I wonder what the threshold is for a niche solution.

You don't want to be, like, the only person with a sand-heat-battery. Well, you might, as an individual, if you like to tinker, but as a municipality it's a pain. All of your problems are unique, you can't just hire a repairman to come in and swap out a standard part for another, you don't have a standard payscale for your Sand Management Technician, etc etc.

How many installations do you need before it becomes a good idea? 1% of the total? 50 total in a service area?

No, I'm implying that the headline claim "transform clean energy" is overstated. It mostly applies for far-flung, low-density areas. This is still useful, but not transformative for most of us.