Characterisation and mapping of daily sky conditions based on ground measurements of solar irradiance in mainland USA

The power produced from solar systems depends strongly on the prevailing weather conditions and more precisely on solar irradiance. Due to the dependence of such systems on the weather conditions the electricity injected into the grid is intermittent in nature with potentially negative impact on the grid operation. The quality and quantity of the electrical power produced from solar systems is directly related to the available solar irradiance and therefore any disturbances to the latter affect the produced power. In this paper, the characterisation and classification of the daily solar irradiance from 12 locations in mainland USA is presented based on 4 years of ground measurements. The K-POP method used evaluates the quantity and quality of solar irradiance for a day and as a consequence classifies the day based on these two indices. The statistical analysis of the results shows high correlation through the years for each station. This is a strong indication that the sky conditions are predictable and that with the utilisation of mechanisms, such as storage, spinning reserve, etc. solar systems can become a major contributor to the energy mix. Finally, this analysis provides better insight into the solar resource of a given location.