Comment by adrian_b
3 days ago
Flywheels can have much higher power density than any kind of battery.
There is no competition between batteries (low power density, high energy density, low storage cycle efficiency) and flywheels (high power density, low energy density, high storage cycle efficiency).
Flywheels (preferably levitated in vacuum) compete only with supercapacitors and superconducting rings (SMES = Superconducting Magnetic Energy Storage).
Supercapacitors/flywheels/SMES have their high-power applications, for which batteries are not appropriate.
Using them where batteries are the right solution is of course not a good choice.
Note that it's typically far easier to just teach the grid behavior aspects to anyways-existing battery interfaced AC/DC converters than to buy separate high power AC/AC converters, along with a flywheel and high power motor for said flywheel.
It's rare that you actually have good reason not just through easily avoidable architectural choices to have dedicated 1~100 second range energy storage. Below that you can just use aluminium wet electrolytic capacitors, and above that you can just use [very] high power lithium ion batteries.
Inherently mechanical systems are different, of course.