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A new material can help cool buildings, even on sunny days, by radiating heat away from them and sending it directly into space.

The heart of the invention is an ultrathin, multilayered material that deals with light, both invisible and visible, in a new way.

Invisible light in the form of infrared radiation is one of the ways that all objects and living things throw off heat. When we stand in front of a closed oven without touching it, the heat we feel is infrared light. This invisible, heat-bearing light is what the new invention shunts away from buildings and sends into space.

Of course, sunshine also warms buildings. The new material takes care of that, too—it’s a stunningly efficient mirror that reflects virtually all the incoming sunlight that strikes it.

Shanhui Fan, professor of electrical engineering at Stanford University, call it photonic radiative cooling—a one-two punch that offloads infrared heat from within a building while also reflecting the sunlight that would otherwise warm it up. The result? Cooler buildings that require less air conditioning.

“This is very novel and an extraordinarily simple idea,” says Eli Yablonovitch, professor of engineering at the University of California, Berkeley. “As a result of Professor Fan’s work, we can now (use radiative cooling), not only at night but counterintuitively in the daytime as well.”

Cooling without power

The material was designed to be cost-effective for large-scale deployment on building rooftops. Though still a young technology, researchers believe it could one day reduce demand for electricity. As much as 15 percent of the energy used in buildings in the United States is spent powering air conditioning systems.

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In practice the coating might be sprayed on a more solid material to make it suitable for withstanding the elements, researchers say.

“This team has shown how to passively cool structures by simply radiating heat into the cold darkness of space,” says Nobel Prize-winning physicist Burton Richter, professor emeritus at Stanford and former director of the research facility now called the SLAC National Accelerator Laboratory.

A warming world needs cooling technologies that don’t require power, says research associate Aaswath Raman, lead author of the paper that is published in the journal Nature.

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

A window into space

Researchers say the real breakthrough is how the material radiates heat away from buildings.

Heat can be transferred in three ways: conduction, convection, and radiation. Conduction transfers heat by touch. That’s why you don’t touch an oven pan without wearing a mitt. Convection transfers heat by movement of fluids or air. It’s the warm rush of air when the oven is opened. Radiation transfers heat in the form of infrared light that emanates outward from objects, sight unseen.

The first part of the coating’s one-two punch radiates heat-bearing infrared light directly into space. The ultrathin coating was carefully constructed to send this infrared light away from buildings at the precise frequency that allows it to pass through the atmosphere without warming the air, a key feature given the dangers of global warming.

“Think about it like having a window into space,” Fan says.

Aiming the mirror

But transmitting heat into space is not enough on its own. The multilayered coating also acts as a highly efficient mirror, preventing 97 percent of sunlight from striking the building and heating it up.

“We’ve created something that’s a radiator that also happens to be an excellent mirror,” Raman says.

Together, the radiation and reflection make the photonic radiative nearly 9 degrees Fahrenheit cooler than the surrounding air during the day.

The multilayered material is just 1.8 microns thick, thinner than the thinnest aluminum foil. It’s made of seven layers of silicon dioxide and hafnium oxide on top of a thin layer of silver.

The layers aren’t a uniform thickness, but are instead engineered to create a new material. Its internal structure is tuned to radiate infrared rays at a frequency that lets them pass into space without warming the air near the building.

“This photonic approach gives us the ability to finely tune both solar reflection and infrared thermal radiation,” says coauthor Linxiao Zhu, a doctoral candidate in applied physics.

“I am personally very excited about their results,” says Marin Soljacic, physics professor at the Massachusetts Institute of Technology. “This is a great example of the power of nanophotonics.”

Cosmic cold source

Making photonic radiative cooling practical requires solving at least two technical problems.

The first is how to conduct the heat inside the building to the exterior coating. Once it gets there, the coating can direct the heat into space, but engineers must first figure out how to efficiently deliver the building heat to the coating.

The second problem is production. Right now the prototype is the size of a personal pizza. Cooling buildings will require large panels. The researchers say there currently are large-area fabrication facilities that can make their panels at the scales needed.

More broadly, the team sees this project as a first step toward using the cold of space as a resource. In the same way that sunlight provides a renewable source of solar energy, the cold universe supplies a nearly unlimited expanse to dump heat.

“Every object that produces heat has to dump that heat into a heat sink,” Fan says. “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.”

Source: Stanford University