Improving the Accuracy of Using Pyranometers to Measure the Clear Sky Global Solar Irradiance

Pyranometer users have customarily applied one responsivity value when calculating the global solar irradiance. Usually, the responsivity value is reported by either the manufacturer or a calibration facility. Many pyranometer calibrations, made both at NREL and elsewhere, have shown that the responsivity of a pyranometer changes with the change in solar zenith and azimuth angles. Depending onhow well the pyranometer sensor is radiometrically leveled, these changes can exceed +/-5% of the reported responsivity, which means that errors in the calculated global solar irradiance can exceed +/-5% from the nominal values. This paper describes a method to decrease the errors resulting from the change of the solar zenith angle under clear sky conditions. Two responsivity functions, morningand afternoon, were used instead of one responsivity value. The two functions have been chosen because of asymmetry of the morning and afternoon cosine responses demonstrated by some pyranometers.