New type of solar structure cools buildings in full sunlight

Mar 27, 2013 by Andrew Myers
Professor Shanhui Fan (center) with graduate students Aaswath Raman (left) and Eden Rephaeli (right). (Photo: Norbert von der Groeben)

(Phys.org) —A Stanford team has designed an entirely new form of cooling panel that works even when the sun is shining. Such a panel could vastly improve the daylight cooling of buildings, cars and other structures by radiating sunlight back into the chilly vacuum of space.

Homes and buildings chilled without air conditioners. Car interiors that don't heat up in the summer sun. Tapping the frigid expanses of to cool the planet. Science fiction, you say? Well, maybe not any more.

A team of researchers at Stanford has designed an entirely new form of structure that cools even when the sun is shining. Such a structure could vastly improve the daylight cooling of buildings, cars and other structures by reflecting sunlight back into the chilly vacuum of space. Their paper describing the device was published March 5 in Nano Letters.

"People usually see space as a source of heat from the sun, but away from the sun outer space is really a cold, cold place," explained Shanhui Fan, professor of electrical engineering and the paper's senior author. "We've developed a new type of structure that reflects the vast majority of sunlight, while at the same time it sends heat into that coldness, which cools manmade structures even in the day time."

The trick, from an engineering standpoint, is two-fold. First, the has to reflect as much of the sunlight as possible. Poor reflectors absorb too much sunlight, heating up in the process and defeating the purpose of cooling.

The second challenge is that the structure must efficiently radiate heat back into space. Thus, the structure must emit very efficiently within a specific wavelength range in which the atmosphere is nearly transparent. Outside this range, Earth's atmosphere simply reflects the light back down. Most people are familiar with this phenomenon. It's better known as the —the cause of .

Two goals in one

The new structure accomplishes both goals. It is an effective a broadband mirror for solar light—it reflects most of the sunlight. It also emits thermal radiation very efficiently within the crucial needed to escape Earth's atmosphere.

Radiative cooling at nighttime has been studied extensively as a mitigation strategy for climate change, yet peak demand for cooling occurs in the daytime.

"No one had yet been able to surmount the challenges of daytime radiative cooling—of cooling when the sun is shining," said Eden Rephaeli, a doctoral candidate in Fan's lab and a co-first-author of the paper. "It's a big hurdle."

The Stanford team has succeeded where others have come up short by turning to nanostructured photonic materials. These materials can be engineered to enhance or suppress light reflection in certain wavelengths.

"We've taken a very different approach compared to previous efforts in this field," said Aaswath Raman, a doctoral candidate in Fan's lab and a co-first-author of the paper. "We combine the thermal emitter and solar reflector into one device, making it both higher performance and much more robust and practically relevant. In particular, we're very excited because this design makes viable both industrial-scale and off-grid applications."

Using engineered nanophotonic materials the team was able to strongly suppress how much heat-inducing sunlight the panel absorbs, while it radiates heat very efficiently in the key frequency range necessary to escape Earth's atmosphere. The material is made of quartz and silicon carbide, both very weak absorbers of sunlight.

Net cooling power

The new device is capable of achieving a net cooling power in excess of 100 watts per square meter. By comparison, today's standard 10-percent-efficient solar panels generate the about the same amount of power. That means Fan's radiative cooling panels could theoretically be substituted on rooftops where existing solar panels feed electricity to air conditioning systems needed to cool the building.

To put it a different way, a typical one-story, single-family house with just 10 percent of its roof covered by radiative cooling panels could offset 35 percent its entire air conditioning needs during the hottest hours of the summer.

Radiative cooling has another profound advantage over all other cooling strategy such as air-conditioner. It is a passive technology. It requires no energy. It has no moving parts. It is easy to maintain. You put it on the roof or the sides of buildings and it starts working immediately.

A changing vision of cooling

Beyond the commercial implications, Fan and his collaborators foresee a broad potential social impact. Much of the human population on Earth lives in sun-drenched regions huddled around the equator. Electrical demand to drive air conditioners is skyrocketing in these places, presenting an economic and an environmental challenge. These areas tend to be poor and the power necessary to drive cooling usually means fossil-fuel power plants that compound the greenhouse gas problem.

"In addition to these regions, we can foresee applications for radiative cooling in off-the-grid areas of the developing world where air conditioning is not even possible at this time. There are large numbers of people who could benefit from such systems," Fan said.

Explore further: Pinpoint laser heating creates a maelstrom of magnetic nanotextures

More information: pubs.acs.org/doi/abs/10.1021/nl4004283

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User comments : 38

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Alexander Riccio
3.4 / 5 (5) Mar 27, 2013
HOLY SH!T

If only it was open access. :(
cyberCMDR
3.7 / 5 (3) Mar 27, 2013
So, it reflects back most of the sunlight? What does this do for anyone looking at the panels at the wrong angle?

