Trams on the run

Trams could run without overhead contact lines and emit significantly less CO₂ using a new hybrid energy-storage system.

Siemens, the developer of the Sitras HES hybrid energy-storage system, claims trams equipped with these systems consume up to 30 per cent less energy per year and produce up to 80 metric tons fewer CO₂ emissions than vehicles without them.

According to Michael Meinert, the development manager at Siemens responsible for the new system, trams with hybrid energy-storage systems can operate without an overhead contact line over distances of up to 2,500m. This, he claims, enhances the appearance of the urban landscape and is environmentally friendly.

The Sitras HES hybrid energy-storage system consists of two energy-storing components, a double-layer capacitor and nickel-metal hydride battery.

The system is mounted on the roof of a tram and electrically connected to the feed-in point of the vehicle with a DC/DC-chopper, which converts one source of direct current (DC) from one voltage level to another.

The system works similar to most hybrid systems, where energy-storage units charge up during braking. In many hybrid cars, the kinetic energy during braking is transformed into electricity by a generator and then stored by a battery for later use.

By recovering the surplus energy generated during braking, Siemens estimates a tram equipped with the Sitras HES can reduce energy demand by up to 30 per cent under optimum operating conditions. Siemens suggested the energy-storage units could also be recharged on routes with overhead contact lines or stationary charging stations at stops.

The company also claimed that this system could make line voltage more stable, because the voltage drop along the overhead contact lines would be reduced — especially during peak hours. The operators of routes without any overhead contact lines would also be likely to save money in respect of electrification costs.

Siemens added that the hybrid energy-storage system could make designing tram transport systems easier. Built structures such as tunnels, under bridges, system changeover points and major intersections can complicate overhead contact-line installation.

The Sitras HES can be directly integrated into new vehicles or retrofitted into existing ones without affecting tramway infrastructure.

Siemens' hybrid energy-storage system has already been tested out on a tram belonging to the Portuguese company Metro Transportes do Sul. The tram, which operates between the cities of Almada and Seixal, has been running since November 2008.

The system has also been certified according to BoStrab (German Construction and Operating Code for Tramways) for use in the public transport system.

Siobhan Wagner