Hacker News Reader

Comment by foxyv | Hacker News Reader

A 1 meter square heat exchanger in a vacuum at 20C will emit about 1 kilowatt at -173C. So about as much as a small space heater per small panel. So a 1 megawatt datacenter would need about 300,000m^2 or 0.3 km^2 of surface area to cool it.

But geothermal cooling would be great on the moon too. Run a pipe 2 meters under the lunar surface and it is -21C.

I think the whole idea though is to make a low wattage space-stead so you can store copies of Moana out of reach of Disney cease and desist letters.

View on HN

> geothermal cooling would be great on the moon

Surely it's selenthermal cooling at that point.

Thank you for the correction. Shows my geocentric thinking.

> But geothermal cooling would be great on the moon too. Run a pipe 2 meters under the lunar surface and it is -21C.

Isn't the moon geologically dead though - no water or geological movements?

I worry this would just result in the ground absorbing the waste heat and eventually becoming too warm to effectively cool anything. Especially because the ground itself would eventually still be limited by the rate of radiative cooling into space, right?

You have to worry about changing the ground temperature even on earth FYI. When designing district heating/cooling systems with borehole fields, one of the things that you check for is to make sure that you don’t inject too much heat (or extract too much) seasonally - ideally it’s roughly balanced so any drift year over year is small.

Obviously things like the diffusivity (so conductivity, mass, density etc) of the ground matter a lot, as does the rate of heat exchange at the surface for it to reject (or absorb) heat to the environment.

Right, I'm roughly aware that's a concern on Earth too which is why I was wondering. How's the thermal conductivity of the moon?

Looked up some papers, and seemingly super low compared to what I would have initially guessed - probably because it’s porous/fluffy/sharp dust with lots of small voids/less compacted I’m guessing. Like, orders of magnitude less than the ground on earth. Not my area of expertise though and was just cursorily skimming papers for values. Specific heat cap and density seem like what you would expect for any rocky materials.

Might need to pour a bunch of water on the soil to increase the thermanl conductivity and make sure to never heat it above freezing?

Water would boil on the moon, any ice would sublimate. You would be better off just compacting the regolith.

Geothermal heat transfer is greatly affected by the moisture of the soil, which on the moon would be pretty low as you can imagine.

No one ever drilled the Moon to any useful extent. All we know is that it's mostly dusty on top.

Geological activity is usually a problem for geothermal cooling. Conductivity of lunar subsurface is your main problem. Generally what makes geothermal really effective is the monstrous thermal mass of solid and liquid material compared to air as well as it's much higher conductivity. While that is true on Earth, on the Moon things are very different.

The problem isn't so much geological activity or lack thereof, as the nature of lunar regolith. Lunar regolith has a conductivity of 0.004W/mK. That is lower than aerogel! So unless the subsurface has a much higher conductivity, using subsurface cooling would be doomed.

Edit: Lunar Regolith is only the first 4-5 meters of the lunar surface.

> Run a pipe 2 meters under the lunar surface and it is -21C.

I would expect the regolith to be a poor thermal conductor. Not useful for heat exchanger

Probably orders of magnitude better than hard vacuum, though.

Their point is sure, you locally dump the heat. Where does it go then? There's not ground water to act as a sink so you're stuck with basically a big regolith insulator, and there's barely any atmosphere, so you're back to the black body of the ground with extra steps and a large local sink. That sink isn't infinite when you're talking even house scales, much less thermal scales of a large datacenter. Cooling works on earth because the atmosphere moves the heat.

For conductive cooling, sure.

But for radiative cooling, vacuum with a clear view of the night sky is orders of magnitude better.

The regolith is 0.004W/m2K. Less than aerogel. Less than an open vacuum! Ouch!

Although the regolith is only 4-5 meters thick, so you could probably just go under it and see what the subsurface is like with regards to conductivity.

> But geothermal cooling would be great on the moon too. Run a pipe 2 meters under the lunar surface and it is -21C.

It won't stay -21C for very long, if you pump heat into it.

Really, radiative cooling is your only longer term option.

Btw, you can make your radiative cooling a lot more efficient than you gave in your example, if you run it at a higher temperature. Radiated power grows with the fourth power of (absolute) temperature. So, run your chips at something closer to eg 100C and you radiate more than 2.5x as much power.

> It won't stay -21C for very long, if you pump heat into it.

You are right, and also it would require digging which is a lot harder than laying out panels on the surface. Back of the napkin it's a tossup depending on the conductivity of lunar sub-surface material and how much pipe you lay. Just like on Earth.

> Btw, you can make your radiative cooling a lot more efficient than you gave in your example

This is true too, heat pumps could even get higher radiator temps than 100C if you like.

> This is true too, heat pumps could even get higher radiator temps than 100C if you like.

I'm not sure that actually works out to your advantage, because the heat pumps themselves produce extra heat. But I haven't done the math on that.

Heat pumps are typically 200-400% efficient. That is, they move 2-4x as much heat as they produce to move that heat. Although that is usually radiating energy to a fairly warm atmosphere. If they are pumping into an extremely cold environment you will get insane efficiencies since you are 'going with the flow' with regards to entropy increase.

Reply on news.ycombinator.com