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Comment by JumpCrisscross | Hacker News Reader

> nuclear power plans by Russia not so much

Why? Any lunar base without nuclear power plans is not a serious effort.

The moon is fairly unique (relative to Earth) in that the exact same spot will go from -200C to +100C on a two week cycle (day and night). It seems "obvious" that there must be some clever way to exploit this to generate energy in a simple and novel fashion.

> seems "obvious" that there must be some clever way to exploit this to generate energy in a simple and novel fashion

Sure, once you have two-week power-storage infrastructure. (And the scale to harvest a useful amount of energy once a month on average.) In the meantime, i.e. our lifetimes, you have countries that can build space nuclear reactors and countries being performative.

I see a couple of issues here. The first is that creating, maintaining, and operating a nuclear facility on the Moon would almost certainly be far more challenging than "just" maintaining a couple of weeks of power storage. But that kind of implicitly leads into the other issue. That is that you only really need the scale of power that nuclear can offer once you've already substantially industrialized the Moon. For some simple habs and research areas, even something as small/simple as a radioisotype generator [1] would be more than fine.

Beyond all this, I meant novel when I said novel. The regular extremes of heat and cold offer all sorts of interesting ideas. You've got room for predictable and endless convection on basically an arbitrarily large scale there. There is certainly going to be some clever way to exploit this in a novel fashion.

[1] - https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

Good news! Thermal cycles are caused by the sun, and we can harvest sunlight directly! Storage is an issue...

Storage is not an issue. It's been solved for decades. In fact, energy storage is amazing and cheap now, thanks to smartphones and EVs. You can buy consumer level batteries for very very cheap.

Battery tech is awesome, but I think you're overselling it a bit. We've barely started figuring out grid-scale battery storage on Earth. There's 2 big reasons the Moon is going to be much much harder:

1. Batteries are heavy, and space ain't cheap. Current launch pricing is about $1.5k/kg to LEO. The Moon will be more, it's further away. Even if Starship brings that down by a factor of 10, transportations costs are still going to be astronomical.

2. The day-night cycle on the Moon is slow. Your batteries are going to need to be able to store half a month worth of power. You'll need 15x more batteries on the Moon than you would on Earth.

I have always assumed that the first moon colony would be on the north or south pole to avoid this issue. Not too hard to imagine a solar array set up to track the sun with a very slow rotation. The colony itself would be in a crater to avoid direct sunlight and provide a place to dump the heat from the solar array.

Otherwise the need to bring enormous power storage to handle the half month of darkness and bitter cold makes solar a bit impractical and the only other reasonable alternative is nuclear power.

The big problem with attempting to exploit the temperature differential is that it happens on such a slow cycle that the total amount of energy available is quite low.

Could you exploit the temperature differentials between the either hot or cold surface, and the presumably in-between temperature at the bottom of a drill hole?

In theory the temperature underground should be the average of the surface temperature, so you could use that gradient to generate energy at night. Someone smarter than me would have to do the math on the energy density of a scheme like this, my gut says it's going to be fairly low and you will need a really big power plant to supply even a modest colony.

I often wondered, how do you deal with all the waste heat from a fission power reactor, in a vacuum?

Giant radiators?

edit: fixed typo, derp. fusion=fission

Perhaps? Any reason that sounds silly to you?

No, and this is not some attack on nuclear power. I am just curious. On earth, nuclear power plants use lots of water and cooling towers. How does that work in a place with no spare water?

I also meant to write fission in my original question, not fusion.

Mostly cooling towers are not likely to work in a place with no atmosphere.

Radiators though, are constantly used in spacecraft, and seem to work well. Low gravity, no motion might let the thing be mostly the radiator, with not much support structure. Except it might need to be in shade, in the shadow of a building? hill? solar panels?

> How does that work in a place with no spare water?

I wonder if recycling human liquid waste through evaporation could be of some use for that purpose.

It would likely be a thermal reactor, I guess? So no waste heat, since the little heat it gives is used to generate electricity.

Thermal power plants don't use up the heat. They make energy through its transfer (like a dam with water).

Most nuclear reactors are thermal power plants and they all need to dump waste heat. Thermal power plants convert heat into electricity with turbines and are limited by dumping the waste heat.

