We are on the verge of the bifacial solar tracker era. Projects are being quoted with many starting to break ground later this year and early 2019.
Only two issues remain in the way of serious wide-spread adoption. First is the price of bifacial modules, which sits at about 30 cents a watt on average right now. The premium price makes sense because the market hasn’t formed yet, and it won’t form until there are bankable production estimates for the technology. That would be issue No. 2: the data set for bifacial tracker performance is incomplete, but this is about to change in a hurry.
Several big-time partnerships between tracker companies, module companies and PV research and testing labs have formed within the last year to understand this new bifacial module + PV tracker paradigm, test theories and build a complete data set on bifacial tracker production.
“This is a fundamentally different paradigm than before because the tracker and module are all intertwined with the site conditions in a way they weren’t before,” says Ron Corio, founder and CIO of Array Technologies.
The splashiest of these partnerships is Spain-based single-axis tracking supplier Soltec teaming with the National Renewable Energy Labs (NREL), Black and Veatch and RETC to build BiTEC, the world’s first evaluation center specialized in bifacial trackers, in Livermore, Calif. We visited the facility in July, and the site has a variety of configurations structured to isolate and measure any site or system design effect, such as:
- Terrain surface
- Types of bifacial technology (from Hanwha-QCells, Jinko, Canadian Solar, LG and more)
- String design
- 2x modules in portrait versus 1x.
One variable not changed throughout the field is tracker height, with all 2x configurations standing at 7.71 ft (with 1x configuration trackers at a height of 4.43 ft). The reason is Soltec’s preliminary electric performance measures over bifacial modules reveal a short-circuit current difference of over 2.3 percent between 1x and 2x trackers, meaning that height has a significant influence over the energy output of bifacial panels. The capture of diffuse irradiance below and around the tracker is increased with height, while the shadow cast on the ground is softened.
Array Technologies is taking a different approach. Also working with a U.S. national laboratory, Array is mapping the backside irradiance at a half cell resolution as well as testing various configurations at string level. Array’s testing is focused primarily on the module tracker interaction, varying module mounting techniques as well as testing module design variance. An important objective of this testing is to validate ray tracing simulation programs which will aid in the accurate modeling of bifacial performance in site-specific applications.
“We are working closely with the module manufacturers in a way we’ve never done before for exactly that reason,” Corio says. “When you design the module and the tracker as one system, you get a better result.”
In a comparative one year test, conducted in 2017, Array saw a 9 to 10 percent yearly gain for bifacial over monofacial at the same test site.
The difference in Soltec and Array’s testing approaches is in line with the difference in their tracking approaches — Soltec uses distributed tracker rows and Array supplys centralized drive. All of the performance gains reported will need to be considered within the already established LCOE of each tracker design.
Example: The torque tube impact is an early point of differentiation depending on who you ask. The Soltec testing team has seen the shadow from the torque tube in a 1x configuration hurting irradiance harvest in a way it does not when positioned in a 2x configuration, so its SF7 tracker includes an intentional gap between modules at the torque tube location that avoids shadowing on the backside of the module. Preliminary measurements have shown that up to 38 percent of reflected light does not reach the center of the bifacial modules compared to the edge due to the torque tube shadow of the 1x configuration. Array is quantifying the impact of the torque tube to harvestable rear side irradiance and testing modules with design characteristics that may use the torque tube as a performance advantage. All of Array’s test data will be compiled in an LCOE comparison.
The difference in testing isn’t really the point. The Lawrence Berkley National Lab noted during the Market Trends panel at Intersolar that the cost premium associated with tracker projects is all but gone with 79 percent of newly installed capacity being trackers. When all of this testing is done, the choice will still be the same centralized or distributed tracker decision its always been, just with these new bifacial performance gains to plug into the equation.
— Solar Builder magazine