That's really neat! I've mostly seen explanations about Hydrogen as a vehicle fuel so I've never stumbled on cavern storage. It makes sense to use isothermal compression. Especially if you can recover the energy from decompressing the hydrogen using a turbo expander which would be obscenely difficult on a vehicle.
A turboexpander in a car would add an obscene amount of cost to an already expensive fuel cell vehicle. Also, it would be a fire and explosion hazard requiring a ton of maintenance.
That sounds completely unsubstantiated. Nothing involved here is that complicated or expensive. In fact, the whole thing should cost less than a comparable BEV given how little raw material you need. And if properly designed, it should be perfectly safe.
