Special requirements for traction batteries in multiple units
Buses, trams and trains operate around the clock and in every weather condition: They are exposed to wind, rain, heat and cold. Even though their transport capacity, as expressed in passenger kilometers, is much higher than that of buses and cars, they are in use for around 30 years. The requirements placed on traction batteries for multiple units are correspondingly high, particularly with regards to charging and discharging currents, safety, low-temperature performance, operation time and cycle stability.
The increasing requirements on low-emission vehicles suggests that in the future the focus will be stronger on diesel hybrids or vehicles with alternative drives based on fuel cells or batteries. Which drive concepts or hybrid combinations will actually be in demand and prevail also depends on the structure of the railway network.
Future trains: batteries, fuel cells or hybrids?
In extensive, non-electrified networks the range and the possibility of rapid refueling (as appropriate) will be the most important decision-making criteria. If it is merely a question of bridging the gaps on routes without catenary between electrified regions, hybrid multiple units are suitable. They are equipped both with pantographs for overhead wires and with alternative drive systems. In this case, drives with batteries that can be recharged on routes with catenary may be useful. Depending on the distances to be bridged, it may even be possible to dispense with charging stations along the lines. If greater distances have to be bridged, it might make more sense to rely on fuel cells which can accommodate longer ranges and the required amount of hydrogen can be refueled in a few minutes.
Similar considerations also apply to the so-called last mile, i.e. the non-electrified section to the destination station where a battery charging station or catenary or H2 filling station is provided.
The right battery technology matters
The high operational requirements of rail transport reduce the number of suitable cell candidates which are available on the market significantly. Lithium-ion cells with LTO anodes offer a safe but costly solution. They are offered as a special technology by a handful of cell manufacturers. A significant disadvantage is the relatively low energy density of this technology. For multiple unit manufacturers – depending on the specific range – this can cause serious weight problems on the vehicle.
Pragmatic alternatives to LTO-based battery cells
A further objective of the study was to find out whether and to what extent any of the requirements could be relaxed or compensated with the help of alternative approaches at the system level. The idea behind it: the requirements do not always have to be met at cell level. The manufacturers of electric cars have opted for such a pragmatic approach within the framework of the National Platform for Electric Mobility (NPE). They are also relying on a cross-industry generation roadmap. It provides information on the development of energy densities and related costs up to the year 2030. The present study concludes with courses of action and recommendations for possible alternative battery concepts.
As part of the study, which was commissioned by the Federal Ministry of Transport, the partners also analyzed the results of the Siemens Mobility GmbH`s development project X-EMU. The project addresses the development and validation of a high-performance fuel cell-drive for hybrid-EMU multiple units in a traction modular system. The VDE study “Battery Systems for Multiple Units” marks the starting point for more planned publications in which the general conditions and success factors for multiple units with alternative drives will be analyzed.