Smart windows keep heat out – but let light in

Charlotte LoBuono, contributor

(Image: Roy Kaltschmidt/Lawrence Berkeley Nat'l Lab)

Sweltering at your desk as the sun beats down? Don't worry, smart windows that change how much heat they let in will soon be able to keep you nice and cool.

"We have are developing a coating that comprises a thin layer of nanocrystals that transmit visible light and can reject near infrared light," Delia Milliron, deputy director of the Molecular Foundry at Berkeley Lab, told New Scientist.

The near-infrared transmittance of the nanocrystals can be tuned by applying a few volts of an electric charge. On a cold day, both visible and the near-infrared light would be allowed through to let the heat in; on a hot day, however, a few volts of electricity are applied so that the windows still transmit visible light, but block light in the near-infrared spectrum, keeping the sun's heat out.

Milliron and her team have synthesised indium tin oxide in nanocrystal form. The crystals are dispersible in liquids and solvents and can be used to coat glass. The tin brings with it an additional electronic charge, and it is those electrons that are actually responsible for the infrared light absorption; when a voltage is applied, the concentration of electrons in the nanocrystals is changed, thereby changing the amount of infrared light absorbed.

Being able to dynamically change the infrared absorption in this way is completely new, says Milliron, who presented the idea at an MIT Energy Showcase on March 16th.

But smart windows are not just about keeping your average office worker nice and comfy. Such technology could reduce the energy and economic impact of windows, which for building owners in the U.S. is about $40 billion per year, Stephen Selkowitz, head of building technologies at Lawrence Berkeley National Laboratory, told National Public Radio last November.

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