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).