Better than shade: Rooftop material sheds heat into space

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by Scott K. Johnson

Stanford's Linxiao Zhu, Shanhui Fan, and Aaswath Raman, reflected in a prototype disk of their rooftop cooler.

Transforming our electricity generation to renewable sources is rightly the focus of most discussions about the future of energy, but the greenest kilowatt-hour is the one not used in the first place. Yes, there are all kinds of ways to reduce energy consumption, and smarter building designs that do more with less are among those. But buildings use a tremendous amount of electricity to shield us from the summer heat via energy-hogging air conditioning systems. What if we could get some of that cooling for free?

“Passive” heating and cooling is a common approach in green buildings; approaches include things like shading windows from summer sun and floors that absorb and store solar heat in the winter. One new, clever idea is a little more ambitious: just dump some of the summer warmth back out into space.

Using rockets for this is probably out, practically speaking, so the main problem with this approach is that the atmosphere is in the way, and it will absorb many convenient wavelengths. But if you radiate the heat, there’s a small window between the infrared wavelengths of 8 to 13 microns where the atmosphere is transparent. Prototype devices have been built capable of shedding a building’s heat by emitting it in that window. But they can only work at night; during the day, they heat up in the sun, eliminating their ability to reduce the temperature of the building below the outside air temperature. Of course, it’s during the middle of the day that cooling is needed most, so that’s a deal-breaker.

To make this approach work, you would need a peculiar set of characteristics. You need to emit heat energy quite effectively without absorbing much thermal energy from the atmosphere. And, critically, you also need to reflect over 90 percent of sunlight. Aaswath Raman and his Stanford colleagues have managed to develop a material with exactly that peculiar set of characteristics, and it's all described this week in Nature.

The cooler is made of seven incredibly thin, alternating layers of silicon dioxide and hafnium dioxide. These sit on top of a layer of silver and a layer of titanium, with a silicon wafer base. In total, it’s less than a millimeter thick, and almost all of that is the silicon wafer. The thickness of each layer varies, combining to produce just the right optical and thermal properties. While hafnium isn’t very abundant, the researchers say it could be replaced with cheaper titanium dioxide.

To test the cooler, the researchers set up a little rooftop experiment. A disk of the material was mounted on Styrofoam and placed inside a small air space sealed by a thin film of polyethylene. That was placed on a California roof for a sunny winter day and tilted to ensure it received direct sunlight. Over the course of the day, they monitored its temperature, along with the air temperature, the temperature of a black painted surface, and some aluminum.

Air temperature hit about 18° Celsius (mid-60s Fahrenheit) during the afternoon, while the aluminum heated up to around 38°C, and the black paint reached 80°C. The cooler, meanwhile, stayed about five degrees cooler than the air temperature all day.

In order to measure just how much heat this could shed if you connected it to the building’s air system, the researchers wired up an electric heater to the base of the disk and slowly cranked it up. A square meter of the material—in this experimental setup—could provide about 40 watts of cooling, in addition to reflecting rooftop-warming sunlight.

However, using a mathematical model of their cooler, they found it had potential to perform even better. With a housing that better prevented the warming of air or other surfaces from bleeding into the structure, it could reach temperatures as much as 20°C cooler than the outside air.

What kind of savings are we talking about here? The researchers ran the numbers for a hypothetical, three-story office building in Phoenix. Covering the roof with the cooler material, monthly electricity savings were as great as 16,000 kilowatt-hours in the summer. It won’t cool a building on its own, but it will shoulder some of the burden.

With a simple economic analysis, they calculated the cost of this cooling to be less than rooftop solar photovoltaic panels could provide via an air conditioning system. “More broadly,” the researchers write, “our results point to the largely unexplored opportunity of using the cold darkness of the Universe as a fundamental renewable thermodynamic resource for improving energy efficiency here on Earth.”

Nature, 2014. DOI: 10.1038/nature13883  (About DOIs).

http://arstechnica.com/science/2014/11/better-than-shade-rooftop-material-sheds-heat-into-space/