Comparison of Calibration Responsivity of Radiometric Data as Obtained from Various Calibration Service Providers
“Accurate solar resource data is essential for solar energy systems. In order to finance these systems, banks need assurances that these systems will produce the energy predicted. Failing to meet the minimum energy performance requirements can result in large financial penalties requiring expensive risk mitigation measures. Additionally, utility planners and grid system operators need to understand how high penetrations of solar energy systems dependent on a variable solar resource will impact the grid. Therefore, accurate solar radiation data sets are the foundation of successful solar energy projects because they are critical in reducing the expense associated with mitigating this performance risk and assist to understand the impacts of variable generation from high penetration levels of solar energy systems.
The accuracy of solar radiation measured by radiometers depends on the specifications of the instrument, calibration procedure, measurement conditions, maintenance, location, and environmental conditions. This study will address the calibration methodology and the resulting calibration responsivity provided by radiometric calibration service providers such as NREL and manufacturers of radiometers. Some of these radiometers are calibrated indoors and some outdoors, therefore, to establish or understand the differences in calibration methodology requires processing and analysis of the field measured data by these radiometers. This study will investigate calibration responsivities provided by NREL’s broadband outdoor radiometer calibration (BORCAL) and a few prominent manufacturers. The BORCAL method provides outdoor calibration responsivity at 45 degree solar zenith angle and responsivity as a function of solar zenith angle. Furthermore, the BORCAL method also employs a thermal offset correction to the calibration responsivity. On the other hand, indoor calibration by manufacturers is performed using a stable artificial light source in a side-by-side comparison between the test radiometer under calibration and a reference radiometer of the same type. This study will discuss the difference in measured irradiance resulting from the calibration responsivity values using indoor and outdoor calibrations. Analyzing these values will ultimately assist in determining the uncertainties of the radiometers.”