Principle

Modern energy storage devices permit the storage of braking energy on-board for use in subsequent acceleration phase. Especially in DC systems, where energy losses in the distribution network are high, this could be an interesting alternative to feeding back energy into the supply system.

Without on-board energy storage much of the potential for brake energy recovery is not exploited because the system is often not able to take up the power. By means of energy storage, brake energy recovery becomes independent of load situation in the supply system. This way the recovery rates and energy savings may be raised substantially.

Furthermore the peak loads of the supply system are reduced, with results beneficial both for system capacity, voltage stability and energy costs.

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 energy type (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 energy 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 (an important condition to downsize energy supply! cf. below). When driving at maximum speed the storage device should be completely discharged.

Integration into drive system

Figure 2: Integration into drive system

Source: IZT

Choice of energy storage device

The choice of the best 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 1 gives the relevant characteristics for some trains.

Table 1: Relevant characteristics of some trains Regional Multiple Unit (EMU) Suburban EMU

Source: Hentschel et al. 2000

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 charging times. They are labelled by train types with corresponding braking times.

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

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

Batteries have very high energy densities but low power densities leading to very high charging times. Nickel metal hydride batteries have sufficient power densities but do not meet the lifespan requirements. Batteries are therefore presently not the first choice for on-board storage.

Flywheels have both high power and high energy densities and are therefore ideally fitted to most situations, especially regional trains.

Supercaps which have made considerable development progress in recent years are beginning to become attractive for railway applications especially light rail urban vehicles, but there are also high-energy supercaps that could be interesting for storage solutions on heavy vehicles.

Superconducting magnetic energy storage (SMES) is of the extreme low-energy high-power type and is therefore not an attractive option for brake energy storage.

Peak load levelling

The load peaks of the supply system are equalized and thus reduced, which improves voltage stability.

Energy price

Furthermore reduced peak demands save energy costs since peak load energy is especially expensive.

Technological

Storage technologies are starting to become a mature technology but lifespan is often still a problem.

Vehicle mass

Storage system increases vehicle mass.

Costs

High investment costs and long payback periods.

Complexity

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

Figure 4 shows the energy saving effect of energy storage on different train types. However, these figures have to be treated with some care. They are a result from theoretical calculations by manufacturers and are quite optimistic. Even so, the figures indicate the huge saving potential offered by energy storage in DC systems.

As can be seen in Figure 4, the energy saving potential comes from two effects:

• recovery and reuse of brake energy ("vehicle only" in the figure)
• reduction of losses in the distribution network due to reduced peak currents (Losses are proportional to the square of the current!).

Figure 4: Energy savings by energy storage for different trains (% of total energy consumption)

Source: IZT, data from: Hentschel, Müller 2000

Amortisation of energy storage systems is in the range of 10 to 30% of the vehicle lifetime, that is 3 to 10 years, depending on the vehcile and the storage type. It could be appreciably reduced by scale effects.

In many cases, amortisation is more favourable for stationary storage.