On the clear sky model of the ESRA - European Solar Radiation Atlas - Withrespect to the Heliosat method

This paper presents a clear-sky model, which has been developed in the fram ework of the new digital European Solar Radiation Atlas (ESRA). This ESRA m odel is described and analysed with the main objective of being used to est imate solar radiation at ground level from satellite images with the Helios at method. Therefore it is compared to clear-sky models that have already b een used in the Heliosat method. The diffuse clear-sky irradiation estimate d by this ESRA model and by other models has been also checked against grou nd measurements, for different ranges of the Linke turbidity factor and sol ar elevation. The results show that the ESRA model is the best one with res pect to robustness and accuracy. The r.m.s. error in the estimation of the hourly diffuse irradiation ranges from 11 Wh m(-2) to 35 Wh m(-2) for diffu se irradiation up to 250 Wh m(-2). The good results obtained with such a mo del are due to the fact that it takes into account the Linke turbidity fact or and the elevation of the site, two factors that influence the incoming s olar radiation. In return, it implies the knowledge of these factors at eac h pixel of the satellite image for the application of the Heliosat method. (C) 1999 Elsevier Science Ltd. All rights reserved.