A Spatial Irradiance Map Measured on the Rear Side of a Utility-Scale Horizontal Single Axis Tracker with Validation using Open Source Tools

This work presents measurements from 40 individual 125 mm x 125 mm crystalline silicon (c-Si) cells placed on the backside of a horizontal single axis tracker (HSAT) located in Roskilde, Denmark (55.6°N, 12.1°E). The measurements are used to validate a general set of conclusions gathered from recent literature, to compare to simulated backside irradiance results from view factor and ray-trace based methods, and to estimate the electrical losses caused by nonuniform illumination at the module and array level. In this work, all simulations are performed using the open source tools bifacialvf, bifacial_radiance, and pvmismatch. The tracker studied is 45 m long with 60-cell bifacial photovoltaic (PV) modules mounted 'two-in-portrait' - a configuration commonly implemented in utility scale PV parks. Our measurements corroborate the conclusions from several simulation-based studies made by other authors. The measurements and simulations indicate that the irradiation-nonuniformity-induced electrical mismatch of the bifacial array is not higher than 0.25% when mounted above grass (albedo 0.22) on a clear sky day. But the array-level mismatch can go up to 3% when the PV park is uniformly covered by a white polymeric material (albedo 0.60). During a cloudy day, the mismatch of the bifacial system over grass is as high as 1%, but is lower than 0.25% around solar noon. Above the white ground cover on a cloudy day, the mismatch is around 1-2%, even at solar noon.