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Comment by Retric | Hacker News Reader

Don't discount the actual energy stored in the rock or capacity factor. Granit has 790 J/kg per degree C and 1 cubic meter is ~2700 kg.

So cooling 10 cubic kilometres of rock by say 50C releases 790J * 2700 * 1,000,000,000 * 50 * 10 = 10^18J or 600MW of heat over 50 years. At 33% efficiency your talking 200 MW of electricity for 50 years assuming 100% capacity factor and ignoring how fast it recharges.

If it’s a backup for solar and wind at 50% capacity factor then you could double the power output or double the lifespan.

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I think your maths is off by 1000x... The specific heat capacity of Granite is 790 J/kg/C

So replace megawatts with kilowatts in your answer... Still not nothing, but you're gonna have to have super cheap drilling to make it viable.

EDIT: Ah - you made another mistake... there is 1e9 cubic meters in a cubic kilometer. So your original answer is correct again!

But how do you access the heat of a cubic km of rock three km below ground?

It would take more than a well. A lot of wells.

I love this idea in general but it seems to me it will only work if you find water underground to carry the heat. I am not a geologist

Same basic idea as a well. When you extract water from underground it gets to your borehole through lot’s of tiny cracks greatly increasing the surface area.

In some places you can tap into an underground very hot aquifer. In others with sufficient permeability need only need to supply your own cold water. Worst case you also need to crack the rock: https://en.wikipedia.org/wiki/Enhanced_geothermal_system

Iceland just drills a new well every 6 months 50 yards from the previous one.

The heat still needs to travel horizontally from this rock to your borehole. A 30 cm diameter borehole drilled 20 km deep with the bottom 12 km being used for heat collection would give you 12000 m^2 of surface area and an average temperature gradient of 135 Kelvin, which at a heat flux rate of 7 W/m^2/K would only give you 13 MW thermal, or about 4.4 MW electric when the well is first drilled and about 3 MW after 50 years. Comparable to a single windmill. You can increase your surface area through more drilling and fracking, but that adds to cost and environmental concerns.

As a battery for storing heat energy generated by other methods though it's great.

Fracking with water (which is what geothermal would do) has pretty limited environmental concerns, and has enormous gains in increasing surface area and reach, so it almost certainly what we're going to end up doing. Iirc this is part of Quaise's plan: drill 20km down then frack the bottom to get a really big volume and surface area at those very high temperatures.

My guess would be that this is a poor choice for backup energy for similar reasons that nuclear is a bad choice: drilling deep is rather expensive, so geothermal costs are mostly capex. That means you want to run the power plant at max output as much as possible to maximize profits. To use it as a backup plant for renewables however you would need to leave it mostly idle. Depending on how much renewable energy you have available you need full backup only for a few days to a few weeks per year.

I'm thinking the reverse... since the thermal conductivity of rock is low, you can either have the well operating at a very low level continuously, or you can have bursts of much higher output for a few hours/days with the same drilled boreholes.

Obviously you will still have to pay the capex for the turbines and generators for whatever peak level you decide, but I'd guess they're a smallish chunk of the total budget. If you think energy prices will be more volatile in the future (more extreme climate means more peak-usage days, and more renewables means more days/hours with a shortfall in generation), then it makes sense to overbuild turbines so you can rake in the $$$'s in that 1-2 days per year when energy prices spike up 1000x.

That is a very good point, thanks for bringing it up. I have changed my opinion.

Geothermal has serious issues if water flow is not kept constant - dissolved minerals in the borehole water are just waiting for an excuse to crystallize/precipitate, and it’s already an issue at most active running plants, even when run continuously. Silica crystallization on heat exchangers, for instance, is a constant maintenance cost.

Stopping the flow (and letting it cool in the pipes ) for any length of time or with any frequency could mean not being able to ever start it again.

You could continue cycling the water, filtering out contaminants, without extracting heat from it.

That would likely require frequent filter replacements, which would could be quite costly.

You’re saying we should pull heat out faster than it’s replenished, but what do we do in 50 years? Didn’t we learn our lesson from oil that we can’t just kick the can down the road?

Drill a new hole? Takes far fewer resources that replacing a wind turbine or solar panel (that lasts about half that time)

The oil and gas people said the same thing until there were no more places to drill holes!

Two differences, first with the right technology any location works, you can even drill deeper at the same location. Second they refill, globally at a rate of 44TW which is a lot faster than oil.

Is granite common enough that far underground for this to make sense?

Granite is common, but other rocks have similar numbers.

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