Quasi-Physical Model for Converting Hourly Global Horizontal to Direct Normal Insolation

Numerous research studies have shown that the use of a single regression function does not adequately characterize the relationship between direct beam transmittance (Kn) and the effective global horizontal transmittance (Kt). Therefore, the Direct Insolation Simulation Code (DISC) employs an exponential relationship between ..delta..Kn and air mass which is parametric in Kt. This algorithm seems to provide a satisfactory Kn-Kt relationship for a wide range of stations and seasons. Validation of the DISC Model was accomplished with data from three widely separated stations (albuquerque, Bismarck, and Brownsville) which were not used for model development. Comparisons with the ETMY model (used to generate direct normal data for 222 of the 248 stations in the national data base) showed substantial improvements in the accuracy of hourly values, significant reductions in monthly RMS errors, and equivalent monthly mean bias errors. Furthermore, modification of the DISC model to incorporate cloud-cover, water vapor, and albedo terms appears to be straightforward.