Quote:
Originally Posted by NiHaoMike
For large amounts of fixed energy storage, wouldn't pumped water be cheaper?
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But underground flywheels don't freeze. Also, with pumped water I can see many potential efficiency losses in the sequence: Electricity --> Pump --> Water flow --> Water flow --> Hydro generator --> Electricity. Not to mention leaks and evaporation, but those can be minimal. But yes, I know of six places here in Poland where water is pumped to an upper reservoir when supply in the grid is greater than demand. I doubt this would work on a small (household) scale.
[EDIT: I found that pumping water to a higher reservoir and then turning it into electricity again has an efficiency of about 70%.]
I think flywheels are still in the expensive zone, mostly because of the carbon fiber they are made of (that is going down) and reinforcements to the outer shell. I would think, though, that putting one in a concrete box under your lawn, with the spin axis vertical, and adding something (either mechanical or electronic) to keep the rpm's at least 10% below the safe max should be OK. I wonder how big a 5kW flywheel would be, or a 25kW. If you wanted to buy large amounts of cheap electricity at night and sell it back during the day, then how long would it pay back?