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I'm working on the story of someone born in the tunnels after the earth has gone though a runaway greenhouse effect.

I want these tunnels to be real, with real infrastructure to keep the inhabitants alive.

There's a mountain of research to do, but the big question I have to start with: "Is this even possible?"

I was thinking that the tunnels would need to be completely sealed from their surrounding environment, like an underground spacecraft.

I am hopeful that you can still exchange heat with incinerating temperatures.

Is such a heat exchange possible?

In my imagination I'm seeing giant metal arm nearly glowing deep red, moving a soft metal short-lived piston though a harder metal cylinder. But I don't know what it's pumping, or at what temperature and pressure.

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    $\begingroup$ What's the timeline for survival? The answer massively changes if you want 10, 100, 1000, or 10,000 years. Also what's the temperature of the bedrock the tunnels are in? $\endgroup$
    – abestrange
    Apr 29, 2021 at 0:08
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    $\begingroup$ @abestrange - eventually, the minimum temperature of bedrock will be whatever the surface is. $\endgroup$
    – jdunlop
    Apr 29, 2021 at 1:04
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    $\begingroup$ Not sure you need reactors. There are a lot of ways to generate energy on the surface which would require minimal maintenance, (possibly mostly robotic) and as the planet is already in runaway greenhouse state I think you can ditch envirnomental concerns for pollution. :-) $\endgroup$ Apr 29, 2021 at 5:19
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    $\begingroup$ @StephenG Weirdly I think non-polluting wind power would be the way to go. Reactors that rely on a thermal gradient (like coal/oil/nuclear) will be much less effective in such a hot environment, but there’s a shedload of energy being thrown around by the atmosphere itself... $\endgroup$
    – Joe Bloggs
    Apr 29, 2021 at 8:41
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    $\begingroup$ @StephenG Time to break out the high altitude tethered zeppelin farms! $\endgroup$
    – Joe Bloggs
    Apr 29, 2021 at 18:54

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There is no reason not to live above-ground in well-insulated and cooled buildings. As far as the infrastructure goes, that's clearly much simpler. The reason it's not crazy is that the effect of insulation has an exponential relation to the thickness: Say, your house is in Canada and its insulation reduces the heat flow from 20°C inside to -20°C outside — a temperature difference of 40K — to 10% compared to being uninsulated (probably conservative). Now that the outside temperature rises to 260°C, the difference is 240K. Heat flow is roughly proportional to the temperature difference, so with a 240K difference you'll have 240/40=6 times the heat flow. Less than doubling your insulation, cutting the heat flow by another factor 10, should deal with that. Of course you need an A/C that can shed the same heat than your old heater generated, much more than an A/C typically can, but you can just add another layer of insulation. You now have 1m or so rock wool in your walls, and the outer surface is most likely neither wood nor PVC nor asphalt shingles, but that's OK.

You also don't need to insulate yourself from the atmosphere; once chilled it will still be breathable, potentially after filtering out smoke from the still-burning Tundra. You could use heat exchangers with outgoing air to pre-chill the incoming air before actively chilling it further to room temperature, effectively using a technology in reverse which is already in use today in low-energy houses.

Because of the cube-square law there would probably be more larger apartment buildings and fewer single houses. You could even have windows; they would likely be small and infrared absorbing, and would need internal cooling, probably by an embedded mesh of cooling fluid. The rest of the house would have reflective coating. Looking out through a normal glass pane would feel a bit hotter than looking into your kitchen oven at full throttle: Doable, but not for long. You also wouldn't want combustible materials close to the window; so it's safer to have infrared absorbing glass. At night and during storms (and there will be storms) you'd close tightly fitting shutters for more insulation and protection, just like you do in harsh cold climates today (except you wouldn't use wood).

Insulated and cooled vehicles or even something resembling bulky spacesuits (also with insulation and cooling) would make it possible to still enjoy the great outdoors. The infrared-absorbing windows and visors would also need internal cooling, like the windows in the house; or you switch to cameras and monitors/VR.

