In the summer of 2010 five public transport organisations joined forces in Ticket to Kyoto, a four-year project co-funded by the EU’s Interreg IVB programme.Part of the project focused on the recovery of braking energy. Different available technologies (flywheels and reversible substations) were assessed, their characteristics were studied, and they were eventually put to the test.This case study presents three of the partners’ approaches, their assessments of the available technologies and results of the different tests.
The recovery of braking energy is when modern rail vehicles recycle the kinetic energy they produce when braking. However, a lot of this energy is wasted because the majority can only be re-used by a vehicle accelerating nearby. Ticket to Kyoto consists of five partners, three of which worked together on solutions to make the recovery of braking energy more efficient. They were:
- Brussel’s STIB (Belgium)
- Bielefeld’s MoBiel (Germany)
- Rotterdam’s RET (The Netherlands).
Between them, the partners invested in and tested reversible substations and a flywheel. A reversible substation uses an inverter allowing the gathered energy to flow in both directions between the high voltage grid and the vehicle electrical grid. The recovered braking energy is not stored but sent back into the network.
A flywheel is a rotating wheel spinning around an axis, used for storing energy mechanically in the form of kinetic energy. Instead of feeding it back to the network, the flywheel stores it and sends it back to another vehicle.
All three partners applied a two-step process when implementing the brake-energy technology. First, extensive simulations were performed to assess the potential energy savings of different technologies and to determine the locations for the substations given the specific network of the respective public transport provider and their different public transport timetables. Second, a European tender was launched.
MoBiel invested in and tested two reversible substations and a flywheel. The flywheel was supplied by Germany’s PILLAR and is located at the end of Line 2, a light-rail line. The reversible substations were provided by Spain’s INGETEAM (Spain) and are located by the sides of Lines 1 and 6. The total investment cost for the systems was € 825 000.
RET invested in and tested two reversible substations built on two different metro lines. These substations fed braking energy from the metro back into the RET power grid. Following a European tender, IMTECH Traffic and Infra B.V. from the Netherlands was selected to supply the substations. The substations are located in Vlaardingen en Schiedam. The total cost of the systems was € 478 000.
Following a European tender, three inverters, each built by a different supplier, were installed on the same substation (Houbba-Brugmann at the end of line 6). The systems were tested for several months to compare their efficiency and the delivered savings. All three systems performed well. However the INGETEAM invertors matched best with the STIB’s needs and its transport system. The total investment cost of the INGETEAM inverters was € 1 800 000.
By implementing the inverters, MoBiel reduced energy consumption by over 900 000 KWh per year. The flywheel storage reduced energy consumption of 320 000 KWh per year – or a saving of 5.6 per cent in annual electricity consumption for the whole light-rail system in Bielefeld. The three systems recover 10 per cent more than the savings calculated in the network study.
Both systems performed above expectations. However, the flywheel, at 10 t, is very heavy and required structural changes to the substation. The system is also very loud, producing 96 dB(A) of noise. This is why MoBiel invested in an additional reversible substation provided by INGETEAM (installed in June 2014) and ultimately chose the reversible substations over the flywheel.
Work on the two reversible substations has finished - however, they are not operational yet. During the first tests malfunctions occurred in the existing substation installations. Consequently, the new reversible substations have to be adapted to the existing RET installations. It is expected that both substations will be put into operation before the end of July 2014.
Building the two reversible substations will potentially save the RET 300 000 kWh energy per year, representing a total reduction of € 54 780 in energy costs, and a reduction of 9480 kg CO2 per year (15,8gr CO2/kWh).
Based on the results of its tests, the STIB extrapolates that the traction-energy consumption of metro lines 2 and 6 will be reduced by 9 per cent. This saves approximately 3 400 000 kWh and 568 tCO2 per year. The STIB also estimates that, thanks to the future savings it will make in energy costs, the money spent on the inverters will be recouped in five years. The company considers this acceptable given that another benefit will be the reduction in CO2 emissions.
The tests revealed some interesting insights. François-Olivier Devaux, technical co-ordinator of the Ticket to Kyoto project, said:
‘While the project has been very successful and deployment continues, those considering implementing braking energy recovery systems should keep in mind that these are technically complex projects. The expertise that is required is important and it can take up to a year for the supplier to fine-tune the systems to the transport network and its specific characteristics.’
The malfunction in Rotterdam is a case in point, where further implementation was put on hold for a few months until old and new systems were aligned. Extensive modelling and specific knowledge were necessary to come up with a solution. This is why, Devaux says, performing simulations of the systems before real-world implementation are crucial. It prevents malfunction and allows for the selection of the right braking-energy recovery technology.
‘Brussels University (VUB) performed the simulations, which allowed us to calculate the impact of the technologies on the electrical network and the vehicles. The calculations showed that energy savings were sometimes significantly different from the suppliers’ calculations. This helped us to better evaluate their product’s potential.’
Devaux cited outside temperature, a neighbouring substation’s energy consumption and fluctuating passenger numbers as factors that can affect the system’s operation. Understanding better the impact of these factors enables operators to improve the recovery systems parameters and maximize the energy recovered. However, Devaux warns, this requires a long period of fine-tuning.