Analysis of the principle and application prospect of flywheel energy storage
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Author: Wang Mingju Wang Hui
Summary: Flywheel energy storage is a high-power, fast-response, high-frequency, long-life mechanical energy storage technology, which is suitable for transportation (rail transit, automobile), emergency power supply, power grid quality management (frequency regulation) and other fields. Flywheel energy storage is an integrated technology, and high-speed, composite rotor, internal fixed and external rotation structure are its future development direction. Flywheel energy storage has broad application prospects, but it is currently in the early stage of market development.
At present, the domestic and foreign energy storage markets present the characteristics of large scale, fast speed, multiple policies and hot market. China's demand for energy storage is huge and urgent, and it is in the early stage of demonstration to commercialization, and the state has a series of policy support. The guidance issued by China's National Energy Administration proposes that by the end of the "13th Five-Year Plan", energy storage will be from engineering demonstration to the early stage of commercialization; By the end of the "14th Five-Year Plan", from the initial stage of commercialization to large-scale application. Flywheel energy storage is a kind of high-power, fast response, high-frequency, long-life mechanical energy storage technology, which has broad application prospects.
1 How flywheel energy storage works
Flywheel energy storage is an advanced physical energy storage technology originating from aerospace, which refers to the use of electric energy to drive the flywheel to rotate at high speed, convert electrical energy into mechanical energy, and generate electricity by the flywheel inertia drag motor when needed, and turn the stored mechanical energy into electrical energy output (the so-called flywheel discharge). Different from other battery technologies, its advantages are reflected in the characteristics of short-time, high-frequency, high-power charge and discharge, mainly used in power grid frequency regulation, new energy power station integration, rail transit, high-power UPS (Uninterruptible Power Supply) and other fields.
Flywheel energy storage systems typically include flywheels, motors, bearings, sealed housings, power controllers, and monitoring instruments, as shown in Figure 1.
Figure 1 Structure diagram of flywheel energy storage system
2 Analysis of flywheel energy storage technology
2.1 Technical characteristics of flywheel energy storage
Flywheel energy storage is a kind of power energy storage technology with fraction and second, high power, long life and high efficiency. Compared with other technologies, flywheel energy storage is characterized by almost no friction loss and low wind resistance. The specific power can reach more than 8 kW/kg, which is much higher than that of traditional electrochemical energy storage technology. Its life depends mainly on the fatigue life of the flywheel material and the life of the electronic components in the system. At present, the service life of flywheel energy storage can reach more than 20'a, and the service life is not affected by the depth of charge and discharge. No harmful substances are produced during operation; Virtually no maintenance is required during operation; Good adaptability to the working environment, -20~50'°C can work normally. The technical characteristics and application scenarios of flywheel energy storage are shown in Table 1.
Table 1 Technical characteristics and application of flywheel energy storage
2.2 Analysis of the development direction of flywheel energy storage
The calculation formula of flywheel energy storage is:
In equation (1), E is kinetic energy, J; J is the moment of inertia, kg·m²; ω is the angular velocity, rad/s.
It can be seen from Equation (1) that under the condition of a given rotor, the rotor mass and energy storage are positively correlated, and the speed increase and energy storage are positively squared. Rotor mass improvement and speed increase can achieve energy storage increase, and the speed increase effect is more obvious. The motor input and output power formulas are:
In equation (2), P is the power, W; Tmn is torque, N·m; Nmn is the rotational speed, r/min.
It can be seen from Equation (2) that under the condition that other conditions of the motor are established, the torque, speed and power are positively correlated with each other. The comparison of the two technical routes of increasing speed and increasing mass is shown in Table 2.
Table 2 Comparison of technical routes
Increasing the speed is an effective way to increase the energy storage capacity of the flywheel. Based on the theoretical formula, it can be seen that increasing the quality and speed of the rotor can increase the power and energy storage, but the increase of the speed has a more obvious effect on the energy storage and power, which is more compatible with the characteristics of the high-speed motor, which is the most effective technical route.
The key to speed improvement is the rotor material. Compared with alloy materials, carbon fiber has the characteristics of low density and high tensile strength, which is more suitable for matching high-speed motors to achieve high speed, high power and high energy storage of flywheel, while the edge line speed that alloy flywheel can withstand is low, which limits the improvement of product energy storage.
Therefore, the composite rotor flywheel is the main direction of future development. In terms of cost, the critical point of the cost of alloy flywheel and composite flywheel is about 53kW·h3, and the energy-based flywheel made of composite rotor has more cost advantages.
2.3 Analysis of the development direction of flywheel structure
The flywheel has two main structures, namely "internal fixed external rotation" and "external fixed internal rotation". A comparison of flywheel structures is shown in Table 3.
Table 3 Comparison of flywheel structures
It can be seen from Table 3 that the structure and technical requirements of "internal fixed external rotation" of flywheel are high, but it is easy to achieve miniaturization and high speed, which is the main development direction of flywheel in the future.
