A common language for bifacial PV

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Bifacial is no longer the future of PV. It is the present. That much was clear from the start of the BifiPV workshop held on Monday and Tuesday in Amsterdam.

At the event, module manufacturers including Longi, Jolywood and Canadian Solar stated plans to devote gigawatts of manufacturing capacity to bifacial panel technology while tracker companies Soltec and Nextracker boasted of the gigawatts worth of bifacial PV projects they had already installed or contracted for construction.

Though the speed of bifacial’s growth has outpaced what most industry insiders predicted, there are questions that still need to be answered to fully unlock the technology’s potential. Those matters were very much up for discussion over the two days of the workshop and pv magazine has compiled these five takeaways from the event.

Collaboration and standardization

A wide range of approaches to working with bifacial PV was presented across the two-day workshop and conversation frequently returned to the need for a clear set of standards and parameters to express the benefits of each given approach so they could be compared.

Several attendees noted disparities in the datasheets of various manufacturers when it comes to expressing the advantages afforded by bifacial, with some stating figures for ‘bifacial gain’ or ‘monofacial power’, and others referring to ‘equivalent efficiency’, ‘output rearside gain’ and various other terms. “Bifacial currently has a lot of opportunities to provide creative numbers,” said Jenya Meydbray, of testing body PV Evolution Labs.

Defining ‘albedo’ also proved problematic with some debate over the type of data that can be used to measure it as well as dispute over the term’s meaning. Many of the models proposed relied on satellite data from various sources for albedo measurement but such data may not be granular enough to give an idea of light behavior at a given site. Since albedo refers only to the ratio of sunlight reflected by a surface, data that includes the effect of shading and seasonal factors might be better termed ‘local irradiance profile’.

“We need to avoid overuse of the term albedo,” Joshua Stein of U.S. nuclear deterrent R&D firm Sandia National Laboratories told the audience, “otherwise it can end up confusing.”

Modelling – the next challenge

Nowhere is the need for a common language clearer than when it comes to modelling energy yield. The lack of a comprehensive model for predicting the annual output of a bifacial installation was identified by many as the biggest factor holding the technology back.

In a presentation on the second day of the workshop, Itai Suez, from U.S. module maker Silfab, explained that with a monofacial module only two types of light need be considered – direct sunlight and diffuse irradiation on the front side. With bifacial, six types become relevant: direct and diffuse sunlight to the rear side and the same types of light reflected from the ground to the rear side also enter the equation, rendering modelling much more complex.

Several leading research institutes presented modelling systems in Amsterdam, including the Netherlands Energy Center (ECN.TNO), the International Solar Energy Center Konstanz and the U.K. National Physics Laboratory. Many models rely on satellite data and it’s clear more data from bifacial installations is needed to sharpen accuracy – almost every presentation on yield modelling began with a request for collaboration and more field data.

Cell concepts

The question of what will follow PERC has been a frequent one for those working in cell technology and while there are several innovations in the running, going bifacial is a key concern for all.

Bifacial cell concepts based on TOPCon, IBC, heterojunction and perovskite/silicon tandem cells were presented at BifiPV, alongside innovations in transparent electrode layers and passivated contacts, which could push efficiency and energy yield even further.

Bifacial could also extend the pre-eminence of PERC, by keeping energy yield rising even as innovations in the cell structure reach their practical efficiency limit. Presenting a bifacial perovskite/silicon tandem cell, ECN.TNO’s Gianluca Coletti noted efficiency for a single-sided module would need to be above 30% to match the yield of today’s bifacial models. “Tandem modules can kick off at around 29%,” he told the audience during a panel discussion. “Heterojunction can play a role but bifacial will fill this gap with its additional yield potential.”

More optimization possible

While commercial bifacial installations are already a reality thanks to fast moving innovation from module and balance-of-system (BoS) component manufacturers, further optimization is possible at the system level to raise energy yield. More work is needed though, to determine which can be achieved cost effectively.

Increasing the height modules are installed at is one way to get more light hitting the rear side of modules – though steel consumption in the racking system/tracker will inevitably rise and there may also be wind resistance complications. Experiments with ground covering materials were also presented and white plastic sheeting or white gravel in various configurations below installations were shown to significantly increase energy yield.

Plastic sheeting was shown to provide the largest yield boost and offer the advantage of being easy to clean and replaceable. White gravel tends to darken and become less reflective over time.

Calculations presented by Michael Woodhouse of the U.S. National Renewable Energy Laboratory found that for every $0.04 increase in a PV system’s cost per watt, an additional 3% in annual energy yield is required to balance the cost. A separate study by solar manufacturer Canadian Solar reached similar conclusions.

However, the closing presentation in the Netherlands – by Beth Copanas of U.S.-based engineering, procurement and construction services provider RES Americas – noted that while bifacial reduces BoS costs by having more power from fewer modules, the additional costs of increasing the installation height or adding ground cover material have so far proved prohibitive.

New uses

The ability to install vertically-oriented bifacial panels alone offers new use cases for solar. ECN.TNO discussed a project where bifacial modules have been integrated into a motorway noise barrier and revealed there are plans to extend it across the Netherlands.

Vertical modules were displayed in various agrivoltaic applications, including being integrated into fences between fields and providing shade for farm animals. Research presented by Jean Garcin, from French agrivoltaic specialist Sun’R demonstrated PV installations above crops can offer benefits and that while market models have yet to be developed, such dual use concepts could theoretically greatly reduce land costs. Farmers might even be convinced to pay for PV to protect crops, suggested Garcin, though a tracker system permitting crops to receive sunlight would need to be developed.

While PV has made plenty of progress in the world’s less sunny regions, a study presented by Swiss developer and system supplier SolarSpar found bifacial modules installed vertically in Yakutsk, Siberia could avoid being covered in snow and take advantage of the bright white surroundings to achieve 45% higher yield than a standard tilted installation in the region.