5 ms·

One thing I have never quite understood about geothermal, maybe someone can enlighten me: the energy flow from the Earth's core to the surface is not that huge,

One thing I have never quite understood about geothermal, maybe someone can enlighten me: the energy flow from the Earth's core to the surface is not that huge, less than 1 watt per square meter. Doesn't that fundamentally limit the usefulness of geothermal power as a general solution outside of exceptional spots where this gradient is locally much higher, or there is an opportunity to collect from a wide area with a single small borehole? And if I drill a hole and collect 500 watts from it on a 100 sqm plot, am I effectively siphoning the heat from my neighbors plots?

brightball 2y ago
That has always been my understanding. It’s useful for HVAC for a home for example, where you aren’t trying to do a conversion to electricity but instead are directly leveraging the consistent temperature to reduce strain on the system.
- phreeza 2y ago
  Yea that is a heat pump scenario where you are also putting energy back in at certain times, that makes sense to me too, it doesn't have much to do with extracting geothermal energy from the core iiuc.
meindnoch 2y ago
The cited value is the energy flow through the surface, which is about 0.1 W/m^2. But this is conducted through kilometers of rock and soil, which acts as insulation. Geothermal power works through convection instead of conduction. You inject cold water into a borehole, and hot water (steam) comes back, and spins a turbine. Convection can extract energy at a much higher rate than conduction through the crust. If you boil a pot of water, you can still comfortably hold the pot's handle if it's long enough, indefinitely. This is heat conduction. On the other hand, if you try drinking from the pot with a straw, you'll find it very painful. This is convection.
- archon1410 2y ago
  Perhaps a stupid question, but... what are the risks? Wolndn't extracting too much energy from the earth's core cool it down, at least a little bit? Or does it contain so much energy that extracting it to replace all of 'surface generation' won't make even a little difference?
  - whatshisface 2y ago
    We can't drill anywhere near the mantle, geothermal extracts energy from the upper part of the crust.
    - archon1410 2y ago
      Wouldn't drawing energy from the crust cool it down, and wouldn't a cooler crust in turn 'draw' more heat from the lower layers? I guess the earth already radiates out a lot of energy, the process of extracting geothermal energy will presumably lead it to radiate more energy. I don't know by how much though, or if it will make any real difference, or if that's how it even works.
  - __turbobrew__ 2y ago
    I guess if you could extract enough energy from the core it would reduce convection which would in turn reduce the strength of the earth’s magnetic field.
  - numpad0 2y ago
    Earthquakes! There are couple comments mentioning "fracking" around, that's destabilizing the land by injecting acid to get energy out. The acid is dangerous, and so are breaking up soil deep down.
  - tylerhou 2y ago
    > Wouldn't extracting too much energy from the earth's core cool it down, at least a little bit? The earth generates ~50 terawatts of energy through radiation/other processes, while global energy consumption over the last year was 0.003 terawatts. I think we're fine.
cyberax 2y ago
> And if I drill a hole and collect 500 watts from it on a 100 sqm plot, am I effectively siphoning the heat from my neighbors plots? Pretty much. But the Earth's crust has a lot of thermal mass, so there's enough energy stored there to last for a long while.
epistasis 2y ago
Enhanced geothermal uses fracking to expose absolutely massive amounts of m^2 through fracking between two parallel, long, horizontally drilled bore holes. Current efforts seem to show a minimum of 30 years before there will be loss of heat quality. The sorts of drilling talked about for enhanced geothermal are on the scale far far above the needs of a house, IIRC about 5MW per bore hole pair, with many connecting from a single point on the surface. It's also at distances kilometers into the earth.
- micro2588 2y ago
  There is no way to tell yet what the longevity of the resource will be as it's too early. In fracked resources the main issue is "short circuiting" where increased flow rates travel along preferential paths between the doublet wells as the source rock cools and cooling rate of the source rock in general. This causes the MWt of the resource to decline per injection / production well. Fervo is getting around this by drilling extra wells per pad to be turned on in response. Many geothermal resources decline over time as heat is slowly extracted and these declines are somewhat manageable by tuning the injection production well rates and drilling new wells. They are built into the economics of existing plants. Geothermal is kind of extractive and not "renewable" in this way over medium term time scales, you need to continuously keep drilling at a certain rate. Rock is a good insulator and it takes a long, long time for it to heat back up.
pfdietz 2y ago
Geothermal is mining heat stored in the crust. It can extract heat at a much higher rate than the average crustal heat flow, at least for a time.

