Development and comparison of HP-41C software to predict solar irradiation of tilted surfaces, based upon cloud cover factors

Summarized is a comparison between predicted and measured solar radiation and cloud cover data from NOAA weather stations and Solar Energy Meterological Research and Training Sites at various locations throughout the US, based upon a semiempirical method which was used recently to generate synthetic insolation data for correlation with measured solar performance under the SUEDE program. This method is unique, since it predicts irradiation of a tilted surface from sky cover estimates by weather observers, using a modified ASHRAE method to compute direct and diffuse insolation on a clear day. Data comparisons were made using an HP-41C programmable calculator, card reader, printer, and eight magnetic cards. Although a more detailed study with a larger data base is desirable, these particular findings, using hourly summations to obtain monthly averages in 1980, indicate that approximately -5 to 15% difference between measured and calculated monthly results is typical of continental US sites. A sensitivity study indicated that monthly percentage differences are reduced by centering ASHRAE constants and earth declination on the 15th day of each month instead of the 21st day.