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In my fiction I have a city deep underground on the planet of Venus. I know this is deeply impractical, and there are far easier places to build an underground city, but that is not what I have done.

The question is this, could I protect the city inside a shell of suitable material that uses the Peltier effect to cool down the internal space from the extreme temperatures outside of the shell. And if so, presumably I would need to generate an enormous amount of energy to power the Peltier effect, so, any ideas on the best method of generating such energy deep underground on Venus.

Any other ideas for alternative heat exchangers that would work on such a large scale, and under such extreme temperatures would be appreciated.

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    $\begingroup$ What is the constraint which makes you require the use of the (grossly inefficient) Peltier effect instead of more reasonable refrigeration methods? $\endgroup$
    – AlexP
    Mar 10, 2023 at 13:42
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    $\begingroup$ Have you never seen a refrigerator or an air conditioning unit? Anyway, Wikipedia has a decent article on refrigeration, and detailed articles on refrigeration cycles, compression refrigeration and absorption refrigerators (the last one only if you have something against moving parts). $\endgroup$
    – AlexP
    Mar 10, 2023 at 14:06
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    $\begingroup$ The key to any refrigeration method is having a place to dump the excess heat. The hotter the area outside the refrigerated zone, the harder it is going to be to get rid of the heat. Can it be managed at all on Venus? $\endgroup$ Mar 10, 2023 at 14:09
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    $\begingroup$ @AlexP that's not a limit, just the point where half the heat you need to reject comes from running the heat pump. There is no physical limit preventing you from stacking heat pumps to achieve the needed temperature difference between a habitat and the surface temperature of Venus...it's just going to require huge amounts of power. (and Peltier junctions would be an unusually inefficient way to do it) $\endgroup$ Mar 10, 2023 at 17:23
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    $\begingroup$ Unlimited electrical energy, same problem as always everywhere, -1. What is the coolant needed for a heat exchanger (at 900f and 90 bar) on Venus? +1. en.wikipedia.org/wiki/Molten_salt ? $\endgroup$
    – Mazura
    Mar 10, 2023 at 22:47

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Peltier coolers are what you do when you need something solid-state. Based on your comments, it sounds like you'd take any solution, but you just didn't know how to ask, so I'm going to suggest "multi-stage cooling".

Your standard air conditioner works by de-compressing a liquid, turning it into a gas, which sucks heat out of the environment. You then move the gas somewhere else and compress it back into a liquid, letting off that heat. On our planet, air conditioning is almost always looking to move heat between something that's above 50 degrees to something that's below 120, so that's what our coolant fluid is optimized for.

Coolant fluid isn't a special magical substance. There are a few qualities that make something like freon a better coolant than others (non-reactivity and high heat capacity, for instance), but all you need is a system that can compress gas into a liquid at the hot end, and let that liquid re-evaporate on the cool end.

You can facilitate higher temperatures with the same coolant by increasing the compression pressure. You could also use a coolant with a higher boiling point.

You would likely need to nest environments so that the temperature steps down in phases, using a different coolant at each transition.

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    $\begingroup$ Thanks, that is a wonderful idea. I like the Russian doll nature of nested environments, each being a different gradient of cooling. Do you know of more gases which exists naturally on Venus which could be compressed in this way? $\endgroup$ Mar 10, 2023 at 20:24
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    $\begingroup$ @EversonThomas I know that there are search engines that can probably answer that question. $\endgroup$
    – Ian Kemp
    Mar 10, 2023 at 21:58
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    $\begingroup$ @EversonThomas, No, that's outside of my domain. For the higher temperature environments, you're probably actually looking for things that are liquid at room temperature, or maybe even solid. I suggest handwavium or trimethybutylmadeitup. You're going to have to import tons of hydrogen to do anything useful on Venus, so a little extra coolant won't be noticed. $\endgroup$ Mar 10, 2023 at 23:40
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    $\begingroup$ The nested enviroments might even a good story point. The elite is living closer to the colder center and the outcast have to survive in the hottest outer ring $\endgroup$
    – Zibelas
    Mar 11, 2023 at 8:54
  • $\begingroup$ @IanKemp What would you type in a search engine to find out what gasses on Venus might be used as refrigerants? $\endgroup$ Mar 11, 2023 at 13:10
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I am afraid not.

The Peltier effect in a nutshell is that when current flows through a junction of different conductors, one gets hotter and the other colder.

While this can and is used in thermoelectric cooling, Venus is a completely different league than Earth.

