[afarrell]

Of those, I believe only gravity storage (at facilities like Blenheim-Gilboa) are currently in use.

[jerven]

Flywheels are as well. But at a much smaller scale than gravity storage.

[afarrell]

Oh I wasn't aware. Link?

[jerven]

http://beaconpower.com/operating-plants/ but there are others, some even quite old. More often selected for niche reasons than market peak load balancing. e.g. site one at long end line of insufficient frequency stability.