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.