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.