A monthly probability distribution function of daily global irradiation values appropriate for both tropical and temperate locations

In this paper it is shown that the generalized cumulative distribution curves of Liu and Jordan are not suitable for tropical locations. However, the Bendt et al. probability density function, originally proposed to predict the cumulative distribution curves of North American locations, can still be used for tropical locations provided the value of the maximum clearness index is suitably adjusted. But this probability density function is unrealistic at high irradiation values, which may result in an overestimation of calculated solar system performance. This shortcoming can be overcome by using a higher order probability density function proposed by Saunier. This function is a generalised single parameter function, namely the ratio of the monthly mean clearness index to the maximum clearness index, and seems to be suitable without ad hoc modification for any type of climate. Finally, the problem of estimating the value of the maximum clearness index has also been addressed and a simple model is proposed to evaluate it.