Passive radiative cooling of surfaces below ambient air temperature

In summary, it is possible to cool surfaces below ambient air temperature through passive radiative cooling, which can be used for water harvesting from the atmosphere. This effect has been studied in a Nature paper, but is behind a paywall. Despite appearing to violate the zeroth law of thermodynamics, this effect is actually possible due to the surface emitting thermal radiation to space and being in convective contact with the surrounding air. This is not a violation of thermodynamic laws, and further research can be done on optimizing this process for water harvesting.
  • #1
greypilgrim
547
38
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?
 
Science news on Phys.org
  • #2
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.
 
  • #3
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?
 
  • #4
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.
 

FAQ: Passive radiative cooling of surfaces below ambient air temperature

1. What is passive radiative cooling?

Passive radiative cooling is a process by which a surface can emit thermal radiation into the atmosphere, effectively cooling itself below the ambient air temperature without the use of external energy sources.

2. How does passive radiative cooling work?

Passive radiative cooling works by utilizing the natural properties of materials to emit thermal radiation. Certain materials, such as polymers and metals, have the ability to emit thermal radiation at a higher rate than they absorb it, allowing them to dissipate heat and cool down.

3. What are the benefits of passive radiative cooling?

Passive radiative cooling can provide several benefits, including energy savings, reduced environmental impact, and improved comfort in hot climates. It can also be used to cool buildings, electronics, and other objects without the need for electricity or other external energy sources.

4. Are there any limitations to passive radiative cooling?

Passive radiative cooling is limited by certain factors, such as the surrounding environment and the material properties of the cooling surface. It is most effective in dry and clear conditions, and certain materials may not be suitable for use in all climates or applications.

5. How is passive radiative cooling being used in real-world applications?

Passive radiative cooling is currently being used in various real-world applications, such as cooling buildings in hot climates, improving the efficiency of solar panels, and preserving food and medicine. It is also being researched and developed for potential use in other areas, such as cooling clothing and vehicles.

Similar threads

Back
Top