Principle

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:

• 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

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

Gyro-effects

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

Manufacturers

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

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.

Maturity

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

Lifetime

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.

Costs

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.

• 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

• No on-board in-service application yet.
• Very few stationary applications.

• 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.

Energy costs

Energy costs are significantly reduced.

Maintenance

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).

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.

Lirex

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.