Electric vehicles using chemical batteries have been tested for decades now, but have not yet entered the practical stage. Solar energy, wind energy, tide energy, and wave energy all have storage problems. At present, they mainly rely on chemical batteries, but due to the limitations of chemical battery life and efficiency, they have not been widely used so far. The above problems have prompted people to seek a green energy storage device with high efficiency, long life, multiple energy storage, convenient use, and no pollution. Unexpectedly, the ancient "flywheel" became the first choice.
The "flywheel" energy storage element has been used by people for thousands of years, from the ancient spinning wheel to the steam engine during the Industrial Revolution. In the past, its inertia was mainly used to balance the speed and pass the "dead spot" The working cycle is very short, less than one second per revolution, in such a short period of time, the energy consumption of the flywheel is negligible. Now I want to use the flywheel to balance the energy for a period of 12 to 24 hours, the energy consumption of the flywheel itself has become very prominent. Energy consumption mainly comes from bearing friction and air resistance. People have changed the bearing structure, such as changing the sliding bearing to a rolling bearing, a liquid dynamic pressure bearing, a gas dynamic pressure bearing, etc. to reduce the bearing friction, and to reduce the air resistance by vacuuming. 3. Even if it is so small, 25% of the energy stored in the flywheel is lost within a day, and it still cannot meet the requirements of efficient energy storage. Another problem is that conventional flywheels are made of steel (or cast iron) and have limited energy storage. For example, to make a power plant with a generating capacity of 1 million kilowatts equalize power generation, energy storage wheels need 1.5 million tons of steel! In addition, to complete the conversion of electrical energy and mechanical energy, a complex set of power electronic devices is required, so the flywheel energy storage method has not been widely used.
In recent years, the breakthrough progress of flywheel energy storage technology is based on the rapid development of the following three technologies: one is the emergence of high-energy permanent magnet and high-temperature superconducting technology; the second is the advent of high-strength fiber composite materials; the third is the power electronics technology improve dramatically. To further reduce bearing losses, people had dreamed of removing the bearings and suspending the rotor with magnets, but the test results failed again and again. Later, a British scholar theoretically stated that the object cannot be completely suspended by permanent magnets (Earnshaw's theorem), which made the experimenter frustrated. Unexpectedly, the dream of full suspension of objects is realized in superconducting technology, which is really like the comfort of nature to the explorer.
The principle of superconducting magnetic levitation is this: When we align one pole of a permanent magnet with a superconductor and approach the superconductor, an induced current is generated on the superconductor. The magnetic field generated by this current is just opposite to the magnetic field of the permanent magnet, so the two generate a repulsive force. Since the resistance of the superconductor is zero, the intensity of the induced current will remain unchanged. If the permanent magnet approaches the superconductor in the vertical direction, the permanent magnet will be suspended in a position where its weight is equal to the repulsive force, and it will have resistance to the interference of up, down, left, and right. After the interference force is eliminated, it can still return to the original position, thereby forming a stable magnetic suspension. If the superconductor below is replaced with a permanent magnet, a repulsive force will also be generated between the two permanent magnets in the horizontal direction, so the permanent magnetic levitation is unstable.
Using the characteristics of superconductivity, we can put a flywheel with a certain quality on the permanent magnet, and the flywheel doubles as the motor rotor. When the motor is charged, the flywheel increases the speed of energy storage and converts the electrical energy into mechanical energy; when the flywheel slows down, it releases the energy and changes the mechanical energy into electrical energy. Figure 1 is a schematic diagram of the energy storage flywheel device. The superconductor is made of barium yttrium copper alloy and cooled to 77K with liquid nitrogen. The flywheel cavity is pumped to a vacuum of 10-8 Torr (Torr is a vacuum unit, 1 Torr (Torr ) = 133.332Pa), the energy consumption of this flywheel is very small, only consumes 2% of the energy storage every day.
Quality, v is speed. Since the speed of each point on the flywheel is different, its kinetic energy can also be expressed as:
Where ∑ is the expression of "summation", mi is the mass of each point on the wheel, and vi is the speed of each point on the wheel. It can be seen from the above formula that the energy storage size of the flywheel is related not only to the mass (weight) of the flywheel, but also to the speed of each point on the flywheel, and it has a square relationship. Therefore, increasing the speed (speed) of the flywheel is more effective than increasing the mass. However, the speed of the flywheel is limited by the material of the flywheel itself. If the rotation speed is too high, the flywheel may be torn by strong centrifugal force. Therefore, the use of high-strength, low-density high-strength composite fiber flywheel can store more energy. The currently selected carbon fiber composite material has a rim linear speed of up to 1000 meters per second, which is higher than the bullet speed. Because of the advent of high-strength composite materials, flywheel energy storage has entered the practical stage.
The following is a brief introduction to the progress of foreign flywheel energy storage.
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