A cool way to cool: Engineers invent high-tech mirror to beam heat away from buildings into space

November 26, 2014, Stanford University
In this illustration a panel coated with a multilayered material designed by Stanford engineers is shown to help cool buildings without air conditioning. The material works in two ways. It reflects incoming sunlight (the sun's reflection seen on the panel) that would otherwise heat the panel. And it sends heat from inside the structure directly into space as infrared radiation (reddish rays). The blue areas on an otherwise warm roof show the cooling effect. Credit: Nicolle R. Fuller, Sayo-Art LLC

Stanford engineers have invented a revolutionary coating material that can help cool buildings, even on sunny days, by radiating heat away from the buildings 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 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 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 .

"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 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.

Stanford electrical engineering professor Shanhui Fan (center) gazes into the pizza- sized prototype with co-authors Linxiao Zhu (left) and Aaswath Raman (right). The high-tech mirror reflecting their faces beams heat directly into space. Credit: Norbert von der Groeben, Stanford Engineering

Using a window into space

The real breakthrough is how the Stanford 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 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," said Fan.

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," said Raman.

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."

From prototype to building panel

Making photonic radiative cooling practical on a large scale requires solving at least two remaining 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 there exist large-area fabrication facilities that can make their panels at the scales needed.

The cosmic fridge

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 said. "What we've done is to create a way that should allow us to use the coldness of the universe as a sink during the day."

Explore further: New type of solar structure cools buildings in full sunlight

More information: Nature, dx.doi.org/10.1038/nature13883

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1.8 / 5 (5) Nov 26, 2014
Typical short sighted "reasoning", often masking craven machination, to be found in the acts of many, if not all, "scientists". A building does not radiate hear away into space, because miles and miles of atmosphere are in the way! The heat will got to the air, first! Warming dust, even preventing the formation of clouds to moderate the weather. Concreted streets, the elimination of trees already create immense heat islands where cities are. This will only make it worse.
5 / 5 (1) Nov 26, 2014
They also have to keep it clean and protect it from caustic stuff like smog and bird droppings.
4.8 / 5 (4) Nov 26, 2014
julianpenrod, The article says that the IR that is radiated from this device is at a frequency that is not absorbed by the atmosphere. But you do have a point that on overcast days, the air above a large city full of these mirrors could heat up rather quickly.

3 / 5 (4) Nov 26, 2014
1 Hafnium is stupidly expensive (and rare)
2 How many pilots will it blind if this mirror is on the roof of every tower.
3 How effective a radiator is it compared to conventional AC? Since a heat pump up to the roof would still be required to cool the bulk of a tall building by this method.
4 How much benefit is lost by replacing rooftop solar panels with this device.

This is clearly a university project devised to test a student rather than create a useful product.
5 / 5 (1) Nov 26, 2014
Sounds great, but what about aircraft flying above?
Will the mirrors surface reflect the light into their eyes?
If so it would seem to be a problem.
5 / 5 (2) Nov 26, 2014
Given all of the above, maybe just having some some solar panels up there to help power the AC would be a better bet.
not rated yet Nov 26, 2014
It appears that high efficiency PV panels would be better, converting the incoming sunlight into electricity. Then backing these panels with heat absorption panels to heat water, provide air conditioning, etc. Whatever IR heat is left, they could pump back into space. Also, I wonder what frequency they convert the IR to - can it be a higher frequency (physically I don't think so, but if so, they could pump that into PV cells also and maybe they could get greater than 100% efficiency), or are they lowering it to miss some water absorption bands?

What this really sounds like is work that has been done for satellite thermal control. Someone had a brain f*rt and thought they could simply apply it to the heat island problem here on the mother ship.
5 / 5 (1) Nov 26, 2014
"the precise frequency that allows it to pass through the atmosphere without warming the air, "
Yes, there are significant IR windows into space.
This material has high emissivity in a designed spectral band.
Most natural materials have broadband spectral emissivity which is why it cools quickly at night in dry climates.

Paint the roofs a diffuse low emissive paint and reflect all heat from the building.
5 / 5 (2) Nov 26, 2014
I prefer they captured the potential energy.
3 / 5 (2) Nov 26, 2014
I prefer they captured the potential energy.

