Busbars. When the largest debate in the last few years has been whether it is one word or two, it may come as a surprise that the solar cell component is suddenly a popular product buzzword. The industry standard has been to use three busbars or ribbons to connect solar cells to each other and allow for the flow of electrons. As R&D for solar cell improvement is hitting its perceived limit, the next best thing to focus on is limiting cell failures and boosting power output by switching up the number of busbars doing the grunt work.
Companies like Suntech, Mitsubishi Electric, Canadian Solar and REC moved to four busbars in the last two years as a logical first step. Others made more drastic moves.
SolarWorld is the latest manufacturer to move right into five-busbar designs, along with others like Trina and CSUN. Solaria’s technology (which has been tapped by SunEdison) abandons busbars entirely.
What’s the best path to take?
Busbars wire solar cells together to create higher voltages. As you add more busbars, the idea is that more electrons are able to pass through, and power and efficiency increases. Suntech’s one busbar improvement generates five more watts per panel than its traditional three-busbar models.
For SolarWorld, the decision to skip four-busbar production and go straight into five came down to getting that competitive edge.
“SolarWorld has decided that we want to be on the leading edge of power and efficiency,” said Steve Pecis, U.S. vice president of operations at SolarWorld. “If you look at some of the big manufacturers, our product portfolio compares pretty well with them for our power performance. We saw that moving to five busbars fit well on our technology roadmap and gave us very good technology performance for what we need.”
SolarWorld’s new solar panels use passivated emitter reach cell (PERC) technology and two additional busbars to boost power (by 2%) and increase efficiency. SolarWorld’s five-busbar, 60-cell panels have approached the nameplate power of competitors’ 72-cell (presumably three- or four-busbar) panels. Five-busbar product is in the field already, but the company sees 2016 as the transition year to switch all its manufacturing lines to the new cell design. Besides adding additional busbars, SolarWorld rethought the whole module to improve the overall process.
“Think about it as an integrated system—everything from the wafer that’s going into the panel to the cell process that you use in the panel to how you connect all those cells together to make a finished module,” Pecis said. “You can improve the efficiency [and] performance of your panel by working on all the different pieces of that value chain. We’re making sure the actual cell design, the layout, the spacing, the solder pads are absolutely specifically optimized for the five-busbar implementation. The ribbon is a different dimension. Those changes can’t be ignored. If they’re ignored, they’re at the risk of taking risk with the long term performance.”
With all panel manufacturers trying to get more power out of their products and using new technologies like PERC, busbars had to be improved to keep up.
“The industry is finding that the interconnection, connecting the cells together in the module, how you’re wiring them together, is starting to turn into a limiting factor,” Pecis said. “It’s the next obvious step because they’ve gotten those other optimizations in the other part of the module.
“For manufacturers that are pursuing a high efficiency roadmap, adding more busbars and improving the series of resistance of the modules is definitely an appropriate strategy.”
It wasn’t a surprise to hear global panel manufacturer SunEdison make its own announcement about busbars last year. It was surprising to hear it was going a different route and getting rid of the traditional ribbons entirely.
Before the company filed for bankruptcy, SunEdison began working with module and technology producer Solaria to produce its new 400-W ZERO WHITE SPACE line of modules, which were expected to hit the market by 2017 (that road map could still be in place, but the industry is in wait-and-see mode right now). Solaria’s technology—which is also being shopped by other panel manufacturers—overlaps solar cells, and the connection is made directly between cells without using busbars. Solaria said this provides a 15% improvement in performance, mostly because there are more cells occupying the surface area of the module. There’s also no shading from the eliminated busbars and no threat of microcracks caused by busbar soldering.
“Silicon panels are very reliable, but when they do fail, it’s because these ribbon wires expand and contract more than the silicon cell,” said Solaria CEO Suvi Sharma. “Over time, that stress can have an effect on the impact on the cell and cause microcracks, and you have reliability issues there because you get hotspots. We eliminate the biggest failure mode in the module.”
Solaria’s busbar-less cell design allows for more flexibility in module size. Modules can be as big or as small as a customer wants.
“Silicon cells are 6-in. by 6-in. for the most part, and you’re limited to the size of modules you can do—traditionally 60 cells or 72 cells,” Sharma said, because cells have to be spaced in even amounts. “If you want to go bigger you have to go much bigger, like 96 cells. We can go to any size and you can do bigger modules more reliability.”
SolarTech Universal, a new module manufacturer in Florida, uses a similar busbar-less design with its SmartWire technology. Citing greater light penetration and reduction of cell shading, SolarTech’s solar panels have a high peak watt power and a lower cost per kilowatt hour.
LG revamped its cell connection process, swapping the traditional three busbars for multiple tiny wires (like SolarTech). The company uses Cello technology—which stands for cell connection, electrically low loss, low stress and optical absorption enhancement. Twelve circular-shaped wires scatter light, which should lead to better absorption and a reduction in electrical loss by spreading the current. The LG NeON 2 product also produces energy from the panel’s backside, doubly absorbing light.
Seraphim Solar also has a no-busbar module with its Eclipse line. Similar in size to mainstream 60-cell panels, the company cites reduced hotspot effects and increased power output as the module’s selling points. Because of their higher power, Seraphim Solar says the Eclipse modules save 8% roof/ground space for the same size power installation using traditional modules.
Solaria’s focus has always been on R&D to make improved modules, Sharma said. Now the company has come up with a module that is similar in cost to traditional panels, but it has higher power and is more reliable—all without busbars.
“We believe that in the future, if not all, most silicon modules will be made the way we’re doing today,” Sharma said. “Our approach—rather than be a pure module maker, we want to enable the industry to make better, more reliable panels with the technology.”