An equation for separating daily solar irradiation into direct and diffuse components

An equation suitable for separating daily total solar irradiation into direct and diffuse components is presented. It incorporates an easily-determined physically based coefficient which reflects the maximum clear-sky transmissivity at the study site. Data collected at Pullman, Washington State, U.S.A. were used to establish the separation equation, and it has been tested on an independent data set from Townsville, Queensland, Australia. Results of these tests indicate that this separation equation can account for more than 90% of the variation in diffuse radiation incident on a horizontal surface. Application of the separation equation for estimating daily total solar radiation incident on non-horizontal surfaces is discussed.