Solar radiation evaluation based on cloudiness data

Abstract

The solar radiation reaching the earth’s surface is essential for a wide range of human activities. However, observations of this important meteorological variable are scarce. Having this said, this paper proposes a methodology to estimate the solar radiation upon a given area from easily obtained data. Both radiation components were analyzed: direct and diffuse. To do so, monthly climatic cloudiness data and the digital elevation model (DEM) were used. The atmospheric transmissivity and the diffuse ratio were estimated from an adaptation of an empirical relation involving cloudiness. Based on the DEM, the solar radiation was evaluated taking into account the slope and orientation of the surface and the influence of surrounding topography. These estimates were compared with direct global radiation measurements along the year of 2009. Due to the use of monthly cloudiness data, no good correlation was obtained for daily radiation. However, daily radiation integrated in a monthly base showed better results, especially in the months in which the observed rainfall was similar to the climatic precipitation expected. Furthermore, analyzing 2009 as a whole year, the proposed methodology showed good results, overestimating the global radiations in 7.25%.