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From: John De Armond
Subject: Re: Flywheels Not Common?
Date: Sun, 04 Mar 2007 18:01:44 -0500
Message-ID: <>

On Sun, 04 Mar 2007 09:00:07 -0500, "(PeteCresswell)" <x@y.Invalid>

>Per Anthony Matonak:
>>I suppose you could just forget about energy density and build it
>>large and slow.
>That's what came to my own mind as soon as I saw the OP.   For residential use,
>the size/density issue seems to be mitigated to a great extent.   Something 10
>feet high could easily be housed in something like a garden shed or other

No.  first off energy storage goes up as the square of velocity but
only linearly with mass so increasing mass is a loser.

Second, any flywheel with useful energy storage capability will
operate in a vacuum.  Otherwise windage losses rapidly dissipate the
stored energy.

Small little concrete example.  Awhile back I bought one of these high
tech gyro toys:

Slick little booger.  Streamlined shape, nice polish, close fitting
band to reduce boundary turbulence, instrument-grade ball bearings.
The supplied little motor spins the thing up to about 15,000 RPM.  The
blurb says that it'll spin for 7 minutes and technically it will but
the gyro action is mostly gone in about 2 minutes.

Put that same gyro in a bell jar and pull a vacuum on it and it'll
spin for hours.  Mine was still going strong after 4 hours when I let
the air back in.

In a vacuum only bearing losses are present.  Magnetic bearings can
reduce that to essentially zero.

All this is old news to those familiar with flywheels which is why
efforts are expended toward stronger materials that can be spun faster
and the device still be small enough to operate in a reasonably sized
vacuum chamber.

With due regard for making blanket statements :-), I seriously doubt
that flywheels will ever compete with batteries for long term energy
storage.  Even if material science comes up with something strong
enough to let the flywheel spin fast enough to equal the energy
density of plain old lead-acid batteries, there remains the
practically insurmountable problem of explosive energy release during
failure.  Unlike a battery that gives up (part of) its energy over
several seconds or minutes even when directly shorted, a failed
flywheel releases the stored energy practically instantly as anyone
who's experienced a flywheel explosion in a dragster knows.

For the few thousand joules of storage the currently available
flywheel surge batteries are capable of storing and releasing, the
manufacturers are taking the expedient (and perhaps only) course of
making the housing heavy enough to contain the fragments and dissipate
the energy as heat.

Reach the hundreds of thousands or millions of joules stored and the
situation becomes quite serious.  To put this in perspective, a 100
amp-hour, 12 volt storage battery (1200 watt-hours) stores 4,320,000
joules.  Another point of perspective, a pound of exploding TNT releases
about 2 million joules of energy.  How many pounds of TNT do you want in
your back yard? :-)


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