THE DETERMINATION OF MONTHLY MEAN HOURLY DIFFUSE RADIATION ON HORIZONTAL SURFACES USING EQUATIONS BASED ON HOURLY CLEARNESS INDEX, SUNSHINE FRACTION AND SOLAR ELEVATION

Abstract

Several years of measured hourly diffuse and global data for four locations in Spain are used to establish correlations for estimating monthly mean hourly diffuse radiation Dh for locations in Spain and to test the influence of various climatic and geographic parameters on the diffuse to global fraction. A correlation connecting the monthly mean hourly diffuse to global fraction Dh/Gh with mean hourly clearness index KT, mean monthly hourly sunshine fraction S/So, and solar elevation at mid hour y, is found most suitable, and an equation of the form Dh/Gh=i+jkT+lsin γ+m s/sO is recommended for estimation purposes for any location in Spain. When the solar altitude and sunshine fraction are added to the basic equation Dh/Gh= a+bKT the accuracy of the estimated data improves to a good extent and this improvement is more significant when correlations are developed from a group of stations in a region rather than for a single isolated location.