Passive Cooling Uses Outer Space as a Heat Sink
Mark Atwater posted on November 30, 2014 |

Air conditioning is a major burden on the electrical grid and our energy independence. If we can keep cool without spiking the meter, we help out our wallets and our planet. Short of living and working in hillsides like hobbits, we don’t have many good choices yet. Stanford engineers have developed a very thin coating which may have a big impact.

Roofs are ground zero for heat absorption. The solar energy pounding down on them can be harnessed for energy (solar roofs), reflected away (white roofs), or it can be absorbed by vegetation (green roofs). All of these approaches address the solar impact, but they aren’t active in radiating out the heat buildup within a structure. That’s where a research team led by electrical engineering Professor, Shanhui Fan, and research associate Aaswath Raman have gone one step farther.

As described in a Stanford News article, the team has come up with a one-two punch for reducing heat, and thereby energy consumption, by applying a very thin coating. The coating, which is only 1.8 microns thick, allows radiative release of heat from inside a building and also reflects 97% of incoming sunlight.

The material is comprised of seven alternating layers of silicon dioxide and hafnium oxide topped with a layer of silver. The material allows the release of infrared radiation to achieve passive cooling, but the layers are carefully graded in thickness to “tune” the properties of the emitted energy.

This specific tuning of the infrared wavelength allows the heat to be released without warming the air near the building. Rather, it travels directly into outer space. Outer space provides a nearly inexhaustible dumping ground for heat. The ability to selectively radiate heat which doesn’t readily absorb in the atmosphere creates a “window to space.”

This “photonic radiative cooler” is able to reduce the air temperature by 9 degree Fahrenheit from ambient during the day. That’s no small accomplishment from a thin film. As the team acknowledges, however, there are hurdles. One is scaling up manufacturing, but that is something they believe is achievable with current technology.

More fundamental issues exist. First, how do you get the infrared heat to the thin film? Unless the building is not insulated, the transfer of heat won’t be especially efficient. What would a city with reflective roof tops do to air travel? How do you “turn it off” in the winter so you can retain heat? This tunable radiative approach was first conceptualized to increase the efficiency of solar panels, but there are obviously many other uses. Maybe as a coating on a vehicle’s sunroof? You can start with mine, if you’re interested in the idea.

The group published a letter in Nature that can be read here.


Images: Stanford News

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