The Importance of Capturing the Full Value of Bifacial PV with Accurate Modeling and Measurement

Bifacial PV Technology

Bifacial PV technology represents the single largest levelized cost of energy (LCOE) improvement opportunity since the introduction of solar trackers. The challenge is to fully capture the value of bifacial PV power plants. To realize this opportunity, investors need to be able to accurately and confidently predict bifacial performance. For bifacial gains to be financeable, they must first be predictable.

Those introductory words come from Jenya Meydbray, CEO of PV Evolution Labs (PVEL) and contributor to Nextracker’s new white paper titled “Quantifying Your Bifacial Gains: Using Calibrated PVsyst Model Input Parameters to Accurately Predict In-Field Performance.” Jenya captures both the bifacial zeitgeist eloquently and provides much of the rationale behind our publication of the paper. As game-changing as bifacial PV is likely to be, the mass deployment of the technology is still in its early stages, albeit at a rapidly accelerating clip. For bifacial to achieve its market potential, the ability to accurately model and measure bifacial performance is crucial. And to do that, we need lots of field testing data and a laser focus on getting the modeling dialed in.

Actionable Content for Stakeholders

The white paper presents our test methodology and findings with the goal of providing actionable content for developers, financiers, independent engineering firms, and performance engineers struggling to characterize fielded bifacial system performance. This post is the first in a blog series that will excerpt highlights and major takeaways from the paper. In future installments, we will discuss our real-world bifacial test results, take a deep dive into the calibration of PVsyst inputs for our NX Horizon trackers, and share data that demonstrates NX Horizon’s bifacial gain advantage over other trackers.

Bifacial PV modules convert irradiance captured on both the front and back sides of PV cells into electrical power. From a manufacturing perspective, the evolution from monofacial passivated emitter and rear-cell (PERC) to bifacial PERC PV modules is incremental, requiring only a modified solar cell back-side metallization pattern and certain module-packaging adjustments. In the field, this one small step for module companies is potentially one giant leap for the global energy transition.

Bifacial on Solar Trackers Achieves Lowest LCOE Values

A recent techno-economic assessment evaluates yield potential and cost effectiveness based on LCOE for large-scale PV power plant architectures around the world and hints at the disruptive potential of tracker-mounted bifacial PV systems. The report concludes that “bifacial single-axis tracker installations achieved the lowest LCOE values for 93.1% of the total land area.” Sensitivity analyses indicate that these results hold “over a wide range of parameter changes,” emphasizing “the potential of bifacial one-axis tracking systems to transform the PV market.”

If stakeholders cannot model bifacial gains with confidence, the difference between monofacial and bifacial performance is interesting but not impactful. The industry’s ability to precisely and consistently model monofacial PV system performance has reduced project risk, cost of debt, and barriers to adoption. To the extent that bifacial performance models carry comparatively uncertainty and perceived risk, financiers may subject these projects to a higher debt-service coverage ratio, reducing profitability.

Accurate Modeling of Bifacial PV Gains Is Crucial

Accurately modeling bifacial gains using industry standard software requires a combination of product-, design-, and location-specific model inputs:

  • The bifaciality factor, for example, is a module-specific PVsyst input that quantifies the ratio of back-side power to frontside power under standard test conditions.
  • Row-to-row spacing and tracker height are design-specific.
  • Variables that influence irradiance on the ground and the rear side of the PV modules.
  • Weather and ground albedo are fluctuating location-specific variables that significantly affect in-field performance.

Missing from the above are tracker-specific inputs to the PVsyst model, a subject of considerable interest to Nextracker, its project partners, and other industry stakeholders.

To help eliminate barriers to the global energy transition, Nextracker operates a state-of-the-art testing laboratory in Fremont, California. At our Center for Solar Excellence, we have studied bifacial performance, in some capacity, since 2013. We commissioned a dedicated bifacial testbed in Q1 2019, which integrates commercially representative mono-PERC PV modules on the latest version of the NX Horizon single-axis tracker.

High-level takeaways from this world-class bifacial testing laboratory are encouraging, both in terms of the magnitude of real-world bifacial gains as well as our ability to model these empirical gains in PVsyst.

Amir Asgharzadeh Shishavan

Amir Asgharzadeh Shishavan, PhD, serves as Nextracker’s PV Systems Performance Engineer, focusing on R&D and advancements in bifacial technology.

Testing indicates that industry standard mono-PERC bifacial electrical strings fielded on one-in-portrait (1P) single-axis trackers provide additional DC-side energy yield gains as compared to equivalent monofacial strings. These additional bifacial gains are on the order of 5% to 7% under low-albedo conditions (~20%) and 10% to 12% under high-albedo conditions (~50%). These grid-connected test results are in good agreement with data from bifacial test sites that the National Renewable Energy Laboratory (NREL) and PVEL operate.

In the next blog series installment, we will present and compare real-world test results seen at the Center for Solar Excellence, PVEL’s outdoor testing lab at PV-USA, and NREL’s bifacial testbed. To download the entire “Quantifying Your Bifacial Gains” white paper, click here.