Basic knowledge of flywheel battery

The flywheel battery is a device that uses a mechanical flywheel to store energy. The development of the flywheel device has been relatively mature. Because its specific power and specific energy are far greater than that of chemical batteries, it has become the research focus of many scientific researchers. American Flywheel Systems (AFS) has produced the AFS20 flywheel battery car based on the Chrysler LHS sedan. This is an electric car powered by a flywheel battery. It is driven by 20 flywheel batteries and each battery has a diameter of 230mm. The mass is 13.64kg. It takes 6h to charge the battery with city electricity, while fast charging only takes 15min. The driving range can be up to 560km on a single charge, while its prototype LHS petrol car has a full driving range of 520km at a time. Its acceleration performance is also very good, 0~96km/h acceleration time only needs 6.5s, and its life span exceeds 3.21 million kilometers.

(1) Structural principle
A typical flywheel battery structure is shown in Figure 1, and its working principle is shown in Figure 2. The electric energy transmitted from the outside is converted into the kinetic energy of the flywheel rotation through the motor and stored. The kinetic energy of the wheel is converted into electric energy, which is output to an external load, and the loss of material during empty cycle operation is very small. In fact, in order to reduce the loss during idle operation, improve the speed of the flywheel and the efficiency of the flywheel battery, the design of the flywheel electric other sales generally uses non-contact magnetic bearing technology, and the motor and the flywheel are sealed in a vacuum container. To reduce wind resistance.

Basic knowledge of flywheel battery
Figure 1 Flywheel battery structure

Generators and motors (see Figure 2) are usually realized by using a single motor (see Figure 1), which are connected to each other through bearings and flywheels. In this way, in the actual commonly used flywheel batteries, they mainly include flywheels, shafts, bearings, motors, Vacuum container and power electronic device. The schematic diagram of the Vycon flywheel battery is shown in Figure 3.

Basic knowledge of flywheel battery
Figure 2 Working principle of flywheel battery
Basic knowledge of flywheel battery
Figure 3 Vycon flywheel battery composition

When external equipment supplies power to the motor through the power electronic device, the motor is used as a motor. Its function is to accelerate the flywheel and store energy: when the load needs electrical energy, the flywheel applies torque to the motor, and the motor acts as a generator. The power electronic device supplies power to external equipment. In the entire flywheel battery, the flywheel is undoubtedly the core component, it directly determines the energy storage of the flywheel battery, and the energy stored by it is determined by the following formula:
Where: E—energy stored by the flywheel battery;
j—The moment of inertia of the flywheel, which is related to the shape and mass of the flywheel;
ω—Angular speed of wheel rotation.

It can be seen from the above formula that the amount of energy stored in the flywheel battery is determined by the shape, quality and speed of the electric flywheel. Power electronic devices are usually two-phase inverters and control circuits composed of FETs or IGBTs, which determine the energy input/output of the flywheel battery. the size of.

(2) Features
The flywheel battery charges quickly and discharges completely, making it very suitable for hybrid energy-propelled vehicles. The vehicle charges the flywheel battery during normal driving and braking. The flywheel battery provides power to the vehicle when accelerating or climbing, ensuring that the vehicle runs in a stable and optimal state, reducing fuel consumption, air pollution and noise pollution. The specific energy of the flywheel battery can reach 150W·h/kg, which is more than twice that of the Ni-MH battery; the specific energy of the flywheel battery can reach 10000W/kg, which is higher than ordinary chemical batteries and internal combustion engines, and its fast charging can be completed in 18 minutes and energy saving time long. In addition, the flywheel battery can be charged super fast, and there is no problem of shortening the service life of the chemical battery. The service life of the whole battery is much longer than that of various batteries. The service life is up to 25 years and can be used for electric vehicles to travel 5 million kilometers. The flywheel is a purely mechanical structure, which does not produce exhaust pollution like an internal combustion engine. At the same time, there is no chemical reaction process of a chemical battery, no corrosion, and no waste disposal and recycling problems.

(3) Application
As far as current technology is concerned, flywheel battery electric vehicles cannot be widely used. Due to the characteristics of flywheel batteries, it is more suitable for hybrid electric vehicles and hybrid electric vehicle technologies. Hybrid vehicles rely on the internal combustion engine and the electric motor to provide driving force. The battery is charged when the car is running and braking, and the car is climbing and accelerating, and the battery is discharged when the power is high.

