Test results of 2-kWh flywheel using passive PM and HTS bearings

Toward demonstrating the potential of flywheel energy storage systems that use high-temperature superconductors (HTSs) and permanent magnets (PMs) as passive rotor bearings, a flywheel system was developed and tested with a 165-kg cylindrical carbon- and glass-fiber rotor to rim speeds of 400 m/s (19,000 rpm) and stored energies of >2.25 kWh. The main bearing´s internal stack of PM rings was passively stabilized by HTS bearings at each end of the rotor. The stator portion of the HTS bearing consisted of an array of melt-textured YBCO pellets bathed in liquid nitrogen inside a nonconducting cryochamber. The motor/generator (M/G) was based on an internal-dipole Halbach array and could produce 1.5 Nm of torque. Each bearing and the M/G included multipiece banded PM rings secured to the rotor inside diameter with flexible urethane rings. In a vacuum enclosure at 10^-4 Pa pressure, rotational drag on the rotor was hysteretic and at low speeds the coefficient of friction was well below 10^-6