Energy storage technology is a key technology for achieving energy-efficient management on the demand side and effectively improving the access to renewable energy. The status quo of the development of physical energy storage technologies such as pumped energy storage, compressed air energy storage, and flywheel energy storage for power storage applications are introduced. Problems that need to be addressed are urgently addressed. Electrochemical energy storage technologies with broad application prospects, including lithium, are highlighted. Ionic batteries, lead-carbon batteries, flow batteries, sodium-sulfur batteries (ZEBRA batteries), and liquid metal batteries, etc., have their operating principles, technical advantages, and their applications and challenges in the power grid, providing references for the development of electric energy storage technologies.
In recent years, China has carried out transformation and upgrading of power grids throughout the country, carried out power-saving reforms on industrial enterprises, and subsidized life-saving and energy-saving for residents throughout the country, indicating that China's power industry has entered the era of demand-side management. The introduction of electric energy storage technology will effectively reduce the peak-to-valley difference of load, reduce the cost of power supply, and effectively achieve demand-side management. At the same time, the extensive application of the large-scale energy storage technology will greatly enhance the capacity of the grid to accept large-scale renewable energy, realize predictable, controllable, and dispatchable intermittent renewable energy generation, promote the upgrading and transformation of traditional grids, and achieve power generation. The decoupling between electricity and electricity in terms of time and space completely changes the construction mode of the existing power system and promotes the transformation of the power system from the expansion type to the connotation efficiency type, and improves the power supply reliability and power quality. Therefore, energy storage technology plays a decisive role in modern power systems (as shown in Figure 1).
Existing energy storage technologies include physical energy storage technologies such as pumped energy storage, compressed air energy storage, flywheel energy storage and superconducting magnetic energy storage, and lithium ion batteries, lead-carbon batteries, flow batteries, sodium-sulfur batteries, and liquid metal batteries Electrochemical energy storage technologies such as super capacitors. Different energy storage technologies have different operating principles, conversion efficiencies, and energy storage characteristics such as cost, life, and so on. Therefore, their applications in power systems are also different. In general, flywheel energy storage, superconducting magnetic energy storage, and supercapacitors are suitable for high-rate power applications, and other technologies are suitable for energy-based energy storage applications. The following will briefly introduce different types of energy storage technologies, in order to provide a certain reference for the development of electric energy storage technology.
1 Introduction to Physical Energy Storage Technology
1.1 Pumped Storage
Pumped-storage technology is mature, reliable, and economical. It is suitable for peaking, frequency modulation, phase modulation, black-start power supply for power grids, and emergency backup. It is the most mature large-scale energy storage technology. However, its requirements for the site are relatively high, and most of them are built in hilly areas in the mountains, which are restricted by geographical factors. At this stage, China urgently needs to overcome the design and manufacturing difficulties of high-headed, high-capacity units to achieve the localization of unit design and manufacture, and fundamentally reduce the project cost of China's pumped-storage power stations, and realize the further development of pumped-storage technology.
1.2 Compressed air energy storage
Compressed air has large energy storage capacity, high efficiency, and high energy density, and is more flexible than pumped energy storage. Therefore, it has received extensive attention and is expected to become one of the important technical directions for large-scale energy storage in the future. At present, compressed air energy storage still faces some problems that need to be solved, such as: low efficiency, limited gas storage room, limited fuel, huge initial investment, and weak investment power. In the future, the development of compressed air energy storage will overcome the above problems and will develop toward supercritical compressed air energy storage systems with large scale of energy storage, high efficiency, low investment cost, and high energy density.
1.3 Flywheel Energy Storage
The flywheel energy storage technology has the advantages of high power density, high energy conversion efficiency, insensitivity to temperature, environmental friendliness, long service life, and short charging time. However, the energy storage density of the flywheel energy storage is relatively low, and the self-discharge rate is relatively high. Therefore, its application and development are greatly limited. China started late in the technology field and has insufficient investment. It is still in the primary research and development stage. At this stage, it is necessary to vigorously strengthen the research and development of the failure protection of the rotor of the flywheel rotor, the design and preparation of the high-speed motor, and the acquisition and maintenance of low-energy vacuum to realize the industrial application of the flywheel energy storage system.
