Solar resource classification in South Africa using a new index

This paper introduces a solar resource index that responds to site-specific sky conditions resulting from stochastic movement and evolution of clouds. The developed solar resource classification index called probability of persistence (POPD) had limited capabilities to distinguish persistent clear-sky conditions from persistent overcast-sky conditions. The metric proposed in this investigation, referred to as the solar utility index (SUI), seeks to extend the POPD index to a simple enough index that can singly discriminate different states of a solar resource. It gives a measure of the fractional time during which a solar resource exhibits predefined characteristics over a specific time period not exceeding the time interval between sunrise and sunset. These solar resource qualities, which are user-defined, measure: (1) the fluctuation characteristic of the solar resource magnitude, and (2) the solar resource diffuse and beam composition. Values of the indexes computed over daily time intervals of 7:00–17:00 apparent solar time were tested for their solar resource classification qualities. Five distinct classes using K-means clustering algorithm were identified for the solar radiation resource measured at eight stations in South Africa. The SUI was found to have superior solar resource discriminating and grouping abilities when compared with other indexes like POPD and fractal dimension.