A new material that requires no electricity uses the universe as a heat sink—even when the sun is shining.
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 counter-intuitive 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.
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.
Courtesy: MIT Technology Review