Did not read the article, but the abstract is misleading.
It does not matter if the atmosphere absorbs the radiation from your device or it reaches space, after a few feet it's irrelevant. The important bit is the underside of the tarp which is not going to be below ambient temperature. Making the absolute best it can do the equiviemnt of a fan on the roof which can't replace AC.
It absolutely does matter, because the beads that are tuned to radiate particular frequencies of infrared will also be tuned to absorb those frequencies as well. If the frequencies aren't transparent to the atmosphere, you'll be reabsorbing just as much as you transmit and end up at equilibrium with the atmosphere.
It is essentially similar to the way in which you can freeze water in a shallow dish on clear nights that are nonetheless slightly above freezing temperature - you are facing a black-body radiator that is well below ambient local temperature.
That's a great explanation. I wish the article had put it in these terms... because reading through it, I kept thinking there was no way to cool the building to below the ambient temperature. But if you're equilibrating with space instead of the atmosphere, that's a whole different kettle of fish.
At night sure, in the day time the sun is adding heat across a wide band of frequencies. This heats up less, but does not drop below ambient temperature in full sunlight.
The advantage of targeting the IR window is that it would work even with clouds and water vapor in the atmosphere whereas, plain old thermal radiation needs a clear night to be effective.
Of course it won't pump heat backwards into the sun but they propose a water circulation system, which I imagine could store some daytime heat to release it at night. That and masonry of the walls of the building would store even more.
Reading the article helps. One of these tarps cooled a container of water underneath it by 8°C during an overnight test:
"We further demonstrate the effectiveness of radiative cooling for a relatively large thermal mass using water as a cold storage medium. A plastic water tank was placed underneath the radiative cooling glass-polymer hybrid metamaterial, putting water in close contact with the heat-conducting copper plate. Since the water is stationary in the experiment, its large heat capacity substantially slows down the cooling process. We therefore used a 10-µm-thick HDPE film on top of the Polystyrene foam box in this setup to reduce convective heat loss and improve thermal isolation. The water temperature continuously dropped, reaching more than 8° C below ambient after two hours of exposure."
A sealed container? I'm not following the physics on this. I can only assume you're talking about evaporative cooling of some sort. This was not an open container.
No, you need to insulate the container from the ground. Then you have conductive and radiative heat gain and loss with the air. Works better on top of a hill, in the middle of a field, under a clear sky. The container has to have minimal thermal mass, and be a food conductor but that's it.
PS: An easy demo is a car roof with the windows open at night in a field.
This has actual cooling capacity. From the publication:
"A 72-hour continuous measurement of the ambient temperature and the surface temperature of an 8-in-diameter hybrid metamaterial under direct thermal testing. A feedback-controlled electric heater keeps the difference between ambient and metamaterial surface temperatures less than 0.2°C over the consecutive three days. The heating power generated by the electric heater offsets the radiative cooling power from the hybrid metamaterial. When the metamaterial has the same temperature as the ambient air, the electric heating power precisely measures the radiative cooling power of the metamaterial. The continuous measurement of radiative cooling power over three days shows an average cooling power > 110 W/m2 and a noon-time cooling power of 93 W/m2 between 11am – 2pm. The average nighttime cooling power is higher than that of the daytime, and the cooling power peaks after sunrise and before sunset. The measurement error of the radiative cooling power is well within 10 W/m2 (32)."
Fans won't drop temperatures below ambient. This will.
I saw this and smirked internally of course, but noticed in the article itself they did refrain from trivialising the names of the inventors in this way, by referring them to Dr Yang and Dr Yin throughout.
Most homes in hot countries are not cooled with air conditioning because most people in hot countries are too poor to afford it. Even the rich 1% tend to use air conditioning only on a room by room basis, almost never the whole house.
Having lived for 30 years in SE Asia, with temps in the mid-forties C during the hot season (which will start very soon) we seldom turn on the aircon for more than 1-2 hours at the most at night, and then only in the bedroom.
