HOURLY DIFFUSE FRACTION CORRELATION AT A TROPICAL LOCATION

Five years of hourly global and diffuse radiation data on a horizontal surface at a tropical location (Madras, India) were used to establish the relationship between the hourly diffuse fraction and hourly clearness index. Two types of correlations-on a yearly and a seasonal (wet and dry months)-have been established. Piecewise and single polynomial curves have been fitted to the data. These correlations have been compared to those developed by Orgill and Hollands(1977), Erbs et al. (1982), and Reindl et al. (1990), which have been obtained from data of temperate locations. The comparison has been performed in terms of standard deviation and relative standard deviation. The results indicate that the proposed correlations are better. The best fits were obtained when the seasonal effects were taken into account. It is also shown that the hourly diffuse fraction is larger at a tropical location at higher hourly clearness indices.