From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: sci.electronics,sci.energy,rec.autos.tech
Subject: Re: Flywheel batteries as EV power source
Date: 21 Dec 92 23:20:36 GMT
Followup-To: sci.energy

In article <1h5afbINN1hs@access.usask.ca>, choy@skorpio.usask.ca (I am a terminator.) writes:
> In article <1gqhdqINNjot@gap.caltech.edu>, carl@SOL1.GPS.CALTECH.EDU (Carl J Lydick) writes:
> |> In article <1992Dec17.170002.28389@u.washington.edu>, basiji@stein.u.washington.edu (David Basiji) writes:
> 
> |>  Perhaps you'd care to review the title of the
> |> thread: "Flywheel batteries as EV power source."  That's why I was talking
> |> about enough energy to travel for ~100 miles at freeway speeds.  Now, if you
> |> simply want to use the flywheel for load averaging, then we're talking about a
> |> LOT less energy stored in the flywheel.  In that application, I'd agree with
> |> you that the dangers of a catastrophic failure of the flywheel would be quite
> |> acceptable.
> |> 
> |> =It is the flywheel which provides the motive force
> |> =for the vehicle via some clutch and gearbox or torque converter.
> |> 
> |> The idea in this thread was that the flywheel is part of a generator which
> |> provides electrical power to the vehicle's electric motor.  That's what the
> |> "EV" stands for in the title of the thread, and why it refers to the flywheel
> |> as a "battery."
> 
> I've seen massive rotating objects being used in electrical power generation.
> For instance, a turbine shaft in a hydroelectric generator that looks the size
> of a spinning boxcar. If the water was shut off, the thing would grind to a
> halt pretty quick because of the counter torque from a loaded generator (Some
> generators are run as motors so that the power production is kept up; you don't
> want to have to stop and restart this spinning boxcar). This shaft isn't
> running all that fast so there isn't as much fear of explosive breakdown, but
> I bet there's a lot of kinetic energy.

Just for comparison, if the 92000 kCal for the car flywheel battery are
dumped in one millisecond (which is probably far too long an estimate, it's
probably closer to 10 microseconds) that's 385 GW.  How many turbines you
know of rated to that kind of power?  Then the kinetic energy is not very
comparable.  Not only that, your big spinning boxcar is spinning much slower,
slowing the rate of energy dissipation in failure, and it is much bigger,
spreading that energy release over a greater mass and volume, dissipating it.

> A flywheel that has the power to run a car at highway speeds (I read in the
> paper that 8 hp can maintain the speed on average) can be compared to giving

8 hp seems way low.  That can't be highway speed, I know of no practical
car design that would experience only 8hp of road friction and air resistance
at 55 mph.  But in addition, let's not forget:  hills, curves, rough roads,
other traffic, etc., things that make you less efficient than travelling
on a glassy-smooth surface perfectly flat and perfectly straight and perfectly
uniform speed.  Supposedly, this flywheel will drive a real car over real
roads for 600 miles (well, 20 of them will according to Popular nonSense).

> I've had little toy cars with flywheels and toy cars with windup springs.
> How about spring energy or gravitational energy?

The energy density is still the problem.  Whether you store that energy
in a spring or a flywheel, it still can be released suddenly and it's still
a lot.  No spring could handle the required energy - the material would
undergo torsional failure.  However, another situation could be devised that
is like a spring - hypercompression.  Then recover the energy through
controlled release of the material.  This would be very dangerous.
It would involve pressures larger than that at the center of the earth.
I have been in a room when a simple high-pressure "belt apparatus" failed.
This is only under a few tons of compression and it blew apart a 10-inch
diameter, 1 inch thick steel confining belt, including the tungsten carbide
die in the middle.  The sample under compression is only a few milligrams
in size.  I understand that devices that can confine significant quantities
of material (grams?) up to 800,000 atmospheres have been built.  That
still isn't enough to power a car.  And because the energy is stored in
a form that can be released suddenly, it is just as explosive as the
problematic flywheel.

As for gravity, how big a weight do you think you have to lift, and how high
to get that kind of energy?  92000 kCal implies for mass M, height h,
M*h=38 million kg-m.  So you'd have to lift 1000 kg 38 km (lift your car
23 miles into the air).  That too high?  maybe you prefer something only
about 10 meters up.  That would be 4 million kg, about 2000 tons.  Try
driving that around.


--
Richard Stead
Center for Seismic Studies
Arlington, VA
stead@seismo.css.gov