Measuring and modeling solar irradiance on vertical surfaces

Abstract

This report presents the first evaluations of 5 algorithms for converting global horizontal and direct normal solar irradiance components to global irradiance on tilted surfaces using measurements from the Solar Radiation Research Laboratory (SRRL) at SERI. High resolution (15-min) measurements from 8 thermopile radiometers oriented vertically in the 4 cardinal directions were available for a 169-day period ending 31 December 1984. Concurrent measurements of global horizontal and direct normal solar irradiance components were also used. The pyranometers used were intercompared on horizontal, north-facing, and south-facing vertical orientations. The conversion algorithms each attempt to account for the very complex atmospheric interactions with incoming solar radiation. The sky and ground radiance contributions to tilted surface are assumed to be isotropic, anisotropic, or a combination of the two depending on the algorithm. Results from the analysis of measurements on vertical surfaces for 22 July through 4 September indicate a general overestimate (18% to 46.5%) of solar irradiance on north-facing surfaces by all 5 of the algorithms. The best agreement with measured data was demonstrated for south-facing surfaces where all but one algorithm were within the present measurement by as much as 23% of the observed 15-min data.