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Public transport remains a difficult position for communities wishing to move to more sustainable transport modes with reduced carbon footprint. Often the preferred solution is simply perceived as a move to electric drives or hybrid electric vehicles.
However, often overlooked in the conversation on electric vehicles is the reality that vehicles need both energy (to achieve range) and power (for acceleration and load following) to function properly. Whilst the internal combustion engine delivers this functionality well, batteries cannot be optimised for both power and energy at the same time. Thus, there is always a trade-off between the two features... or are flywheels the solution?
Flywheels are an old means of storing energy but with modern materials and improved design new possibilities and applications are emerging. There has been much recent work done on the use of flywheel technology for vehicle application, including public transport. Compared to the alternative battery energy systems, flywheels offer much higher power densities as well as higher reliability, longer cycle life without degradation, and reduced ambient temperature concerns. Flywheels are also free of environmental pollutants.
Moreover, integration of these technologies into urban bus routes (which involve large periods of deceleration and acceleration for potential energy recovery and re-use) have proven the technology suitable for high utilisation paths. When combined with inductive power, the modern flywheel can be readily reenergised at passenger stops; thereby offering continuous drive power throughout the day.
Examples of flywheels optimised for vehicles include power buffers in cars where a constant power flow from the primary energy source is advantageous, such as hybrid cars with combustion engines (higher efficiency, higher torque and less emissions with constant power outtake) and fuel cells (slow response times, low power density). Flywheels are seen to excel in high-power applications, placing them closer in functionality to super capacitors than to batteries.
Examples of the use of flywheel technology include the deployment of a buffer flywheel systems for London buses, which resulted in fuel savings of over 20%. The 0.4 kWh flywheel used provides enough power to accelerate a bus from 0–50 kph whilst recovering the energy when braking. More recently Citadis (Alstom Transport) and AutoTram of Fraunhofer Institute Dresden have prototyped a 300kW storage / 4kWh flywheel power line to buffer a Bollard Fuel cell (80 kW) prime mover city tram.
With improved performance, flywheel systems with traction power derived from ‘refuelling’ at stopping points, have gained more prominence. Flywheel systems were demonstrated as part of the “Ultra Low Emission Vehicle-Transport using Advanced Propulsion” (ULEV-TAP) consortium under the EU Brite-Euram program. Examples include the Great Western Railway Test of its “Parry People Movers” (PPM) flywheel tram concept on the Stourbridge line and the CAF catenary-free tram operating system in Zaragoza.
Catenary-free trams are gaining prominence, attracting more companies to market. The technology enables transfer of electricity from infrastructure buried in the street to either power electric motors in a vehicle or to charge energy storage devices such as super-capacitors, batteries or flywheels.
Although the technologies available for catenary-free trams have a limited history, longer term demand for these systems is expected to exceed overhead powered systems, due to the multiple advantages offered. It is likely we will see the accelerated deployment of the modern flywheels, delivering an alternate solution to the need to renew existing aged electric supply assets, and the associated system constraints that arise for hybrid and electric propulsion.
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