Guest Post By Willis Eschenbach [See two Updates at the end]
Here’s an oddity. Some very clever folks have invented a plastic film that cools surfaces by as much as 10°C. From Science magazine:
Cheap plastic film cools whatever it touches up to 10°C
Here’s the innovative part, according to the article. The tiny glass spheres act as resonators for the infrared emitted by the underlying surface. By choosing the right size spheres, the frequency of the resonators is tuned to be that of the so-called “atmospheric window”. This is the band of frequencies that is not significantly absorbed by any of the greenhouse gases. Infrared (IR) at that frequency pretty much slides right past the water vapor, the carbon dioxide, the methane, the ozone, it misses everyone and goes straight out to space.
In other words, it dodges the greenhouse effect …
Now, I’m left with some questions.
First, is it possible-to frequency-shift infrared radiation in this manner?
Next, what does the emission curve for this material look like? As an example, here’s a typical curve from MODTRAN showing the absorption of upwelling longwave radiation:
The smooth colored lines in the upper right panel show the Planck blackbody emission curves for various temperatures. The uppermost green curve is the warmest, 300 kelvin. The lowest yellow curve is 22oK. The “atmospheric window” is the area from wavenumber 750 to 1250, interrupted in the middle by the ozone absorption band just above wavenumber 1000.
As you can see, the warmer it is, the more the peak of the Planck curves (smooth colored lines) is shifted to the right. Now, with the resonator the peak radiation is supposed to be shifted by the resonators to a wavenumber of around 1000. That’s just below the ozone absorption band.
So I’m very curious about the shape of that curve. If the peak shifts towards the right it would have the characteristics of a warmer surface … can you mess with the Planck curve like that, shift the peak? Not saying it’s impossible, metamaterials have bizarre properties, I’m just out of my wheelhouse here.
Finally, what can this be used for? Well, I had a scheme a while ago for solar distillation of water. This would have been very useful to cool the condensing side of the still.
More directly it seems like it could cool buildings. A coating that could cool a large building by even one degree would translate into big savings in air conditioning. Ten degrees would be marvelous.
Anyhow, that’s what I’m calling a reverse greenhouse effect … it concentrates the radiation on the band where there is minimum atmospheric absorption by greenhouse gases.
Best to all,
w.
My Usual Request: If you comment please QUOTE THE EXACT WORDS YOU ARE DISCUSSING. That way we can all understand your subject.
[UPDATE] Thanks to a tip from the commenter Johanus, the underlying paper is here. It has what I asked for above, the actual emissivity curve in the thermal IR range. Fascinating. Here’s a preview, a graph of the temperatures throughout the day:
Now, that is a beautiful thing for a couple of reasons.
One is that I love real data. It is so much more interesting that a computer model of the same thing. Facts. Observations. If I stick to the facts I know I can’t go far wrong.
Next, look at the photonic radiative cooler. Throughout the day it is running cooler than the ambient air temperature by something on the order of 5°C … so for all the folks who said it was impossible, there’s an old Soviet joke about a Political Commissar berating someone and saying “Yes, yes, Comrade, you’ve proven that it works in practice … but it will never work in theory!” …
[UPDATE 2] After many helpful comments I’m finally understanding what’s happening. It’s not so much related to the selective emission of longwave radiation (thermal infrared). Instead, Kirchoff’s law says that frequency by frequency, emissivity equals absorptivity. So selective emission in a narrow band also means selective absorption in the same band.
The selective absorption is important because the “atmospheric window” also means that there is very little downwelling radiation in that window. Here’ MODTRAN again, showing the downwelling radiation from the viewpoint of the surface looking up:
Now, we can see that as expected, we have a lot of downwelling radiation. With the given parameters shown at the left, it’s shown at the top right as “Iout”, about 260 W/m2.
But notice … almost none of that is in the atmospheric window. The photonic material selectively absorbs mainly in that window … but there’s almost nothing in that window to absorb.
This is how they get the large temperature differences shown in the underlying papers. The material simply absorbs poorly where the incoming longwave radiation is, and absorbs well in the window where there’s little radiation.
