Estimation of the error caused by the circumsolar radiation when measuring global radiation as a sum of direct and diffuse radiation

Abstract

The directional (cosine) error of pyranometers must be eliminated if precise global radiation data are required. One possible way to do so is to measure the global radiation as a sum of the diffuse radiation and of the vertical component of the direct radiation. Due to geometrical reasons, the direct radiation excluded by the diffusometers is not equal to the vertical component of the direct radiation measured by pyrheliometers. The difference comes from the different amount of circumsolar radiation measured with the actual direct beam. With the changing solar elevation, the viewing geometry of the diffusometer composed of a shading-disk and a pyranometer also changes. In this paper the penumbra function of diffusometers is approximated by taking into account the exact slope and limit angle values. Instead of the actual circumsolar sky radiation the statistically expected one has been considered. Using these approximations the circumsolar difference values have been calculated for a cavity type absolute pyrheliometer and a Normal Incidence Pyrheliometer (NIP) as well as for the NIP and two geometrically different shading disk diffusometers. The difference is in the range of up to 5 W m −2 and should be taken into account in both meteorological and solar energy investigations.