For starters, to have a cooling device work you need to be able to bring the energy at the temperature higher than the environment where you are going to dump it. This means that the bar on Venus will be set pretty high: the surface temperature is 470 C, and I don't expect it to be much lower anywhere below the surface, considering that Venus seems to be still geologically active and that on Earth temperature increases with depth thanks to the heat flux coming from the inner of the planet, so something similar will happen on Venus.

Now, can you get a Peltier cell whose hot side is hotter than 470 C?

From what I have found with a quick search, 90 C is the upper limit for good performances, while until 150 C the performances are acceptable.

enter image description here

And we are still far from the 470 C of Venus.

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    $\begingroup$ Mostly correct, peltier coolers made with silicon semiconductors won't work because they tend to physically breakdown above 150-200C. You could however make a semiconductor out of different materials which could use the same principle. We just never have because our semiconductor industry primarily uses silicon chemistry, and it works for our normal ambient environments on Earth. $\endgroup$
    – Josh King
    Mar 10, 2023 at 22:17
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    $\begingroup$ Note that the MMRTG uses PbTe/TAGS junctions, where TAGS is an alloy of tellurium, silver, germanium, and antimony. The design hot side temperature is 541 °C. It uses these as thermoelectric generators, of course, but they will work in reverse as heat pumps as well. (Not that it wouldn't still be a horrible idea due to the inefficiency.) $\endgroup$ Mar 11, 2023 at 23:31
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Mercury would be a gas on Venus' surface. This could be the the first stage of a condensation-evaporation refrigerator. I think you would need two more stages before you get to habitable temperatures.

The only reference I can find for cooling a Venus lander is this one. This vents ammonia and helium to the atmosphere, so it only lasts 24 hours.

See also Stirling Engines

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  • $\begingroup$ Thanks, this is a great response. Please forgive my ignorance, but if the nested levels were insulated from each other, could Mercury be used in each level to bring the temperature down multiple times, or can it only be used once? $\endgroup$ Mar 10, 2023 at 20:29
  • $\begingroup$ @EversonThomas what locks down it's usage is the temperature (and pressure) it boils at. Different pressures would change the boiling temp. So maybe. Equipment used puts it's own constrains on whatever refrigerant you use. It must withstand temp, pressure, and not react. We're exploiting that it takes a lot of energy to make the refrigerant change phase. But we're using this old trick in an unusual situation. That's a good way to get surprised. $\endgroup$ Mar 10, 2023 at 22:05
  • $\begingroup$ As the Stirling engines mentioned at the end of the answer demonstrate, you don't necessarily need a phase transition, though it's helpful for moving large amounts of heat and you'll most likely want it for most stages. A Stirling cooler operating near ambient pressure in the surface level atmosphere might perform pretty close to the same level due to the density of that supercritical atmosphere, and might get the cold side temperatures low enough for a phase-change refrigerator using CO2 as a working fluid. $\endgroup$ Mar 11, 2023 at 23:38
  • $\begingroup$ Many cooling systems are efficient over a finite temperature and temperature difference range. Peltier and magnetic systems have a particularly narrow range, so you may have a cascade of many systems to do a large temperature drop. I do not know what sort of a temperature drop you might get, but 100C would be a lot for something with a large heat throughput. That would get you from 475C to 375C. You would then need three more jumps of 100C to get to something comfortable. Or just some out at night where the temperature may be lower. $\endgroup$ Mar 12, 2023 at 13:37
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To ammend the answer from @Robert Rapplean:

A thermodynamic cycle can be done with gases alone. The liquid phase improves efficiency and simplifies the design, but is not absolutely required.

What one could use: The coldest stage: R600 or R600a in order to bring the temperature down to ~15C at the cold side and ~120C at the hot side.

The intermediate stage: Ordinary water to transfer heat between ~100 and ~350C. You will need water stockpile anyway.

The hottest stage: gas phase only, using either the atmospheric mixture or somewhat purified carbon dioxide, going from 300C to the external red-hot temperature (~550C). This part will have significant leaks because no sealant is effective at these temperatures, on the other hand the fluid is readily available from the atmosphere.

If you happen to fix the leak problem (and a host of other problems, anyway) liquid / vaporized sodium is a good candidate for a phase-change refrigerant at the hottest stage.

All of the above is pretty much used today in various fields here on Earth (well, sodium is used as a liquid only). As we know from the IT field, at scale everything breaks, but your mileage may vary.

Venus has strong and sustained winds so the energy will not be an issue even with this rather complex cascade.

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