Electromechanical battery

An electromechanical battery^[1] is a device to store energy in a flywheel, with some motor to accelerate it, and some generator to draw energy from it.

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Figures of merit

Devices to be used as batteries are qualified through the following elements of appreciation:

• volumic stored energy (J/m³)
• efficiency of charge and discharge processes
• long term retention
• emissions (vibrations, noise, radio frequency)
• usage safety.

As of 2007, compact designs (automotive) mostly use electrochemical batteries.

Modern design

In the latest years, technological improvements gave new arguments in favor of this application of the flywheel concept.

While ball bearings or active magnetic bearings would cause a significant power leakage, a suspension with passive magnetic bearings (similar to Inductrack) is workable. By letting the flywheel spin in vacuum, energy loss only occurs by eddy currents and allows typical half-life of a few months when unused.

The passive loops can also be powered to spin up the rotor, and conversely used to draw electrical energy from the device.

Under these assumptions, you would spin the rotor at the highest possible speed before it breaks.

Issues

Two problems remain though: coping with gyroscopic effect and safety in case of rotor explosion. When the flywheel is stationary, and can be buried, it does not really matter.

In the hypothesis it would be used in a car, two contra-rotative vertical axis flywheels may cancel their effects when car tilts back/forth or left/right, and just increase the stress on magnetic bearing.

Risk of rotor explosion at 60 000 to 200 000 rpm can be moderated by using a material that pulverises rather than creates shards at rupture. Counter intuitively, carbon fiber is more adapted than dense metals such as lead for this purpose.

References