A methodology for evaluating and reducing rotor losses, heating, and operational limitations of high-speed flywheel batteries

Abstract

Flywheel batteries are machines that store kinetic energy in the form of a rotating flywheel. Energy is transferred to and from the flywheel via a motor-generator mounted on the flywheel rotor. For mobile systems in particular, it is important to maximize the stored energy while minimizing the mass and volume of the flywheel battery. This requirement leads to the use of high rotational speeds which in turn necessitates the use of composite materials for flywheel construction, magnetic bearings for reduced wear and friction losses, and a low pressure environment to reduce windage losses. With the flywheel rotor suspended on magnetic bearings and operating in a partial vacuum, radiation becomes the primary heat transfer mode for removing losses incurred on the rotor. Radiative heat transfer from the rotor to the flywheel battery housing is limited by the relatively low maximum allowable temperature of the composite materials and the permanent magnets which are often used in the motor-generator. In order to ensure the feasibility of a high-speed flywheel battery design it then becomes paramount to properly manage the total rotor losses as well as the heat removal strategy. This dissertation develops a methodology for accurately modeling the components of rotor heating in high-speed flywheel batteries with a focus on mobile systems employing an integrated design whereby the motor-generator is integrated with the flywheel into a common vacuum housing. The methodology makes it possible to reduce losses through design, construction, and operation so that high-speed flywheel batteries made with temperature sensitive components such as permanent magnets and composite materials can be operated without serious overheating. The rotor loss origins are investigated with respect to windage, magnetic bearing, and motor-generator sources in general, and with specific regard to a metropolitan transit bus flywheel battery system developed by the University of Texas Center for Electromechanics. Methods are provided to reduce the contribution from each source and measured temperature data is provided to confirm the effectiveness of many of the methods. Finally, thermal finite element models are utilized to determine the operational limitations placed on a flywheel battery by the incurred rotor heating.