At least that’s my current understanding …
w.
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It seems to be a very efficient reflector of incident sunlight. That stops it from being heated up like its surroundings.
It seems to be an efficient IR emitter, too. It reminds me of the way some clear plastics with parallel sides trap incident light by internal reflection and the light is emitted at the edge of the plastic.
I suspect that in this case the IR gets trapped inside the glass beads by internal reflection then preferentially emits at the “top” of each bead where the covering of encapsulating plastic is minimum. This may be fortuitous rather than by design, but it means that the photons are mostly emitted away from the material.
So it seems to work by reflecting incoming energy very efficiently and emitting outgoing energy directionally rather than in all directions.
I don’t think that greenhouse gases, atmospheric windows or phase shifts have anything to do with it.
Agreed
I also don’t see why the IR must be at the “window” wavelengths. Although on the face of it it seems to make sense that the energy is “dumped” in outer space, but why would it make any difference to the object if it were “dumped” into the atmosphere? As long as it emits IR would it not cool, whatever the wavelength.
On directionality, it would not matter if the balls emitted downwards as well as upwards as long as some of the photons were emitted away from the object.
“I also don’t see why the IR must be at the “window” wavelengths.”
Because IR radiation at other frequencies is blocked by the atmosphere. 8-13um is called the sky window is it is one of the few frequency ranges where IR does not get bounced back by the atmosphere.
Chris, thanks for the reply, but it is not quite the answer. Say the photon from the box was intercepted by “the air” 100m from the surface. The box does not know this and it has still lost a photon, so would still have cooled. It would not “bounce back” to the box. Why does it matter that the photon travels all the way to space? I mean, if it were absorbed by the ozone layer we would not expect that to be re-emitted back to the box.
Update 2 seems to have answered this point. There is little downwelling radiation at these wavelengths (the window), so because emissivity = absorptivity (for a given wavelength), the frequencies where the material is a good emitter (and hence absorber) there is no incoming radiation to absorb.
This means the “escape to space” argument is a bit of a red herring, although possibly related to the reason why there is little downwelling radiation at this wavelength.
Some people don’t seem to understand the principle behind this.
I will try to explain:
First of all, forget about the resonating balls thing because it is science speak to dazzle the ignorant,whatever effect of the size of the balls will be lost because of their inclusion in the TPX (film). Some may disagree.
The principle is quite clever. Over a highly reflective surface (Au, Ag, Al), really poor emitters, you place SiO2 (glass), which is quite transparent to SW radiation. At longer waves , glass is quite poor at transmission, which makes it an excellent emitter (Kirchoff’s Law).
The metal backing picks up the ‘heat’ and passes it to the glass because it can’t emit but the glass can. It’s not a conspiracy, it’s just basic science. The glass then emits.Glass balls wil ,of course, make the film more flexible than a glass sheet.
Practical application for domestic use:
Check your bank about applying for a second mortgage.
For the film to be effective it has to be in intimate contact with the area it is cooling.
Good luck doing that with your tile or corrugated roof.
It would need massive preparation before application, think film on your smooth car windows.
As a tradesman, I would tell you to go f@ck yourself.
If I took the job I would only give you a 3 month guarantee that most of the film would stay on the roof.
Willis and I could make millions on instalation of this stuff but we would need some south pacific hideaway.
Alex,
I like your humour. Just one small point…
Whether or not the device is viable, I don’t think you are correct to say that the cooled object under the shield has to be in intimate contact with it. If the device works at all it would also cool the air below by convection. Thus it could (in principle!) be used to cool a (sealed) room to below ambient.
Alex February 13, 2017 at 8:21 am
Some people don’t seem to understand the principle behind this.
I will try to explain:
First of all, forget about the resonating balls thing because it is science speak to dazzle the ignorant,whatever effect of the size of the balls will be lost because of their inclusion in the TPX (film). Some may disagree.
I disagree, the size of the balls will have an effect if the refractive index difference between the media is non-zero, the Mie scattering pattern of the beads in the paper clearly shows a significant difference.
