Principle

Stationary storage of braking energy is an option in DC systems, where direct use of braking energy is difficult. The stored energy can be used for traction purposes either by the same or by other trains.

Apart from energy savings, stationary storage smoothens the power demand over time and has thus a stabilising effect on catenary voltage.

Location

Storage systems can be installed in substations or along the track.

The location for an energy storage system has to be chosen very carefully to ensure maximum efficiency of the measure. The track topology in the vicinity of a storage location has a decisive influence:

The highest energetic benefit of energy storage systems can be realised in parts of the network with a low degree of cross-linking (low probability of direct use by other trains), with slopes and high velocities (high amounts of braking energy).

In contrast, tightly meshed parts of the network with low velocities favour a direct interchange of braking energy.

Energy management

The energy flows in the system are managed in a way that braking energy is stored only if no other train can use the energy directly. In other words there is a clear hierarchy:

• direct use by other train
• storage

Technological realisation

The choice of the best storage technology has to take into account the following requirements:

• Sufficient power (during charging and discharging) and sufficient energy content in order to serve as an effective booster for accelerating trains.
• High cycle stability: the storage medium has to allow for 500-1000 charging and discharging cycles a day without undergoing any performance changes.

These conditions clearly favour flywheels and supercapacitors.

Apart from the storage unit itself, the storage system consists of

• power electronic equipment (inverters etc.)
• cooling unit (peaks of up to ~ 50kW heat generation have to be cooled, with restrictions to be met concerning the noise emission of cooling unit in urban areas)
• resistor (needed for safety reasons, only used for discharging of storage unit in failure scenario)

An important issue is the layout of the storage system. There is a complex trade-off between technological and economic needs. On the one hand, the storage unit should be dimensioned in such a way that it supplies enough energy and power for a train to accelerate without additional energy supply. Assuming a 50 t light rail vehicle and a maximum speed of 80 km/h, the critical energy is 3,4 kWh. On the other hand, storage systems have high investment costs and no unnecessary storage capacity should be installed.

The following table gives the technical data of the flywheel storage system used in Cologne light rail network.

Technical data of storage system used in Cologne light rail network
Catenary voltage       rated voltage       variation	750 V 450…980 V
Power       continuous       maximum	280 kW 600 kW
Maximum speed       in-service       test	15 000 min-1 21 000 min-1
Energy content for maximum in-service speed	6,6 kWh
External dimensions       Height       Diameter	0,75 m 0,77 m

Source: Gunselmann, Höschler, Reiner 2000