A Study on Flywheel Energy Storage System Mounted on Electric Vehicle

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Flywheel energy storage system (FESS) works by accelerating flywheel to high speed rotation and maintaining the energy in the system as kinetic energy. The energy is converted back by slowing down the flywheel. A typical system consists of a rotor suspended by bearings inside a vacuum chamber to reduce friction, connected to a combined electric motor/generator. Active magnetic bearings (AMBs) are necessary to improve total energy efficiency. In conventional mechanical bearings, viscous damping is directly proportional to speed, and at high speed, too much energy would be lost. From this background, we have been focusing on the use of AMB in FESS due to the significant advantages such as contactless and frictionless bearings at high speed rotation. Usually, magnetic bearings are mostly used only in systems with immovable environment. Here on the contrary, we developed a vehicle with flywheel using magnetic bearing and gimbal mechanism as energy storage system.Flywheel-power assisted car … More s (mostly mechanical, or with mechanical bearings) have been developed since long time ago and in ongoing researchs in effort to make flywheel systems smaller, lighter, cheaper and have greater capacity. Proposed flywheel systems would eliminate the disadvantages of existing battery systems such as low power density, long charge times, heavy weight, short lifetimes, and lead pollution. The weakness is difficulty to store energy for a long time in flywheel. And, high speed rotation implies that the safety concerned with burst failures should be guaranteed. From this consideration, carbon fiber reinforced polymer (CFRP) is chosen as the material for the flywheel, since it is lighter and yet stronger than steel.In vehicle applications, flywheels also act as gyroscopic body, since the angular momentum is typically of a similar order of magnitude as the forces acting on the moving vehicle. This property may be detrimental to the handling characteristics. Besides, this property could be utilized to improve stability in curves. Conversely, the effect can be almost completely removed by mounting the flywheel within an appropriately applied set of gimbals, where the angular momentum is conserved without affecting the vehicles. We achieved good performance of flywheel supported by zero-bias AMBs by means of controllers which significantly compensate gyroscopic effects. The flywheel can rotate up to 300Hz without any gyroscopic effect. We mounted FESS on an electric vehicle (EV) and designed electric power converter to charge/discharge the energy. We developed and implemented new algorithm to compensate gyroscopic effect while EV is turning. This report describes experimental results including maneuverability and overall energy efficiency, including the results of outdoor field experiments such asfeasibility test of steer-by-wire system, implementation of input shaping to reduce vibration and gyroscopic effects, simple adaptive control method for flywheel attitude control, and the efficiency measurement of the energy conversion system. Less