Considering the need for increased economic efficiency and improved service delivery, local transportation companies are faced with the need for measures to upgrade existing systems in order to improve performance and reduce costs. Energy storage could make an important contribution to meeting these new requirements. Currently, the braking energy of a vehicle is normally converted into heat. However, a system for energy storage would allow the storage of this braking energy and its supply at a later time in the form of acceleration power. In this way, energy that up to now was lost, could be harnessed and reused.
The aim of the project was to use flywheel energy storage to regenerate the braking energy of vehicles. The anticipated reduction in energy consumption was up to 10% of the total energy for the mass transit system. This reduction in energy consumption would result in a direct reduction of CO2 emissions, as less energy has to be produced for the same transportation. The reduced energy consumption, as well as the long-term reliability and effectiveness of the process, were to be proven using a prototype. The use of energy-storage-units for the power supply of regional light rail systems reduces total energy consumption and can bring several advantages for the operation and the dimensioning of the power supply:
- optimised regeneration of the breaking energy of vehicles.
- Smaller dimensioning of substations with constant peak power in the system.
- Rescue of trains from tunnels during power failures in the public supply network.
- Stabilising of the system voltage on distant feeders.
The optimised regeneration of the braking energy can save up to 10% of total energy requirement of a substation. The energy-storage-unit consists of a carbon-fibre flywheel rotating at more than 10.000 rpm. The energy-transport to and from the flywheel was managed by a special synchron motor-generator-unit, which was fed by a special power-converter. The control unit for the power converter was developed especially for this project.
The project included the development of basic tools to determine the optimal location for energy storage for the energy storage system, to be used in complicated mass transit systems. Thanks to an intelligent software, even with only a few data inputs it is possible to estimate in advance the optimal location of a system and the saving potential, and therefore to make a statement about economic efficiency.
During the project the optimal location for the first fast rotating flywheel energy storage in the system of the Transportation Company of Cologne was determined. This first prototype of energy storage has been operating successfully and continuous at the substation in Ensen since June 2000. Measurements at the prototype storage system in Cologne confirm the high-dynamic characteristic of energy storage and the predicted energy saving potential. One single energy storage system in a mass transit substation can reduce the annual demand for electrical energy by up to 340,000 kWh. To generate this amount of energy, a modern coal fired power plant would burn about 100 tons of coal and emit about 370 tons of C02. Assuming ten years of operation, more than 3700 tons of CO2 emmission would be prevented, which equates to a financial vaue of EUR 75,000 (based on the average price of EUR 20 per ton of prevented CO2-Emission.
Considering all the advantages and disadvantages, as well as the costs of the different storage systems, it was concluded that a high-speed flywheel is the most suitable technique for application in public transportation systems. The electrical machine as the basis of the flywheel has practically no limiting effects as regards lifetime (millions of cycles are possible, as compared with thousands for batteries). This high number of cycles is necessary a train will stop every 1 to 2 minutes, which can cause more than 1,000 cycles per day. Discharging to extremely low energy content is not a limiting factor for flywheels. For batteries this is a decisive factor in reducing limetime.
A follow-up questionnaire was completed by Siemens in 2005. It showed that the originally chosen mechanical flywheel technology had not proved to be mature enough for the tough conditions in the railway system. However, lessons were learned from this first experience in terms of reliability and operational costs, and follow-up work was able to address these obstacles. The successful technology could not be applied to the LIFE project, which closed in March 2000, as the requirement for development of the new capacitors couldn’t have been foreseen in the preparatory phase of the project. As a result, a completely new concept for energy storage was chosen: and the prototype installation of this second generation storage system was installed at the substation Cologne Ensen, which had been also the site of the flywheel energy tests. Based on the experience with the new prototype, Siemens continued developing the energy storage technology, using double-layer capacitors (DLC) as storage media. This is an innovative component based on electrostatic energy storage.
In February 2001, the world’s first energy storage system equipped with double-layer capacitors was installed in the Brueck substation of the Cologne transport company. Double-layer capacitors are the most innovative energy strorage technology at extremely high power cycling capability. The small voltage of the individual capacitor cells is adapted to any traction system voltage by series connection and paralleling of the capacitors. First measurements and operational results realized at the DLC storage system showed the reachable energy saving potential was visibly higher then with the prototype of the first generation in Cologne Ensen. Therefore, the group concluded that the economic and environmental advantages have proven to be much better than with the flywheel system. In the follow-up questionnaire, Siemens reported that the second generation of its storage system, with supercapacitors, was already installed at five sites - in Germany, Spain, and in the US, and five further installations were planned for the near future.