Storing energy in a rotating flywheel, that is, storing energy in the form of mechanical energy, is a very old energy storage idea. As a simple mechanical energy storage element, it has been used by human beings for thousands of years. The flywheel was used in the making machinery of zhandai Potter, the ancient spinning wheel, the steam engine invented during the industrial revolution in the 18th century and later the automobile engine. However, some of these large and small flywheels aim at uniform speed, and some meet the needs of real-time energy storage. They store less energy and short time, so they can not be called energy storage flywheels in the real modern sense. The modern flywheel energy storage system with the purpose of large capacity and long-term energy storage appeared in the 1950s, but limited to the technical conditions at that time, the energy storage flywheel did not make great progress. Until the 1990s, the flywheel energy storage technology brought new vitality and opportunities mainly due to the breakthrough progress of the following technologies.
The appearance of various high-strength composites, such as carbon fiber and glass fiber, makes the allowable linear speed of the flywheel up to 500m / S ~ 1000m / s, which greatly increases the energy storage density of the flywheel, that is, the kinetic energy storage per unit mass.
In recent years, the development of magnetic levitation technology, combined with vacuum technology, high temperature superconducting technology and the emergence of permanent magnet materials with high magnetic energy product, has greatly reduced the mechanical friction and wind resistance of the flywheel support system and improved the flywheel speed.
The emergence of various power electronic components and high-speed motors provides great flexibility for the power conversion and control of flywheel batteries.
Among the above technologies, the development of magnetic levitation technology is an important factor to promote the research and development of energy storage flywheel. Most of the early flywheels used mechanical bearings (such as gem bearings). The friction loss of mechanical bearings is large, so the energy loss is serious. The application of magnetic bearing in modern high-performance flywheel battery makes it possible for people to use the high-speed rotating flywheel rotor to obtain the dream of large capacity and long-term energy storage.
Ultra high speed flywheel, also known as flywheel energy accumulator or flywheel battery, uses the ultra-high speed rotating flywheel to store energy, and realizes the mutual conversion of mechanical energy and electric energy through electromechanical energy conversion device. Based on its high specific energy, high specific power, high conversion efficiency between electric energy and mechanical energy, fast charging, maintenance free and good cost performance, ultra-high speed flywheel has a wide application prospect in electric vehicles, aviation, aerospace, power grid peak shaving, uninterrupted power supply of wind power generation system, military and other fields. The research shows that the vehicle weighing 2200kg needs about 78kw · h energy storage and 94kw power generation to maintain the journey of 200250km and the acceleration process of 1096km / h in 10s. In the early 1980s, Oerlikon engineering company of Switzerland successfully developed the first bus powered entirely by flywheel. The flywheel has a diameter of 1.63M and a weight of 1.5T. It operates at 3000r / min in hydrogen environment to reduce wind loss. The car has 70 passengers and a journey of about 0.8km. When parking at each stop, the flywheel will need to be charged for 2min. After years of research and testing, AFS has produced the flywheel battery car afs20, which is based on the Chrysler LHS car. It is an electric vehicle completely powered by the flywheel battery. It is driven by 20 flywheel batteries, each with a diameter of 230mm and a mass of 13.64kg. It takes 6h for the battery to be charged with mains power, while it only takes 15min for fast charging, and the driving distance of one-time charging can reach 560km.
- Working principle of super high speed flywheel
Modern flywheel energy storage system (FESS) is an energy storage device that converts energy between mechanical energy and electrical energy, so it is also called flywheel battery or electro mechanical battery. The flywheel battery stores energy in the form of the rotational kinetic energy of the flywheel rotor rotating at high speed. Similar to the traditional chemical battery, the working process of flywheel battery can also be divided into “charging” and “discharging”. However, different from the chemical battery, the “charge and discharge” of the flywheel battery is completed by the power generation / motor and the flywheel rotor connected with it, that is, the same motor works in its “electric” and “power generation” states respectively. Specifically, the working process of flywheel battery can be divided into the following three stages.
