# Research on Magnetic Coupling Flywheel Energy Storage Device for Vehicles
## 1 Introduction  
As a green energy storage method, flywheel energy storage has attracted widespread attention and has been explored and applied in many fields, such as electric vehicles, wind power generation, uninterruptible power supply (UPS), aerospace and rail transit [1; 2; 3]. [PERSON] et al. [4] used the working characteristics of flywheel energy storage to propose an optimized charging control strategy, which effectively suppressed the influence of motor loss power and load power. [PERSON] et al. [5] proposed a microgrid coordinated control strategy based on a battery/flywheel electromechanical hybrid energy storage system, which adjusts the charging and discharging currents of the flywheel and battery according to the remaining capacity of the flywheel and battery, so as to reduce the power fluctuation in the microgrid caused by new energy access. [PERSON] et al. [6] used a flywheel energy storage system to realize AC-DC-AC conversion by using the internal frequency converter, which can urgently meet the power supply demand of the shock load, thereby preventing the ship's power grid from large frequency fluctuations and accelerating fault self-healing. [PERSON] et al. [7] proposed a two-stage flywheel energy storage system, and the feasibility of the flywheel energy storage system for electric vehicles was verified after simulation and analysis of flywheel parameters. [PERSON] et al. [8] proposed the application of flywheel energy storage technology in the field of electric vehicle braking energy recovery and increased energy utilization efficiency by storing vehicle braking energy recovery in the energy storage flywheel to provide auxiliary power when the car starts, climbs and accelerates. Studies in recent years have shown that flywheel energy storage systems have great potential for the application of electric vehicles. Automotive brake energy recovery devices are mainly divided into electrochemical energy storage and mechanical energy storage [9]. Electrochemical energy storage converts kinetic energy into electrical energy in brake recovery and stores it. [PERSON] et al. [10]optimized the braking force by hierarchical control of braking energy regeneration under the premise of considering braking intent and braking compensation. [PERSON] et al. [11] proposed a distribution strategy for the braking force of the front and rear axles after considering the regenerative braking torque as limited by the charging power of the battery. [PERSON] et al. [12] coordinated the motor feedback braking force and hydraulic braking force to meet the control requirements of the braking process. [PERSON] et al. [13] proposed a regenerative braking force distribution strategy based on a composite power supply system after combining the operating conditions of pure electric buses. However, due to the bottleneck in the development of batteries at this stage, this braking energy recovery strategy was limited by the material requirements of the vehicle. Considering mechanical energy storage devices is an effective way to improve the efficiency of braking energy recovery. Vehicle braking energy recovery technology seeks to add a set of energy storage devices to the vehicle transmission system [14; 15; 16; 17]. When the vehicle brakes, the braking energy can be recovered and stored; when the vehicle starts or accelerates, the energy stored is released, thereby improving the energy utilization efficiency. Flywheel energy storage is a common method of mechanical energy storage. The vehicle flywheel energy storage system proposed achieves the recovery and release of vehicle braking energy through the combination and separation of clutches; however, the belt drive used has the disadvantages of high wear rate, short life and large energy loss [18]. The electromagnetically coupled flywheel energy storage system proposed achieves the recovery and release of automotive braking energy through no passage of current in the electromagnetic differential clutch; however, the energy transfer path is complex, which increases energy loss [19].