ESTIMATION OF HOURLY SOLAR-RADIATION FOR INDIA

The ASHRAE constants predict high values of the hourly beam radiation and very low values of the hourly diffuse radiation when used to predi ct radiation at Indian locations. Hence a procedure has been developed for the estimation of direct, diffuse and global hourly solar radiati on on a horizontal surface for any location in India. To calculate hou rly solar radiation, an exponential curve, similar to the one used by ASHRAE, was fitted to the measured solar radiation data of six cities from different regions of India. The statistical analysis was carried out for the data computed using ASHRAE constants and the set of consta nts obtained for India using the measured data of four different India n cities selected randomly. Three statistical indicators were used to compare the accuracy of the developed procedure. The results show that ASHRAE constants are not suitable to estimate hourly solar radiation in India. Hourly solar radiation estimated by constants obtained for I ndia are fairly comparable with measured data. The mean percentage err or with Indian constants for these four Indian cities was found as low as 2.27, -6.29 and -6.09% for hourly beam, diffuse and global radiati on, respectively. (C) 1997 Elsevier Science Ltd.