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Hybrid energy storage system

Saft has supplied Ni-MH (nickel-metal hydride) integrated traction battery systems to Siemens Mobility for use in the development of a new generation of trams equipped with the Sitras HES hybrid energy storage system. The first tram with Sitras HES, equipped a Saft battery system onboard, has completed more than one year of successful passenger service trials in Portugal with the Siemens Mobility customer MTS (Metro Transportes do Sul, S.A.), Since the end of 2008, it has been operating services between Almada and Seixal, two cities to the south of Lisbon. 

The trams with Sitras HES can operate autonomously, without external power from an overhead contact line (OCL), over long sections of track. The concept is ideally suited for areas where impact on the local landscape must be minimized, such as in historic city centres, and is especially environmentally-friendly as well as saving energy.  

The Sitras HES concept 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 a very efficient energy storage system. Saft's Ni-MH traction batteries for the Sitras HES system was primarily chosen by Siemens Mobility because they were commercially available and could be integrated in the test programme quickly, without a long development period. Furthermore, they met Siemens' stringent specifications in terms of weight, size, reliability and high energy performance.

Regenerative braking 
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 2,500 metres over sections of track where no OCL is available. This distance represents a very significant increase in autonomous operation compared with the typical 500 metre 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 new 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 percent, 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. To ensure maximum reliability and safety, 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 105 kW.
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