The long read: A new test for trackers

A handful of competing tracker manufacturers are teaming up with independent engineers and national science lab partners, among others, to field test the performance of a variety of current bifacial panel designs on their latest tracker models. These field tests should confirm the accuracy of the mathematical software models used to predict the product performance, and to prove the precision of the predictions in the field.

To further compare the independent certification of bifacial panel models, some independent engineers, including U.S. EPC company Black & Veatch, are working rapidly to build up a bifacial ranking database. DNV GL, Fraunhofer ISE, and TÜV Rheinland are among other independent engineers pursuing this capability.

Beyond the melding of separate panel and tracker product analyses to show the performance potential of a specific tracker/panel pairing, there are also efforts underway to test the best panel and tracker product performers with one another in the field. DNV GL plans to develop a multi-client study with such data in the near future, says Colleen O’Brien, Principal Engineer for Solar Technology for the company in California.

Variations complicate testing

However, this design, model, and confirm task is neither quick nor easy. Several of the tracker companies on this quest – including Array Technologies (ATI), NEXTracker, and Soltec – are just now building out test fields where a half a dozen or more different bifacial panels at each site will yield initial data sets ranging from four to six month periods. All these companies aim to accumulate several years’ worth of data, of which some elements are expected to be public information.

The field data will be compared with product performance software analysis models, including PVsyst, which predicts backside performance. The independent engineers will report the proximity between the predicted and actual performance in bankability studies for the tracker companies.

“The first company to prove that a bifacial panel is the best performer on their tracker, even with only a 10% bifacial gain, will win big in the market,” reckons Ron Corio, Founder and Chief Innovation Officer at Array Technologies. ATI suggests that the bifacial boost could range from 5 to 30% – one test with LG bifacial panels yielded a 9.5% gain.

His company is using PVsyst to predict backside performance of its latest DuraTrack HZ v3 tracker model using bifacial panels from Canadian Solar, FirstSolar, Jolywood, LONGi, Hanwha Q Cells, and Trina Solar at a test site in Albuquerque that will start at 1 MW and grow to 2 MW. The PVsyst algorithms will be checked for accuracy by a national laboratory, which has launched a new bifacial solar tracker testing site in New Mexico. The national lab’s testing project with Array Technologies will include the development of a model for analyzing bifacial panel performance optimization on a tracker. The field performance data will also be analyzed by DNV, which will produce a bankability report naming the top two panel performers in combination with the tracker.

Similarly, NEXTracker is augmenting its Center for Solar Excellence test facility in Fremont, California, this summer to undertake multi-year testing of a handful of different bifacial panels on its Horizon tracker, which was designed especially to use bifacial panels, says NEXTracker CEO Dan Shugar. Leidos prepared a bankability study for the tracker. His company will also work with an as yet unidentified independent engineer to confirm the field data. The company has registered gains from bifacial panels ranging from 5.6 to 15%, he says.

Soltec is testing half a dozen bifacial panel models at its Bifacial Tracking Evaluation Center (BiTEC) test facility in Livermore, California, utilizing the company’s new SF7 single-axis tracker. Soltec is working with the Renewable Energy Test Center, a certification and bankability laboratory, and with Black & Veatch to assess the field data. The U.S. National Renewable Energy Lab (NREL) will also collaborate in the project to confirm algorithms and other field test values for variables.

Soltec is now examining two years’ worth of data from its La Silla project, a 1.72 MW bifacial installation in Coquimbo, Chile. The company cites a 30% theoretical gain for bifacial panels on trackers.

Arctech Solar is also testing bifacial panels in the field, according to CEO Guy Rong, although not enough data have been analyzed to present publicly, he notes. But panel makers are not waiting for results from the tracker maker’s tests. Among those that have themselves been testing bifacial panels on different trackers are GCL, FirstSolar, JA Solar, and LONGi.

GCL is testing bifacial modules at multiple tracker sites in China and is currently seeking a tracker manufacturer partner for a longer field test, said one company official during the Intersolar North America show in July. The company was test-marketing its new lower cost glass-glass multicrystalline Mars Series bifacial panels off-site during the show. GCL predicts a 45% gain from the use of its bifacial panels on single-axis trackers, in comparison with fixed monofacial arrays.

The roster of bifacial panel makers continues to grow. Other companies with bifacial modules currently on the market include Trina Solar, BYD, Hanwha Q Cells, LG, LONGi, Lumos Solar, Panasonic, Prism Solar, Risen Energy, Silfab, Sunpreme, and Yingli Solar.

The International Technology Roadmap for Photovoltaic (ITRPV) predicts that the global market share for bifacial technology will increase from less than 5% in 2016 to 30% in 2027. Bifacial tracker installations already have been completed in China, Chile, Mexico, the United States, and across Europe.

Variables challenge test standardization

The problem with analyzing the field performance of bifacial panels on single-axis trackers is that tracker designs by different companies incorporate all sorts of different mechanisms to achieve the best tilt at a given time.

