Hourly diffuse and global solar irradiation: Further correlation

Abstract

Following the correlation equations between hourly diffuse and global irradiation established in an earlier study(1) the work has been extended by analysing data for a further six locations. Plots of hourly diffuse ratio (ratio of diffuse to global irradiation) against clearness index (ratio of global to extra terrestrial irradiation) were obtained in the first instance for eleven UK sites. The plots were further examined for seasonal trends by plotting separately data for winter, summer and autumn/spring and also obtaining regression curves. While the plots for summer and autumn/spring showed definite trends, the winter plots showed more scatter. Some factors which cause this scatter have been identified. Plots of the regressed curves for the three seasons showed the summer and winter curves lying at the top and bottom respectively with the autumn/spring curve intermediate. This behaviour has been traced to the higher and lower turbidities during the summer and winter months. An error analysis was carried out for the seasonal model developed for each location. This exercise confirmed the adequacy of the model. The seasonal curves for different locations when superimposed disproved a latitude trend. The curves for Lerwick and Stornoway departed from the rest. Furthermore, the two locations were found to have almost identical characteristics. Finally equations were obtained for the summer and the rest of the year period for the UK by pooling data from all mainland locations and carrying out regression analysis. Statistical analysis of these models confirmed their validity. The model established in the earlier study based on data from only five locations proved satisfactory for the prediction of the diffuse ratio for all additional mainland locations.