Oh, apparently because of "dramatic improvements in power plant performance":
> Starting with the
traditional 2400 psi / 1000 F (165 bar / 538 C)
single-reheat cycle, dramatic improvements in
power plant performance can be achieved by
raising inlet steam conditions to levels up to
4500 psi/310 bar and temperatures to levels in
excess of 1112 F/600 C. It has become industry
practice to refer to such steam conditions, and
in fact any supercritical conditions where the
throttle and/or reheat steam temperatures
exceed 1050 F/566 C, as “ultrasupercritical”.
Anyway, those are the plants that Standard Thermal wants to sell their product/service to. And once the hot dirt falls below 600°, it can no longer heat the water to 600°. So I think they have to be aiming far above that temperature, which is also why heating element reliability is a challenge and why the clays in the soil are firing (a phenomenon which only happens at 600° for the lowest-firing terra-cotta clays, more typically requiring 1000°–1400°).
Here's an article giving the state of such plants in the US in 2011. Since then I imagine some of the smaller/older plants have been retired. There is no new coal capacity coming online in the US.
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.
Oh, apparently because of "dramatic improvements in power plant performance":
> Starting with the traditional 2400 psi / 1000 F (165 bar / 538 C) single-reheat cycle, dramatic improvements in power plant performance can be achieved by raising inlet steam conditions to levels up to 4500 psi/310 bar and temperatures to levels in excess of 1112 F/600 C. It has become industry practice to refer to such steam conditions, and in fact any supercritical conditions where the throttle and/or reheat steam temperatures exceed 1050 F/566 C, as “ultrasupercritical”.
https://www.gevernova.com/content/dam/gepower-new/global/en_...
Anyway, those are the plants that Standard Thermal wants to sell their product/service to. And once the hot dirt falls below 600°, it can no longer heat the water to 600°. So I think they have to be aiming far above that temperature, which is also why heating element reliability is a challenge and why the clays in the soil are firing (a phenomenon which only happens at 600° for the lowest-firing terra-cotta clays, more typically requiring 1000°–1400°).