A Technique for Determining Solar Irradiance Deficits

A novel analytic technique, involving the use of computer-assisted data-reduction techniques, has been developed to determine the variation of solar irradiance from long-term averages. The objective of the study was to develop a tool that would enable solar system designers to assess and improve overall system reliability by determining the amount of backup and/or storage capability required to supplement a baseline system sized according to long-term averages. The technique allows the determination of variations for intervals of time up to 60 days. This paper presents results of the analysis of 13 sites in the continental USA. Cost-effective accommodation of solar irradiance deficits includes both increasing the collector area by about 15% to cover long-term year-to-year variations from the average, and adding energy storage or backup to cover short-term deficits such as those caused by local storms. Site-to-site dependency exists, causing a range of required short-term storage capacities of 1 to 7 no-sun days to accommodate the weather-caused deficits for this broad variety of sites.