A new way to cool buildings without electricity may be emerging.

Researchers from Stanford University have developed a cooling system that involves specialized optical surfaces that can cool flowing water to a temperature below that of the surrounding air without using electricity.

“This research builds on our previous work with radiative sky cooling but takes it to the next level,” Aaswath Raman, a former research associate at Stanford, said in a statement. “It provides for the first time a high-fidelity technology demonstration of how you can use radiative sky cooling to passively cool a fluid and in doing so, connect it with cooling systems to save electricity.”

Radiative sky cooling is a natural process that results from the movements of molecules releasing heat. It is particularly noticeable on cloudless nights where the radiating heat can more easily make it through the Earth’s atmosphere, all the way to the cold reaches of space.

“If you have something that is very cold—like space—and you can dissipate heat into it, then you can do cooling without any electricity or work,” Shanhui Fan, professor of electrical engineering, said in a statement. “The heat just flows.

“For this reason, the amount of heat flow off the Earth that goes to the universe is enormous.”

The researchers used a multilayer optical film that reflects about 97 percent of the sunlight while simultaneously being able to emit the surface’s thermal energy through the atmosphere. Without heat from sunlight, the radiative sky cooling effect can enable cooling below the air temperature, even on a sunny day.

“With this technology, we're no longer limited by what the air temperature is, we're limited by something much colder:  the sky and space,” Eli Goldstein, a former research associate, said in a statement.

The researchers originally used small wafers of a multilayer optical surface—about eight inches in diameter—to show how the surface itself cooled.

They then created a system where panels covered in the specialized optical surfaces sat atop pipes of running water. They tested it on top of the roof of a building at Stanford in 2015.

With the water moving at a relatively fast rate, the researchers found that the panels were able to consistently reduce the temperature of the water by three to five degrees Celsius below ambient air temperature over the course of three days.

The researchers also simulated how the panels could cool a two-story commercial office building in Las Vegas and calculated how much electricity they could save if in place of a conventional air-cooled chiller, they used a vapor-compression system with a condenser cooled by the panels. They found that they could save 14.3 megawatt-hours of electricity in the summer months, a 21-percent reduction in the electricity used to cool the building.

The three researchers created SkyCool Systems, a company focused on further testing and commercializing the new technology.