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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.

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