Engineers overcome passive cooling technology challenge

Air conditioning is said to account for around 15 per cent of total primary energy consumption in the United States and can be as high as 70 per cent in countries like Saudi Arabia.

Technologies that use radiative cooling to control the temperature of buildings, such as planar multi-layered photonic films and hybrid metamaterial films, are attracting attention because they do not use electricity. A major drawback is that they are complicated and costly to manufacture.

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Now, a team led by Qiaoqiang Gan and graduate student Lyu Zhou from The State University of New York at Buffalo, Jian-Wei Liang and colleagues from Boon Ooi's Photonics Laboratory at KAUST in Saudi Arabia, working with researchers from the University of Wisconsin, have developed a passive cooling technology made from a polydimethylsiloxane (PDMS)/aluminium film structure. The research is described in Nature Sustainability.

"PDMS exhibits very high absorption in the Earth's 'atmospheric window' range and low absorption in the solar visible wavelength range," Jian-Wei said in a statement. "These properties make it an ideal material for passive radiative cooling."

By exploiting the spectral overlap between this atmospheric window, corresponding to wavelengths between 8-13 microns and the range of thermal radiation emitted by buildings at typical ambient temperatures, the PDMS/aluminium film can effectively cool buildings during daytime.

To fabricate the film, the researchers used a blade coating process to first coat the surface of an aluminium sheet with a layer of PDMS resin and then a metering blade to control its thickness, heating the structure in an oven at around 60^oC for two hours to complete the process.

"Although the PDMS has low absorption in the solar wavelength range, we found that its radiative cooling ability was significantly impacted by the surrounding environment when tested outdoors, especially in crowded urban settings," said Qiaoqiang.

To address this, the team developed a spectral-selective shelter that directs the thermal radiation skyward and achieved a daytime temperature reduction of up to 6.5 degrees Celsius in the outside environment.

The PDMS/aluminium film is claimed to provide a low-cost and greener solution to cooling buildings in urban environments and can also be manufactured on a large scale.

"We are now working on the optical structure of the film to enhance its radiative cooling, as well as its application in vapor condensation and water cooling," said Jian-Wei.