Interrelations Between Hourly and Daily Global and Diffuse Illuminance

In the present article, interrelations between hourly and daily global and diffuse solar illuminance have been developed. By examining a large database, 56 primary locations, of TMY2 [1], it has been found that the values of ratio of hourly global or diffuse illuminance to corresponding daily values, (r_vt = I_v/H_v ; r_vd = 1_vd/H_vd), are very close to the corresponding values for global and diffuse solar radiation, r_t = I/H; r_d = I_d/H_d). Thus, it has been proposed that the existing correlations for r_t and r_d be employed to calculate r_vt and r_vd. The performance of the existing correlations for r_t and r_d to predict r_vt and r_vd has been evaluated employing TMY2 data and data of 6 IDMP (International Daylight Measurement Programme) [2] locations. It has been found that the correlation for r_t due to Collares-Pereira and Rabl [3] predicts r_vt satisfactorily. Correlation due to Satyamurty and Lahiri [4] for r_d predicts r_vd satisfactorily after suitable modification.