Measurement and modeling of solar irradiance components on horizontal and tilted planes

In this work new measurements of global and diffuse solar irradiance on the horizontal plane and global irradiance on planes tilted at 20° and 30° oriented due South and at 45° and 65° oriented due East are used to discuss the modeling of solar radiation. Irradiance data are collected in Padova (45.4°N, 11.9°E, 12 m above sea level), Italy. Some diffuse fraction correlations have been selected to model the hourly diffuse radiation on the horizontal plane. The comparison with the present experimental data shows that their prediction accuracy strongly depends on the sky characteristics. The hourly irradiance measurements taken on the tilted planes are compared with the estimations given by one isotropic and three anisotropic transposition models. The use of an anisotropic model, based on a physical description of the diffuse radiation, provides a much better accuracy, especially when measurements of the diffuse irradiance on the horizontal plane are not available and thus transposition models have to be applied in combination with a diffuse fraction correlation. This is particularly significant for the planes oriented away from South.