She blinded me, with science!
PPihkala
5 / 5 (1) Mar 27, 2013
I wonder what will be the cost of these panels per area? Many technologies are not used because they are too expensive. Used at walls they would probably not blind anyone.
phys314
not rated yet Mar 27, 2013
Am I missing something? Most heatpumps pump more thermal energy around than they consume in electrical, so 100 W electrical can "cool" much more that 100W thermal. Doing it passively and sending the energy "away" may have advantages, but overall seems not a game changing issue...?
TheGhostofOtto1923
1.8 / 5 (10) Mar 27, 2013
Its still throwing away energy. Thermoelectrics combined with solar cells which turn heat and light into electricity would be better.

Maybe you could configure surfaces to concentrate the reflected light for a solar furnace.
frog55
3 / 5 (2) Mar 27, 2013
I agree; instead of wasting the energy by sending it back out into the void, turn it into usable electricity.
robeph
5 / 5 (2) Mar 27, 2013
A second consideration is the seasonal difference in interior temperatures and the energy consumption differences during daylight hours.

Specifically and anecdotal, last night it was 27ºF outside. Were I to turn off my heat pump at night, the temperature would drop rather quickly, probably into the 40s or 50s inside my home. However, this morning around 10am when the sun had fully risen above the mountain I'm at the base of, I actually had to switch from heat pump to air conditioning. Mind you, it's still 48ºF outside right now, but the temperature in my home increased to around 77ºF just from sun before I turned the AC on, the heat pump disengaged at 68ºF the temperature I have it set to.

Now, while in the summer this may be useful in energy reduction, unless there is a method to quickly disengage the reflective surfaces, the winters would likely increase in heating costs dramatically, so they'll definitely need to consider the seasonal performance of such methods.


christophe_galland1
not rated yet Mar 27, 2013
The summary is somehow misleading: The structure has not been fabricated nor measured. The paper reports numerical calculations. Very interesting though.
Parsec
not rated yet Mar 27, 2013
Add some fans to circulate outside air when it's beneficial to do, and shut it off when it isn't.
Myno
5 / 5 (1) Mar 27, 2013
Nanostructures on the exterior surfaces might be hard to keep clean?
harryhill
not rated yet Mar 27, 2013
Is this thing 'mirror like'? Wonder what the birds will think.
Maybe 'crashing' was not considered part of the wonder.
homedoc
not rated yet Mar 27, 2013
I do not understand how it could actually produce cooling per se unless it is pumping heat from inside the building to the environment. If I aimed one of these at the sun and put my hand on the bottom surface, would I feel cold?
thatsitalright
2 / 5 (4) Mar 27, 2013
One step closer to Photafine steel...
GraemeMcRae
not rated yet Mar 27, 2013
Does the 100w per square meter of cooling continue throughout the night? If so, then a cooling panel is better than a solar panel/heat pump combo for areas (and times of year) where cooling is needed throughout the night.
KeithMcC
not rated yet Mar 28, 2013
Add some fans to circulate outside air when it's beneficial to do, and shut it off when it isn't.

It seems Prof. Fan is not a fan of fans.
_traw_at
4.5 / 5 (2) Mar 28, 2013
My architect acquaintances are certainly going to want to see some working samples of this one as soon as possible. :-) And so do I.
FastEddy
1 / 5 (8) Mar 28, 2013
Compared to light brown and medium brown beach sand? Coating darker roofs with blond beach sand does ~~ 10-15% improvement, reflecting heat ... same, same with coating road surfaces with blond sand = cooler tires, better smilage, less overall local heat retention.
If these new panels do a 30-35% better job = super duper. I'll bet their color has a bit to do with it.
Of course getting g'ment to kill their office building lights and A/C overnight will do even more. The real savings come when the A/C is killed in g'ment buildings all together.
TheGhostofOtto1923
1.5 / 5 (8) Mar 28, 2013
Is this thing 'mirror like'? Wonder what the birds will think.
Maybe 'crashing' was not considered part of the wonder.
Glass on bldgs already kills 100s of millions of birds a year, making things like solar panels and wind turbines and cats minor by comparison. This would replace some of that glass on vertical surfaces.
Sean_W
1 / 5 (7) Mar 30, 2013
So you would want to have poor insulation under the panels so the heat in the building can be radiated out through them, right? Otherwise, wouldn't you just be cooling the outside skin of the building?
TheGhostofOtto1923
1.6 / 5 (7) Mar 30, 2013
So you would want to have poor insulation under the panels so the heat in the building can be radiated out through them, right? Otherwise, wouldn't you just be cooling the outside skin of the building?
It would be nice to have a tunable insulating material... You could decrease the R value when you wanted to transfer interior heat to the panels, and increase it when you wanted to retain heat.

