[infiniteparamtr]

I haven't researched the overhead costs, but there are alternative kinetic "batteries" that have been proposed, some of which are already being used:

- Use excess energy generation to drive a gravel-filled train up a few miles of 2% grade, back down the track to drive turbine(s) for energy to be tapped

- Run water up an incline into a reservoir. This can simultaneously serve irrigation purposes. Water level is indicative of available energy to be sent through turbines

- Multi-ton flywheels. I've seen that some fusion experiments use this for massive, quick energy bursts. Magnetism prevents excessive wear/tear on bearings

[basicplus2]

composite thin cylindrical flywheels have the best energy storage characteristics

"greater than 400 Wh/kg can be achieved by certain composite materials"

https://en.m.wikipedia.org/wiki/Flywheel_energy_storage

[karterk]

Flywheels sound exciting but their energy density is about the same as Li Ion batteries (even when carbon composites are used). There are very few large scale deployments of flywheel energy storage systems in the world despite lots of active interest in them. Safety is also a huge concern since if there is a mechanical failure, a huge amount of kinetic energy will have to be safely disposed or you are going to have shrapnels flying everywhere.

[jakeogh]

You can recharge the flywheel over 10M times. Containment is designed in.

[basicplus2]

Flywheel systems last considerably longer than batteries, and are generally installed dug into the ground where failures cause no safety issues. Even when not installed underground the vessels fully contain any failure in any case.

[Terr_]

In certain failure scenarios I think an energetic flywheel is predictable: Most of the momentum is within the plane of rotation, which is where you can place whatever kind of shielding or catching-material.