Design optimization and fabrication of a hybrid composite flywheel rotor

A novel approach to composite flywheel rotor design is proposed. Flywheel development has been dominated by mobile applications where minimizing mass is critical. This technology is also attractive for various industrial applications. For these stationary applications, the design is considerably cost-driven. Hence, the energy-per-cost ratio was used as the objective function. Based on an analytical approach for calculating stresses in multi-rim hybrid composite rotors, the nonlinear optimization problem was solved using a multi-strategy optimization scheme that combines an evolutionary algorithm with a nonlinear interior-point method. The problem was solved for a sample rotor with varying cost ratio of the rim materials. Instead of an optimal solution per cost ratio, only four optimal designs were obtained with a sharp transition between designs at specific cost ratios. This sharp transition is explained by the intricate interplay that exists between the objective function and the nonlinear constraints imposed by the applied failure criteria.

Cost optimization of hybrid composite flywheel rotors for energy storage

This paper presents a multiscale modeling approach for the progressive failure analysis of carbon-fiber-reinforced woven composite materials. Hierarchical models of woven composites at three different length scales (micro, meso, and macro) were developed according to their unique geometrical and material characteristics. A novel strategy of two-way information transfer is developed for the multiscale analysis of woven composites. In this strategy, the macroscopic effective material properties are obtained from property homogenizations at micro and meso scales and the stresses at three length scales are computed with stress amplification method from macroscale to microscale. By means of the two-way information transfer, the micro, meso and macro structural characterizations of composites are carried out so that the micromechanisms of damage and their interactions are successfully investigated in a single macro model. In addition, both the nucleation and growth of damages are tracked during the progressive failure analysis. A continuum damage mechanics (CDM) method is used for post-failure modeling. The material stiffness, tensile strength and damage patterns of an open-hole woven composite laminate are predicted with the proposed multiscale method. The predictions are in good agreement with the experimental results.

A Modeling Approach Across Length Scales for Progressive Failure Analysis of Woven Composites

The applicability of a macroscopic time-temperature superposition principle (TTSP) to unidirectional composite strength is discussed based on the microscopic Simultaneous Fiber-Failure (SFF) model that has been presented by Koyanagi et al. (J. Compos. Mater. 43:1901–1914, 2009a). The SFF model estimates composite strengths as functions of fiber, matrix, and interface strengths. This paper first investigates the applicability of SFF to the complicated temperature dependence of composite strengths, i.e., one composite exhibits significant temperature dependence and another does not, considering the temperature dependence of the components, which results in successful estimations for the two composite systems used in the present study. The long-term durability predicted by the SFF and that predicted by the TTSP are then compared. They typically correspond to each other in various cases; accelerated testing methodology (ATM) employing TTSP is thus proved to be valid from the micromechanical viewpoint, assuming the SFF applicability.

Tensile strength at elevated temperature and its applicability as an accelerated testing methodology for unidirectional composites

A procedure to find the optimal design of a flywheel with a split-type hub is presented. Since cost plays a decisive role in stationary flywheel energy storage applications, a trade-off between energy and cost is required. Applying a scaling technique, the multi-objective design problem is reduced to the maximization of the energy-per-cost ratio as the single objective. Both an analytical and a finite element model were studied. The latter was found to be more than three orders of magnitude more computationally expensive than the analytical model, while the analytical model can only be regarded as a coarse approximation. Multifidelity approaches were examined to reduce the computational expense while retaining the high accuracy and large modeling depth of the finite element model. Using a surrogate-based optimization strategy, the computational cost was reduced to only one third in comparison to using only the finite element model. A nonlinear interior-point method was employed to find the optimal rim thicknesses and rotational speed. The benefits of the split-type hub architecture were demonstrated.

Cost optimization of a hybrid composite flywheel rotor with a split-type hub using combined analytical/numerical models

The effects of geometric scaling on the static and dynamic behavior of a multi-rim hybrid press-fitted composite flywheel rotor are investigated and applied to its design procedure. It is verified that the stresses among the scaled multi-rim hybrid rotors remain the same as long as press-fit interferences are linearly scaled, curing temperature remains unchanged, and the rotational speed is inversely proportional. With the linearly scaled supporting stiffness of the scaled rotor and shaft, a relationship between inversely scaled critical speeds and rotational speeds remains unchanged. Double rim rotors of 5 kWh and 100 kWh energy storage are designed by scaling an optimized 0.5 kWh multi-rim rotor. Then, the rotors of 5 kWh were manufactured by wet-winding E-glass and T-700 fibers with a low-temperature cured epoxy system.

Jae H. Kim

Papers

Design of a Hybrid Composite Flywheel Multi-rim Rotor System using Geometric Scaling Factors: