Performance Testing using Silicon Devices - Analysis of Accuracy

Accurately determining PV module performance in the field requires accurate measurements of solar irradiance reaching the PV panel (i.e., Plane-of-Array - POA Irradiance) with known measurement uncertainty. Pyranometers are commonly based on thermopile or silicon photodiode detectors. Silicon detectors, including PV reference cells, are an attractive choice for reasons that include faster time response (10 microseconds (μs)) than thermopile detectors (1 s to 5 s) and lower cost and maintenance. The main drawback of silicon detectors is their limited spectral response. Therefore, to determine broadband POA solar irradiance, a pyranometer calibration factor that converts the narrowband response to broadband is required. Normally, this calibration factor is a single number determined under clear-sky conditions with respect to a broadband reference radiometer. The pyranometer is then used for various scenarios including varying airmass, panel orientation, and atmospheric conditions. This would not be an issue if all irradiance wavelengths that form the broadband spectrum responded uniformly to atmospheric constituents. Unfortunately, the scattering and absorption signature varies widely with wavelength and the calibration factor for the silicon photodiode pyranometer is not appropriate for other conditions. This paper reviews the issues that will arise from the use of silicon detectors for PV performance measurement in the field based on measurements from a group of pyranometers mounted on a 1-axis solar tracker. 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.