I first read about mechanical flywheels as a means of electrical energy storage more than thirty years ago when I was in graduate school. The amount of progress in practical applications during the intervening three decades has not been impressive. Steel flywheels in vacuum chambers using magnetic levitation bearings have found a market niche in uninterruptible power supplies (UPS) where the flywheels supply 10 to 15 seconds of electrical energy during a power interruption, allowing time for some other backup source (e.g. a diesel gen set) to come on line.
Flywheels made out of carbon fiber are mechanically stronger than steel flywheels and thus can spin faster and store more energy (The energy stored is proportional to square of the angular speed) and can thus provide longer time periods of energy backup in a given physical foot print. However, carbon fiber flywheels are more expensive than steel flywheels. A company called Pentadyne introduced carbon fiber flywheels into the UPS market but ultimately declared bankruptcy. A company called Beacon Power introduced carbon fiber flywheels into the power grid frequency regulation market, but they subsequently joined Pentadyne in bankruptcy. Neither of these flywheel designs is completely dead since Pentadyne was acquired by Phillips Service Industries and Beacon Power was acquired by Rockland Capital LLC. However, the long term succes of both of these enterprises is still questionable.
A new entry into the field of carbon fiber flywheels for grid energy storage is a German company called Stornetic. Interestingly Stornetic is a spin off from a company called Enrichment Technology Company (ETC) whose core technological expertise is in high speed gas centrifuges used to produce enriched uranium fuel for nuclear reactors. Apparently ETC felt that their expertise in high speed rotating machinery was a good match with flywheel energy storage.
In this brochure Stornetic states that their basic flywheel unit can deliver 22kW of power and store 3.6kWh of energy. These numbers imply 10 minutes worth of energy storage at the maximum power rating which is much longer than the 10 to 15 seconds delivered by steel flywheels in UPS system. These kind of discharge times open up the application space of grid power frequency regulation if the cost is low enough. Whether or not ETC and Stornetic can deliver carbon fiber flywheel energy at significantly lower costs than has been achieved in the past remains to be seen.