Bifacial Solar Module Performance – Technical Commentary

The benefit of bifacial modules is in the additional solar irradiance captured and resulting gains in energy output for a given site and solar plant design. Module manufacturers may advertise gains “up to 25%” or more, and while this may be technically achievable, in practice the gains are highly variable and depend on many project site and design characteristics (Romero et al., 2018). Based on ArcVera’s modeling of bifacial systems and our review of third-party models and test systems, typical ground-mount tracking systems in North America more likely exhibit annual energy gains in the range of 3% to 10%, with some opportunity for further optimization.

The primary drivers of bifacial energy production gains are:

• Module “Bifaciality”: This aspect denotes the efficiency of the “bottom” side, relative to that of the “top”. Current bifacial modules have bifaciality factors of 60% to 95%. On the higher end of that range are higher-cost modules more typical of smaller/niche applications. Typical utility-scale bifacial modules are currently in the range of 60% to 70%. Multiplying the bifaciality fraction by the percent of back side solar radiation to front side irradiation gives the bifacial percent gain.
• Surface Albedo: The primary variable associated with bifacial gains is surface albedo, or the amount of solar radiation reflected by the ground (Stull, 1988). Surface albedo exhibits significant differences throughout regions and groundcovers, across the different seasons, and even throughout the day. The albedo factor is quantified as a number between 0 and 1 (0.0 indicating 0% reflection and 1.0 indicating 100% reflection). The reflected irradiance and resulting bifacial energy gains are directly proportional to this value. Light colored surfaces have higher albedo, and dark colored surfaces have lower albedo. Snow, for example, is more reflective and may have an albedo factor between 0.4 and 0.9, whereas grass may have an albedo factor between 0.1 and 0.2. Quantifying time-varying albedo is of primary importance when estimating bifacial gains.
• Ground Cover Ratio (GCR): GCR is the ratio of the module array footprint to the total project land area. This number depends on the module size and racking orientation (i.e. two modules in portrait, one in landscape, etc.), and the spacing between rows. A lower GCR causes less shading as a percentage of the project area, leaving more ground area to reflect solar irradiance to the module’s back side, whereas a higher GCR would imply less exposed ground surface (more shading), and less irradiance available to the back side.
  Array Height: The array height will influence how much reflected light makes its way to the module’s back side. A higher rack will allow more of the light reflected by the surrounding ground surface to reach the back side, increasing the potential gains.
• DC:AC Ratio: The DC:AC ratio is an important design choice to be optimized at any site, and bifacial module gains need to be considered in this optimization. For example, during peak sunshine hours, additional bifacial gains may not be realized if the system is already clipping power, perhaps allowing for a lower DC:AC ratio at the site.

There are several technical considerations which can complicate the accurate estimation of bifacial gains. Albedo alone has fairly high uncertainty and variability, and until recently has not been measured by on-site albedometers. An albedometer is typically comprised of one up-looking and one downward-facing pyranometer. Satellite-based (e.g. NASA Clouds and the Earth’s Radiant Energy System, or CERES, satellite measurements) or other methods of estimating albedo without ground-based measurements are sufficiently coarse (Stephens et al. 2012, Rutan et al., 2009) or approximate with respect to solar plant footprint, so as to introduce questions regarding applicability and adequacy for analysis.

Other more nuanced impacts, though perhaps of second order, include bottom-side soil accumulation, snow accumulation on the ground (increasing albedo but reducing effective array height), site footprint variance of albedo, bifacial array mismatch losses, module degradation, under-side racking and wiring obstructions, and other factors.

These specific considerations are the focus of those pushing bifacial design and energy modeling forward. Several field tests have shown significant performance gains under a range of conditions, and in some cases have validated modeled results. Bifacial modules are opening up very interesting opportunities, including model improvements like incorporation of time-series albedo and more detailed racking specifications, optimization of ground cover such as genetically engineered reflective plant species or ground-surface modification, vertical fixed-tilt applications, new racking and wire management design solutions, and new optimization of tracking algorithms.