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 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).
• In some systems substations can feed energy back into the national grid.

Feeding back into the national grid

This is in principle possible in AC networks. However, is is often not an attractive option for railways since most energy suppliers do not reimburse the recovered energy or only at a much lower price than for the intake.

Influence of supply system

The electric supply system has a considerable influence on the feasibility of energy recovery:

AC systems are generally better fitted for recovery than DC systems due to higher catenary voltage leading to lower losses. On the other hand, there is a strong limitation in 50 Hz systems consisting in the electric separation of feeding sections (cf. General criteria - Barriers).

• Energy saving
• Reduced wear of mechanical brakes.

Wear on mechanical brakes

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

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

Feeding sections and „non-receptive“ catenary

Catenary voltage (25 kV) is very high which allows for a transmission at low losses over relatively big distances. Since they share a common frequency, 25 kV 50 Hz systems receive their power supply directly from the public grid. The primary power supply is therefore taken from different points without any connection between the points resulting in different phases in the network. As a consequence, the feeding sections have to be electrically separated to avoid short circuits. This lack of interconnection strongly reduces the probability of having two trains (one braking while the other one is in traction).

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.

In the case of heavy freight trains only a small fraction of the kinetic energy can be recovered, since tractive force is supplied only by the locomotive and (mechanical) braking force is distributed along the entire train. The situation is somewhat improved in double traction, i.e. with a train hauled by two locomotives.

In loco-hauled stock, there is a general limitation to the braking of the locomotive. If the locomotive brakes, the following cars exert a longitudinal force on the rear of the locomotive. In order to avoid an increased derailment risk, this force must not exceed a certain limit. Especially in freight trains this is a strong limitation for the braking performance of the locomotive and thus for regenerative braking.

Running time

Regenerative braking slightly prolongs running time compared to trains using mechanical brakes. This effect is small, but may lead to the use of mechanical brakes (or blending) in case of tight running schedules.

Acceptance

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

Operation concept of the locomotive

Operation concept of the driver cabin may not be optimised for the use of regenerative braking. For example, in most locomotives at DB the brake handles are usually coupled for blended braking. For an exclusive use of electric braking, an extra effort is required to decouple the handles.

The future potential may be exploited by removing some of the present obstacles. This includes

• Training programs and incentives
• Favourable operational concepts for driver cabins of locomotives: Manual braking concepts should be optimised in order to facilitate electric braking whenever feasible. On the other hand, electronic control of brake blending is to be preferred generally. This way the maximum permitted use of recuperation brakes can be ensured.
• Installation of more high-powered traction systems (which will be too expensive in most cases)
• Migration towards EMUs

However, the main obstacle for regenerative braking in 50 Hz systems which is the electric separation of feeding section cannot be eliminated.

Share of recoverable energy:

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

The following are typical values (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 (~ 85%).
• Receptivity of catenary: The supply system may be „non-receptive“ because no other train is close enough to use it. This is frequently the case in 50 Hz systems.
• 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 50 Hz systems:

Source: IZT

Since modern stock usually has the capacity to recover energy during braking, a big part of this potential is already exploited in today’s railway operation. Nevertheless, some potential is not exploited due to the following reasons:

• Some old electric stock is not equipped with regenerative braking.
• In some vehicles (especially locomotives) the choice of the brake is up to the driver.

The remaining potential is extremely dependent on the situation of an individual railway company but may be up to half of the above values, i.e. ~ 2 - 9%.