This works if you have good fans and a lot of icewater. Using aircons too much ends up making you sick, going in and out of the heat. In Thailand we had two golden retrievers who had their own fans in the coolest parts of the house, they would sleep through the heat and then be very active in the early morning and evening.
It should also be mentioned that we are far enough south that the length of day and night doesn't change that much over the year -- the night is always long enough for radiational cooling to do it's job. That is not the case in places like New York or Boston where the nights are so short in August that it never cools down. It's always comfortable in the morning here before the mid-morning heat hits.
Contrast this with visiting hot parts of the United States where you go from your airconditioned house to your airconditioned car to your airconditioned big box store to your air conditioned office. That's a terrible way to live.
Where I'm from(Poland) summers can get stupidly hot, 40C on some days, it's difficult to sleep at night, yet I don't know anyone who has air conditioning at their house. It's a huge luxury, due to the cost of the unit + cost of electricity to run it.
Can confirm: ~100M Americans run A/C all summer, and also in spring and fall, too. Most American homes have central A/C, so you cannot easily "enable" it only for the room you're in. The typical supermarket or office is ~20C in summer, many homes are kept 20-22C.
I never realized how much I hated this until I started spending a lot of time in a warm country that uses AC sparingly.
However in the hottest parts of Texas the summers are brutally hot and humid even at night, and the insects are a force of nature. AC is kind of a requirement. I would imagine that if AC were magically outlawed tomorrow, 90% of the population or more of most of the South would pack up and leave by the end of August.
Not only that, but the energy is radiated in wavelengths that aren't absorbed by the atmosphere (I suppose it's different with cloudy skies). If this solution becomes massively widespread, it could have a good effect on global warming.
With caveats: If we imagine a city radiating all its energy as infrared, it could cause weather abnormalities.
Wow, that is an awesome result. With a cooling panels on the shade side of the roof and water pipes behind your PV panels on the sun side you can boost solar PV production by 3 - 5%. PV panels become less efficient when they are hot, we put some drip irrigation drippers on roof behind our panels (the panels are about 4" above the roof surface) and the evaporating water improved production by 2% on otherwise hot days.
It wouldn't be useful on a spacecraft because it takes advantage of a transparent band in the Earth's atmosphere. In space, you have that already at all wavelengths.
More to the point, a heat shield is used for re-entry and this stuff would decompose almost instantaneously. It's be like trying to use a handheld fan to cool yourself while standing in a kiln.
Probably. A cursory google search shows that TPX is prone to thermal degradation. But glass microbeads do not have the same negative environmental impact as plastic ones. They are already naturally omnipresent in the biosphere.
Does light in other wavelengths pass through with little transmission losses? Then it might be an elegant way to cool green houses in the summer. Yes, they can simply be vented, but it makes sense to keep them closed in order to keep CO2 fertilization inside and bugs outside.
Patents will ensure it sells for $100 per square-meter though, or whatever ludicrous price can be burn by the market and still make it valuable to the consumer.
I'm not sure I get how the beads re-radiate energy at 8 microns that came in at other frequencies. If the energy didn't come in at a different frequency, then the building isn't going to end up any cooler. If it did, then the beads are essentially phosphorescing in the IR region, aren't they? Or exhibiting some other form of nonlinear conversion?
I would believe that the heat energy gets transferred to the beads by conduction and or radiation.
I tend to think this is like a gas mantle in a gasoline (propane) lantern. The mantle is doped with rare earth that emit more strongly in the visible section than a black body would. On this case the temperature is much lower of course and the glass beads are sized to emit strongly in an the infrared band that isn't absorbed by the atmosphere.
So, couple this, in terms of the dispersion of energy:
"Our hybrid metamaterial is extremely emissive across the entire atmospheric transmission window (8-13 μm) due to phonon-enhanced Fröhlich resonances of the microspheres."
And this in terms of the absorbtion:
"In the case of large microspheres, modal interference between higher order modes makes the hybrid metamaterial strongly infrared-absorbing."
