Proposed high altitude long endurance (HALE) unmanned aerial vehicle (UAV) concepts for solar powered aircraft indicate that energy storage devices will be required that significantly improve power, energy density, efficiency, and depth-of-discharge over state-of-the-art electrochemical (NiH2) batteries, without which these aircraft systems cannot become a reality. Flywheel energy storage systems offer the potential for making these systems concepts practical. However, current concepts for flywheel energy storage systems rely on energy conversion and power generation approaches that limit the available energy for aircraft use to near 60% of the fully charged capacity of the fly wheel, with efficiencies below 90%. With useable specific energy capacities below 50 Whr/kg, these systems are in capable of enabling solar-powered HALE UAV technology.
In this paper, the performance of a high-power, ultra-low loss, “ironless” motor/generator design is outlined as part of a supercritical self-balancing flywheel energy storage module (FESM). The super high-speed “ironless” generator design enables a 99% energy conversion efficiency and a 90% depth-of-discharge (DOD). The design approach outlined combines the ironless stator design with a super sonic magnet array (> Mach 1.5) to achieve a cruise power output of 9 kW for 10 hours, and peak power output capability of 275 kW over the full range of the module. Analytical trade studies indicate stator power losses well below 10's of watts at cruising power, with maximum losses of 4 kW associated with 275 kW pulse output at minimum generator speed. Stress analyses indicate critical components affecting energy storage capacity are flywheel hub stress, hub-to-rotor interface stresses, and magnet contact stresses. Analysis indicates a low stress flywheel hub approach provides a flywheel specific energy of 120 Whr/kg, and unit specific energy of 84 Whr/kg. Electromagnetic analyses indicate useable specific energy of 75 Whr/kg, which is enabling for HALE UAV technology.