An Average Slope Factor for Solar Insolation

Abstract

Design methods for solar collector sizing (such as f-chart) usually require a multiplying factor (the slope factor) to correct insolation data taken on the horizontal for the tilt angle of the solar collector. The slope factor is a function of the haze factor (K T ) at each location which varies month to month. A simplification (with attendant error) can be made for space heating by taking the 6 month average winter haze factor as a constant to get a single set of slope factor curves. For hot water heating a 12 month average of the haze factor is used to find the correct slope factor chart. Available slope factor equations using average haze factors were plotted to result in a chart whose parameters are latitude and month of year. Correction charts for off-average locations are presented. The error in the given slope factor charts is estimated to be accurate ± 7 percent for most cities.