And finally this in terms of reflectance:
"The hybrid metamaterial strongly reflects solar irradiation when backed with a 200-nm-thick silver thin film prepared by electron beam evaporation... The measured spectral absorptivity (emissivity) of the sample (Fig. 3D) indicates that the 50-μm-thick film reflects ~ 96% solar irradiation..."
I don't know about this particular technology but as a parallel all Florescent Lights operate on a similar concept. They absorb radiation at frequency X and emit in frequency Y where Y is visible light.
It appears that the researchers have tuned the emission to a frequency that is not reabsorbed by the atmosphere but emitted unimpeded into space. Pretty clever engineering.
In Phoenix AZ, July/Aug/Sept pool parties are often BYOBOI (bring your own block of ice). Everybody brings a 10 lb. block of ice to throw in the pool and within an hour it's cool enough to enjoy, even in the 115F heat.
The new film works by a process called radiative cooling. This takes
advantage of that fact that Earth’s atmosphere allows certain wavelengths
of heat-carrying infrared radiation to escape into space unimpeded. Convert
unwanted heat into infrared of the correct wavelength, then, and you can
dump it into the cosmos with no come back.
the work is done by the huge temperature difference, about 290°C, between
the surface of the Earth and that of outer space.
If you floated thousands of square miles of it on the surface of the oceans, yes. But that first assumes a reasonable time payback of the CO2 burden of manufacturing the stuff in the first place, which is not a given. It's obviously not practical for oceans, but it might protect ice caps and glaciers.
Seems innovative and awesome if true, but it is not so awesome as to be without issues. Clearly this is great for places that are always warm, slap this stuff on every building in Texas.
But what about places with erratic weather, or even just seasonal changes? It seems like this will need to be some way to flip this stuff.
The article says that the material is only half the solution. The other half is pipes with water to move the heat to the material so it can be dissipated. I assume that if you stop moving as much water, it doesn't cool as much. (It can't radiant heat that isn't there.)
Have another film above it that can be switched from reflective to transparent? These already exist, but are made of exotic materials (vanadium oxide, if I recall correctly) and thus expensive.
Wow, this is such a beautiful example of basic science turning into powerful technology. Technology which looks like it will be both cheaper and better for the environment than any current alternative.
Does anyone have an idea of how long this might take to get to market as a rooftop heat exchanger for domestic use? I guess it largely depends on durability of the film; presumably the heat exchanger part of the system is a known quantity.
"50 micron thick polymethylpentene mixed with tiny glass beads"
As an achievement in science this is cool, but... we should be reducing energy consumption while also not polluting the surface of the planet. Where's this stuff going to end up when houses covered with it are torn down in 30-100 years?
As I understand it, "microplastics" are small particles of plastic, not thin sheets of it. The sheets should be easier to reprocess than small particles would be.
But then, after thinking a bit about it (in my semi frozen office because someone turned out the heating during the night), won't this increase the heating costs a lot during the winter?
Go read the other comments. This is not an insulator. It will take a source of heat and start radiating that heat at a certain frequency where our atmosphere happens to be transparent. And because our atmosphere is transparent, there isn't at heat at that frequency outside so this "window" in your house is effectively one way.
Also, the demonstrated energy transfer density of this stuff is high enough that the area of your roof is enough cooling to make a series dent in your cooling cost.
This only works because the sky does not transmit much energy at the particular infrared frequency this operates at.
In contrast ordinary objects (the computer case, a tree, a wall), do transmit those frequencies.
This would have no effect - it's absorbing the frequencies emitted by the wall, and transmitting to the wall, the net effect is zero.
i.e. it only works under open sky. I suspect even clouds would block this from functioning. (Not sure about this - maybe water has a "window" at this frequency?)
8 to 14 microns is called the "atmospheric window", and a large portion(up to around 80%, from what I've seen) of energy at these wavelengths escapes the atmosphere. This window is a result of weak absorption by carbon dioxide and water, thus the IR emissions will pass right through clouds.
I appreciate your concern for the quality of HN! but I don't think this is fair. The user was expressing delight at an unexpected observation, which seems perfectly in keeping with intellectual curiosity.
