PERFORMANCE EVALUATION OF SEVERAL VERSIONS OF THE PEREZ TILTED DIFFUSE IRRADIANCE MODEL

From experimental data acquired in Valencia (Spain), the performance o f the Perez model is evaluated in its original and simplified circumso lar (alpha = 25-degrees) forms. First, the coefficients of both models obtained from irradiance measurements in Valencia are presented. To e valuate the model's performance, the deviations between estimates from the model and measured diffuse and global irradiances on four differe ntly oriented vertical planes are analyzed. The results show that the simplified circumsolar model more closely approximates measured values than the original Perez model and that the approximation improves whe n local coefficients are used.