Design and experimental study of flywheel energy storage system

China energy storage Network News: Chen Daixingjian, Jiang Xinxin, Wangqiunan, Wangyong, Wangshanming, and Li Guoxian, researchers of the Department of Electrical Engineering of Tsinghua University and Tsinghua University, wrote in the 21st issue of the Journal of Electrical Technology in 2017. A 1MW/60MJ flywheel energy storage system was developed for the drilling rig hybrid drive system. Experimental research on dynamic balance, charge and discharge, loss and efficiency of axial system was carried out.

The high strength alloy steel variable section flywheel is used to store kinetic energy. The flywheel motor shaft is a vertical support structure. The heavy assembled permanent magnetic ring bearing bears 97 % of the axle weight. The rotor of permanent magnet motor adopts germany-shaped Silicon steel embedded with magnetic steel to achieve high speed and safe operation.

Based on the high-capacity power electronic transformation and motor control technology, the hardware and software of the control circuit for 100-300kW charging and 500-1000kW discharge were developed. In the process of charging, the double-closed loop control strategy of weak magnetism, speed and current was adopted. Voltage closed-loop and current feed-forward control strategies are used in discharge process. After dynamic balance, the vibration of flywheel energy storage unit is reduced to within 0.07 mm. The charging and discharging efficiency of the flywheel energy storage power supply reaches 86 % -88 %.

The basic principle of flywheel energy storage is the conversion between electrical energy and kinetic energy of the rotating body: In the energy storage stage, the flywheel is accelerated by the motor to convert electrical energy into mechanical energy; During the energy release phase, the motor operates as a generator, the flywheel motor slows down, converts mechanical energy into electrical energy and outputs.

Modern flywheel energy storage system integrates advanced composite rotor technology, magnetic bearing technology, high-speed motor and power electronics technology to greatly improve performance. Around 2000, modern flywheel energy storage products began to be promoted to uninterrupted power supply, power quality management, and power grid FM. The flywheel storage energy applications under development include wind power generation and solar power generation. In recent years, China has made breakthroughs in the research and development of key technologies for flywheel energy storage.

Oil drilling rig is the main equipment for drilling construction. Its power is usually provided by diesel or natural gas engine units. The output characteristics of the power unit are not flexible enough to adapt to the frequent and large fluctuations of the rig load, and the power system has more power redundancy capacity.

The peak adjustment motor and flywheel energy storage device are introduced into the drilling power system to realize the peak adjustment operation. At low load, using the redundant power of the power unit to drive the peak adjustment motor to generate electricity and charge the flywheel energy storage system; When peak load occurs, the flywheel energy storage system discharges, drives the peak regulating motor to do electric operation, and rapidly releases power to the power system to balance the load. The power unit running on peak is running smoothly, and can reduce redundant capacity, which is conducive to energy conservation and emission reduction.

The technical indicators of the energy storage unit proposed by the rig hybrid transmission system are: 100-300 kW of charging power, 500-1000 kW of generating power, 42 MJ of usable energy, and 3-5 min of frequent charge and discharge cycle. The research shows that no mature products can be provided by the manufacturers. For this purpose, a 1MW/60MJ flywheel energy storage system has been developed, and an experimental study on axial dynamic balance, charge and discharge, loss and efficiency has been carried out.

conclusion

Using the finite element strength analysis method, the lattice structure of variable section large alloy steel Flywheel and embedded magnet steel permanent magnet motor rotor is modeled and optimized. After optimized design, the bearing capacity of the assembled permanent magnetic bearing reaches 50 kN, which is equivalent to the weight of the flywheel motor shaft system.

Based on high-capacity power electronic transformation and motor vector control technology, the control circuit hardware and software for small power charging and high-power power generation are developed. The combined control strategies of weak magnetism, current feed-forward, voltage closed-loop and speed closed-loop are adopted. The vibration of the flywheel energy storage unit was reduced to within 0.02 mm in the high rotation speed area after the maneuver balance, and the whole speed range was realized to run smoothly.

The theoretical analysis of mechanical loss and iron loss of motor is consistent with the experimental results. The system standby loss is 18 kW, which is 2 % of the maximum power of power generation. The charge and discharge cycle efficiency of the flywheel energy storage system is 86 % -88 %. Experimental results show that flywheel energy storage is a high-power and high-efficiency energy storage technology.

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