How much energy is lost by using a wheel vs battery? Meaning, how much energy can be extracted from the amount of energy put in? How does that compare to a battery?
How long would a charge on a wheel last (eventually it would spin down on its own) compared with a charge on a battery?
Motor efficiency is proving elusive on the web. But let's say 90%-95% for brushless DC motors. (square that for in and out)
The trick about batteries is that if you only want to use them for intense bursts you will need a huge energy capacity. You don't normally want to charge at a rate faster than say, 1 hour to get full. If you need to store a 3 second brake stomp and give it back 5 seconds later then you are going to have about 1200 times more batteries than you needed just to store the energy (to support the current).