Flywheel storage is used in many places where the desirable properties are high reliability, low maintenance, long performance life. Some examples are hospitals and server farmsFlywheels are also used in telecoms for providing power storage for land lines in remote sites. Still I think this is kind of old tech. I expected something better. I don't know, something solid state like ultracapacitors, or something else like that. I guess they still don't have enough power density. With flywheels you either make them heavier or you make them spin faster. Supposedly you can even make them out of carbon fiber composites and spin them really quickly. But this is a kind of niche application.