(1) When the flywheel is charging, the electric energy from the power grid is converted by power electronics to drive the motor connected with the flywheel rotor to drive the flywheel to rotate at high speed. The flywheel stores the energy in the form of electric energy, so as to complete the energy storage process of electric energy mechanical energy. At this time, the motor is in the working state of the motor.
(2) In the energy holding phase, the motor maintains almost a constant speed until a control signal for energy release is received.
(3) When it is necessary to supply power to the load, the high-speed rotating flywheel rotor is used as the prime mover to drive the motor to generate electricity, and the current and voltage suitable for the load are output through the power converter. At this time, the motor is in the state of generator, so as to complete the energy release process of mechanical energy electric energy conversion. Thus, the whole flywheel energy storage system completes the process of primary electric energy input, storage and output. Flywheel battery is generally composed of high-speed rotating flywheel rotor, motor / generator, bearing support system, power conversion device, electronic control equipment and additional equipment (such as vacuum cover and emergency standby bearing).
There are no chemically active substances and no chemical reactions in the flywheel accumulator. The flywheel in rotation is a pure mechanical motion. In the kinetic energy formula of the flywheel in rotation: J is the moment of inertia of the flywheel; ω Is the angular speed at which the flywheel rotates; M is the mass of flywheel; R is the radius of the flywheel.
There are two ways to improve the energy storage of flywheel battery: one is to increase the moment of inertia J of flywheel rotor, and the other is to increase the rotation speed of flywheel ω。
The former method is more suitable for fixed applications when there are no strict requirements on the quality and size of flywheel battery. The moment of inertia of the flywheel is directly proportional to the diameter and mass of the flywheel. When the flywheel is too large and heavy at high speed, it will be subjected to great centrifugal force, which often exceeds the ultimate strength of the flywheel material and is very unsafe. Therefore, it is limited to increase the kinetic energy of the flywheel by increasing the moment of inertia of the flywheel. The kinetic energy of the flywheel is directly proportional to the square of the angular velocity. Increasing the angular velocity of the flywheel rotation without increasing the flywheel diameter and flywheel mass can significantly improve the kinetic energy of the flywheel. The flywheel used in modern flywheel accumulator is generally made into a small flywheel with small size, light weight and ultra-high speed rotation. The speed of the flywheel can reach more than 200000 R / min. Increasing the speed can greatly improve the energy storage of the flywheel. Increasing the speed of the flywheel is an effective method to realize large-capacity energy storage. Of course, the maximum speed of the flywheel is limited by the bearing stress of the material.
1.1 energy storage and release of ultra-high speed flywheel
Flywheel stored energy:
Pc（t）= [ Jf ω （t）+ C1 ω 2（t）+ C2] ω 2（t）
Flywheel releases energy:
Pd（t）= [ Jf ω （t）+ C1 ω 2（t）- C2] ω 2（t）
Where: PC is the stored power; PD is the released power; JF is the moment of inertia of the flywheel rotor; ω Is the angular velocity of the flywheel rotor; C1 is the air resistance coefficient (generated by the air around the flywheel rotor); C2 is the rotational resistance coefficient (generated by the inertia of the flywheel rotor).
Resistance in the process of energy storage and release of ultra-high speed flywheel.
In the process of storing and releasing energy, the flywheel will be affected by the resistance of the surrounding air. Therefore, the ultra-high speed flywheel generally rotates at high speed in a sealed vacuum shell.
But the flywheel angular velocity is determined by ω Min gradually increased to ω Max, the resistance torque generated by the change of flywheel kinetic energy increases with the change of flywheel angular velocity.
1.2 moment of inertia of super high speed flywheel
Generally, the ultra-high speed flywheel mostly adopts the structural form of rotating around the vertical axis, because in the earth’s gravity field, the ultra-high speed flywheel rotating around the vertical axis is less affected by the earth’s gravity field than the ultra-high speed flywheel rotating around the horizontal axis, and the gyro effect of the ultra-high speed flywheel rotating around the vertical axis is more conducive to the stable operation of the flywheel energy accumulator.
