A regenerative brake is an apparatus, a device or system which allows a vehicle to recapture part of the kinetic energy that would otherwise be lost to heat when braking and make use of that power either by storing it for future use or feeding it back into a power system for other vehicles to use.
Regenerative braking should not be confused with dynamic braking, which dissipates the recaptured electrical energy as heat. In that respect, dynamic braking behaves much like an electromagnetic brake, which employs eddy current losses to produce the braking effect. None of these methods of braking are capable of completely stopping a vehicle, and therefore are not a substitute for friction brakes.
Brakes as an Electrical Generator
Regenerative brakes are a form of dynamo generator, originally discovered in 1832 by Hippolyte Pixii. The dynamo's rotor slows as the kinetic energy is converted to electrical energy through electromagnetic induction. The dynamo can be used as either generator or brake by converting motion into electricity or be reversed to convert electricity into motion.
Using a dynamo as a regenerative brake was discovered co-incident with the modern electric motor. In 1873, Zénobe Gramme attached the wires from two dynamos together. When one dynamo rotor was turned as a regenerative brake, the other became an electric motor.
It is estimated that regenerative braking systems in vehicles currently reach 31.3% electric generation efficiency, with most of the remaining energy being released as heat; the actual efficiency depends on numerous factors, such as the state of charge of the battery, how many wheels are equipped to use the regenerative braking system, and whether the topology used is parallel or serial in nature.
The system is no more efficient than conventional friction brakes, but reduces the use of contact elements like brake pads, which eventually wear out. Traditional friction-based brakes must also be provided to be used when rapid, powerful braking is required, as well as to hold the vehicle stationary.
Dynamic and Regenerative Electrical Brakes
Electric brakes have been used in vehicles with electric motors since the early-20th century. On record, The Warner Electric Brake Corporation was using electric brakes in 1927; but it is possible that they were using electric brakes even earlier.
Dynamic brakes (rheostatic brakes in the UK) convert the electric energy back into heat by passing the current through large banks of variable resistors. Vehicles that use dynamic electrical brakes include forklifts, Diesel-electric locomotives, and streetcars. If designed appropriately, this heat can be used to warm the vehicle interior. When the energy is meant to be dissipated externally, large radiator-like cowls can be employed to house the resistor banks.
Regenerative brakes in electric railway vehicles feed the generated electricity back into the grid. In battery electric and hybrid electric vehicles, the energy is stored in a battery or bank of capacitors for later use.
The main disadvantage of regenerative brakes when compared with dynamic brakes is the need to closely match the electricity generated with the supply. With DC supplies this requires the voltage to be closely controlled, and it is only with the development of power electronics that it has been possible with AC supplies, where the supply frequency must also be matched (this mainly applies to locomotives where an AC supply is rectified for DC motors).
It is usual for vehicles to include a 'back-up' system so that friction braking is applied automatically if the connection to the power supply is lost. Also, in a DC system or in an AC system that is not directly grid connected via simple transformers, special provision must also be made for situations where more power is being generated by braking than is being consumed by other vehicles on the system.
A small number of mountain railways have used 3-phase power supplies and 3-phase induction motors and have thus a near constant speed for all trains as the motors rotate with the supply frequency both when giving power or braking.
This technology, having been around in many forms for many years, is now part of the future technical regulations of the Formula One racing. The governing body of motorsport, the FIA, have included the use of regenerative braking systems, known as "Kinetic Energy Recover Systems" (KERS) in the regulations for the 2009 Formula One season.