Evaluation of Photodiode and Thermopile Pyranometers for Photovoltaic Applications: Preprint

Accurately determining photovoltaic (PV) module performance in the field requires measuring solar irradiance reaching the PV panel at a high level of accuracy and known uncertainty. Silicon detectors used in various solar energy measuring instruments that include reference cells are a potentially attractive choice for multiple reasons, including faster responsivity than thermopile detectors, cheaper cost, and lower maintenance. The main drawback, however, is that the silicon detectors are only spectrally responsive in a narrow part of the solar spectrum; therefore, to determine broadband solar irradiance, a calibration factor that converts the narrowband response to broadband is required. Normally, this calibration factor is a single number determined under standard conditions but used for various scenarios, including varying airmass, panel orientation, and atmospheric conditions. This would not have been an issue if all wavelengths that form the broadband spectrum responded uniformly to atmospheric constituents. Unfortunately, the scattering and absorption signature varies widely across wavelengths, and the calibration factor computed under certain test conditions is not appropriate for other conditions. This paper lays out the issues that will arise from the use of silicon detectors for PV performance measurement in the field. We also present a comparison of simultaneous spectral and broadband measurements from silicon and thermopile detectors and estimated measurement errors when using silicon devices for both array performance and resource assessment.