A Tunable Material for Capturing and Releasing Heat

The sun is a lovely, ecofriendly, renewable source of warmth and there are many ways to use it to heat our buildings and homes. But it’s not always winter. Any passive material that we might use to soak up the sun’s rays for heat becomes a nuisance when we’re trying to cool down. 
 
But now researchers at Duke University are developing a material for our walls, and perhaps someday our windows, to use the sun when we want warmth and block it when we don’t. 
 
We’ve had the materials to do both for centuries. Black things absorb radiative heat and mirrors reflect it away.

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“Why don’t we combine them and provide tunabilty,” asked Po-Chun Hsu, a professor in the university’s department of mechanical engineering and materials science and co-author of the paper “Ultra-Wideband Transparent Conductive Electrode for Electrochromic Synergistic Solar and Radiative Heat Management.” The paper appeared in American Chemical Society’s Energy Letters in October. “We have seasons and you do want to utilize whatever that’s renewable and sustainable around you, as much as possible.” 
 
Hsu and his colleagues initially created a surface that could mechanically switch between two materials, rotating them like an ad at a bus top. Though their creation proved the effectiveness of the method, they found that architects and building engineers didn’t want to rely on moving parts that are both bulky and prone to breakage. 
 
So they turned to the relatively new world of electrochromic windows. These smart panes can switch from opaque to clear when a material, sandwiched between two electrodes, is zapped with a bit of electricity. Unfortunately, they’re only effective for visible light, and are unable to block the solar to mid-infrared wavelengths that bring heat. For that they needed a new material. 
 
“It’s hard to manipulate such a large spectral regime and also in the exact opposite way,” said Hsu. “Turning to nanoparticles was definitely one of the ‘aha’ moments that we had.” They used a monolayer of graphene with a gold micro grid to get wideband transmittance as well as the conductivity needed to induce the electrochemical reaction needed to go dark. When in a cooler environment that layer can be turned transparent, revealing a mirror which reflects away the sun’s radiation. 

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“Because it’s intrinsically reversable, we can create this solar absorber and make it disappear and create it again and then make it disappear,” said Hsu. 
 
A material with such a highly tunable opacity would be useful in space, where temperature differentials are stark. “I’m definitely very very excited about any possibility that it can help us either maintain the temperature of devices in space missions, or one day help human on mars,” said Hsu. 
 
Right now, the material has only been made and demonstrated in the lab, and the first prototype is just a few centimeters wide. But its size is not all that will have to change before our walls—and satellites—are covered with it. For one thing, to produce the metal, they use silver and an organic solvent, making application to large swaths of building exteriors both impractical and expensive. And, currently, when in cooling mode, the material still traps some heat (in part because the electrolyte they are now using is a bit reddish). Much of that can be fixed by using different materials and Hsu has some hope that switching to aqueous solvents and electrolytes will both lower the cost and increase the ability to cool.
 
“I think that the good thing is really to demonstrate that we have this platform to continue,” said Hsu. “By changing the material, hopefully we can improve the performance, little by little.”
 
Michael Abrams is a technology writer in Westfield, N.J.