The Charm Of Bifacial Solar

It is impossible not to fall in love with bifacial technology. Its charm and promise are simply so enticing.

While there are different opinions about the bifacial path to go, it's clear that the technology has now started to become rapidly every one's favorite – from cell makers, module makers to system installers. Even developers are very interested on the promise of even lower LCOEs.

Why? Because bifacial solar simply opens up for sunlight absorption the rear surface of a PV device, which is otherwise useless. Depending on cell and module technology, and most importantly the installation site's albedo, a bifacial system can generate a 5 to 30% higher power yield.

The technology is actually pretty old. But several factors coming together have now made bifacial ready for prime time:

• First, on the cell technology front: All advanced cell architectures are capable for bifacial technology. Recently, the industry started quickly to move away from long-year standard BSF towards high-efficiency.

• Over the last few years, PERC has rapidly developed into the new cell standard in the p-type monocrystalline segment. While PERC is not bifacial intrinsically, the effort to go bifacial is very minor, offering companies to boost their PERC investment to a higher level. In fact, PERC has a relatively low bifaciality of about 65 to 70%, but with improved aluminum pastes, the fingers widths have been reduced considerably – 50 μm in R&D. This is enabling PERC to reach a bifaciality close to 80%, as attained by LONGi in its pilot lines.

• Other advanced cell architectures, such as n-PERT and heterojunction are mainly based on n-type wafers. They come with naturally higher bifaciality of up to 97% for HJT and some even offer a higher efficiency promise than PERC. With the price gap between n-type and p-type narrowing, and low cost alternative processes becoming available, these developments will also support the transition to bifacial in the mid-run. In fact, the number of companies looking into n-type wafer based advanced cell architecture is quickly growing. While its high bifaciality is not the deciding factor, it is definitely a motivating aspect to look into n-type and its high efficiency and low cost potential.

• On the module level, the major change to bifacial means to opt for a transparent rear cover, which offers module makers now another argument to push glass-glass products, which have not really found a large market so far despite the promise of 30-year warranties. While glass is the current state of the art for nascent bifacial solar, backsheet suppliers are now working hard to push transparent solutions, promoting low weight and several other advantages, which are the reasons why glass- backsheet configurations dominate over glass- glass today. Leading backsheet component supplier DuPont, for example, has just released next generation transparent variants of its well- known Tedlar product.

• Also balance-of-system suppliers have quickly reacted and have been introducing new products adapted to the needs of bifacial solar.

Of course, there are many issues that need to be overcome. One of the major bottlenecks for bifacial modules is the lack of a testing standard and no common ground for labelling. Half of this problem could be solved soon. An IEC standard on how to test bifacial modules and how to label is in the last round and now expected to be published in early 2019. But a few questions are still hovering. For example, how to ensure bifacial gain? Because, the gain is highly sensitive to installation site, especially its albedo. There is apparently also an issue with system simulation software capabilities to model the right yields for bifacial systems.

However, these are all just teething troubles. The more bifacial systems will be installed, the more companies will gain experience and much needed data about the technology. And that's now happening rapidly. Many companies are setting up large-scale test systems to evaluate the performance of bifacial systems under different operating conditions. This will not only help in improving all sorts of system products but will establish bifacial as a new standard solar technology that is bankable like many others. It's not a question if but only when bifacial solar will fully fly – its charm and promise to cut LCOEs are simply too enticing.

The article was originally published in the TaiyangNews Bifacial Solar Module Technology Report 2018, which was released at Intersolar Europe 2018 and can be downloaded for free here.