Cool meta materials technology to cool buildings

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werepossum

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Jul 10, 2006
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Thought this was cool. http://www.designfax.net/cms/dfx/opens/article-view-dfx.php?nid=4&bid=420&et=featurearticle&pn=03

Stanford engineers have invented a revolutionary coating material that can help cool buildings, even on sunny days, by radiating heat away from the structures and sending it directly into space.

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

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 Stanford invention shunts away from buildings and sends into space.

Of course, sunshine also warms buildings. 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 -- 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 is cooler buildings that require less air conditioning.

SNIP

"This is very novel and an extraordinarily simple idea," said 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. "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 United States 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.

"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 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, according to Raman, lead author of the Nature paper. "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," he said.

Using a window into space
The real breakthrough is how the Stanford material radiates heat away from buildings.

As science students know, 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 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 said.

Aiming the mirror
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.

Together, the radiation and reflection make the photonic radiative cooler 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 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.

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

"I am personally very excited about their results," said Marin Soljacic, a physics professor at the Massachusetts Institute of Technology. "This is a great example of the power of nanophotonics."
SNIP
More at the link. Harvesting and transmitting unwanted heat within the building is not exactly a trivial problem, but the prospect of having a roof 9 degree F below ambient is pretty tempting. Also potentially even more attractive for cooling towers and industrial processes. Best to harvest the waste heat, but if you can't effectively harvest it, sending it into space is a pretty darned good idea.

There is absolutely nothing more exciting today than meta material engineering.
 

Jaskalas

Lifer
Jun 23, 2004
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So... does it come as a removable cover - so the sun can warm us in the winter?
 

werepossum

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Jul 10, 2006
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So... does it come as a removable cover - so the sun can warm us in the winter?
No, but you wouldn't have to pump heat up to it. You'd still lose the solar gain, though I doubt on commercial buildings that does you much good.

If someone wanted to get real froggy they could create a rotating sectional array to mount on the roof, one side with this radiating material and one with a flat black painted surface to absorb solar radiation. Pump your working fluid through it and simply reverse the flow to heat or cool.
 

Moonbeam

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Nov 24, 1999
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Great, we solve global warming by pumping heat into space and create Universe Warming instead. That should get our asses invaded if the Universe isn't controlled by Republicans.
 

realibrad

Lifer
Oct 18, 2013
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Great, we solve global warming by pumping heat into space and create Universe Warming instead. That should get our asses invaded if the Universe isn't controlled by Republicans.

That may be one of the most scientifically ignorant statements I have ever seen. It shows a complete lack of any understanding about entropy. I get it was an attempt at a joke, but your understanding of science is depressing. You should stop talking about this subject until you understand this subject better.

Wow.
 
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