The principle is quite clever. Over a highly reflective surface (Au, Ag, Al), really poor emitters, you place SiO2 (glass), which is quite transparent to SW radiation. At longer waves , glass is quite poor at transmission, which makes it an excellent emitter (Kirchoff’s Law).
The paper is about plastic not glass.
The metal backing picks up the ‘heat’ and passes it to the glass because it can’t emit but the glass can.
The metal backing reflects visible but transmits IR, and the thermal IR from the surface interacts with the beads which then emit in the ‘window’ region.
There is already a reverse greenhouse on the surface of water, its called surface tension and it works a treat.
Well you still have convection to deal with. The surrounding air will warm the surface to air temperature.
And now that I think about it I don’t think this thing can work. A black body will radiate more heat than any other surface. Whether the atmosphere radiates it back or not doesn’t matter much. Since most of the re-radiation would occur high in the atmosphere and would not make it back to the this surface.
I’ll believe it when I can buy it at Home Depot.
They sell a lot of stuff at home depot that doesn’t work, not saying this doesn’t just saying you might want to find a better test methodology.
and you still should be skeptical.
It’s possible this same effect is what causes the decrease in net radiation, only instead of little glass sphere’s, it’s little water sphere’s.
As the condensing water becomes later and larger collections of molecules, at some time they will cross the proper dimension to radiate at 15u, a large increase in emitted IR during the middle of the night would be one explanation for the drop in this graph.
Very interesting plot. However, I doubt that CO2 has anything to do with it. The high cooling rate is due to heat leaving through the atmospheric window. When the relative humidity reaches 100%, a very thin fog forms and closes the window. (Fog is a full spectrum radiator.)
Using a green laser, it is possible to see the nearly invisible fog several feet above your head.
Unfortunately, the blue curve does not have a scale and there is no information on where, or how, the “net radiation” is determined.
First, the scale is the same, just W/m^2. And it’s not just the optical window at high speed, it’s much wider (though without a spectrometer..), because when the cooling rate has dropped, the temp in the window is still about the same delta to ground.
I suspect something very much like the magic foil. Except instead of silica, it’s water blobs floating mid-air. As air temps bang into dew point, you get that slight fog (visible or not, I’ve just taken pic’s of galaxies during the slow rate many times).
I note co2, as it’s radiative forcing would still be embedded into either rates, it’s just that doesn’t matter in switching regulators, though as also noted it will leave a little of the residual energy until the nights get longer. And generally, co2 has minimal impact on morning minimum temps, the warming of the last 40 years is maybe 5% co2, the rest is just changes to global water vapor distribution.
The only way to cool a surface exposed to direct sunlight is to make the surface more reflective. The silver does that. These people have invented “The Mirror.” Wow.
There is mention made of a grant for dry cooling of power plants. So, some fool at the DOE got run by these clowns. Down with DOE! Up with Perry! Up with Trump!!!
Read the article.
They also claim that the product is more efficient at radiating in the IR frequencies.
I did read the article.
Then you didn’t understand what you were reading.
Somebody doesn’t understand much about heat transfer…
“The only way to cool a surface exposed to direct sunlight is to make the surface more reflective”
No,it is not the only way. Read up on black body radiation and emissivity. Why do you think deserts get so cold at night? It has nothing to do with reflectance, it has everything to do with high emissivity.
deserts cool so much because they have very low dew point temperature. And they are tan, not black.
Heat is so cool.
Xiaobo Yin’s report in Science is available at this DOI: 10.1126/science.aai7899 .
Thanks, Kip.
There is also a free review article from 2016 on this technology area here-
onlinelibrary.wiley.com/doi/10.1002/advs.201500360/epdf
Also, this is an old idea in general. US Patent 3043112 was issued in 1962- “Method and means for producing refrigeration by selective radiation”.
Willis,
“Infrared (IR) at that frequency pretty much slides right past the water vapor, the carbon dioxide, the methane, the ozone, it misses everyone and goes straight out to space.
In other words, it dodges the greenhouse effect …”
I don’t see any reason to think a small area would be significantly cooler because the IR from it was not intercepted by the atmosphere . . I don’t think that’s the “trick” involved here . .
