Energy storage for traction
The installation of energy storage systems for the vehicle traction management presents a great interest. Today the engineers have to face two major problems: the increase of the mean vehicle power request and the increase of the number of vehicles circulating on the network. The distribution of energy storage systems in or close to the vehicles allows a reduction of the peak power request of the network. It is possible to supply more vehicles without infrastructure investment because the electrical conductors don't need to be oversized [i]. Beside this cost saving the energy storage provides a reduction of the financial penalties in the electricity bill based on the maximum peak power request.
The storage systems brings also a global energy saving. The energy stored during the vehicle braking is delivered for its acceleration after the stop at the station. It has been shown that with today electricity cost, the storage system has a return on investment of five years only with the energy saving. In the future this performance will be improved because of the energy cost increase.
The manufacturers propose either fixed or on board systems. Both solutions fulfill the same function. For the fixed solution the supercapacitors are placed in an electrical network sub-station. The advantages are:
With 192 supercapacitors connected in series and 3 parallel, the energy content is 1.5 kWh. The 14 F module is rated at 518VDC. It is mounted on the roof of the LRV in a container which has a size of 1.9 m x 0.95 m x 4.5 m and has a weight of 450 kg. The energy cost saving are estimated at 30'000 per year for each vehicle thanks to the 30% energy consumption reduction.
The Siemens SITRAS SES energy storage system (see fig. 2) has a capacitance of 64 Farads and a nominal voltage of 750VDC. It is composed of 1300 BCAP0010 pieces of 2600 F.
With the cabinet, the inverter and the control power electronic, the system has a volume of 2.9 m x 2.8 m and has a weight of 5.5 tones. It is able to supply a maximum power of 1 megawatt in an area of 3 km. The supercapacitor module allows a reduction of the mean power consumption of 50 kW.
[i] Rufer A, Barrade P, Hotellier D. Power-Electronic Interface for a Supercapacitor-Based Energy-Storage Substation in DC-Transportation Networks. EPE Journal 2004; 14(4):43-9.