Estimation of hourly direct normal from measured global solar irradiance in Spain

The availability of a good data set, registered in six Spanish locations, i ncluding several radiometric variables, has been used to test different app roaches for estimating hourly direct normal irradiance by decomposition mod els. Models proposed by different authors have been tested. Following this preliminary study, to improve the k(b)-k(t) correlations, another geometric variable has been used as a predictor of hourly beam transmittance, k(b), by means of piecewise correlations. The new beam transmittance correlations , which include additional geometric information, reduce the root mean squa re deviation. In addition, they show a better performance in terms of the d etermination coefficient of the regression analysis of measured vs calculat ed values, providing an improved capture of the real world effects than mod els that are function of the clearness index only. A new model that uses on ly two ranges of clearness index is proposed. The proposed model shows seas onal dependence and thus we have developed a seasonal version of it. Howeve r, the performance of the seasonal version has proved to be similar to the corresponding annual model. (C) 2000 Elsevier Science Ltd. All rights reser ved.