Short term forecasting of solar radiation based on satellite data

Forecasting of solar irradiance will become a major issue in the future integration of solar energy resources into existing energy supply structures. Fluctuations of solar irradiance have a significant influence on electric power generation by solar energy systems. An efficient use of solar energy conversion processes has to account for this behaviour with respective operating strategies. Examples are the management of electricity grids with high penetration rates from solar sources and the thermal control of buildings. This paper focusses on forecasting the surface solar irradiance in a short term time range of 30 minutes to 6 hours. As far as short term horizons are concerned, satellite data are a high quality source for irradiance information because of excellent temporal and spatial resolution. Due to the strong impact of cloudiness on surface irradiance, the description of the temporal development of the cloud situation is essential for irradiance forecasting. As a measure of cloudiness, cloud index images according to the Heliosat method, a semi-empirical methode to derive radiation from satellite data, are calculated from the satellite images. To predict the future cloud index image in a first step motion vector fields are derived from two consecutive images. The future image then is determined by applying the calculated motion vector field to the actual image. Finally, solar surface irradiance is derived from the predicted cloud index images with the Heliosat method. Figure 1 gives an overview of these steps to derive the irradiance forecast. For an effective application of forecast information, it is important to estimate the uncertainty of the forecast accuratly. The accuracy of the forecasted irradiance depends on the meteorological situation. Thus, it is possible to distinguish different levels of accuracy. It will be shown that there are good results of the forecast for situations with high irradiance, where as it is more difficult to forecast situations with variable cloud cover or low sun elevations. The assessment of the forecast quality was performed for single metoerological stations and for a regional forecast. The first section of this paper gives a brief introduction to the calculation of cloud and radiation maps from satellite data. In the next section the algorithm to forecast cloud index images is described. Finally in the last section a detailed error analysis of the forecast is performed.