Direct Normal Irradiance from Satellite VIS Images
The calculation of the direct normal irradiance is performed in different steps. In a first step cloud information is derived from the satellite measurements using an enhanced version of the Heliosat method . As a measure of cloudiness a dimensionless clear sky index k* is derived. Using a clear sky model the global irradiance is immediately related to the clear sky index, while our new beam fraction model allows for calculating the DNI. Fig. 1 represents the necessary process steps, which are described in this section.
Fig. 1: Significant process steps of the Heliosat method.