I How does the cooling effect of this "white-est paint" work?

[jg167]

A new result from Purdue https://www.purdue.edu/newsroom/rel...-is-here-and-its-the-coolest.-literally..html
Is about the development of a new paint that is the whitest yet, reflecting 98.1% of solar radiation. That is so much that they claim that it causes a cooling effect unseen with previous white paints. This is pretty significant, in the range of 8-19 degrees F. This teases the possibility of very low power (fan only) air-conditioning, which seems like it must be violating basic thermodynamics but hey its Prudue making these claims.

The key seems to be that the reflectivity is so high that the energy absorbed is less than the energy radiated by the painted plate (or any mass above absolute 0) but I might well be misreading that. If that is all it takes wouldn't we expect a very good mirror (99.9%) to do the same thing, and I've never heard of a a mirror being cooler than its surroundings all by itself (but maybe they do?).

Anyway I'd love to hear some elucidation on this.

 

[hutchphd]

There are two competing processes: (1) absorption of the directly beamed sunlight and (2) semi-isotropic thermal radiation characteristic of the temperature of the roof surface.
The enhancement of "whiteness" means that more of the visible and near visible light from sun (98.1%) gets reflected . They also claim that this stuff looks "black" at the 10 um (far IR) blackbody roof radiation peak, so the 300K roof is free to radiate into cold (3K) dark space. According to their reckoning this radiation part is bigger than the 1.9% absorption of sunlight. Seems possible to me.
Your point about the mirror is well taken but it will not absorb (and therefore will not radiate) the 10 um infrared.

 

[sophiecentaur]

Your point about the mirror is well taken but it will not absorb (and therefore will not radiate) the 10 um infrared.

I am trying to compare / reconcile this with how IR coating on large area window glass reduce incoming IR on sunny days and reduces net loss of IR when it's cold outside. I can confirm that the expensive windows we bought for our conservatory are very effective in reducing extremes of temperature inside. But, of course, the glass is transparent to visible light.

The 'Science' behind my windows is claimed to be Nano Particles. They get everywhere these days, don't they? The self cleaning roof glass is said to use them too.

 

[hutchphd]

From this (random) article I see that the nanoparticle coating is absorbing the near infrared and so is mostly just blocking (by absorption apparently) incident solar flux $\lambda>1000nm$.
The radiative processes for a blackbody at 300K peak in the far infrared $\lambda=10\mu m = 10,000nm$ and probably not very important. This was a pretty small effect for the really white stuff too.