It's been done already! Beacon Power operates a 20 MW / 5 MWh flywheel plant in Stephenstown, NY that does nothing but store grid energy and release it when nee

It's been done already! Beacon Power operates a 20 MW / 5 MWh flywheel plant in Stephenstown, NY that does nothing but store grid energy and release it when needed. And ancillary services such as spinning reserve carry a price on the electricity market, so there is definitely an opportunity there. I'm not sure if frequency regulation is currently considered an ancillary service in electricity markets right now though - from what I know about NY state, only 10- and 30-minute reserve are priced.