https://www.energiestro.net/technology/
These guys are serious: Concrete flywheel storage system for residential PV – pv magazine International
It has to be very precisely balanced. They don’t mention a system price yet.
Metaphorically, this idea won’t take off, literally though, it will.
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I like this one …
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If you have a wife, installing something like that in your shower might yield a few kWhs every morning before sunrise and evening after sunset… 
This is fascinating!
Very cool idea! Spin up the flywheel when you have excess solar, harvest when you don’t.
In theory, how long would such a flywheel be able to spin if no power is harvested from it?
It talks about a frictionless sealed environment and perfectly balanced. A week? Two?