1.3 working cycle of energy storage and release of ultra-high speed flywheel
The energy storage of ultra-high speed flywheel is an important design index. The energy storage and release working cycle of ultra-high speed flywheel. When the flywheel stores energy, the flywheel rotor accelerates, and then the flywheel keeps rotating at a uniform speed. When the flywheel releases energy, the flywheel rotor decelerates.
Under normal circumstances, if the minimum speed is selected as 1 / 2 of the maximum speed, 3 / 4 of the total stored energy can be utilized, and the discharge depth can reach 75%.
An important index to measure the performance of any energy storage device is its energy storage density, that is, the energy stored by the energy storage device per unit mass. For the miniaturization and portability of flywheel battery, it is not expected that its mass can not be very large when designing flywheel. At the same time, it is limited by working space, so it is required to have high energy storage density. In order to obtain large energy storage and high energy storage density, the material with high strength and low density is the ideal material for flywheel rotor.
Depending on the shape of the flywheel, the effective stress distribution of the flywheel (the shape of the flywheel) depends on the shape of the flywheel and the shape of the rotor (the shape of the flywheel) of the flywheel, and the effective stress distribution of the flywheel (the shape of the flywheel) depends on the shape of several materials. For the flywheel made of metal, because the metal is an isotropic material, there is no problem of equal stress design. Therefore, the shape of equal thickness disc is generally selected to facilitate processing.
- Structure of super high speed flywheel
Ultra high speed flywheel is mainly composed of flywheel, motor / generator system, rotating shaft, bearing and vacuum chamber.
2.1 flywheel material
The calculation shows that when a flywheel with a diameter of 230mm and a mass of 13.5kg reaches a specific energy of 150W · H / kg, the flywheel speed will reach 150000.2 million R / Ming. At such a high speed, the centrifugal force on the materials for manufacturing the flywheel is 12 times greater than that of steel. When the stress generated by the centrifugal force is too large, the fragments after the flywheel is broken will produce great destructive force. Therefore, the material and shape of the flywheel are the focus of the design. The shape of flywheel usually adopts the principle of equal stress design, that is, each part of flywheel rotor has equal stress. Therefore, the flywheel thickness should decrease with the increase of rotor radius. Moreover, the material of flywheel rotor is required to be absolutely uniform and balanced, and must have very good dynamic balance accuracy.
2.2 motor / generator system
There is a built-in motor in the ultra-high speed flywheel energy storage device, which is both a motor and a generator. When charging, it acts as a motor to accelerate the flywheel; When discharging, it acts as a generator to supply power to the peripherals. At this time, the speed of the flywheel decreases continuously; When the flywheel is idle, the whole device operates with minimum loss. The flywheel motor must meet the requirements of flywheel energy storage system. The specific requirements mainly include:
(1) The flywheel motor shall be reversible and can operate in both electric and power generation states.
(2) The flywheel energy storage is directly proportional to the square of the speed, so the flywheel motor is required to have a high running speed.
(3) Energy storage and release require that the motor can adapt to a wide range of speed changes.
(4) Long time uninterrupted operation requires a long service life of the motor.
(5) Long time energy storage operation requires that the no-load loss of the motor should not be too large.
(6) The motor is required to have large torque output capacity and power capacity.
(7) The motor is required to have high operation efficiency and good speed regulation performance.
Starting from the above requirements, the commonly used motors now include permanent magnet brushless motor, three-phase brushless DC motor, reluctance motor and induction motor. These motors are used in flywheel design outside China. Among them, permanent magnet brushless DC / AC motor is mostly used. It has the advantages of convenient regulation and control, wide speed regulation range, low rotor loss, easy realization of two-way power conversion, and simple structure, It can be made into various shapes according to the design requirements of flywheel battery, so it has great attraction in flywheel energy storage device, especially in flywheel system with speed above 30000r / Ming. At present, the speed of permanent magnet motor can reach 200000 R / Ming. The flywheel battery of indigo energy company adopts three-phase high-efficiency permanent magnet brushless motor, and its energy conversion efficiency is greater than 95%.
