New Spray-On Coating Can Make Buildings, Cars, and Even Spaceships Cooler (bgr.com)

Long-time Slashdot reader davidwr and Iwastheone both submitted this story about "a paint-like coating that facilitates what is known as 'passive daytime radiative cooling,' or PDRC for short...when a surface can efficiently radiate heat and reflect sunlight to a degree that it cools itself even if it's sitting in direct sunlight." BGR reports on research from the Columbia School of Engineering: Their newly-invented coating has "nano-to-microscale air voids that acts as a spontaneous air cooler," which is a very technical and fancy way of saying that the coating is great at keeping itself cool all on its own. "The air voids in the porous polymer scatter and reflect sunlight, due to the difference in the refractive index between the air voids and the surrounding polymer," Columbia writes in a post. "The polymer turns white and thus avoids solar heating, while its intrinsic emittance causes it to efficiently lose heat to the sky."

It sounds great, but the best news is that it can be applied to just about anything, from cars to spaceships and even entire buildings. The team believes their invention would be an invaluable resource for developing countries in sweltering climates where air conditioning is impractical or unavailable.

Comments

Does it work on people?

[MrKaos]

For example if someone is being a bit of a dick, you just spray some of this on them and they become cooler?

You'd be the life of the party.

Re:It's called "white paint"

[drinkypoo]

Seriously, how much better is this than using plain old white paint?

It's better in that a white surface doesn't radiate much, so even though it reflects most of the energy that strikes it, it still heats up. It's worse in every other way. These nanoscale structures are always fragile and turn into nanoscale dust, and which don't break down easily (because of what they're made of) which makes them persist in the environment. Auto paint is expected to last for decades in harsh conditions, but they gave that as an example anyway.

Re:Useful in the Developed World

[Mr D from 63]

Questions that come to mind are "how much emittance occurs which there is a high delta-T? How long does the coating last and how easily is it's emittance reduced by normal environmental conditions (dirt, microbials, etc)? How much would it cost? Any toxicity concerns?

Any concerns about nanoscale particles?

[Aristos Mazer]

How well does this paint clump together? As a spray, it seems likely that some of the particles would linger in the air. Medical researchers are still trying to get a handle on the risks of nanoparticles to living beings -- we just aren't made to filter stuff at that level because its not common in the natural environment for there to be free-floating bits at the nanoscale. It might be safe, but we don't currently know. Here is one report from the UK government just listing the research unknowns about nanoparticle exposure. Before we start spraying whole buildings in stuff like this, we should know the environmental and health impacts. I hope the inventors of this stuff start working on those questions before they try to productize this.

Re:It's called "white paint"

[Iwastheone]

I've done a lot of roofing in my life, one type being a rubber roll that gets melted down with a propane torch for flat roofs. An aluminum coating in a can needs to be applied to the rubber roof in order to reflect the ultra-violet rays of the Sun. If this special coating isn't applied, the rubber rots away within 5 years. Reapplying this silver coating every 5 years can almost indefinitely extend the life of the rubber roof. Does this material do this also?

reciprocity, emissivity reflectance

[goombah99]

This is a tricky point of physics which is seldom explained well, and I don't understand it much more than the basic idea. But in nearly everything emissivity at a given wavelength is the compliment (1-x) of the reflectance.

It's a frustrating idea because it says that while white can keep the sun off you (reflecting) it also doesn't radiate either keeping you hot.

Now the escape clause here is that if you are not as hot as the sun, then your black body emission is in the mid IR while the sun is peaked in the visible. Thus if you can make something white looking in the visible but black in the Mid IR it can beat the system and help cool by both reflection of the sun and thermal emission.

Now this device seems to be arguing for something different it sounds like they are saying they use a dark (emissive) material but then make it so granular and scattering that it looks white (which indeed is why clear crystals look white when you powderize them (e.g. sugar).

But my understanding of this, possibly wrong which is why I'm asking, is that it doesn't matter why something looks white to the emissivity/reflectance relation. If a nominally black material "looks" white to the eye then it's going to act identically to an object that is actually pigmented white. It doesn't matter if its refraction, reflection of pigment that causes it to be white. White is white.

THus the explanation seems like B.S. However it still might be working just fine for the old boting reason of being a mid IR black object. that just would not be news.

The reason I'm wobbly on this is when you think about absorbption down at the atomic transition scale then the whole emissivity, reflectance thing gets confusing. It becomes unclear how discrete spectral lines actually add up to that. It's much easier to treat the relation at the macroscopic black body level.

SO is it really possible for this nano scale voodoo to do something other than white paint does?

Re:reciprocity, emissivity reflectance

[Ramze]

I've been reading about this daytime cooling through infra-red emissivity to space stuff for a while now. The biggest benefit isn't the color (or the reflective value), but that it will absorb heat from whatever the source and radiate that energy away in the mid-IR band that will allow it to leave Earth's atmosphere without being re-absorbed by anything nearby. That makes it effectively a way to remove heat from the surface of the earth by emitting that energy into space.... heat that would otherwise be trapped by our atmosphere's greenhouse effect.

This is the first time I've seen this expressed as a coating for everyday consumer items rather than as a heat sink layer added to an exterior A/C unit or a potential roofing material, though.

My understanding is that generally these coatings are white in the visible spectrum to reflect sunlight, but emit light in the mid-IR range. There's a startup company for using this to improve efficiency in A/C units I read a while back, and they tested the material in the hot sun on a roof in India -- you could put your hand on it after it had been baking in the sun, and it was cool to the touch. That's relative term, though. I don't recall the actual temperature readings.

Re:It's called "white paint"

[hankwang]

In thermal properties it's not different from most other white materials or white paint: they absorb and emit well in the thermal infrared range (5 to 15 micrometers) and reflect well in the visible and near-IR (400 nm to 1500 nm). Even plain white paper would do the job. The effect of this fancy coating is possibly a marginal improvement in the 1500-2500 nm range, but sunlight does not carry that much energy in that range.

The opposite is much harder: absorb sunlight but don't emit or absorb thermal infrared. That's what you need for solar water heating. It can be done (silicon layers on top of reflective metal) but it is expensive and sensitive to contamination.

Re:It's called "white paint"

[mspohr]

Seriously, if you read TFA, you'll find out that:
1. It's more reflective than plain white paint
2. It emits IR which can actually bring the surface temperature BELOW ambient.

HOAs!

[TechyImmigrant]

I'd love to see what crap the HOA comes up with when I paint my house white.

Re:I Need this Yesterday

[Pinky's Brain]

They only claim to do it better, they are not the first. Skycool in Australia has been selling their wavelength selective reflecting/absorbing paint for over a decade now. Unfortunately they don't seem to want to get into the consumer or even small contractor supply chain, or at least they don't hint at it on their latest website. On their old website they had a link to a contractor, but that one is dead now.

https://www.skycool.com.au/