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    $\begingroup$ I like this - this makes a lot more sense than compounding your refrigeration problems with the problems of digging underground. $\endgroup$
    – jdunlop
    Apr 29, 2021 at 19:13
  • $\begingroup$ "There is no reason not to live above-ground in well-insulated and cooled buildings." Isnt that what would really happen anyway? Why live in tunnels? $\endgroup$
    – Len
    Apr 29, 2021 at 19:16
  • $\begingroup$ @Len - because the OP specified tunnels. $\endgroup$
    – jdunlop
    Apr 29, 2021 at 20:41
  • $\begingroup$ @jdunlop, Of course. That was rhetorical. I thought we were agreeing that the better option was to not live underground. $\endgroup$
    – Len
    Apr 29, 2021 at 21:26
  • $\begingroup$ I'm glad the transition from surface habitable to not didn't cause any wars to happen. $\endgroup$
    – Josiah
    Dec 7, 2021 at 1:17
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Heat can be dumped into an area hotter than the area being cooled; if that weren't true, we wouldn't have refrigerators.

Now, these would have to be spectacularly good refrigerators. Normal refrigerants probably would be insufficient, and they'd be on permanent cycle, and their failure wouldn't just involve a call to Maytag.

Happily, 500F isn't all that hot. It's oven-hot, but well below the melting point of most metals. Iron won't even be red-hot (sorry about your mental image). So you won't need any unusual materials in construction.

It would be an existence balanced on a knife edge, and "diggers" would still need somewhere to dispose of their spoil, but it doesn't seem impossible.

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    $\begingroup$ Normal refrigerants probably would be insufficient Why do you think that? Specialist refrigerants are typically only needed for accessing particularly low temperatures. Otherwise, it's just a working fluid and if you want a space to be a livable temperature then the refrigerants we use in normal AC units are probably exactly what you want. It's not the refrigerant selection that dictates how much heat you can remove - the only limitation there is the size of the refrigerator and its power requirements. $\endgroup$
    – J...
    Apr 29, 2021 at 10:55
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    $\begingroup$ Normal refrigerants will exceed their critical temperature well below 260 C, so if you wanted to avoid using a transcritical cycle, you would need to find a refrigerant that wasn't yet critical at 260 C. But you could certainly use a different cycle and be fine. $\endgroup$
    – Aliden
    Apr 29, 2021 at 14:59
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    $\begingroup$ @Aliden Fair point. Transcritical cycles can actually be more efficient, though, depending on the circumstances, and certainly everything else about the system design would need to change. I don't think we know of any single refrigerants that could work other than in a transcritical cycle, but it would be pretty simple to make a two-stage conventional system with normal refrigerants - one high critical to the environment and cooling an intermediate exchanger (primary expansion, secondary condenser) to below the critical point of the secondary fluid. $\endgroup$
    – J...
    Apr 29, 2021 at 18:04
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    $\begingroup$ @J... For sure. If you could solve the compressor problem and could get a good seal, you could use water as the refrigerant, although there's no guarantee that would be any better than a transcritical cycle. Would make for a neat design problem though... $\endgroup$
    – Aliden
    Apr 30, 2021 at 12:14
  • $\begingroup$ Also, moving heat from a cooler area to a hotter one takes energy. Energy generates heat. Ultimately you're going to need to have a way to dump that heat outside the closed system you're trying to cool. $\endgroup$ May 1, 2021 at 20:50
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Short and imprecise summary of refrigeration: A refrigerant is compressed, causing it to heat up. The heat is shed by pumping the compressed refrigerant through a radiator. The cooled compressed gas is then pumped into the cooled area and depressurized. In order to expand it must heat up, which it does by absorbing heat from it's surroundings. This is why deodorant is cold. The refrigerant is then pumped out and compressed again, allowing it to reject the heat it absorbed

Engineering a system like this for such a situation is difficult, but by no means impossible, probably even with current day technology. (I was thinking of a multi stage system with a couple different refrigerants hooked up to a coolant loop, but nevermind that.) What would be more difficult is heat management.

