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Recently, there was a thread on Hacker News about building a nuclear power plant on the moon.

Humans being what they are, I could imagine some dark outcomes. Instead could we build a steam engine on the moon and drive it with energy from the sun?

Granted a solar array ON the moon is not going to work half the time. But how about a solar array on a satellite around the moon? It would be cheaper to deploy. Already exists. 10 kw can be generated by an array the size of a small rooftop. Getting the power to the moon is the remaining engineering challenge. And learning how to do this would have tremendous benefits for the exploration of space in general. Although current tech using masers exists, I cannot imagine why we could not use recently developed high frequency lasers to boil water on the moon to run a steam turbine. (Which we would need to get from a comet). Besides the obvious steampunk attraction of a steam engine on the moon, the presence of a large reservoir of heat energy at a stable temperature could facilitate the temperature regulation of a nearby colony. And the constant supply of energy could drive ion engines that use the dust of the moon as a reaction mass making trips to lunar orbit free. (This is a wild idea because you’d have to charge the dust with high voltage and accelerate it out of a nozzle)

I just don’t like the idea of concentrated points of failure that are easy to subvert into weapons like a dirty bomb. We need to be cognizant of human’s ability to turn even the best invention to evil when someone gets disgruntled.

Although this thread was closed for lack of clarity

Could a steam Engine outperform other electric producing devices in space?

And this thread explored the use of steam for propulsion in space

Could steam be used as a efficient power source for a space craft?

Of course solar panels on the moon could also be driven be mirrors in orbit around the moon

What is the status of concentrated solar energy (CSE) in space exploration?

My question is specifically aimed at the the thermodynamics of boiling water on the moon in the absence of air. If the steam generator were put on the dark side, or in a cave anywhere, the temperature gradiant achievable would be awesome. And in the absence of air, the boiling point would be a lot lower (20degrees C?)

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  • $\begingroup$ I've voted to close this as needing clarity. The laws of physics apply on the moon so a steam engine could work there as long as there is heat. However, the OP seems to be aiming at some sort of vacuum system which doesn't make sense. Without more detail on the system this question cannot be answered. $\endgroup$ – GdD Jul 26 '20 at 17:33
  • $\begingroup$ @ GdD. I think my question is sufficiently detailed - specifically - can a simple steam engine work on the moon. Designing a machine that exploits temperature differences and phase change of a liquid does not necessarily require an atmosphere if the system is closed. $\endgroup$ – aquagremlin Jul 26 '20 at 17:35
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    $\begingroup$ In order for this question to be well received I strongly recommend that you 1) narrow it down to one thing (you can ask additional questions) and 2) don't try to sell your idea as a good idea. Try not to use a question post to push your own opinions. Just ask "What would be the biggest challenges to get X to work?". Don't ask "Would X work?" by itself because the answer is often "It depends... on how much time and money you have". $\endgroup$ – uhoh Jul 26 '20 at 18:12
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – gerrit Jul 27 '20 at 9:14
  • $\begingroup$ Are you only interested in a STEAM engine? Could be this is more achieveable with another gas/liquid that can work a Carnot cycle at a lower pressure, that would be a poor choice at Earth-level pressures. $\endgroup$ – Criggie Jul 27 '20 at 10:55
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Note that I am answering only the question in the title, not most of the body of the question.

Yes you can run a steam engine on the Moon if you have a source of energy and a way of dissipating waste heat. Steam engines do not boil their water in air: they boil it in boilers which contain water and steam: there is no air in the boiler of a steam engine.

Obviously you would not want to vent the exhaust steam from the engine to space since water is a scarce resource, but this is nothing new: steam ships also do not want to vent their steam into the atmosphere since fresh water is a scarce resource for them. So they use condensers to condense the steam back to water. Nothing prevents you doing this in a vacuum, although you will need some way to dissipate enough waste heat, which will be harder than it typically is on Earth.

Note again: I am addressing only the technical possibility of doing this, not the desirability of it.

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    $\begingroup$ You also have the problem of shedding heat. In a vacuum it is very difficult to shed heat, and steam engines make huge amounts of heat. You would either need massive radiator fins for the condesnser, or perhaps fields of underground piping to conduct the heat into the ground. $\endgroup$ – Dan Hanson Jul 26 '20 at 19:42
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    $\begingroup$ The Diviner instrument on NASA's Lunar Reconnaissance Orbiter measured temperatures of minus 396 F (minus 238 C) in craters at the southern pole and minus 413 F (minus 247 C) in a crater at the northern pole. Clearly there are regions not as cold that could be great heat sinks. $\endgroup$ – aquagremlin Jul 26 '20 at 22:13
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    $\begingroup$ Low temperature doesn't mean the same thing as a large reservoir of moderate cold to dissipate massive amounts of heat into. And that would only work at the pole. $\endgroup$ – ikrase Jul 26 '20 at 23:06
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    $\begingroup$ steam ships and some locomotives $\endgroup$ – uhoh Jul 27 '20 at 3:49
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    $\begingroup$ @uhoh: I've never understood why more locomotives did not have condensers: any kind of long-distance travel means having to stop for water or do heroic things to pick up water with scoops. I think perhaps its because having a big enough cold reservoir on a locomotive is hard (so, same problem a steam-engine in space has...). $\endgroup$ – tfb Jul 27 '20 at 10:00
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If you are planning to boil water at low pressure, it will happen at low temperature, which means you need an even lower temperature to condense the spent steam back to water. An efficient heat engine will operate across a significant temperature difference - the bigger the better. Dissipating heat in space can only be done by radiation; the higher the temperature, the higher the rate for a given surface area. That means a practical steam engine would work more efficiently using a high temperature (therefore high pressure) boiler and a high temperature radiator to condense the spent steam (also necessarily operating at significant pressure). Ambient pressure (or lack of it) is irrelevant. There is nothing about the lunar environment that could be uniquely exploited by steam power; indeed the lunar environment poses numerous challenges to the feasibility of steam power.

