A SIMPLE PROCEDURE FOR CORRECTING SHADOWBAND DATA FOR ALL SKY CONDITIONS

A model is presented that corrects the horizontal diffuse irradiance measured using a polar-axis design shadowband. Ratios of true diffuse to uncorrected diffuse measurements are categorized and averaged according to parameters describing the associated geometric screening and current sky conditions. The screening parameter is a geometric calculation for the amount of the sky hemisphere eclipsed by the band. The sky condition parameters account for the anisotropic distribution of the diffuse irradiance across the sky hemisphere. An independent set of shadowband diffuse data is corrected using this parameterization technique. Comparison of these corrected values with concurrent true diffuse irradiance values gives a root mean square error of 6.9 W/m2 and a regression slope of 0.99. The anisotropic contribution to the total shadowband correction is examined for seven different levels of sky clearness and indicates the importance of its consideration in the model's development. Since the correction ratios are developed from parameters that are not site specific, the model should be generally applicable. © 1990.