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   Fly-wheels (storage technology)  evaluated  
The fly-wheel is an electro-mechanical energy storage system based on rotating masses. It is a powerful storage technology which may be used both for on-board and for stationary applications.
Technology field: Regenerative braking and energy management
close main section General information
  close sub-section Description


The fly-wheel is an electro-mechanical energy storage system based on rotating masses. It is a powerful storage system which may be used in a number of application contexts in railways, mainly:

Comparison to other storage technologies

As can be seen from the Ragone diagram in Figure 1, fly-wheels are characterised by both high energy and high power densities making them an attractive storage technology for braking energy storage in rail vehicles. Compared to double-layer capacitors have a good cycle life and thus long lifetime.

Figure 1: Ragone diagram


Source: Schneuwly 2002

Technical details

The fly-wheel system consists of the following main components: rotor in an almost frictionless bearing motor/generator power electronics.

Rotor: Since the stored energy is proportional to the rotor mass and to the square of the rotational speed, the rotor needs to combine high mass and high speed tolerance. The rotor of state-of-the-art fly-wheels is a hollow cylinder primarily made of carbon fibre composite. Advantages of this material (as compared to steel rotors) lie in its tearing stability allowing much higher rotation speeds and its favourable crashing behaviour saving difficult protection measures. Drawbacks of carbon fibre composites are: relatively small mass (limiting storing capacity since energy content is proportional to mass) and difficult manufacturing process.

Bearings and vacuum housing: In order to minimise bearing friction, most of the rotor weight can be borne by magnetic forces. The rotor housing is evacuated, thus minimising air friction losses. In some fly-wheels inert gases are used instead of a vacuum.

Motor/generator unit: For an optimum compact system design the motor/generator (M/G) unit is integrated inside the hollow rotor.

Rotation speed: 25.000-30.000 rpm

Energy content: typically between 6 - 12 kWh of which only about 75 % can be used since the generator is not operable at very low rotor speeds. Energy densities of current fly-wheels attain 20 kWh/m3.

Charging and discharging times: medium (between double-layer capacitors and batteries).

Efficiency: >90%.

Figure 2 shows the technical data of the fly-wheel used in the LIREX experimental train.

Figure 2: Technical data of the LIREX fly-wheel

Manufacturer WTZ Rosslau
Energy content 6 kWh
Maximum power 350 kW
Duration of the complete charging cycle For 350 kW
Efficiency including frequency converter (charging/discharging) > 90%
Idling losses 2,5 – 7 kW
Voltage [V] 550 – 750
Rotor material Carbon fibre / epoxy resin
Diameter of the rotor 700 mm
Maximum speed 25.000 r/min
Minimum speed 12.500 r/min
Type of bearing Precision ball bearings with lubricating oil circuit
Type of motor Synchronous motor, permanent excitation
Life 20 years
Suspension of storage fly-wheel Resilient mountings
Working temperature range From –40°C to 60°C
Dimensions of the complete system 1900 x 1625 x 1080 mm3
Mass including the carrying frame 1300 kg

Source: Witthuhn 2001


Due to rotational mechanics, fly-wheel operation theoretically has an impact on the wheel-set forces. However, calculations made for the Lirex experimental train show that these gyro-effects are negligible.

Fields of application

  • Diesel-electric busses (problem: diesel busses have to be refitted for diesel-electric operation first)
  • Trolley busses
  • Discussed for hybrid-electric cars (application hardly profitable due to low number of cycles)
  • On-board and stationary use in railways (DC mass transit and diesel-electric regional trains)
  • Industrial applications


Magnet-Motor GmbH Starnberg (Germany), WTZ Rosslau (Germany), etc.

close main section General criteria
  close sub-section Status of development: in use

Stationary storage

Fly-wheel storage was/is used in stationary applications for local trains in Cologne, Hannover and few other European cities. Experience from Cologne showed technical problems which led to the abandoning of the system. The application in Hannover based on a steel fly-wheel is operated successfully.

