Predicting solar irradiance on inclined surfaces using sky radiance data

Abstract

The availability of more comprehensive solar irradiance data is invaluable for the reduction of cooling load in buildings and for the evaluation of the performance of photovoltaic plants. In many parts of the world, however, the basic solar irradiance data are not always readily available. This paper presents an approach to calculate the solar irradiance on sloped planes by integrating the sky radiance distribution. Sky radiance data recorded from January 1999 to December 2001 in Hong Kong were used to estimate the solar irradiance for the horizontal and four principal vertical surfaces (N, E, S and W). The performance of this approach was assessed against data measured in the same period. Statistical results showed that using sky radiance distributions to predict solar irradiance can give reasonably good agreement with measured data for both horizontal and vertical planes. The prediction approach was also employed to compute the solar irradiance of a 22.3° (latitude angle of Hong Kong) inclined south oriented surface. The findings indicated that the method can provide an accurate alternative to determine the amount of solar irradiance on inclined surfaces facing various orientations when sky radiance data are available.