Representative Terrestrial Solar Brightness Profiles

Abstract

Solar thermal energy systems often use optical imaging concentrators. The image size and shape produced in the focal plane of the concentrator system depends on the solar brightness distribution. Therefore, the forward scattering of solar radiation by the Earth’s atmosphere modifies the solar brightness distribution and creates a circumsolar aureole. The circumsolar ratio, the energy contained in the solar aureole compared to total energy, can impact the performance of these concentrating systems. Based on about 2300 sunshape measurements from sites in France, Germany, and Spain made with a camera system developed by the German Aerospace Center (DLR), average solar brightness profiles with a circumsolar ratio of about 0%, 5%, 10%, 20%, 30%, and 40% were generated. These profiles are compared to the measurements taken by the Lawrence Berkeley Laboratory (LBL) in the late 1970s and a commonly used limb-darkened solar brightness profile, as known from astronomy. A statistical analysis gives information on the frequency of occurrence of each of the average profiles. The profiles combined with the statistical weight should offer a numerical database for calculating the influence of variable conditions of the sunlight scattering on solar concentrating systems. Furthermore, a single average profile was calculated from the DLR data.