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points by pfdietz 3 days ago | 0 comments
The cost of putting dirt in a pile is $1-2/ton.
[–] cyberax 3 days ago
The biggest expense will be the pipework inside that pile. That you won't be able to service easily. And you'll also need a LOT of insulation around it to prevent heat from leaking out too quickly.
[–] adrianN 3 days ago
Thermal storage benefits from the square/cube scaling, so the amount of insulation goes down as you scale up the storage.
[–] pfdietz 2 days ago
Even better than that: the thermal time constant is proportional to the square of the radius. There's the square/cube effect that causes a factor of r ratio in the volume to surface area, but there's also a factor of r from the temperature gradient (which, from the center of the pile to the surface, is inversely proportional to r).
[–] pfdietz 3 days ago
The insulation is just more dirt.
[–] cyberax 3 days ago
Homework: compute the heat flux from a 40 meter radius sphere, given the surface temperature 600C and the ambient temperature 0C. Now compute the amount of insulation needed to keep its temperature above 500C for 3 months, given that dry sand has an R value of 0.25 W/mK.
[–] pfdietz 2 days ago
Note that the thermal time constant of a given geometry scales as the square of the radius. This is why subsurface geothermal heat is there even though it was "charged" gradually over millions of years. So it's not hard to achieve very long time constants just by making the pile a bit larger. They've done these computations. The result is the minimum practical size is a pile that provides ~100 kW(th) constant output.

So the idea is not practical for storage for an individual home, but could work on the scale of a village or small industrial enterprise. It would allow a solar microgrid to be completely disconnected from the larger grid, even at high latitude (assuming one could get a sufficiently good heat engine to work at that scale; it's a bit small for steam turbines.)

If you look at very large piles of dirt, like large earthen dams, if arranged in a suitable geometry they would have thermal time constants (assuming no fluid flow through them) in the thousands of years.

A computation for you: compute the cost of a given R value per m^2 for various materials. Dirt is very cheap, so even if it's not the best insulator it comes out on top. You need the thing being insulated to be large enough that you can fit the thick dirt layer around it, of course.

[–] PaulDavisThe1st 3 days ago
so $36-72k for days of 1GW of power? Sounds pretty cheap to me ...
[–] pfdietz 3 days ago
They also state the cost per kWh(th) of storage capacity is about $0.10, or $100,000/GWh(th). This is thermal energy, which is converted to work at maybe 40% efficiency, mind you. There are costs over and above the dirt (for example, pipes laid through the dirt to carry heat in/out).