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Bifacial shines as PV power booster in cloudy conditions

US solar tracking company Alion Energy explains the results of research that shows how premium equipment can reap respectable yields from non-premium sites.

As solar edges its way into grids around the world the PV industry has turned into a research giant, always looking to eke more watts from a square metre of sunlight. A fairly recent entry has been bifacial module technology, which generates front and back. Bifacial works, especially with tracking, but testing has shown it really proves itself in environments where direct sunshine dawn to dusk isn’t always a sure thing.

Or, as Mark Kingsley puts it, don’t expect the same results in Chile’s cloudless Atacama Desert as you would get in overcast Malaysia.

“The second you get some cloudy, diffuse weather, they’re really going to sing,” says Kingsley, the CEO of US-headquartered Alion Energy, which has seen its tracking and O&M gear deployed in large-scale solar projects around the world.

Bifacial modules and single-axis tracking go hand-in-hand, Kingsley tells EcoGeneration. “We’ve been waiting for bifacial for seven years – it took five years longer than I’d hoped, frankly.”

Low-cost dual-glass panels pushed out the expectations for useful lifetimes of solar, but the emergence of bifacial technology has added muscle to the shoulders, so to speak. Reflected light that hits the rear of a bifacial panel will of course boost yield above what’s to be expected from a one-sided panel, but the real art of deploying bifacial PV comes down to how that light is harvested.

With the Alion Energy tracker, a semicircular cog is turned on an axis secured by A-shaped bracing fixed to extruded concrete runners. The runners, whether separate rails or joined by a low concrete floor, are the secret to reflecting light in the early or late stages of the day. The trick with Alion’s solution is to plot your PV plant so that neighbouring arrays cast minimal shade on the concrete foundations. When the sun is low, there should be plenty of room for sunlight to hit the foundations and bounce up onto the module’s underside. For extra boost, paint your foundations white.

Not so shady

How much difference can it make? On a painted, 1-metre-wide Alion foundation, a test by PV Evolution Labs of a Californian PV plant showed a 10% boost in generation over monofacial modules; a site in Chile showed an 8.2% gain over monofacial for a plant that used unpainted split rail foundations, where there is no concrete between the two runners.

Kingsley applies a “very simplified” rule when estimating the energy that can be yielded from the rear of a panel, which he says is a function of global horizontal irradiance, albedo, shading from neighbouring arrays and the inverse of the tracker height squared. “We needed a durable, paintable, serviceable reflector underneath the bifacial module, and we don’t want to be too far away from that,” he says. “You want to get the best from the energy that you are collecting on your diffusion reflector. That’s why we stuck on concrete.”

If Alion’s tracker looks low to the ground, it’s because anything higher ignores the rules of high school physics. “The inverse law of all radiation says that as all radiation travels it attenuates and loses energy; a flashlight at 1m might have 100% illumination but at 2m it might have 25% illumination,” he says.

“We took a Goldilocks approach, so the reflector and the height are just right.”

Cost control

Bifacial panels come at a premium, and tracking isn’t cheap. The extra yield can make the investment worthwhile, however, even in locations with high direct irradiance. Data from Rystad Energy shows bifacial panels are expected to account for nearly 20% of solar projects commissioned in 2021. Alion’s product offering includes a panel-cleaning robot that works through the night.

In Australia, Kingsley figures bifacial with tracking can really prove its worth in cloudy spots, such as Darwin or Hobart. “The things we’ve been watching for years with bifacial, even on fixed tilt, is it’s always better at cloudy sites.” Direct irradiation on the front of a panel is so effective that bifacial won’t add much gain for the extra cost, “but if you’re in more dusty, humid or diffused-light situations, the bifaciality really sings.”

The reflective property of the groundcover is the next determining factor. Grass has an albedo around 0.18-0.2, whereas concrete can range between 0.5 and 0.6.

Testing by PVEL at a test site with Alion tracking in Davis, California, in May this year showed a more-than-25% spike in energy gain over monofacial between 8am and 10am, as low morning light hit the painted foundations and reflected on the rear of the modules, and again between about 4pm and 6.30pm, although less pronounced. The result for June was nowhere near as good, because the grass had grown, but after a mow the morning spike was up to 20% by July. Overall, bifacial at the site yielded between 8-10% over monofacial over the three months.

Solar is right for Australia – and once a few obstacles are out of the way developers will enjoy the sunshine. Kingsley is eyeing the big game. “If you’re going to build a big array out in Western Australia to power hydrogen, you’re going to want that reflector.”


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