We were looking into flywheel UPSes for my company and I read up on this a bit. Of note in recent developments:
Instead of traditional bearings, they're using magnetic bearings to mostly eliminate friction.
To be able to suspend the flywheel on magnets, that requires a lower weight/mass.
To counter the lower weight/mass, you have to replace that with ...
Some years ago I read of an off-grid farmstead in Australia that used a conventional chest freezer as a refrigerator. The way he did it was to use a probe and temperature controlled relay to turn the freezer on and off.
He added 2" of closed cell XPS polyurethane sheet to the front, ends and top, but not the back, as the radiant grid was under the skin ...
AFAIK not yet, at least not any that are open to a large audience, but with so many pilot projects on-going it's only a matter of time. Basically there are 3 things that make V2G hard / expensive to do right now:
The limited amount of electric vehicles on the market (that support bi-directional charging)
Keeping track of returned energy
Providing enough ...