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Comment by dgacmu | Hacker News Reader

This article is a fairly terrible summary, particularly when it comes to explaining the really neat thing about these coatings: It's not just that they reflect a lot of inbound light, they also radiate heat in a part of the infrared spectrum that can pass transparently through the atmosphere.

https://en.m.wikipedia.org/wiki/Passive_daytime_radiative_co...

Which is one of the major reasons that these specialized coatings are better than just white paint or mirrors or things like that. They can actually cool the structure to below ambient temperature.

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Even the wikipedia page is confusing for someone who knows basic thermodynamics.

> PDRCs can be broadband in their thermal emittance capacity, meaning they possess high emittance in both the solar spectrum and atmospheric LWIR window (8 to 14 μm), or selective emitters, meaning they narrowband emit longwave infrared radiation only in the infrared window

It's not really that you want to avoid emitting thermal radiation per se at the frequencies where the atmosphere is opaque. It's that by the second law your thermal (i.e., passive) emissivity is equal to your absorptivity, and you want your absorptivity to be low in frequencies where the atmosphere is opaque and hence is warm. In other words, by basic thermo your passive behavior at each frequency is some value ranging between insulative (reflective) to absorbtive (transparent). If insulative, you will neither thermally emit nor absorb at that frequency. If absorbtive, you will both thermally emit and absorb at the frequency. So you want to be insulative at the frequencies where you would be thermally coupled to the warm atmosphere and you want to be conductive at the frequencies where you would be thermally coupled to cold outer space.

It would be interesting to put these out at night and have them drop a designated area below the dew point and collect water.

As I understand it, nighttime temps get near the dew point but often don’t hit it because water in the air releases so much heat.

Mold/algae will grow on the wet surfaces in a matter of days, destroying the main promise of the material.

Im not so sure. Mildew has to eat something. Assuming the surface is kept clean and water is collected regularly, it shouldn't be a major problem.

You don't see mildew growing on cars very often, do you? Yet, growing up in Florida, I'd see few covered cars in the morning all the time.

Cars not, they go to wash. Those who don't are indeed visibly dusty/moldy.

Wet roofs and other surfaces -- pretty much all the time, especially in humid climates (like Hong Kong, where the research is made). Same for glass windows (we have window-washer job for that reason).

Surfaces can be cleaned with pressure washer, but unlike purely aesthetical reasons, I'm saying that the main feature of this material may stop working in a matter of days due to even minuscule amount of growth.

At night they would do the opposite what you suggest. A very white surface would stay warmer, a black (in the sense of IR black) surface would radiate heat better and cool below dew.

Wouldn't a black surface be absorbing heat from above it?

The question then is, what heat? From the stars and moon is almost nothing. No idea what it exactly is, but assume that a clear sky at night has about -170 C temp (you are looking at the cold universe, only some CO2 in between to absorb). So the net radiation is always outbound.

And the best way to radiate are black (not the color but as in the definition of heat radiation) objects.

The question I was really asking was answered by this: https://physics.stackexchange.com/questions/159856/why-is-bl...

I.e. surfaces which are good at absorbing radiation are also good at emitting, which I didn't know.

This is because the blue sky is very "cold". Point an infrared thermometer up there and see... Very cold indeed.

Which usually objects can't fully exploit, because the heat they radiate away is absorbed by their immediate surroundings, which radiate part of it back towards them. You can "cheat" that by radiating away heat in a spectrum that isn't absorbed by anything, removing it from the area.

Basically. To be clear, it's not that you are worried about getting "the same" heat reflected or otherwise radiated back toward you. (The radiation emitted by your roof doesn't appreciably change the radiation incident on your roof from the atmosphere.) It's that if you are passively emitting at some frequency you are necessarily absorbing at that frequency too (by the 2nd law). And you don't want to be absorbing at the frequency where the atmosphere is opaque and hence warm. In contrast, it's fine to be absorbing at the frequency where the atmosphere is transparent, because then you're just exposed to cold outer space.

I had no idea what you were talking about until I remembered that infrared is just a Radio wave and therefore everything is an antenna.

Antennas that are good at transmit at X frequency tend to also be good at receiving X frequency necessarily. And different 'colors' are nothing more than different antenna resonance points.

Wow. I've never thought of heat in that way. Mind blowing.

You could use chimneys that are coated by the irradiative material on the inside, pointing all the IR up into the sky.

I had to look this up - apparently it's around 0C if you do this! https://mynasadata.larc.nasa.gov/sites/default/files/2018-12...

Most materials, aside from metals, of sufficient thickness have high high infrared emissivity. All white paints, for example, already did radiate heat upwards through the atmospheric window. These more reflective films are no more emissive than previous paints (they're all quite emissive to begin with!)

The only difference, optically speaking, really is that they are better solar reflectors.

The difference is that the wavelengths in which they reflect and emit are different. Where you emit, you absorb. White paint has a lower solar reflectance because it emits and absorbs across a broader band. The better PRDC coatings absorb/emit more tightly in the atmospheric IR window.

And literally cool the planet. That ir that passes through the atmosphere is heading into space, at least in clear days.

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