Monthly Reference Frequency Distributions of Daily Clearness Index Data from Burundian Stations

Abstract

A 6-years period (1994-1999) data of the daily global solar irradiation (G) from six Burundian stations have been statistically analyzed in this paper. The observed monthly frequency distributions of the daily clearness index (kt) exhibit the following features: (i) most of them are asymmetrical and have only one maximum; (ii) frequency distributions with higher monthly averages ¯(k_t )  and lower standard deviations are noticed in the dry season, i.e. from June to August or September; (iii) at the opposite, broader fluctuations and lower ¯(k_t ) values are observed in the remaining period of the year (rainy seasons); (iv) the yearly averages ¯(k_t ) range from 0.41 to 0.59. This indicates that in average the sky over Burundi is a fairly sunny one. Moreover, a statistical test (the F-test) has been used to check for each station if the monthly samples with the same ¯(k_t ) had also the same variance. The null hypothesis has been accepted for all the ¯(k_t ) intervals of interest (excepted one) for the six stations. Those monthly samples must belong to the same mother population, and have therefore the same statistical distribution, referred to as a monthly reference frequency distribution. Such reference distributions have been built for the six stations and the relevant ¯(k_t ) intervals of interest. They would be useful for designing solar energy systems at the Burundian stations of this study and for predicting the long-term average performance of those systems.