ABSTRACT
Global horizontal irradiance (GHI) is the most common measurement because it uses the simplest measurement equipment: a pyranometer on a horizontal surface. Measuring diffuse horizontal irradiance (DHI) irradiance with shading bands is not complex or very expensive, except that the data must be corrected for the obscuration of the sky by the shading band. The diffuse sky radiation on a horizontal surface DHI is a function of cloud cover, the scattering properties of the atmosphere, and the path length. The parameterizations discussed in this chapter, were based on estimating hourly average DHI and used the ratio of hourly DHI to GHI as the dependent variable. In an effort to improve the performance of DISC, Perez worked with the American Society of Heating Refrigerating, and Air Conditioning Engineers to develop versions of DISC, called DIRINT and DIRINDEX. The DIRINDEX model modifies the DIRINT model result with information from the Bird model or another clear sky model.
