From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: sci.energy
Subject: Re: Flywheel for
Date: 7 Jan 93 15:41:27 GMT

In article <1ij0sfINN2vc@zephyr.grace.cri.nz>, srgxbhh@grace.cri.nz writes:
> The following details of the flywheel come from the Nov 1992 IEEE 
> Spectrum  which includes a drawing of the proposed 
> " Electromechanical battery " ( EMB ) on page 100.

I am distressed.  IEEE Spectrum is definitely a respectable journal.  This
flywheel thing is taking on the dimensions of the old cold-fusion claims.
I'll analyze the claims:

> material, and rotates at a velocity in the order of 10,000 radians per
> second. An EV would be powered by 20-30 such units, each storing about
> 1 kWh of energy." .... 

Ok, the housing is cubical, I think it is safe to assume the rotor is
equidimensional.  In addition, the design is reported as cylindrical and
the walls are 10% of the radius.  The energy stored is given. We can now
computed the dimensions:

E = 0.5 * I * w^2
w=10000
I = M * R^2 for a cylindrical shell.
M = 2 * pi * (H/2) * H * (0.1 * H / 2) * D where H is height and R is H/2
    and D is density (2.7 g/cc for graphite)
E = 1 kWh = 3.6e6J
Solve for H:  H = 0.23 m

So the rotor is about 10 inches tall and 5 inches in radius, cylindrical shells
half an inch thick.  They weigh 5.2 kg.  He claims the housing, etc., is
about 4 times as massive, so it weighs 20 kg.  It is true that a 20 kg cube
only about a foot on a side is pretty small and 30 of them might fit
in a reasonable vehicle.  Also, 30 would store about 100 MJ, not incredibly
far from the 400 MJ under discussion in this thread (for 600 mile range
that Popular Science claimed for these, 100 MJ would be 800 mpg.  allowing
for 100% electric efficiency and 30% gasoline efficiency, that's 240 mpg.
Pretty impressive efficiency, but PS may have incorrectly quoted the 600
mile range, and the IEEE Spectrum article excerpts posted do not mention
a range.)

Is it reasonable?  Maybe.  Assuming maximum composite strength of 1 Mpsi
(a generous assumption, as far as I can tell, but it is not my number -
it was posted), we need a minimum of 125 kg of composite rotor to store
100 MJ.  The 30 5 kg rotors in question are 150 kg of composite.  I
personally doubt that a flywheel can be engineered to approach that closely
its material strength limit.  Also, these flywheels are not just carbon
composite - they are heterogeneous, containing some sort of metal to
permit magnetic coupling for spinning up and for electric generation,
as well as magnetic suspension.  This will greatly affect the strength
limit.  It will also provide for projectiles amid the fluff.

How hot is the fluff?  If we convert 3.6MJ to heat and heat graphite,
it will raise the graphite to a temperature of 516 C.  That's pretty
hot, but not incredible.  We might reasonably expect this to be split
evenly with an equal mass (5 kg) of graphite composite housing.  Then
you'd have 10kg of fluff at 258 C.  That should be containable in a vacuum,
since the heat available can only vaporize a very small amount of the
carbon (carbon takes a lot of energy to vaoprize).  The only problems
then are 1) does the rotor really turn into fluff, permitting efficient
and rapid conversion of KE to heat?  2) what happens to the non-composite
portions of the rotor?  3) are 30 of these really enough for significant
range?  4) can it really be designed that close to the material strength
limit?  5) What are the losses?

> " for a chamber pressure of 10^-3 - 10^-4 pascals, Post calculates 
> the rotor should have a rundown time constant of several months."...

I'd like to know how these were done.  Losses at a given pressure are
different for a rotor spinning in infinite space than for one spinning
within a few centimeters or a fractions thereof of a containment wall.
There will also be vibration losses, both internal and external.

> about 150 Wh/kg ( the rotor itself should be able to store 540 Wh/kg; 

A little high - implies material strength of about 4 Mpsi.  (and assumes that
you can run right up to the material strength).

> The drawing shows a cross-section of a cubical outer containment made 
> from 3-D fiber composite anchoring springs providing a gimbal mounting 

Good design, would eliminate significant torques on the vehicle.

Overall, many of the claims given here are self-consistant and consistant
with known material properties.  I am still skeptical, however.  I am not
convinced that all design considerations have been included, in particular
the heterogeneous nature of the rotor and just how much energy will be
needed to power the vehicle to a useful range.  If the design is so complete
as this article apparently demonstrates, why will it take 2 years to build
the prototype?  Surely these parts can be one-offed in a matter of
a month or so and testing begun.


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