Cooling is very power intensive, probably even more so in the setting you're describing. Using any method of power production that produces significant waste heat (i.e. heat not converted to electricity or motion) will eventually become a problem. Using the ground as your heat rejection medium will also work only for the first couple of decades, until the bedrock slowly gets to temperatures similar to those of the atmosphere. Going deeper will find you getting increased temperatures as seen in the gold mines in south Africa. Basically you need a system that produces enough electricity and a small enough quantity of waste heat to cool itself. (This is not a perpetual motion machine, it is still being fueled externally)

Any kind of thermoelectric generator would likely be a bad idea, therefore burning biomass, geothermal and the temperature of the surface are all out. You will have to consider using thing like photoelectric (solar panels), wind and possibly aneutronic fusion, depending on how science- fictiony your setting is.

Another option would be to have your nuclear plant run hotter than usually practiced today and have it out on the surface, cooling down its coolant to the temperature of the surface at which your coolant is liquid, and then heating it back up to insane temperatures again, boiling, running through a turbine etc. This would give you an excuse to have red hod tungsten carbide pistons pumping around gaseous corrosive salt or tin.

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  • $\begingroup$ For cooling, radiative sky cooling might be an option: osti.gov/pages/servlets/purl/1424949 $\endgroup$ Apr 29, 2021 at 15:11
  • $\begingroup$ @Colin Young, while this is an interesting method of cooling you must consider two things: a planet with a runaway greenhouse effect (such as Venus) may not have the same atmospheric invisibility window. Second, the power is mostly used to concentrate the heat. If the temperature of your radiator is higher than that of your coolant, it will be less effective. $\endgroup$ Apr 29, 2021 at 15:49
  • $\begingroup$ the original question did specify Earth. It's been a while since I've done any heat transfer calculations (a long, long while), so I don't have a good sense for what the actual temperatures would be, but it seems reasonable that with the level of tech proposed in the question it would be possible to insulate the radiative surface from direct heat transfer from the planet $\endgroup$ Apr 30, 2021 at 2:01
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No intrinsic problem:

Given enough expense, the only limitation to a refrigeration system is that the place where you dump your heat must not be so hot that your hot-side of the refrigerator gets destroyed.

Also, your power generation method must not create more waste heat than can be handled, either by dumping into the environment (the usual way) or be pumped out using your refrigeration system using the same power that was just generated.

As your system uses Thorium reactors, which run at 650C++, they will work just fine using the outside ambient of 260C to dump heat. No problem there. None of the power generated by your reactor need to be wasted in an inefficient refrigeration scheme.

The whole setup will be silly expensive, of course. And will require really, tremendous huge number or size of power reactors, but it can be done. One advantage of having the surface being a hell-land, is that you might not mind contaminating it with radioactive and chemical wastes, which will serve to simplify your industries and power reactors a lot.

As for the visual effect you are looking for: The scheme by thewildnobody above is a perfectly valid excuse/reason to have such. One mode of Thorium reactors works with redhot molten salts as the reactor core, and if you don't mind the radiation you can quite efficiently pump the actual reactor core material around.

P.s. I hereby UN-volunteer myself for a surface inspection and maintenance squad. That would be one of the least desirable jobs in the whole universe!

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Mountains

As others have noted, cooling 250 C is quite feasible using existing technology. After all, scientists quite regularly cool experiments to near absolute zero, which is more than 270 C below (water) freezing. Most MRI machines use liquid helium* cooled superconducting magnets and can operate essentially 24/7 in a hospital environment. Note that the insulation required to maintain this temperature is not absurdly bulky, as Peter observes.

Even so, I would suggest that your inhabitants, rather than digging underground, instead burrow into mountains. There are several advantages. First, the mountain provides a large natural thermal barrier, reducing the amount of insulation required for human structures. Second, we already have a ton of tools and experience digging into mountains. And third, the coldest heat sink accessible in such an environment is most likely going to be high-elevation atmosphere.

