We noticed you're browsing in private or incognito mode.

To continue reading this article, please exit incognito mode or log in.

Not an Insider? Subscribe now for unlimited access to online articles.

Sustainable Energy

The Sky May Hold the Secret to Efficient Air-Conditioning

An unconventional approach to cooling sends heat to the cold sky.

Air-conditioning accounts for almost 15 percent of all energy use by buildings in the United States. One way to cut that is to send heat to outer space, according to Aaswath Raman.

Raman is the cofounder of a new company called SkyCool Systems. The startup, spinning out of Stanford University, is one of several groups commercializing an energy-efficient cooling approach that takes advantage of the coldness of space.

The idea is to exploit a natural phenomenon called radiative cooling. All objects emit thermal radiation. When it’s emitted toward the sky, a portion of it is absorbed and reflected by the atmosphere. Another portion, which falls within a particular range of frequencies, escapes into the upper atmosphere and outer space, where conditions are much colder. This can cause the object emitting that radiation to cool to below the temperature of the surrounding air.

SkyCool is developing a technology meant to exploit this phenomenon, based on relatively recent advances in the ability to manipulate light at the nanoscale. Engineers have known for a while that radiative cooling is useful for cooling buildings at night. During the day, however, the sun’s radiation counteracts the cooling effect. But a few years ago Raman and a colleague at Stanford determined that it should in fact be possible to achieve radiative cooling during the day.

SkyCool's radiative cooling prototypes.

In 2014, the group published a paper in Nature in which they showed that a device designed to combine the optical properties of three different materials, arranged in stack of multiple layers, cooled to nearly 5 °C below the ambient air temperature. This proved that “the cold darkness of the Universe” can be used as a renewable resource, “even during the hottest hours of the day,” wrote the researchers.

In the past two years, several more groups have begun pursuing daytime radiative cooling. Three teams recently got funding from ARPA-E to develop technologies for thermal power plants, which right now consume huge amounts of water.

A conceptual illustration of a future rooftop system featuring SkyCool’s radiative cooling technology.

Raman says his company is exploring a range of potential applications, in buildings as well as many kinds of structures beyond “what we think of as conventional buildings”—in the developing as well as the developed world.

An existing model for SkyCool’s general approach, he says, is the fairly recent development and commercialization of techniques for applying optical coatings to make windows more energy efficient.

Raman says the company has also shown that its prototypes can significantly lower the temperature of water, meaning it should be possible to “plug this into a wide range of cooling and refrigeration systems” that use cooled water to remove heat from the air. For typical buildings in North America, he adds, “you will want to use this in conjunction with an existing cooling or refrigeration system.”

Srinivas Katipamula, a staff scientist at the Pacific Northwest National Laboratory who studies advanced heating and cooling concepts, recently conducted a modeling study with three colleagues at PNNL and found that daytime radiative cooling could reduce the energy consumption of a medium-sized office building by 30 to 50 percent. Crucially, though, it’s not yet clear how much the technology will cost, so it’s hard to know how long it would take to pay back the investment, he says.

Be the leader your company needs. Implement ethical AI.
Join us at EmTech Digital 2019.

Register now
SkyCool's radiative cooling prototypes.
A conceptual illustration of a future rooftop system featuring SkyCool’s radiative cooling technology.
More from Sustainable Energy

Can we sustainably provide food, water, and energy to a growing population during a climate crisis?

Want more award-winning journalism? Subscribe to Insider Plus.
  • Insider Plus {! insider.prices.plus !}*

    {! insider.display.menuOptionsLabel !}

    Everything included in Insider Basic, plus the digital magazine, extensive archive, ad-free web experience, and discounts to partner offerings and MIT Technology Review events.

    See details+

    Print + Digital Magazine (6 bi-monthly issues)

    Unlimited online access including all articles, multimedia, and more

    The Download newsletter with top tech stories delivered daily to your inbox

    Technology Review PDF magazine archive, including articles, images, and covers dating back to 1899

    10% Discount to MIT Technology Review events and MIT Press

    Ad-free website experience

You've read of three free articles this month. for unlimited online access. You've read of three free articles this month. for unlimited online access. This is your last free article this month. for unlimited online access. You've read all your free articles this month. for unlimited online access. You've read of three free articles this month. for more, or for unlimited online access. for two more free articles, or for unlimited online access.