Racking Reflection and Shading Effects on Single Axis Tracked Bifacial Photovoltaic Modules

Bifacial photovoltaics (PV) is predicted to comprise 80% of the silicon PV share within the next ten years. However, bifacial energy yield models are still undergoing validation, and their uncertainty may slow adoption. One of the challenges of single-axis-tracked (SAT) bifacial PV performance modelling is accurately accounting for the effects of racking elements, such as the frame, module supports, and torque tube, on the rear irradiance. In this work, we calculated front and rear irradiances for the center modules of a 2-in-portrait SAT bifacial photovoltaic system from hourly typical meteorological year data for the Bifacial Test Evaluation Center (BiTEC) site in Livermore, California using bifacial_radiance ray tracing software. For every hourly timestamp, we calculated 2D front and rear irradiance maps in three cases: with no racking, absorptive racking, and reflective racking. From these, we calculated three racking effects: shading, reflection, and shading and reflection combined. We also calculated shading and reflection factors as well as rear irradiance non-uniformity for each case. For the PV system modelled, racking reflection is focused in the same areas of the module as racking shading, partially counteracting shading-induced irradiance reduction and irradiance non-uniformity. For example, for a winter day at noon, racking reflection reduces the rear shading factor from -18.4% to -10.8% and the irradiance non-uniformity from 14.8% to 10.8%. The effects of racking, including both shading and reflection, vary by time of day and year. Accounting for these variations, rather than using annual average correction factors, will improve energy yield prediction accuracy for bifacial PV, especially over short time periods.