The specific storage capacity (kJ/kg) of some batteries is much greater than that of even a high-speed flywheel so surely it will be less effective?
Whilst the useable flywheel storage capacity is less than some batteries all of it can be used at full power. The rated capacity of batteries can be misleading as they cannot deliver (or accept) anything like the rated power over the full depth of charge. They also have a very short life if you use all the storage capacity. Storage capacity is useless if you cannot charge it up and during typical vehicle usage the hybrid system is almost always power limited. Just think about how long you spend on the throttle compared to how long you spend on the brakes and you will immediately see that power during energy storage is THE limiting factor.
