Effect of rainfall on the estimation of monthly mean hourly solar radiation for India

The constants in the empirical equations to predict hourly solar radiation on a horizontal surface recommended by ASHRAE were modified by the authors [1] for Indian locations. In further studies, India is divided into four regions of rainfall, namely, region of heavy, medium, low and very low rainfall. Using ASHRAE equations with the modified constants, monthly-mean-hourly solar radiation values are estimated for ten cities from different regions of India. From the comparative data analysis of the measured and estimated solar radiation of these cities, empirical correction factors for the four regions of rainfall were obtained. The statistical analysis is carried out for the computed data with and without considering correction factor for rainfall and the measured data for four randomly selected Indian cities. Two statistical indicators, namely, mean percentage error and normalized root mean square error, are used to compare the accuracy of the developed procedure. The results show that the yearly average normalized root mean square error with Indian constants, considering effect of rainfall for these four Indian cities, is found to reduce to 14.86, 12.15 and 7.61% for monthly-mean-hourly beam, diffuse and global radiation respectively on horizontal surface, as against the corresponding values of 23.29, 22.2 and 16% without considering the correction for rainfall. (C) 1998 Elsevier Science Ltd. All rights reserved.