Principle

Modern energy storage devices permit the storage of braking energy on-board for use in the subsequent acceleration phase. This offers the possibility of an effective brake energy recovery in diesel-electric vehicles.

Furthermore the peak demands on the diesel engines are reduced allowing for both downsizing of engine layout and better load management of diesel engines.

Energy storage in the driving cycle

Figure 1 shows the use of energy storage in a typical driving cycle consisting of the phases acceleration – constant speed – braking – standstill.

Figure 1: Driving Cycle and Storage Cycle. Line 1: Speed v, tractive effort T. Line 2: Power P, Energy E. Line 3: Voltage U, current i of a supercapacitor for energy storage

Source: Hentschel et al. 2000

During braking phases the kinetic energy of the vehicle is transformed into another form of energy (e.g. electrostatic energy in the case of a capacitor) and stored in the storage device. When the vehicle stands still the energy storage device should be fully charged to be able to deliver energy during the subsequent acceleration phase. The power supply during acceleration is supported by the stored energy. The energy management system should be designed in such a way that the external energy supply never needs to deliver the full accelerating power (important condition to downsize energy supply!). When driving at maximum speed the storage device should be completely discharged.

Table 1 shows the operating modes discussed at DB AG for trains equipped with an on-board energy storage.

Table 1: Operating modes for dieselelectric vehicles with energy storage

Source: Witthuhn, Hoerl 2001

Integration into drive system

Figure 2: Integration into drive system

Source: IZT

Choice of energy storage device

The best choice of an energy storage device heavily depends on the individual vehicle and service type. The following table shows the main characteristics to be looked at in an individual application context and the corresponding storage parameters. 

Table 2 gives the relevant characteristics for a DMU.

Table 2: Relevant characteristics of a DMU in local service

The Ragone diagram plotting energy density against power density is a convenient means to compare different storage technologies and assess their suitability for different vehicles. Figure 3 shows the position of relevant storage technologies in the Ragone diagram and the corresponding charging times. They are labelled by train types with corresponding braking times. Since most diesel-electric vehicles are used in the regional or local range but hardly as LRVs, it is clear from the Ragone diagram that fly-wheels and in some cases double-layer capacitors are the first choice for energy storage.

Figure 3: Ragone diagram and charging times (corresponding to braking times of different trains)

Source: IZT, data mainly from: Hentschel, Müller et al. 2000.

A wide-spread integration of on-board storage systems into diesel-electric vehicles seems doubtful since payback is critical for regional service with relatively long distances between stations.

Downsizing of energy supply

The load of the supply system (diesel engine) is equalized and thus reduced, which allows for a smaller layout of the diesel engine/generator system.

Technological

Storage technologies are starting to become mature but lifespans are often still a problem.

Vehicle mass

The storage system increases vehicle mass.

Costs

High investment costs and long payback periods.

Complexity

Operators are reluctant to purchase trains with higher complexity, since this could decrease availability and reliability and always requires additional know-how.

In principle, there are two energy efficiency effects obtainable through energy storage in diesel-electric vehicles:

• recovery and reuse of brake energy
• demand equalizing in order to operate diesel engine only at the operation point of highest efficiency

Various simulations of brake energy recovery on diesel-electric vehicles exist giving a quite heterogeneous picture. Anything between 10 and 35 % saving potential can be found in literature.

It is estimated that a saving potential between 10 and 15 % for regional service and up to 30 % for DMUs in some suburban networks with very frequent stops could be realistic.

DB Simulations of LIREX

The Research Center of Deutsche Bahn AG carried out simulations of the operation of the LIREX with energy storage, showing an energy saving potential of about 11% for a vehicle with storage fly-wheels compared with a similar vehicle without storage fly-wheels. As can be expected, the simulations showed that the use of storage flywheels in vehicles produced the greatest saving effects on routes with short distances between stops.

The simulation was carried out for the German line Magdeburg - Halberstadt - Thale having a total length of 87 km and 8 stations. It was based on operating mode 1 ("reduction of the fuel consumption by recovering the braking energy", cf. table 1). The simulations allowed for the power losses of the storage flywheel as well as the maximum available coefficient of adhesion between wheel and rail. The optimum driving style was assumed.

DB Simulations of Shunting Locomotive

Earlier calculations by the DB environmental center simulating a shunting locomotive with brake energy recovery yielded 14%. If in addition, an idle speed cut-off and fuel supply cut-off during slow down phases are assumed, energy consumption can be reduced by up to 35% compared to a reference vehicle.

Other estimates

Hentschel, Müller et al. (DaimlerChrysler / Adtranz) give a saving potential of 35 % for a suburban DMU. Even if this is too optimistic in a practical context, it hints at the huge theoretical saving potential offered by energy storage in diesel-electric vehicles.