2.3 flywheel shaft support system
One of the characteristics of flywheel battery energy storage is that the flywheel still rotates at high speed for a long standby time. Therefore, to preserve the rotation function of the flywheel and eliminate the friction loss of the bearing (which is also necessary to prolong the service life of the bearing) is the key to realize the high-efficiency flywheel battery. Therefore, the bearing support system of the flywheel rotor is the key problem to realize the high-efficiency energy storage of the flywheel. The requirements of flywheel battery for bearing support system are to bear heavy load, less loss, long service life, convenient maintenance and so on. The supporting methods of flywheel battery mainly include superconducting magnetic levitation, electromagnetic levitation, permanent magnetic levitation and mechanical support, as well as their two or two combinations.
In the early flywheel battery devices, mechanical bearings such as ceramic bearings, gem bearings and rolling bearings are mostly used. Due to mechanical friction and wear, this kind of bearing mode has low speed, low service life and low operation efficiency. It is generally suitable for rapid charge and discharge systems. Therefore, most modern new flywheel energy storage systems do not use mechanical bearings. However, because of its simple, compact and firm structure, it is generally used as the auxiliary bearing of flywheel rotor in emergency.
In recent years, represented by the ultra-high speed maglev train, the technology of suspending objects by magnetic force has tended to the practical stage. For the rotating body, instead of the past mechanical bearing, the use of non-contact magnetic bearing has become an ideal choice for the rotor support system of flywheel battery. The magnetic bearing technology has the following characteristics.
(1) Non contact, no wear, long service life and unchanged working performance.
(2) There is no need for lubrication and lubricating medium, so there is no need for pumps, pipes, filters and seals, and the environment will not be polluted due to lubricant leakage. It can work in special environments such as high temperature or very low temperature (- 253 ℃ ~ 450 ℃).
(3) The speed of the magnetic suspension flywheel is only limited by the centrifugal force of the rotor, and the circumferential speed is high. Therefore, the ratio of rotor angular momentum to mass can be greatly improved, so as to reduce the mass of the flywheel.
It is a very good choice to use magnetic bearing as the supporting element of energy storage flywheel. In recent years, the vigorous development of energy storage flywheel research and the improvement of performance also benefit from the use of magnetic bearing to a great extent. The development of magnetic bearing opens up a new way for the research and development of energy storage flywheel.
2.4 vacuum environment
When rotating at an ultra-high speed of 200000 R / Ming, the air around the flywheel will form a strong eddy current, resulting in huge air resistance and loss of flywheel energy, which is very unfavorable to the movement of the rotor. In order to reduce the wind loss in the flywheel chamber, the modern high-speed flywheel accumulator operates in a highly sealed environment. The vacuum degree of the vacuum container is below 10-3pa ~ 10-4Pa. The key is to solve the sealing of the vacuum container to prevent external gas from penetrating into the vacuum container and solve the gas escaping from the materials of the flying wheel system, which will also destroy the vacuum degree of the vacuum container.
2.5 power conversion device
The function of the power conversion device is to control the motor and realize the mutual conversion of electric energy and mechanical energy. When the flywheel stores energy, the electric energy of the AC distribution system is converted into DC through the rectifier, and then the power conversion device controls the acceleration of the motor according to the principle of constant torque or constant power. After reaching a certain speed, turn to the low-voltage mode, and the power conversion device provides a low-voltage to offset the motor loss and maintain the speed of the flywheel. During energy release, the motor operates as a synchronous generator. The generated electric energy is first rectified into DC, and then transformed into stable DC by the voltage stabilizing device. The electric energy is transformed into AC by the inverter and transmitted to the AC distribution system. If the distribution system is a DC distribution system, the inverter link connected to it is not required.
2.6 additional equipment
Additional equipment includes auxiliary bearings and vacuum chambers. Among them, mechanical auxiliary bearings are also called holding bearings or landing bearings. Auxiliary bearings are used for two reasons:
(1) During schedule maintenance, the electromagnetic bearing does not work, but to keep the rotor rotating at low speed, auxiliary bearings are needed to support the rotor.
