Flywheel energy storage
 

I've had flywheel energy storage in my head ever since I read in a PopSci issue (somewhere in 1995-1999) about plans to use flywheels buried next to houses for backup energy when the grid goes down, or to relieve grid load. I've been waiting for this to happen and... nothing :(
So today I searched around and found this great blog: Damn Interesting - The Mechanical Battery. The comments are actually the best part. I'm still going through them, but thought I'd go ahead and share my find now.

Here's more info: One megawatt of grid storage, 10 big flywheels.

Yeah, flywheels are definitely a means of energy storage. I wonder how price compares to other forms of energy storage though.

For large amounts of fixed energy storage, wouldn't pumped water be cheaper?

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?

Flywheel "batteries" have high power density but low energy density. Said another way... they can deliver large amounts of power for very short periods but are lousy for delivering small/medium amounts of power for a long time. It's better to think of them like capacitors than batteries. They cover or absorb spikes but do not last long.

Only very specialized flywheels have anything approaching battery performance over long time periods. They are made out of carbon fiber and spun in hard vacuum over magnetic bearings. It isn't something a DIY'r could rig up in his basement.

If I remember correctly, those air wrenches work by spinning a tiny flywheel and using a centrifugal clutch to connect it to the chuck.

Flywheel sizing
 

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.