Technical Aspects of Critical Components for High-Temperature Superconductor Bearings-Based Flywheel Energy Storage System

Abstract

Flywheel-based energy storage systems are gaining prominence in present-day energy-deficit situation. For energy storage system, the bearings and motor cum generator, for charging and discharging energy to and from the flywheel, form the vital components which have to be given due consideration. The low coefficient of friction of high-temperature superconductor bearings used in flywheel-based energy storage systems decides the energy loss factor. Yttrium Barium copper oxide (YBCO) which is one of the popular high-temperature superconductors is widely being used in high-temperature superconductor bearings. The planar type of bearings can be materialised by building the stator of the bearing with YBCO bulk high-temperature superconductor discs mounted in a circular array and rotor part can be realised by Nd–Fe–B permanent magnet ring which is to be embedded with the flywheel. The levitation force is proportional to the mean magnetization of high-temperature superconductor and the horizontal gradient of the vertical component of the magnetic field density in the superconductor. The magnetization is proportional to the product of critical current density and the grain diameter of the bulk superconductor. The high-temperature superconductor bearings outweigh the magnetic bearings because their passive nature, by virtue, does not require any external active stability balancing devices. The paper discusses of development, fabrication and load testing of prototype planar type of high-temperature superconductor bearings. For charging and discharging energy to and from the flywheel energy storage system, Halbach array-type motor cum generator is a promising candidate. The motor, with an outside rotor design, is a dipole Halbach array, establishing a uniform flux within the stator area. A uniform flux is needed for operating the motor on a superconductor bearing, where slight movement of the stator inside the air gap is inevitable. The salient construction features of the prototype Halbach array motor generator will be discussed.