Energy storage flywheels are quickly becoming an attractive alternative to electrochemical batteries. Their advantages of long cycle life, high turnaround efficiency, and insensitivity to operating temperatures have led to growing interest in terrestrial and space applications, including International Space Station and low-Earth-orbit satellites. This effort to develop composite arbor technology greatly increases the flywheel rotor specific energy potential and has the added advantage of relieving the high radial compressive stresses that limit durability, life, and operating temperature range.
Composite flywheels store energy by rapidly spinning a large wheel to ultrahigh speeds. This is made possible by the large load-carrying capacity of high-strength filament-wound composites (FWC). These composites are wound into large rings, focusing mass at the outer periphery to maximize the stored rotational energy. These large spinning rims must be mechanically connected to shafts, bearings, and motors and must interface with these metallic components without compromising on specific energy. Traditionally, the composite rims are pressed over metallic hubs, which also act as shafts for bearings and motors. Work has been ongoing at the University of Texas, Center for Electromechanics (CEM), under NASA Research Announcement funding to improve the interface between these rims and shafts. Composites are being used to traverse the small monolithic shaft to a larger rim while minimizing compressive stresses and achieving the full-speed operating temperature range of -45 to 85 °C necessary for satellite applications.
Prototype test rotor being lowered into the armored spin pit.
The arbor itself is an intricately wound FWC tube (see the preceding illustration) smaller at one end to accept the metallic shaft and larger at the other end to interface with the main energy storing element, the composite rim. The shear, transverse, and hoop properties are traded off throughout the profile to efficiently optimize mechanical safety factors throughout. Plastic deformation was removed from the design by tailoring the winding angles and thicknesses of the helical layers. Both high-strength carbon fiber with tensile strength approaching 106 psi and intermediate strength carbon and glass fibers were interspersed in the arbor and rim builds to optimize mechanical properties in corresponding areas and stress states.
Conceptual cutaway view of composite arbor flywheel rotor.
The recently completed year two of this 3-year development program saw several test arbors built and tested to anchor anisotropic modeling techniques and characterize material design allowables. Finally a prototypical arbor/rim spin test rotor (see the preceding photograph) was spun to burst speed. The rotor was tested on September 3, 2003, and successfully reached 1337 m/sec (2990 mph) tip speed, corresponding to 60,070 rpm for the 16.7-in.-diameter wheel, which stored 1.4 kW·hr at that speed. This represents the highest known attained speed in any useable flywheel configuration. The specific energy of this rotor was 73 W·hr/kg, which rivals most present-day electrochemical battery units. Under year three of this program, further increases in specific energy will be realized as the shaft, arbor, and rim are optimized for additional reduced weight. The analytical results agreed quite well with the strain response measured with eddy current and laser proximity probes around the rim and arbor (see the graph).
Analysis results and laser probe growth data of arbor during low-speed test run to 50,000 rpm.
Long description.
A further benefit of this program is a thorough understanding of the composite arbor's fundamental scalability, which will permit this technology to be useful in a broad range of applications. This technology is being used in a next-generation flywheel battery being designed and built at the NASA Glenn Research Center. In addition, a duplicate prototype arbor is being shipped to Glenn for cyclic spin testing and life verification.
Glenn contacts: Kevin E. Konno, 216-433-8373, Kevin.E.Konno@nasa.gov; Kerry L. McLallin, 216-433-5389, Kerry.L.McLallin@nasa.gov; and Raymond F. Beach, 216-433-5320, Raymond.F.Beach@nasa.gov
CEM contact: Richard C. Thompson, 512-232-1615, r.thompson@mail.utexas.edu
Authors: Kevin E. Konno and Richard C. Thompson
Headquarters program office: OAT
Programs/Projects: Propulsion and Power, ISS
Last updated: January 21, 2005
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