Boosts and boasts: The newest information on solar tracker + bifacial PV module manufacturing yields
For a long time, improving the electricity yield in photovoltaic solar energy was a game with gradual additions to new technologies. In recent years, the introduction of supply-scale bifacial panels has been tested and the designs have been refined to the point where high single-digit profit from a uniaxial tracker + bifacial array is bankable. It is now clear that using dual-axis tracker technology instead of a single-axis design can produce another 20% profit, if only for commercial-scale installations.
A third incremental technology gain – engineered reflective albedo – is now also being tested on uniaxial two-axis arrays, with the promise of another gain of 10% or more, which will allow far more solar projects to achieve a favorable ROI, especially on sloping, irregular ones or limited land footprints.
The main factor influencing the performance of bifacial panels is the albedo, or the ratio of light reflected from the floor to direct sunlight captured by the back of a bifacial panel. The natural albedo ranges from around 30% with grass surface to almost 100% with snow. Developed albedo from reflective surfaces of paint, white stone and mirrored plastic or polished metal, the latter with the bonus of year-round availability coming close to the albedo value of snow.
Bifacial panels on one axis give a two-digit boost
The bifacial thrust from uniaxial tracker arrays has been studied by several national renewable energy laboratories as well as private engineering firms. For example, in a field test in New Mexico with a national research partner, Array Technology found that bifacial gains for modules inside a supply-scale array were in the range of 5% to 6%. The rest of the results showed that modules rated higher bifaciality scores returned 7%, according to Jon Sharp, vice president of Array’s program administration office.
Similarly, Nextracker’s NX Horizon was found to produce high single digit to low double digit profit from bifacial panels. “Overall, Nextracker has a mono-PERC bifacial gain of 5% to 7% under low albedo (~ 20%) conditions and 10% to 12% under high albedo (~ 50%) conditions,” the company says. “These Center for Solar Excellence results are in line with PVEL (PV Evolution Labs) and NREL,” adds the Fremont, CA-based company.
Dual-Axis + Bifacial Yields Highest Boost
Sandia Labs reported in mid-2020 that biaxial trackers were seeing significantly higher yield gains during the winter months when snow reflects light on bifacial panels. “Two years of empirical data from experimental systems in Vermont support this claim, showing that bifacial modules on a biaxial tracker produced 14% more power in one year than their monofacial counterparts and 40% more during the peak winter months,” writes Joshua Stein, who senior investigator for Sandia.
“These bifacial gains are in addition to the estimated 35 to 40 percent energy gains of a two-axis tracker compared to a fixed incline system,” Stein reported. His experiments included Williston, VT-based AllEarth Renewables two-axis trackers that support a 24-panel table.
Improved albedo improves the boost
Technical albedo has been widely found to increase bifacial thrust. The construction of floor surfaces to improve albedo has been studied for a variety of materials, from white gravel to white vinyl roofing to mirrored plastic to polished metal.
For example, a test of Alion Energy’s concrete rail-based uniaxial tracker showed increased yields with albedo enhanced by white paint. PVEL evaluated the performance of bifacial modules mounted on single-axis Alion trackers in a year-long outdoor study. “An average bifacial gain of 8% from five minutes was achieved on a low-albedo concrete foundation in February 2020, which increased to 11% in March 2020 after the foundation was painted white,” reported PVEL.
Similarly, in January, Gamechange Solar launched its patent-pending BifacialReflector in combination with the Solar Genius Tracker and Bifacial modules. The technology is a 95% albedo obtained from a solid self-cleaning material that has not yet been identified. The design can increase profits by as much as an estimated 15 to 20% when using bifacial modules, the company said.
Another reflective surface that is being tested to increase yield with bifacial panels is water: Taiwan’s Big Sun claimed in 2018 that its uniaxial tracker had a 20% to 30% gain. “By installing our iPV Tracker over a surface of water with bifacial modules, the yield increase over fixed slope systems that use monofacial modules can be as high as 60%,” the company said.
Terra Pave, a reflective, albedo-enhancing soil binder for bifacial solar locations, recently passed Sandia National Laboratories’ tests for heavy metal contaminants, organic contaminants and water pollution and compared them with the results for AEP (Asphalt Emulsion Product). Terra Pave products are liquid soil stabilizers that permanently bind the soil and turn it into a pavement-like layer. Terra Pave Top-Seal is designed to reach the albedo of snow – for use with bifacial solar panels. This year it was recognized as a finalist in the AMERICAN-MADE SOLAR PRISE competition by NREL. Terra Pave is available exclusively through Beckett Solar Energy.
The next step: Engineered Albedo + Dual Axis
Stacking the boosts from two axes and engineered albedo provides the highest overall yield yet.
A biaxial + bifacial systems provider, Point Load Power, claims to have found test results of a 70% bifacial boost on a commercial white vinyl roof with its PV booster tracker compared to a fixed slope monofacial array. One of the system’s limitations is the distance between the panels and the roof, where the wind lift is a critical factor.
More recently, Mechatron Solar of Stockton, Calif., Has begun testing a new reflective metal surface that is mounted directly under the two-axis tracker table that supports 90 panels. This monolithic structure enables the distance between the reflective surface and the panels to be optimized.
“We expect a double-digit increase in yield from the new, patent-pending reflective substrate design that is mounted under our gearless two-axis M18kD tracker,” says Michael Fakukakis, CEO of Mechatron. “The two-axis design already delivers 17 to 20% more power than a single-axis tracker or 40% more power than a fixed-slope array with the same number of panels,” he says.
The improved mechatron albedo design is currently being tested on an existing California customer array. Preliminary dates for the public review are expected this spring. “The bottom line is that our technology and bifacial panels achieve a performance-guaranteed gain of more than 50% compared to conventional fixed-slope arrays and monofacial panels. Take that to the bank, ”says Fakukakis.
Is the sky the limit?
How far can solar go with low cost bifacial profits? Only time can tell. “In highly optimized systems with ideal ambient conditions, profits of 20% to 30% can be achieved,” says Jenya Meydbray, CEO of PVEL. “However, certain system optimization techniques, such as the use of special high albedo land cover, can increase system-level construction costs and ongoing operational and maintenance costs. The challenge for the industry is to identify the most cost-effective tactics to optimize system design. “
Charles W. Thurston is a Solar Builder employee based in Northern California.
Keywords: Alion Energy, AllEarth Renewables, Array Technologies, Commercial, GameChange Solar, Mechatron Solar, NEXTracker, Point Load Power, PVEL, Sandia Labs, Terra Pave, Utility Scale