Steam engines on Earth are practical and efficient because they boil water at high temperature and typically have access to an effective heat sink in the form of a large river (for stationary power plants) or the body of water on which they operate (ships) and can condense spent steam at relatively low temperatures because of the efficacy of the available heat sink.

Concepts for power in a space environment that are more practical would use solar electric panels. On the Moon, aside from its poles, any given point on the surface is illuminated for about 14 days and in darkness about 14 days. That means you have to either collect solar power at the surface and store it for when you need it, or use devices in lunar orbit. There are concepts for large-scale chemical batteries which might fit the bill for storing energy. Alternatively, solar power could be collected in lunar orbit and beamed to the surface as microwave energy, or simple orbiting reflectors could shine onto solar collectors on the surface during lunar "night" at a given location.

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  • $\begingroup$ These are all good points! $\endgroup$ – uhoh Jul 27 '20 at 3:51
  • $\begingroup$ One of the big advantage of the moon is that you can use other working gases. It doesn't matter if they're toxic. If they escape, they'll escape to space. $\endgroup$ – MSalters Jul 27 '20 at 11:45
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My question is specifically aimed at the the thermodynamics of boiling water on the moon in the absence of air. If the steam generator were put on the dark side, or in a cave anywhere, the temperature gradiant achievable would be awesome. And in the absence of air, the boiling point would be a lot lower (20degrees C?)

In a conventional steam power plant on earth, the steam is not usually exposed to atmosphere, but instead condensed again and re-injected into the boiler in a closed loop. A different source of water is used to cool the water in the closed loop.

It is important to recognise that the water in a steam engine situation boils under pressure: the pressure is what's doing the work! Higher efficiency is achieved with higher pressure at the turbine, which means there must be an equal or greater pressure at the surface of the boiling water.

On a lunar steam engine the closedness of the system would be even more important, since water would be very precious. Leaks would be a disaster. But other than that, there's no requirement for an atmosphere outside the engine. Steam turbines work just as well underwater in nuclear submarines, for example.

Carnot efficiency depends on the cold side as well as the hot side. Since you have no atmosphere or river for cooling, you'd have to find a way to dump all the heat back into space through just radiation. Cooling is a major problem for the ISS.

concentrated points of failure that are easy to subvert into weapons like a dirty bomb

The only risky bit is getting the fuel to the moon. Once it's there, a huge point in favour of safety of nuclear power on the moon is that there's no air or water: nothing to contaminate. Radioactive particles cannot drift in smoke, they'd fall immediately back to the lunar surface. You could have a lunar chernobyl and then just .. leave it.

Mind you, I feel we're missing the big picture that solar photovoltaic is the obvious solution on the moon!

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  • $\begingroup$ Not sure if a lunar Chernobyl will be easier. No wind, but also no atmosphere - a lot of particles will fly ballistic trajectories and fall over larger area. And your power plant will likely be near a settlement that you also have to leave (no power and the radioactivity seeping in). $\endgroup$ – fraxinus Jul 27 '20 at 10:31
  • $\begingroup$ @fraxinus: Any habitat has to be shielded anyway. There's an unshielded big nuclear reactor at the center of our solar system, and on the moon you're not protected by the Earth's atmosphere or magnetic field. A few extra bits of radioactivity on the outside won't hurt that much. $\endgroup$ – MSalters Jul 27 '20 at 11:48
  • $\begingroup$ @MSalters Sun core unshielded? Well, most neutrinos escape. Sun has some unpleasant UV/X-ray radiation from its photosphere/corona, but nowhere near a working fission reactor. $\endgroup$ – fraxinus Jul 27 '20 at 12:33
  • $\begingroup$ RTGs are only. 10% efficient. Most of their heat is wasted. NASA interestingly designed a stifling engine to capture that heat and turn it into mech energy. So instead of send heavy, expensive RTGs to the moon, the sun is an obviously better source of energy. $\endgroup$ – aquagremlin Jul 27 '20 at 23:31
  • $\begingroup$ The sun is a better source of energy .. while you can see it. A "solar day" on the moon lasts one earth month, so either way you need storage. But there's no advantage to solar-thermal compared to solar-steam. $\endgroup$ – pjc50 Jul 28 '20 at 8:05
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Note that nuclear power and steam are not alternatives to each other.

Nuclear power is source of heat. Steam turbine and steam engine are ways turn heat into motion and energy.

Most of nuclear power plants still nowadays boil water to produce steam which is then ran through steam turbine. Steam engines are not used because of their lower efficiency.

If you are going to moon, you will want steam turbine, and yes it will work, if you only find source of heat.

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  • $\begingroup$ It's still amazing to me the extent to which we till rely on steam power in the 21st century. $\endgroup$ – user16338 Jul 27 '20 at 13:45
  • $\begingroup$ I think the OP was proposing solar thermal as the heat source for a steam power plant to avoid nuclear. $\endgroup$ – Anthony X Jul 27 '20 at 23:07

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