Technical Commentary: Bifacial Solar Module Performance
Bifacial solar photovoltaic modules convert solar radiation received on their front and back sides into electricity. Calculating the solar energy generated by solar radiation received by the back side is complicated by shadows, varied weather and dust conditions by day and season, back side equipment and reflections, and albedo.
May 05, 2021. By News Bureau
Based upon ArcVera Renewables’ technological assessment, we conclude that bifacial solar module technology will find beneficial application in a significant number of solar energy power plants. While advertised 25% or greater gains in energy yield are unlikely for most projects, typical project energy production gains are 3-10%. Our analysis finds that many projects under the scrutiny of informed technical due diligence, proper measurement, and site design, will reap meaningful net present value. Although financing institutions have recently not awarded significant debt financing value to bifacial projects, we expect full credit for bifacial benefits to accrue rapidly as experience and market penetration increases.
As with any emerging commercialized technology, the solar industry is just beginning to grapple with how to value the project impact of bifacial solar module technology. What is a bifacial module? Bifacial simply means that the solar module is receiving irradiance and generating power from both sides of the module, whereas a traditional monofacial module only accepts irradiance incoming on one side, with an opaque back side. Bifacial technology is not necessarily new – designs were presented as early as 1960, and installed systems have been tested since the 1990s (Guerrero-Lemus et al. 2016). However, with advances in module manufacturing, cell efficiencies, and the overall rapid growth of the global solar photovoltaic industry, bifacial technology has recently emerged with the potential to capture a significant portion of the module market.
Bifacial photovoltaic module technology has seen strong growth in recent years, with significant interest throughout the industry and many more product offerings available. Several Tier 1 module manufacturers, including Trina, ongi, Jinko, Canadian Solar and others currently offer bifacial products, and racking manufacturers have also developed designs to optimize gains and minimize racking costs specifically for bifacial systems. Solar resource assessment campaigns are more commonly incorporating albedo measurements to better quantify the potential bifacial gains, and solar energy modeling software such as NREL’s System Advisor Model (SAM) and PVsyst have incorporated algorithms to simulate bifacial energy output.
Based on rapid developer adoption, ArcVera Renewables expects bifacial designs to eventually dominate the utility-scale module market.
Bifacial Energy Production Considerations
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 groundmount 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 downwardfacing 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.
Bifacial Project Economics
Energy gains from bifacial modules should be considered with due respect to the associated change in cost of implementation. Therefore, design optimization should be thoroughly evaluated with respect to determining the cost of energy in financial models.
The bifacial module market is evolving, and pricing has fluctuated with the larger module market and more recently with changing trade tariffs. Manufacturers may quote costs in the range of 6% to 10% increase from their equivalent monofacial modules. Given that modules are only a fraction of the overall installed cost (albeit a significant fraction), a fairly modest increase in energy output can provide substantial economic benefit and justify the implementation of bifacial modules (aside from other considerations like product availability, warranty, and other project- or developer-specific factors). In general pricing terms, the increased cost of the bifacial panel adds an estimated five cents per watt. In a project where debt financing is utilized, an increase in the production value of a project will generally increase the amount of debt the project can support. Depending on the lender’s evaluation of the risks associated with bifacial technology, the increased production value may not be answered by the same increase in debt. However, under a typical project model using debt financing, an increase of 3-4% in energy production value is generally found to result in in a more valuable project, with lower cost per installed watt.
In terms of technological risk, the electrical components of a bifacial panel are not much different than a monofacial panel (Siason and Kedir, 2018). The challenge is how to accurately model and estimate the production value of bifacial panels to help stakeholders in their acceptance and adoption of the technology. With its potential for significant energy yield benefit, the methodology to technically assess and accurately estimate bifacial value panel technology is rapidly converging to a best practice.
A key consideration to financially valuing bifacial technology is determined by how well the project developer designs the project and measures the albedo, given that different modules and project designs will respond differently to the same albedo. To understand its value risks and optimize potential gains, ArcVera Renewables recommends deployment of met stations including albedometers to gather site-based albedo. This, along with design certainty, is a key strategy to underwrite the technical value of a prospective project considering the use of bifacial panel technology.
Deployment of a met station to understand albedo is relatively low-cost when compared to the value they deliver and provide actual GHI and DNI measurements which ‘true up’ the long-term, satellite TMY3 datasets. Met stations also provide other important data that reduces associated resource uncertainties, such as measuring the rate of panel soiling and site design loads. Importantly, without on-site albedo measurements it is possible that the calculated energy production gain will be offset significantly by the uncertainty of those calculations, reducing debt value depending on the P-Value utilized by a given financial institution. More than one albedometer measurement site may be advisable if ground conditions (and therefore albedo) vary significantly across the site footprint.
Bifacial technology and energy estimation are evolving, and with them the level of comfort from financial institutions. To realize the expected project value increase associated with 3-10% in bifacial solar project energy production, ArcVera Renewables recommends diligent and accurate measurement and modeling of site conditions, equipment specification, and project design. Ultimately any estimated gains in energy output and offtake, changes to system costs, and associated changes to project economics would be subject to review for project finance.
- Kevin Lichtenstein, Solar Energy Group Director, ArcVera Renewables
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