Team Germany's Solar Home Turns to High-Tech Phase-Changing Materials: Solar Decathlon

Team Germany's members may all be new to the Solar Decathlon this year, but they learned from the previous team's house—and though it won first place in 2007, they strived to improve upon it. The team increased the size of the building but decreased the area of the windows in order to reduce solar gain; they also placed them more strategically within the structure. As a result, Germany's house is a two-story cube with windows concentrated on the northern and southern sides. The windows are also shaded by automated louvers and triple glazed to have a U value below 0.8.

"We were able to increase the space inside by 10 square meters [108 square feet] by reducing the thickness of the walls," Martin Zeimer, the team's architectural adviser from Technische Universität Darmstadt, says. The facade is made of custom vacuum insulation panels covered with CIGS (copper indium gallium diselenide) thin-film solar panels. "The whole building is solar active," Zeimer says. "We wanted to increase solar collection, and the surface of the building is normally the most expensive aspect of the wall, so we were able to spend all that money on photovoltaics instead."

The roof supports 11.1 kilowatts of photovoltaic panels, too, but the team expects 200 percent of the home's energy needs to come from the thin-film on the facade—even though CIGS efficiency is only 11 percent, lower than the 18 percent efficiency of the monocrystalline rooftop array. If the thin-film heats up, it doesn't lower its efficiency as much, Zeimer says, and it also copes better with indirect solar radiation, which meant that they were able to cover the house's north wall as well.

Inside, the house is one large room with a second-story gallery. "One of the main goals of the building is not to go out as architects and tell people how to live, but to let people use the building to reflect how they live," Zeimer says. An electric air-source heat pump for heating and cooling the house and providing hot water is concealed in a closet by the bedroom—essentially, a bed that pulls out from a low step leading up to the center of the room.

Supplementing the heat pump for cooling the room is 1 ton of phase-change material in cavities in the ceiling. Warm air is absorbed up into the salt hydrate, which makes it fluid. After about a day and a half, that energy needs to be released, so PV panels in the facade open, exposing the cavities to the cool night air. The air moves through them, deloading the system, and the salt hydrate returns to a phase that resembles ice.

After the Solar Decathlon, the house will be shipped back to Germany. The team hopes to do more research on the building. "We know the technologies run well individually," Zeimer says, "but we don't know how they run as a system."

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