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Brian Westenhaus

Brian Westenhaus

Brian is the editor of the popular energy technology site New Energy and Fuel. The site’s mission is to inform, stimulate, amuse and abuse the…

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Nanophotonic Material Beams Excess Heat Into Space

Stanford University engineers have invented a material designed to help cool buildings. The material reflects incoming sunlight, and it sends the heat from inside the building directly into space as infrared radiation.

Its sure to cheer up the global warming crowd. For everyone else, it affords the opportunity to save some serious money on air conditioning.

A new ultrathin multilayered material can cool buildings without air conditioning by radiating warmth from inside the buildings into space while also reflecting sunlight to reduce incoming heat.

Artist's rendering

Photonic Radiative Cooler Artists Rendering. 

A team led by electrical engineering Professor Shanhui Fan and research associate Aaswath Raman reported this energy and money saving breakthrough in the journal Nature.

Related: Scientists Improve Efficiency Of Organic Solar Cells

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 the infrared light. This invisible, heat-bearing light is what the Stanford invention shunts away from buildings and sends into space.

Sunshine also warms a building. The new material, in addition to dealing with infrared light, is also a stunningly efficient mirror that reflects virtually all of the incoming sunlight that strikes it.

The result is what the Stanford team calls photonic radiative cooling that offloads infrared heat from within a building while also reflecting the sunlight that would otherwise warm it up. The result is cooler buildings that require less air conditioning.

Eli Yablonovitch, a professor of engineering at the University of California, Berkeley, and a pioneer of photonics who directs the Center for Energy Efficient Electronics Science said, “This is very novel and an extraordinarily simple idea. 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 researchers say they designed the material to be cost-effective for large-scale deployment on building rooftops. Though it’s still a young technology, they believe it could one day reduce demand for electricity. As much as 15 percent of the energy used in buildings in the U.S. is spent powering air conditioning systems.

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

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

According to Raman, lead author of the Nature paper, a warming world needs cooling technologies that don’t require power, “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.”

The real breakthrough is how the Stanford material radiates heat away from buildings.

Quick refresher: heat can be transferred in three ways: conduction, convection and radiation. Conduction transfers heat by touch. That’s why you don’t touch a hot 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 operation 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 said.

But transmitting heat into space is not enough on its own.

This 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 said.

The multilayered material is just 1.8 microns thick, thinner than the thinnest aluminum foil. It is made of seven layers of silicon dioxide and hafnium oxide on top of a thin layer of silver. These layers are not 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.

Linxiao Zhu, doctoral candidate in applied physics and a co-author of the paper explained, “This photonic approach gives us the ability to finely tune both solar reflection and infrared thermal radiation.”

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

For background, making photonic radiative cooling practical requires solving at least two technical problems. The first is how to conduct the heat inside the building to this 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 Stanford team’s prototype is the size of a personal pizza. Cooling buildings will require large panels. The researchers say large-area fabrication facilities can make their panels at the scales needed.

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 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.”

Related: Chemists Find Material To Both Generate And Store Hydrogen Fuel

At this stage of development talking about the costs is likely far from being accurate or even near a reasonable estimate. The press release noted that one problem is going to be getting the internal heat to the mirror; The technology has a long way to go. But once some costs are in it will be very interesting to see how a building with such a system built in would fare on its air conditioning expenses.

The tech might be awaiting other technological developments. After all, once a building’s heat is under control, there might be other uses that come ahead of simply pitching it to outer space.

It’s exciting technology with a good future out in the general sense. We’ll be watching this, for the sunnier parts of the world where ambient heat is a major expense this idea is sure to get some market legs.

By Brian Westenhaus

Source - www.newenergyandfuel.com

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