Some tracker makers are betting on a row of single panels in a portrait orientation, with only half an inch (1.25 cm) between panels to maximize bifacial performance. Others use two panels in portrait, leaving a six inch (15 cm) space between the panels over the torque tube. Some tracker designs hold the panel table above the torque tube, while others straddle the vertical mast fixtures.

Engineers who write algorithms for such variations face a daunting task. Apart from position, spacing, and table design, the component structure of the balance of system components also plays into optimizing light capture. Some tracker makers use square torque tubes, others use round tubes, and at least one uses an octagonal tube, playing different roles in backside capture. Indeed, the differences in design are a large part of the tracker makers’ sales mantras.

Among the laundry list of key variables in field testing that need to be considered are: site condition and ground color; the seasonal effect of vegetation; the ground coverage ratio of the array; the height of the panel table; the position of the torque tube relative to the panel table; direct and diffuse shading; and cumulative albedo, for starters.

Some tracker companies are testing over a single ground color condition with basic gravel. Others are testing with white, green, and grey gravel surfaces. And at least one, Big Sun Energy Group, based in Hukou Township, Taiwan, is testing a floating bifacial array over water, that they predict will bump up performance by 50%, according to Marketing Director Angel Liu. And ATI is testing torque tubes that range from 2.5 to 14 cm in diameter to determine the optimal design.

The degree to which a tracker controller is able to back-track or front-track to capture the maximum available diffuse light will also play into the field performance testing. And whether the tracker utilizes photo sensors or not can affect the performance. Weather condition data will be acquired from third parties to compare the bifacial/tracker performance.

Performance standardization under study

Universities and laboratories have been studying bifacial panel performance for several years, as the manufacturing costs have dropped to a commercially attractive level. Sandia National Laboratories, NREL, and the University of Iowa, for example, have been working together for two years to better understand the performance characteristics of bifacial PV modules and systems.

Sandia won a $3 million grant from the Department of Energy’s SunShot Initiative to provide the data, standard test methods, and validated models to allow developers to evenly evaluate the potential benefits of bifacial PV technologies for specific projects, according to DOE. The grant was part of the SunShot National Laboratory Multiyear Partnership (SuNLaMP) funding program.

Sandia observes, “A standard module rating condition for bifacial PV modules would be a boon to the PV community as it would provide a common, accepted basis for measurement and nameplate rating of bifacial PV products. In particular, conditions that also harmonize with existing PV module rating standards are desired.”

The lab adds, “Through simulation and experiment, we are investigating back side irradiance conditions that are appropriate for the power rating of bifacial modules. We are also field testing proposed procedures that are intended to allow a 1 sun rating of bifacial modules using indoor flash test equipment.”

Other engineering research centers, like the International Solar Energy Research Center Konstanz (ISC Konstanz), have been at work to improve the commercial manufacturing process for bifacial cells. ISC Konstanz also offers performance testing at its outdoor test sites in Egypt, Germany, and Malaysia.

Drawing on its work with Soltec, Black & Veatch plans to produce a performance ranking for bifacial modules, says Ralph Romero, the Senior Managing Director and Head of Bankability Practice.

In September, NREL, SNL, ISC Konstanz, the French National Solar Energy Institute (INES), Zurich University of Applied Sciences (ZHAW), and the Netherlands Energy Research Centre (ECN) will jointly present the fifth and last Bifacial PV Workshop in September in Denver “to review all existing technologies on the market, and to begin setting standards, to identify the market potential, and improve bankability of bifacial PV.”

NREL, which has a large body of bifacial research, produced a study, “Bifacial PV Performance Models: Comparison and Field Results,” in October 2017, focusing primarily on fixed-tilt installations of bifacial modules. The workshops are expected to yield a new wave of white papers on bifacial performance.

Cost/benefit balancing

Bifacial panel technology is not new. Bifacial panels were first patented by Hiroshi Mori in 1966, but it wasn’t until 1981 that Isofoton was the first company founded to produce bifacial cells. Isofoton cells were used in the 20 kWp power plant in San Agustín del Guadalix, built in 1986 for Iberdrola.

While bifacial panels have been available for years, the cost of producing them only recently dropped low enough for the technology to be commercially competitive. The additional cost of using a bifacial panel rather than a monofacial panel is widely estimated at about 3% today. With energy production gains ranging from a high single digit to double digit percentages, the benefits already clearly outweigh the cost.

The field testing of bifacial panels on single-axis trackers that is now underway should produce the first wave of confirmations of the actual value of the bifacial/tracker combination, which is widely expected to set an industry benchmark in solar technology advancement.

“Bifacial PV systems can sometimes see gains of more than 30% and, combined with tracking, the total electrical gain can be close to 50%, which will reduce LCOE to below $0.02/kWh,” according to a Sandia report.

Bifacial technology is estimated to boost PV project income by 1% thus far. Once field testing is complete and at least some performance data become public, the race will be on to benchmark project ROI with bifacial tracker technology.

By Charles W. Thurston