Correlation between hourly diffuse and global solar irradiation for the UK

Abstract

Computation of hourly solar irradiation incident upon a surface of any orientation and tilt is the prerequisite for detailed solar energy simulation. Since the relevant meteorological data available are usually the horizontal global irradiations, a method is needed to obtain the diffuse component. Irradiation on a tilted surface is then obtained by solar geometry. Hourly correlations between the diffuse ratio (ratio of diffuse to global irradiation) and the clearness index (ratio of global to extraterrestrial irradiation) have been developed by investigators for various regions of the world; here we present correlations for the UK sites. The correlations are based on three years' data for five sites in the UK covering the whole latitude range. Computer plots for each location showed a definite correlation between the hourly diffuse ratio and the clearness index. The data points were then used to obtain correlation equations for each of the five sites. A cubic fit was found to be the best in all cases. Comparison of these correlations shows a latitude effect with the Lerwick (latitude 60° 8′) and Easthampstead (latitude 51° 23′) curves lying at the top and bottom respectively. However, the closeness of the curves suggests the possibility of a single correlation for the UK; the data for all the sites were pooled to obtain such a correlation, and a cubic fit was again found to be the optimum. The effects of fractional possible sunshine and solar altitude on the correlation were investigated. While the former was found to be insignificant in reducing the scatter, the latter showed some bearing on the correlation, but for only two of the five sites. An error histogram for the UK correlation showed that the error distribution is symmetrical and that many estimates fall within small error bands.

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