Design and fabrication of hybrid composite hubs for a multi-rim flywheel energy storage system Seong J. Kim, Khazar Hayat, Sana U. Nasir, Sung K. Ha ⇑ Dept. of Mech. Eng., Hanyang University, 1271, Sa 3-dong, Sangnok-gu, Ansan, Kyeonggi-do 426-791, Republic of Korea article info Article history: Available online 18 July 2013 Keywords: Hybrid composites Press-fit interference Finite element analysis Filament winding abstract A composite hub was successfully designed and fabricated for a flywheel rotor of 51 kWh energy storage capacities. To be compatible with a rotor, designed to expand by 1% hoop strain at a maximum rotational speed of 15,000 rpm, the hub was flexible enough in the radial direction to deform together with the inner rotor surface. This hub is also stiff in the conical deformation mode to increase the vibration fre- quency for high rotational speed. A dome type hub of carbon-glass/epoxy has been developed to be press-fitted into the rotor with interference in order to offset the hoop strain. A series of parametric study were sequentially performed to determine fiber fractions, layer thickness, winding angles, interference, and the shape of the geodesic dome contour. A safety factor of two was secured in the final design to take into consideration a long term fatigue life of the hub. The hub was fabricated by wet filament winding- process, followed by press-fitting into a surrogated rotor, which has the same inner and outer diameters and stress states as those of original rotor except height. The strains were measured during the press-fit and found to be in agreement with the stress analysis results. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Flywheels have been developed for energy storage and retrieval in various applications, including frequency regulation, uninter- ruptible power supplies, hybrid electric vehicles, and space sta- tions [1,2]. When compared to other energy storage devices (e.g., electrochemical batteries), flywheels can be viable alternatives due to a high power density, no degradation during the entire de- sign life, and superior energy discharge rates [1,2]. A flywheel rotor system consists of a metallic shaft, a rotor, and a hub. The rotors of these flywheels are made of composite materials since they usually operate at high speeds, whereas conventional metals (steel or aluminum) are seldom applicable. Low weight, high strength and stiffness, longevity, and anisotropy are characteristics of the com- posite materials, which often make them excellent candidates for the fabrication of flywheel rotors for energy storage [3–6].A flywheel rotor system is levitated mostly by magnetic bearings [7–9], which offer very low friction, enabling low internal losses and a long service life. The rotor is connected to the metallic shaft by a hub, which transmits torque between the rotor and the shaft during the fly- wheel operation. During the rotation, the hub is subjected to stres- ses due to the centrifugal forces and differential growth of the metallic shaft and composite rotor in the radial direction. As the rotational velocity increases, the hub can be subjected to the reso- nant frequencies. Thus, the hub should be stiff enough to overcome the critical speeds. Moreover, the hub should be tightly connected to the metallic shaft, which is very stiff, and the outer surface of the hub that is attached to the rotor should be flexible enough to ex- pand and hold onto the inner surface of the rotor. The flexible hub is intended to facilitate the radial deformations of the hub while securely attaching the rotor to the shaft. Such radial defor- mations cause compression at the interface between the rotor and the hub, lowering the radial tensile stresses [4]. Several investigations have been performed regarding the de- sign and material of the hub [3,4,10–14]. Xingjian et al. [3] de- signed and performed tests on a prototype flywheel system with an aluminum alloy hub formed by a thin plate and shell, where the low rigidity of the hub developed flexible modes at a lower fre- quency than the rated frequency, requiring complex methods to control the nonsynchronous vibrations of the flywheel system. Ha et al. [4] determined the effect of the hub expansion on the ro- tor performance using both conventional ring-type and newly introduced split type aluminum alloy hubs. Though the strength ratios were decreased using the split type hub, the hub weight may increase significantly for large capacity flywheels due to the high density ofaluminum. Herbst et al. [10] performed spin tests on a sub-scale model of a 10 MJ rotor with a conical steel alloy hub, and measured the rotor growth and strains; a complex struc- ture comprised of a composite arbor, a conical metallic hub, and a metallic sleeve was deployed, which connected the composite ro- tor with the metallic shaft. 0263-8223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compstruct.2013.07.032 ⇑ Corresponding author. Tel.: +82 31 400 5249; fax: +82 31 407 1034. E-mail address: sungha@hanyang.ac.kr (S.K. Ha). Composite Structures 107 (2014) 19–29 Contents lists available at ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct