[Porsche-O-Phile]

There was an article in Wired a few years ago about something similar using flywheels to store energy in the form of rotational kinetic E. When you want to store it, you spin up the flywheel faster. When you need to tap it, you convert the rotational KE to electrical or other forms. The flywheels are many times more efficient than any electric motor casing, having virtually no internal friction. NASA is using them on spacecraft - very, very lightweight, precision-balanced flywheels in evacuated cannisters suspended with magnetism.

The intention is to someday (hopefully) replace chemical batteries, which are always a problem.

But for short term, sure a motor casing would probably work. The flywheel systems won't spin down for DECADES however.