Validation of models for global irradiance on inclined planes

The accuracy of models to estimate irradiance on inclined planes is tested by comparing the predictions to measurements taken with four instruments of various tilt and azimuth angles in Sede Boqer, Israel. The three models investigated are: the Perez model, Hay's anisotropic model, and the isotropic model. The Perez model is found to perform significantly better than the other two, with residual errors that are comparable to the accuracy of the measuring instruments themselves. The same data are also used to evaluate the precision of empirical correlations to estimate the direct component from global horizontal radiation, and to assess the sensitivity of the predicted irradiance on tilted surfaces to the errors associated with these correlations.