(I mean the “straight out to space” aspect is not the trick)
Can I order 45 squares of shingles, and posibly some opaque windows please? I would also like a canopy/tent to use at the beach made from this also. TIA
Will it increase the gas millage of my car if I tape some of it to my hood?
If I understand it right, it does not cool the object down. it reflects incoming IR. Tuning to the atmospheric IR window has the only sense – it reflects preferably the band where sun can warm the object up directly. So where it helps is preventing objects in direct sunlight to warm up.
Nope they are saying it cools the object not just keeps it from getting warmer.
So…what happens when it gets dark, will the miracle blanket allow the object it has been protecting from the heat to cool down? Nope? Dang!
Yes – it works at night. This is why we get ground frosts. The trick was to make it work in the day.
The first clue this BS:
“Cooling is a significant end-use of energy globally and a major driver of peak electricity demand. Air conditioning, for example, accounts for nearly fifteen per cent of the primary energy used by buildings in the United States. A passive cooling strategy….”
Everyplace I can think of ‘air conditioning’ includes heating. In Virginia we hand a mild winter climate with a 30 degree differential temperature (70 inside, 40 outside) for heating but some places could 60 degree differential temperature. On a hot humid summer 15 degree differential temperature on average.
So, the first thing you need is insulation to reduce energy use. Then you have to account for moisture. Moisture condensing on colder surfaces will reduce efficiency of insulation and cause mold. In other words, it is important to do it correctly.
We bought our split level house in Virginia in the spring but did not move in July when we discovered the old system could not keep up. I found that the attic was not properly vented and 130 degrees. The vertical wall were not insulated either. You feel the heat going upstairs. First I installed a thermostatically controlled power vent. Next I installed reflective covered foam insulation board on the vertical walls that is also a fire barrier. Then I put a reflective barrier on top the insulation over the ceiling joist. After completing the project, the a/c could keep up and upstairs felt cooler.
Reflective heat barrier material were developed for space suits. Think of shiny aluminium foil with bubble wrap in between. For the attic get the kind that lets moisture escape. Added to existing insulation, it is more effective for summer than winter based on test results.
The bottom line is that there are products that already serve the same purpose. Nothing to see here folks, keep moving.
I am failing to see the mystery here… The material is absorbing mechanical energy (heat) from the material it touches, and emitting the energy as IR photons tuned to a narrow range. The reflective surface keeps the IR photons from being reabsorbed by the underlying material, hence improving the “cooling effect”. Why is this violating any natural laws? You are converting mechanical heat (vibrating matter) into radiant heat (IR photons), and then directing (reflecting) the photons away.
It would seem, at first glance, to violate the 2nd law of thermodynamics.
It doesn’t, of course, but that’s the way the Science article kind of reads when it says that the material is capable of cooling the underlying surface by up to 10 C. At a minimum, an explanation is needed.
“Cheap plastic film cools whatever it touches up to 10°C”
I think a more useful metric would be maximum BTU/hr over a given surface area.
In the underlying paper:
“and has a cooling power of 40.1 watts per square metre”
That’s about 136 BTU/hr, 30 or 40 square meters of the stuff and you’ve put a 5,000 BTU window AC out of work. I’m impressed (if it does work that well).
Willis,
This is a very interesting post. The ‘magic sheet’ looks to me like a scam. But whether it is or not, it is a neat challenge in the application of thermodynamic principles. So let’s assume that it really works…
If I read it correctly, the underlying assumption is that some object X situated at or near the ground is at some steady-state temperature T1. This steady-state is maintained by a balance between the incoming energy flow to the object (convection from the surrounding atmosphere + direct absorption of the Sun’s SW radiation) and the outgoing energy flow (convection from the object + LW radiation towards the atmosphere).
We now interpose the Magic Sheet between the object and the Sun.
