LR1130 battery.Supercapacitor energy storage technology and its applications

　　Supercapacitor, also called electrochemical capacitor, is a new type of energy storage component developed in the 1960s. In 1957, Becker in the United States first proposed that capacitors could be used as energy storage components, with an energy density close to that of batteries. In 1962, Standard Oil Company (SOHIO) produced a capacitor with an operating voltage of 6V and carbon materials as electrodes. Later, the technology was transferred to NEC Electric Company, which began producing supercapacitors in 1979 and was the first to market them in 1983. Since the 1980s, supercapacitors using metal oxides or nitrides as electrode active materials have aroused great interest among scientific researchers because they have several advantages that electric double layer capacitors do not have.

　　1 Energy storage principle of supercapacitor

　　Supercapacitors can be divided into electric double layer capacitors and Faraday quasi-capacitors according to the energy storage principle.

　　1.1 Basic principles of electric double layer capacitors

　　The basic principle of the electric double layer capacitor is a new type of electronic component that uses the interface electric double layer formed between the electrode and the electrolyte to store energy. When the electrode and the electrolyte come into contact, due to the action of Coulomb force, intermolecular force or interatomic force, two stable layers of charges with opposite signs appear at the solid-liquid interface, which is called the interface double electric layer. The energy storage of this capacitor is achieved by electrochemical polarization of the electrolyte solution without producing an electrochemical reaction. This energy storage process is reversible.

　　1.2 Basic principles of Faraday quasi-capacitor

　　Following the electric double layer capacitor, Faraday quasi-capacitor, or quasi-capacitor for short, was developed. The capacitance is a two-dimensional or quasi-two-dimensional space on the electrode surface or in the bulk phase. The electroactive material is deposited under potential, and a high degree of chemical adsorption and desorption or redox reaction occurs, resulting in a capacitance related to the charging potential of the electrode. For Faraday quasi-capacitors, the process of storing charges includes not only storage on the electric double layer, but also storage of charges in the electrode due to oxidation-reduction reactions of ions in the electrolyte in the electrode active material.

　　2Characteristics of supercapacitors

　　Supercapacitor is a better energy storage component between traditional physical capacitors and batteries. Its huge advantages are as follows: ① High power density. Supercapacitors have very low internal resistance and can achieve rapid charge storage and release both at the electrode/solution interface and within the electrode material itself. ② Long charge and discharge cycle life. Supercapacitors do not undergo electrochemical reactions during charging and discharging, and their cycle life can reach more than 10,000 times. ③Short charging time. Full charging takes just minutes. ④Achieve high specific power and high specific energy output. ⑤Long storage life. ⑥High reliability. Supercapacitors have no moving parts and require minimal maintenance. ⑦Ambient temperature has little effect on normal use. The normal operating temperature range of supercapacitors is -35~75℃. ⑧It can be used in any parallel connection to increase the capacitance; if voltage equalization is adopted, it can also be used in series to increase the voltage level.

　　3 Application of supercapacitor energy storage technology

　　As a high-power physical secondary power supply, supercapacitors are widely used in various fields of the national economy. All developed countries have listed the research on supercapacitors as national key strategic research projects. In 1996, the European Community formulated a development plan for supercapacitors. Japan's New Sunshine Plan listed the development of supercapacitors. The U.S. Department of Energy and the Department of Special Affairs also formulated a research plan for the development of supercapacitors. my country's National 863 Plan has formulated a major electric vehicle project (2001) supercapacitor project. The following introduces the current application status of supercapacitor energy storage technology.

　　3.1 Tram power supply

　　Because supercapacitors have very high power density, they can well meet the power requirements of electric vehicles when starting, accelerating, and climbing hills. They can be used as acceleration or starting power sources for hybrid electric vehicles. General Motors of the United States has used Power Cache supercapacitors produced by Maxwell Technologies to form parallel hybrid power systems and series power systems for use in automobiles. Research in literature [3] shows that using supercapacitors and batteries in parallel as power sources can reduce the size and weight of the battery and extend the service life of the battery.

　　In July 2004, my country's first supercapacitor bus and its fast charging waiting station system were put into trial operation. This system solves the problems of visual pollution, poor maneuverability and difficult planning caused by trolleybuses. With its zero-emission and low-noise performance, it improves the air pollution caused by bus exhaust emissions in urban areas and avoids the secondary damage of traditional batteries. pollution, extending the service life.

　　3.2 Electronic power supply

　　Supercapacitors can not only be used as power sources for small devices such as optoelectronic electronic watches and computer memories, but can also be used on satellites. The power supply used on satellites is mostly a hybrid power supply composed of solar energy and batteries. Once supercapacitors are installed, the satellite's pulse communication capabilities will definitely be improved. Due to the fast charging characteristics of supercapacitors, supercapacitors are undoubtedly an ideal power source for devices that require fast charging, such as power tools and toys. Literature [4] introduces that in the field of mobile communication power supply, electrochemical electric double layer capacitors will be mainly used to mix with other power supplies to form a power supply due to their high power density and low energy density characteristics. They can also be used for short-term power backup. , used to protect memory data. Literature [5] studies the use of electric double layer capacitors as emergency power supplies for implantable medical devices. Since electric double layer capacitors do not require transitional charge and discharge protection circuits and have long service life, they will replace traditional batteries.

