UPDATE: What I'm hearing is that the major problem is containment; the flywheel has to be in a vacuum, and that's awfully hard to maintain efficiently at large sizes (and all failure modes are spectacular—picture a fifty-foot-diameter concrete barrel rolling through downtown LA on the way to the ocean). And yeah, I do remember reading about the experiments they did on flywheels in cars; if I recall correctly, they went very fast down the straightaway on the test track, but when the driver tried to turn, the car had other ideas and torpedoed through the embankment in a perfect straight line...

UPDATE: SDB responds:

A house-sized flywheel made of concrete would fly apart if spun fast enough
to store the amount of energy you're describing. Concrete has tremendous
compressive strength, but terrible tensile strength. (That's why it has to
be steel-reinforced when used in bridges and similar structures.) You'd have
to make such a flywheel out of something which had decent tensile strength,
likely steel. So let's run some numbers, based on the simplification
assumption of a wheel with virtually all its mass on the outer rim, storing
its power as kinetic energy. And because we're just trying to get an idea of
the problem, we'll use rough numbers and estimates for a first order
approximation.

The state of California uses power at rates which vary between about 25
gigawatts and about 40 gigawatts (and that's daily fluctuation). So in order
for an energy storage system to make any significant difference, it would
have to be able to store enough energy to be able to produce two hours of
power at 2 gigawatts. In other words, about 14 terajoules.

Just to pull a number out of my ear, let's assume that the rim mass of the
wheel is 50 metric tons, or 50,000 kilograms. The formula for kinetic energy
is:

e = 1/2*m*v^2

v = sqrt((2*e)/m)

So the rim velocity turns out to be 23.7 km/s. That's 63 times the speed of
sound. It's also twice the escape velocity of the earth. That's really
cooking.

Pulling another number out of my ear, let's assume that the radius of the
flywheel is 10 meters. Then the circumference is about 63 meters, which
means the flywheel would rotate 375 times per second. What kind of bearing
can spin that fast, for hours (or weeks) at a time with negligible energy
loss, supporting that much weight, without failing? I don't think anyone
knows how to design such a thing.

And what kind of containment housing do you put that sucker in, which is
capable of preventing anything from escaping if the bearings fail or any
other kind of catastrophic failure takes place? Pretty much any significant
mechanical failure of this system will be catastrophic. For instance, that
wheel has better be damned well balanced, because any imbalance at all will
cause the system to shake itself apart at speed.

Flywheels which can be supported by feasible bearing technology wouldn't
store enough energy to make any difference unless you used thousands of
them, which would be grossly expensive.

This is another idea which looks really good as long as you aren't the one
who has to implement it. (And I haven't even talked about the efficiency,
the percentage of the energy going into the system which can come back out
again. Nor have I bothered to calculate whether the centripetal force on the
flywheel described above even exceed the tensile strength of steel, though I
would bet that it would.)

As to hydro power, I'm afraid you've got it totally wrong. It is extremely
easy to control the output of a hydro plant. In fact, it's easier than for
any other large source of power we currently use.

Shows what I know.

UPDATE: Be all that as it may, here's the story on what sparked the discussion in the first place. (And here's a company that might be able to make it work.)

UPDATE: Chris M. says:

SDB's back-of-the-envelope calculations are OK, but designs for modern flywheels for power storage actually *don't* put "virtually all its mass on the outer rim". The modern designs are somewhat bell-shaped and run at extremely high speed. As for bearings, the usual bearing is a magnetic one for almost zero friction.

He also notes this press release on recent advances made in the field.

Meanwhile, Bob P. says:

I agree with SDB...to make a decent difference you would need an energy storage system that stores 2gigawatts for 2 hours, not 120kw for 20 secs !

If you calculate how many of the pentadyne systems it would take to store 2GW for 7200 seconds you get

2GW/120KW * 7200/20 = 6 MILLION

With 33 million residents that means one of these pentadyne thingies for every 6 people !

Yeah right as we say down here...pehaps pentadyne has been set up to extract research grants from gullible lefties who can't do math

Heh. (Boy, this must be how Glenn feels.)