Energy in the Future series: Regenerative braking

Regenerative braking is an energy recovery system that provides traction control by harnessing the otherwise wasted kinetic energy from braking and storing it for later use. Rather than allowing the energy to be transferred to heat energy via the brakes and dissipated into the atmosphere, the energy is either stored on-board the rolling stock or in track-side equipment and can be used for traction power as well as powering infrastructure. The energy is stored chemically in a battery, electrically in capacitors, or mechanically in a rotating flywheel.

Regenerative braking can be used to reduce harmful emissions and ​costs. For example, London Underground has utilised regenerative braking on newer stock, allowing 20% of used energy to return to the network. Lower friction braking reduces brake wear and dust build-up due to friction, leading to a reduction in maintenance costs and improved air quality, respectively. The technology can also be used in conjunction with storage systems to shift the use of energy to a time where consuming energy directly from the grid may be more expensive (peak shaving), resulting in cost savings. Reversible substation systems have also been successfully used on electrified lines, thus limiting wasted energy, providing voltage stability and reducing tunnel temperatures through the removal of on-board brake resistors. By using reversible substation systems, London Underground has been able to store enough energy to power a medium-sized station for two days a week and could save approximately £6m. a year. Some reversible substation system suppliers state that the number of traction substations could reduce by 20%.

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