Solar radiation model for Europe

This study is concerned with the evaluation of the isotropic and four anisotropic solar radiation models for inclined surfaces. The evaluation procedure was split in two stages— abbreviated and detailed analysis. Physical reasoning was used in the abbreviated analysis to demonstrate the shortcomings of the older models, while detailed evaluation was carried out for the newer models, using at least one-year's measured hourly data from each of five European sites. Plots displaying the hourly estimated radiation against measured values have been prepared along with computation of the mean bias error and the root mean square error for each slope/ azimuth. The isotropic model has been found to perform very poorly and in view of its impact on the thermal energy and daylight related calculations it is not recommended for further use. In this work further refinements have been suggested for the treatment of the background sky-diffuse radiance. The newer models including the presently proposed one have shown good agreement with the measurements.