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

Considerable optimisation expected on the basis of present materials and construction principles and improvement by < factor 2 in all fields

The development of more powerful and more efficient storage systems will improve stationary storage solutions.

• Energy savings
• Stabilise catenary voltage
• Smoothen power demand

Smoothened power demand

The demand peaks are cut down by the use of a storage system. This saves energy costs, not only due to a reduced energy demand but due to a price effect as well. The energy price is determined by electricity suppliers in a way that basic demand is much cheaper than peak demands. Therefore, a smoothened power demand substantially reduces the price per kWh.

Stabilisation of catenary voltage

The use of a storage system has a stabilising effect on catenary voltage. Tests in Cologne indicate that an energy storage system may even replace a rectifier of a substation, under specific conditions.

Increased peak performance of DC system

With storage systems acting as boosters for accelerating trains, the system gains a peak performance not achievable by the DC system alone.

Stationary energy storage in Cologne light rail network

Since 2000 an energy storage system is tested in service in the Cologne local transportation network. The flywheel with an maximum energy content of 6,6 kWh and a maximum power of 600 kW was installed in a substation of the DC supply grid. Braking energy which would otherwise be lost in brake resistors is stored and can be used later for an accelerating train. Comprehensive tests demonstrate that energy storage saves about 24 % of the total energy consumption. Additional cost effects can be realised due to the fact that energy storage reduces power peaks and thus the energy price. Encouraged by positive experience with the first energy storage, it was decided to equip a second substation with an energy storage system, this time realised with a double-layer capacitor developed by Siemens. Experiences are as positive as in the case of the flywheel.

Status quo in DC local networks:

~ 40% of energy demand goes into kinetic (or potential) energy and could theoretically be recovered.

If vehicles are equipped with regenerative brakes, 10-30% are recovered and used by other trains (in dense networks). The rest is converted into heat in on-board resistors.

The installation of stationary energy storage systems is an interesting option on peripheric routes with low train density and/or high velocities and slopes. In such areas of the network, a direct use by other trains is rare, so recovery rate will usually be < 10% (of total energy demand), leaving > 30% of theoretical potential for energy savings through stationary energy storage. There are however further limitations:

Losses in energy transmission, conversion and storage

Layout of storage system is not sufficient to store the entire braking energy (e.g. in the Cologne case, the average braking current is ~1000A, the flywheel can only take 450A). The layout is usually a trade-off between technological and economic considerations

In the Cologne case, the average power demand of the substation equipped with a flywheel was only 160 kW compared to 210 kW without energy storage. This means energy savings of about 24%. On has to keep in mind that these savings were realised at a carefully chosen location and can by no means be generalised to an entire network.

The system-wide effect achievable by putting storage systems at all “hot spots” of the network is very difficult to assess and strongly dependant on the specific network layout. Given the above figures and the limitations mentioned, it seems reasonable to assume that the overall effect for a local DC network could be between 2-10% depending on the density of "hot spots".

Energy costs are substantially reduced by two effects:

• Energy demand is reduced
• Energy price is reduced due to reduced demand peaks