A simple model for the calculation of global solar radiation using geostationary satellite data

A model for the calculation of global solar radiation incident on the Earths surface is presented. It is a simple model which allows operative calculations to be carried out merely by using visible METEOSAT images. As a result, hourly and daily radiation values are obtained over wide areas and with a spatial resolution of around 50 × 50 km, suitable for the values to be integrated into vegetation studies. It is a statistical model based on the Tarpley method. Hourly global solar irradiance is related to the solar zenithal angle, the digital level of the visible images and the cloudiness, which is obtained by applying a threshold technique to the visible histogram. Different regression equations based on cloudiness conditions are established and the coefficients are obtained using radiation values acquired by the Spanish Meteorological Service network. The equations can be updated in a simple manner and the model provides results with errors similar to those found in the bibliography: 19.5% for hourly values and 9.2% for daily values, with the advantage that it is easy to apply and does not require the use of meteorological data.