When an ordinary car is driving normally, the power is only about 1/4 of the maximum power. The addition of batteries and motors in a hybrid vehicle can just solve this problem. In this way, when designing a hybrid vehicle, it is not necessary to design according to the maximum power of the vehicle, which can avoid the phenomenon of “big horse-drawn trolley” during normal driving, and greatly improve the performance of the vehicle. Hybrid power vehicle technology has long attracted the attention of scientists. The United States and many European countries have begun to apply it. Many large buses have begun to use two energy sources to provide power, and they are also widely used in many military vehicle equipment. Hybrid energy technology has been developed, but in this type of vehicle, the requirements for the battery are very high, which limits the development and wide application of hybrid vehicles. With the development of magnetic levitation technology, the number of charge/discharge times of flywheels is far greater than the needs of car batteries, and the charge/discharge of flywheels is the mutual conversion of chemical energy and mechanical energy, and its depth of discharge can be large or small, and will never affect Battery life. At the same time, the flywheel system driven by multiple drive motors can reach tens of thousands of revolutions per minute in a short period of time. In addition, in the flywheel battery, the device that determines the input/output is its external power electronic device, which has nothing to do with the external load. It can also be easily controlled by controlling the speed of the flywheel to control the charging of the flywheel. This feature is in chemistry. It is much more difficult to implement in batteries.

The principle of a hybrid electric vehicle is similar to that of a hybrid vehicle. It adds a flywheel battery to a chemical battery or other batteries to form a battery (called a flywheel hybrid battery) to drive the car motor together. The typical representative is the Porsche 911 GT3 R Hybrid oil. Electric hybrid vehicle, as shown in Figure 4. This hybrid power system developed for racing uses a hybrid four-wheel drive mode with front-wheel electric drive and rear-wheel engine drive. The two motors on the left and right front wheel drive shafts each have an output of 60KW, and the output is assigned. The 35kW rear-drive six-cylinder horizontally opposed engine adopts a small, high-efficiency electronically controlled flywheel battery design, and uses flywheel physical energy to replace the current mainstream nickel-metal hydride and battery pack design. The maximum speed of the flywheel battery pack can reach 4000rimin, and it is equipped with two motors on the front axle to form a charge/discharge structure. During braking, the front-wheel motor will become a generator, which converts the front-wheel braking kinetic energy into electrical energy and recharges it to the flywheel battery. When the accelerator pedal is to be stepped on to output power, the flywheel battery can supply power to drive the two motors. In a full discharge, the total output power of the front wheels up to 120kW can be maintained for 6~8s.

Basic knowledge of flywheel battery
Figure 4 The flywheel battery assembly of a Porsche 911 GT3 R Hybrid hybrid electric vehicle

In the early 1980s, the Swiss company Oerlikon Energy successfully developed an electric bus powered by a flywheel battery. The flywheel has a diameter of 163mm and can carry 70 passengers. In 1987, Germany developed a flywheel battery hybrid electric vehicle, which uses the flywheel battery to absorb 90% of the braking energy, and outputs electric energy to supplement the power of the internal combustion engine under short-term acceleration and other working conditions. In 1992, American Flywheel Systems (ASF) used fiber composite materials to manufacture flywheels and developed a flywheel battery electric vehicle, which has a driving range of 600km on a single charge.

Volvo’s kinetic energy recovery system used in racing cars uses a mechanical flywheel energy storage structure. As shown in Figure 5, the kinetic energy from the car body is stored in a flywheel module composed of a carbon fiber with a mass of 6kg and a diameter of 200mm. When energy needs to be released, the energy is transferred to the rear axle through a continuously variable transmission (CVT) transmission module to directly drive the wheels. According to official test results, a four-cylinder turbocharged engine using this technology can reach the level of a six-cylinder turbocharged engine, while reducing fuel consumption by 25% compared to a six-cylinder turbocharged engine.

Basic knowledge of flywheel battery
Figure 5 Mechanical flywheel energy storage structure used by Volvo KERS