A modeling study of the total irradiation reaching a tilted surface facing towards the Equator at a place with the same latitude (-22.85º), using the Daily partitioning energy, is presented. In this study three total irradiation incident on tilted surface estimate models (LIU; JORDAN, 1963; HAY, 1979; PEREZ et al., 1987) were modified in two different manners, to improve irradiation estimative model for tilted surface knowing only the global irradiation. The first modification consisted to apply empirical functions to determine the diffuse irradiation by the global radiation measurements. These empirical functions were tuned using a linear regression technique, the fraction of the diffuse irradiation (Kd) from the global irradiation, was correlated with the clearness index (Kd). In this case the highest correlation found was a fourth order polynomial function. In the second modification the fraction of the direct irradiation K BH from the global irradiation was correlated assuming a linear correlation at given intervals of Kt. Model results showed that the empirical relationship K BH=f(Kt ) presented better results and the Hay model presented a better performance compared with the two others models for the Botucatu region.
Solar irradiation; Tilted surface; models
