Marcus Hoff wrote:does anyone have any knowledge, opinion or comments on this?

I understand it's impractical.

Here's the inimitable Tom Murphy, whom

**everyone should read**:

http://physics.ucsd.edu/do-the-math/2011/09/got-storage-how-hard-can-it-be
__Flywheel Storage__

Let’s put a massive spinning disk in our energy-storage “bedroom.” These might end up being popular in Malibu, as the gyroscopic stability inherent in the spun-up system could be very handy in a mudslide—keeping the house level on its way down the hill, mimicking the surfers it’s been watching all these years.

The kinetic energy stored in the rotation of a cylindrical-shaped solid disk is ¼mv², where m is the mass of the spinning cylinder and v is the velocity at the outer edge. For a fixed mass, it is better to put as much of the mass as possible on the outer edge, in a hollow cylinder (supported by spokes, for instance), which can deliver a factor of two more energy per mass. But in the case where space, not mass, is the constraint, the solid disk has more mass than the hollow version would, making it a net win to just go solid.

How big do we make this thing? Let’s give it a diameter of 2.5 m and a height of 2 m (need room for mounting, and surrounding container/structure) yielding a 10 m³ volume. At the density of steel—about 8× that of water—we get 80 tons (now even more important to reinforce that floor!).

How fast do we spin it up? Let’s pick the speed of sound—345 m/s—and see where that puts us. Go big, or go home! We get 2.4 GJ, or about 650 kWh of energy stored in this scary flywheel. That’s somewhat comparable to a similar volume of lead-acid batteries (though four times as massive). We would want to evacuate the air around the spinning disk or we will suffer a drain rate of something like 1 kW (consuming 24 kWh/day just to keep it spinning; the room would also get warm-ish).

The acceleration at the outer radius is about 10,000 times that of gravity, and it turns out the geometry and speed we picked indeed approaches the yield strength of steel. Structural weaknesses then risk breakup, which would dump the unwelcome energy equivalent of half a ton of TNT in your house. We would need to slow to a speed of 250 m/s at the outer rim to provide an adequate material safety factor, resulting in 250 kWh of storage. Another safety concern: if the flywheel comes off its support, it could barrel through the neighborhood, popping through houses like they weren’t even there. Not ideal in earthquake country.

Obviously, we can afford to scale things down a bit, since our first cut provided three weeks of storage capacity. The same cylinder spinning at 125 m/s (275 m.p.h.) at the edge gives about 90 kWh of storage, and may be somewhat more tolerable from a safety point of view. Scaling down the size/mass in addition to velocity begins to result in a less useful storage solution for the average house. If you’re going to go through the effort, expense, and sacrifice of space for a scary flywheel, you’d better feel like it provides enough energy storage to be worthwhile.

A recent $53 million flywheel storage facility in Pennsylvania uses 200 large units storing 25 kWh each, working out to $10,000 per kWh of storage capacity. Each unit’s vacuum chamber looks to be about 1.5 m in diameter and 3–4 m tall. If we raise the ceiling and squeeze four into our bedroom, we could get 3 days of electricity storage for the typical American house for a cool million bucks. But the frictional losses—while painstakingly minimized—likely preclude these units from being useful over periods of days.

http://physics.ucsd.edu/do-the-math/2011/09/got-storage-how-hard-can-it-be