Space-radiated cooling cuts power use 21%

Radiative sky cooling sends heat from buildings out into space to be chilled. Electricity use ultimately will be slashed compared to traditional air conditioning, scientists say.

stanford university cooling system
Aaswath Raman

Using the sky as a free heat sink could be a solution to an impending energy crunch caused by increased data use. More data generated in the future will require evermore electricity-intensive cooling — the data centers will be getting bigger.

Researchers at Stanford University think they have a solution to cooling creep. They say the way to reel in the cost of getting buildings cold enough for all the servers is to augment land-based air conditioning by sending excess heat into space and chilling it there.

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The scientists say cost savings will be in the order of 21 percent through a system they’ve been working on, and up to 70 percent, theoretically, by combining the kit with other, newer radiant systems, according to an article in IEEE Spectrum this week.

Also, by beaming heat into space with specialized optical surfaces mounted on roof panels, entire buildings could be cooled without using any electricity at all, they claim.

“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,” says Shanhui Fan, professor of electrical engineering at Stanford, in a Stanford University press release. “The heat just flows.”

At the root of the Stanford system is a set of mirror-like panels mounted on the building's roof.

The panels act similar to a blacktop road surface that dissipates heat stored from the daytime sun overnight — the baked road accumulates the heat during the day in full sun, then as night falls, and as the surrounding air begins to cool, the road-stored heat passes from the blacktop to the cool air, and consequently the road surface becomes cold.

On a cloudless night, the effect is more pronounced because heat radiates into space without obstruction.

The potential problem is that the air temperature must be lower than the road temperature for the heat to dissipate — the heat needs to have somewhere to go. It’s the reason the road doesn’t release its heat during a warm day.

To counter that issue and make the proposed system function 24 hours a day, the university's rooftop system ingeniously reflects interfering sunlight away from the panels during the day, allowing the pent-up heat to radiate all the time.

“Without heat from sunlight, the radiative sky cooling effect can enable cooling below the air temperature even on a sunny day,” the Stanford press release says.

Scaling the space-based system to cool data centers

Stanford University has been working on the electricity-free, space-based system for some years but recently said it has made significant progress scaling it.

In 2015, the researchers added water pipes under the reflective material on an electrical engineering school building, which allowed them to reduce the water temperature below the ambient air temperature by three to five degrees Celsius.

More recently, they transported the experiment to a commercial building in the Las Vegas desert and added a vapor-compression system with condenser, as opposed to a standard air-cooled chiller, and they saved 14.3 megawatt hours of electricity during the scorching, summer months there. That was a 21 percent reduction in cooling electricity use.

With global “demand for cooling expected to grow tenfold by 2050,” according to their academic paper, the researchers “are particularly excited at the prospect of applying their technology to the serious task of cooling data centers,” the university says. They’ve set up a company, SkyCool Systems to promote it, too.

The researchers say their system design allows for integration with existing air conditioning setups, such as those found in data centers.

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