What is a super capacitor
Super capacitor (super capacitor), also known as electric double layer capacitor (Electrical Double-Layer Capacitor), gold capacitor, farad capacitor. It is a new type of energy storage element between traditional capacitors and rechargeable batteries. Its capacity can reach hundreds to tens of thousands of methods. The power is more than 10 times that of the battery, and the storage capacity is higher than that of ordinary capacitors. It has the characteristics of wide operating temperature range, fast charge and discharge, long cycle life, no pollution, and zero emissions.
The basic structure of the supercapacitor energy storage system is shown in Figure 1. Supercapacitors are mostly electric double-layer structures. The space between the activated carbon electrode and the electrolyte is a spatially distributed structure. Multiple capacitors can be used in series and parallel to describe the characteristics of supercapacitors.
During the charging and discharging process of the super capacitor bank, the terminal voltage range varies greatly, and a DC / DC converter must generally be used as an interface circuit to adjust the energy storage and energy release of the super capacitor. The DC / AC converter can use a bidirectional DC / AC inverter, or an AC / DC rectifier and a DC / AC inverter. The supercapacitor energy storage system is connected in parallel to the bus or feeder in the microgrid.
The supercapacitor energy storage system uses multiple sets of supercapacitors to store energy in the form of electric field energy when the energy is urgently lacking or needed. Then the stored energy is released through the control unit to accurately and quickly compensate the active and reactive power required by the system, so as to realize the balance and stable control of electric energy. The advantages of the supercapacitor itself make it win the competition with other energy storage methods when it is applied to distributed power generation.
Super capacitor classification introduction
It is generally believed that supercapacitors include electric double layer capacitors and electrochemical capacitors.
(1) Electric double layer capacitor
The electric double layer capacitor is a new type of component that stores energy through the interface double layer formed between the electrode and the electrolyte. When the electrode is in contact with the electrolyte, the solid-liquid interface is caused by the effect of Coulomb force, intermolecular force, and interatomic force. The appearance of stable double-layer charges with opposite signs is called interfacial double layer.
The electrode materials used in electric double layer capacitors are mostly porous carbon materials, including activated carbon (activated carbon powder, activated carbon fiber), carbon aerogel, and carbon nanotubes. The capacity of the electric double layer capacitor is related to the porosity of the electrode material. Generally, the higher the porosity, the larger the specific surface area of ​​the electrode material and the larger the electric double layer capacitance. But it is not that the higher the porosity, the greater the capacity of the capacitor. Keeping the pore size of the electrode material between 2-50 nm and increasing the porosity can increase the effective specific surface area of ​​the material, thereby increasing the capacitance.
(2) Principle of pseudocapacitors
Pseudo-capacitors, also called Faraday quasi-capacitors, are the under-potential deposition of electroactive substances on the surface of the electrode material or the two-dimensional or quasi two-dimensional space of the bulk phase, and highly reversible chemical adsorption / desorption or oxidation / reduction reactions A capacitance related to the charging potential of the electrode is generated. Since the reaction is carried out in the entire bulk phase, the maximum capacitance value that can be achieved by this system is relatively large, for example, the adsorption quasi-capacitance is 2 000 & TImes; 10-6 F / cm2. For redox capacitors, the maximum achievable capacitance value is very large, and the specific volume of carbon materials is usually considered to be 20 & TImes; 10-6 F / cm2, so under the same volume or weight, the pseudo-capacitor The capacity is 10-100 times the capacity of electric double layer capacitors.
The current pseudo-capacitor electrode materials are mainly metal oxides and conductive polymers. The electrode materials used in metal oxide supercapacitors are mainly some transition metal oxides, such as: MnO2, V2O5, RuO2, IrO2, NiO, WO3, PbO2 and Co3O4 metal oxides as supercapacitor electrode materials, the most successful research is RuO2 , The specific volume energy in H2SO4 electrolyte can reach 700-760 F / g. But RuO2's rare resources and high price limit its application. The researchers hope to find electrode materials with excellent electrochemical performance from metal oxides such as MnO2 and NiO to replace RuO2.