On-board storage

Deutsche Bahn AG plans to integrate an on-board fly-wheel in their Lirex experimental train. However, the development of the 6 kWh fly-wheel has run into difficulties. Therefore the fly-wheel version of the Lirex will be delayed and Deutsche Bahn plans to start regular service of the Lirex in December 2002 without an energy storage system. It is planned to integrate the fly-wheel system later.

  Time horizon for broad application: 5 - 10 years
    According to WTZ Rosslau, stationary applications in light city rail systems could reach 30% market diffusion within 5 years (as of 2002).
  Expected technological development: dynamic
    cf. Application outside railway sector - Expected technological development outside railway sector
    If used as a storage technology for braking energy, the motivation is saving energy. Other possible applications include catenary-free operation of city trams.
  Benefits (other than environmental): not applicable


Despite technological challenges still to be mastered, fly-wheel technology is relatively mature.


According to WTZ Rosslau, fly-wheels have a cycle life of about 5 million which corresponds to a lifetime of twenty years in a railway application, ten times more than today’s double-layer capacitors.

  Barriers: high


Investment costs of fly-wheels available on the market are still high.

Technological maturity

Small fly-wheel systems (~ 2 kWh, ~ 150 kW) as needed for busses are available on the market (Magnet-Motor Starnberg). Early failures (such as false system reactions due to sensor levels adjusted too low or mechanical problems with fixation of certain subcomponents) have been resolved in the meantime.

Reliable higher power/energy classes based on steel technology exist for stationary applications. The operation of a stationary fly-wheel (by Magnet-Motor) in the Cologne KVB network has been stopped because of low reliability.

Technological competition

Recent progress in the development of double-layer capacitors makes a wide-spread diffusion of fly-wheel technology uncertain.

    Success factors:
    (no details available)
  Applicability for railway segments: medium
    Type of traction:  electric - DC, electric - AC, diesel
    Type of transportation:  passenger - main lines, passenger - regional lines, passenger - suburban lines, freight
  • On-board use in diesel-electric vehicles to store braking energy.
  • On-board use in DC systems to raise recuperation rate
  • Stationary use in DC systems to raise recuperation rate
    Grade of diffusion into railway markets:
  Diffusion into relevant segment of fleet: 0 %
  Share of newly purchased stock: 0 %
  • No on-board in-service application yet.
  • Very few stationary applications.
  Market potential (railways): low
  • An economic use of fly-wheel technology will be mainly possible as stationary installation in light rail and mass transit systems.
  • An on-board application in diesel-electric vehicles may be profitable in some networks with frequent stops.
    Lirex experimental train (planned for in the future).
close main section Environmental criteria
  close sub-section Impacts on energy efficiency:
  Energy efficiency potential for single vehicle: > 10%
  Energy efficiency potential throughout fleet: (no data)
    Depends on application context. Cf. Stationary energy storage in DC systems, Diesel-electric vehicles with energy storage and On-board energy storage in DC systems.
  Other environmental impacts: neutral
    (no details available)
close main section Economic criteria
  close sub-section Vehicle - fix costs: high
    WTZ Rosslau that the market price of the fly-wheel currently developed (6 kWh / 350 kW) will be roughly 200.000 EURO. This is the market price and does not cover the total development costs of the joint project between Alstom and WTZ Rosslau.
  Vehicle - running costs: significant reduction

Energy costs

Energy costs are significantly reduced.


The only known in-service use of fly-wheels in railways is a stationary fly-wheel storage system in a substation at Kölner Verkehrs-Betriebe AG (KVB), Cologne, Germany. There is however no data available on maintenance experience.

Maintenance experience with Magnetmotor fly-wheels used in Basel trolley busses

The maintenance intervals of the fly-wheels used in trolley busses in Basel are defined to 3500 operation hours. The manufacturer Magnetmotor claims that specific tests with bearings and lubrication prove that maintenance intervals of 6,000 h are possible and that for the power electronic the statistic MTBF (mean time between failure) is more than 40,000 hours.

