Model Development for Predicting Solar Radiation under Clear and Overcast Skies

Abstract

Models to enable estimation of monthly mean global solar radiation from air temperature, relative humidity and rainfall measurements were developed. The multivariable models were developed for days on which the probabilities of clear and overcast skies are maximum using a range of five years meteorological data (2002–2006) obtained from Nigerian Meteorological Agency (NIMET) for the city of Kaduna in Nigeria. Also, a one year data of (2010) for the city was used to test the performance of the models with some results giving equality, underestimation or overestimation compared to the measured values. The months of clear skies were identified to be October through March and that of overcast skies to be April through September. The highest and lowest atmospheric transmittance, k, values were 0.42 and 0.12. 2002 clear sky model gave 21.66 MJm^-2day^-1 as January 2010 predicted solar radiation which is an underestimation when compared to the measured value of 26.30 MJm^-2day^-1 while the overcast sky model gave 24.01 MJm^-2day^-1 for April 2010 which compared to the measured value of 22.30 MJm^-2day^-1 is an overestimation. In other instances, equality between the predicted and measured values were observed. The highest and lowest values of the coefficient of determination, R², obtained are 0.99 and 0.56. However, rain data were used in the development of overcast skies models only. The highest and lowest values of the MPE were 0.02 and -1.76. Other statistical indicator tests (RMSE and MBE) were carried out to further validate the models showed that the estimated values of the global solar radiation were in good agreement with the measured values. These models are therefore recommended for utilization by designers and engineers of solar energy systems in Kaduna and other localities having similar weather patterns.

Keywords: model, predicting, solar, radiation, clear, overcast, skies.