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

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: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.