The silver coating on the underside of the sheet cuts out all the Sun’s direct radiation. So the energy flow into the object X reduces its steady-state temperature significantly to some lower level T2. That’s great – so far. It’s called a sunshade.(I will need a grant to pursue the idea of a silver surfaced sunshade further…)
But there is more…because this is no ordinary silver-surfaced sunshade. Instead of radiating across a broad spectrum it radiates more efficiently through the atmospheric window directly to space. So T2 is somewhat lower than it would be if the sunshade consisted just of an ordinary silver sheet.
My problem is: how much lower? I strongly suspect that in any practical application the convection effects (which remain the same in both cases) would considerably outweigh the differing radiation effects. Because (as usual in most real situations) convection effects tend to dominate.
Willis,
I found the paper. It was on the lead author’s (Shanhui Fan) website at Standford:
“Passive radiative cooling below ambient air temperature under direct sunlight”, Aaswath P. Raman, Marc Abou Anoma, Linxiao Zhu, Eden Rephaeli & Shanhui Fan
web.stanford.edu/group/fan/publication/Raman_Nature_515_540_2014.pdf
A quick scan revealed no claim of “pulling heat” from objects, so that particular phrasing was apparently invented by the journalist, otherwise it does indeed seem to work as described, by using a “phototonic radiative cooler” which optimizes solar reflectance and IR transmittance.
On page 3 is a picture of the device, equipped with a 200mm photonic cooler disc, sitting a on Stanford rooftop. As a scientific control (page 4) they built two more devices, identical to the first except the cooler disc was replaced by plain 200mm discs covered with black paint and aluminum resp.
Not surprisingly the black disc reached temperatures about 40C higher than the ambient air temperatures. The aluminum disc was 20C higher than the ambient air temperatures, while the unit with the photonic device was 4-5C cooler than the ambient air temperatures.
I have to say I’m impressed by these results.
Let’s try that paper URL again:
http://web.stanford.edu/group/fan/publication/Raman_Nature_515_540_2014.pdf
Johanus, you are THE MAN!
The paper is everything I’d hoped for, including just what I’d asked—the planck emission curve in the thermal IR region.
I’ll add it to the head post, credited to you.
w.
johanus-
Thanks for the link to the stanford paper.
“As a scientific control (page 4) they built two more devices, identical to the first except the cooler disc was replaced by plain 200mm discs covered with black paint and aluminum resp.”
It is unfortunate that they did not use as a control a silicon wafer coated with a state of the art white roof paint which both reflects highly in the visible and also has high emissivity in the infrared. The aluminum they used has good reflectance in the visible but an absorption band in the near infrared (dropping to 75% R at 800 nm) and poor emissivity (0.01 – 0.2) in the infrared. Also, why not use a silicon wafer coated with the same silver film but not the photonic layers? That would have about the same solar absorption but have a much lower infrared emissivity compared with the photonic structure.
The white roofing paint reference sample would have been a much better comparison to determine the effect of narrowing the emissivity band to coincide with the atmospheric window, as compared with a conventional solution that can be purchased at your local hardware store.
From the paper –
“Further, the cold darkness of the Universe can be used as a renewable thermodynamic resource, even during the hottest hours of the day.”
Yeah. Right. A couple of years have passed. Still no wondrous avoidance of thermodynamic laws.
Amateurish experimental setup. By now, I expect these “researchers” to realise where they made their mistakes.
Oh well, on to the next piece of silliness. Maybe someone could try to heat a thermometer using CO2!
Cheers.
See the update on the head post …
w.
I may be wrong but I don’t think the claim is to frequency-shift infrared radiation. Rather it is to change the shape of the energy distribution wrt wavelength.
Peter, if you shift the frequency you shift the wavelength, and vice-versa.
‘w.
“if you shift the frequency you shift the wavelength, and vice-versa.”
Yes, I know. My point was that it is shifting the energy distribution to wavelengths that radiate more strongly through the atmosphere. No magic, entirely consistent with thermodynamics.
