While the flywheels at the Bitter Magnet Lab are 10 tons each, around 10 feet in diameter, take hours or days to spin up to speeds of a few hundred rpm and dim the lights in Cambridge while running up, they store a very large amount of energy, which is fed into the magnet coils to bring the flywheels to a halt in a few milliseconds. They "simulate" briefly directing approximately the full generating capacity of the Cambridge MA grid to one user.

There was a spurt of publicity several years ago though about flywheels for vehicle "load leveling" where they used fiberglass/composite flywheels encased in vacuum enclosures, generally with "dynamic bearings" of a couple of kinds, with peak speeds of up to 100,000 rpm or so. These flywheels were on the order of 10 to 30 pounds, and under about 100 pounds for the whole "storage assembly." A few prototype buses were built and were used in test/demonstration street use. Laboratory efficiency numbers looked good; but I don't recall ever hearing of a "final report" on the street testing; and haven't heard of any continuing advocacy for these schemes among transportation engineering companies or groups.

Someone mentioned flywheel bearing problems earlier. In either kind of flywheel storage system, there are adquate bearing designs, if intelligently used. (But good bearing designers are really scarce.)

There is a very real problem with containment in a failure for the casual experimenter, but with the lighter weight very high speed flywheels used in the vehicle tests the cases required to hold the vacuum were almost automatically provable (at least by analysis) as being adequate to contain the energy dump in case of failure. (Flywheels of this kind would be very inefficient due to "windage" without the vacuum.) The flywheel itself of course self-destructs in almost any failure, but for small amounts of energy storage fail-safe design isn't too much of a problem.

The amount of energy to be stored dictates whether the small high-speed flywheel or the massive low speed one is more efficient, and unfortunately the difference between energy storage requirements for a small bus (or single home) and storage requirements even for a local municipal power grid are like the difference between a flea-fart and a hurricane.

John