Sounds like it was 80% subcritical at the time. I hadn't realized. It sounds like even "regular supercritical steam" is at like 580°, though. Maybe dirt at only 600° could still provide a substantial fraction of its stored energy to subcritical steam circuits, if they're much colder than that?

I think "storage at 600 C" could provide most of its heat output at close to 600 C. It's not like the entire thermal store is tapped at the same time.

Imagine a pile with long pipes through it. Cool fluid is introduced at one end; the steam is gradually heated as it travels down the pipes, emerging as hot steam at the end. If needed, gang two of these together with the second pile acting to top off the temperature from the first one (or more than two).

So, during discharge, a wave of cold sweeps down the pile(s), while the pile near the outlet end stays pretty hot. Only when most of the piles are discharged does the temperature decline.

The Standard Thermal approach is described as heating the piles with embedded resistive heaters, but it could also use an external heater that sends in steam in the opposite direction from when it discharges. This would turn the system into a giant counterflow heat exchanger. Counterflow heat exchangers are known for their high performance, enabling almost all the delta-T between two fluid streams to be interchanged.

Hmm, you're right that a counterflow heat exchanger could indeed provide 600° output by cooling a hot dirt pile most of which is well below 600°. I hadn't thought of that, even though I knew it in other contexts. I mistakenly thought that only the heat over the output temperature would be available.

The only drawback is that, when you're heating the pile, most of the time you are only heating part of it (the cold part), so your heating elements need to have a significantly higher power capacity, assuming you're heating the pile with embedded resistors as described, rather than with a heat transfer fluid as you suggest.

I don't think you want the heat transfer fluid in the dirt pile to be steam; that would involve making the pipes through the hot dirt resistant to both pressure and steam corrosion, which would make them expensive. It also means you'd have a high-pressure steam leak deep inside the dirt pile when they did fail, which would probably cause damage to other pipes and to resistors and a significant uncontrolled heat transfer from the zone of the leak to other parts of the dirt pile. I think you want a more relaxing atmospheric-pressure heat transfer fluid that allows you to use cheap pipes, or even no pipes. Air, for example. Solar salt can reach those temperatures, and it has a much larger heat capacity, but it's fairly corrosive.

For a seasonal thermal store, also, we're talking about astoundingly low power densities, so low-density heat transfer fluids like air should be fine. If we're cooling the hot dirt from 600° down to 100° over a 3-month low-sun season, and the dirt is 1J/g/K, we're only extracting 63 watts per tonne, which at 1.3 tonnes of dirt per cubic meter is 80 watts per cubic meter. My human body is largely air-cooled and generates about 800 watts per tonne in normal operation, even without forced air.

I don't think it's necessary for the pipes in the piles to be operating at the pressure of the steam in the turbines. There could be an intermediate heat exchanger to transfer heat from the low density steam (or air) in the pipes to the high pressure steam in the turbine loop.

Yes, that is what I meant to propose, but when you have that additional heat exchanger, why would you use steam rather than air (or some other fluid like helium or argon) in the low-pressure pipes?

An expensive gas would present the problem of leakage. If that could be cheaply addressed, great, otherwise go with something where making up small leaks is not expensive. That probably means steam or air.

I think steam would have better heat transfer properties than air, maybe?

I do wonder how much we should worry about oxidation of the pipes. Stainless is certainly fine at 600 C in an oxidizing atmosphere, but what about cheaper steel?