Prediction of hourly diffuse solar radiation from measured hourly global radiation on a horizontal surface

Abstract

A statistical procedure has been employed to develop correlations between the hourly global horizontal radiation and its diffuse component. Several years', hourly radiation data from three Canadian stations and two French stations have been employed for this purpose. The relationships have been developed in dimensionless form which predict I d I o for particular solar altitudes when I I o is given. Under heavily cloudy conditions or when the sky is completely covered ( I I o < 0.35 ), diffuse radiation increases linearly with the global radiation. In this region, solar altitude has no bearing on the fraction of diffuse radiation. As I I o goes beyond 0.35, the effect of solar altitude begins to appear and the region immediately following this may be considered as partly-cloudy-skies conditions. In the beginning of this region, the diffuse component increases briefly with the increase in global radiation and then begins to decrease as the partly cloudy skies become clearer. At particular solar altitudes, a minimum value of the diffuse radiation is reached. The value of I I o where I d I o reaches its minimum value varies with solar altitude. The region beyond which a minimum value of I d I o is reached may be considered as mainly-clear-sky conditions. In this region, I d I o increases again with I I o , lower solar altitudes giving a higher percentage of diffuse radiation. Under partly cloudy skies and under clear skies, solar altitudes lower than 30° had a marked effect on the fraction of diffuse radiation. However, solar altitudes greater than 30° had minimal influence on the fraction of diffuse radiation.