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https://beaconpower.com/carbon-fiber-flywheels/

These guys have two projects (check their operating plants) using flywheels. They're used for frequency regulation. About 20MW each.

In the past they have been made to work, but they were not much better than batteries. With the price of batteries dropping like a stone I suspect they are going to have an even harder time competing.

There are several differences from batteries. Generally the benefits are faster charge and discharge rates, unlimited cycles without degradation, no lithium or rare earths needed, and wider operating temperatures.

They are not good at long term storage though, as the self discharge rate is high. And historically they have been expensive.

They have become more viable thanks to higher efficiency motors/power components and magnetic bearings, among other things. The biggest drawback is still cost, which is what we're tackling.