Accuracy of empirical angular variation model for solar absorptance of architectural glazing

A new empirical way of predicting solar absorptance in windows has been proposed. Solar light is traditionally assumed to have near-normal incidence. This is an approximation since light reaches the window from all angles between 0° and 90°. The approximation leads to an incorrect estimate of the solar absorptance, since the absorptance actually varies with angle of incidence. In this study, international standards and Fresnel calculations have been used to calculate the error in the total solar absorptance with this near normal approximation, with a clear pane approximation, as well as with our new angular variation model. The new model implies knowing to which of nine groups the window belongs and using a set of polynomial fits to the angular absorptance, one set for each window group (one polynomial fit for each window pane). It is shown that the error with the new model is considerably smaller than with the other two approximations, especially at high angles of incidence.