Green Design

Ongoing Power Performance Research to Advance Electromobility

The AutoTram® is as long as a streetcar and as maneuverable as a bus. It doesn’t need rails or overhead lines because it rolls on rubber tires and simply follows white lines on the street. In “Fraunhofer’s System Research for Electromobility,” the AutoTram® was used as an experimental platform. It was a component of the research collaboration of more than 30 of Germany's Fraunhofer institutes.

“We provide functioning solutions for advancing electromobility in Germany. We use these two demonstrator vehicles – AutoTram® and a passenger car – to demonstrate that the recently developed components function interactively,” said Dr. Ulrich Potthof, the department head at the Fraunhofer Institute for Transportation and Infrastructure Systems. The research topics in this group project were vehicle designs, power generation/distribution/conversion, energy storage engineering, technical system integration, reliability, testing and launch, as well as social issues. New modules installed in the vehicle such as an energy storage device, double layer capacitors, and clutches can show what they do in real-life practice.

The battery system consists of a whole series of cells and they do not always discharge at the same speed. The entire battery can suffer if individual ones fail or no longer provide the planned output. This is the reason why these packs are controlled by a higher-level energy management system. Dr. Matthias Vetter is from the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, Germany. As project director for coordinating the project, he describes the basic principle: “The electronic system measures the current, single-cell voltage, and temperature in fractions of a second for calculating the charging and aging state. This is how we can find out whether each cell is about to overcharge, undercharge, overheat, or age prematurely.” Unlike cars that park an average of 23 hours a day, buses and trains are on the road the whole day so there isn’t much time to charge batteries. A potential solution is fast-charging stations at bus stops in combination with dual storage units in the vehicle itself.


The tram taps the quantity of energy it needs to make it to the next stop or the stop after that in only 20-30 seconds. During this time passengers are getting in and out. Scientists at the Fraunhofer Institute for Transportation and Infrastructure Systems are working on the modules needed such as the energy storage device, high-performance converters, and contact systems for transmitting the current. When the tram stops, the silvery current collector on the roof moves upward and docks onto the source of electricity. Then small but strong electromagnets turn on. They generate enough power to press the contacts together enormously. This is where the resistance (and therefore heat development) is lower when high levels of energy of more than 1,000 amperes and 700 volts are transmitted. But where does the needed electricity come from? There is an energy storage device at the tram stop, and it slowly collects small amounts of electricity to avoid load peaks for the power generator wherever possible.

The engineers are working at linking the battery system and capacitors in urban transportation for this application. They are coming up with dual storage units and also testing combinations with other storage models and fuel cells. Researchers at the Fraunhofer Institute for Integrated Systems and Device Technology are contributing new developments in high-performance electronic components such as a DC converter that adapts the voltage level. These specially built DC/DC converters are needed to couple the double-layer capacitors with the drive train. However, the materials that stand up to high power conversion are also of pivotal importance. The surface of these contacts has to be very sturdy and resistant to wear. This is where the researchers from the Fraunhofer Institute for Material and Beam Technology IWS come in – they have developed the materials needed and the way they are processed.

These Fraunhofer institutes have all contributed their experience to developing a new type of magnetorheological motor generator clutch. This electrically switchable clutch functions in the following fashion - an integrated fluid changes its consistency from liquid to solid under the influence of a magnetic field. That means that the coupling process can be quickly and precisely controlled.

(Fraunhofer)