>You also need uninterruptible power supply to maintain safety and control systems when grid power is unavailable since you've still gotten a huge spinning mass that you can't slow down without somewhere to send the energy (it's possible to use braking resistors, but it's another cost).

The braking resistors are however a much lower cost, which for this (infrequently used) application is mostly what matters.

It seems like spending >10x more on a contingency system just to save a few hundred kWh (costing a few tens of dollars) every few years seems suboptimal. Especially since those batteries could be in a daily cycling installation, so the opportunity cost (compared to using those batteries elsewhere) is very high.

Why buy X kWh of batteries to sit idle 24/7/364, plus X kWh of flywheel storage? Why not A) eliminate the flywheel, use the battery, and be done with it? Or B) use the braking resistor? It seems like either A or B should always be preferable to a "hybrid" given reasonable assumptions.

Favoring energy recapture over resistor heat dump seems like very suboptimal high-level design coming from a flywheel storage engineer, so what am I missing here?