This paper develops a semi-empirical model to calculate direct solar shortwave irradiance on a horizontal surface for any cloudiness condition. The model is based on radiative transfer simulations combined with empirical relationships; it makes use of the global solar irradiance, experimentally...

Using experimental data collected at Uccle (Belgium) in 1980, new relations are proposed which allow the spectral density distribution of diffuse solar radiation on a horizontal surface to be estimated for clear skies, as a function of the commonly measured broadband global and diffuse solar...

Several years of measured data for 17 European locations have been used to develop models for estimating monthly mean daily values of diffuse radiation (Hd) from combinations of the following: clearness index, sunshine fraction, and solar declination. Two models giving the highest correlation...

The classical equation of Liu and Jordan for estimating the monthly mean hourly to daily diffuse ratio (rd) was tested for applicability to locations in Spain. While the model predicts the rd ratios and the monthly mean, hourly diffuse (Id) values accurately at low hour angles close to solar...

In the present work we have modelled mean hourly values of the luminous efficacy of direct solar radiation for clear skies in Madrid using solar elevation as the only independent variable. Models of the polynomial type, similar to those given by different authors, and a model for the direct...