As an energy storage device, the main technical of flywheel battery indicators are: extractable energy; charge and discharge voltage; charge rate or power and discharge rate or power. The extractable energy of the flywheel battery is related to the maximum safe operating speed of the flywheel, the minimum stable operating speed and the moment of inertia of the flywheel. The charging rate of the flywheel battery mainly affects the charging time. The higher the charging rate, the shorter the charging time. The size of the discharge rate determines the ability of the flywheel battery to drive the load. The greater the discharge rate, the greater the power to drive the load.
The main performance indicators to measure the flywheel battery are: energy conversion efficiency; energy storage density; idle loss; operating temperature, life, reliability, safety, etc. The higher the energy conversion efficiency, the smaller the loss of mutual conversion between kinetic energy and electrical energy, and the better the economy. The energy storage density of a flywheel battery generally depends on the tensile strength of the flywheel material. The higher the tensile strength, the higher the speed at which the flywheel can operate safely. And high reliability and high safety are the goals pursued by all instruments and equipment.
Generally speaking, in addition to meeting the main technical indicators proposed by users, a good flywheel battery should also have low idle loss, long life, high energy density, high energy conversion efficiency, high reliability, and high Safety and good economy. Some of these technical performance indicators are reflected by some components of the flywheel battery alone, and some are reflected by several or all components.
1. Composition of flywheel battery
A typical flywheel energy storage system is generally composed of three main bodies, two controllers and some auxiliary parts: energy storage flywheel, integrated drive motor, magnetic suspension support system, magnetic bearing controller and motor variable frequency speed regulation controller and auxiliary. pieces.
2. How the flywheel battery works
A flywheel battery is similar to a chemical battery, and it has the following two working modes.
(1) "Charging" mode of the flywheel battery. When the plug of the flywheel battery charger is inserted into the external power socket, turn on the start switch, the motor starts to run, absorbs electric energy, and increases the speed of the flywheel until it reaches the rated speed, and the motor controller cuts off the connection with the external power supply. During the entire charging process, the motor acts as a motor.
(2) The "discharge" mode of the flywheel battery. When the flywheel battery is connected to a load device, the generator starts to work and supplies power to the outside, and the speed of the flywheel decreases until it drops to the minimum speed, and the motor controller stops discharging. During discharge, the motor acts as a generator.
The two working modes of the flywheel battery are all completed by the motor controller.
The rotor of the flywheel battery is supported by the magnetic bearing without contact when it is in motion, and the contact bearing is mainly responsible for the auxiliary support when the rotor is stationary or there is a large external disturbance, so as to avoid the direct collision between the rotor of the flywheel battery and the stator and cause catastrophic damage to the vacuum. The equipment is used to keep the inside of the shell always in a vacuum state and reduce the wind consumption of rotor operation. The flywheel battery cooling system is responsible for cooling the motor and magnetic bearings. Safety containers are used to avoid accidents if the rotor bursts or the stator collides with the rotor. The display meter is used to display the remaining power and working status.
The flywheel battery is an energy storage device. When "charging", the motor gradually increases the speed of the flywheel rotor through variable frequency speed control, converting the electrical energy into the kinetic energy of the flywheel and storing it; Stable output of electrical energy, so that the speed of the flywheel gradually decreased.
3. Flywheel battery system structure
The overall structure scheme of the flywheel battery system is closely related to the structure of the flywheel, the support scheme of the flywheel battery rotor, the structure of the integrated motor/engine and some other accessories. It stands to reason that the flywheel battery should be determined after the structural scheme of all parts is formed.
A new type of magnetic levitation support system is proposed for the fixed application of the flywheel battery, and the structural scheme of the flywheel battery is constructed considering the structure and arrangement of the key components inside the flywheel battery. The biggest feature of this scheme is that the magnetic suspension support system of the flywheel battery adopts a new type of magnetic bearing as the radial support of the rotor, and combines the axial hydromagnetic magnetic bearing to form a non-contact magnetic suspension support for the flywheel rotor.
For the flywheel battery used in fixed applications, since there is no gyroscopic effect, the bearing mainly bears the dead weight of the rotor of the flywheel battery. In order to make full use of the ability of the hydromagnetic bearing to withstand static or stack static loads, the flywheel rotor is arranged vertically, and its weight is completely borne by the hydromagnetic bearing, while the radial electric magnetic bearing is mainly used to withstand the radial displacement of the rotor due to the rotor. The dynamic load generated by the movement, and to ensure the self-centering effect of the rotor of the flywheel battery during the movement, that is, to ensure that the rotor always rotates near the predetermined center.
