STATISTICAL PROPERTIES OF HOURLY GLOBAL RADIATION

The statistics of hourly global radiation are analysed for various climates and locations, at time scales ranging from one month to one hour. The probabilistic and sequential properties of the hourly clearness index k(t) are related with a variety of parameters, including the daily clearness index K(t) and the solar altitude angle h, thereby displaying the nonstationary and time-inhomogeneous nature of hourly radiation. Analytical expressions for the probability distribution of k(t) are proposed (truncated Gaussian functions depending on K(t) and h). The results reported in this paper provide a deeper understanding of hourly solar radiation statistics, and explain some of the features displayed by radiation data at longer time scales. From the practical point of view they have implications, for example, for the construction of models for generating synthetic radiation data, an important and often indispensable tool for designing and simulating solar systems.