Measuring Irradiance With Bifacial Reference Panels

The heterogenous nature and spectral distribution of rear plane-of-array irradiance R_POA presents challenges when measured by small-area sensors such as pyranometers. Bifacial reference modules serving as large-area sensors can simplify irradiance monitoring because their electrical response follows that of the power generating modules in an array. This article compares R_POA and effective irradiance G_E measured by calibrated reference modules against three commonly used small-area sensors including pyranometers, reference cells, and photodiodes. A technology-matched monofacial module is mounted side-by-side with the bifacial reference to decouple effective irradiance measurements into front and backside contributions. The results show that R_POA and G_E measurements made with reference panels have the best correlation to reference cells. The mean absolute errors between the two measurement approaches are 9% relative, 4 W/m² absolute for R_POA and 4% relative, 7 W/m² absolute for G_E. When G_E measurements from the four sensor types are used to predict string-level power, the reference panel measurements show a 3.4% prediction error, which is comparable to that achieved when using G_E measurements from pyranometers (3.0%) and reference cells (2.9%) thereby suggesting that reference modules can be used to accurately measure R_POA and G_E in bifacial systems.