As part of its research into the public transport of tomorrow, researchers at Fraunhofer have developed the AutoTram – a vehicle as long as a streetcar and as agile as a bus. Combining the best of both vehicles it has no need for rails or overhead contact lines, instead the “bustrolley” rolls on rubber tires and follows a simple white line on the road surface. It was constructed to serve as a research platform in the institute’s “Fraunhofer System Research on Electric-Powered Mobility” project – a large-scale research cooperative involving 33 Fraunhofer institutes that focuses on developing mobility solutions for the future.
The project is broken down into four areas of focus: Vehicle concepts; energy generation, distribution and conversion; energy storage technology; and technical system integration and social issues. The AutoTram was first mooted several years ago and was built to provide a platform for the researchers to test new developments in these areas, not only in simulations but in the real world. New modules for energy storage, double-layer capacitors and coupling coming directly from the Fraunhofer research laboratories are installed in the vehicle to allow them to prove their capabilities in the field. They have now presented their first results.
Accomplishing this in such a short period of time requires super-capacitors. Researchers are working to develop the modules required: for instance on energy storage units based on double-layer capacitors, on high-performance converters and on contact systems for the transmission of current. Unlike batteries, double-layer capacitors – also known as ‘supercaps’ – have a high power density. Those capacitors ensure that the charge can be quickly stored.
"Batteries take their time charging. You can compare this to a big bathtub with a small spigot. Capacitors, on the other hand, quickly take up the charge, like a small bathtub with a large spigot. However, they can only store a smaller quantity of energy, “ explains Dr. Ulrich Potthoff, department head at the Fraunhofer Institute for Transportation and Infrastructure Systems IVI in Dresden. Engineers are working on linking the battery system with the capacitors for this application in city traffic.
“We’re developing dual storage units and testing their features in combination with other storage types and fuel cells” Potthoff adds. His colleagues at the Fraunhofer Institute for Integrated Systems and Device Technology IISB are contributing innovative developments for the power-electronic components, such as a direct voltage converter that adjusts the voltage level. These DC/DC converters are needed to link the double-layer capacitors with the drive train.
Also decisive in this regard are materials that can withstand transmission of high levels of current. The surface of the contacts must be very stable and wear-resistant. In this regard, researchers at the Fraunhofer Institute for Material and Beam Technology IWS came up with suitable materials and the methods for processing them.
These packages are being developed both for passenger vehicles and for the AutoTram. Usually, the battery system consists of several hundred cells, and these do not always discharge at the same rate. And if individual cells should fail or no longer deliver the intended performance, this can take a toll on the entire battery. The individual cells are controlled using an overarching energy-management system.
“Within a few fractions of a second, the electronics measure the current, the voltage in the individual cells and the temperature and use these parameters to derive values for the battery’s state of charge and health. This way, a determination can be made for each cell as to whether there are any threats of overload, deep discharge, excessive heating or premature aging,” explains Project manager Dr. Matthias Vetter of the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, who coordinates the project.
These are just the first results to come from the “Fraunhofer System Research on Electric-Powered Mobility” project and the Fraunhofer team says there are still some technological hurdles to clear before passengers waiting at the bus stop are no longer inhaling clouds of exhaust fumes.
See the stories that matter in your inbox every morning