[kragen]

Hmm! Interesting! I would have thought that 600° would be close to the minimum for producing supercritical steam, so any energy stored up to 600° would be "overhead" that couldn't be effectively recovered—only the heating above that. And I assumed they would have to use cheap ceramic for the pipes, because oxidation is usually a problem for cheap steel even below 600°.

[pfdietz]

The critical temperature of water is 374 C.

[kragen]

You're right; I wonder why they operate supercritical steam turbines at 600° instead? https://direns.minesparis.psl.eu/Sites/Thopt/en/co/cycles-su... https://www.mdpi.com/2673-7264/5/1/1 https://www.modernpowersystems.com/analysis/is-700-c-steam-t...

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°).

[pfdietz]

Most coal plants in the US aren't ultrasupercritical. The first one only went operational in 2013, in Arkansas (the John W. Turk, Jr. Power Plant).

[kragen]

Oh, really? What temperatures do US power plants operate their steam at?

[pfdietz]

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.

https://www.powermag.com/coal-fired-generation-cost-and-perf...

[kragen]

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?