‘Photonic radiative cooling’ uses an ultra-thin mirror-like material to cool buildings, even on hot, sunny days
Engineers at Stanford University in the US have invented a remarkable way to cool buildings, even on hot, sunny days, without consuming electricity. Their technology uses an ultra-thin mirror-like material that radiates warmth from inside the building into space while reflecting back incoming heat from sunlight.
“We’ve created something that’s a radiator that also happens to be an excellent mirror,” says Aaswath Raman of Stanford. The invention of what the team calls “photonic radiative cooling” is published in the journal Nature.
The micro-engineered cooling material is just 1.8 microns thick – less than the thinnest aluminium foil. Yet it contains seven layers of silicon dioxide and hafnium oxide on top of a silver sheet, with an internal structure tuned to radiate heat from the building into space as efficiently as possible without warming the atmosphere. At the same time the mirror reflects 97 per cent of incident sunlight back into space.
“This photonic approach gives us the ability to finely tune both solar reflection and infrared thermal radiation,” says Linxiao Zhu, another member of the team.
Other experts praised the research. “This is very novel and an extraordinarily simple idea,” says Eli Yablonovitch, professor of engineering at the University of California, Berkeley. “We can now [use radiative cooling] not only at night but counter-intuitively in the daytime as well.”
Tests of a prototype system during sunny weather on a Californian rooftop showed that it could maintain a temperature 5C below the ambient air. In contrast, reflective aluminium sheets were 20C above ambient (and a black painted surface was 60C higher)....MORE
