Flywheel Batteries for Vehicles
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Energy storage flywheels are useful in power conditioning applications, i.e. when there is a mismatch between the power generated and the power required by the load. Two examples of this mismatch are a temporal mismatch and a mismatch in magnitude. The use of a flywheel in a hybrid vehicle, for example, permits the engine to be designed to provide only the power needed to overcome steadystate losses and not have the inefficiencies that result when the engine must also provide power for maximum acceleration. While power management has provided the opportunities for flywheel batteries in vehicles, advances in technology have made the systems more practical. The key advance is the development of very high strength, long-life composites. These materials have significantly improved the energy density in the system over what could be achieved with a steel wheel by permitting much higher rotational velocities. So, smaller, lighter wheels can store energy in the range from less than a kilowatt-hour to more than 100 kilowatt-hours. Other important advances have been in magnetic bearings that allow reliable high-speed operation and in power electronics to control the output power. Vehicular operation does produce new issues that were less significant in the more traditional stationary applications. One of the most obvious among these is torque management. In charging or discharging a flywheel, the rotational velocity is changed and a torque is produced. For example, systems intended for the International Space Station, where torque management is critical, the initial plan is to cancel torque by using two counter-rotating flywheels. Once confidence is gained in this mode of operation, energy will be distributed among various flywheels to produce the net torque needed for stable operation. For terrestrial vehicles, the flywheel is in a gimbled compliant mount with the axis of rotation orthogonal to the plane of vehicle motion. This orientation permits torque to be compensated by the magnetic bearings and the mount. Tests show that the mount and bearing system can accommodate the shock and vibrations, as well as traveling up or down grades, expected under on-road operation.