Bending the Laws of Thermodynamics

[VEBill]

Tech News: Materials that emit more energy at the ‘IR window’ into space than they absorb in the visible. Even placed in direct sunlight, they reportedly manage to passively cool themselves below ambient. I wonder if they depend on clear skies, as opposed to any overcast clouds? Still, this seems to be a counter-intuitive and very interesting technology, possibly with very big implications.

Related links:

https://www.ted.com/talks/aaswath_r...cold_of_outer_space_into_a_renewable_resource

(TED Talk)

https://www.nature.com/articles/nenergy2017143

(Abstract)

https://www.technologyreview.com/s/...o-space-could-soon-reinvent-air-conditioning/

https://www.technologyreview.com/s/600964/the-sky-may-hold-the-secret-to-efficient-air-conditioning/

https://www.skycoolsystems.com

 

[moltenmetal]

If I understand correctly, these materials work for the same reason that dew and frost form on car windows etc. But yes, the effect must be dependent on atmospheric conditions.

 

[IRstuff]

It works because the sky's blackbody temperature is actually well below freezing. This is one reason you can get frost on upward point surfaces at night, even with an above-freezing air temperature. Any, yes, clear skies would be better, although higher clouds are possibly close to, or below, freezing as well. It looks like the "quirk" is that even in direct sunlight, the sun only occupies occupies a tiny portion of the field of view of any surface with a clear view of the sky. The LWIR band was chosen specifically because 300 K-ish temperature blackbodies have their peak at 10 um. For the 8-13 um band cited, the sun's input is only about 1 W/m^2, while the emission is about 59 W/m^2


A couple of the Nature articles cited are on the Stanford website:

https://web.stanford.edu/group/fan/publication/Raman_Nature_515_540_2014.pdf

https://web.stanford.edu/group/fan/publication/Goldstein_NatureEnergy_2_17143_2017.pdf

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[VEBill]

Solar incident radiation is famously about a kilowatt per square meter. Somewhere Raman mentions reflecting about 97% it.

Thanks for the links.

 

[IRstuff]

It's paradoxical in a way, that you can't actually reflect 100%, because you then can't emit anything; so 97% gives you 3 percent to play with.

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