The long read: Bottom-optimization

From pv magazine 09/2020

A new PV test facility has been installed at one of the earth’s sunniest locations, and it is adopting an approach that may shed additional light on the promising combination of bifacial modules with single-axis tracking. The 120 kWp Bifacial Test and Innovation Center in Chile’s Atacama Desert was commissioned in May 2020. Engineers from Engie Laborelec are now looking to find the sweet spot between various bifacial module designs, trackers, smart power electronics and cleaning robots to deliver the lowest PV LCOE in Latin America and beyond.

“We are taking a holistic or system approach,” explains Thore Müller, who heads bifacial PV R&D and solar services for Engie Laborelec Latin America. “To get rid of as many assumptions and sources of error as possible we are comparing different combinations of trackers and bifacial panels side-by-side, without relying on simulations as a reference.”

The Atacama Bifacial Test site is composed of 10 rows of bifacial modules, installed on two different single-axis tracking systems – five rows of the Soltec SF7 system and five of the Nextracker NX Horizon. The Engie Laborelec team specifically selected “big commercial” bifacial tracking systems, in Müller’s words, to make the results directly applicable to real-world project developer considerations. The selection of the two systems is also notable in that the Soltec system deploys a two-in-portrait (2P) configuration, while Nextracker opts for one-in-portrait (1P).

Five different bifacial modules have been installed on each of the two tracker systems. On each of the ten trackers, several panels have had their rear side covered with black vinyl – providing a “monofacial” reference. Temperature differences are accounted for.

“What we are trying to do is verify certain claims of manufacturers,” continues Müller. “We used the same ground cover ratio, the same number of panels per area, for both [tracking] systems. The big advantage of our setup is what we want to do with the parameters, in that they are the same, under the same condition, with our findings lying in the relative difference between the two.”

Müller and the Laborelec team will present some initial findings from the Atacama testbed at the EU PVSEC 2020 digital conference, although as it is an Engie project the data will not be released publicly for researchers.

Boosting bifaciality

The Atacama Desert not only provides extremely sunny conditions in which to test the bifacial+tracking systems, but also a relatively high albedo – a function of the reflectivity of the surface below the array. The Chilean desert provides an albedo approaching 30%, Müller reports. But there are ways to boost the ground albedo and one recent study by PV Evolution Labs (PVEL) has brought this into focus.

PVEL carried out some testing at the behest of California-based Alion Energy. Alion, like the Laborelec test site, takes a full-system approach to its ballasted single-axis tracking and SPOT automatic cleaning system. It argues that this allows its tracker system to be deployed on both difficult geological and topographic sites, while also delivering attractive yields.

Alion reports having supplied 65 MW of projects to date, making it a relatively new entrant. Project sizes to date range from 4 MW to 10 MW in size, with Alion having supplied six key solar markets. A noteworthy feature of the Alion system is that its concrete base structures do away with the need for support pile driving, while also providing a base along which its robotic cleaning system can travel.

The PVEL testing of the Alion system began in February of this year. The month of January was used as a reference, with the tracking system maintained at zero degrees. In terms of measuring the albedo, the concrete foundations of the structure were left unpainted through February, after which they were painted white. The ground was grassy at the test site.

“We were very happy with the results,” says Alion Energy President and CEO Mark Kingsley. “It really demonstrated what we were seeing at our other sites, the results from which we couldn’t share because they were confidential. It confirmed that our general product planning was in the right direction.”

The headline takeaway from the PVEL testing of the Alion system was that the bifacial boost increased from around 7% when the foundations were left gray, to slightly more than 10% once they had been painted white the following month – on a 1P tracking system. In a two-in-landscape (2L) configuration, the bifacial boost increased from just 7% to more than 12%.

“The main finding that was a bit different to the other sites was that if we have a high-quality reflector, then it isn’t necessary for the rear side of the module to be too far away from that [concrete ballast] reflector,” says Kingsley. “And it doesn’t require huge pieces of land to space rows out to capture that light. You can control your destiny by having a more stable and reflective foundation color.”

Additional findings

The testing was carried out at PVEL’s long-running outdoor test site in Davis, California. The Alion testing is running alongside PVEL’s 1,500 V bifacial+tracking testbed that was established in December 2019 and is partly supported by a U.S. Department of Energy grant. PVEL founder and CEO Jenya Meydbray says that the Alion results indicate that the painting of the concrete foundations white, “has a pretty material impact” on the bifacial boost – and may make economic sense for a project operator.

Meydbray adds that the different bifacial boost observed for the 1P and 2L configuration on the Alion tracker is noteworthy. The Alion tracker utilizes dual tube purlins on the rear side of the module, resulting in some rear-side shading and light refraction. By contrast, the Nextracker NX Horizon has a single torque tube with a larger circumference.

“The difference between the landscape versus portrait performance on Alion is interesting,” says Meydbray. “I would chalk that up to the cell layout relative to the diodes in the modules themselves. On the Alion system, there is uniform left to right rear-side irradiance, but not up to down. When the module is in landscape, then uniformity is consistent over the diode layout.” The PVEL head notes that split or dual module designs, which are becoming more commonplace with half-cut cell modules, would likely exhibit different behavior again.

The DOE-supported portion of the Davis testbed deploys the Nextracker NX Horizon, with modules from four different module suppliers. Modules are characterized before installation onto the trackers, “because if you don’t characterize that beforehand then it ruins the whole study,” says Meydbray. And even at this pre-installation phase, variations from specifications were observed.

“The bifaciality factor [observed for PVEL’s outdoor and indoor testing] was often 5% off what the data sheet stated,” observes Meydbray. “It was always below what the data sheet said, sometimes quite a lot below, sometimes less.” He says this could very well be a result of inconsistent methodologies for the rear-side flashing of bifacial modules in production. As modules are sold and warranties applied only to the front side output, such variation could not be seen as being crucial, despite the fact that project developers could be basing their financial models on a certain level of bifaciality. The front and rear side of the modules could degrade at different rates – further complicating matters.

Front-side reigns

Efforts to brighten the ground and boost rear-side illumination may even trigger module faults or premature degradation. Tracker supplier Array Technologies’ Kyumin Lee notes that if the light is too intense, hotspots could form over time and bypass diodes could be triggered. Lee adds that such effects were observed at the CFV Solar Test Laboratory in Albuquerque, New Mexico, at which he worked before joining Array in February.

“Modifying the albedo to boost the [rearside] gain might help, but it is not a game changer,” says Lee. He adds that while there is considerable hype in the industry at present regarding bifacial+tracking technology, that the effect of single-axis tracking in boosting front-side production remains the main game.

“In testing, it can be observed that for single-axis trackers, the highest bifacial gain occurs at midday, and at this point the inverter may be clipping in any case,” says Lee. The frontside production, Lee argues, remains key and is clearly far better understood, and can be very accurately modeled during the project design phase – a development that appears some distance away with bifacial+tracking arrays.

As for a production boost during the early morning or evening, the “shoulders” of the power production curve when wholesale power prices may be higher, backtracking, to avoid row-to-row shading when the sun in low in the sky, delivers just this “shoulder” production, with the unintended consequence that the module orientation means that there is very little bifacial gain at these times while backtracking. “There is a very hard limit on how much you can widen the shoulders,” says Lee.