While bifacial module technology increases energy production, a variety of factors can affect the rate of this increase. Among the most critical factors to consider when predicting bifacial module yield are module mounting height and albedo, or the fraction of light reflected by the surface.
Module mounting height — The closer a bifacial PV array is to the ground or a roof surface, the less chance reflective light will reach the back of the array. A significant bifacial energy boost is possible, however, with a relatively modest height increase. In one simulation, the energy boost curve was steepest between 0 and 7.9 inches. After about 20 inches, the curve flattened out and additional energy gains were negligible. This data from Solar Pro suggests that bifacial modules are appropriate for most ground-mounted applications, given that the leading edge of these arrays is usually from 18 inches to 36 inches above grade.
Modeling bifacial modules to predict their increased energy production, however, continues to be a combination of art and science. PVsyst, a software program used to study PV systems, has limited modeling capabilities, with module manufacturer and user inputs remaining a critical part of the modeling process. Currently, hand calculations are typically needed to compute the equipment shading factor. Extra modules and cell mismatches are, at this point, also largely a judgment call by the installer.
As long as inputs are accurate, however, software modeling results tend to be relatively accurate. Field testing is needed to verify these results. Keep in mind that a 10% error on the back-of-module calculation can result in a 1% error in annual production, according to Solar Pro.
Albedo — Annual energy production increases of 5% to 10% are typical with bifacial modules. In most areas, imported ground cover is often needed to push the increase much over 10%. The question is, does it pay to bring in light-colored gravel or roofing to control albedo? The answer to this question is both location- and project-specific, and a cost evaluation is needed to make the final determination. The estimated albedo for various surfaces is outlined in Figure 2.