I first read about Flywheels as an energy storage system for EVs in a Discover article about Jack Bitterly - it seemed to make sense to me as power storage goes up squared as rotational speed is increased- and limits on rotational speed are governed by material strength, which seems to increase every year. The last article I read about the subject was about someone who had invented a duel flywheel where a super fast flywheel spun inside a slower metal flywheel that acted as encasement for the super fast flywheel, with the result being that the overal system had a higher energy storage per weight than other flywheel systems.

I think the most promising energy capture devices are ones like the hydrolic systems where energy is store as compressed gas.

The energy of the fly wheel would be lost if you are parked in a parking lot during the day where as gas stored in canister is not lost over time.

The fly wheel could help with lateral stability though with gyroscopic properties. Or maybe depending on the axis of rotation you would be able to turn at all?

Flybird System in the United Kingdom looks very interesting too. Hydraulic system has been researched some too such as UPS and DOE plus a few others that I have an interest into.