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.
‘Supercaps’
Unlike cars, which remain parked for an average of 23 hours a day, buses and trams are in motion all day long. Which doesn’t leave much time to recharge the batteries. One solution approach the AutoTram takes involves fast-charge docking stations positioned at the stops along the route. Current can then be drawn at every third or fourth stop. The requisite amount of energy must be recharged in just 30 to 60 seconds at more than 1000 amperes and 700 volts.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.
Getting it together
“These newly developed concepts must be coordinated with one another so they will harmonize with all of the other components. At IVI, we are incorporating the modules into the overall AutoTram system and configuring the interfaces” Potthoff explains. This also includes the lithium-ion battery systems for electric vehicles. This is an effort to which experts at 11 different Fraunhofer institutes have been working to advance – not an easy task, as the batteries and electrical systems are subject to extreme demands. These systems need to be safe, durable and efficient.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.
Coupling
The project is also looking to develop a new type of magnetorheological engine-generator coupling. Electrically switchable coupling works by altering the consistency of an integrated fluid from liquid to solid under the influence of a magnetic field. This reaction can be harnessed to provide precision control of the coupling process. Equipped with highly efficient electric drive motors and control units as well as high-performance batteries and supercapacitors, Fraunhofer says the AutoTram can transport its guests with nearly zero emissions.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.
