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Mirror

Published on January 9th, 2015 | By: April Gocha, PhD

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Multilayered oxide mirror heats up space to help cool buildings

Published on January 9th, 2015 | By: April Gocha, PhD

 

[Image above] (From left) Doctoral candidate Linxiao Zhu, professor Shanhui Fan, and research associate Aaswath Raman are members of a team that invented a new energy-saving material. Credit: Norbert von der Groeben

 

 

Stanford researchers have developed a new material that they hope will someday heat up space and cool down rooftops.

 

The multilayered material—which the scientists describe as a “one-two punch”—works by reflecting both visible and infrared light away from buildings and into space.

 

The cooling strategy may someday make buildings much more energy efficient by reducing unwanted heat and thus decreasing energy needs. Those energy needs are staggering, as cooling requirements were estimated a couple of years ago to total 1 trillion kWh per year, a significant fraction of the approximately 144 trillion kWh of annual total worldwide energy use (as estimated in 2008).

 

That’s a lot of energy—and a lot of money—that’s spent to cool the places where we live, work, and play.

 

The Stanford scientists think their new material may be able to ease that energy consumption and cost, through a process they call photonic radiative cooling.

 

That material, just 1.8 micrometer-thick, is composed of seven layers of silicon dioxide and halfnium oxide overlaid onto a thin silver film. “These layers are not a uniform thickness, but are instead engineered to create a new material,” according to a Stanford press release.

 

The material’s structure is devised to radiate infrared light at a particular frequency that pushes that energy directly into space, rather than warming the air around the building. It essentially uses space as a heat sink, rather than allowing that heat to be absorbed into the atmosphere.

 

“Think about it like having a window into space,” lead researcher and electrical engineering professor Shanhui Fan says in the release.

 

But that’s not all—the material also functions as a mirror, reflecting up to 97% of sunlight.

0109ctt mirror illustration lo res

The material reflects incoming sunlight and sends heat from inside the structure directly into space as infrared radiation (represented by reddish rays). Credit: Fan Lab; Stanford

“Together, the radiation and reflection make the photonic radiative cooler nearly 9 degrees Fahrenheit cooler than the surrounding air during the day,” the release states.

 

Although the team has focused on making the material cost-effective for large installations, such as on roofs, a few challenges remain. One is how to direct heat from within buildings to the coating for dispersal into space. And the second—a familiar one—is production and scaling. According to the release, the prototype is now the size of a personal pizza, but much larger dimensions will be needed for building-scale cooling.

 

The paper, published in Nature, is “Passive radiative cooling below ambient air temperature under direct sunlight” (DOI: 10.1038/nature13883).

 

 


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