A material that simultaneously reflects light and radiates heat at frequencies that vent it through the Earth’s atmosphere could one day help cool buildings on hot days. The material cools itself to a temperature below the ambient air, and has been tested on a rooftop at Stanford University by its inventors, who are now working on scaling up the design.
The new material uses optical engineering tricks to behave in ways that are counterintuitive and, at first glance, appear to violate the laws of thermodynamics, says Stanford electrical engineer Shanhui Fan, who developed it.
Usually the way to let something cool off is to put it somewhere cold; the hot object will radiate its excess heat into the surroundings. Fan’s material becomes cooler than its surroundings by reflecting light and emitting heat at carefully tuned frequencies. The material emits heat at frequencies that match the planet’s “thermal window”—from eight to 13 micrometers—which lets it pass through the atmosphere and into space. It effectively cools down by using outer space as a heat sink.
For decades, researchers have been exploring this effect, called passive radiative cooling, to try to make systems that cool buildings more efficiently by radiating heat at night. And like Fan, some have succeeded in making materials that emit heat in the thermal window. But no passive cooling system has previously worked during the day, because all such materials also absorb sunlight. In fact, materials that are very good at emitting heat tend to be black—meaning they absorb a lot of sunlight.
Fan figured out a way to make a material that not only radiates in the thermal window, but reflects light like a mirror. He made the material by layering thin films of alternating layers of silicon dioxide (glass) and hafnium oxide deposited on an eight-inch silicon wafer. The material reflects 97 percent of sunlight and releases heat in the thermal window.
“Something that cools down rather than heating up in the sun is counterintuitive, but that’s what the device is designed to do,” says Fan.
To demonstrate the weird properties of this layered material, the Stanford researchers compared it to silicon painted black—a good thermal radiator—and aluminum—a good reflector. The passive cooler design is described today in the journal Nature.
“These guys have pushed the limits with some clever optical engineering,” says Geoff Smith, an applied physicist at the University of Technology in Sydney, Australia. Smith and others have demonstrated materials and systems for passive cooling of buildings at night. But previous systems could not also reflect sunlight, so they didn’t cool off during the day.
Fan says covering an entire roof with the material should eliminate the need for air conditioning. The group plans to leverage manufacturing technology that’s used to make coated windows, and it might be possible to make the material, which is only about two micrometers thick, on lightweight plastic films for easier installation. But the next step is a modest one: they’ll to go from the eight-inch demo to a square-meter tile of the material.