NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/electron/21483

Internet Message Format | 1992-12-23 | 3.5 KB

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Subject: Re: Flywheel batteries as EV power source
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Date: Wed, 23 Dec 92 18:01:40 GMT
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In article <[e-mail]>, [e-mail] (Mark Wilson) writes:

All right we've made it beyond energy density calculations!!!

>
> The vaporized carbon-carbon will deposit itself on the first thing that
> it encounters that is cold enough to cause it to condense.
> In this case, the inside of the containment vessel.  By condensing the fluff
> will transfer the heat of vaporization to the containment vessel.
> By vaporizing the flywheel, you have not diverted the energy, but mostly
> just delayed it a bit.
>
> The only way to get rid of this energy is to vaporize material.
> If the containment was made out of carbon-carbon as you suggest, then it
> must be massive enough that after the requisite amount of material has
> vaporized, there is still a thick enough wall to withstand the pressure
> of all of that hot gas inside the containment vessel until the unit
> as a whole has had enough time to radiate the energy away as heat.
>
> As I see these are the data points we need for carbon-carbon.
>
> Heat of vaporization.  (how much material is needed to absorb the energy)

I don't have the numbers but a good guess would be a compromise between the
heat of vaporization of graphite and diamond.

> Vaporization temperature of carbon-carbon.

Again choose the temperature of graphite, which is close to the nature of
the fibers of carbon carbon.  NOTE if there is any O2 present the fibers
burn long before they reach the vaporization temperature.

> Tensile strength of c-c.

The tensile strength of carbon carbon fibers is currently > 1 million psi,
the producers won't tell you the real numbers as this field is very competative.
This means that if a carbon carbon composite is 80% fiber the tensile strength
of the composite is 800 kpsi.  The ultimate tensile strength is the tensile
strength of the graphite plane, meaning one has made a perfect single crystal
coiled graphite sheet fiber.  This perfect fiber would have a tensile strength
greater than that of perfect diamond which is 14 million psi.

> It might be nice if we knew the thermal resistance of c-c.

The thermal conduction of carbon carbon depends on the direction.  Conduction
down the fibers is probably at most 4 W/cm K while between the fibers it is
about 0.04 W/cm K.  This means that the temperature of the exterior and
interior of the containment vessel would be relatively uniform, while supporting
a massive temperature gradient across the vessel wall.  If one could make
fibers out of diamond, not as hard as it might sound since diamond fibers
have been grown, the intra fiber thermal conduction jumps to 20 W/cm K.  Note
that these are at room temperature.

I'm not sure if the massive thermal gradient helps us or hurts us.

Hope this helps,

Darin Olson
R&DD
LMSC
Palo Alto, CA