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.