While I think what Standard Thermal is doing is very interesting, and in particular may be very helpful for already-built thermal plants, I don't think they've solved the fundamental problem that large heat engines are really expensive to build compared to solar.

But that's not a problem they're trying to solve, or one they need to solve.

As I understand it, their market is dependent on it. They can't store electrical energy, only thermal energy, and their system is designed to store it at fairly high temperatures (they don't say explicitly, but I'm guessing 800° and up from the problems they report having to solve) which you can avoid doing if you're just targeting the process heat market. So turning their stored heat back into electrical energy is necessary for their process to make sense, and that requires a heat engine, such as a steam turbine.

But utility-scale steam circuits cost more per watt than solar panels, and much more than batteries, the electrochemical kind.

No, their market is not dependent on it. Generation of power from stored thermal energy is significantly different from directly using solar: it is completely dispatchable. As such, it serves a role even in a situation where most solar energy (or most solar + wind) is used directly. It enables solar to be used in places, like at high latitudes, where it is otherwise strongly disadvantaged by seasonality (something batteries cannot fix).

Long term storage of this kind reduces the overall cost of providing steady solar/wind output in Europe by half.

They are also addressing markets where the need is for heat. If you are going to make heat from the solar energy, storing it as heat is much cheaper than storing it beforehand as electrical energy and then converting it to heat later.

Generally speaking, steam circuits have large thermal masses, resulting in ramp-up times measured in hours, so most thermal energy is nowhere close to "completely dispatchable". Completely dispatchable thermal power is internal combustion engines (diesel or Otto) and open-cycle gas turbines, and Standard Thermal is not targeting temperatures high enough to operate those machines. Adding Standard Thermal to a baseload coal plant will not make it dispatchable; you will still have a baseload plant, not a peaker. It just won't consume coal.

I agree that it would make solar usable in situations where it would not otherwise be usable, and high latitudes are a good candidate.

For reasons like these I do not think that they will result in a cost reduction.

Standard Thermal has been bending over backwards to store their heat at the high temperatures I mentioned, resulting in a lot of engineering challenges that a lower-temperature thermal store (say, 400° or below) wouldn't have to deal with. For the most part, process heat is lower in temperature than 400°, so I think that isn't their market either.

Batteries would handle the higher frequency components of the supply-demand mismatch curve, so steam sources wouldn't have to dispatch faster than on an hours timescale.