[OrganicMSG]

> A heat sink will not cool the air below ambient. This is required for condensation. No configuration of heat sink + device will decrease the temperature unless we put some "work" into the system.

A heat sink surrounded with foam with an opening left that looks down the focus of a satellite dish that has been carefully covered in tinfoil will get colder than ambient air temperature, if the dish is pointed at a clear sky.

[nkurz]

I'm not sure if you looked at the paper, but they take advantage of this phenomenon to lower the nighttime moisture collection temperature, and say it gives about a 3C benefit even without a satellite dish:

Operation in such arid regions also opens an interesting avenue for increasing water harvesting output with passive radiative cooling by leveraging the typically clear sky. The clear night sky and low vapour content in the atmosphere enables dissipation of long-wavelength (infrared) thermal radiation from the device to the cold sky to cool it below its ambient temperature. By facing the device to the sky during adsorption, a ~3 K temperature drop was achieved, which corresponds to an increase in 5–7% RH experienced by the adsorbent. This passive cooling can lead to opportunities to utilise other adsorbents that have their adsorption steps located beyond the typical levels of RH in specific regions.

https://www.nature.com/articles/s41467-018-03162-7

[cwkoss]

Why does the clearness of the sky matter?

Wouldn't cloud cover just cause the heat energy to be absorbed by the clouds or reflected back to a much larger area of the ground (so only a small percentage would return to the device itself)?

[nkurz]

I agree that the common explanation is confusing. If I understand it right, it's not the transparency of the sky to the outgoing radiation that matters, but the absence of a (relatiely) high temperature emitter like a cloud. The two are closely tied (if there is no absorber, then there is no emitter) but the "clear sky" explanation mixes up cause and effect.

Rather than emphasizing the transparency, a clearer explanation would emphasize the absence of a counterbalancing warm mass whose emissions can be absorbed. Objects are always losing energy by black body radiation, but normally are gaining thermal energy from their similarly temperatured environment. One a clear night, there is a greater imbalance.

[cwkoss]

Fascinating, thank you for the explanation

[cwkoss]

I understand why this works, but still feels like magic. Very cool.