>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?
It's not X kWh of batteries for X kWh of flywheels, it's 0.01 kWh of batteries to provide enough power to run the flywheel's protection and control systems while it 'freewheels'. Ideally, you don't want to brake the flywheel while the grid is down, because then you're losing all the storage energy and need several minutes to spin back up when the grid is back.
Surely you can power the control electronics from the flywheel, right? That should always work except when the thing is stopped, which is a safe state.
The grid is the primary power source. Flywheel when no grid. Battery would be third. It might be enough to have a passive resistor load for the third option. But at the scale of these things, a battery to run a few hours of controller is not a big deal.
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?