NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/electron/21720

Internet Message Format | 1992-12-30 | 4.0 KB

Xref: sparky sci.electronics:21720 sci.energy:6497 rec.autos.tech:17241
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Path: sparky!uunet!think.com!enterpoop.mit.edu!bloom-picayune.mit.edu!pierre.mit.edu!chuck
From: [e-mail] (Chuck Parsons)
Subject: Re: Flywheel batteries as EV power source
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Date: Wed, 30 Dec 1992 05:15:00 GMT
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In article <[e-mail]>, [e-mail] (Ma writes...
>In <[e-mail]> [e-mail] (Carl Smith) writes:
>
>|I am not going to argue the above point, but consider this.  If you are
>|going to measure danger by the amount of stored energy, then which is
>|more dangerous when it explodes?  A flywheel that has enough energy to
>|move a car 100 miles at freeway speeds, or a tank of gasoline with enough
>|energy to move a car at greater-than-highway speeds for 300 miles?
>
>Except that under rare circumstances, gas tanks don't explode, they burn.
>

>|The point I am trying to make is that flywheels may be dangerous, but driving
>|around with 20 gallons of gasoline strapped to the bottom of your car in a
>|thin sheet metal container is A LOT MORE dangerous.  But we consider that

>
>Now if the flywheel fails, all of the energy will be dumped in a matter
>of milliseconds.  While there may be more energy in the gas tank, the
>time compression makes the effects of the flywheel failure much more
>dramatic.
>--

  People have been considering this problem from the standpoint of
how much energy a gas powered car uses and missing a few points.

 First a car engin is only about 25% efficient under good running conditions.
electric systems can be 95% efficient. 75% efficient should be redily
obtainable so divide gasoline_energy by three.

  Second mechanical strength of materials is _not_ greater than the
strength of the chemical bonds forming the material.
Consider a thin hoop flywheel. Any flywheel can be thought of as being made
up of many such hoops. Let the hoop have a fixed cross-sectional area A
and the average mass of each atom in the material be M.

  The energy stored in each atom of the hoop is E=.5 M*V**2.

  When the hoop spins it tries to pull itself apart. It requires
  on the order of .25*M*V**2/R  _per_ atom  of force to keep the loop intact.

  The number of atoms in a loop is 2*pi*R/D where D is the spacing between
atoms. Thus the force required is on the order of

      .25*M*V**2/R  * 2*pi*R/D = .5*pi*M*V**2/D


   Which is about 3*E/D! but if E is greater than about 1/3 the
chemical binding energy then the molecules/material will come apart,
because enrgy = force * distance

   Since for really bitchin materials the heat of vaporization
is similar the the binding energy, and because real materials have
defects and fall apart well before the strain (or is it stress)
aproaches the atomic limit. The flywheel will at _most_ have
enough energy to vaporize itself, probably much less. The flywheel
and enclsoure (assuming the enclosure mass is greater thant the flywheel)
only has to get hot it doesn't have to vaporize.


  Relating this to the previous arguments I think the following
numbers are plausable.


  6 gallons of gas *50mpg =300 miles range

  6gallons *.25/.75 = 2gallons  equivalent energy (assuming 25% vs 75% eff.)

  2gallons*2.7kg/gallon=  5.4 kilograms of gas 'energy'

  5.4kg/.02 = 270kg flywheel if

        chemical_energy_density/pratical_mechanical_energy_density=50


   Add another 270kg of shelding and you have a mass comparable
to a gasoline engine or the batteries in an electric car. Properly
designed it will get damn hot but need not explode when it fails.


Regards, [e-mail]