Solar diffuse irradiance: Estimation using air mass and precipitable water data

The influence of meteorological variables on the hourly fraction of diffuse solar irradiance was studied. The goal was to determine whether including variables other than the clearness index (ratio of hourly global to hourly extraterrestrial irradiance) will further improve the results of existing correlations. Two new models were developed in this regard. In the PAC model (for precipitation, air mass and clearness index) the diffuse fraction was predicted as a function of three independent variables: the precipitable water, air mass and clearness index. No further improvement was observed. An entirely new approach is proposed on the basis of a relationship between diffuse irradiance and the product of air mass and precipitable water. The new correlation shows significant improvement under all sky conditions and reduces the mean bias error by an average of 37% under overcast conditions, 28% under intermediate conditions and about 33% for clear sky conditions. The proposed correlation has been fitted against data for several locations in the United Kingdom and outperforms the other models on all statistical criteria.