Hey they have windows with tunable reflectance so who knows?
http://www.innova...orp.com/
Steven_Anderson
1 / 5 (8) Mar 30, 2013
I would suggest making highway median strips with the stuff if its cost effective enough which is a big if. but then why not use solar panels to convert it to electricity unless solar panels cost more. In the US alone we have roughly 46,000 miles of highways with median strips. If you could reflect 100 Watts per meter back into space it would be huge numbers. I did the calculation Here: http://rawcell.co...-planet/ (Save Planet #1) for painting the roads with UV reflective coatings but this would be better since the energy would be reflected back into space. If my calculations are correct it would be the equivalent of taking 22 coal plants out of commission.
Steven_Anderson
1 / 5 (7) Mar 30, 2013
Here is my calculations on that:
1 meter=10.7639 square feet
46,876 miles of interstate highways X 8 foot breakdown lane OR 1,980,042,240 square feet of area or 183,951,943.43 square meters
which at 100 watts /square meter would give us 18.395 billion watts, multiplied by 60 minutes and divide by 1,000,000 gives us 11,037,116.605 MWh and one plant is about 500 MWh. So it would be equivalent of removing 22,074 Coal Plants from commission. (sorry my calculations were wrong in the first post)

Steven_Anderson
1 / 5 (8) Mar 30, 2013
oops another mistake 2207.4 coal plants...sorry I wish I had a calculator that had comma's every three zeros!
gear
not rated yet Mar 31, 2013
As my comprehension on the panel, it is only a perfect mirror, but it cannot absorb heat from air and emit these heat into space, I donot think it will of great use in life.
TheGhostofOtto1923
1.5 / 5 (8) Mar 31, 2013
Here is my calculations on that:
1 meter=10.7639 square feet
46,876 miles of interstate highways X 8 foot breakdown lane OR 1,980,042,240 square feet of area or 183,951,943.43 square meters
which at 100 watts /square meter would give us 18.395 billion watts, multiplied by 60 minutes and divide by 1,000,000 gives us 11,037,116.605 MWh and one plant is about 500 MWh. So it would be equivalent of removing 22,074 Coal Plants from commission. (sorry my calculations were wrong in the first post)
You forgot to factor in all the crud buildup and subsequent deterioration from trying to keep them clean, plus vandalism, tire damage, wrecks, snow removal, etcetcetc.

Plus the fact that they DON'T GENERATE POWER. Read the article numbnuts.
CQT
1 / 5 (1) Mar 31, 2013
Anyone who wears glasses already knows intuitively the principle.
In a broad sense glasses channel electromagnetic radiation.
Future houses will wear the 'glasses' described here. In fact meta-materials will channel any of the wavelengths of the electromagnetic spectrum for the desired effect - heating, cooling, electricity, cloaking etc. The most important aspect of all this channeling is a passive function - to quote Andrew Meyers' article above "It is a passive technology. It requires no energy. It has no moving parts. It is easy to maintain."

The meta-materials will go under high sounding names such as photonic and phononic layered crystals or lattices.

By any other name a rose is a rose is a rose.
Kudos go out to the researchers.

@gear
The heat in air is the wavelength of the electromagnetic spectrum that is being channeled (or reflected), not absorbed.
Steven_Anderson
1.5 / 5 (8) Mar 31, 2013
TheGhostofOtto1923 I would resort to name calling, but I am a higher life form than you that has evolved past personal attacks in an intellectual argument. Your completely wrong. FIRST OF ALL the highways are generally kept clear, and I am sure we could develop the surface material of the device to absorb the road wear (paving roads with solar panels has been proposed previously) Secondly, Here is where you injected your personal attack. THE DEVICE REFLECTS THE HEAT from the air and ALSO transfers the heat on one surface to launch into space as photons in the right frequency so it's not absorbed by the atmosphere. IN OTHER wordsit acts as a heat pump and the ground benieth it is NOT at absolute 0 Therefor it should work even on the ground.
Steven_Anderson
1.6 / 5 (7) Mar 31, 2013
Its even possible with the heat pump part that you could make the panels like blinds and flip them to a inward positive heat pump from the air to the cooler inside when the temperature gradient is favorable. As for the other part with shooting off into space, I haven't thought that through nearly enough, considering the scarcity in the article about the actual workings, presumably due to patent application timing. http://rawcell.com
TheGhostofOtto1923
1 / 5 (7) Mar 31, 2013
So steve why are you heating and cooling roads again?
since the energy would be reflected back into space
-and why do you not consider the 60 miles of atmosphere between your road coolers and space?

