Actually I’ve worked on a system similar to this. It’s a flywheel generator. Magnetic bearings, vacuum enclosure, 6ft diameter steel fly wheel. You spin it up really really fast, and it spins for years. There is some very slight energy input to counteract any loses, but it’s very minimal. But when you need power the fly wheel could power a house for a couple days (with the right inverter). Totally impractical but a cool prototype not work on.