Are you thinking of radioisotope generators? Those aren't reactors. They use thermocouples and need to get rid of the waste heat. The Voyager RTGs have radiator fins.

Yeah, that's what I meant, sorry about the incorrect naming.

> Giant radiators?

Yup, that's the thing on top [1].

[1] https://www.nasa.gov/tdm/fission-surface-power/

Thanks, I have seen a lot of pages on nasa.gov, but not that one. Neato!

Heat is what you use to make steam and drive the turbines with. I don't understand your question.

On the Moon you could also use the Moon itself as a heat sink. But yeah, any near-term solutions would use giant radiators.

Solids don't conduct too much heat. I think most of the cooling in a nuclear plant is generated by evaporation of water.

Yes, what I mean is that theoretically you could do something like have a very very wide base to passively dump heat into the ground (since we're still only looking at kW scale reactors in space), or actively cycle large amounts of regolith through the cooling loop (say, via a heat exchanger from a closed water loop) and dump it out. Depending on how much heat the system can handle, you could maybe even extract stuff boiling off from the regolith.

The nuclear plant nearest to me uses sea water, which is put back into the ocean.

Interesting. I had to look for more details. I found this link https://nuclear.duke-energy.com/2013/11/13/why-don-t-all-nuc...

Oversimplifiying: All new plnats have cooling towers, so the water they return to the environment is not too hot.

maybe not so hot indeed due to the scarce air that's why it's so cold at night on the moon. Only turn on the reactor at night to generation and use capacitors at day?

You can't really turn fission reactors on and off, they're always producing decay heat even if not being used for power generation. But why not radiate the excess heat into the crust of the moon with a subsurface loop, kind of like a ground source heat pump.

The day is 28 times longer than on Earth. You'd need very big capacitors.

Solar PV linked to battery storage is the obvious energy source for a moonbase. All reactors require coolant circulation and water is going to be among the most valuable commodities on the moon, not something you want to circulate through a reactor (where you inevitably get tritium formation, making the water unsuitable for other uses).

Maybe you could use helium or liquid sodium metal as the primary coolant, but then you still need to generate electricty via secondary water coolant loop that runs a steam-powered turbine. Really not plausible on the moon.

You'll need an insane amount of battery storage, since the moon only rotates once per month and you'll need to power through a long period of no sunlight.

Or just very long power lines and multiple solar stations, it's not like the moon is impossible large. But that would be harder than a singular location using nuclear fuel, it's true.

Or alternatively, for polar bases, very tall panels, which would be able to be in permanent sunlight.

> very tall panels, which would be able to be in permanent sunlight

Or normal panels on the rim of Shackleton [1]. (You'd still want to bootstrap with fission.)

[1] https://en.wikipedia.org/wiki/Shackleton_(crater)#Potential_...

A belt of solar panels encircling the moon would solve that problem though perhaps a bit ambitious at present.

> Solar PV linked to battery storage is the obvious energy source for a moonbase

It's not serious with chemical propulsion and the Moon's day/night cycle. Put another way, if one team uses nukes and is, as a result, power unconstrained, while the other spends all its energy launching solar panels and batteries to keep life support online, it's obvious who's going to be doing any science.

> All reactors require coolant circulation and water is going to be among the most valuable commodities on the moon

Sodium and sterling, no water [1]. There is a reason even NASA is only seriously considering nuclear power [2].

[1] https://www.nasa.gov/directorates/stmd/tech-demo-missions-pr...

[2] https://www.nasa.gov/tdm/fission-surface-power/

You'll need a massive amount of batteries for solar night. People severely over-estimate battery storage.

Even on earth, giant grid-scale storage system are not able to supply all the power for region they serve alone for more than some 4 or 6 hours. In the moon, they would have to supply power for some two weeks.

Battery storage is not a real solution on earth, it is even less plausible in the moon.

Solar PV battery combo is just a giant spatial transmission line system for a fusion reactor. For high density applications, fission and/or fusion make more sense.

It's just that we're close enough to the Sun, and that a lot of us are living in low-density residences.

Solar ray density reduces to half of what we get on Earth on Mars our closest outer planet, simply from inverse square law. And deep space people always said it goes down so fast a space nuke is a hard requirement for Jupiter and beyond. There might be ways to make it work on Moon, but PV plus battery is just rural inner planets thing, not the way forward or anything. It's just temporary hype technology.

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