Would the lunar analogue of geothermal energy be practical on a lunar base?

Question

Given sufficient liquid in the form of water or other suitable medium, would the use of the lunar analogue of geothermal energy be practical or feasible for powering a lunar colony?

Wikipedia (referenced) gives an internal temperature for the Moon's core of 1600–1700 K, which would seem to easily be sufficient for creating steam to power turbines. Unlike Earth, there is no volcanic activity near the surface so drilling would likely have to be much deeper, how deep I am not sure.

Other challenges are moonquakes and the lack of water in the ground. Which could lead to higher risk of fracturing a pipe and leakage of heat transferring fluid (a parched Luna would more quickly leach away fluids).

Answer 1

The "much deeper" drilling would have to be roughly 900km to get near to the core. No drilling of this scale has been ever attempted on Earth. Lower gravity will make it easier but not much easier, and you will have to have pumping stations from time to time - no pipe will withstand 900km long column of water, even in lunar gravity.

On top of the difficulties you list, there's filling the pipes with water - to make it worthwhile and not cool down to zero within first 100 meters the pipes should have some reasonable diameter. Say, 1m² cross section $\times$ 900,000m length, times two (up and down) that's 1,800,000 tons of water just to fill it. How do you plan to bring it there?

Sure, you might just try to generate electricity deep down there and send it up by wires. This requires the heating$\rightarrow$cooling cycle for the water though and while you have heating available locally, cooling might pose some problems. You can't use cheap evaporation, or convection. You're pretty much stuck with dissipation into native rock, a huge grid of thin pipes conducting the heat away into the rock far enough from the "hot zone" to make it worthwhile. The further you get the more water you need.

On the other hand:

• there is no air to disperse solar heat;
• there are no clouds to obstruct it;
• the dust flies very short way without wind to carry it, to cover the panels;
• there is one rotation per month, so automatic tuning systems are idle most of the way;
• the moonquake risks are minimal on the surface;
• there is no need for bulk amounts of expensive materials like water. Quartz is abundant on the surface.
• Sure, you have a half-month-long night (when you might need to use stored energy or pull it by wire from the other side of the Moon... or at least use an orbital reflector to bring it to the panels) but then you get half a month of constant energy.

In other words, if we didn't have the option of abundant, efficient, cheap solar energy we might feel compelled to seek other options like geothermal on the Moon. Currently though, there is simply no reason to pick a solution several orders of magnitude more expensive when you have something so cheap and efficient as solar power.

Answer 2

The first question is, how far do you have to drill? I found an article that provides a reasonable temperature profile. Given that profile, how far would we need to go? It seems that most geothermic energy sources rely on water near the boiling point, or ~400 K. Given that as our benchmark, the model seems to say we'd need to go about 70 km deep to pull it off.

So, how far have we gone on Earth? This Wikipedia article references a 12 km hole dug. That's a far cry from 70 km! And that requires lots of heavy machinery to achieve this feat!

Bottom line, it could be done, and probably will be done some day, but there are far more practical ways of getting power for now.

Answer 3

Not the same kind of geothermal but - Ground source heat pumps, aka geothermal heat pumps could have potential uses for heating and cooling of habitat buildings. These things use the ground as a heat sink and heat source. They are also reversible so that cooling of buildings adds heat to rock and earth around the pipework, which can later be recovered (with some losses) for heating. They do require a power source for pumps.

I would expect the lack of moisture in Lunar "soil" would reduce heat conductivity so more densely arranged may be required. Buried pipework is commonly used, however boreholes in solid rock are also used and might be more appropriate. Less conductivity may actually reduce heat loss beyond the effective range of heat exchangers.

(PS added) These kinds of systems can work purely as thermal energy storage but they more often utilise seasonal heating and cooling of subsoil. At the correct depth they should be able to use heat from Lunar day that persists long enough to last through a Lunar night.

Answer 4

You would not need to use water! You could use the methane and other hydrocarbons that could be used. They have much lower freezing points.

Answer 5

The Moon really doesn't have enough of an internal dynamo to make lunarthermal (the Moon's equivalent to geothermal) to be viable.