2 Introduction to Electrochemical Energy Storage Technology
Electrochemical energy storage has advantages such as power and energy that can be flexibly configured according to different application requirements, fast response speed, free from external conditions such as geographical resources, and is suitable for mass production and large-scale applications. It has broad development in power storage. prospect. Electrochemical energy storage includes flow batteries, lithium ion batteries, sodium-sulfur batteries (ZEBRA batteries), lead-acid (carbon) batteries, nickel-metal hydride batteries, and liquid metal batteries. Table 1 shows the relevant parameters of various electrochemical energy storage technologies. . The following focuses on several types of energy storage battery technology with good application prospects.
2.1 Lithium-ion battery
Lithium-ion batteries are used in mobile electronic devices, power tools and new energy vehicles because of their high specific energy/specific power, charge and discharge efficiency, and output voltage, long battery life, low self-discharge, and no memory effect. Other fields have been widely used. The working principle of lithium-ion battery is shown in Figure 2. During the charging process of the battery, lithium ions in the positive electrode are released, passed through the electrolyte and embedded in the negative graphite interlaminar lattice, and discharging performs the opposite process. Lithium-ion battery reaction mechanism is called "rocking chair" mechanism.
In recent years, lithium-ion battery energy storage systems of different sizes have been established in Washington, California, New York, and Michigan, where they are used to cut peaks and fill valleys, improve grid reliability, and realize micro grid renewable power generation. Countries such as Chile and South Korea also use lithium-ion battery energy storage technology for grid frequency regulation and improvement of power quality. China has been a major producer of lithium ion batteries. Since 2010, Lithium-ion battery energy storage systems have been established in Fujian Anxi, Ningde, Zhengzhou, Henan, Dongguan, Guangdong, and Changzhou, Jiangsu, where they have been successfully used to cut peaks and fill valleys to increase power grid acceptance of wind power. Ability and so on. In the future development of lithium-ion batteries, it is necessary to further develop key electrode materials with high specific capacity, excellent cycle performance, and low cost, optimize the matching technology of positive electrodes, negative electrodes, electrolyte solutions, and battery manufacturing processes to significantly improve the cycle life of lithium-ion batteries. And safety features further reduce battery costs.
2.2 lead carbon battery
Lead-acid batteries have a long history of development, rich in raw materials, low cost, and good safety. They have an irreplaceable position in the battery market, but the negative sulfation of lead-acid batteries leads to a short cycle life, which limits the long-term development of batteries. As a new type of lead-acid battery, lead-carbon battery only needs to add a proper amount of carbon material to the anode of lead-acid battery, effectively inhibiting the sulfidation phenomenon of the negative electrode, and its rate performance and cycle life have been significantly improved, and is expected to be in the field of energy storage. widely used. The structure of the lead-carbon battery is shown in Figure 3.
Lead-carbon battery energy storage technology has a wide range of demonstration applications at home and abroad. Around 2011, the United States established a grid-level lead-carbon battery energy storage project with a capacity of 3MW/1 to 4MWh in North America, and a lead-carbon battery wind power storage capacity of 15MW/10MWh and 10MW/20MWh in Oahu and Maui, Hawaii respectively. Energy system, used in grids to assist in energy storage, frequency regulation and energy demand management. Australia will invest a 3MW/1.6MWh lead-carbon battery energy storage system into the King Island project to ensure that new energy is connected to the power grid. Areas of Colombia and Antarctica have also established energy storage systems based on lead-carbon battery technology. At present, lead-carbon battery energy storage technology is applied in 14 micro-grid energy storage projects in China's Hebei, Qinghai, Tibet and Zhejiang provinces. In the future, the production process of lead-carbon batteries should be optimized, and the optimal addition amount of carbon materials should be discussed. The carbon materials should be modified or hydrogen evolution inhibitors should be added to suppress the hydrogen evolution phenomenon of the negative electrode so as to further improve the cycle life of lead-carbon batteries.
