A new kind of cloudy sky model to compute instantaneous values of diffuse and global solar irradiance

A model that uses two parameters to describe the state of the sky is presented. The parameters are the total cloud amount and a new two-value parameter - the sunshine number - stating whether the sun is covered or uncovered by clouds. Regression formulae to compute instantaneous cloudy sky global and diffuse irradiance on a horizontal surface are proposed. Fitting these relationships to Romanian data shows low bias errors for global radiation but larger errors for diffuse radiation. The model's accuracy is significantly higher than one based on total cloud amount alone. The model is used to generate time-series of solar radiation data. A first approximate relationship, neglecting auto-correlation of the sunshine number, is used in the computations.