Diffuse, global and extra-terrestrial solar radiation for Singapore

SYNOPSIS In this paper, equations have been developed to estimate diffuse fraction of the hourly, daily and monthly global insolation on a horizontal surface. These correlations are expressed in terms of Kd, the ratio of diffuse-to-total radiation, and KT, the clearness index. The hourly correlation equations, show a fairly similar trend to that of Orgill and Hollands (1) and Spencer (5) and support, their views on latitude dependence. The correlation equation for daily diffuse fraction of the global insolation agrees well with LIU and Jordan (6) and the latitude dependence is not very clear in this case. An equation has also been developed for monthly diffuse fraction of the global radiation. These correlation equations have been developed with limited data and apply to Singapore meteorological conditions.