Investigation of the relationship between solar radiation and cloud cover using a total sky imager.

Abstract

Solar radiometric studies have been done in South Africa, but no research studies have been performed to understand how the cloud fraction (CF) affects the amount of solar radiation received by radiometers. This work focused on studying the relationship between radiometry readings and cloud fraction at the University of Kwa-Zulu Natal (UKZN) at Howard College in Durban. This study used a pyrheliometer to measure beam irradiance (DNI), a shaded pyranometer to measure diffuse irradiance (DHI), an unshaded pyranometer to measure global irradiance (GHI) and a Total Sky Imager to give CF percentage. When we studied 1-minute data of radiometric profiles versus CF with respect to time, we found that an increase in CF resulted in a decrease in DNI except for the rare moments when the clouds did not obstruct the sun. An increase in CF resulted in an increase in DHI only if the sky was not overcast. For an overcast sky, there is no simple relationship between CF and DHI. When we studied centred moving averages of DNI against clearness fraction (1 – CF) as a function of time for different time-averaging scales, it was found that DNI had strong linear correlation with 1 – CF. When we studied centred moving averages of DHI and CF as a function of time for different averaging scales, it was found that DHI increased as CF increased but only if the sky was not overcast. Scatter plots of different clearness indices against CF for different averaging scales were studied and it was found that linear correlation increased with averaging scale, and beam-related clearness indices had a strong linear relationship with CF at a time-averaging scale of 4 hours.