Optical Radiation Measurements for Photovoltaic Applications: Instrumentation Uncertainty and Performance

Evaluating the performance of photovoltaic (PV) devices in the laboratory and in the field requires accurate knowledge of the optical radiation stimulating the devices. We briefly describe the radiometric instrumentation used for characterizing broadband and spectral irradiance for PV applications. Spectral radiometric measurement systems are used to characterize solar simulators (continuous and pulsed, or flash sources) and natural sunlight. Broadband radiometers (pyranometers and pyrheliometers) are used to assess solar resources for renewable applications and develop and validate broadband solar radiation models for estimating system performance. We describe the sources and magnitudes of uncertainty associated with calibrations and measurements using these instruments. The basic calibration and measurement uncertainty associated with this instrumentation are based on the guidelines described in the International Standards Organization (ISO) and Bureau Internationale des Poids et Mesures (BIPM) Guide to Uncertainty in Measurement. The additional contributions to uncertainty arising from the uncertainty in characterization functions and correction schemes are discussed and illustrated. Finally, empirical comparisons of several solar radiometer instrumentation sets illustrate that the best measurement accuracy for broadband radiation is on the order of 3%, and spectrally dependent uncertainty for spectroradiometer systems range from 4% in the visible to 8% to 10% in the ultraviolet and infrared.