In this illustration, the reflective panel is coated with a material invented by Stanford engineers. They designed it to help cool buildings without air conditioning. The material works in two ways--It reflects incoming sunlight (yellow splotch) that would otherwise heat the building. And it sends heat from inside the structure directly into space as infrared radiation (reddish rays). The blue areas on an otherwise warm roof show the cooling effect.
The way we cool buildings in the summer has a lot of middlemen. What if we could cut out the middlemen and just eject the heat out to space?
Technology being developed at Stanford University could slash the need for summertime air conditioning, which currently uses up about 15 percent of the $180 billion spent on energy for commercial buildings in the U.S.
Lowering a building's temperature typically requires burning coal or gas to generate electricity, which runs the AC, which lowers the mercury. Far easier and cheaper than burning fuel to cool hot places would be to both stop the heat before it comes in and suck it out when it's too hot.
Research published today in the journal Nature describes a new, experimental material that can do those two things. Thin, silicon-based wafers act as both a rooftop mirror, reflecting sunlight and heat skyward, and as a kind of thermal funnel, drawing a building's internal heat up through the roof. Neither requires a volt of electricity.
The Stanford material can reduce the temperature by up to 5 degrees Celsius (9 degrees Fahrenheit), below the outside air temperature, according to the study. Experiments have proven successful on an eight-inch-diameter wafer. The researchers' next job is to scale up the effort. "We need to get to a point where I can cover part of your roof," said Shanhui Fan, a co-author and professor of electrical engineering. "So there's still a way to go."
The reflective panel and coating can probably be manufactured using existing fabrication techniques, Fan said. Scaling-up will require more time, testing and money. The work so far has been supported by the U.S. Energy Department's high-end research agency, ARPA-E. The scientists haven't yet formed a company to commercialize their research.
Reflecting heat and light is easy enough to imagine for anyone familiar with mirrors, or with efforts, like New York City's, to paint roofs white so they heat up less. The real trick is "venting" a building's heat out to space, which has to do with a peculiar feature of the Earth's atmosphere.
Trace gases, such as carbon dioxide and water vapor, have an outsize influence on the Earth's climate, because they prevent heat from re-radiating back out to space. The research takes advantage of the fact that there are wavelengths of heat, or infrared energy, that are not blocked by the atmosphere.
There's a kind of heat window in the atmosphere that no naturally occurring substance, trace gas or otherwise, can block. Set atop a roof, the new material teases this heat out of the building. It could be applied to cars, too. The silicon and hafnium oxides in the wafer absorb and emit heat at the same wavelength that the atmosphere can't block. So it just harnesses energy coming up out of the building and shoots it out of the atmosphere, into the great beyond.
The great beyond is where it gets fun.
For decades, environmental futurists have pointed up at the Sun as the obvious answer to all of our needs. It provides the energy living things need to live and grow. And ever-improving technologies can harness it for residential and commercial power.
The Stanford researchers suggest that what's really interesting isn't the Sun -- but everything else in the universe: the emptiness. The universe is absolutely freezing. Really: It's a few notches above absolute zero. The new Nature paper suggests that humanity should harness what it calls "the cold darkness of the Universe" as a renewable resource. Heat seeks out cold. And there is just no limit to the amount of heat that space would take off our hands.
The technology belongs to a field called nanophotonics -- a wing of nanotechnology that concerns structures tiny enough to interact with light. The Stanford invention appears to successfully reflect solar energy and shuttle the building's heat up, up and away during the hottest daylight hours, when it's needed most.
"I was surprised that you could actually accomplish that, because it's very challenging," said Marin Soljacic, a physics professor at the Massachusetts Institute of Technology. He is familiar with the Stanford research but did not participate in it. "It has all the right ingredients that make it exciting."
A journal research article does not an energy revolution make. It is however, an intriguing technology with backing from a high-profile U.S. agency. It exposes how backwards our current system is of burning carbon minerals to make power to cool buildings. And, to hit the trifecta, it performs the rarest of all feats: it introduces into energy debates a new, renewable resource and a new way of thinking about, literally, the universe: It's the heat equivalent of an infinite garbage dump.