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.
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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.
Flywheels are a potential solution to many of the concerns around public transport investment, including the commercial readiness of batteries, their environmental footprint and the inherent disadvantages of charging times and supply availability for conventional electric vehicle modes.
Buffer flywheel systems have a demonstrated performance with lower operational costs than conventional drive modes.
Flywheels are strongly enabling of fuel cell application and support the uptake of renewable energy.
Incorporated with inductive power systems, catenary-free systems are opening up light rail solutions to revitalise urban environments.
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