High-tech mirrors could slash cooling needs

A new ultra-thin, multilayered coating material that beams heat from buildings into space could greatly reduce the need for airconditioning, researchers from Stanford University have said.

While it looks like a conventional mirror, the invention reflects visible light while also offloading infrared heat from inside the building into space, allowing radiative cooling to occur even during the day. Importantly, this radiative cooling has been designed to occur at a frequency that allows heat to pass back through the atmosphere without warming it up, a key feature given the importance of not contributing to global warming.

The authors of the research published in Nature, led by Professor Shanhui Fan and research associate Aaswath Raman, have named the process “photonic radiative cooling”. The result is cooler buildings that require less airconditioning.

“This is very novel and an extraordinarily simple idea,” Professor Eli Yablonovitch, director the Center for Energy Efficient Electronics Science at the University of California, Berkeley, said. “As a result of Professor Fan’s work, we can now [use radiative cooling] not only at night but counter-intuitively in the daytime as well.”

The nanophotonic material, the researchers say, has been designed to be cost-effective in order for a large-scale deployment over rooftops. This could see the energy requirements of buildings drop dramatically. Both the radiative cooling and sunlight reflecting properties of the photonic radiative cooler make the device around 5°C cooler than the ambient temperature.

“This team has shown how to passively cool structures by simply radiating heat into the cold darkness of space,” said Nobel Prize-winning physicist Burton Richter, professor emeritus at Stanford.

Dr Raman said a warming world needed cooling technologies that didn’t require power to operate, especially in developing countries.

“Across the developing world, photonic radiative cooling makes off-grid cooling a possibility in rural regions, in addition to meeting skyrocketing demand for airconditioning in urban areas,” he said.

The researchers said a system for delivering internal building heat to the device would need to be developed, as well as a large-area production process.

The team sees the project as a step towards tapping the cold depths of space as a resource.

“Every object that produces heat has to dump that heat into a heat sink,” Professor Fan said. “What we’ve done is to create a way that should allow us to use the coldness of the universe as a heat sink during the day.”

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