Techniques for obtaining the monthly mean hourly diffuse radiation from daily values

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 noon, errors involved in the estimation increase rapidly as one approaches high angles away from solar noon. A technique is suggested to overcome this discrepancy. The proposed method, which employs the monthly mean daylength So as the only input parameter, may be used to evaluate the rd ratios accurately for locations in Spain when measured data are not available. A set of second-order polynomial equations relating rd and So, obtained from the analysis of longterm, measured hourly diffuse radiation data for several stations in Spain, is recommended for estimation purposes.