Engineering prototype of a superconducting flywheel for long term energy storage

We built a flywheel system with superconducting magnetic bearings. The bearing consists of six melt-textured YBCO pellets mounted inside a continuous flow LN2 cryostat. A disk measuring φ 190 mm × 30 mm was safely rotated at speeds up to 15 000 rpm. The disk was driven by a high speed three phase synchronous homopolar motor/generator. Maximum energy capacity was 4.8 Wh, maximum power was 1.5 kW. The dynamic behavior of the prototype was tested, characterized and evaluated with respect to axial and lateral stiffness, damping, decay torques (bearing drag), vibrational modes and critical speeds. Experimental data were found to be in agreement with a structural damping model. Rotor unbalance together with the hysteretic nature of the superconducting magnetic bearing gave a significant contribution to the overall losses. At a background pressure of 6×10-4 mbar, the coefficient of friction (drag-to-lift ratio) was measured to be μ = 9×10-6. The experiments demonstrate the applicability of superconducting magnetic bearings in highly efficient, kinetic energy storage systems.