For years solar companies have wanted to make lightweight, flexible panels that are cheap to ship and easy to install (by unrolling them over large areas). But they’ve been held up by a lack of good and affordable glass substitutes.
Now 3M thinks it’s found a solution. This week the company unveiled a plastic film that it says can rival glass in its ability to protect the active materials in solar cells from the elements and save money for manufacturers and their customers.
The protective film is a multilayer, fluoropolymer-based sheet that can replace glass as the protective front cover of solar panels, says Derek DeScioli, business development manager for 3M’s renewable energy division. Manufacturers laminate the sheets onto the solar panels to seal them tight and shield them from moisture and other weather elements that can be deadly to the solar cells inside.
The film is 3M’s answer to demand by solar-panel makers–particularly manufacturers of certain thin-film solar cells–for an alternative to glass. Glass has been the armor of choice because it’s cheap, weather-resistant, and durable enough to last decades. The vast majority of the solar panels made today rely on glass as the top cover. But glass also adds weight and bulk to solar panels, and it must be packaged carefully to keep it from breaking, adding to shipping costs. By replacing glass, the new film can do away with the need for supporting racks, which is particularly useful on roofs that can’t bear a lot of weight. Blending solar panels into roofs also can overcome aesthetic objections by homeowners.
“Flexible solar panels have all these great-sounding benefits, but then you come to the question of how you encapsulate them. For many years people didn’t appreciate this problem,” says Steven Hegedus, a scientist at the Institute of Energy Conversion at the University of Delaware.
Using plastic to protect solar cells isn’t a new idea. You can find plastic-covered solar cells in camping gear and novelty gadgets such as backpacks with built-in solar-energy chargers. But this type of plastic film isn’t designed to withstand continuous outdoor exposures for 20 to 25 years, which is how long solar panels are supposed to last, Hegedus says.
Several other companies have recently started to market plastic front covers for flexible solar panels, but 3M’s material has orders of magnitude higher performance in terms of keeping out moisture, which is key to long life.
United Solar Ovonic is the only major thin-film maker that has been shipping flexible panels for years. The Michigan company uses amorphous-silicon, which also isn’t as sensitive to moisture as other emerging compounds. Its amorphous-silicon solar panels can only convert about 7 percent of the sunlight that falls on them into electricity, a low efficiency rate that has rendered Uni-Solar’s products less desirable. Uni-Solar uses a fluoropolymer-based resin from DuPont as the top coat for its panels.
Solar cells made of a semiconductor material containing copper, indium, gallium, and selenium (CIGS) can be almost twice as efficient as amorphous silicon, but are more sensitive to moisture and so require better protection from the elements. Many CIGS manufacturers already have come up with manufacturing processes to deposit the compound onto rolls of pliable metal foil or plastic. The companies say they can make solar panels faster using this roll-to-roll process; achieving higher production volumes is crucial for reducing manufacturing costs. But they have relied on glass for the front sheet because of a lack of suitable plastic. Aside from CIGS solar-panel makers, 3M also is targeting developers of cadmium-telluride and organic thin films, DeScioli says.
Minnesota-based 3M set out to develop a more durable front casing, and the result is a plastic film that is 23 micrometers thick, much thinner than the 3,000-micrometer glass typically found on solar panels today, DeScioli says. The company uses fluoropolymer because the material doesn’t allow water to seep through easily, and it is resistant to high temperatures and ultraviolet radiation. 3M also engineered the film to prevent it from reflecting much sunlight. 3M says its film can achieve water vapor transmission rates of less than 0.0005 grams of water per square meter per day. Other front barrier films can let in hundreds of times more moisture.
DeScioli says the film can be laminated in the same roll-to-roll process used to deposit thin-film semiconductors, and that can shave production costs. Flexible solar panels also can be larger than glass panels because the flexible variety doesn’t require the support of a racking system and can be easier to transport. The time and costs for assembling an array of large panels can be significantly less than putting together many small panels, he says.
“The primary value of our film is, it enables our manufacturers to make larger modules with the roll-to-roll process,” DeScioli says. He says that while the material itself will not cost less than glass, it will reduce overall costs by saving on manufacturing and installation.
3M is making the film at a pilot production line and plans to mass-produce it next year. The company says it has lined up customers, but DeScioli declines to disclose them.
Source: http://www.technologyreview.com/energy/26494/