　　3.3 Applications in power systems

　　The vast majority of high-voltage substations and switch stations use electromagnetic-operated switch mechanisms, which are specially equipped with capacitor energy storage silicon rectifier opening and closing devices as DC power supplies for opening and closing operation, control, and protection. However, electrolytic capacitors in capacitor energy storage devices have limited capacity and poor reliability. Research in literature [6] shows that supercapacitors ensure the absolute reliability of opening energy supply, while retaining the advantages of traditional capacitor energy storage silicon rectifier opening and closing devices.

　　UPS often plays a decisive role in the first few seconds or minutes when the power grid is out of power or the grid voltage drops instantaneously. The battery provides power during this period. Due to the shortcomings of the battery itself (requires regular maintenance, short life), the UPS needs to always pay attention to the status of the battery during operation. Literature [7] studied that in a data protection backup system, the time required by UPS is relatively short. At this time, the advantages of supercapacitors are particularly obvious. Its output current can rise to hundreds of amps with almost no delay, and the charging speed is very fast. Fast, energy can be stored within minutes so it can be used again the next time the power fails. Although the energy storage time of supercapacitor can be maintained for a short time, when the energy storage time is about 1 minute, it has unparalleled advantages. It has 500,000 cycles and does not require care for 10 years, making the UPS truly maintenance-free.

　　Literature [8] proposed a static synchronous compensator (STATCOM) based on double-layer capacitor energy storage, which can be used to improve the voltage quality of distributed systems, especially at the 300-500kW power level, and will gradually replace traditional superconducting energy storage. . In terms of economics, the cost of electric double layer capacitor energy storage and superconducting energy storage devices of the same capacity are almost the same, but the electric double layer requires almost no operating costs, while superconducting energy storage requires considerable cooling costs.

　　Literature [9] introduces the application of supercapacitors in photovoltaic power generation. Supercapacitors can be charged at a state only higher than their leakage current. This feature enables photovoltaic cells to charge supercapacitors even on cloudy days in photovoltaic power generation, improving the effectiveness of photovoltaic power generation and weak current charging.

　　In 2005, the 863 project’s key technology research on supercapacitor energy storage systems for renewable energy power generation, undertaken by the Institute of Electrical Engineering of the Chinese Academy of Sciences, passed expert acceptance. This project completed the research and development of a 300Wh/1kW supercapacitor energy storage system for photovoltaic power generation systems.

　　The frequency converter is very sensitive to voltage, and due to various faults and operations of the power grid, instantaneous low voltage phenomena will occur. The rapid charging and discharging characteristics of supercapacitors can be used to achieve the low voltage leap of the frequency converter and ensure the normal operation of the frequency converter. .

　　In 2005, a 450kW supercapacitor energy storage device was built in California, USA, to reduce the fluctuation of power delivered by a 950kW wind turbine to the grid. In Singapore, ABB's DVR device using supercapacitor energy storage is installed in a 4MW semiconductor factory. The device can achieve a low voltage span of 160ms.

　　The voltage of a single supercapacitor is low, and the modular version does not exceed 100V, so it cannot be directly used in power systems. Two methods can be used to increase the voltage level: directly connecting supercapacitors in series to increase the voltage level; literature [10] connects the supercapacitor module to a BoostDC/DC converter, and then connects it to the power grid through an inverter. In order to achieve a higher voltage level, A step-up transformer can also be added between the inverter and the grid. The first method has the problem of voltage equalization and the voltage boost range is limited. The second method is usually used to achieve energy storage and power supply.

　　At present, supercapacitors are mostly used in high peak power and low capacity applications. With the development of supercapacitor materials and the continuous improvement of power density and energy density, their application scope in power systems will be broader.

　　4 Issues to note during application

　　Supercapacitors have fixed polarity, and the polarity should be confirmed before use. Supercapacitors should be used at nominal voltage: when the capacitor voltage exceeds the nominal voltage, the electrolyte will decompose. At the same time, the capacitor will generate heat, the capacity will decrease, the internal resistance will increase, and the life will be shortened. Supercapacitors cannot be used in circuits with high frequency charging and discharging. High-frequency rapid charging and discharging will cause internal heating of the capacitor, capacity attenuation, and increase in internal resistance. In some cases, the performance of the capacitor will collapse.

　　When supercapacitors are used in series, there is a problem of voltage balance between cells. Simple series connection will cause overvoltage of one or several individual capacitors, thereby damaging these capacitors and affecting overall performance.

　　5 Conclusion

　　The emergence of supercapacitors solves the contradiction between the power density and energy density of energy systems. With the further development of supercapacitors, they will replace the current batteries that require frequent charging and replacement for electric vehicles, and household energy storage supercapacitors are also possible. Non-polluting energy sources such as solar energy, wind energy and fuel cells will be stored in supercapacitors to continuously provide electrical energy. There is no need to invest in large power stations or complex transmission grids. It is an energy-saving measure that uses renewable energy and requires little investment.

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