Re: Technology Prediction

From: Charles D Hixson (
Date: Tue May 02 2006 - 15:00:29 MDT

Bob Seidensticker wrote:
> Dani: And fuel cell technology was invented in 1839. Our proposed Hydrogen
> Economy has been a long time coming.
> Your comment about hybrids got me thinking.
> The benefits of a hybrid car are regenerative braking, having extra
> acceleration on startups (engine + motor), and being able to turn off the
> engine when idling or coasting, knowing that you have your motor available
> for instant power if necessary. But couldn't we replace the expensive and
> heavy motors and batteries with a flywheel? The energy stored in a flywheel
> probably wouldn't be as much as that stored in the batteries, but the car
> would be much cheaper and lighter. Seems like a nice approach to a poor
> man's hybrid.
> I read an article about flywheel cars in the 70s, I recall (Scientific
> American?). Apparently, a flywheel can in theory hold roughly as much
> energy as a tank of gas.
> (Sorry for the tangent.)
> ...
Flywheels depend on being heavy AND on fast rotation to store energy.
Also, they are merely a store, not a primary source. (Well, primary
source is a bit vague...but gas is rather easy to add and fairly energy
dense given an oxygen atmosphere.) So you can't use flywheels +
electric unless you can recharge the things fairly often. I've heard of
some electric buses that made effective use of them...but they could
frequently connect to overhead wires, and only used the flywheels when
stepping from area to area. Improved batteries may have made them obsolete.

I can accept that a flywheel can hold as much energy as a tank of
gas...tell me it's weight (and mass distribution) and the amount of
energy it needs to hold and I'll tell you how fast it needs to spin.
For anything very large you'll need to manage frictionless rotation. (I
suggest magnetic levitation in a vacuum as the easiest method to achieve
this.) Then you'll need to devise some way to transfer momentum from
the flywheel to the powertrain. A magnetic link is probably the only
feasible way, though perhaps something based on static electricity could
be made to work...unless that were better used for the levitation. At
some point the flywheel will need to be spun-up to's that
magnetic coupling again (think electric motor). If you spin faster than
a certain amount, the flywheel will disintegrate. This whole thing
sounds like a bear to design for durability, remember that roads tend to
have bumps in them. Now consider what will happen if the road
tips...OK. Gyroscopic bearings are needed, so that it can stay in
alignment (a flywheel IS a gyroscope), and the case needs room to turn
(well, to stay stable while the vehicle turns).

To me flywheels sound like a "best choice" only for certain specialized
circumstances. I'm no mechanical engineer, so I could easily be wrong,
but I haven't been surprised by the limited success such vehicles have had.

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