The Regenerative brake or Recuperation brake recovers kinetic energy as electrical energy when a vehicle brakes. It is used, for example, in electric locomotives, railcars, trolleybuses, conveyor belts in mining and cable cars, in particular ore transport and material cable cars, electric cars, hybrid electric vehicles and combustion cars with power storage and electric bicycles.
The regenerative brake works like any electrodynamic brake without wear. The braking effect comes about when the traction motors are operated as electrical generators. Such a regenerative brake is a special form of the electromotive brake. In contrast to the pure resistance brake, the electrical energy recovered from the kinetic energy is not converted into heat in the regenerative brake in braking resistors, but either fed back into the catenary network or stored in a memory in the vehicle, for example an accumulator or high-performance capacitor.
rail vehicles
Electric drive
Even in the early days of the electric railways, some locomotives were equipped with regenerative brakes, for example the SBB Ce 6/8 "Crocodile". When braking, the traction motors are switched to generators. In conventional AC locomotives and multiple units, the electricity produced is routed back to the transformer via complex circuits and fed into the catenary. With this technology, it was initially only possible to recover around five percent of the energy consumed, and the braking force was also weak and irregular.
Modern vehicles equipped with traction converters can make better use of braking energy. The traction motors of the locomotive feed the converters with three-phase current. These in turn convert the energy into alternating current, which is stepped up and fed into the contact line. This circuit works in the entire speed range up to the full power of the traction motors and allows the recovery of around 25 to 30 percent of the energy required for the drive. Modern systems can recover up to 40% of the energy required for acceleration.
In three-phase railway systems, especially the northern Italian network from around 1902 to 1976, but also some mountain railways such as Gornergrat near Zermatt, significantly higher feedback gains (around 50%) were achieved with a very reliable braking force with simple asynchronous motors (see history of the electric drive of rail vehicles (Italy )).
AC overhead line networks can normally always absorb the electricity produced by locomotives, as they can be fed back and the electricity can be used in the entire traction current network (only in the event of massive further disruptions can overloads and thus a power outage occur, for example in Switzerland on June 22, 2005). If they are not capable of being fed back into the national grid, direct current grids are only able to absorb to a limited extent; the electricity fed in can then only be used locally. If there is no consumer, for example a vehicle moving uphill, in the same feed section, braking current cannot be fed in either. Otherwise the overhead line voltage would rise in an impermissible manner. In order to enable intermediate storage of electrical energy in direct current networks, there are experiments, for example, with flywheels (Hanover tram network) or super capacitors (Warsaw tramway). Modern direct current and direct current capable multi-system locomotives have braking resistors so that the wear-free electric brakes can also be used in situations in which the electrical energy cannot be fed back.
Tram trains can be equipped with capacitors (double-layer capacitors) that store the braking energy on board so that it can be used the next time the vehicle starts up. There is also the option of setting up capacitor stations on the lines in order to be able to absorb the energy.
Road vehicles
Cars with electric, hybrid or gyro drives are usually capable of regenerative braking. They feed the braking energy back into their accumulators, into accumulators buffering supercapacitors or into a flywheel.
Electric bicycles are also occasionally capable of regenerative braking.
In 2007, BMW introduced braking energy recovery for many of its gasoline and diesel vehicles under the heading Efficient Dynamics. This is not a matter of recovery in the strict sense of the word, rather the on-board battery is charged, as far as possible, only when the vehicle is overrun (engine brake). This reduces the energy consumption of the generator and thus the fuel consumption when traveling. This is technically implemented by actively varying the voltage of the generator: When the battery is charged above a certain threshold value, the on-board voltage is regulated to 13,0 to 13,2 volts. The charging current of the battery is reduced to a minimum. In overrun mode and when the battery is low, the on-board voltage is increased to up to 14,8 volts. In a test with an approx. 80% full battery, the following charging currents could be measured depending on the voltage: 13 volts - 5 amps, 13,2 volts - 10 amps, 14 volts - 20 amps and 14,8 volts - 30 amps. This means that in typical operating conditions, recuperation takes place with up to 500 watts. The statements known from advertising such as “the alternator is only active when the driver brakes or takes off the accelerator” or “the alternator is disconnected when accelerating” do not apply; the on-board voltage is regulated to at least 13,0 to 13,2 volts shortly after the engine is started and only drops slightly for a short time when larger consumers are switched on.
Car racing
A variant of the regenerative brake, the so-called Kinetic Energy Recovery System, has been used in Formula 1 since the 2009 season. Porsche installed a regenerative brake in the 911 GT3 R Hybrid racing car (2010), in which the energy generated is fed into a flywheel storage device.