From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: sci.electronics,sci.energy,rec.autos.tech
Subject: Re: Flywheel batteries as EV power source
Date: 17 Dec 92 17:31:32 GMT
Followup-To: sci.energy

In article <1992Dec17.010235.986@cmkrnl.com>, jeh@cmkrnl.com writes:
> In article <1goebdINNik@gap.caltech.edu>, carl@SOL1.GPS.CALTECH.EDU (Carl J Lydick) writes:
> > In article <1992Dec16.192456.6261@news.cs.brandeis.edu>, andyh@chaos.cs.brandeis.edu (Andrew J. Huang) writes:
> > =2. There are several places for the energy to go during an explosion.
> > =Since the energy is kinetic, a lot of it will end up as particle
> > =velocity.
...
> > =What residual energy escapes these will have to be determined by testing.
> > 
> > Er, the amount of energy that escapes is exactly the amount of energy you
> > started with.  Law of conservation of energy comes into play.
...
> > It didn't just vanish.  And that's a LOT of energy.  And it got dumped FAST.
> 
> Carl is absolutely right.
... 
> Therefore, if the flywheel comes to a stop
> quickly, ALL of that energy gets released as heat.  
> 
> Yes, all of it.  Including that part that momentarily goes to breaking up the
> flywheel, hitting the inside of the container, maybe breaking the container,
> etc.  Ultimately *all* the released energy ends up as heat.  
> 
> > In other words:  BOOM!
> 
> Couldn't have said it better myself. 

Nor could I.  I find it hard to fathom that people are actually arguing about
this.  Let's say that car is real efficient, and that 100 miles represents
the equivalent of 2 gallons of gas.  That's a lot of energy, equivalent
to about 50 pounds of TNT (if I did the math right).  TNT is the proper
comparison, because it dumps the energy fast, unlike liquid gasoline that
will burn for a long time.  I can't conceive of a Kevlar jacket strong
enough to withstand the blast of 50 pounds of TNT.  That blast will take
down a moderate size building, let alone a little kevlar.  And that same
energy dump will occur in any failure - if the flywheel has any microscopic
flaws made during manufacture, or acquired during use - these could induce
fracture.  A fracture in a material with that kind of rotational energy means
instantaneous and complete failure - BOOM!

> > 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.
> 
> And even if the container does hold, we're not out of the woods yet.  
> If the container doesn't break, it is because it is able to absorb the energy.
> Which means that the energy STILL gets turned into heat -- it's just that it
> isn't enough heat to break the container.  The container, nonetheless, is going
> to get damned hot.  

Absolutely - I get about 92,000 kCal for my 2 gallon example (I am not a
chemist, so I could have screwed that up).  So let's say the flywheel is
steel-jacketed, how much iron would be required to prevent the iron from
melting?  I get 300 kilograms.  That's a lot of iron at 1500 degrees C.
The same heat would completely melt 250 kilograms of iron.  It would raise
140 kg of iron to 3000 C - the vaporization temperature; and could vaporize
40 kg of iron.  So if you wrapped it in 40 kg of steel, and none of the energy
went beyond the container, it would vaporize entirely.  It doesn't matter how
much energy it briefly took to deform the container, etc., it all goes to
vaporizing it in the end.
Anyone who knows the heat of vaporization (or heat of formation) of Kevlar
want to figure out how many kilograms the flywheel can vaporize?

> The only way around this is to make the container so sturdy that the flywheel
> keeps spinning in the worst collision imaginable.  Right. 

And design a flawless flywheel, so it doesn't ever blow up on its own.
Actually, I could imagine a pretty sturdy container.  Afterall, 300 kg
of steel around a little flywheel would be nearly indestructible at
mere highway speeds.  However, the flywheel axle would probably fail
in a sharp enough jolt, destroying the flywheel anyway.

The only good answer to this problem that I can come up with (and this
takes too much wild imagination):
1) develop a sensor that can sense something wrong with the flywheel, and
   respond in less than one revolution.  (No such technology currently)
2) have the sensor close a circuit that permits the flywheel to become
   a very powerful dynamo, essentially braking the flywheel within only
   a handful of revolutions.  (This can be done on big flywheels - it is
   used as a technique to get a lot of energy in a very short time).
3) dump the power to something that can use it that quickly.  I don't
   know of many such things - some big lasers can eat up power fast, but
   where would you shoot the laser, and how could any mechanism capable
   of eating up the power quickly survive a collision?  Maybe you could
   design a coil that generates an EMP, and wipe out everyone's unshielded
   electronics for a few blocks around (it'd be a big coil).


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