Modelling irradiance on inclined planes with an anisotropic model

The circumsolar (alpha = 25 degrees) and point-source versions of the simplified Perez model were developed to estimate irradiances on inclined planes from values for horizontal planes through the determination of sets of empirical coefficients. This model has been evaluated by us at Madrid. Mean hourly values of irradiances on vertical surfaces facing north, east, south and west were used. A modification in the structure of the moder was made by analyzing separately the data obtained when the surface "sees" or does not "see" the circumsolar disk (for the point-source version, the centre of the disk). The validity of the different versions is assessed statistically using experimental and estimated values for the irradiance. For both the point-source and circumsolar versions, with data for all orientations, the model performs better with local coefficients than with those given by Perez. The accuracy was increased when local coefficients were calculated independently for each orientation. When the surface does not "see" the circumsolar disk (the centre of the disk in the case of the point-source version), the specified data separation produces an equation that is simpler and easier to use than the corresponding equation in the original model while yielding some increase in accuracy. (C) 1998 Elsevier Science Ltd. All rights reserved.