brightball 2y ago

That has always been my understanding. It’s useful for HVAC for a home for example, where you aren’t trying to do a conversion to electricity but instead are directly leveraging the consistent temperature to reduce strain on the system.

phreeza 2y ago
Yea that is a heat pump scenario where you are also putting energy back in at certain times, that makes sense to me too, it doesn't have much to do with extracting geothermal energy from the core iiuc.

meindnoch 2y ago

The cited value is the energy flow through the surface, which is about 0.1 W/m^2. But this is conducted through kilometers of rock and soil, which acts as insulation. Geothermal power works through convection instead of conduction. You inject cold water into a borehole, and hot water (steam) comes back, and spins a turbine. Convection can extract energy at a much higher rate than conduction through the crust. If you boil a pot of water, you can still comfortably hold the pot's handle if it's long enough, indefinitely. This is heat conduction. On the other hand, if you try drinking from the pot with a straw, you'll find it very painful. This is convection.

archon1410 2y ago
Perhaps a stupid question, but... what are the risks? Wolndn't extracting too much energy from the earth's core cool it down, at least a little bit? Or does it contain so much energy that extracting it to replace all of 'surface generation' won't make even a little difference?
- whatshisface 2y ago
  We can't drill anywhere near the mantle, geothermal extracts energy from the upper part of the crust.
  - archon1410 2y ago
    Wouldn't drawing energy from the crust cool it down, and wouldn't a cooler crust in turn 'draw' more heat from the lower layers? I guess the earth already radiates out a lot of energy, the process of extracting geothermal energy will presumably lead it to radiate more energy. I don't know by how much though, or if it will make any real difference, or if that's how it even works.
- __turbobrew__ 2y ago
  I guess if you could extract enough energy from the core it would reduce convection which would in turn reduce the strength of the earth’s magnetic field.
- numpad0 2y ago
  Earthquakes! There are couple comments mentioning "fracking" around, that's destabilizing the land by injecting acid to get energy out. The acid is dangerous, and so are breaking up soil deep down.
- tylerhou 2y ago
  > Wouldn't extracting too much energy from the earth's core cool it down, at least a little bit? The earth generates ~50 terawatts of energy through radiation/other processes, while global energy consumption over the last year was 0.003 terawatts. I think we're fine.

cyberax 2y ago

> And if I drill a hole and collect 500 watts from it on a 100 sqm plot, am I effectively siphoning the heat from my neighbors plots? Pretty much. But the Earth's crust has a lot of thermal mass, so there's enough energy stored there to last for a long while.

epistasis 2y ago

Enhanced geothermal uses fracking to expose absolutely massive amounts of m^2 through fracking between two parallel, long, horizontally drilled bore holes. Current efforts seem to show a minimum of 30 years before there will be loss of heat quality. The sorts of drilling talked about for enhanced geothermal are on the scale far far above the needs of a house, IIRC about 5MW per bore hole pair, with many connecting from a single point on the surface. It's also at distances kilometers into the earth.

micro2588 2y ago
There is no way to tell yet what the longevity of the resource will be as it's too early. In fracked resources the main issue is "short circuiting" where increased flow rates travel along preferential paths between the doublet wells as the source rock cools and cooling rate of the source rock in general. This causes the MWt of the resource to decline per injection / production well. Fervo is getting around this by drilling extra wells per pad to be turned on in response. Many geothermal resources decline over time as heat is slowly extracted and these declines are somewhat manageable by tuning the injection production well rates and drilling new wells. They are built into the economics of existing plants. Geothermal is kind of extractive and not "renewable" in this way over medium term time scales, you need to continuously keep drilling at a certain rate. Rock is a good insulator and it takes a long, long time for it to heat back up.

pfdietz 2y ago

Geothermal is mining heat stored in the crust. It can extract heat at a much higher rate than the average crustal heat flow, at least for a time.