4 hour energy storage flywheel

I thought I would never see a flywheel with 4 hours of energy storage.  Amber Kinetics has managed to develop a 6.25 kW flywheel with 29 kWh of energy storage.  The traditional flywheels I was aware of stored less than one minute of energy.

Here is what we know:

- Weight of flywheel = 5,000 lbs

- Speed of rotation = 8500 rpm

- Flywheel is a monolithic steel rotor

- Flywheel is also used as rotor for the motor/generator

- Stator is 8 pole permanent magnet

- Full power conversion to get 60 Hz AC

Although no cost is mentioned, one of their key objectives is low $/kWh cost and second is low self discharge rate.

Details are available at: http://www.energy.ca.gov/2015publications/CEC-500-2015-089/CEC-500-2015-...

Hawaii Electric seems to be the first customer.