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What does drilling a Hole for Water cost here in Europe?

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nctnico:

--- Quote from: Marco on March 29, 2022, 10:41:00 pm ---
--- Quote from: Someone on March 29, 2022, 10:02:26 pm ---deeper isnt getting access to more flux

--- End quote ---
Of course it does, the volume of rock you can steal flux from before it escapes to space grows quadratically with depth.

--- End quote ---
Yes and no. At some point you either need insane pressures to prevent the water from boiling from the wall and not getting heated at all. Heating or evaporating a liquid is actually quite an interesting process in itself. Many years ago I worked on a smoke generator that evaporates 'smoke liquid'. You can't keep increasing the temperature to evaporate more liquid. At some point the liquid will form a gas layer between the wall of the heating element and the liquid which prevents heat transfer. The liquid will come out as is; not even very hot.

In practical geothermal systems you see that water gets injected at one point and being pumped up through a different hole. That gives a much larger surface area with a constant temperature compared to a single, deep hole.

Marco:

--- Quote from: nctnico on March 29, 2022, 11:16:38 pm ---In practical geothermal systems you see that water gets injected at one point and being pumped up through a different hole. That give a much larger surface area with a constant temperature compared to a single, deep hole.

--- End quote ---

Ignoring practicality for a moment, this ain't going to work for really deep wells. Nothing to pump water through in bedrock, that only works in porous sedimentary rocks and aquifers.

But even with essentially a point source of extraction at the bottom of a well, you will still be stealing flux from a larger top soil surface area the deeper you go.

Someone:

--- Quote from: Marco on March 29, 2022, 06:30:51 pm ---If you go deep enough the thermal resistance to the core shrinks and the core has defacto infinite energy.
--- End quote ---

--- Quote from: Marco on March 29, 2022, 10:41:00 pm ---
--- Quote from: Someone on March 29, 2022, 10:02:26 pm ---deeper isnt getting access to more flux

--- End quote ---
Of course it does, the volume of rock you can steal flux from before it escapes to space grows quadratically with depth.

Depth is a trade off for area.

--- End quote ---
You're arguing sideways....

Its very very simple:
1. assume the majority of the geothermal flux is coming from the core of the earth (you framed this, and its a reasonable measure for generalised geothermal outside the highly active areas)
2. the deepest bore hole ever made ever, 12km, earths radius 6000km. So far from the core its effectively a point source.
3. by going this deep you've increased the flux (W/m2) by 60002/(6000-12)2, 0.4% increase in flux

Yes, you gain access to a larger thermal mass which can have higher extraction ("stealing") power, but the energy coming back in to balance what is being taken out has not increased. Exactly what was said in the paper I linked you to:

--- Quote from: Someone on March 29, 2022, 10:02:26 pm ---"Longevity and power density of intermediate-to-deep geothermal wells in district heating applications"
https://link.springer.com/article/10.1140/epjp/s13360-021-01094-8
Its still energy in < energy out or you're going to run out. Their simple explanation:

--- Quote ---One of the most important things to keep in mind is that, once the well has been depleted, its rather modest ability to recover leads effectively to a single-use solution unless additional charging is provided. This, however, would merely convert the well from a heat source into a heat storage, incurring additional, perhaps significant electricity costs. Hence, in sizing the well, one should consider choosing a configuration which guarantees a sustainable level of longevity and does not require re-drilling of the bore holes every few years.
--- End quote ---

--- End quote ---

Trying to claim the first (n) km of the earths depth is radically different in thermal conductivity to the rest is not supported.

Marco:

--- Quote from: Someone on March 30, 2022, 01:05:08 am ---1. assume the majority of the geothermal flux is coming from the core of the earth (you framed this, and its a reasonable measure for generalised geothermal outside the highly active areas)

--- End quote ---

Flux does not go straight up, it follows thermal gradients, the deeper you drill the larger thermal gradients you can create across a larger volume. Or to put it another way, the low temperature spherical disruption of thermal equilibrium around the bottom of the borehole casts a shadow on the surface, the deeper the borehole, the larger the shadow.

You could have say a 10 km deep borehole capture the same flux as 3 3km deep boreholes with 3km between each of them ... the depth can be traded off against area, a single deeper borehole can capture more of the flux across a larger area of the surface.

If you look at figure 2 in the paper you linked you can clearly see it going to an equilibrium after about 200 years, an equilibrium which has to be based on flux and not stored heat. An equilibrium at around a quarter of the initial peak, but still on the same order of magnitude. 0.054 Wm^−2 is small, but with a large enough shadow it can be large enough.

Someone:

--- Quote from: Marco on March 30, 2022, 02:41:35 am ---
--- Quote from: Someone on March 30, 2022, 01:05:08 am ---1. assume the majority of the geothermal flux is coming from the core of the earth (you framed this, and its a reasonable measure for generalised geothermal outside the highly active areas)

--- End quote ---

Flux does not go straight up, it follows thermal gradients, the deeper you drill the larger thermal gradients you can create across a larger volume. Or to put it another way, the low temperature spherical disruption of thermal equilibrium around the bottom of the borehole casts a shadow on the surface, the deeper the borehole, the larger the shadow.

You could have say a 10 km deep borehole capture the same flux as 3 3km deep boreholes with 3km between each of them ... the depth can be traded off against area, a single deeper borehole can capture more of the flux across a larger area of the surface.

If you look at figure 2 in the paper you linked you can clearly see it going to an equilibrium after about 200 years, an equilibrium which has to be based on flux and not stored heat. An equilibrium at around a quarter of the initial peak, but still on the same order of magnitude. 0.054 Wm^−2 is small, but with a large enough shadow it can be large enough.

--- End quote ---
Fig 2 is their example of approaching an equilibrium by limiting the average power extraction. Also note their idea of a lifetime: "time it takes for the system to reach the water inlet–outlet temperature difference of 3 [degrees] C" extremely low grade heat. Its not surprising that they predict lifespans of hundreds of years to draw down several km depth of rock to close to ambient temperature. Seasonal variations disappear after just 10m of depth, so hundreds of years from hundreds of times the depth are right where they would expect to be. Its drawing down on the enormous stored thermal resource and not an infinitely sustainable power draw, just like drawing on fossil fuels, massive stored resource, slowly accumulated.

Humans have been around for 10's of thousands of years, so planning to use up a resource in just hundreds is very short sighted. Might as well suggest once through fission reactors....

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