The bottom line is that bifacial panel use on trackers is expected to grow to a double digit share within a year, and eventually become the dominant design. “Next year there will be explosive growth in bifacial tracker installations,” predicts Guy Rong, CEO of Arctech Solar, based in Kunshan. Others also agree. “The bifacial tracker is one of most exciting untapped opportunities in the solar industry; there will be a huge drive toward this,” suggests Dan Shugar, Founder of NEXTracker.
Tweaking the cost/benefit design
Today’s leading bifacial panels are based on rear-side efficiency enhancement schemes widely focused on rear passivation layers. Bifacial panels have been around for decades, but experimentation with photovoltaic chemistries and manufacturing processes have finally yielded a generation of products that seem difficult to ignore. Overall, bifacial panels now add only about three percent to the total cost of a tracker system, and even that number will be crunched down to the cost of monofacial panels within a few years, several PV industry officials say.
While some bifacial panel makers add the performance component of the backside to the price calculation, others are offering it as a bonus. “Some suppliers like LONGI quote the back side for free, which means their bifacial module has the same cost per watt as a standard module,” notes Angel Lu, the marketing director for Bigsun Energy Group, based in Hukou Township, Taiwan.
Performance on the ground
The beneficial use of bifacial panels is highly dependent on the site, the officials caution. “Not all players understand that bifacial is best used for specific environmental situations,” says Matteo Demofonti, the chief commercialization officer for Rome-based Convert Italia.
The specifics list is long. “There are many factors, poorly understood in general, that go into the successful use of bifacials on trackers, including albedo, a low ground cover ratio, strong irradiance on site, a high ratio of direct sun, the rear side effect of the panel, and the aspect ratio of design,” explains Shugar.
Albedo is the largest factor in bifacial performance, and some sites are far better than others. “The higher the albedo the better, and snow, sand, water provide that. We are designing a floating bifacial iPV Tracker system, which is projected to increase energy by 50 percent compared with a conventional ground mount monofacial system,” says Lu.
Module advances will also help maximize back side performance. “For bifacial modules we use p-type crystalline silicon, but in the future, n-type will represent the largest portion of the market. N-type helps the back performance but it is getting more expensive now,” says Parjanya Rijal, Product Marketing Manager for Trina Solar.
P-type silicon is produced by adding atoms like boron or gallium that have one less electron in their outer energy level than does silicon. Because boron has one less electron, a vacancy or hole is created, according to the American Chemical Society. N-type silicon is made by adding atoms that have one more electron in their outer level than silicon, such as phosphorus. It bonds with its silicon neighbor atoms, but the electron that is not involved in bonding is free to move inside the silicon structure, aiding in electricity flow, the ACS says.
Whatever it takes to optimize the design of the bifacial tracker combination, it will be up to the tracker company to do virtually all of the system optimization work. “The customer wants everything figured out from the foundation to the BOS,” says Preston.
Framed or not
One question that remains open to design solutions is whether framed bifacial panels or unframed glass-glass bifacial panels are best for tracker installations.Unframed modules tend to cause less backside shading. However, some mounting designs for unframed bifacial modules may cause additional shading, so it may be up to the engineers to say which approach makes the most sense.“Unframed module makers are trying to shift some of the module cost to the mounting system, but the total BOS may wind up costing more than if a panel frame was used,” said one executive.
“We have looked into different ways to tab glass-glass modules, but in terms of total system cost, it may be cheaper if you add the frame to the module,” says Rijal. Another issue is that framed bifacial modules can trap sand or other soiling materials, unlike frameless bifacial modules. In Chile, however, Convert Italia found little problem with framed bifacials on an installation after two years, according to Demofonti.
Testing in global markets
Bifacial tracker testing has been ongoing for about a decade. NEXTracker installed part of the field at Nellis Air Force Base, in the Las Vegas Valley, with bifacials on trackers in 2008 as a test, says Shugar. “Within the 14 MW single-axis project, 2 MW was bifacial, and the custom solution we developed with Sanyo had exceptionally good performance, with gains of 14 to 17%,” says Shugar.
Now NexTracker is testing bifacial trackers in a 20 MW California project within which there is 1 MW of bifacial trackers. Another test is underway in Asia with a 1 MW bifacial tracker as a controlled experiment, Shugar says.
Convert Italia has been studying its 2 MW bifacial tracker installation in Chile for two years now, as well as several test installations in Italy, according to Demofonti.
ArcTech solar, which has been studying bifacial panel use on trackers for years, installed two 40 MW tracker projects in China last year, and found a 7 to 10% gain, says Rong. “We’re going to do several installations this year in India, beginning with a 2 MW test plant, and a marketing promotion there in March. And we likely will be testing in Mexico and Arizona soon,” he says.
Soltec has also installed bifacial trackers in Chile and has begun installing a project in Israel, says Marketing Manager Victoria Vestal.
Array also has been testing bifacial rows of trackers, for over a year, and found that a portrait orientation is best, considering that it is far enough from the torque tube that reflected light is not shaded, says Preston.
Independent ranking of bifacial module performance
While tracker makers are generally relaxed about whose panel is used in a project, there may be enough of a performance difference to change that stance. “It’s buyer beware with the panels. Customers need to understand how they were tested and proven,” says Preston.
A significant limit to testing bifacial performance on trackers is the capability of current mathematical models in solar. “The main thing to resolve is getting better data on bifacial energy gain, and some standard industry tools for modeling just can’t do it,” says Rijal.
One effort to bring independent performance ranking to bifacial panels is the partnership between Kansas City-based Black & Veatch and the Fremont-based Renewable Energy Test Center. The two are now testing modules anonymously from various manufacturers, first in the lab, then in the field for a year’s time. “There is a lot of horsepower being put in this,” says Ralph Romero, the senior managing director of the company’s management consulting arm in New York. We can field test at our sites all over world, including Chile, China, India and the United States,” he says.
Although the joint venture is now testing different manufacturer’s modules on a single tracker design, for data reference, there may ultimately be tests of a lead module brand on different tracker designs. Array also is working with Sandia National Laboratory to develop a model for analyzing bifacial panel performance optimization on a tracker, relying on advanced analytic algorithms. A tracker that always points directly at the sun may not be maximizing performance without considering albedo sufficiently, as well as near and outer field shading.
Similarly, Convert Italia has won a multi-year award under a European Union research project to design the next generation bifacial tracker.
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