Passive radiative cooling of surfaces below ambient air temperature

[greypilgrim]

Apparently, it's possible to cool surfaces below ambient air temperature by passive (no input of energy required) radiative cooling to harvest water from the atmosphere:
https://www.science.org/doi/10.1126/sciadv.abf3978

There's also a Nature paper about this effect, but behind a paywall:
https://www.nature.com/articles/nature13883

This looks like a textbook example of a device violating the zeroth law of thermodynamics, but apparently it's not. Why?

 

[jrmichler]

The key is in this quote from the first paper you linked:

A conventional radiative cooling foil that is used for dew harvesting emits thermal radiation in the wavelength range where the atmosphere is mostly transparent (8 to 13 μm) and can therefore directly emit heat radiatively to space. This effect cools the foil below the dew point temperature of air, causing water to condense upon it, which is termed “dewing” (20). Fortunately, most regions affected by high water scarcity have a natural advantage of abundant sunshine and hence a clear sky, which are optimal conditions for radiative cooling (21).

The surface is losing heat by radiation to the night sky while gaining heat by convection. It can only gain heat by convection when it is cooler than the surrounding air. A practical means of harvesting water is then a matter of optimizing the details. It helps that in a desert climate, the day to night temperature change is about 40 deg F.

Thus, there are not violations of the laws of thermodynamics.

 

[greypilgrim]

But still, thermodynamics states that two systems in contact will reach the same temperature in thermodynamic equilibrium. So what's not the case here: is this not equilibrium, or is the surface somehow not in complete contact with the surrounding air because it's partially transparent?

 

[jrmichler]

The surface is in convective contact with the air.
The surface is in radiative contact with the night sky.
The night sky is at lower temperature than the air.
Therefore, the surface temperature will be between the air temperature and the night sky temperature.

Good search terms to learn more are radiation heat transfer.