Superconducting magnetic energy storage technology
In the 1970s, Superconducting Magnetic Energy Storage (SMES) began to be used in power systems. Superconducting magnetic energy storage uses the characteristic that the resistance of a superconductor approaches zero when the temperature is close to absolute zero to store electrical energy in the superconducting coil without loss. The core element of the superconducting magnetic energy storage system is the superconducting coil. When the energy is abundant, the power grid uses the converter to supply power and excite the superconducting coil, which converts electrical energy into magnetic field energy and stores it. When needed, the stored energy is released through the converter and sent back to the grid or used for other purposes. Since the current density of the superconducting coil is 1 to 2 orders of magnitude higher than that of the conventional coil, its energy storage density is very high, and the response speed is very fast, which other types of energy storage devices cannot match.
The superconducting magnetic energy storage system is mainly composed of a low-temperature refrigeration system, superconducting coils, power electronic devices, and measurement, control and protection systems. Since the superconducting characteristics can usually only be maintained at very low temperatures, once the temperature rises, the resistance of the superconductor increases significantly, the power loss rises rapidly, and the energy storage effect no longer exists. Therefore, the superconducting coil must be placed in a very low temperature environment, usually the superconducting coil is immersed in a very low temperature liquid (liquid hydrogen, liquid nitrogen, etc.) and enclosed in a container. Since superconducting coils store energy in a direct current manner, they must undergo power electronic conversion to achieve energy exchange with the power frequency AC grid.
Superconducting energy storage has many advantages, its response speed is fast, usually only a few milliseconds, the conversion efficiency is very high, the specific capacity is large, the specific power is large, and it can carry out real-time large-capacity energy exchange and power compensation with the power system. At the same time, it is light in weight, small in size, low in loss, and does not require energy form conversion during power transmission. However, the cost of superconducting energy storage systems is much higher than other energy storage systems, about 20 times the cost of lead-acid batteries. This is also the main reason why the superconducting magnetic energy storage system has not been used on a large scale in the current distributed power generation system.
Flywheel energy storage technology
Flywheel energy storage is a mechanical energy storage method that stores energy in the form of kinetic energy. Figure 1 shows the schematic diagram of its energy conversion. When the load is low, the electrical energy of the power grid drives the motor through the power electronic input device, and the motor drives the flywheel to rotate, which converts electrical energy into mechanical energy for storage. When the peak load occurs, the flywheel drives the generator to generate electrical energy, converts the mechanical energy into electrical energy, and realizes voltage and frequency conversion through power electronic output equipment to meet the electricity demand.
The actual flywheel power generation system, its basic structure is mainly composed of five parts, namely the flywheel rotor, vacuum chamber, bearing, motor/generator and power electronic conversion equipment. Among them, the flywheel rotor is often composed of solid steel structure and high-strength composite fiber materials. Bearings are used to support high-speed rotating flywheel rotors, and magnetic suspension bearings are generally used to eliminate friction loss and increase system life. The flywheel system is sealed in a protective sleeve with high vacuum to reduce wind resistance loss, ensure high energy storage efficiency, and prevent accidents caused by high-speed rotating flywheels. The motor/generator usually adopts a DC permanent magnet brushless electric/generator reciprocal two-way motor. The power electronic conversion equipment is used to connect the flywheel with the electric motor and generator to realize the adjustment of the speed of the flywheel and the power exchange between the energy storage system and the grid. In addition, the flywheel energy storage system must also add monitoring equipment to monitor the speed, position, vibration, vacuum degree, and motor operating parameters of the flywheel.
Flywheel energy storage technology has many advantages, such as high efficiency (over 90%), fast charging, many times of charging and discharging, long cycle life, large energy storage ((maximum capacity can reach 5kWh, energy storage power density is more than 5kW/kg, energy density is higher than 20Wh/kg), and its construction period is short, maintenance is simple, and environmentally friendly and pollution-free. However, the flywheel energy storage system will self-discharge. If the charging is stopped, the energy will be released by itself within a few to tens of hours and cannot be stored for a long time. It is more suitable for grid frequency modulation and power quality assurance.
At present, the cost of flywheel energy storage is still relatively high, and it cannot yet be applied to distributed power generation systems on a large scale. At present, it is mainly used as a supplementary equipment for battery energy storage to better improve the performance of the energy storage system. With the marketization and industrialization of flywheel energy storage, its application prospects in distributed power generation systems are broad in the future.