What are the energy storage technologies in the smart grid?

　　Energy storage technology has a wide range of applications in many fields, including power systems. In recent years, the reorganization of the power industry worldwide has brought new development opportunities to a variety of energy storage technologies. The use of these technologies can be more effective. This effect is particularly obvious in realizing the energy management of the power system, especially in the field of renewable energy and distributed power generation. In traditional power generation and transmission and distribution networks, these new technologies can also be applied. The following briefly introduces the basic principles and development status of various energy storage technologies.

　　1Pumped water storage 　　Pumped storage power station must be equipped with two reservoirs upstream and downstream. During the low load period, the pumped energy storage equipment works in the state of the motor, pumping the water from the downstream reservoir to the upstream reservoir for storage. At peak load times, the pumped water energy storage equipment works as a generator and uses the water stored in the upstream reservoir to generate electricity. Some high-dam hydropower stations have water storage capacity and can be used as pumped storage power stations for power dispatch. It is also technically feasible to use mines or other caves to achieve underground pumped energy storage. The ocean can sometimes be used as a downstream reservoir. In 1999, Japan built the first pumped-storage power station using seawater.　　 Pumped water storage was first applied in Italy and Switzerland in the 1890s. In 1933, reversible units (including pump turbines and electric motors and generators) appeared, and now there are adjustable speed units to improve energy efficiency. The pumped storage power station can be built with any capacity, the release time of the stored energy can be from a few hours to a few days, and its efficiency is between 70% and 85%.

　　Pumped water storage is the most widely used energy storage technology in power systems. Its main application areas include energy management, frequency control, and provision of system reserve capacity. At present, there are more than 90GW of pumped energy storage units in operation worldwide, accounting for about 3% of the world's total installed capacity. An important constraint restricting the wider application of pumped storage power stations is the long construction period and large project investment.

　　2 Advanced battery energy storage

　　It is estimated that the global annual market demand for storage batteries is approximately US$15 billion. In terms of industrial storage batteries, such as UPS, power quality adjustment, backup batteries, etc., the total market can reach US$5 billion. In the United States, Europe, and Asia, companies that produce high-performance storage batteries for power systems are being established. In the past 12 to 18 months, a battery production line with a production capacity of 300MW per year has been put into operation.

　　Lead-acid battery is the oldest and most mature battery technology. It is a low-cost universal energy storage technology that can be used for power quality adjustment and UPS. However, due to the short life of this battery, its application in the field of energy management is limited. ZnBr battery was successfully developed by Exxon in the early 1970s. After years of research and development, many ZnBr battery energy storage systems with a capacity of several kilowatt-hours have been built and tested, with a net efficiency of 75%. In the early 1980s, the University of New South Wales in Australia took the lead in developing VRB (Vanadium Redox Flow Battery) batteries. Currently, a 500kW/5MW?h VRB energy storage system has been installed in Japan, with a net efficiency of 85%.

　　In recent years, various new types of storage batteries have been successfully developed and applied in power systems. Regenesys Technologies in the United Kingdom is using PSB (Polysulfide Broe Flow Battery) batteries to build a 15MW/120MW?h energy storage power station with a net efficiency of about 75%. The NaS battery has a high energy storage efficiency (about 89%), and also has the ability to output pulse power. The output pulse power can reach six times the continuous rated power value within 30s. This feature allows the NaS battery to be used at the same time It is used for the two purposes of power quality adjustment and load peak-shaving and valley-filling adjustment, thereby improving the economy of the overall equipment. In Japan, there are currently more than 30 energy storage demonstration projects using NaS battery technology, with a total energy storage capacity of more than 20MW, which can be used for 8h daily load peak and valley adjustment.

　　Compared with other batteries, the main advantages of lithium-ion batteries are high energy storage density (300～400kW?h/m3,130kW?h/t), high energy storage efficiency (close to 100%) and long service life (every discharge It can be recharged 3000 times when it does not exceed 80% of the energy storage). Due to the above advantages, lithium-ion batteries have been developed rapidly. However, although lithium batteries have accounted for 50% of the power supply market for small mobile devices within a few years, there are still some challenging tasks to be done to produce large-capacity lithium-ion batteries. The main obstacle lies in its high cost. Mainly because it needs special packaging and necessary internal overcharge protection circuit.

　　Among all the storage batteries, the Metal-air battery has the most compact structure and is expected to become the lowest cost storage battery, which is a kind of storage battery that is harmless to the environment. The main disadvantage is that the charging of this battery is very difficult and the efficiency is very low.

　　3 flywheel energy storage　　Most modern flywheel energy storage systems are composed of a cylindrical rotating mass and a support mechanism composed of magnetic suspension bearings. The purpose of using magnetic suspension bearings is to eliminate friction loss and increase the life of the system. In order to ensure a sufficiently high energy storage efficiency, the flywheel system should be operated in a high vacuum environment to reduce wind resistance loss. The flywheel is connected to a motor or a generator, and through some form of power electronic device, the speed of the flywheel can be adjusted to realize the power exchange between the energy storage device and the grid.

