Estimating diffuse solar radiation in India: performance characterization of generalized single-input empirical models

Solar radiation data obtained from Indian Meteorological Department, Pune over the span of 1986-2000 were used to establish generalized models for estimation of monthly mean diffuse solar radiation over India. Four categories of single input models were established by rationalizing the diffuse component which correlated diffuse fraction and diffuse coefficient with relative sunshine period and clearness index. 52 models (13 models under each defined category) were selected from the literature. 28 new generalized model with 7 in each category were also developed from the data. The accuracy of all the 80 models was assessed using error analysis. Global Performance Indicator was calculated and models were ranked accordingly with the highest value representing best performance. GPI variation under first category ranged from -6.1035 to 0.5416, where top five models were T4, T3, T2, T1 and M13. In second category, GPI varied from -7.8812 to 0.0839 with top five models as T11, T10, T9, T13 and T12. Third category exhibited variation of GPI from -6.8551 to 0.4354 and top five model were T18, T17, T16, T15 and T19. The last category, involved GPI variation from -6.1323 to 0.1221 with T24, T23, T22, T25 and T26 as top five models.