Flywheel Background

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's MLC flywheel is electrically driven.

Further reading

• H. Liu and J. Jiang, Flywheel energy storage—An upswing technology for energy sustainability. Energy and Buildings. Volume 39, Issue 5, May 2007, Pages 599-604. doi:10.1016/j.enbuild.2006.10.001