(2) During operation, the magnetic bearing is suddenly powered off for some reason. For safety reasons, the auxiliary bearing is required to temporarily support the rotor until the rotor stops completely.
In order to reduce wind loss and prevent safety accidents of high-speed rotating flywheel, the flywheel system is generally placed in a high vacuum sealed casing. Although increasing the vacuum can reduce the wind loss, the heat dissipation function is weakened due to the thin gas environment, and the temperature of the rotor is easy to rise.
- Flywheel battery characteristics
As a new member of the battery family, this new kinetic energy battery has the following outstanding advantages compared with the traditional chemical battery.
(1) High energy conversion efficiency, working efficiency up to 90%.
(2) The energy storage density is high, the energy storage density is greater than 0W · H / kg, and the power density can reach 5000W / kg.
(3) Long cycle life, no overcharge and over discharge problems. Chemical batteries generally cannot be deeply discharged or overcharged, otherwise their service life will decline sharply. When the flywheel battery is deeply discharged, its performance is not affected at all, and it is very easy to prevent overcharge (actually limiting the maximum speed of the rotor) with the help of power electronic devices. The service life of flywheel battery mainly depends on the service life of its power electronic device, so it can run repeatedly for millions of times, generally up to about 20 years.
(4) It is easy to measure the discharge depth and the charging time is short. As long as the flywheel battery measures the rotating speed of the rotor, it can accurately know the discharge depth, while the chemical battery is not so easy. In addition, the charging of a flywheel battery is generally completed in a few minutes, while the chemical battery takes a few hours, usually seven or eight hours.
(5) It is not sensitive to temperature. The performance of chemical battery will decline sharply at high or low temperature, while that of flywheel battery is not.
(6) Friendly to the environment, chemical batteries will have a bad impact on the environment after scrapping, and the recovery cost is high. Flywheel battery is a kind of green battery, which will not have any impact on the environment, so it has great potential in the application of electric vehicles.
(7) Low loss, unique bearing system and vacuum working environment make the mechanical loss negligible, and other losses can also be minimized to a negligible level through design.
Of course, flywheel energy storage system also has its limitations, such as high manufacturing cost, poor stability, poor seismic performance and so on. However, with the continuous development and research of flywheel energy storage technology, these shortcomings will be gradually improved and solved.
- Problems and solutions of ultra-high speed flywheel
Different from the fixed energy storage device, the ultra-high speed flywheel currently faces two major problems. Firstly, when the running direction of the system changes (such as turning or bumping, deviating from the straight line), the flywheel will produce gyro torque, which will seriously affect the handling performance of the system; Secondly, when the flywheel fails, the energy stored in the flywheel in the form of mechanical energy will be released in a short time, and the high-power output will cause great damage to the system and the use matrix. For example, if 1kW · h flywheel fails, 7203600kw power output will be generated within 15s. Therefore, fault suppression has always been a huge obstacle to the use of ultra-high speed flywheel. A simple measure to reduce the gyro torque is to use multiple small flywheels and connect them into groups. Half rotate clockwise and the other half rotate counterclockwise. In theory, the total gyro torque acting on the operating system is zero. But in fact, there are still many problems in the distribution, arrangement and coordination of these flywheels. Moreover, the total specific energy and specific power of these flywheels may be less than that of a single flywheel. Although this method has been applied, it needs to be improved. Another new suppression measure is to increase the thickness of the flywheel rotor edge instead of reducing the flywheel edge thickness according to the equal stress design principle. When the flywheel rotor fails, the thicker part of the rotor edge will fall off first and act as a fuse. The flywheel rotor named “Kavlar” invented abroad uses carbon fiber reinforced epoxy resin composite with high strength and low density. When the flywheel is damaged by centrifugal force, this material will disperse into flocculent fluff and will not cause harm. Therefore, selecting super strong composite material is one of the safety measures to solve the problems faced by the flywheel.