Irradiance on the upper and lower modules of a two-high bifacial tracking system

We examine the illumination incident to each of the modules in a two-high bifacial tracking photovoltaic system. Experiments and simulations indicate that the upper module receives more diffuse sunlight, whereas either the upper or lower module can receive more direct sunlight depending on the configuration and conditions. We use a view-factor model to illustrate how the fraction of illumination received by the upper and lower modules depends on albedo, the sun's position in the sky, module tilt, and the fraction of direct sunlight. We also use ray tracing to illustrate how edge effects can significantly change the amount of sunlight reaching the upper and lower modules; these results underscore the complications of using measurements from small test facilities to predict the behavior of large PV fields.