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).

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:

	Total potential	Correction due to traction efficiency	Correction due to catenary receptivity	Correction due to blended braking	Corrected potential
Main lines	15%	0,85	0,4	0,8	4%
Regional lines	35%	0,85	0,5	0,8	12%
Local lines	45%	0,85	0,6	0,8	18%
Freight lines	20%	0,85	0,4	0,5	3%

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%.