The generation of hourly diffuse irradiation: A model from the analysis of the fluctuation of global irradiance series

An analysis of models for the estimation of hourly diffuse irradiation based on the interrelations between the hourly diffuse fraction kd and the hourly clearness index kt, has concluded that kt is not a sufficient variable for parametrizing the effect of clouds on diffuse irradiation. A detailed study of the dispersion recorded by this diffuse component for a specific clearness index under partly cloudy sky conditions has led to analyzing how the variability in the instantaneous clearness index influences this dispersion. The data sets correspond to 10 years of hourly and instantaneous value records of global and diffuse radiation collected in Córdoba, Spain.In addition to the inclusion of the sine of solar elevation as a variable into the kd–kt correlations, this model propose the inclusion of others parameters related to the variability in the normalized clearness index within an hour and with the fluctuations presented by the time series of the instantaneous values of that index.Also presented is the implementation of an algorithm permitting both the determination of the hourly diffuse irradiation and the discrimination between the different sky conditions in those situations known by the designation partly cloudy sky.