These metapanels on median strips would be heating atmosphere from below in order to cool dirt. Until they got coated with snow, road salt, and tire dust that is. Whereas when installed on bldgs they would be preventing interiors from being heated, just like white roofs only better. And how would they work in conjunction with photovoltaics again? I missed that.
Steven_Anderson
1.5 / 5 (8) Mar 31, 2013
The idea of the device is that it emits the radiation at a frequency that isn't absorbed by the atmosphere. You really should read the article BEFORE coming up with an opinion. You cool ANY part of the system by reflecting the energy back into space or transferring it back into space, you effectively cool the system. Of course my premise is going on what the article is saying that it transfers the energy back into space. If you have a problem with the logic then you have a problem with the article's premise. Here is where it says so, "Thus, the structure must emit thermal radiation very efficiently within a specific wavelength range in which the atmosphere is nearly transparent"
Steven_Anderson
1.5 / 5 (8) Mar 31, 2013
Secondly, roads and median strips are generally kept clear on ALL highways except during actual snow fall. It's a requirement that the roads be clear by LAW. Thirdly, I made the second comment about the blinds when applied to buildings not to the roads. (Sorry to confuse you. I should have been more specific)
TheGhostofOtto1923
1.5 / 5 (8) Apr 01, 2013
You cool ANY part of the system by reflecting the energy back into space or transferring it back into space, you effectively cool the system
And so how does cooling median strips cool bldgs again?
46,876 miles of interstate highways X 8 foot breakdown lane OR 1,980,042,240 square feet of area or 183,951,943.43 square meters
You are proposing to replace the entire road surface including breakdown lane with solar panels? What does this have to do with the material in the article? How much energy would it take to maintain this delicate material with the constant wear from traffic and weather?
Steven_Anderson
1 / 5 (7) Apr 01, 2013
TheGhostofOtto1923 You aught to take a look at the research that was done about 2 years ago where a gentlemen came up with the idea of paving the roads with solar panels (the electric heating would keep the roads clear without plowing) but the study that was done said they could make it so it would last for 50 years, so it is possible. Secondly, It has nothing to do with heating or cooling the building. It's simple another possible use for the devices. Only proposing to replace the break down lane, since it is relatively low traffic use(in most places) and is still plowed so snow doesn't accumulate. It would allow one to take energy out of our planets system and reflect it back into space regardless of the quantity of CO2 in the atmosphere. It would serve to reduce the carbon footprint of the USA until coal plants can be taken out of commission with a LFTR Reactor Conversion that I can only hope will happen http://rawcell.com. In other words just another gun in the arsenal.
TheGhostofOtto1923
1.5 / 5 (8) Apr 02, 2013
2 years ago where a gentlemen came up with the idea of paving the roads with solar panels
I saw that as it was here on physorg. Very limited applicability.
Secondly, It has nothing to do with heating or cooling the building
"New type of solar structure cools buildings in full sunlight"

-is the title of the article. What are you selling?
rawcell.com
Ah. Spam.
ValeriaT
1.5 / 5 (8) Apr 02, 2013
..the reflector has to reflect as much of the sunlight as possible... the structure must emit thermal radiation very efficiently within a specific wavelength range in which the atmosphere is nearly transparent
It's a waste of tax payers money - the plants are doing it already without any research. And they produce oxygen and they're beautiful. One of possible solutions
Steven_Anderson
1.5 / 5 (8) Apr 03, 2013
TheGhostofOtto1923 Try to write coherently please, it would be easier to follow.
As far as if it heats or cools the building, to make it clear, the device they propose DOES have to do with heating or cooling the building, my application for the roads has NOTHING to do with heating or cooling the building its about COOLING the atmosphere!

Really Ghost, you try my patience.

http://rawcell.com
TheGhostofOtto1923
1.6 / 5 (7) Apr 03, 2013
As far as if it heats or cools the building, to make it clear, the device they propose DOES have to do with heating or cooling the building, my application for the roads has NOTHING to do with heating or cooling the building its about COOLING the atmosphere!
Right. So how does this stuff in your road application, which cools only dirt and prevents vegetation from doing the exact same thing, generate the equivalent power of 20,000 power plants or something?
Steven_Anderson
1 / 5 (7) Apr 04, 2013
TheGhostofOtto1923, O.k., I will give it one more try just before I simply ignore you. The vegetation does not not have the power conversion density of these devices. I would challenge you to work the numbers. http://rawcell.com

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