　　One of the outstanding advantages of 　　 flywheel energy storage is that it requires almost no operation and maintenance, has a long equipment life (20 years or tens of thousands of deep charging and discharging energy processes) and has no adverse effects on the environment. The flywheel has excellent cyclic use and load tracking performance. It can be used in applications that are between short-term energy storage applications and long-term energy storage applications in terms of time and capacity.

　　In the realization of flywheel energy storage device, solid steel structure flywheel or composite material flywheel can be used. The specific flywheel needs to be compared with economy and technology, and the system cost, weight, size, material performance and other indicators are compromised. Using high-density steel materials, its edge linear velocity can reach 200-375m/s, while using lighter weight and higher strength composite materials, its edge linear velocity can reach 600-1000m/s. The actual output energy of the flywheel depends on its speed range, it is impossible to output the rated power at a very low speed.

　　At present, high-power flywheel energy storage systems have been developed and applied to aviation and UPS fields. Research institutions with BeaconPower as the leading level are working on the optimized design of flywheel energy storage so that it can be used for long-term energy storage services (up to several hours) while reducing its commercial costs. At present, a 2kW/6kW?h flywheel energy storage system has been used for power supply of communication equipment. The use of a flywheel group (FlywheelFarmApproach) can realize an energy storage device with an output power of megawatts and a duration of several minutes or several hours.

　　4 Superconducting magnetic energy storage 　　 Although people discovered superconductivity as early as 1911, it was not until the 1970s that it was first proposed to apply superconducting magnetic energy storage as an energy storage technology to power systems. Superconducting magnetic energy storage has quickly attracted the attention of the power industry and the military due to its fast electromagnetic response characteristics and high energy storage efficiency (charge/discharge efficiency exceeding 95%). The application of SMES in the power system includes: load balance, dynamic stability, transient stability, voltage stability, frequency adjustment, power transmission capacity improvement, and power quality improvement.

　　The 　　SMES unit is composed of a superconducting coil placed in a low temperature environment. The low temperature is provided by a cryogenic device containing a liquid nitrogen or liquid helium container. The power conversion/regulation system connects the SMES unit with the AC power system, and can charge and discharge the energy storage coil according to the needs of the power system. Two power conversion systems are usually used to connect energy storage coils with AC power systems: one is a current source converter; the other is a voltage source converter.

　　Compared with other energy storage technologies, SMES is still very expensive. In addition to the cost of the superconductor itself, the cost of maintaining low temperatures is also considerable. However, if the SMES coil is combined with the existing flexible AC transmission device (FACTS), the cost of the converter unit can be reduced, which generally accounts for the largest share of the overall SMES cost. Existing research results indicate that for transmission and distribution applications, micro (<0.1MW?h) and medium-sized (0.1-100MW?h) SMES systems may be more economical. The use of high-temperature superconductors can reduce the energy storage system's requirements for low temperature and cooling conditions, thereby further reducing the cost of SMES. At present, many SMES projects are underway or in the development stage around the world.

　　5 super capacitor energy storage　　Capacitor is a kind of equipment widely used in power system. Compared with conventional capacitors, supercapacitors have a higher dielectric constant, a larger surface area, or a higher voltage withstand capability. For example, ceramic supercapacitors have a fairly high withstand voltage level (about 1kV) and dielectric strength, which makes them a good candidate for future energy storage applications.

　　At present, super capacitors are mostly used in occasions with high peak power and low capacity. Since it can work normally for more than ten years in a fully charged floating state, the super capacitor can increase the power supply level during voltage drops and transient disturbances. Supercapacitors are simple to install, small in size, and can operate in a variety of environments (hot, cold and humid), and can now provide commercial services for low-power level applications.

　　6 compressed air energy storage

　　Compressed air energy storage is not a simple energy storage system like battery energy storage. It is a gas turbine power plant for peak shaving. For the same power output, it consumes 40% less gas than conventional gas turbines. This is because conventional gas turbines need to consume about 2/3 of the input fuel to compress air when generating electricity, while CAES can use the cheap electric energy when the grid load is low to pre-compress air, and then release the stored energy plus some gas as needed. To generate electricity. Compressed air is often stored in suitable underground mines or caves under lava. It takes about one and a half to two years to build such a cave through lava.

　　The first CAES put into commercial operation was a 290MW unit built in Hundorf, Germany in 1978. The United States built the second commercial CAES in McIntosh, Alabama in 1991, with a unit power of 110MW. The entire construction took 30 months and cost US$65 million. This unit can be connected to the grid within 14 minutes. The third commercial operation CAES, which is currently the world's largest capacity CAES, is planned to be built in Norton, Ohio. The entire power station has an installed capacity of 2,700MW and a total of 9 units. The compressed air is stored in an existing limestone that is 2200ft deep underground. In the mine.