New equations for the calculation of the horizon brightness irradiance in the model of Perez

In this technical note authors propose new expressions to calculate the diffuse irradiance coming from the horizon brightness band on a tilted plane. As in the model of Perez, it is assumed that the radiance from this band is a constant. New expressions are more exact than previous ones as they eliminate some of the simplifications considered in the model of Perez. For small slope angles new expressions give irradiance values up to 32.98% higher than with the cited model. Notwithstanding, when slope angle increases obtained differences sharply decrease at the beginning and this decrease becomes practically linear from about 40 on. 2005 Elsevier Ltd. All rights reserved