ABSTRACT
The models discussed in this chapter, provide examples of the differences that arise from different approaches to clear sky solar radiation modeling. Earth’s atmosphere is a continuously variable filter operating on the quasicollimated extraterrestrial direct beam, or direct normal, solar radiation, the extraterrestrial radiation (ETR), or direct normal irradiance. The sunlight reaching the top of the atmosphere or ETR is the starting point for models based on the physics of the interaction of matter and radiation, called physics-based models. Empirical models are based on correlations, or relations derived, usually through linear or multilinear regression analysis. This approach assumes that measured solar radiation data can be described as a function of some other independently measured or available variables or parameters. The clear sky total hemispherical irradiance on a horizontal surface global horizontal irradiance is just the combination of the computed direct beam and the computed sky diffuse irradiances.
