The FINANCIAL — Siemens is in the process of launching the new Sitras HES hybrid energy storage system on the market. It consists of a Sitras MES mobile energy storage unit and a traction battery.
“Trams with hybrid energy storage systems can operate without an overhead contact line (OCL) over distances of up to 2,500 meters. They not only preserve historical buildings and enhance the appearance of the urban landscape, they are especially environment-friendly and save energy,” said Michael Meinert, development manager responsible for the new system at Siemens Mobility. Vehicles equipped with energy storage systems consume up to 30 percent less energy per year and produce up to 80 metric tons less CO2 emission than vehicles without energy storage systems. Thanks to a new connection concept, Sitras HES and Sitras MES can be retrofitted in existing vehicles without any difficulty; the tramway infrastructure remains completely unaffected. In Portugal, the hybrid energy storage system has been successfully used in passenger services since November 2008. It has also been certified according to BoStrab (German Construction and Operating Code for Tramways) for use in the public transport.
The Sitras HES hybrid energy storage system consists of two energy-storing components: the Sitras MES mobile energy storage unit (double-layer capacitor, DLC) and a nickel-metal hydride battery. Sitras MES enables energy-saving operation. The Sitras HES hybrid concept combines the advantages of the DLC with the properties of a traction battery. The length of line sections on which driving is possible without an OCL can therefore be increased to up to 2,500 meters. The systems are mounted on unused roof surfaces of a tram and electrically connected to the feed-in point of the vehicle by means of a DC/DC-chopper. Thanks to this new autonomous connection concept, the energy storage system can be directly integrated into new vehicles or built into ones that already exist. When the vehicle is in operation, the energy storage units are charged during braking. A tram can use this stored energy to travel relatively long distances without having to be supplied with power from the contact line. The energy storage units can also be recharged on routes with OCLs or stationary charging stations, for example at stops. The high energy content of the traction battery also allows operation in case of an OCL-failure or maintenance work on the OCL as well as when unforeseeable problems arise on routes without OCLs.
Saving money by operating without an OCL
By recovering the surplus energy generated during braking, a vehicle equipped with Sitras HES or Sitras MES can reduce its energy demand in future by up to 30 percent under optimum operating conditions. And a vehicle that requires less energy also produces up to 80 metric tons less CO2 emission per year. Moreover, the line voltage becomes more stable because the voltage drop along the OCL is reduced, especially during peak hours. The operators of routes without an OCL also save money in respect of electrification costs. Driving without an OCL is especially suitable where there are difficult built structures that complicate OCL-installation, for example in tunnels, under bridges, at system changeover points or major intersections used by different transportation modes.
Sitras HES is already being used in passenger service
Last year, Siemens Mobility installed a Sitras HES in a tram belonging to the Portuguese company Metro Transportes do Sul S.A. (MTS). Since November 2008, the system has been used in passenger service and has proved to be highly successful. The re-designed Combino vehicles now operating between Almada and Seixal, two cities to the south of Lisbon, are not only able to travel over distances of up to 2,500 meters without being supplied with power from the OCL, they are also saving energy at the same time.
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