Technology

Flywheels have been used to store and stabilise energy for hundreds of years. Early examples include the potter's wheel and spinning wheels. More recently advances in bearing technology, power electronics and vacuum enclosures have substantially improved their performance characteristics. The first modern flywheel systems were large stationary installations used to provide uninterruptible power supply and the production of very large pulses of electricity for scientific or industrial use.

Only in the last two decades has flywheel technology been seriously considered for use in mobile applications. It was held back by prohibitive weight and unwanted precession forces. Both of these characteristics are determined by the specific tensile strength (the ratio of the hoop stress to material density) of the flywheel. Advances in carbon fibre composite technology has allowed the specific tensile strength to be greatly improved leading to the development of light, high-speed flywheel systems. Test vehicles, particularly buses, have been produced using mechanical flywheel systems with a continuously variable transmission (CVT) to transfer power to and from the flywheel. The next evolution was electrically-driven flywheels which do not require a CVT system thus avoiding added weight and reduced efficiency. Electrically-driven flywheels have another important advantage over their mechanically driven relatives in that vacuum integrity is easier to maintain as no high speed mechanical seal is needed.

WHP has taken the electrically powered integral motor flywheel design and radically improved its performance characteristics by incorporating Magnetically Loaded Composite (MLC) technology. The MLC technology, which was developed in the nuclear industry by Urenco, incorporates the permanent magnets of the integral motor/generator into the composite structure of the flywheel itself by mixing magnetic powder into the resin matrix. In the event of a burst failure, the containment has to withstand only the crushing force of the composite material, which is far less than the load of discrete metallic fragments. The reduced containment requirement minimizes the overall weight of the system.

The magnetic particles in the composite are magnetised as a Halbach Array after the rotor is manufactured avoiding the need for backing iron to direct the flux. As the magnets in an MLC flywheel are comprised of tiny particles and there is no additional metal in the structure, the eddy current losses of the machine are significantly reduced. This can result in one-way efficiencies of up to 99%. The ultra-high efficiency means thermal management of the system is easier and it can be continuously cycled with no detriment to performance or reduction in life.