By Anne Cosgrove
Published in the February 2006 issue of Today’s Facility Manager
Imagine a facility where the building envelope was equipped to heat or cool the interior air. Warmth would literally radiate from the walls and windows when it was cold outside; on a hot day, the building would emit cool temperatures inside. In this scenario, heating and cooling costs would be drastically reduced and the facility’s contribution to greenhouse gas emissions would also decline dramatically.
Such a system—an Active Building Envelope (ABE)—has been developed by a team of researchers at Rensselaer Polytechnic Institute in Troy, NY. Led by Steven Van Dessel, Ph.D., assistant professor of architecture, the team includes Achille Messac, Ph.D., professor of mechanical, aerospace, and nuclear engineering (MANE), as well as students in Rensselaer’s architecture and MANE programs.
The patented ABE system consists of photovoltaic (PV) solar panels which collect and convert solar energy into electricity; there is also a series of thermoelectric heat pumps which receive the electricity and emit the warmth or coolness. The heat pumps, about 1″ square and 1⁄4″ thick, can be placed on the roof, within walls, or in windows.
Whether the ABE heats or cools the building depends on the direction of the electric current supplied to the thermoelectric heat pumps. “A thermoelectric heat pump is a direct current (DC) device,” says Van Dessel. “Two wires—one for positive and one for negative—extend out of the pump; these wires connect to the PV system through a regulating device. If you apply current in one direction, then one surface becomes warm and the other becomes cold. If you reverse the polarity, then there is the opposite result.” To collect energy for use when little or no sunlight is available, an energy storage mechanism is also integrated.
While the technology works, implementation of the current system can be costly and impractical, since it is made from bulk materials. The most feasible application to date, says Van Dessel, is the incorporation of the ABE system into windows. Currently, a prototype of the window system is in operation at Rensselaer. By monitoring the temperature inside the box-shaped unit, the research team continues to learn how to optimize the system.
“We control the temperature inside the box,” explains Van Dessel. “It’s not yet optimal, but we can change the temperature inside more than what occurs with a natural response to outside conditions. With the system running, we can increase the temperature by about 8°C [46.4°F] in the heating mode, and we can decrease the temperature the same amount in the cooling mode.”
Fine tuning interior temperatures can be a challenge. “We’ve found that it’s easier to raise interior temperatures than it is to reduce them. To address this, shades can be placed on the window system to minimize heat gain inside.”
Pioneering facility managers will want to keep an eye out for an even more practical solution on the horizon. The Rensselaer team, with the help of a $300,000 grant from the National Science Foundation (NSF) along with recent advances in materials, is embarking on the development of an ABE system on a much smaller, micrometer scale using thin-film technology.
“In developing the current system, the main focus was to use off-the-shelf components,” explains Van Dessel. He points out, however, “We knew from the outset there were new technologies emerging for the components within our system. There has been a lot of research on thin-film photovoltaic cells with a focus on making these devices less expensive. The second, more recent development is with thin-film thermoelectric devices.” In addition to lower costs and a smaller size, an advantage of thin-film thermoelectric heat pumps is that they are solid state, with no moving parts.
In terms of performance, Van Dessel says, “More critical for this particular technology is that the thin-film materials have a faster response time. Temperatures would rise and fall more rapidly than with the current, bulk materials system, which can take up to 20 minutes to reach a desired temperature.”
In speaking on what building applications might lie in store for a thin-film ABE system, Van Dessel says the most promising would be to integrate the components into double paned windows. “The PV system—which you would be able to see through—would sit on the outside pane of glass, and the thermoelectric heat pumps would be integrated into the window frame. The most successful scenario would probably be for the system to be installed at the manufacturing level.
“Initially, I think it would be most successful in buildings with large surfaces of glass,” Van Dessel explains. He also mentions the possibility of an exterior cladding system manufactured with the ABE technology.
With research into the thin-film ABE system set to begin, facility managers will have to wait and see what develops. In the meantime, the success of Van Dessel’s team to date proves that technology continues to push the limits of how sustainable buildings can be.
Information for this article was provided through an interview with Professor Van Dessel.