A COMPARATIVE-STUDY OF SOME SIMPLE-MODELS FOR GLOBAL SOLAR IRRADIATION IN IBADAN, NIGERIA

After an analysis of ten different empirical relations utilizing common solar radiation parameters, it was found that the quadratic form utilizing relative sunshine gave the best overall estimate of the total solar radiation in Ibadan, Nigeria. It is shown that monthly averages of relative sunshine SIS, can be used to estimate the monthly average insolation H/H0 as H/H0 = 0.281 + 0.490(S/S0) - 0.188(S/S0)2 which fits the available ten-year (1979-88) insolation data with a coefficient of multiple correlation r1,23 = 0.76. This equation, when applied to data outside the ten-year data set used to produce it, generally yields overall average total errors of the order of 7% or less. Besides the above overall single correlation, individual monthly coefficients for the quadratic form are also provided with their associated correlation coefficients which are generally higher than 0.95. Comparison of the estimations of the above equation with Ibadan measurements inside and outside the ten-year data set indicate relative errors generally of the order of 5% or less. Furthermore, comparison of the estimations of the above equation with measurements from a different locality in Southern Nigeria indicates that less than 10% relative error is possible for most months of the year, thus suggesting wider applicability of the recommended equation.