I may be wrong but I don’t think the claim is to frequency-shift infrared radiation. Rather it is to change the shape of the energy distribution wrt wavelength. The report doesn’t say, at least in the one I read, what the temperature of the surface was when the film was laid on top and statements like “the radiation drifts away into space” and the film “sucks heat from the surface” are just daft. It does seem possible, however, that resonance at a particular wavelength will cause faster cooling at the point of resonance (a glass bead) to which heat will then be transferred from its surroundings (the plastic film and surface underneath). I suspect this film only has any effect when it is applied to a surface already hotter than the air. Obviously the silver foil reduces absorption of solar energy by the surface under the film but the sunlight also stimulates the resonance. The film would only work if more energy is radiated by the resonant beads than is absorbed from the sun, stated to be 4%. I wonder whether it works in the dark.
I bought a flower vase at a second hand shop. Nice round belly with a thin neck, fitting perfectly for one red rose. Put in some water and set it on the living room table. In the afternoon, the sun shone in through the window, through the vase and a spot about 20cm behind the vase started smoking.
So, whenever I see round glass things, I see lenses. The area behind this material may well be 10° C cooler, until the plastic stuff starts melting or burning.
Yes this can work, See Pictet’s experiment of 1800. Which also explains why the cold gases in the atmosphere cannot warm the surface of the Earth permanently.
See here;
http://www.tech-know-group.com/papers/Pictet-Apparent_Radiation_and_Reflection_of_Cold.pdf
Willis, with all due respect, you have simply disproved the “Radiative Greenhouse Effect”.
This new film is merely optimized for maximum emission at a wavelength band that is most transparent.
This film is also optimized to block transmission of incoming radiation.
Thus it allows minimal radiative heating and maximum radiative cooling, much like the mirrors in Pictet’s experiment.
The practical problem is durability, any plastic that is expected to see long term exposure to sunlight and is specifically designed to be an excellent absorber of sunlight will fade.
Cheers, KevinK
KevinK,
You say “See Pictet’s experiment of 1800. Which also explains why the cold gases in the atmosphere cannot warm the surface of the Earth permanently.”
On the contrary, the document you refer to is a wonderful description of the thermodynamic work of the 17th/18th century on the physics of electromagnetic radiation that culminated in showing that when two radiating objects at different temperatures face one another, the only flow of energy between them is equal to the DIFFERENCE between their two facing radiative potentials – and is ALWAYS in the direction from the warmer to the cooler object.
This is exactly the situation in the atmosphere, where the higher temperature of the ground compared with the air above causes a net flow of radiative energy upwards, but at a reduced rate compared with the rate that would occur if there were no atmosphere, but only space at a temperature of 3K.
So just because the air exerts a radiative potential downwards, you don’t have to believe that any energy flows backwards towards the ground from colder to hotter.
it’s more fun than that, it’s active 🙂
KevinK,
Pictet’s experiment led the way to under standing that the rate of energy transfer between a hotter and a cooler radiating body is the difference between the radiative potential exerted by each body in the direction of the other. Because the radiative potential from the hotter body is greater than that from the cooler body, it follows that energy transfer is ALWAYS from hotter to cooler in full conformance with the 2nd law of thermodynamics.
In the case of the ground-atmosphere interface, the energy flow from the warmer ground to the cooler atmosphere will be less than would be the case if there were no atmosphere. There is no need to postulate that energy flows from the atmosphere to the ground, only that there is a POTENTIAL exerted in that direction which is overcome by the larger POTENTIAL exerted by the ground towards the atmosphere, the difference representing the magnitude of the energy flow.
So Willis has in no way “disproved the greenhouse effect” due to radiative atmospheric gases (which is real but most probably of negligible amplitude).
Killer Marmot February 13, 2017 at 10:35 am
See the update to the head post … I see no violation of the laws of thermo.
w.
You don’t see violation of the laws of thermo in a thought experiment where more energy comes out of a steel shell than is in it.
I fear you’ve misunderstood something. The steel greenhouse example is balanced at all levels—the same amount of energy enters the steel shell as leaves the steel sheel. The difference is that the shell has ~ twice the surface area as does the planetary surface, so the shell radiates at half the intensity per square metre.
Take a careful look, and multiply the emission in W/m2 by the areas. You’ll se it all balances exactly.
w.
I’ll take a roof’s worth. Maybe a car roof, too.
Car roof is interesting … seems like you could do it as a paint …
w.