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points by cyberax 3 days ago | 0 comments
I just don't see it with thermal. You have to pay the Carnot tax, and you also need humongous amounts of material.

E.g. 100 GWh (4 days of 1GW thermal power) with a 100C temperature swing (you can't go down all the way to room temperature) will need around 3.6*10^4 tons of sand. That's a lot.

Honestly, power-to-gas sounds like the only reasonable way for now. In theory, with SOFC (Solid Oxide Fuel Cells) for power generation, it can beat the steam cycle.

[–] Panzer04 3 days ago
36000 tons... doesn't actually sound like that much? That's a ~30x30m cube of dirt.

But I have no reference for the cost of setting up such a storage and how it compares to battery or hydro.

[–] adgjlsfhk1 3 days ago
The ~grid scale design of this for reference is somewhere around 200mx200mx50m for above ground, or ~100m cube (or bigger) for quarry infill. Such a design is going to get crushed by battery/hydro over the hours/days, but wins on cost for seasonal usage.
[–] pfdietz 3 days ago
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).
[–] pfdietz 3 days ago
The temperature swing can be much greater than 100 C. Heat can be extracted as a wave passing through the store, then heat added back by reversing the flow. It would operate like a counterflow heat exchanger.

View another way: if we hook several piles in series, then lukewarm air from a first mostly discharged pile can be directed into later piles for more heating. In the continuous limit this is the concept I described above.

[–] cyberax 3 days ago
> The temperature swing can be much greater than 100 C.

No, because your steam will be at a lower temperature as you draw down the heat, bringing down efficiency.

[–] adgjlsfhk1 3 days ago
Realistic designs for this can achieve 450C delta t. Your energy store can store at ~1000C (slightly below rock melting point), and run steam turbines with a hot side of 600C and a cool side of 150C.

Over the season, your sand drops from ~1000 to ~600 (which is fine since your power needs drop in the spring)

[–] cyberax 3 days ago
Sorry, but 1000C is wildly unrealistic. There are no bulk materials that can be used to construct heat exchangers for this kind of environment. 600C is already a stretch.

At 1000C steel will react with _water_.

To be clear, I'm not talking about cooling a small crucible but a structure that is at least 20 meters in size.

[–] pfdietz 2 days ago
At 600 C one can use stainless steel, but things rapidly become expensive and problematic beyond that. 1000 C would require Inconel or something.
[–] pfdietz 3 days ago
That is wrong, and I already explained why.