A simple method for calculating the annual insolation on solar collectors

The annual average insolation incident upon a variety of principal solar collector types-including shading effects-is calculated by approximating the annual variation of daily radiation by its value on one average day, equinox. The rationale for the selection of equinox is discussed, and results are presented for concentrating collectors with two-axis, one-axis polar, one-axis east-west and one-axis north-south tracking, and for flat plate collectors. We compare our results with the Rabl correlations[1], which are based on actual weather data from a variety of locations throughout the U.S.A. Values predicted from the coarse approximation of treating the entire year as one specific average day (equinox) turn out to be surprisingly accurate. For the specific cases considered, the predictions deviate from the Rabl correlations by 5 per cent at most.