Are you suggesting to basically harvest the thermal energy in the rock in a non-renewable fashion? I don't think that is very promising, the heat capacity of rock is not that huge.
Back of the envelope calculation is drawing 1 GW from a cubic Kilometer of rock would lower the temperature by 1 degree C every 25 days. So I think you'd deplete a typical borehole quite quickly?
Re:Geothermal's Time Has Come
by griffzhowl on Friday November 21, 2025 @08:15AM
(#46004222)
> Are you suggesting to basically harvest the thermal energy in the rock in a non-renewable fashion?
As I understand it, that's how many geothermal plants work, effectively mining the heat underneath them, but at a rate of extraction that means they would become uneconomical over a span of decades rather than months.
Doesn't that deep down rock reach equilibrium with the system and is then limited by the flow rate?
Exactly. The only exception to this are very rare sites like the one in Iceland where you can get close to a magma cell which has a much higher thermal gradient and possibly magma convection replenishing it.
It's a misleading comparison. This is only the average amount of heat that diffuses through an ordinary patch of surface, and has more or less nothing to do with how a geothermal plant works, since they don't harvest heat by covering a large area of surface with conducting material.
The surface heat flow is low because rock acts as an insulator. If you drill down to where it's hot and draw the heat up you obviously get orders of magnitude larger flows of energy to the surface.