Diffuse Sky Measurements and Determination of Corrected Shadow Band Multiplication Factors

Abstract

A meteorological monitoring station was operated at Blytheville, Arkansas, from April 1978 to April 1980. Direct normal, global, and diffuse sky radiation were monitored. From these data, models have been developed for the prediction of solar radiation, and discussions of several diffuse solar radiation models are included herein. Comparisons are made with these current diffuse models, and the correlation is quite good. In addition, instantaneous shadow band correction factors are presented which will allow a more accurate correction to be applied to the measured diffuse sky reading. The instantaneous correction factors are keyed to the global radiation measurement. Instead of applying a fixed correction factor to the diffuse measurement, regardless of sky condition, a variable factor can be applied. This will solve some of the current errors observed in diffuse measurements, because the current factors overpredict the diffuse radiation on cloudy days and underpredict the diffuse on clear days.