2.3 flow battery
The flow battery is a kind of high-performance energy storage battery that uses the positive and negative electrolytes to be separately stored and circulates separately. The active material is present in the electrolyte, which allows the electrode and the active material to be separated in space. The battery power is determined by the size of the electrodes and the number of batteries in the stack. The battery capacity is determined by the concentration and volume of the electrolyte. Therefore, the battery power and capacity can be designed separately, which is flexible and convenient. The structure of the schematic diagram shown in Figure 4. There is no solid-phase electrode process and morphology change during charging and discharging, and the theoretical life is longer and the safety performance is higher. At present, the more mature liquid flow battery system includes iron-chromium system, iron-titanium system, vanadium-bromine system and all vanadium system. The positive and negative active materials of the vanadium redox flow battery are all vanadium, which can avoid the element cross-contamination caused by the diffusion of the active material through the ion exchange membrane, and has obvious advantages. It is the most important commercial development technology direction at present.
The flow battery will play an important role in improving the access of renewable energy, balancing the stability of the power grid, and so on, and has received extensive attention at home and abroad. The United States has adopted flow batteries as an important energy storage technology development direction in its 2011 energy storage development plan, and has funded the establishment of 12 liquid battery energy storage systems within one year. In 2015, Ontario, Canada also launched four flow battery energy storage projects. In 2012, China established the world's largest 5MW/10MWh all-vanadium vanadium flow energy storage system for the wind-powered Wanniu wind farm in Liaoning, and it has taken the lead in implementing the demonstration application of this technology at home and abroad. At present, China is establishing a national 200MW/800MWh vanadium flow battery national energy storage demonstration project. At the present stage, the main problems facing the development of flow batteries are the development of high-performance electrolytes, the optimization of separators and plate materials, further reduction of costs, and improvement of performance, thereby better promoting its industrial development.
2.4 Sodium-Sulfur Battery and ZEBRA Battery
The positive and negative materials of the sodium-sulfur battery are respectively molten sulfur and sodium, and the electrolyte is an alumina ceramic tube, and the working temperature is 300-350°C. Sodium-sulfur battery schematic diagram shown in Figure 5a, sodium ions through the electrolyte membrane and sulfur between the reversible reaction occurs in order to carry out energy release and storage. The theoretical specific energy of sodium-sulfur battery is as high as 760Wh/Kg, and it can reach 150Wh/Kg, which is 3-4 times that of lead-acid battery, high charge and discharge efficiency, and long cycle life. The structure of ZEBRA battery and sodium-sulfur battery is very similar, but the difference is that ZEBRA uses solid porous nickel chloride (NiCl2) as the positive electrode and liquid NaAlCl4 as the secondary electrolyte. The schematic diagram is shown in Figure 5b. When the ZEBRA battery is discharged, the metal sodium in the negative electrode is ionized and passes through β"-A12O3, diffuses to the positive electrode and reacts with NiCl2 to generate Ni metal and NaCl, and charging performs the opposite process. The energy density of the ZEBRA battery can reach 100Wh/Kg, and the lifespan is Longer, lower energy storage costs, better resistance to overcharge and overdischarge, and higher safety performance.
