IMPROVEMENT OF THE PREDICTIONS OF A TEMPERATURE-BASED SOLAR RADIATION MODEL USING EMPIRICAL ADJUSTMENT FACTORS AT A TROPICAL STATION ILORIN, NIGERIA
Adjustment factors for the predictions of the temperature-based Hargreaves solar radiation model were derived using historical (1996 to 2001) daily weather data for Ilorin, Nigeria. The adjustment factors (ρ) were estimated as ratiosof observed to predicted global solar radiation. The average daily ρwere determined as a Fourier series function of Julian day by fitting the historical average dailyρ to the function. The year was also delineated into three seasons namely, Cool dry Harmattan, Hot dry with Early Rains and Cool Rainy seasons and the average ρ for each season determined. The annual average ρ was also obtained. The periods of the seasons were considered to be November through December to February for the first, March to May for the second and June to October for the third. Their determined ρ-values were 0.869, 0.987 and 0.856 for the Cool dry Harmattan, Hot dry with Early Rains and Cool Rainy respectively. The annual average value was 0.894. The adjustment factors were tested by applying them to the predicted daily solar radiation for the period 2002 to 2005. The daily observed solar radiation for the period were compared with both the adjusted and unadjusted Hargreaves model predictions. The mean and root mean square of the prediction errors were found to be substantially reduced by the application of the adjustment factors. The seasonal factors were therefore recommended for locations with weather conditions similar to Ilorin but with different timings for the seasons.