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Material Developed to Reflect Sun’s Heat, Cool Buildings

Shanhui Fan, center, with graduate students Aaswath Raman, left, and Eden Rephaeli.

Shanhui Fan, center, with graduate students Aaswath Raman, left, and Eden Rephaeli. (Norbert von der Groeben, Stanford University)

Engineers at Stanford University in California developed a new type of cooling material based on nanotechnology that radiates sunlight back into space. The team led by Stanford engineering professor Shanhui Fan published its findings earlier this month in the journal Nano Letters (paid subscription required).

Fan, with doctoral candidates Eden Rephaeli and Aaswath Raman, engineered nanoscale photonic materials, electronic conductors that sense and process light waves. The quartz and silicon carbide material is a weak absorber of sunlight, but efficiently radiates heat in the key frequency range needed to escape Earth’s atmosphere.

This new kind of material turns the natural process of radiation cooling upside down. Instead of the normal radiation cooling — where heat escapes into the atmosphere — that takes place at night, this new material makes it possible to radiate heat in bright sunshine, during peak demands for cooling, such as from air conditioning.

“We combine the thermal emitter and solar reflector into one device,” says Raman, “making it both higher performance and much more robust and practically relevant.” The design of the material, Raman adds, “makes viable both industrial-scale and off-grid applications.”

The authors estimate that their new device can achieve a net cooling power in excess of 100 watts per square meter. The cooling panels could replace conventional solar panels on rooftops and achieve much of the same net energy impact. For example, the engineers calculate that a typical one-story, single-family house with just 10 percent of its roof covered by radiative cooling panels could offset 35 percent its entire air conditioning needs during the hottest hours of the summer.

In addition, says Fan, “we can foresee applications for radiative cooling in off-the-grid areas of the developing world where air conditioning is not even possible at this time. There are large numbers of people who could benefit from such systems.”

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