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Comment by rfrey | Hacker News Reader

You can go deeper, but in most regions you'll be into impermeable rock by the time you get usable temperatures. You need the rock to be permeable so brine can flow from your injection site to the extraction well.

Companies like Eavor claim a closed loop in impermeable rock will work, but then you're limited by the thermal conductivity of the rock around your pipe. You can only pull out as much heat as can flow through the rock around the pipe. Our simulations on Eavor's system showed they would work for about a couple months before they started dropping off.

Fracking seems like a more promising avenue, and it might work. A ton of political challenges there though.

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Interesting, when I studied these a long time ago they talked about mining geothermal energy in these kinds of deep rocks but having to move the "heat mine" after it got cool due to the lack of heat transfer through the solid rock. I got the impression that heat flows into the hot rock are orders of magnitude lower than the heat flows out required to make power generation efficient and useful.

The radiator design is interesting but ultimately it's a heat sink and power output will be limited. Initially I bet the power output is great, and as the area cools they tap it out. Who knows how long it'd last, you'd think the higher the power capacity the faster it would cool down underneath.

So really you need to be able to build new mines cheap enough to justify doing such a hard mine knowing it'll only last for 22 years.

I don't have a good feel for the geology of hot spots. But remember seeing a paper on hot springs. Typically you have a situation when water seeps down a deep fault and then comes back up. Would seem you are correct. Heat transfer via conduction will be really slow. Also know that the geothermal plants in The Geysers Geothermal plants in Sonoma County have lower output then they did decades ago because the rocks have cooled somewhat.

Thermal conductivity is one of the parameters that varies by more orders of magnitude than most properties in everyday materials.

Aerogels 0.024 W/m·K

Aluminum 240 W/m·K

https://en.wikipedia.org/wiki/Caprock says that sandstone is a common type of rock there, and has a thermal conductivity of 2.4 W/m·K

So I guess on a log scale it's half way between! So I guess imagine it's drawing heat from a source that is 100x smaller cross section and made of aluminum. Not that that is particularly helpful...

Conduction is sort of dismal. Materials tend to conduct either too well when you don't want them to. And too little when you want them to.

If you want to move heat from point a to b usually you want to use a fluid. That why I tend to assume for geo thermal plants you want to drill into rock where there is a lot of hydrothermal action going on.

Something absolutely weird is Japan has basically zero installed geothermal power.

Yes, I agree, I was surprised they bothered to mention trying to seriously mine heat from hard rock, it seems like running fluid through a bore horizontally barely solves the conductivity issue since heat still has to get to the walls of the bore.

I'm not sure if you are unaware but:

"In 2007, Japan had 535.2 MW of installed electric generating capacity, about 5% of the world total."

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

Which sure, is the total equivalent of like two nuclear cores so small by any absolute measure, but they seem to be trying?

Consider Northern California has about 900MW of geothermal installed and the state isn't really known for geothermal activity. Japan which is known for that, what are they doing?

To give California some credit, it is slightly physically larger than Japan, has far more open space without people despite the bay area (my first thought was "would they really be willing to wreck the onsens and such that have been there for a thousand years?"), is on active tectonic plates, and has been pretty focused on this for a while.

Against by local onsen owner is a main blocker. It's hoped and some develop is ongoing, but we also like onsen.

The geothermal plant in Basel, Switzerland was AFAIK among the very first production ready plants using fracking. I remember the news of them turning it on and then a series of 2-3.5 Richter earthquakes happening. The city was leveled by earthquakes before so people got scared and immediately turned it off. The tiny damage that happened in the very old housing stock was costlier to the insurances than the geothermal plant itself. It did produce quite a bit of clean electricity though.

Buildings getting damaged by 2-3.5 Richter earthquakes seems really dodgy from where I sit in California.

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