Quote:
Originally Posted by Piwoslaw
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?
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I'm not sure about one so small but here is a 300Kw version: w w w(dot)apc(dot)com/products/resource/include/techspec_index.cfm?base_sku=FWP78VXEGOL]Flywheel Energy Storage System, 300kW Rating for; EPS7000, EPS8000, Galaxy PW, Optional Level 2[/url]
It looks like it's somewhere in the range of 76K cubic inches in a very narrow column. Not too big for a home installation. It does cost $80K US so is still kind of pricey for an average income.
I also agree that using a reinforced concrete poured structure buried with a vertical axis should allow some of the flywheels housing reinforcement to be reduced. The purpose of the heavy housing is to prevent a catastrophic failure from exploding. A buried installation would also prevent this as the earth and concrete would absorb the energy. That would just leave a vessel strong enough to withstand a vacuum. Magnetic bearings and evacuated operation are important to avoid energy losses to friction and with something heavy moving at high RPMs friction loss adds up very quickly.
As to your last point I'm not sure that's even possible without very specific licenses and prior approval from the utility company, but as a thought experiment: If you bought one of these and were able to exploit the difference in day/night costs of about $0.04/KWhr for the entire 300Kwhr then you'd be getting $12/day and it would take 18 years for ROI. If you could find a higher day/night difference it would reduce the ROI. That is about the same as ROI on a solar installation so it would probably be better to just invest in a large solar installation as less complex and having less legal issues.