A model for determining the spectral quality of daylight on a horizontal surface at any geographical location

Abstract

Using solar irradiance data gathered over 8 yrs (1968–1976) a very simple model for determining daily and hourly totals of solar irradiance has been derived. The simplified model works extremely well for broad spectral regions, i.e. the total irradiance, the visible (400 nm–700 nm), and the total IR (800 nm–2800 nm). Smaller regions (100 nm bands) are not computed with the same accuracy because of the spectral dependency of such parameters as the albedo, water vapor and ozone. Also, this model does not give good results around sunrise and sunset. The computational accuracy of the model of daily totals and hourly totals on clear days is within ±10% when compared to values obtained from Eppley precision pyranometers. The accuracy for 100 nm bands is considerably less being greater than ±10 per cent on clear days and cloud corrections will increase the errors to ±25 per cent.