Japan's NGK Corporation is the only sodium-sulfur battery supplier in the world. As early as the 1980s, it cooperated with Tokyo Electric Power Co., Ltd. to develop sodium-sulfur batteries for energy storage applications. In the late 1990s, a megawatt-hour energy storage system was successfully developed. It was mainly used to cut peaks and valleys, assist in standby and stabilize the power grid. In 2002, the United States built a 100kW/500kVA demonstration power plant using sodium-sulfur batteries provided by NGK in Ohio. In 2006, it established a sodium-sulfur storage power station in West Virginia and successfully ensured the energy supply of residents in surrounding areas. In China, sodium-sulfur batteries have also received more and more attention. At present, Shanghai Electric Power and Chinese Academy of Sciences Institute of Silicate conducted a series of studies on the large-scale preparation and consistency control of β-alumina ceramic tube electrolytes. The 650Ah monomer has been successfully developed and a 2MW battery cell pilot line has been established. In 2010, the 100kW/800kWh sodium-sulfur battery energy storage system was successfully applied to the Shanghai World Expo smart grid project. In 2013, Shanghai Power Company passed the acceptance of three sodium-sulfur battery energy storage projects. However, the sodium-sulfur battery has a high manufacturing cost, a poor rate performance, a limited practical life, and a large potential safety hazard, which severely limits its application in energy storage systems.
Swiss MAS® DEA and GE in the United States have already realized the industrial application of tubular design ZEBRA. ZEBRA batteries have been successfully used in Mercedes-Benz, BMW 3 and Clio, and have good application prospects in communications and military. In the future, it is necessary to increase the research and development of ZEBRA batteries, further increase the energy density and power density of batteries, and promote the localization and commercial application of ZEBRA batteries.
2.5 Liquid Metal Battery
In recent years, a cheap and efficient new liquid metal battery energy storage technology has been rapidly developed. Figure 6 shows the working principle of the liquid metal battery. The battery consists of two layers of liquid metal and inorganic molten salt electrolyte. The three layers of the liquid are immiscible with each other and are automatically layered according to the density difference. The operating temperature of the battery is 300-500°C. Liquid metal batteries have no electrode deformation and dendrite growth in long-term use, exhibit good safety performance and long cycle life (estimated life expectancy of up to 10,000 cycles, 15 years). The liquid metal battery does not require special diaphragms, making the battery system easy to scale up and production free from critical technology constraints, energy storage costs (less than $250/kWh). The excellent characteristics of the liquid metal battery can meet the requirements of the large-scale energy storage market, so it has a broad application prospect in the field of energy storage.
At present, Huazhong University of Science and Technology and other units are dedicated to the research and development of liquid metal batteries. A lot of research has been done on key electrodes and electrolyte materials, which has effectively improved the battery's safety characteristics. The battery cells have been successfully scaled up and the liquid metal battery energy storage technology has been rapidly promoted. development of. To realize the application of liquid metal battery scale, it is necessary to effectively solve the problem of high temperature sealing and corrosion of the battery, and at the same time develop new materials and new systems, further reduce the operating temperature of the battery and reduce the cost of energy storage.
3 Conclusion
The importance of the energy storage industry in the transformation of traditional power grids and smart grid construction has become increasingly prominent. At present, the development of pumped-storage energy storage is the most mature, but it is still necessary to further realize the design and manufacture of high-performance generating units and to reduce the cost of energy storage. Compressed air energy storage, as an important-scale energy storage technology, will develop in the direction of higher-efficiency, higher-stability, and lower-cost supercritical-compressed-air energy storage. The flywheel energy storage technology started relatively late and is still in its infancy. It needs to increase research and development efforts.
Electrochemical energy storage technology has been applied in peak load reduction, grid stability improvement, and microgrid renewable power generation. At present, lithium-ion batteries are the most widely used, but it is necessary to further improve the safety performance of batteries and reduce the cost of batteries. Lead-carbon batteries are expected to become an important technology in the development of large-scale energy storage systems, and their production processes and anode hydrogen evolution problems still need further optimization and improvement. Flow batteries have a wide range of applications in terms of increasing renewable energy grid-connection rates and balancing grid stability. However, they need to further optimize key materials and reduce costs. The manufacturing cost and safety performance of sodium-sulfur batteries still need to be researched. It is necessary to increase the research and development of ZEBRA batteries and realize the localization of batteries as soon as possible. As a new type of inexpensive and highly efficient battery system, the liquid metal battery has low energy storage cost and long service life. After technical research, it is expected to obtain better results in the field of energy storage.
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