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Solar shines through

Semi-transparent arrays expand the possibilities for integrating solar with glass

July 01, 2002 |

When students enter the Wintergarden sunroom at the University of Wisconsin, Green Bay's new Mary Ann Cofrin Hall, most won't realize that the daylight illuminating the study space also helps to power the building. That's because the room's 2,000 square feet of glass curtain wall integrates the latest technological advancement in building-integrated photovoltaics (BIPV): semi-transparent solar arrays.

Called amorphous-silicon (a-Si) thin-film, the modules generally allow 5% light transmittance, depending on the manufacturer, and can be "etched" to meet higher light-transmittance requirements. The technology has been used overseas for years, especially in Europe and Japan, but a-Si is rather new in the U.S. It provides owners and designers with even more possibilities for integrating solar with glass exterior applications, especially when daylight control is an issue, because a-Si can potentially replace traditional ceramic fritted and low-e glass.

"Solar is no longer a support module; it's now being embraced by architects as part of the building's design," says Steven Strong, president of Solar Design Associates, Harvard, Mass. The firm served as consultant to the Cofrin Hall project, which was completed last September.

Micro-thin solar sandwich

The a-Si modules at Cofrin Hall consist of 13 layers of a-Si film — all together thinner than one piece of paper — sandwiched between two panes of glass and installed in a curtain-wall framing system. A total of 252 solar modules replace traditional, double-pane glass, generating about 12,500 kWh of energy annually.

The system was custom manufactured by Viracon Inc., Owatonna, Minn., using 43-watt, thin-film modules from BP Solar, Linthicum, Md., and was installed in a curtain-wall framing system from Kawneer Co., Norcross, Ga. The modules were etched by BP Solar to achieve the 10% light transmittance desired by design architect HOK, St. Louis.

Solar gas stations

One of the latest and largest installation of a-Si thin-film is actually part of an ongoing self-promotion by BP Solar's parent company, British Petroleum, to install photovoltaic canopies above its gas-station pumps at locations worldwide. Approximately 600 2-by-4-in., 40-watt modules make up each canopy, which produces about 10% to 15% of the site's overall energy needs.

Crystalline vs. amorphous

The idea of combining solar with glass is not a new one, according to Strong. In fact, building teams have been integrating small (4-in.-sq.) crystalline-silicon solar wafers into glass systems for more than a decade to collect energy while diffusing daylight. Case in point is the Thoreau Center for Sustainability at Presidio National Park, San Francisco, where a skylight with crystalline cells spaced 1/2-in. apart allows 17% light transmittance.

Crystalline is the more widely used and efficient variety of PV at 10% to 14% efficiency. Thin-film is less expensive and less efficient, at 4% to 6%. Both types, however, have already tested at much higher efficiencies — around 30% — in R&D labs, pointing to a future where PV will offer more significant paybacks.

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