VP7-07 A Flywheel Energy Storage System with Double High-Strength Steel Rotors
Creators: Pingli Zhang
Jiangtao Yang
Zhenyu Wang and Shoudao Huang
Publisher: name: IEEE
Year: 2025

Flywheel energy storage (FES) has attracted much attention due to its merits of no environmental pollution, fast response time, high power density, and high reliability [1]. However, the idling loss caused by the stator core loss would reduce the energy conversion efficiency, especially for the long idling operation state. Addressing this issue, this paper proposes a novel FES system (FESS) with double high-strength steel rotors, as shown in Figs. 1(a) and 1(b). The inner and outer rotors are connected to each other and rotate synchronously. The inner surface of the outer rotor is equipped with halbach array permanent magnets. The armature windings are placed on the stator processed from nylon material. Meanwhile, the rotors are supported by magnetic bearing. It has the merits of low idling loss, high charge/discharge efficiency, and high energy storage density. The key structure parameters of the electrical machine are optimized and electromagnetic performance is analyzed. Due to the ironless structure of stator, the air-gap length of the proposed machine is larger than of the electrical machine with the iron core structure [2][3]. The amplitude of the air-gap flux density is unequal along the radial direction. To accurately calculate the electromagnetic performance of the electrical machine, an analysis is proposed and analyzed. Figs. 1(c) and 1(d) give the back electromotive force (Back-EMF) and electromagnetic torque obtained by analytical and finite analysis methods. The amplitudes of the Back-EMF are 228V and 226V, and the average torques are 7.75 Nm and 7.62 Nm. It can be found that the results obtained by the analytical calculations are well coincide with those obtained by the finite analysis method. The magnetic bearing is designed and the magnetic force is analytically calculated using the Maxwell tensor method. The magnetic bearing provides the magnetic force that counteracts the gravity of the rotors to reduce friction loss in mechanical bearings. Finally, a prototype of the proposed FESS with a 24-slot and 8-pole electrical machine is being manufactured. The machined outer rotor with halbach array permanent magnets is shown in Fig. 2. The tested results will be presented in full paper.References: [1] Z. Zhang, C. Wang and W. Geng, "Design and Optimization of Halbach-Array PM Rotor for High-Speed Axial-Flux Permanent Magnet Machine With Ironless Stator," in IEEE Transactions on Industrial Electronics, vol. 67, no. 9, pp. 7269-7279, Sept. 2020, doi: 10.1109/TIE.2019.2944033. [2] Z. Wang, J. Yang, H. Chen, C. Ye and S. Huang, "Electromagnetic Performance Analysis of a Dual-rotor Ironless Stator PM Machine," 2022 International Conference on Electrical Machines (ICEM), Valencia, Spain, 2022, pp. 874-879, doi: 10.1109/ICEM51905.2022.9910693. [3] Z. Q. Zhu, D. Howe, and C. C. Chan, “Improved analytical model for predicting the magnetic field distribution in brushless permanent-magnet machines,” IEEE Trans. Magn., vol. 38, no. 1, pp. 229–238, Jan. 2002.
Flywheel energy storage (FES) has attracted much attention due to its merits of no environmental pollution, fast response time, high power density, and high reliability [1]. However, the idling loss caused by the stator core loss would reduce the energy conversion efficiency, especially for the long idling operation state. Addressing this issue, this paper proposes a novel FES system (FESS) with double high-strength steel rotors, as shown in Figs. 1(a) and 1(b). The inner and outer rotors are connected to each other and rotate synchronously. The inner surface of the outer rotor is equipped with halbach array permanent magnets. The armature windings are placed on the stator processed from nylon material. Meanwhile, the rotors are supported by magnetic bearing. It has the merits of low idling loss, high charge/discharge efficiency, and high energy storage density. The key structure parameters of the electrical machine are optimized and electromagnetic performance is analyzed. Due to the ironless structure of stator, the air-gap length of the proposed machine is larger than of the electrical machine with the iron core structure [2][3]. The amplitude of the air-gap flux density is unequal along the radial direction. To accurately calculate the electromagnetic performance of the electrical machine, an analysis is proposed and analyzed. Figs. 1(c) and 1(d) give the back electromotive force (Back-EMF) and electromagnetic torque obtained by analytical and finite analysis methods. The amplitudes of the Back-EMF are 228V and 226V, and the average torques are 7.75 Nm and 7.62 Nm. It can be found that the results obtained by the analytical calculations are well coincide with those obtained by the finite analysis method. The magnetic bearing is designed and the magnetic force is analytically calculated using the Maxwell tensor method. The magnetic bearing provides the magnetic force that counteracts the gravity of the rotors to reduce friction loss in mechanical bearings. Finally, a prototype of the proposed FESS with a 24-slot and 8-pole electrical machine is being manufactured. The machined outer rotor with halbach array permanent magnets is shown in Fig. 2. The tested results will be presented in full paper.References: [1] Z. Zhang, C. Wang and W. Geng, "Design and Optimization of Halbach-Array PM Rotor for High-Speed Axial-Flux Permanent Magnet Machine With Ironless Stator," in IEEE Transactions on Industrial Electronics, vol. 67, no. 9, pp. 7269-7279, Sept. 2020, doi: 10.1109/TIE.2019.2944033. [2] Z. Wang, J. Yang, H. Chen, C. Ye and S. Huang, "Electromagnetic Performance Analysis of a Dual-rotor Ironless Stator PM Machine," 2022 International Conference on Electrical Machines (ICEM), Valencia, Spain, 2022, pp. 874-879, doi: 10.1109/ICEM51905.2022.9910693. [3] Z. Q. Zhu, D. Howe, and C. C. Chan, “Improved analytical model for predicting the magnetic field distribution in brushless permanent-magnet machines,” IEEE Trans. Magn., vol. 38, no. 1, pp. 229–238, Jan. 2002.