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

The availability of a good data set, registered in six Spanish locations, including several radiometric variables, has been used to test different approaches for estimating hourly direct normal irradiance by decomposition models. Models proposed by different authors have been tested. Following this preliminary study, to improve the kb–kt correlations, another geometric variable has been used as a predictor of hourly beam transmittance, kb, by means of piecewise correlations. The new beam transmittance correlations, which include additional geometric information, reduce the root mean square deviation. In addition, they show a better performance in terms of the determination coefficient of the regression analysis of measured vs calculated values, providing an improved capture of the real world effects than models that are function of the clearness index only. A new model that uses only two ranges of clearness index is proposed. The proposed model shows seasonal dependence and thus we have developed a seasonal version of it. However, the performance of the seasonal version has proved to be similar to the corresponding annual model.