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:
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,
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 …