On the Estimation of Daily Beam Radiation on Tilted Surfaces

In practical applications, it is essential to calculate the daily beam radiation on a tilted surface. The widely used correlation for estimating daily beam radiation on a tilted surface is the one suggested by Liu and Jordan. However, the method of Liu and Jordan was developed based on the assumption of no atmosphere over the earth, and is a typical approximate method. Results obtained in this work show that the deviation of annual collectible beam radiation on a tilted surface calculated based on Liu and Jordan method from those obtained based on monthly horizontal radiation and radiation predicted from clear sky situation increases with the increase in tilt and azimuth angles, Liu and Jordan method is suitable to estimate daily or annual beam radiation on a tilted surface with an azimuth angle less than 15o, otherwise, other methods should be used.