Advanced design and experiment of a small-sized flywheel energy storage system using a high-temperature superconductor bearing

A small- sized flywheel energy storage system has been developed using a high- temperature superconductor bearing. In our previous paper, a small- sized flywheel was fabricated and successfully rotated at 38 000 rpm under a vacuum condition. However, a large drag torque was present because of the non- axisymmetric magnetic flux of the motor/ bearing magnet and the eddy current loss in the planar stator, resulting in a short spin- down time of 20 - 30 s and a coefficient of friction of 0.15. This paper presents the design, fabrication and electromagnetic analysis of the flywheel in order to reduce the large drag torque. The advanced flywheel designed for solving the non- axisymmetric magnetic flux problem comprises eight motor magnets and a single bearing magnet, a magnetic screening disc, and an aluminium-disc which is 50 mm in diameter and 5 mm in thickness. The eddy current loss in the planar stator is minimized by reducing the contact area between the planar stator and the motor magnets. The maximum rotational speed increases 1.3 times to 51 000 rpm and the spin- down time increases 600 times to 3 h 20 min. On the basis of these results, the coefficient of friction decreases 100 times to 0.001 - 0.002.