There’s a big renewable energy resource we’ve yet to exploit: the cold of the universe.
That’s according to Aaswath Raman, a researcher with the Ginzton Laboratory, at Stanford University. What he means is the capacity of space to be a “thermodynamic resource,” helping us Earthlings to be more energy-efficient. Raman is working on a building material that’s ultra-reflective to the sun’s rays, and able to bounce significant amounts of heat so far away from buildings that it goes beyond the atmosphere.
The technology—passive cooling that works during the daytime—could help reduce the need for wasteful air conditioning. Currently, A/C accounts for 15% of all primary U.S. electricity demand, an enormous amount considering all the other things we use electricity for.
The material exploits the way that all objects throw off heat as infrared radiation. Some of this unseeable light is trapped within the atmosphere, all the more so because of greenhouse gas emissions. But some heat also goes into the upper atmosphere, where it’s dispersed into an enormous heat sink: the freezing cold vacuum of space. Raman’s coating–made up of seven layers of silicon dioxide and hafnium oxide and a layer of silver–does two things. First, it’s able to send the infrared at a particular frequency so that it leaves our atmosphere. And second, it reflects 97% of sunlight, stopping buildings from heating up in the first place.
“The challenge was to continue to send out this heat as infrared light, or radiation, but at the same time not be heated up by the sun,” says Raman.
Scientists have long known about the radiative cooling effect, where buildings become cooler than the air during the night. But Raman’s team wanted to exploit the effect during the day, when the need for cooling is greatest.
They haven’t finalized how to use the coating yet. It could be sprayed across a rooftop. Or, more likely, it could cover tubes of water taking heat away from building cooling systems. “We can cool water below air temperature without having to evaporate it, and only needing electricity to pump water through panels,” Raman says. “You can think of this as a solar water heater, except instead of warming the water, it’s doing the opposite. It’s cooling the water.”
One day, perhaps, we’ll see three types of solar panel on our rooftops. One set for generating electricity. One set for heating water. And another reducing air conditioning costs by expelling heat more efficiently. Raman says the cooling panels can reduce cooling loads by 20% even on one or two story buildings.
“Our goal is to show we can do this effectively and in a way that is commercially compelling,” he says. “This is very cost competitive especially in hot and dry climates where there’s a big need for air conditioning.”