Quantifying the impact of incidence-angle dependence on solar radiometric calibration

Evaluating photovoltaic cells, modules, arrays, and system performance relies on accurate measurements of the solar radiation resources available for power conversion. Measuring solar resources accurately can lead to a reduction in the investment risks associated with installing and operating solar energy systems. The National Renewable Energy Laboratory´s Solar Radiation Research Laboratory collects and disseminates solar irradiance data and provides calibrations of broadband radiometers that are traceable to the international standards. It is essential that radiometric data are traceable to the international system of units, e.g., through the World Radiometer Reference and World Infrared Standard Group. This paper demonstrates the importance and application of an existing approach that ultimately reduces the uncertainty of radiometric measurements. Almost all commercially available broadband radiometers use a single responsivity value that is generated at a 45° solar zenith angle (incident angle) based on outdoor calibrations or transfers between radiometers inside integrating spheres or that responsivity is generated using normal incident radiation based on indoor calibrations using lamps and comparisons to reference radiometers to compute measured irradiance data. However, based on our experience and that of other experts in the radiometric science community, this method introduces increased uncertainty to the data. If a single responsivity value is used, the radiometer will overestimate or underestimate the irradiance data compared to the reference irradiance. This was demonstrated in Myers [1], Reda [2], and Reda et al. [3]. Further, by using responsivity as a function of solar zenith angle, the uncertainty for some instruments in the responsivity value can be reduced by as much as 50% compared to using a single responsivity calculated at 45° [2, 3].