On the characteristic distribution of hourly diffuse and global solar radiation at Ibadan

The potential of a previous model for calculating direct and diffuse solar radiation is further explored by employing the model to study the characteristic distribution of hourly insolation for six years (1980–1985) at Ibadan. The study covers “clear” and “moderately clear” sky conditions for which the turbidity β < 0.175, and for which the range of β, the probability of occurrence in the year is 60.63%. The computed results yield correlations of the hourly-ddaily insolation ratios rh and rd with the sunset hour angle θs which are in satisfactory agreement with, and therefore confirm, the work of Liu and Jordan as well as Collares-Pereira and Rabl. The present results also show that the hourly diffuse ratio kd′ is a function of both the hourly clearness index kT and time t from solar noon: this explains why the previous plot of Orgill and Hollands of kd′ vs kT without accounting for time effect gives an unacceptably wide scatter. Further, the present correlation of the hourly diffuse coefficient kd with kT yields similar trends to those found in the previous work of Iqbal.