Tram, streetcar and light rail systems can provide very efficient, environmentally
friendly urban mass transit.However, they do require a significant investment for tracks and overhead catenary lines to deliver electricity to moving vehicles. Catenaries can be
rather unsightly, especially in congested downtown and historical districts. They also can be quite complicated and expensive to construct and maintain in tunnels, under bridges, at system changeover points or at major intersections used by different modes of transportation.
Siemens Mobility’s Sitras hybrid energy storage (HES) system overcomes this by allowing these electricpowered trams and trains to travel up to 2500 m without having to be connected to an overhead connection line (OCL). Besides eliminating the need for
OCLs, the system reduces emissions by up to 80 metric tons of CO2 annually.
It also can cut electric energy consumption by up to 30% under optimum operating
conditions. The latter occurs because the Sitras HES can be recharged by regenerative braking that recovers energy normally lost during braking and deceleration.
As the title “hybrid” implies, the Sitras HES consists of two energy-storing components. These are the Sitras mobile energy storage (MES) unit, which is a double-layer capacitor
(DLC), and a nickel-metal hydride (NiMH) battery. This hybrid system takes advantage of the strong points of capacitors, which include ultra-high energy for short periods, batteries and the ability to supply moderate energy for extended periods.
The Sitras HES is mounted on unused roof surfaces of tram and train cars. It is electrically connected to the feed-in point of the vehicle by means of a DC/DC-chopper. This autonomous connection allows the energy storag system to be directly integrated into new vehicles or retrofitted to existing ones. Besides charging via regenerative braking, the DCL and NiMH battery can also be recharged enroute from the
OCL or at stationary charging stations during stops.
Power from the Sitras HES can help stabilize line voltages such as OCL voltage drops during peak usage hours. The high energy content of the traction battery can provide a backup capability in case of a failure of the OCL or maintenance work on the OCL.
The Sitras HES has been successfully used in passenger service in Portugal since November 2008. A redesigned Combino low-floor tram belonging to Metro Transportes do Sul S.A. is now operating between Almada and Seixal, Portugal, two cities to the
south of Lisbon. The system has also been certified according to BoStrab (German Construction and Operating Code for Tramways) for use in the public transport.