From: basiji@stein.u.washington.edu (David Basiji)
Newsgroups: sci.electronics,sci.energy,rec.autos.tech
Subject: Re: Flywheel batteries as EV power source
Date: 17 Dec 92 17:00:02 GMT
Article-I.D.: u.1992Dec17.170002.28389
Organization: University of Washington

carl@SOL1.GPS.CALTECH.EDU (Carl J Lydick) writes:

>In article <1992Dec17.035214.15058@u.washington.edu>, basiji@stein.u.washington.edu (David Basiji) writes:
>=>Unless you wrap the flywheel in a massive enough container that it can absorb
>=>all that energy without vaporizing, you're going to have an explosion.
>=
>=I would design a housing of Kevlar or Spectra woven around a thin lead
>=alloy lining. The lead would prevent small particles from penetrating the
>=weave by distributing the force over an area larger than the mean cell
>=size of the fabric.  The weave would contain any larger particles so that
>=the energy would be dissipated within the housing by all the mechanisms
>=mentioned before, as well as a considerable portion from particulate
>=collisions within the housing.  The housing would be relatively light
>=and could be formed into fairly complex shapes. Cost would not be prohibitive
>=, probably on the order of $300, with economies of scale.

>You're saying, then, that you couldn't melt that container with a blowtorch and
>on the order of a gallon of gasoline?  And that you STILL couldn't melt it if
>you managed to apply all that heat in the course of a fraction of a second? 
>Sounds like pretty impressive stuff to me.

I haven't been following this thread for very long, so perhaps I should
state my assumptions.  I'm assuming we're talking about a vehicle with
a relatively low-power/high duty cycle motor that spins a flywheel to
very high speeds.  It is the flywheel which provides the motive force
for the vehicle via some clutch and gearbox or torque converter.  Now
that flywheel is going to have to either be some kind of composite
or some strong but brittle conventional material. It will spin in a vacuum.
Most of the energy stored in the flywheel will be kinetic energy.
The flywheel shatters...how do we safely dissipate the energy?
The energy will still be primarily kinetic so we'll have to contend with
both large and small projectiles at very high speeds. Unless there's a
fire to go along with the flywheel failure, temperatures will be around 
300K, initially. 
The housing I've proposed is suited to these conditions.  Since we're 
encouraging collisions between particles by containing them, much of their
kinetic energy will be dissipated in collision-induced fractures of the 
flywheel material. Heat is generated in the housing only in irrecoverable
deformations of the flywheel materials and the housing itself, not fractures
or elastic deformations. Assuming the flywheel is brittle, there will
be fractures but not much elastic deformation.  If the flywheel is composite,
and therefore pretty tough, there will be elastic deformation along with
fracturing, so the composite should be designed to be strong only
in the radial direction of the flywheel. Current bulletproof vests made of
Kevlar can absorb about 7e3 Nm of kinetic energy.  A 1000Kg car at 60mph
has about 3e5 Nm of kinetic energy. With proper engineering of the flywheel
to shatter into uniformly-sized pieces (for maximum energy xfer in collision)
and an appropriate weave or composite of Kevlar in the housing, it seems
that it's within shooting range (pardon the pun) to develop a system where
there's enough energy loss via particle collision and fracture that the
heat generated in the housing materials will be manageable. 

David Basiji
UW Bioengineering
NeRD #3762
Newsgroups: sci.electronics