Investigation of factors influencing the accuracy of pyrheliometer calibrations

The accuracy of solar cells calibrated as primary reference cells is directly dependent on the accuracy of the pyrheliometer used to measure the direct beam solar irradiance on the cell. Pyrheliometers are also used in measuring performance of concentrating photovoltaic modules. In order to reduce errors in photovoltaic performance measurements, we have investigated the calibration uncertainties for pyrheliometers from two manufacturers. Our calibration comparisons are relative to an absolute cavity radiometer traceable to the World Radiometric Reference. This paper quantifies the effects of aging, temperature, time-rate-of-change of temperature, wind, solar spectral shifts, linearity, window transmission, and solar tracking on pyrheliometer calibrations. Uncertainty remaining after accounting for these factors is 0.8% at the 2-sigma level