Solar Chimney

So, your inhabitants will most likely want to build a mega-project somewhat like a massive solar chimney at the top of the nearest/highest mountain. The chimney should be as large as they can afford to construct it, both in terms of height and cross-section. Unlike a traditional chimney, they actually want bidirectional flow, so I would suggest a coaxial design where an inner pipe moves cold air from the top downwards, and the outer section moves exhaust heat upwards.

The theory here is that exhaust heat will be hotter than cold intake heat, and so you want that in the jacket layer, closer to the hot outside air. You want to protect your cold intake air as much as possible. Note that "cold" is relative, and may still be 100+ C at the altitude they can build it.

The top of the chimney should be forked, so that hot exhaust gas can be blown downwind of the cold air intake, and the exhaust vent needs to be able to rotate in response to prevailing winds.

Being able to dump waste heat into air which is 100 C colder than ambient will be a significant energy savings.

Even if your inhabitants cannot afford to build a tall chimney, they can still use the mountain itself as a chimney, either with a rotating external vent, or with multiple vents built into all sides of the summit, and an internal mechanism which can rotate to vent/intake from different ports. But if they have multiple thorium reactors to power their civilization, then they have the tech and resources to build a pretty big chimney.

Ocean

The bigger problem, IMO is water. You haven't said how long the atmosphere has been 500 C, but eventually, you will boil away the oceans. Until then, you could use the deep sea as a heat sink, if you don't mind accelerating the loss of water. Most likely, the thermal output of your remaining civilization will be a drop in the bucket compared to the atmospheric thermal load.

But the lack of rain means that your civilization will have just as much problem keeping water as it does staying cool. Fortunately, the boiled oceans will mostly stay in the atmosphere, making the air completely saturated with water vapor (and probably quite unpleasant to breathe, as it will be super-heated vapor which likely burns your lungs). However, this also gives you a source of water, since you can just take in outside air at whatever elevation has high vapor pressure, and condense the water out of it. Basically, make your own rain.

Good luck!

* Helium boils at 4 K

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  • $\begingroup$ Yes, water will be the problem. $\endgroup$ May 1, 2021 at 20:07
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Simple real-world example to show it's possible:

Your temperatures are not far from has already been done many decades ago. Specifically, the cockpit of the SR-71 spy plane. It was more efficient to put the pilots in basically spacesuits and cool them than it was to cool the whole cockpit.

It will be much easier on the ground because you aren't meaningfully weight or size limited, you can replace power with insulation.

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In addition to refrigerating the living compartments, you will need to move waste heat from the reactors out (significant amount). Note that 500F is pretty close to normal temperature of a steam cycle (for a nuclear plant). So, you'll probably need to use conventional cooling (say water recirc) and then massive refrigeration of the heated water. This may dwarf the heat transfer required for cooling the insulated dwellings.

[I guess you could also imagine some sort of nuclear process using much higher temp fluids and then directly using 500F air for cooling. But I suspect the materials issues in developing higher temp reactors, turbines, etc. (especially on the secondary side) would be much more challenging than just using a conventional system and then cooling the cooling fluid via (more of) the same refrigeration method you use to keep the people safe.]

P.s. I don't think thorium is needed or optimal. You could use conventional uranium fission reactors. Probably a lot easier than thorium. (After all there is a reason why they win now--read the Rickover memo on "paper reactors".) If you are concerned about exhaustion of ore, breeders are the natural go-to. I would think that option would happen well before thorium. Also, presumably, you posit a much smaller society than the world's current population.

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  • $\begingroup$ If you have any sources it's never a bad idea to link them in. $\endgroup$
    – A Writer
    Apr 30, 2021 at 15:44
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I think going underground is good, because it is colder further down. Then there is no need for air conditioning and special materials for insulation.

The hot air is full of water, which will condensate when the air is pumped underneath. All food production would need to go underground. This needs a lot of water. You need to pump a lot of air/water gas and then you need to pump a lot of water back up. The used water evaporates and drives turbines.

The temperature difference from below ground to atmosphere as well as within the atmosphere (winds) are energy sources. Maybe no nuclear reactor is necessary.

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