Bestimmung der Diffusstrahlung aus Globalstrahlungsmessungen an vertikalen Flächen

Usually in solar energy calculations diffuse and beam radiation on a horizontal plane is needed to calculate the irradiation on any other inclined plane. For non-concentrating collectors or photovoltaics this is very often carried out with the isotropic sky model. However, these calculations are not correct as an important part of the measured diffuse radiation, the circumsolar part, comes from the direction of the sun. Therefore the diffuse part on surfaces not facing South is very often overestimated. This is especially true for North oriented surfaces. One solution of the problem would be the use of an anisotropic sky model. These models are more difficult to use and there are appreciable differences between models. Moreover for a user of a commercial program using the isotropic model it is nearly impossible to modify the source code. The second possibility is to use equivalent diffuse and beam radiation data as input for the isotropic model. Then the circumsolar radiation is treate d as beam radiation. As the circumsolar is dependent on the clearness of the sky, the calculation of equivalent data is not straight forward. Measurements from vertical pyranometers oriented towards the four cardinal points of the compass are used to calculate the equivalent data. The use of a database system for the measurement points facilitates the work.