Solar Engineering of Thermal Processes, 4th Edition

2.11 Beam and Diffuse Components of Daily Radiation

Studies of available daily radiation data have shown that the average fraction which is diffuse, , is a function of , the day's clearness index. The original correlation of Liu and Jordan (1960) is shown in Figure 2.11.1; the data were for Blue Hill, Massachusetts. Also shown on the graph are plots of data for Israel from Stanhill (1966), for New Delhi from Choudhury (1963), for Canadian stations from Ruth and Chant (1976) and Tuller (1976), for Highett (Melbourne), Australia, from Bannister (1969), and for four U.S. stations from Collares-Pereira and Rabl (1979a). There is some disagreement, with differences probably due in part to instrumental difficulties such as shading ring corrections and possibly in part due to air mass and/or seasonal effects. The correlation by Erbs (based on the same data set as is Figure 2.10.2) is shown in Figure 2.11.2. A seasonal dependence is shown; the spring, summer, and fall data are essentially the same, while the winter data show somewhat lower diffuse fractions for high values of . The season is indicated by the sunset hour angle . Equations representing this set of correlations are as follows