Colloquia

Summary

The increasing usage of renewable energy resources, even though they have a fluctuating yield, causes stability problems in the energy grid. With a continuously growing demand for electrical energy, energy storage systems have to be called upon to compensate for the growing difference between energy supply and demand by acting as a buffer. The idea of producing a compact, modular, ultra-high speed flywheel energy storage system (FESS) has come to existence in an effort to increase safety, reliability, and energy density, and make it a viable energy storage system. The flywheel rotor, which stores the energy in a rotational motion, is of critical importance to the proposed system. The implementation of a high-strength rotor retaining sleeve has to prevent the rotor from disintegrating under the high inertial loads. The rotor sleeve is proposed to be made out of thermoplastic matrix material impregnated with stiff fibre reinforcements due to its high strength to weight ratio, low brittleness, and good fracture toughness properties. Defect formation in the thermoplastic composite material, which can have detrimental effect on rotor sleeve performance, has not been researched in literature under small radius geometrical conditions (D<30mm). Therefore, this study focusses on the formation of these defects in thermoplastic composite materials during the wrapping process of small diameter rotors.

In this study, an instrumented setup is developed to investigate the formation of waviness in a rotor-like geometry for a single composite ply, with different diameters, and different processing parameters. Micro-defects in the material such as resin percolation leading to resin rich regions, loose fibres, fibre nesting, and fibre waviness are observed in processing. Fibre waviness is identified as most detrimental to performance, and is therefore characterized in detail. The characteristics of the fibre waviness at inner-radius surface is found to be a function of tape tension. Outer-radius surface, however, shows no sign of periodic fibre waviness due to geometrical constraints. The waviness characteristics identified in the as-received material, particularly the wavelength, is found to be important since this gets transferred to the final part. It is also found that lower pressure, created by reduced tape tension, results in larger amount of waviness formed in the specimens. In the future, this information can be used to determine performance of the wrapped composite rotors. Finally, the knowledge of the process of waviness formation during wrapping can be used to optimize the manufacturing process and material to obtain minimal defects in the parts.