[epistasis]

Hydro storage and flywheel storage have very different use cases, typically.

Flywheels contain tiny amount of energy, but can respond nearly instantaneously To provide frequency regulation on the grid.

Hydro has massive amounts of energy storage (days+ worth of output), but takes a looot longer to respond than flywheels.

I think we will see very little flywheel or dam construction in the coming decades; battery storage is becoming cheap enough that it will hit the non-linear inflection point in deployment in the next few years. First, lots of lithium ion, and likely after that lots of flow batteries.

[ascorbic]

Hydro can be fast. The UK uses pumped hydro for frequency management among other things, and can spin up several GW in seconds.

[epistasis]

Oh interesting! I had heard of some hydro in the US planning to instal batteries to help with faster response. But I guess there is great variety in design.