3 Application prospect analysis
Flywheel energy storage is suitable for high-power, fast response, high-frequency scenarios, typical markets include UPS, rail transit, power grid frequency regulation three major fields, in the future, there will be charging piles, construction machinery and other emerging markets.
3.1 UPS Market
Domestic and foreign data centers, communication base stations, and important activities have clear requirements for uninterrupted power supply, and the combination mode of chemical battery + diesel generator is mainly used. Compared with the current chemical battery, flywheel energy storage has the advantages of faster response, large instantaneous power, small footprint, and long service life, and is more suitable for matching with diesel generators as UPS power supply.
3.2 Rail Transit
At present, the electric energy recovered by subway trains entering the station is consumed by resistance heat dissipation, which has the problems of waste of resources and impact on the power grid. Flywheel energy storage will recover electric energy when the train enters the station, and release electric energy when the train leaves the station, playing the role of energy-saving and friendly grid. Many subway stations in the United States have demonstrated flywheel energy storage, which can achieve the effect of saving electricity by 20%.
3.3 Grid frequency regulation market
The imbalance between power generation and consumption in the grid will cause fluctuations in the frequency of the grid, and in order to smooth out this fluctuation, the grid needs to be equipped with a frequency regulation power station with 2% of the total power generation capacity. At present, China's power grid frequency regulation is mainly undertaken by generator sets, and the proportion of new energy entering the grid will increase in the future, and the demand for power grid frequency regulation will be greater. Flywheel energy storage has the characteristics of large power, fast response speed and strong cycling ability, which can quickly and effectively compensate active/reactive power with the change of the power grid, smooth the fluctuating load, buffer the transients of power generation output, and support the frequency and voltage of the power grid, which has a good application prospect.
3.4 Energy storage electric vehicle charging pile
Energy storage electric vehicle charging pile is mainly used in the occasions of high-power and fast charging of electric vehicles. Limited by the existing grid framework capacity, the construction of an electric vehicle charging station involves a series of issues such as grid capacity increase and urban planning adjustment. The high-power electric vehicle fast charging pile system with energy storage link can reduce the pressure of charging pile on the power grid capacity increase. With the magnetic levitation flywheel as the energy storage device, the typical operating characteristics of low-power charging and high-power output can not only meet the requirements of fast charging of electric vehicles, but also avoid the constraints of grid capacity increase. At the same time, due to the high power density, small size, flexible layout, green environmental protection and pollution-free energy storage flywheel, it can be arranged underground, which eliminates the impact of the construction of charging stations on the city appearance and reduces the pressure of urban construction work.
4 Current application status in China
The technical threshold of flywheel energy storage system is high, and there are still some urgent problems to be solved in composite structure technology, magnetic bearing technology, and high-speed and high-efficiency motor technology in vacuum. High speed, composite rotor, internal fixed and external rotation structure are the future development direction. Chinese enterprises have basically mastered alloy flywheel technology, but there is a significant gap between them and foreign countries in terms of structural design and composite rotor flywheel.
Foreign flywheel energy storage technology is mainly concentrated in flywheel frequency regulation power station, UPS and other fields, among which the representative is Beacon3Power (Lighthouse Power Company), its operation of flywheel frequency regulation power station is an international benchmark, New York 203MW flywheel frequency regulation power station accounts for 3% of the frequency regulation capacity in the region, but undertakes 23.5% of the frequency regulation workload in the area, frequency regulation accuracy rate of 95%; Active3Power's UPS products have been serialized, with cumulative sales of 9,003MW.
At present, domestic flywheel energy storage is in the stage of extensive experimentation, small prototypes have been successfully developed, and a number of demonstration projects have appeared. Regarding the frequent introduction of relevant support policies for flywheel energy storage, only statistics in 2019, flywheel energy storage was listed in the "First (set) Major Technical Equipment Promotion and Application Guidance Catalogue (20193 Edition)", selected into the third batch of technical catalogs of demand-side management of the Ministry of Industry and Information Technology, the 20193 Green Data Center Advanced Applicable Technology Product Catalog, and was listed in the "Made in China 2025 ——— Energy Equipment Implementation Plan", at the same time, the China Electricity Union issued a consultation on the " High-speed flywheel energy storage device for short-term high-frequency shock load". The Zhongguancun Energy Storage Alliance took the lead in formulating flywheel energy storage group standards. Judging from the frequent support policies of the state, the foundation for the start-up of the flywheel market is becoming more and more solid.
5 Summary
Among the many energy storage technologies, flywheel energy storage system has attracted much attention because of its high efficiency, large capacity, fast response and environmental friendliness. As a gradually mature energy storage technology, flywheel energy storage system has been applied to the fields including electric vehicles and electric power, gradually replacing chemical battery energy storage and becoming a force that cannot be ignored in the energy storage industry. The technical reserve of China's flywheel energy storage has been basically completed, and it is in the early stage of commercialization, and demonstration projects in the fields of high-power UPS, grid frequency regulation, and kinetic energy recovery have been promoted.
Analysis of the principle and application prospect of flywheel energy storage
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