In the context of current discussions about climate protection goals and particulate matter emissions, pollution free traffic concepts continuously attain increasing importance. In urban and suburban areas local public transport can provide a crucial contribution to climate protection goals through a substantial reduction of harmful and greenhouse gas emissions. Local public transport has to fulfill the following requirements in the future:

  • Reduction of the primary energy consumption of public transport vehicles,
  • Decrease of climate and health-relevant emissions of public transport,
  • Decrease of the life cycle costs of emission-free propulsion technologies compared to means of local public transport with overhead lines,
  • Long-term reductions of pollutant emissions caused by traffic, particularly in critical metropolitan areas and thus increase of the attractiveness of local public transport as a clean, efficient and low noise transport system.


Since environmental aspects by itself are often not able to motivate the introduction of efficient and emission-free propulsion concepts, the overall economic competitiveness compared to conventional systems is of great importance.

All subsystems for the realization of a new propulsion concept are united under the project title »DockingPrinciple«. The objective is to operate both locally emission-free buses in public transport and trams with low wayside infrastructure costs.

This electric propulsion concept is characterized by an onboard energy storage system, which receives its energy from punctual installed wayside high-speed energy transfer stations (docking stations). The charging processes are carried out during scheduled stops. Furthermore the utilization of the traction power from the onboard energy storage system is optimally controlled through a predictive energy management system. In addition, the onboard energy storage system is also capable to store energy from regenerative braking. The recuperation of kinetic energy leads to a significant increase in energy efficiency of public transport vehicles because of frequent accelerations and braking actions. Therefore the implementation of efficient energy storage systems on buses and trams contributes to the reduction of greenhouse gases and harmful emissions.


A precondition for the development process is the identification of critical parameters for the vehicles concerning power and performance requirements and technical constraints (space, interface). Furthermore suitable storage configurations must be identified. They form the basis for the integration and test of such energy storage systems. A predictive energy management concept, as integral part of the »DockingPrinciple«, ensures the optimal use of stored electrical energy for the driving task and to cover the energy supply to auxiliaries in a predictive manner during the driving process.

The development and the prototypical implementation of product solutions for a power supply device consisting of a wayside energy transfer station (docking station) and a high current energy transmission system are integral parts of the overall project »DockingPrinciple«.

In the context of preexamination extensive investigations as well as LCC and cost-benefit analyses were carried out showing the market potential and competitiveness of the »DockingPrinciple«.

Energy Transfer Station

The energy transfer station is connected to the low-voltage network of a local utility company from where the station energy storage is continuously charged with low power. This energy, which is stored in double layer capacitors, is transferred into the vehicle with high power via the high current energy transmission system. This offers substantial cost reduction potentials compared to conventional overhead contact installation networks and their substations. For the transformation of the alternating voltage of the power network into a defined DC voltage an electronically regulated power supply unit with an additional DC/DC converter is used. In order to transfer the energy from the capacitor block into the energy storage system of the vehicle, another DC/DC converter with an output voltage equal to the vehicle power circuit voltage is necessary.

Further development steps will focus on substantially smaller docking stations which can easily be integrated into urban road space.

High Current Energy Transmission System

A high amount of energy must be transferred in a very short period of time via an interface between a wayside power supply device and the vehicle. This causes very high current loads in the power transmission components. Currently known and available energy transmission systems for urban transport vehicle systems, like pantographs to third rail/catenaries are not able to deal with thermal load caused by the high current loads.

A wayside high current energy transmission system for fast charging of an onboard energy storage system of a tram was developed in cooperation with a tram manufacturer during a research project funded by German Federal Ministry of Education and Research (BMBF). The system consists of spring mounted segments and enables the required power transmission to any badly worn carbon brush of standard pantographs.

An alternative system was developed and prototypically implemented for vehicles without pantograph (buses).

The availability of a fast, save and widely automated transmission system for the charging of onboard energy storages is a fundamental precondition for the demonstration of an all-electric city bus operation.

Funding and Partner

The project was supported by the Federal Ministry for Education and Research BMBF as part of the funding scheme »klimazwei - Research for climate protection and protection against climate impacts« for the duration of three and a half years.

The main project partner was the Bombardier Transportation GmbH. The Association of German Transport Companies VDV and the Dresdner Verkehrsbetriebe DVB AG participated with project-related studies.

Detailed information about the project results are available in the project final report (in German only):

Lehnert, M.; Klausner, S.; Himme, C.; Klohr, M.: Das DockingPrinzip - Klimaschutz durch emissionsreduzierte Nahverkehrssysteme. Gemeinsamer Schlussbericht des Projektverbundes: Fraunhofer-Institut für Verkehrs- und Infrastruktursysteme IVI und Bombardier Transportation GmbH, Dresden, 2010. This Document is online available.