For an energy storage system, you probably don't want a synchronous machine. You want to be able to deliver (or absorb) rated power over a wide range of rotor s

For an energy storage system, you probably don't want a synchronous machine. You want to be able to deliver (or absorb) rated power over a wide range of rotor speeds, since the difference between your lowest and highest speed is what effectively determines your energy storage capacity. To do this with a synchronous generator you'd need a big, expensive high-speed gearbox similar to what you see in synchronous generator wind turbines. So we used an induction motor/generator + back-to-back converter to tie to the grid, which is a common design very similar to what's used in most modern wind turbines. Therefore, from the perspective of the grid, no rotational inertia is coupled to the power system and the machine doesn't exhibit the inertial response of a synchronous generator that I'm sure you're familiar with. However, if you have a fast enough control and communications system, you can provide the desired response for both frequency and voltage control and that's a big part of what we were trying to do. You can ramp from minimum to maximum active or reactive power very, very fast (like 5 AC cycles), which is much faster than any traditional generator. Due to the fast ramping capability, we also researched the possibility of using flywheels for large unit trip contingencies in small power systems (e.g. Hawaii) where operating reserve is very expensive. At the end of the day, the biggest problem is that anything flywheels can do, batteries can do too - and batteries are getting cheaper every year.