[jacquesm]

This company is quite advanced, but now focusing more on research and IP:

http://afstrinity.com/company/

I've seen a system meant for hospital ride-through that packed an amazing amount of power into a relatively small enclosure using carbon fiber wound flywheels. For short durations you can get away with this because you're only going to operate one of those. Larger installations would need multiple of those and then you need to get very serious about containment so that if one of the flywheels ever fails it doesn't set off a chain reaction.

The nice thing about the fiber would units is that when they break they self destruct completely within the enclosure. The fiber acts as energy sink by breaking up, as opposed to a steel wheel that will spit out sizeable chunks across fairly larger distances (many meters).

Another player in that market:

https://vyconenergy.com/products/ups/vdc-products/

Grid stabilization is better done using superconductors rather than with flywheels.