# Q & A: energy reuse?

Q:
Electrodynamics question: Given: Biggest current problem with electric vehicles and hybrids is storage capacity and limited range. Partial solutions include regenerative braking and "flywheel" mechanical energy storage. Why can't one use all or some of the rotating parts of the drive train and turn them into true electrical generators. For example, encircle the drive shaft with a housing made of appropriate material to serve as the stator with the drive shaft as the rotor, generating significant amounts of electricity - more than simple regenerative braking which only works when the brakes are being applied rather than the whole time the parts are rotating? This could be applied to the axles as well. This may be an ignorant question from someone who forgot his rudimentary physics decades ago. Thanks.
- Mark Nissenbaum, M.D. (age 65)
Abington, PA 19006
A:
This is easier to see as an energy flow problem than as an electrodynamics problem, although it could be solved that way too.

If you try to extract energy from the drive shaft, you're just removing energy that would otherwise have helped power the car. By the time you've driven an electrical generator, a motor, etc., with losses at each stage, you'll just have thrown out some of the energy that your engine produced.  From an electrodynamics point of view, those coils are exerting a torque on the drive shaft opposing its rotation.

What's different about regenerative braking? In that case, you were about to throw out some of the kinetic energy of the car anyway, just heating up some brakes. Diverting it into a generator saves some of it from being wasted. Grabbing energy on its way to do something useful has the opposite effect.

Mike W.

(published on 01/10/2011)