Play with this and you'll immediately see it doesn't seem useful. e.g. storing a 50-tonne weight at 10m gives you less than 1.5 kWh of energy.

Flywheels that don't become fly-apart wheels is one problem.

Containment of fly-apart wheels another. Buried concrete pipes are usually what you're looking at. You need mass to contain these things.

The per-kg energy densities just aren't all that high. If you want reactive load to soak up very short-term demand / supply fluctuations, you can get that. And you've got crazy charge-discharge cycles.

Bearings are mostly solvable with magnets, as I understand. But you still lose a given percentage of storage over short time -- ~1% per hour or so, within a few powers of 2. Enough that long-term storage isn't viable.

And the thing that really screws you over is that the Earth moves. Geometric precession of your flywheel is something you've got to deal with.

There's a place, but I don't think it's large-scale, long-term grid storage.

There is a modern effort to use the technology in a massively scaled up fashion by Heindl Energy[1]. Their concept uses pumped water to raise a rock weight 100 meters or more in diameter.

[0] https://en.wikipedia.org/wiki/Hydraulic_accumulator [1] http://www.heindl-energy.com/hydraulic-rock-storage/overview...