Researchers at Stanford University say they have developed a coating material that if applied to the roof of a building could slash the need for summertime air conditioning.
The ultrathin, multilayered material deals with heat in a novel way by both stopping it before it comes into a building and sucking it out when it's too hot, radiating the heat away from the building and sending it into space.
The Stanford researchers have dubbed what their material does "photonic radiative cooling," a double-barreled effect that can reflect the sunlight that would otherwise cause a building to heat up while also pulling infrared radiation out of the building and into space.
Focusing on infrared radiation -- the heat of invisible light -- allows the researchers to take advantage of a peculiar quality of the Earth's atmosphere.
Trace gases, including carbon dioxide and water vapor, can have a significant impact on the Earth's climate, by preventing heat from re-radiating back out into space, the so-called "greenhouse effect."
However, the Stanford material takes advantage of the fact that certain wavelengths of heat, especially infrared energy, aren't blocked by the atmosphere.
"Venting" that heat back out into space would help create a cooler building and require less air conditioning. Air conditioning systems currently account for around 15 percent of the $180 billion energy cost of commercial buildings in the United States, the scientists say.
The material has shown the capability of reducing the temperature in a building to almost 10 degrees Fahrenheit below outside air temperature, they report in their study published in the journal Nature.
After working with small 8-inch-diameter wafers of the material in their study experiments, the researchers say the next task is to scale up the effort.
"We need to get to a point where I can cover part of your roof," says Shanhui Fan, study co-author and electrical engineering professor. "So there's still a way to go."
The material -- alternating thin layers of silicon dioxide and hafnium dioxide over layers of silver and titanium on a base of silicon wafers -- can most likely be manufactured using existing fabrication techniques, Fan says.
Large-scale deployment of the material on building rooftops in a cost-effective manner is what the material has been designed for, the researchers say.
The Stanford material's ability to both reflect solar energy and pull a building's heat up and away during the hottest daylight hours is most impressive, experts say.
"I was surprised that you could actually accomplish that, because it's very challenging," says Marin Soljacic, an MIT physics professor who is familiar with the Stanford research but did not take part in it. "It has all the right ingredients that make it exciting."
