Climate-neutral mobility: Alternative drive solutions for railways

Key Findings

The battery system of a multiple unit must meet the most exacting demands. High power levels need to be provided when setting off and accelerating. At the same time, however, there must be sufficient energy available to ensure an adequate range. There are also additional railway-related requirements which make it necessary to consider completely new battery technologies.

• Supplying an electric motor with energy from a battery represents a good emission-free alternative to diesel engines on lines which are not electrified and have no catenary.
• There are highly exacting demands on the reliability and quality of multiple units, as they are expected to provide more or less 24-hour service (usually on a tight schedule) for up to 30 years. The requirements placed on traction batteries are correspondingly stringent with regard to charging and discharging currents, safety, low-temperature performance, operating time and cycle stability.
• The VDE examines which of the battery technologies and cells currently available are suitable for use in multiple units, and highlights the major development trends up to 2030 and beyond.
• The exacting railway requirements can be met very well by lithium ion cells with anodes consisting of lithium titanate (LTO). However, cells of this type have relatively low energy densities. They are also expensive in comparison to the technologies used by car manufacturers.
• As an alternative to the LTO-based solution, the study draws attention to a battery technology that meets the requirements at both the battery system and cell levels.

Key Findings

The German railway network covers about 40,000 kilometres, only 23,000 kilometres of which are electrified with overhead lines. Roughly 2,900 diesel multiple units are in operation on the remaining 17,000 kilometres, some of them with exhaust systems which have been in use for decades. However, complete electrification of these branch lines would cost at least 11.5 billion euros. In addition, the lengthy planning approval procedures and the infrastructure construction companies' lack of capacity make it more or less impossible to reach the 2050 target.

• The decision to decarbonise transport by 2050 means that hybridisation of diesel vehicles would represent an unsatisfactory and above all expensive interim solution.
• The study therefore focuses on purely electric drive solutions as the most sensible alternatives to diesel engines.
• The study distinguishes between two solutions for on-board provision of the required energy: a large lithium-ion battery that can be charged by means of pantographs via the overhead line, and a fuel cell that converts the energy held in hydrogen into electricity.
• In principle, batteries represent a good solution for infrequently used lines with gaps in the catenary not exceeding 40 to 80 kilometres. Fuel cells are favoured in cases where there is no overhead line at all or where there are gaps of 80 kilometres or more..
• Qualitative evaluation of the alternatives using neutral benefit analysis shows that battery-powered and fuel cell trains are equivalent solutions in systemic terms. The principal finding is: The most suitable alternative depends on the route network in question and the given framework conditions.

Key Findings

In its third study on climate-neutral rail transport, the VDE focuses in particular on evaluating the economic viability of alternative drives. It takes the 'Düren network' as the basis for its in-depth analysis.

• The economic viability study is based on data relating to the year 2026, including timetables, fleet size, operating capacity, speed and stopping times at stations. Also included are the costs for vehicle procurement and maintenance, for replacement of the relevant components, as well as the costs for installation and operation of the additional infrastructure components which are required such as overhead lines, electrification islands or refuelling stations.
• The energy requirement of the multiple units on the 'Düren network' lines is calculated on the basis of simulations which take idling, speed curves, terrain profiles and auxiliary unit performance values into account.
• The VDE takes the typical rail transport mileage of 30 years as its period of observation and uses the net present value method as a dynamic investment calculation procedure for comparing the economic viability of the alternatives.
• The most important finding is that, in the 'Düren network', battery-powered multiple units prove to be much more economical than fuel cell multiple units. The main reasons for this are the energy costs and the relative costs of replacing traction batteries and fuel cells.
• Finally, the VDE reveals the extent to which the findings can be transferred from the 'Düren network' to other diesel networks, and the conditions under which fuel cell or battery-powered multiple units would represent the more cost-effective investment project in the long term.

The VDE does not issue a general recommendation for an alternative drive solution. In the various related studies the VDE experts highlight the importance of assessing each case separately and show how other aspects must also be taken into account in the decision-making process alongside purely economic criteria. In its capacity as an independent technology association, the VDE is committed to neutrality – which is why it evaluates the individual issues based on scientific criteria, e.g. technological challenges (2018 study), systemic potential (2019 study) or economic aspects (2020 study). Furthermore, it is not always possible to transfer the findings from individual case studies (e.g. Düren district) to other regions. Technological progress or changes in the regulatory framework could also result in a shift in the relative advantages of the alternative drive solutions in the future.

In contrast to a systemic comparison of the alternatives to diesel multiple units, a business analysis must be based on a concrete example. Nahverkehr Rheinland (NVR) and Rurtalbahn provided the VDE with the extensive data on the 'Düren network' which is necessary for analysis. They also helped set up test runs for measurement purposes.

The results of the economic viability study carried out by the VDE must be examined separately in each individual case, as many different factors influence the result (see 2020 Study). 

In general, it can be stated that the difference between battery-powered drives and fuel cell drives can be much smaller under certain (primarily network-specific) conditions. The fundamental advantage of battery-powered operation is mainly due to physical factors (energy conversion efficiency) and to the fact that, at present, fuel cells are still disadvantageous in terms of their costs and working life. If the technical or regulatory framework conditions change, the overall outcome may also shift towards fuel cells.

This decision must be examined and assessed on a case-by-case basis. This is primarily because economic criteria are not the only decisive factor in most projects. Ecological goals, public acceptance, the service aspect of public transport and regional factors concerning economic and research promotion must also be taken into account in addition to economic viability. VDE experts can support local authorities and public decision-makers as neutral advisors in this evaluation process.

Economic viability was calculated using a procedure known in the literature as the "net present value method". This is applied in situations where a range of very different technologies are available, the investment covers a very long period of time and details unrelated to the technology itself are irrelevant for the consideration. 

The VDE is a neutral partner in a development project funded by the Federal Ministry of Transport and Digital Infrastructure (BMVI). It conducts analyses and neutral assessments which are published in the form of studies. Within the funding project, the VDE acts as a subcontractor of Siemens Mobility – but is strictly independent in its assessments and analyses. It can also publish its results without any influence being exerted by Siemens Mobility.