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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.

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.

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.

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

Granite is common, but other rocks have similar numbers.

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.

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.

Related comment thread. The OP is a former CTO of a Canadian geothermal company

https://news.ycombinator.com/item?id=24854216

While I think this first-principle analysis is helpful, I fear it may be holding Quaise to too high a standard; their goal (AFAICT) is not to provide 100% of the energy needs of anywhere; it's a huge win if coal's 10-20% contribution of total power generation (or even just a substantial fraction of that) can be in-place replaced with a geothermal generator. Maybe this tech won't be cost-effective in 50 years as solar continues to get cheaper, but as a transitional bridge technology it could be incredibly impactful in the 10-20 year timeframe.

Also, you didn't specify a depth for your calculation; won't the flux be higher the deeper you go (i.e. be proportional to the delta-T)? They are talking about going to 20km which as I understand it is WAY deeper than most geothermal systems contemplate. Their whole bet is predicated on the idea that with the gyrotron they can drill deeper since they don't need to mess with high-temp drill bits.

> Also, you didn't specify a depth for your calculation; won't the flux be higher the deeper you go (i.e. be proportional to the delta-T)? They are talking about going to 20km which as I understand it is WAY deeper than most geothermal systems contemplate. Their whole bet is predicated on the idea that with the gyrotron they can drill deeper since they don't need to mess with high-temp drill bits.

The depth doesn't matter at this scale. 20km deep the heat flux is about 0.3% higher. Things get complicated as you go down into the mantle, but at 20km you're still in the top part of the crust.

The only thing drilling deep does is increase your max temperature, which increases efficiency, but eventually you hit the same limit as with any other steam generating plant where you can only handle steam that is so hot and so high pressure. With current technology, that limits the efficiency percentage to the low 40s. Maybe with some technological improvements this can go up a bit, but the carnot efficiency of a heat engine where the water is heated to the point it will decompose is 87%, and there is no way you're even going to get near that in a real plant, so increasing efficiency isn't really going to make a huge difference.

> Maybe this tech won't be cost-effective in 50 years as solar continues to get cheaper

As we replace base load with less reliable sources we need to come up with some way of expressing a penalty for availability. Does it matter how cheap solar is if it isn’t available when we need it and storage is not practical?

How about inter-seasonal heat storage, a la Drake's Landing?

https://dlsc.ca

What if you dig down deep enough to molten rock? Then you're aren't limited by heat conduction of the solid rock, but the heat convection of the magma

How would you keep the hole open?

But most places that is way, way deeper than 20km.

Wikipedia says the Earth's crust is on average 15-20km thick. Soviets drilled 12km deep with 1980's tech. It's difficult for sure, but might be doable.

https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole

"Doable" is very different from "economically justifiable".

The thickness of the Earth's crust is strongly bimodal, making "average" meaningless: thinner under oceans, thicker on continents. Much thicker. Most of the planet is, of course, sea floor, so mostly thinner. But drilling the sea floor is unpleasant and expensive, and at the end, the hole is all the way down there.

From what I understand, most of the earth's mantle is not molten and you can only find molten rock, magma, in some specific places like mid-ocean rifts and volcanic hotspots. The mantle has convection currents, but those flow on geological timescales. The Earth's outer core is liquid, but that's almost three thousand kilometers below the surface.

Can I cool my house into the well during the summer? With surface heat abundant during the summer, I'd expect storing heat downstairs should make sense even if the efficiency of the underground reservoir is not exactly stellar.

This works very well. The low heat conduction of rock also means that without going too deep you get to a temperature that is almost constant year round, so you can cool your house in summer and heat in winter so long as you're not dumping absurd amounts of heat.

From what I understand, geothermal works both ways, yes

Look up Drake Landing Solar Community - https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community // https://www.dlsc.ca/reports/swc2017-0033-Mesquita.pdf

It is mainly addressing winter heating rather than summer cooling (the cooling degree days are rather minimal - https://okotoks.weatherstats.ca/charts/cdd-weekly.html // https://weatherspark.com/y/2404/Average-Weather-in-Okotoks-C... "Over the course of the year, the temperature typically varies from 13°F to 76°F and is rarely below -12°F or above 86°F.")

That said, there's nothing saying that one can't use a heat pump to as a source of heat in the summer (which would also cool the house) in additional to other sources of thermal energy.

> For context, to provide the electricity demand for kansas would require about 35% of the total geothermal flux into kansas. New Jersey's electricity demand is 7 times higher than what its geothermal heat flux could provide. For the whole US, electricity consumption is about 75% of geothermal heat flux if you ignore variations like the yellowstone hotspot.

It would be great if you can provide a bit more details on those assumptions. Thanks

(Electric energy use per year * 1 year) / (Area * 100 mW/m^2 * 60% thermal to electric conversion efficiency)

In reality electricity capacity needs to meet peak demand, not average demand, the heat flux in continental crust is more like 65 mW/m^2, and the thermal to electric conversion efficiency is going to be closer to 40%, so the situation is actually much worse for geothermal, but maybe with the right technological developments and implementation you could get better performance.

Bore hole open cycle loops probably wont be viable in areas that fracking is happening. Super hot steam mixed with methane doesn't sound like a good idea.

Why not? Sounds like https://en.wikipedia.org/wiki/Syngas

Furthermore! Maintaining a steam turbine is an operational expense not matched by any for wind or solar. There are reasons we are not drilling new regular geo wells.

Literally everything costs more than wind and solar. It is why nukes will soon be mothballed.

I think this was marketed as “geoplutonic” power about a decade ago.

What makes the heat flux in iceland so high? Why doesn't drilling deeper get you to a similar heat flux?

Iceland is over a volcanic hotspot where hot rock from deep in the mantle is physically rising up, so the heat conductivity of the rock doesn't matter as much. It's like the difference between standing in a pool of water and being sprayed by a hose.

That makes sense - thank you for helping me understand. If we drilled down low enough would convection in the mantel increase the availability of heat? I assume the issue is that that would be way too deep to dig, even with new tech like this?