a conversion. This paper presents the performance of five arithmetic models used to estimate solar irradiance on inclined surfaces. Utilizing solar radiation data from June to October measured in Longdong area of Gansu province, Model performance is assessed by an inter-comparison between the calculated and measured solar global radiation on a south-oriented surface tilted at 25°. Three statistical parameters (NMBE, NRMSE and R2) have been used to confirm the calculated values. The results show that the accuracy of the models is related to the sky conditions. Liu & Jordan model shows the best calculated performance under clear sky conditions, and Klucher model perform best under overcast and cloudy conditions. The other models included Perez, Hay and Skartveit & Olseth display an intermediary performance under all sky conditions. In order to estimate the tilt angle simply, two simple approximate equations (-15° model and Soulayman model) based on geographical latitude and solar declination are suggested for predicting daily optimum tilt angle in summertime.