Having delivered stage one of a giant behind-the-meter solar-and-tracking project, Trina explains how the technology fits into Australia’s utility-scale PV market.
Flat land and open skies are the two simple requirements for a solar plant. In the real world, however, developers know they won’t get much back on their investment without a muscly grid connection and an offtake agreement.
As the prime real estate for utility-scale solar projects gets harder to find, developers are working out how to squeeze maximum value from top-shelf technology such as tracking systems and bifacial modules. The extra cost is worth it in many cases, with higher output leading to lower levelized cost of energy.
“We’re seeing LCOE reductions of 5-8% for single-axis tracking and bifacial in pretty much every part of Asia compared with fixed-tilt,” says Trina Solar head of utility solutions Asia-Pacific Andrew Gilhooly, before pointing out the hard part. “There’s always a bit of technology risk with trackers; things move compared with fixed tilt, there’s more maintenance and inherently a higher risk profile.”
Gilhooly spoke to EcoGeneration having just finished checking 4.5MW of Trina solar and tracking at McCain Foods Australia’s operations in Ballarat, Victoria, in what will be Australia’s largest behind-the-meter power plant. When it’s finished, the plant will include 7.5MW of solar and a bioenergy plant that can take tonnes of potato chip waste every day to make energy and recover heat.
The panels at the McCain plant are secured to single-axis tracking system by TrinaTracker, a subsidiary of Trina Solar that joined the family as Spanish company NClave.
On Gilhooly’s estimates, about 80-90% of utility solar projects in Australia use single-axis tracking. The remaining 10-20% are in cyclonic region C, which includes the northern coast of Queensland. Regardless of the risk of strong winds, developers chose single-axis solutions by TrinaTracker for 120MW of projects in cyclone region C in 2018.
“There have been some high wind events pass through that locale in the years since then without incident,” he says. “We’ve got a very solid track record of tracker deployment in high-wind regions.”
The extra yield gains from bifacial with tracking are highly dependent on location, with owners expecting between 15% and 25% extra yield, “depending on the region and the solar resource, especially the proportion of direct diffuser radiance”.
To state the obvious, cloud cover has a big impact on expectations for solar plants. Tracking and bifacial panel technology can be worth the extra effort under these conditions. When working on modelling for monofacial projects in Malaysia and India Trina has found single-axis systems in tropical, quite cloudy locations can generate about 10% more output than a fixed solution. In that situation, the higher above-ground elevation of a tracking solution makes bifacial panels a worthy investment as more reflected light will hit the rear of the array. “You get an even bigger bifacial boost from the modules on the tracker and that tends to restore the tracking-and-bifacial advantage [over fixed monofacial] to at least 15% or more,” Gilhooly says.
In heavy cloud cover, when sunlight is scattering in all directions, reflected diffuse irradiance is maximised when panels are horizontal. When stowed flat under those weather conditions, yield can be boosted by at least 3% on a typical site, Gilhooly says.
The higher you go…
The McCain job uses the company’s 210mm-wafer modules, which are about 20% longer than standard panels. As modules get bigger they need to be elevated higher above ground when tracking is used, so as not to reach the ground when tipped at east- and west-facing extremes. PV arrays catch the wind, of course, and the relationship between height above ground and wind force on an array is non-linear. This means incremental increases in height will result in proportionally greater wind effects.
“That increases the propensity for challenges from second-order dynamic effects like galloping, resonance or fluttering in the wind,” Gilhooly says, listing a few destructive manifestations of high wind speeds on solar farms.
To get around these effects Trina has adopted a multi-point driving system, where a master drive in the middle of the tracker and slave drives at each end stiffen the entire tracker. “That motor at both ends also acts as a self-locking feature,” he says, stiffening the structure so that it increases the natural frequency to a level where it should never lose control.
That sounds good, but winds can be vicious and the fact that tracking is seldom used in cyclone region C is testament to developers’ using caution.
One row or two
TrinaTracker offers two solutions: Vanguard, which uses the 2P (arrays of two panels joined at the short end, or in portrait) configuration, and Agile, which uses two separate arrays of panels in 1P orientation where one array with tracking is linked to a second, separate array.
Gilhooly offers the view that Vanguard offers the best value for most of the country because it uses fewer foundation points per megawatt. As for wind conditions, he says Vanguard is good for all of region A and bits of region B, where extreme wind speeds are less likely, whereas Agile is recommended for region C and most of B. Because Agile trackers are closer to the ground they’ll stand up better at high wind speeds.
“[Agile] is cheaper from a supply perspective and there are fewer critical components that can fail in service, so there is an opex reduction as well,” he says.
In a country the size of Australia it sounds outrageous to suggest we’re running out of land to build solar plants on, but Gilhooly says it’s true that it’s becoming harder to find land that suits all the criteria. “The geotechnical characteristics are rockier or the soil is more reactive, which has a big bearing on foundations,” he says.
It was generally believed that trackers should only be used on flat sites, but it’s no longer the case, he says. “We’re seeing tracking being considered on sites with really quite variable terrain,” he says, “and solar farms being deployed or considered on sites prone to flood inundation.”
Learning on the job
Programming the tracking for bifacial panels is way more complicated than monofacial. The controller takes its cues from an on-site weather station and applies machine learning so the tracker improves its decision-making over time. Gilhooly claims a further 8% boost in yield is possible on non-flat sites by varying the angles on individual arrays to reduce shading on adjacent trackers.
Because the high-power 210mm-wafer modules are low-voltage relative to standard panels, more of them can be fit on a string, which reduces balance of system and capex costs. Trina has been on a tear rolling out ever more powerful modules, with the 500W and 600W Vertex modules launched last year and most recently a 670W Vertex module.
At the McCain site, which was built by Smart Commercial Solar, it plans to test the albedo effects of different ground covers such as crushed rock and sand on bifacial performance. The company is also in talks to source Australian steel for tracking systems installed locally.
The wide spacing of Vanguard trackers dictated by the tall 2P orientation of module arrays makes the tracker-panel combination a good complement to arable crops, he says, with plenty of ground open to the sky but benefiting from partial shade throughout the day. “It doesn’t have to be one or the other – solar or agriculture – we can potentially keep both,” he says. “That’s true sustainability.”