Estimation of global illuminance on inclined surfaces for clear skies

In the present work, we have studied the luminous efficacy of global solar radiation incident on vertical surfaces for clear skies (cloudless and rather clean skies) and mean hourly values of global solar radiation (i.e. averaged over each hour). Luminous efficacy models similar to those previously obtained for a horizontal surface have been developed using solar elevation as the independent variable. Thus, the hypothesis that the slope of the surface does not influence the general formulation of the model has been assumed. However, the final formulation of the models clearly depends on the insolation conditions of the inclined surface and, more specifically, on whether direct radiation is or is not incident on it. As a consequence, two different types of models have been proposed to take both insolation conditions into account. On the other hand, global luminous efficacy models have also been obtained as the best fits of the experimental data. Global illuminance has been estimated for the vertical surfaces considered, facing north, east, south and west, with the models obtained as the best fits of the experimental data and the other two types of models mentioned in the preceding paragraph. Comparison of the performance of models for global illuminance estimation has validated the initial hypothesis that the slope of the surface does not influence the general formulation of the model. The performances of the known Perez model and the proposed models are also compared