This isn't really the same use case.

A basic thing to understand about electricity storage is that there are very different needs for different kinds of storage. Flywheels from what I understand are for very shortterm storage needs, i.e. balancing out shortterm fluctuations in electricity use vs. generation. But they're unsuitable for any kind of longterm storage, because they loose power over time.

In the long run with a high-solar-high-wind-scenario we'll need some seasonal storage to get us over a couple of weeks in some circumstances. This will need some storage that doesn't loose power over time.

(FWIW I have no idea if these gravitational storage techs will play any role in that, and one can be doubtful about it.)

From Wikipedia: Flywheel energy storage systems using mechanical bearings can lose 20% to 50% of their energy in two hours.

Basically flywheel cannot store energy for a long time. It needs to be constantly used to be effective.

Could you not levitate the flywheel on magnets?

Apparently not that easily or the half-life would be longer than two hours... You want to try if storing energy in a flywheel while using a lot of energy for the magnetic field can outperform gravity storage?

> 20 MW storage systems

Megawatts measure the peak discharge rate, not that amount of energy stored.

For a fixed number of dollars of investment, gravity will store far more joules of energy.

Is it with or without an existing hole?

The problem with a flywheel is that you ha w to keep it moving. Bearing wear out, require lubrication...

A weight you can just hang there

A flywheel in a vacuum floating on a magnet also just hangs there. But indeed the weight solution seems more simple.

You would need to supply a lot of power just to maintain a vacuum and levitating your flywheel. Not sure this solution would be very effective. Maybe with superconductors, but then cooling is your problem instead.

Or use normal magnets (non-electro magnets) The vaccuum, sure, this may take some energy depending on the insulation.