The promise and the reality of bifacial modules and solar trackers
Solar power plant developers are a diverse lot. Thanks to the versatility of solar technologies, project developers can focus their attention on everything from residential rooftop to massive, multi-megawatt-sized utility-scale power plants.
Yet all solar developers have one thing in common: Their main priority is to maximize the amount of energy – and revenue – their installations can generate. It’s a fact that helps explain why trackers that move panels to follow the sun have become a required component in utility scale solar.
That perpetual quest for increased generation also explains why one of the hottest topics in the solar industry is the potential benefits of pairing bifacial modules with trackers. On the surface, it seems like a marriage made in heaven. While trackers can boost generation by ensuring panels receive more direct sunlight each day than if they were attached to a fixed-tilt mount, bifacial modules can further enhance production by generating electricity from both sides of the solar panel.
The interest in bifacial modules, particularly when paired with trackers, is understandable: There is genuine potential to increase generation and revenues without adding undue complexity and cost. Some in the industry are already trumpeting production increases as much as 30%. “It’s understandable that our customers are interested in the potential of putting trackers and bifacial modules together,” said Ron Corio, Founder and Chief Innovation Officer of Array Technologies, pioneers of solar tracking. “But at this point what we know about the pairing is not based on hard data outside of a few actual installations. We have been working to change that since installing a test site in Nevada in 2016.”
Indeed, while bifacial technology has been around for decades, scant research has been conducted on how the modules work when mounted on tracking systems. “There’s very little data to draw on about how bifacial performs in large-scale installations,” said Kimberly Weaver, Lead Engineer of Bifacial for Array Technologies. “There has been more scientific research on bifacial and fixed-tilt but very little with trackers. I think the industry is struggling because there is a lot more complexity with understanding the performance for bifacial installations.”
That complexity comes in the form of uncertainty about how production is impacted by factors such as the shading caused by the components of a tracker, albedo and seasonal effects, module technology, etc. Getting more clarity on how much additional production is possible by combining bifacial modules and trackers – and, importantly, whether that production gain is worth the added cost – is one of the main goals of a research project Array and a New Mexico national laboratory is currently pursuing. “We want to be transparent with customers that, yes, you can get more gain but you need to be cognizant of any additional costs,” said Weaver. “We want to be able to substantiate any claims and point to actual results that prove them.”
There are certainly plenty of issues where additional clarity will be needed to accurately estimate the amount of production bifacial modules and trackers can generate. One is around how much shading the torque tube of a tracker creates on the rear side of the module. When bifacial modules are mounted one in portrait, some amount of shading is inevitable, but there are a variety of measures which can mitigate the impact. “One thing we are trying to understand is what the optimal height the module is mounted from both the torque tube and the ground,” said Weaver. “The further away, the less impact there is on rear side shading. But it comes to a point of diminishing gains because you must add a lot more structure to accommodate the module mounted higher. That cost has to be considered from a CAPEX perspective.”
Additional investigation is also needed to understand how production is impacted when bifacial modules are mounted in a two-in-portrait on trackers. “When you look at two-in-portrait, you must have the tracker higher off the ground and that means you must have larger foundations and thicker piles. The structure has to be heavier and more rigid to accommodate the increase in loads,” said Weaver. “The costs increase and the installation of modules becomes more cumbersome. Does the additional gain by installing bifacial in two-in-portrait offset the cost increase, this is what we want to truly understand.”
Other areas in need of investigation include how the albedo, which is the ability of the ground to reflect the light, changes from location to location and season to season. “Current bifacial modeling techniques do not adequately take this important factor into account and this needs to change.” says Weaver.
Testing with the national lab will gather data from multiple rows of trackers with bifacial modules, the information will then be used to model the real-life workings of a large-scale array. “We can take data from the test rows to build a scenario and create an accurate simulation of a large-scale array,” said Weaver. “In the simulation we can then do many iterations and batch runs to understand heights, shading and albedo impacts and how it changes over the seasons.”
Ultimately, Weaver would like to see the insights gained through this and future research used to improve the accuracy of production estimates. “The next big hurdle the industry is facing is simulating the gain of bifacial,” she said. “Right now, tools like PVsyst make assumptions that may not reflect actual site conditions. They don’t have enough data to refine the simulations; it’s another area where the industry is working to improve so that developers to be more confident in the results these simulations are giving us.”
This matters to the industry because accurate projections of the costs and benefits of pairing trackers with bifacial modules is necessary for investors and end customers alike. “The last thing we want to do is over promise about the possibilities of using trackers and bifacial modules and then have those promises not work out to benefit customers,” said Corio. “It’s vital for the ongoing growth of the industry that we deliver on our promises. That’s why we are building on the knowledge we gained from the test sites installed a few years ago with our current testing with national lab.