5 ms·

Yes, most wells are productive for about 30 years, and then you have to wait a bit (another 30) to let the rock heat back up. The NPV past 60 years is pretty ti

Yes, most wells are productive for about 30 years, and then you have to wait a bit (another 30) to let the rock heat back up. The NPV past 60 years is pretty tiny, so it is hard to amortize capex that far out. Worst case, you run them continuously at half capacity, but then your cost per kWh is about 2x.

xoa 9y ago
>Yes, most wells are productive for about 30 years Wait, I haven't looked at geothermal in a long time so I'm not at all up to speed on the current state of the at. But previously you wrote: >and so geothermal plants are generally only built in areas that require shallow (1km) wells That sounds like your "30 years" number there is for shallow wells. Intuitively it doesn't seem like this logic would necessarily be identical for deeper wells: obviously at some depth the rock is always extremely hot (or, in fact, molten). The article seems to indicate that they expect a deeper well to produce higher temperatures and pressures, in turn allowing more energy production, and that seems like it'd make sense. Maybe 1km vs 5km makes no difference in lifetime beyond that, but does your analysis include deep wells already in an equivalent area? Otherwise it doesn't seem reasonable to extrapolate past lifetimes of shallow wells to future lifetimes of deep wells, even ignoring that they say they'll be able to achieve far higher energy generation per well.
- BurningFrog 9y ago
  > obviously at some depth the rock is always extremely hot Not if you keep pumping in water to run a steam turbine. I assume that's how geothermal energy works, and I'm not going to double check.
  - xoa 9y ago
    >Not if you keep pumping in water to run a steam turbine. Are you being serious? In terms of known capabilities the Kola borehole shows that even with older tech we can drill down to over 12km in depth, and there have been decades [1,2,3] of efforts drill all the way through the Mohorovičić discontinuity into the mantle (20+km minimum from a continental starting point, 5-10+km from ocean). Lithosphere/mantle boundary tends to be defined by temperature, in that around 1800°F is when the weakest common mineral (olivine) begins to exhibit viscosity. Well before that it'd reach the point where the amount of pressurized water we could pump through it wouldn't be able to keep up with new thermal load. So yeah, at some depth the rock is always extremely hot. In terms of economics though of course there is some set of balance points for a given level of tech and economics between cost of drilling to a given depth, gains in lifetime/power density, maintenance costs, costs/losses of actually moving working fluid from heat point to generator, reliability advantages, etc etc. Presumably experts go through and consider all of this, it should be tractable within some set of risk bars. If an experienced power company is actually putting down millions on doing a project after evaluation, I see no reason to doubt that they've done the math to their/their investors' satisfaction to a level sufficient for a pilot project. 1. http://www.smithsonianmag.com/science-nature/decades-long-quest-drill-earths-mantle-may-soon-hit-pay-dirt-180957908/ http://www.smithsonianmag.com/science-nature/decades-long-qu... 2. https://en.wikipedia.org/wiki/Chiky%C5%AB https://en.wikipedia.org/wiki/Chiky%C5%AB 3. https://www.extremetech.com/extreme/247370-drill-baby-drill-japanese-scientists-hope-reach-earths-mantle-massive-drilling-project https://www.extremetech.com/extreme/247370-drill-baby-drill-...
    - BurningFrog 9y ago
      I'm just making the simple point that however hot the rock is, it will cool down when you pour enough water on it. I don't know what the Mohorovičić discontinuity is, but I'm sure it doesn't change this physics fact.
      - mmmpop 9y ago
        The rock will radiate heat sufficient enough to boil the water before it can even come close to coming in contact, so no, basic thermodynamics would like a word with your physics fact....
        askvictor 9y ago
        And the water, in boiling, will remove some of that heat that would otherwise radiate back. The entire premise of geothermal energy is removing some heat from deep down in the earth. Whether that amount is big enough to cause a noticeable difference is was is being debated.
- chris_va 9y ago
  Yes, you have a point, though the reality is even more complicated. There are physical limits to the heat capacity of the rock, and the heat flux into the region. Deeper wells are not necessarily hotter. Going hotter adds other costs (drilling mud, water=>co2, casement, etc), so its a big multivariate optimization.

