Bifacial PV and tracking: Q&A with JinkoSolar

JinkoSolar offers two bifacial module product lines: mono-PERC and n-type PERT. Why have you gone for these, specifically, and for which purposes are each most suitable?

Andrea Viaro, JinkoSolar: The P-type silicon technology offers, at present, the most advantageous cost-benefit balance for bifacial modules, as it is based on the reliable and mature mono PERC technology commercially available at GW scale already. On the other hand, the N-type currently represents the high-efficiency solution in the PV industry, which allows to maximize the module peak power and plant performance, thanks to the significantly higher 90% bifacial factor and virtually zero LID.

Our R&D Team is also working on the development of even more advanced solutions such as shingle-cell modules, which are expected to be available soon in the market once the production processes will be optimized.

Why have you used glass-glass for your bifacial modules? Is this necessary to reduce PID vulnerability?

Apart from the obvious necessity of having a clear surface on the rear side of the modules, the double glass encapsulation structure offers the highest protection against cell efficiency decrease phenomena, thanks to the lower moisture ingress into the laminate and reduced cell crack development during the module lifetime. This solution gives us the possibility to extend our Power Warranty up to 30 years, bringing significant and tangible benefits in terms of energy yield and return on the investment.

Polycrystalline modules are very popular in some countries. Are these also suitable for bifacial technology, and does JinkoSolar plan to offer them?

At JinkoSolar we focus on monocrystalline given by the fact that the combination of PERC or PERT technologies used for bifacial with mono cells give the highest efficiency increase and the LID stabilization processes are reliably implemented in mass production since long time already.

Bifacial modules can generate higher yields than other module types in the same region. They are also used in countries with high levels of irradiation. At the same time, they offer the chance to try out new concepts. One idea is to install the modules vertically and, thus, enable the coexistence of PV and agriculture.  What do planners have to consider when choosing bifacial modules for such projects?

Surely the bifacial technology opens the doors for a lot of new applications that were not possible or limited with traditional PV modules, such as vertical installation or advanced level of integration on buildings. The advantage of the glass-glass structure of the modules is not only on the reliability and performance side, but also on the possibility to install modules on greenhouses or farms, where a certain degree of transparency is required and can be, in principle, modulated according to the specific needs of the farming activities.

Tracking systems are also used with bifacial modules. Is it not a disadvantage to have the rear side of the tracked modules in more shade than if they were in fixed rows?

Colin Caufield, Soltec: There is no reason to think that tracker equipment will shade the rear side of the modules more than fixed mounting. This assumption may come from the fact that most other trackers have a torque tube behind the active area of the modules, but our tracker doesn’t. Moreover, our trackers have 46% less piles per MW, no dampeners, 15% less parts count, and 58% less screw connections than our leading competitor, which all results in less rear-side shading factor.

Bifacial module prices have started to come down, close to those of monocrystalline. By combining bifacial modules with solar trackers, it has been said overall gains can increase by 8-9%. Using fixed-tilt, the gain increase is said to be even higher, at around 28%. What’s your opinion on this?

It should be the opposite. With fixed tilt, we obtain an 8-9% increase while up to 35% if modules are mounted on SF7 Bifacial single-axis trackers. In fact, trackers already offer a gain over fixed-tilt systems of 20-30% even with monofacial modules. Then not every tracker is the same. Soltec’s SF7 trackers have a standard height of 7.71 ft (2.35 m), while 1x-configuration trackers have a height of 4.43 ft (1.35 m). Preliminary electric performance measures taken at BiTEC (Livermore, California) over bifacial modules reveal a short-circuit current difference of over 2.3 percent between 1x and 2x trackers.

The additional yields of bifacial modules can differ greatly, depending on the environment. Laboratory situations can also produce different results. Which long-term field analyzes and measurements have you already finished and which yields have you determined?

We have recently established BiTEC, the world’s first evaluation center specialized in bifacial trackers, but our history with bifacial tracking dates back to 2015, when Soltec designed and installed a test-bed for utility-scale tracking in Atacama, Chile. The power plant is comprised of Soltec´s trackers with 2x configuration and has exhibited 14.9 percent of gain over monofacial tracking.

The Atacama area has a consistent ground color and conditions throughout the year, whereas in BiTEC we are experimenting with different ground surfaces that simulate a variety of environments. The intention is that we will get data that allows us to more accurately apply findings and assumptions to different locations and surface types.

Fortunately, our results are indicating that even in areas with lesser albedos that are thought of as unsuitable, or at least not ideal, for bifacial applications are still offering gains that justify the extra expense of the bifacial panels. The decreasing costs of bifacial modules are certainly supporting that conclusion. Customers that requested bifacial trackers to us for their bed-tests 3 years ago have now placed orders for big utility-scale solar plants. It proves that bifacial technology is a good investment.