Das Projekt "Bau eines Schwungradsystems zur Energieeinsparung zur Minderung des Energieverbrauchs in S-Bahn-Systemen von bis zu 10 Prozent" wird vom Umweltbundesamt gefördert und von Siemens AG durchgeführt. The use of energy-storage-units for the power supply of regional light-rail systems reduces the total energy consumption and brings several advantages for the operation and the dimensioning of the power supply: optimized energy regeneration of the braking energy of vehicles; Smaller dimensioning of substations with constant peak power in the system; Rescue of trains from tunnels of power failures in the public supply network; Stabilizing of the system voltage on distant feeders. The optimized regeneration of the braking energy can save up to 10 per cent total energy amount of a substation. The energy-storage-unit consists of a carbon-fibre flywheel rotating at more than 10000 rpm. The energy-transport to and from the flywheel is managed by a special synchron motor-generator-unit, which is fed by a special power-converter. The control unit for the power converter is especially developed for this problem. By the use of flywheel energy storage it will be possible to regenerate the braking energy of the vehicles, which cannot be used by conventional measures nowadays. The reduction in energy consumption will be up to 10 per cent of the total energy for the mass transit system. This reduction of energy consumption will directly result in a reduction of the CO2 emission, as less energy has to be produced for the same transportation. This reduction of energy consumption shall be proven by the prototype as well as its long-term reliability and the effectiveness.