Image Courtesy of Stanford University
Passive rooftop nanostructure would be only tens of microns thick and could eliminate active cooling, say Stanford University researchers.

Stanford University researchers are counting on prototype testing of a nanotechnology-based passive solar cooler to demonstrate that their analytical computer model is more than just smoke and mirrors. If brought to market, the rooftop daytime cooler, which would keep temperatures down in buildings and single-family homes bathed in full sunlight, might eventually make chiller-based air-conditioning systems a thing of the past.

"It is hard to say when [the cooler] will be ripe for the market, but we don't think it is far in the future," says Shanhui Fan, a professor of electrical engineering at the Stanford, Calif.-based school.

The cooler, only tens of microns thick, is best described as an ultra- thin mirror that reflects back most of the sunlight that hits it into the frigid vacuum of space. The idea for the nanostructure belongs to two of Fan's doctoral candidates in applied physics, Aaswath Raman and Eden Rephaeli.

"There is a possibility of eliminating all active cooling," depending on a building's power needs and its roof area, says Rephaeli.

The amount of potential cooling is significant, says Fan. If 10% of a roof is covered, cooling needs can be offset by 35%.

There is a patent filed on the radiative cooler design, which consists of nanostructure photonic materials. Two upper crystal layers, made of quartz and silicon carbide, emit sunlight. Alternating lower layers of magnesium fluoride and titanium dioxide reflect sunlight. The substrate is a layer of silver.

For success, the cooler must reflect as much of the sunlight as possible. Poor reflectors absorb too much sunlight, heating up in the process and defeating the goal of cooling, say the researchers.

The cooler must also emit radiation very efficiently within a specific wavelength range. Outside the range, the thermal radiation interacts with the Earth's atmosphere, in what is known as the greenhouse effect.

The team is currently building a prototype to prove the concept under a $400,000 grant from the U.S. Dept. of Energy. Testing should be done within the year.

Rephaeli says much more work is required before the cooler is ready for development. One key task would be to engineer a protective cover.