MIT Technology Review

Thursday, June 04, 2009

Roll-Up Solar Panels

A startup is making thin-film solar cells on flexible steel sheets.

Xunlight, a startup in Toledo, Ohio, has developed a way to make large, flexible solar panels. It has developed a roll-to-roll manufacturing technique that forms thin-film amorphous silicon solar cells on thin sheets of stainless steel. Each solar module is about one meter wide and five and a half meters long.

As opposed to conventional silicon solar panels, which are bulky and rigid, these lightweight, flexible sheets could easily be integrated into roofs and building facades or on vehicles. Such systems could be more attractive than conventional solar panels and be incorporated more easily into irregular roof designs. They could also be rolled up and carried in a backpack, says the company's cofounder and president, Xunming Deng. "You could take it with you and charge your laptop battery," he says.

Amorphous silicon thin-film solar cells can be cheaper than conventional crystalline cells because they use a fraction of the material: the cells are 1 micrometer thick, as opposed to the 150-to-200-micrometer-thick silicon layers in crystalline solar cells. But they're also notoriously inefficient. To boost their efficiency, Xunlight made triple-junction cells, which use three different materials--amorphous silicon, amorphous silicon germanium, and nanocrystalline silicon--each of which is tuned to capture the energy in different parts of the solar spectrum. (Conventional solar cells use one primary material, which only captures one part of the spectrum efficiently.)

Still, Xunlight's flexible PV modules are only about 8 percent efficient, while some crystalline silicon modules on the market are more than 20 percent efficient. As a result, Xunlight's large modules produce only 330 watts, whereas an array of crystalline silicon solar panels covering the same area would produce about 740 watts.

United Solar Ovonic, based in Auburn Hills, MI, is already selling flexible PV modules. The company also uses triple-junction amorphous silicon cells, and its modules can be attached to roofing materials. But Xunlight's potential advantage is its high-volume roll-to-roll technique. "If their roll-to-roll process allows them to go to lower cost and larger area, that's the central advantage," says Johanna Schmidtke, an analyst with Lux Research, in Boston. "But they have to prove it with manufacturing."

Other companies, notably Heliovolt and Nanosolar, are in a race to make thin-film panels using copper indium gallium selenide (CIGS) cells. These have shown efficiencies on par with crystalline silicon and can be made on flexible substrates. In comparison with amorphous silicon, CIGS is a relatively difficult material to work with, and no one has been able to create low-cost products consistently in large quantities, says Ryan Boas, an analyst with Photon Consulting, in Boston.

Building integrated photovoltaics (BIPV), especially rooftop applications, would be the biggest market for flexible PV technology, Boas says. That's because flexible products are inherently very light, in addition to being quick and easy to install. "Imagine carrying a roll of flexible product on the roof and unrolling it," he says. "Workers are already used to unrolling roofing material."

But there are hidden risks and costs associated with BIPV, Schmidtke says. "BIPV is often touted as low cost," she says, "but in actuality, you've got greater risk in terms of a watertight system [for roofing materials] or fire risk, and that increases total installation cost." However, BIPV does have the advantage of being more aesthetically pleasing, which is important to consumers, she says.

So far, Xunlight has raised $40 million from investors. In December, the state of Ohio gave the company a $7 million loan to speed up the construction of a 25-megawatt production line for its flexible solar modules. The company expects to have commercial products available in 2010.