- Structure of flywheel energy storage device
Flywheel energy storage device, also known as flywheel battery, mainly involves flywheel technology suitable for high-speed working environment, high-efficiency motor technology that realizes mutual conversion between electrical energy and mechanical energy, and power converter technology that realizes switching between various working modes. The flywheel energy storage device obtains electrical energy from the power source, the motor drives the flywheel to rotate, and stores energy in the form of mechanical energy. When the flywheel accumulates energy, the rotational speed increases, and when the energy is released, the rotational speed decreases. The reduced mechanical energy is converted into electrical energy by the generator, and the output circuit generates electricity. The electrical energy of the machine is output to the load, the principle is shown in Figure 1, and Figure 2 is the flywheel energy storage system designed by NASA.
At present, the flywheel energy storage system is mainly composed of four parts: rotor system, motor/generator, input/output circuit and vacuum chamber.
(1) Rotor system
The rotor system includes a flywheel body and a support.
The flywheel body is based on the design principle of the maximum specific strength σb/ρ (σb is the material strength limit, ρ is the material density) required by the flywheel material. Generally, super-strength glass fiber (or carbon fiber, etc.) – epoxy resin composite material is used as the flywheel material. There is also a small amount of literature on the use of aluminum alloys or high-quality steel to make flywheels. From the flywheel shape, there are single-layer cylindrical, multi-layer cylindrical, spindle-shaped, umbrella-shaped, solid disc, belt-type variable inertia and spoke-shaped.
The University of Maryland in the United States has successfully researched a multi-layer cylindrical flywheel that can store 20kW·h of energy. The flywheel material is carbon fiber epoxy resin composite material. The specific parameters are: outer 10.564m, A diameter 0.254m, thickness 0.553m, weight 172.8kg, maximum Speed 46345r/min.
The Texas Center for Superconductivity at the University of Houston in the United States is devoted to the development of a spindle-shaped flywheel, which is an iso-stress design with a shape factor equal to or close to 1. The material is also made of glass fiber composite materials. The 30.48cm American Satcon Technology Company developed an umbrella-shaped flywheel. This structure is conducive to the placement of the motor, which is very beneficial to the stability of the system.
The Department of Mechanical Engineering of Shiraz University in Iran has developed a belt-type variable inertia flywheel for electric vehicles, the purpose of which is to save energy and stabilize the system. There are four main supporting methods for supporting the flywheel: superconducting magnetic suspension, electromagnetic suspension, permanent magnetic suspension and mechanical support, and some combinations of two of the four are also used.
a. There are many research institutes using this method for superconducting maglev, such as Japan’s Mitsubishi Heavy Industries, the United States’ Argonne National Laboratory, etc., but the largest one is Germany, which is developing 5MW·h/100MW superconducting flywheel energy storage power station.
b. Electromagnetic Suspension The University of Maryland has been engaged in the development of electromagnetic suspension energy storage flywheels for a long time, using differentially balanced magnetic bearings, and has completed the development of flywheels with energy storage of 20kW·h, with a system efficiency of 81%. In addition, Lawrence National Laboratory has also carried out electromagnetic suspension. Flywheel research work.
c. Flywheels with mechanical support are generally used in fast charging and discharging systems, such as electromagnetic guns and electrochemical guns developed by Kaman Electromagnetic Company in the United States, which require a discharge current of 200kA to be generated within a few seconds to meet the needs of the load. . The flywheel battery used in hybrid vehicles developed by Newcastle University in the United Kingdom, and the advanced aircraft attitude control system developed by the American Satcon Technology Company have all adopted this support method.
d. The superconducting magnetic suspension is mixed with the permanent magnet support. The University of Houston has used this support method to float the 19kg flywheel rotor. The permanent magnet bearing provides the suspension force, and the superconducting bearing is used to eliminate the inherent magnetic and magnetic instability of the system. . Tests show that under vacuum of 0.93Pa, the power consumption per hour of the hybrid support is less than 5%.
e. Mixing of permanent magnet suspension and mechanical support, the University of Washington in Seattle, USA, is developing 1kW·h hydromagnetic suspension and gem bearing hybrid support flywheel. The permanent magnet suspension is used for the upper support of the vertical rotor and unloaded to reduce the friction power consumption of the lower support; the gem bearing is used as the lower support, and radial electromagnetic support is introduced at the same time as the active control of vibration to ensure the stability of the system.
From the perspective of system structure and power consumption reduction, foreign research institutes generally use permanent magnet synchronous motor/generator reciprocal bidirectional motors. The power consumption of the motor also depends on the armature resistance, eddy current and hysteresis loss. Therefore, ironless stators are widely used, and NdFeB water magnets are selected for the rotor.
The University of Maryland specially designed the magnetic core lamination, the magnet material and the magnetic core winding method, and the total efficiency of the motor can reach 94%. The armature winding is connected by three-phase A, and at the same time, each phase has a 1/3 pole pitch overlap; the laminated material of the armature is CarpenterHymu8o, each piece is cut by laser and insulated with silica coating, and the surface of the stator is magnetically induced by NdFeB. The strength reaches 3.2kT, and the strong magnet in the air gap of the large motor produces a magnetic flux density of 0.4T.
The Lawrence National Laboratory of the United States applies permanent magnet NdFeB rods specially arranged to form a stator, resulting in a rotating dipole area. The rotor is mostly wound in succession with low inductance, and the copper loss of the stator is controlled by cooling.
(3) Input/output circuit
The input/output circuit is the control element of the energy storage flywheel system. It controls the motor and realizes the mutual conversion of electrical energy and mechanical energy. American Beacon Power Company adopts pulse width modulation converter to realize bidirectional energy conversion from DC bus to three-phase variable frequency AC. The flywheel system has the function of constant speed and constant pressure, which is automatically realized by a patented algorithm that does not require specifying the direction of energy conversion.
(4) Vacuum chamber
There are two main functions of the vacuum chamber: one is to provide a vacuum environment to reduce wind resistance loss; the other is to shield accidents. Vacuum level is a determining factor that affects system efficiency. At present, the international vacuum degree can generally reach the order of 10-5Pa. A typical flywheel energy storage system consists of three parts: flywheel assembly (including rotor, support bearing, motor/generator and casing), electronic control equipment (mainly electronic circuit controller), and auxiliary operation system (heat dissipation components, etc.).
- Principle of flywheel energy storage device
The energy E stored by the flywheel is (3):
In the formula, J is the moment of inertia of the flywheel, J=kmR2; m is the mass of the flywheel; R is the radius of the flywheel; k is a constant (related to the shape of the flywheel, for a ring k=1, a solid disk with uniform thickness k=1/2 , the solid ball k=2/5); ω is the angular velocity of the flywheel.
It can be seen from the above formula that the energy stored by the flywheel is proportional to the square of the rotational speed and the moment of inertia, respectively. A low-speed flywheel with a large diameter and small axial dimension and a high-speed flywheel with a small diameter and large axial dimension can store equal energy. The higher the speed of the flywheel, the greater the stored energy, but limited by the speed of the flywheel and the strength of the material used in the rotor, the speed cannot be increased indefinitely. Due to the strength limit acting on the flywheel material. It is related to the geometry, density ρ and working speed of the flywheel. The material with high σb/ρ ratio is the best design for the flywheel rotor, because the theoretical specific energy of the flywheel is proportional to this ratio.
The energy density EG of the flywheel can also be used to measure the energy storage capacity of the flywheel (4):
Table 1 shows the characteristic parameters of the ultra-high-speed flywheel rotor composite material.
Read more: What is a switched reluctance motor?