Currently the maintenance of the MDS fly-wheel is done at Magnetmotor, i.e. the system has to be removed from the vehicle sent to the manufacturer. Typical maintenance includes checking of components and the system as a whole, cleaning and lubricating or exchanging the bearings. The average maintenance time is about one working day.

By 2000, the average repair requirements was down to one repair every 38,000 hours of operation (equivalent to one repair in 8 years per fly-wheel).

  Infrastructure - fix costs: none
    (no details available)
  Infrastructure - running costs: unchanged
    (no details available)
  Scale effects: low
    Scale effects are to be expected from a more wide-spread use of fly-wheels. However, a mass market for fly-wheel technology will not exist in the foreseeable future. According to WTZ Rosslau production of 100 fly-wheels will not yield any price effects. This would require selling at least 10.000 systems, a figure only to be reached in automotive mass market where fly-wheels have little or no potential.
  Amortisation: > 5 years

Payback time depends on application context but is generally long.

Study at NS

In a study made in 1998, NS calculated that the energy savings pay off the investment to a great extent but not completely. The amortisation period is between 17 and 30 years. For an energy price of 13 ct/kWh at NS, the return on investment is 0,35-0,6. NS expects that this situation could improve in the future by growing energy prices. The payback time is quite pessimistic.


The situation should have improved in the meantime due to technological progress. DB AG assumes that the investment for the Lirex fly-wheel system will pay off easily within the lifetime of the vehicle currently planned to be around 15 years.

Diesel-electric vs. stationary applications in DC systems

According to WTZ Rosslau, stationary applications in local DC systems will generally have a better cost-benefit ration than on-board storage systems in regional diesel-electric stock.

close main section Application outside railway sector
  close sub-section Status of development outside railway sector: in use
    Since 1988, small fly-wheel systems (2 kWh, 150 kW) have been in use in electric busses for urban transport in several European cities. Fly-wheels made by the Magnetmotor GmbH have been in use in diesel-electric city buses since 1988. Since 1992, 12 trolley buses with fly-wheels have been running in Basel, Switzerland, with a total of more than 200.000 operating hours.
  Time horizon for broad application outside railway sector: in 5 - 10 years
    (no details available)
  Expected technological development outside railway sector: highly dynamic
    Energy densities of current fly-wheels attain 20 kWh/m3. Experts claim that a 5-fold increase of this figure is possible. The use of superconductors instead of conventional magnets for the bearing of the fly-wheel would lead to a further reduction of losses.
  Market potential outside railway sector: small
    Since an application of fly-wheels in private cars is highly doubtful, there is no mass market for fly-wheel technology in the foreseeable future. The main market is diesel-electric busses and city rail systems. This means that demand will no exceed a few hundred fly-wheel systems.
close main section Overall rating
  close sub-section Overall potential: promising
  Time horizon: long-term
    Fly-wheel technology is a promising solution for energy storage systems. First in-service experience from trolley busses and stationary storage in a light city rail DC system show principal technological feasibility and reliability. High power fly-wheels for on-board storage in DMUs are still under development. Main barriers for fly-wheels are high initial investment and long payback times. Best cost-benefit ratio is reached for stationary storage systems in local DC systems. Scale effects will be small in the foreseeable future since no mass markets exist. Growing technological competition from double-layer capacitors make a wide-spread use of fly-wheel technology uncertain. Nevertheless, due to long life-time and relatively high maturity, fly-wheels are still a promising technology.
References / Links:  Caputo 2000;  Engel et al. 2001;  Hennig, Stephanblome 2000;  NS Reizigers (no year);  Reiner, Weck (no year a);  Reiner, Weck (no year b);  Witthuhn, Hoerl 2001;
Related projects:  Flytrain;  Studies performed on energy storage systems
Contact persons:
 date created: 2002-10-09
© UIC - International Union of Railways 2003
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