Researchers from the Fraunhofer Institute for Transportation and Infrastructure Systems IVI and its several partners in the EDDA Bus project, have developed a pantograph fast-charging solution for electric buses that supports route-based opportunity charging. Operational system testing of a converted bus and its charging station began in Dresden in November last year.
The solution is based on four core technologies: the Dresden-based company M&P GmbH designed a charging station with very high charging capacities; HOPPECKE Advanced Battery Technology GmbH supplied special batteries designed for such high power capacities; Vossloh Kiepe GmbH was responsible for adapting the power electronics; and the contact system located on the roof of the bus was realized by Fraunhofer IVI together with Schunk Bahn- und Industrietechnik GmbH.
Charging station. In order to recharge a passenger bus sufficiently, charging stations with different charging capacities are needed to reflect the different amounts of time available—i.e., when the bus is at a passenger stop vs. being in the depot. Recharging during the passenger exchange at underway bus stops, for example, is only practical if a charging capacity of 400 - 500 kW can be realized.
M&P thus developed two types of charging stations: a fast-charging station with maximum charging capacity of 250 kW; and a pulse-charging station, endowed with 2,6 kWh of supercapacitors (4.0 kWh optional), with a maximum charging capacity of 600 kW (700 kW optional).
|Fast charging station. Click to enlarge.||Pulse charging station. Click to enlarge.|
Batteries. HOPPECKE Advanced Battery Technology GmbH (Zwickau), developed the batteries for the EDDA Bus, with a focus on high charging capacity. They have a modular design and can be combined to different-size packs for the installation in buses.
The cells are Li-ion pouch with an NMC chemistry. Total capacity is 86 kWh (three modules, with charging capacity of 450 kW. The battery system, which is mounted on the roof of the bus at the rear, is liquid-cooled.
|The battery modules for EDDA Bus. Click to enlarge.|
(As an aside, following Elon Musk’s unveiling of the “missing piece” in sustainable energy—Tesla’s PowerWall stationary storage system—HOPPECKE tartly observed that “The missing piece had been found a long time ago.” HOPPECKE has sold more than 5,000 home energy storage systems in the last three years.)
Contact system. The electrical contact system, consisting of a pantograph, a contact head and a roadside contact hood, was developed by Schunk Bahn- und Industrietechnik GmbH in collaboration with the Fraunhofer IVI.
The development aim was to safely transmit high electric capacities, and the currents associated with them, to a stationary vehicle via a compact and robust system. In order to enable the automation of the process, this system was developed as an alternative to plug-in systems, which need to be connected manually.
A single actuator positions the contact head inside the roadside contact hood. The developed contact system allows generous positioning tolerance regarding the roadside contact hood. In addition, the contact system balances out sideway movements of the vehicle during kneeling.
For the contact head, six powerful contacts were housed in a small installation space. There are two contact poles—one positive, one negative—on each side. Fitted to the top, there is a ground contact and a control pilot, which monitors the grounding.
Schunk Bahn- und Industrietechnik GmbH developed heat-stable components for the contact system. This was necessary because, in contrast to streetcars for instance, where energy is transferred while the vehicle is moving, transferring power to a stationary vehicle generates high local temperatures that would damage conventional components.
Safety an interaction of bus and charging station. The development goals were to avoid making additional demands of the driver and to not cause any restrictions for the passengers. Accordingly, the automated charging process identifies all relevant system states and reacts to them appropriately in case of any discrepancies.
Under the monitoring of the TÜV Rheinland, a safety and operating concept was compiled based on the technical standard DIN EN 618511. As a first step towards the overall concept, a risk analysis was conducted in compliance with the standard IEC 615082. The implementation of the derived tasks and requirements was monitored and approved by the TÜV Rheinland.
The following framework conditions for the design of the automated charging process were derived from the knowledge gathered at the Fraunhofer IVI regarding the requirements for its use in regular bus operation. Subsequently, they were implemented by the automation solution.
Each driver must be able to position a bus at the curb with an accuracy of 40 cm in the direction of travel and 30 cm laterally.
The automated process must be easily integrated into the drivers’ regular work routine. The only information that the drivers will be provided with is status information about the charging process, but they will retain the option of canceling or terminating the charging process.
The roadside contact hood has to be installed so that its lower edge remains at a height of at least 4.50 m above the road at all times.
Energy transmission is to start within 0.5 sec of the contact closure.
Energy transmission is to be initiated only if electric safety can be ensured.
If a discrepancy or deviation is detected by one of the subsystems, the charging process is to be terminated automatically.
As demanded by the DIN EN 61851, the charging system is constructed as an electrically insulated system with a galvanic isolation from the grid input. This is realized by the installation of an insulating transformer within the charging station.
During the recharging process, the electric safety for persons and facilities is ensured by connecting the electric grounding in the bus stop area with the vehicle’s chassis and additionally by monitoring the connection during the time of the energy transmission.
Furthermore, an insulation monitor continuously checks the insulation resistance of the closed system circuit consisting of all components involved in the charging process. DC contactors inside the charging station and inside the vehicles serve to ensure the fast and secure electrical separation in fault case. These electrical switches are also used to realize the permanent separation of the contact system’s electrically active components, which can be touched by the maintenance staff while no charging process is going on.
EDDA Bus passed its first practice assignments in regular service in Dresden with flying colors. Following the test phase, the vehicle served a more challenging route of around 20 kilometers (12.4 miles) in length and containing several hills. The average energy consumption is 1.19 kWh per kilometer. (The bus is not heated electrically, but uses diesel heating.) Recharging takes less than 6.5 minutes.
EDDA Bus is part of the Fast Charging Systems for Electric Buses in Public Transport joint research project funded by the German Federal Ministry of Education and Research (BMBF).