Compared with other magnetic suspension support systems, the combined magnetic suspension support system of the flywheel battery uses neither electromagnetic bearings nor superconducting magnetic bearings, thus eliminating the failure of electromagnetic bearings due to the need for position sensors and feedback control systems. The possibility and cost increase, as well as the energy loss of the electromagnetic coil; it also avoids the increase in the structural size of the flywheel battery superconducting magnetic bearing due to the increase in the structure size and the failure of the refrigeration equipment due to the need for liquid nitrogen preparation equipment. Reduced reliability and increased cost, The support system of the flywheel battery is currently the most economical and reliable support system, so it is worthy of vigorous research and development.
4. The flywheel battery supports the magnetic bearing of the flywheel rotor
The magnetic suspension bearing in the flywheel battery is basically the same as the magnetic suspension bearing used in the machine tool spindle, and both are used to support the rotor rotating at high speed, but the work requirements are very different.
For the magnetic bearing supporting the spindle in the machine tool, it mainly bears the radial force and axial force acting on the rotor of the flywheel battery from the cutting tool, as well as the weight of the rotor of the flywheel battery itself, which requires the magnetic bearing to have high rigidity and adjustable. Therefore, active magnetic bearings, namely electromagnetic bearings, must be used, and the bearings are also required to precisely position the spindle to ensure the accuracy of machining. As for the magnetic suspension bearing in the flywheel battery, the flywheel battery mainly bears the weight of the flywheel rotor itself and the dynamic load caused by the offset movement of the rotor, as well as the additional gyroscopic force caused by the movement of the foundation or frame for fixing the flywheel battery, in addition , the flywheel battery rotor does not need precise positioning. If the flywheel battery is used in applications where the frame is fixed, the flywheel battery rotor will not be affected by gyroscopic effects. Current flywheel energy storage systems often choose a combination of several types of bearings, and in high-speed flywheel systems the most The magnetic bearing is still the magnetic suspension bearing. This is because the magnetic bearing has no wear, long life and maintenance-free, which makes the traditional mechanical contact bearing incomparable. At present, the magnetic bearings of the flywheel battery that can be used mainly include: electromagnetic bearing superconducting magnetic bearing; Permanent Magnetic Bearings and Electric Magnetic Bearings. The former is called active magnetic support, and the latter three are called passive magnetic support.
The flywheel battery electromagnetic bearing is to control the size of the suspension force by changing the current of the electromagnet coil, so as to adapt to the change of the external interference force; the flywheel battery superconducting magnetic bearing uses the Meissner effect exhibited by the superconductor below the critical temperature, namely Magnetic lines of force cannot pass through superconductors and show complete diamagnetism to suspend objects; flywheel battery hydromagnetic bearings use the attraction or repulsion between water magnets or between water magnets and soft magnets to suspend objects. In some occasions where the rotational speed is not too high, in addition to the magnetic bearing, an axial ball bearing is sometimes used, but the material of the ball bearing is generally made of a material with a very small friction coefficient and a very wear-resistant material, such as cermet or ruby, etc. .
5. Combined magnetic suspension support system
At present, there are two main types of support system solutions for flywheel batteries: one is an active magnetic suspension support system with electromagnetic bearings; the other is a passive magnetic suspension support system. There are three main combination schemes for the active magnetic suspension support system of the flywheel battery: the first is to use electromagnetic bearings in the radial direction, and passive magnetic bearings or mechanical contact bearings in the axial direction; the second is to use electromagnetic bearings in the axial direction and passive magnetic bearings in the radial direction. ; The third is the use of active magnetic bearings in both radial and axial directions. Since the active magnetic suspension support system of the flywheel battery contains electromagnetic bearings, although the bearing capacity is high and the stability is good, it has the aforementioned shortcomings. Static load, the advantages of electromagnetic bearings cannot be reflected, but the disadvantages become particularly prominent. Therefore, it is not necessary to use an active magnetic suspension support system for the fixed application of the flywheel battery. The passive magnetic suspension support system of the flywheel battery currently has three combination schemes: the first is to use passive magnetic bearings in the radial direction and mechanical contact bearings in the axial direction; Magnetic bearing; the third is the use of permanent magnetic bearings in the axial direction, and superconducting magnetic bearings in the radial direction. For the first support method of the flywheel battery, although the support is reliable, due to the use of mechanical contact bearings, the maximum speed of the bearing will be reduced, and the energy loss will be large and the service life will be low. For the second and third support methods of the flywheel battery, due to the use of superconducting magnetic support, it is necessary to add a liquid nitrogen refrigeration device, which increases the volume of the flywheel battery and reduces the reliability and economy of the flywheel battery system.
Read more: Rectifier in wind power systems