I guess you don't understand how such energy is captured.
5 / 5 (2) Nov 26, 2014
Could the coating be used on interior walls to retain heat?
1 / 5 (2) Nov 26, 2014
hemitite "argues" that the article states that the material will reradiate infrared radiation at a frequency that is not absorbed by the atmosphere. But the "atmosphere" is just the air, the combination of gases. Basically, they are saying that garden variety "greenhouse effect" will not occur. Absorption usually occurs by the dimensions of the gas molecules correlating with the wavelength of light. But there are many other elements in the atmosphere that do not behave the same way. Dust, water vapor are far larger in size, and so, just by that factor, can interact with a broad range of light wavelengths. Don't expect the "hate Mafia" who always give what I say a "1" just out of spite to necessarily realize or have the integrity to admit this fact.
not rated yet Nov 27, 2014
And what about winter? and cool summer nights? How do you switch it off? Really useful to have all the output from your expensive heating system radiated into space!
not rated yet Nov 27, 2014
personal pizza

Is this a regular unit of size in the US? How big is a 'personal' pizza? (what is a 'not personal' pizza?)

But you do have a point that on overcast days, the air above a large city full of these mirrors could heat up rather quickly.

On overcast days you don't need as much air conditioning - and also there's a lot less light to refelcet because it is overcast, neh?

How many pilots will it blind if this mirror is on the roof of every tower.

Dunno. Cockpits are rarely on the bottom of aircraft, though.

How effective a radiator is it compared to conventional AC?

The big point here is that this doesn't dump heat into the atmosphere (unlike a conventionaly AC)

How much benefit is lost by replacing rooftop solar panels with this device

Not all (or even most) rooftops are covered in panels. And it doesn't say that the entire rooftop needs to be covered in any case.
not rated yet Nov 27, 2014
Could the coating be used on interior walls to retain heat?

Yes - but you'd be living in a fully mirrored room. If you kink that way that may be OK.

I prefer they captured the potential energy.

Depends on the math: Does it cool more than an AC run on the equivalent area of PV panels (at equivalent set-up cost).
And: how much is it worth to be able to reflect heat back into space over dumping it into the atmosphere? The more global warming becomes a problem the more the value of the latter factor will rise.
(Remember that slectricity generated by PV panels - just like electricity from all other sources - gets converted to heat eventually)

How do you switch it off?

- Cover it. (or apply it to something like shutters and just turn them away from the sun)
- Switch off the pumps that transfer the heat to the roof
5 / 5 (1) Nov 27, 2014
Dunno. Cockpits are rarely on the bottom of aircraft, though.

Small craft pilots can see the ground from their window, and use it to navigate. Think of a helicopter pilot trying to land with surrounding houses covered in mirrors that shine as bright as the sun.
not rated yet Nov 27, 2014
The only shine as bright as the sun if the helicopter happens to be in a position where the angle to the sun is the same as to the helicopter.

Which is never the case during landing unless the helicopter lands on such a mirrored surface itself.

The 'problem' here is no different than reflections from windows. And I don't see massive amounts of aircraft crashing because of that.
1 / 5 (2) Nov 27, 2014
This appears to be a case of either fraud literature search on a rather major scale either by the journalist who wrote this piece or the scientists or the reinvention of an old, extant technology necessitated by not doing an adequate literature search.

Selective surface technology was invented and developed extensively in the very early 1970s by Drs. Aden and Marjorie Meinel, then at the Optical Sciences Center of the University of Arizona. There was no lack of publications on their part, either. I got to know them, wonderful people, when I was managing a speaking series on emerging solar energy technologies at that time at the University of Texas Department of Architecture.

not rated yet Nov 27, 2014
"Molecular Cooling Fan: Factors for Optimization of Heat Dissipation Devices and Applications"
not rated yet Nov 27, 2014
Reminds me the "snowball earth" effect:
cooling the planet reflecting the solar heat into space like ice does...
1 / 5 (1) Nov 27, 2014
julianpenrod, The article says that the IR that is radiated from this device is at a frequency that is not absorbed by the atmosphere. But you do have a point that on overcast days, the air above a large city full of these mirrors could heat up rather quickly.

