Principle:

The energy put into accelerating a train and into moving it uphill is “stored” in the train as kinetic and potential energy. In vehicles with electric traction motors (this includes electric, diesel-electric and hybrid stock) a great part of this energy can be reconverted into electric energy by using the motors as generators when braking. The electric energy is transmitted “backwards” along the conversion chain and fed back into the catenary. This is known as regenerative braking and widely used in railways.

Braking and safety

Braking safety requires installation of additional brakes besides regenerative brakes, for two reasons:

• Braking power of 3-phase AC motors is of the same order as power installed for traction. Additional braking power is therefore indispensable and provided by mechanical (e.g. disk brakes) or other dissipative brakes. Typically brakes are blended, i.e. when the driver brakes, first the regenerative brakes are applied, if more power is needed (especially in unforeseen situations) additional brakes are applied.
• If the contact between pantograph and catenary is interrupted, regenerative braking is impossible.

Use of recovered energy

The energy recovered by dynamic braking is used for different purposes:

• on-board purposes (auxiliaries or comfort functions). On-board demand is usually far too low to consume all the energy supplied.
• energy is fed back into catenary to be used by other trains motoring close enough (in a section of track supplied by the same substation).
• If DC substations are equipped with thyristor inverter units, they can feed back energy into the national grid.

Influence of supply system

The electric supply system has a considerable influence on the feasibility of energy recovery. In DC systems, the catenary can be interconnected over great distances (since in contrast to AC systems, no phase shifts can occur). This would in principle allow for a long-distance transmission of recovered energy. However, given the low voltage of these systems (1,5 or 3 kV), transmission losses strongly limit the feasible feeding distances. Therefore the probability of having trains braking and trains accelerating close enough to each other to allow for an effective transmission is rather small.

• Energy saving
• Reduced wear of mechanical brakes.

Wear of mechanical brakes

The use of regenerative brakes reduces wear and maintenance of mechanical brakes. It may also be possible to reduce the complexity, weight and cost of mechanical brakes.

Since regenerative braking works without friction, no wearing parts are present.

Low voltage

Due to the low catenary voltage in DC systems (1,5 or 3 kV) transmission losses are high. This reduces the probability of having trains braking and trains accelerating close enough to each other to allow for an effective transmission considerably. Without additional technology to improve the situation, substantial recovery rates can only be achieved in dense suburban networks.

Voltage limits

It may happen that during braking the catenary voltage increases beyond the limits foreseen by the standards. In this case voltage is automatically cut off and no recovery is possible.

Feedback into supply grid

A feedback of recovered energy into the public grid is usually not an option in DC systems. However, if substations are equipped with thyristor inverter units , they become reversible and can feed energy into the supply grid.

Insufficient braking power

The power of regenerative brakes is roughly the same as the one installed for traction. For many situations (trains running late, bad track conditions, unexpected stop signals) this is not sufficient. In this case regenerative brakes are blended with dissipative brakes or completely replaced by them.

Generally, EMUs have a better regenerative braking performance than loco-hauled trains, since more axles are powered. The higher the motor power and the more axles are powered, the more energy may be recovered.

Acceptance

Acceptance is generally high. However some drivers are reported to be reluctant to use regenerative brakes because of safety or timetable concerns.

Inverter units for substations

By installing thyristor inverters in substations of DC systems, a feeding back of recovered braking energy into the public mains becomes a possibility. This can considerably increase recuperation rates in suburban or regional DC systems.

Energy storage

On-board or stationary energy storage are another way of enhancing recuperation rates in DC systems.

Automatic train control

Automatic driver-less systems offer the possibility of introducing a timetable which is optimised for regenerative braking by synchronising the acceleration and braking phases of subsequent trains.

Share of recoverable energy:

Share of recoverable energy heavily depends on speed and stopping pattern.

The following values are typical (referring to total energy demand) for different operation types

Main lines: 15%

Regional lines: 35 %

Suburban lines: 45%

Freight lines: 20%

The recovery rate actually reached in operation only exploits a part of this potential. This is due to several reasons:

• Efficiency of backwards power train: The recoverable energy can never be fully regenerated due to losses in backwards power train. Backwards efficiency is comparable to traction efficiency (~ 90%).
• Receptivity of catenary: The supply system may be „non-receptive“ because no other train is close enough to use it. In DC systems, this is frequently the case (cf. General criteria – barriers).
• Braking power: Many times the electric braking power is not sufficient and blended braking (cf. Description) is applied. Especially in freight operation, the electric brakes are usually insufficient for braking the entire train.

There is little (if any) quantitative data on these effects. The following table gives some estimates (!) for DC systems. Since the main obstacle is limited receptivity of catenary, the table gives the potential to be exploited with additional technologies (cf. General criteria – success factors) and the potential to be exploited without additional technologies:

	Theoretical potential	Correction due to traction efficiency	Correction due to blended braking	Potential if additional technologies are used	Correction due to non-receptive catenary	Potential without additional technology
Main lines	15%	0,9	0,8	11%	0,2	2%
Regional lines	35%	0,9	0,8	25%	0,4	10%
Local lines	45%	0,9	0,8	32%	0,5	16%
Freight lines	20%	0,9	0,5	14%	0,2	3%

Source: IZT

A part of the potential given in the last column is already exploited at present. So the remaining potential without additional technology will be around 1 - 5%. If innovative technology (cf. General criteria – success factors) is implemented, there is a saving potential of 5 – 20 % depending on the specific situation.