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A medieval-tech desert city relies on a nearby subterranean lake for their water source. In lieu of employing thousands of slaves carrying pots up a long series of tunnels, could the intrepid desert settlers boil the lake water and force it through narrow tubes to the surface, where it is cooled, condensed, and collected?

I can think of at least three ways to generate the heat necessary.

1) A vast system of convex mirrors collects solar radiation (plentiful and cheap in the desert) and directs it down a series of tunnels to the lake's surface.

2) The desert-dwellers manipulate magma vents to boil the water.

3) The people have a contract with the local deity of magma, who allows them access to as much heat as they care to generate.

Would this be economical? Would the amount of heat needed to boil a lake (or sections of it) be ridiculously prohibitive? Is there any possible way to keep the water from cooling as it travels up the narrow tunnels? Would the steam arising form the tunnels make the ground surface on which the city sits unbearably hot? Any reason why these people shouldn't just dig wells to tap the lake?

Any other conceivable difficulties?

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    $\begingroup$ See this recent question: worldbuilding.stackexchange.com/questions/106235/… Babylon and other desert countries figured it out millennia ago. $\endgroup$ – RonJohn Mar 12 '18 at 21:26
  • $\begingroup$ I suggest moving the city to where the water is easy to access which is how it works in real life. $\endgroup$ – A. C. A. C. Mar 12 '18 at 21:27
  • $\begingroup$ Option #3 is definitely viable. $\endgroup$ – Alexander Mar 12 '18 at 21:39
  • $\begingroup$ What kind of subterranean lake are you thinking about? Is it an aquifer, that would be pressurized (and have a warm temperature already, compared to surface water), or is it like a lake within a cavern, which wouldn't be pressurized? Is this something the characters are going to explore, like spelunking? $\endgroup$ – KernelOfChaos Mar 12 '18 at 22:49
  • $\begingroup$ Well, if your subterranean water happens to be in Iceland, Yellowstone, or somewhere else where there are geysers :-) $\endgroup$ – jamesqf Mar 13 '18 at 1:35
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Subterranean lakes are often pressurized. Perhaps they drill or otherwise fracture the rock to create a man-made spring. Even if it is unpressurized they could create a good old-fashioned well that makes use of the lake.

Even the smallest body of water would be prohibitively expensive to heat for a medieval tech society. Think of how much firewood it would take to boil a large cauldron of water then times that by a million or billion, depending on the size of the lake. Not to mention getting it to the underground lake, and funneling the smoke.

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    $\begingroup$ In addition to that, it is not only the water that you have to heat up. As water steam rises it will become progressively colder because of heat transfer to the "tubes" and the matter around the tubes. Unless the tubes are very well insulated you are going to waste a lot of energy heating the entire terrain around the tubes. $\endgroup$ – SJuan76 Mar 12 '18 at 21:44
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You're over-complicating things. Turning water to steam and condensing it elsewhere is an attempt to use a distillation setup as a source of mechanical motion. While it's entirely possible to move water this way the vast majority of the energy is going to be wasted turning every unit of water to steam and back.

It's going to be much more energy efficient to have steam powered water wheels, or pumps to move the water than to rely solely on distillation.

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  • $\begingroup$ While this is true enough it may be too optimistic about low-tech steam engines. Efficient steam-power is hard without precision manufacturing. Hero of Alexadria's steam engine was a toy that required more labor to haul wood than at could have produced even with a efficient gear-box (which also wasn't available). $\endgroup$ – dmckee Mar 12 '18 at 21:55
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    $\begingroup$ @dmckee There were already human and animal powered pump systems in use but the OP seems pretty set on using a subterranean heat source to power it. $\endgroup$ – sphennings Mar 12 '18 at 21:57
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    $\begingroup$ Moving water up out of mines was one of the earliest applications of steam engines. en.wikipedia.org/wiki/Cornish_engine There is no reason you could not make the steam from geothermal heat. $\endgroup$ – Willk Mar 13 '18 at 15:06
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Sure you could.

A scheme exactly like this was my answer to this question, linked here: A way of getting the water to the top of the tall building without electricity?. In this scheme solar power is used and the water is moved up a pipe as steam. The pressure of the new hot steam at the bottom will keep the steam above it moving up the pipe. Insulate your pipe!

We are used to prioritizing the energy costs of doing any work. But if heat energy can be had for very cheap (e.g. geothermal) then you could move the water with no moving parts - just a boiler, a pipe and a radiator. You would need to condense the steam back to liquid with a radiator topside, as in the linked answer.

Boiling the water might serve double duty. Suppose your subterranean water was super salty or full of biological contaminants. Distilling water is a great way to get the water away from the solutes and your steam scheme would be distilling the water.

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