xoa 9y ago

>Yes, most wells are productive for about 30 years Wait, I haven't looked at geothermal in a long time so I'm not at all up to speed on the current state of the at. But previously you wrote: >and so geothermal plants are generally only built in areas that require shallow (1km) wells That sounds like your "30 years" number there is for shallow wells. Intuitively it doesn't seem like this logic would necessarily be identical for deeper wells: obviously at some depth the rock is always extremely hot (or, in fact, molten). The article seems to indicate that they expect a deeper well to produce higher temperatures and pressures, in turn allowing more energy production, and that seems like it'd make sense. Maybe 1km vs 5km makes no difference in lifetime beyond that, but does your analysis include deep wells already in an equivalent area? Otherwise it doesn't seem reasonable to extrapolate past lifetimes of shallow wells to future lifetimes of deep wells, even ignoring that they say they'll be able to achieve far higher energy generation per well.

BurningFrog 9y ago
> obviously at some depth the rock is always extremely hot Not if you keep pumping in water to run a steam turbine. I assume that's how geothermal energy works, and I'm not going to double check.
- xoa 9y ago
  >Not if you keep pumping in water to run a steam turbine. Are you being serious? In terms of known capabilities the Kola borehole shows that even with older tech we can drill down to over 12km in depth, and there have been decades [1,2,3] of efforts drill all the way through the Mohorovičić discontinuity into the mantle (20+km minimum from a continental starting point, 5-10+km from ocean). Lithosphere/mantle boundary tends to be defined by temperature, in that around 1800°F is when the weakest common mineral (olivine) begins to exhibit viscosity. Well before that it'd reach the point where the amount of pressurized water we could pump through it wouldn't be able to keep up with new thermal load. So yeah, at some depth the rock is always extremely hot. In terms of economics though of course there is some set of balance points for a given level of tech and economics between cost of drilling to a given depth, gains in lifetime/power density, maintenance costs, costs/losses of actually moving working fluid from heat point to generator, reliability advantages, etc etc. Presumably experts go through and consider all of this, it should be tractable within some set of risk bars. If an experienced power company is actually putting down millions on doing a project after evaluation, I see no reason to doubt that they've done the math to their/their investors' satisfaction to a level sufficient for a pilot project. 1. http://www.smithsonianmag.com/science-nature/decades-long-quest-drill-earths-mantle-may-soon-hit-pay-dirt-180957908/ http://www.smithsonianmag.com/science-nature/decades-long-qu... 2. https://en.wikipedia.org/wiki/Chiky%C5%AB https://en.wikipedia.org/wiki/Chiky%C5%AB 3. https://www.extremetech.com/extreme/247370-drill-baby-drill-japanese-scientists-hope-reach-earths-mantle-massive-drilling-project https://www.extremetech.com/extreme/247370-drill-baby-drill-...
  - BurningFrog 9y ago
    I'm just making the simple point that however hot the rock is, it will cool down when you pour enough water on it. I don't know what the Mohorovičić discontinuity is, but I'm sure it doesn't change this physics fact.
    - mmmpop 9y ago
      The rock will radiate heat sufficient enough to boil the water before it can even come close to coming in contact, so no, basic thermodynamics would like a word with your physics fact....
      - askvictor 9y ago
        And the water, in boiling, will remove some of that heat that would otherwise radiate back. The entire premise of geothermal energy is removing some heat from deep down in the earth. Whether that amount is big enough to cause a noticeable difference is was is being debated.
chris_va 9y ago
Yes, you have a point, though the reality is even more complicated. There are physical limits to the heat capacity of the rock, and the heat flux into the region. Deeper wells are not necessarily hotter. Going hotter adds other costs (drilling mud, water=>co2, casement, etc), so its a big multivariate optimization.