It radiates anyway without them right now. Or is the heat that is radiated by buildings right now into the air not IR?

2 How many pilots will it blind if this mirror is on the roof of every tower.

How many pilots do you know that can see in IR ? :)
3 / 5 (2) Nov 27, 2014
Julian ought to research before trying to defend the uninformed comments he makes.
Could the coating be used on interior walls to retain heat?
Insulation already comes with reflective foil for the purpose of reflecting IR back into the bldg.
5 / 5 (1) Nov 27, 2014
EnricM: Yes, the heat is radiated away now, and is IR. But it's radiated at various frequencies, many of which are absorbed by air or water vapor. This warms the air, which in turn reradiates the heat, also as IR, at least some of which is radiated downward, rewarming the surface one is trying to cool.

This new material produces "tuned" IR, that isn't absorbed, allowing the heat to escape to space. Basically the same thing happens naturally in desert climates, where the very low levels of water vapor allow most IR to escape to space, and allowing the ground to cool even after hot days. In more humid areas, the water vapor in the air traps the heat, keeping the nights warmer. Mars is an extreme case, where tropical days can be above freezing, and the following night near -100 C.

Clouds will interfere, but they also block the sun, reducing the need for cooling in the first place.
5 / 5 (2) Nov 27, 2014
A few years ago I replaced the very darkly oxidized galvanized steel on my roof with white Colourbond(R) steel and an extra layer of aluminium foil directly under it. In South Australian summers the house used to get very hot in a heat wave, like a week or two of around 40C (104F) daily maximums. Now, during comparable external temperatures I need to wear warm clothes indoors. My cooling bill has been reduced to zero, but my winter heating bill has increased.
So, while I applaud the technology in the article, I think a nice additional feature may be to be able to reverse the direction of heat transfer, or switch it off.
5 / 5 (1) Nov 27, 2014
FainAvis: In hot regions such as yours a reversible design would be very handy. One easy way would be to mount the reflectors on rotatable panels, similar to some solar collector designs. In one position, they'd reflect the light and heat, in the other they'd be edge-on to the Sun and the building would warm. It wouldn't have to be automated, a simple manual "flop-over" design could be changed seasonally. A fancier design could be adjustable, reflecting only as much heat as needed.

In much of the northern US, and probably Europe and Asia, many modern buildings generate enough internal heat that the cooling systems run year round, with heating needed only in exceptional cold periods. Especially in areas with warm sunny summers and cloudy winters a reversible system wouldn't help much, so those areas would probably go for the cheaper fixed design.
1 / 5 (1) Nov 28, 2014
OMG they discovered a way to make radiant barrier expensive. Pure aluminum foil reflects up to 97%. You still have to be intelligent enough to use it correctly. It works great as a reflective agent under your roof on top of existing insulation to reflect heat radiated by the roof, back to the roof. It also reflects heat which has been absorbed by your ceiling and conducted by the insulation to the attic, right back into the insulation and back into the room in winter. It requires a 3/4" to 1" air space in front of the foil to reflect properly. It's a fantastic product that's been around for many years now. NASA has used both aluminum and gold foil on satellites. I've used it for years and it works great. best part....... IT"S CHEAP. Two sided reinforced is about 10¢ per square foot. No need to reinvent the wheel. NASA already did it for you.
not rated yet Dec 01, 2014
It is also possible to have solar control coatings that reflect only the invisible light, then harvest part of the rest of photons. Keeps the buildings cool and self-powers many useful devices.
5 / 5 (1) Dec 01, 2014
OMG they discovered a way to make radiant barrier expensive. Pure aluminum foil reflects up to 97%.

Reflection is not the point. It's no use reflecting if the energy gets caught by the atmosphere (while that may cool the building it doesn't help with global warming). You have to reflect and convert into a frequency where the atmosphere is transparent.
not rated yet Dec 03, 2014
Instead of beaming it out into space, why not beam it into the electric grid?

Solar panels can offer your building a lot of shade.

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