A new model for obtaining horizontal instantaneous global and diffuse radiation from the daily values

Abstract A new method for obtaining horizontal monthly average instantaneous global and diffuse radiation from the respective daily values has been developed. The method is based on the two-parameter β distribution. By allowing unequal values of the two parameters, the method can consider morning-afternoon asymmetries of the instantaneous radiation. Otherwise, one parameter is enough to give the instantaneous radiation values. Simple linear expressions for the parameters in terms of the day length have also been obtained for both the global and the diffuse radiation. Using the hourly irradiation data of three French and one Canadian location, the performance of the method has been tested. The method is found to be highly accurate for both the diffuse and the global radiation.