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Saft’s Ni-MH (nickel-metal hydride) integrated traction battery systems is being used in the development of a new generation of trams equipped with the Sitras HES hybrid energy-storage system.

Siemens Mobility is developing the trams, which can operate autonomously without external power from an overhead contact line (OCL) over long sections of track.

The concept is suitable for areas where impact on the local landscape must be minimised and is environmentally friendly.

The Sitras HES comprises two energy-storing components: the Sitras MES mobile energy storage unit based on a double-layer capacitor (DLC) and the traction battery system.

The two components work together to provide an efficient energy-storage system.

Saft’s Ni-MH traction batteries for the Sitras HES system were primarily chosen because they were commercially available and could be integrated into the test programme quickly, without a long development period, according to a Siemens Mobility spokesperson.

Furthermore, they met Siemens’ stringent specifications in terms of weight, size, reliability and high performance, added the spokesperson.

When the tram is running, the energy-storage units are charged regeneratively during braking.

The vehicle can use this stored energy to travel over distances of up to 2,500m over sections of track where no OCL is available.

This distance represents a significant increase in autonomous operation compared with the typical 500m capability of a tram relying on battery power only, without regenerative braking.

The high-energy Ni-MH battery also improves the reliability of the tram service as it allows continued operation when the OCL is temporarily unavailable, such as in the case of failure or planned maintenance.

The Sitras HES system is designed for roof mounting on the tram and is electrically connected to the vehicle feed-in point by means of a DC/DC chopper.

This autonomous connection concept enables the energy-storage system to be integrated into new trams or retro-fitted into existing vehicles.

Under optimum operating conditions, the Sitras HES is expected to reduce the future energy demand of a vehicle by up to 30 per cent, producing up to 80 tonnes less CO2 emissions per year.

The Sitras HES battery unit has been adapted by Saft from its range of fully integrated battery systems developed to meet the needs of the new generation of tramways.

It is based on Saft’s NHP Ni-MH cells designed specifically for high-power applications to offer excellent power storage in a compact maintenance-free package.

Each battery system incorporates an active cooling device and battery management control (BMC).

The unit is supplied to Siemens Mobility as a plug-and-play package in a custom-built tray complete with power and communications connections.

The exact battery system voltage and capacity for future systems would vary according to the specific application.

A typical specification would be 44 cell blocks of NHP10-340 cells connected in series, providing a nominal 528V and capable of storing 18kWh of energy and sustaining a peak power of 105kW.

The total battery weight is 826kg.

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