The use of conductive polymers as electrode materials for supercapacitors has been developed in recent years. The polymer product has good electronic conductivity, and its typical value is 1-100 S / cm. The electrical conductivity of conjugated polymers is generally compared with doped semiconductors, and the terms "p-doped" and "n-doped" are used to describe the results of electrochemical oxidation and reduction, respectively. Conductive polymers introduce positive and negative charge centers on the electron conjugated polymer chain by means of electrochemical oxidation and reduction reactions. The degree of charging of the positive and negative charge centers depends on the electrode potential [9]. Conductive polymers also store large amounts of energy through the Faraday process. At present, only limited conductive polymers can stably perform electrochemical n-type doping at higher reduction potentials, such as polyacetylene, polypyrrole, polyaniline, polythiophene, etc. Research work at this stage is mainly focused on finding conductive polymers with excellent doping properties, improving the charge and discharge performance, cycle life and thermal stability of polymer electrodes.
The composition of super capacitors
Common supercapacitors have three composition methods: series connection, parallel connection, and series-parallel hybrid connection. Series-connected supercapacitor components: Since the working voltage of the single cell of the supercapacitor is not high, it cannot cover the voltage requirement range of the application conditions. Multiple cells need to be connected in series to meet the voltage requirements of the application conditions. Due to the inherent difference between the two, the total voltage acting on the series components cannot be evenly distributed to different capacitors, it will cause asymmetrical voltage distribution.
Supercapacitor in parallel: Supercapacitor components constructed in parallel can output or accept large currents. In the charging process, the voltage distribution between the monomers is ensured by the series charging resistance, but the inherent charging resistance of the supercapacitor itself is a dynamic quantity and has a certain degree of dispersion, making the control circuit to adjust the resistance change extremely complicated and difficult to achieve Point-by-point control; during the discharge process, control the discharge resistance to obtain a high output power, but in order to avoid excessive discharge current and ensure the allowable output power, the energy storage of the components must be properly controlled.
Serial-parallel hybrid supercapacitor components: combine the advantages of series and parallel methods to avoid the shortcomings of the two methods. Each capacitor is assigned a resistor to control the voltage of its charging process. Therefore, the new crane described in this article
In the hybrid system, the combination of supercapacitors is a combination of series and parallel connections.
Application of Super Capacitor in Microgrid
The microgrid is composed of micropower, load, energy storage and energy manager. The forms of energy storage in the micro-grid are: connected to the DC bus of the micro power supply, the feeder containing the important load or the AC bus of the micro grid. Among them, the first two can be called distributed energy storage, and the last one is called central energy storage.
When connected to the grid, the power fluctuations in the microgrid are balanced by the large grid, and the energy storage is in the charging standby state. When the microgrid is switched from grid-connected operation to isolated grid operation, the central energy storage starts immediately to make up for the power shortage. The fluctuation of the load or the fluctuation of the micro-power source when the microgrid is isolated can be balanced by central energy storage or distributed energy storage. Among them, there are two ways to balance the power fluctuation of the micro power supply. Connect the distributed energy storage and the micro power supply that needs energy storage to a feeder line, or connect the energy storage directly to the DC bus of the micro power supply.
Smart grid technology topics
1. Provide short-term power supply
There are two typical operation modes of the microgrid: under normal circumstances, the microgrid and the conventional distribution grid are connected to the grid, which is called the grid-connected operation mode; when a grid fault or power quality is not met, the microgrid will promptly The grid is disconnected to operate independently, which is called the isolated grid operation mode. Microgrids often need to absorb some active power from conventional distribution networks. Therefore, when the microgrid is converted from the grid-connected mode to the isolated grid mode, there will be a power shortage, and installing energy storage equipment will help the smooth transition of the two modes.
2. Used as an energy buffer
Due to the small scale of the microgrid and the small system inertia, frequent fluctuations in the network and load appear to be very serious, which affects the stable operation of the entire microgrid. We always expect high-efficiency generators (such as fuel cells) in the microgrid to always work at its rated capacity. However, the load of the microgrid does not remain constant throughout the day. On the contrary, it will fluctuate with weather changes and other conditions. In order to meet the peak load power supply, the peak load adjustment of fuel oil and gas must be used to adjust the peak load. Due to the high fuel price, the operating cost of this method is too expensive. The supercapacitor energy storage system can effectively solve this problem. It can store the excess power of the power supply when the load is low, and feed back to the microgrid to adjust the power demand at the peak of the load. The characteristics of high power density and high energy density of supercapacitors make it the best choice for handling peak loads, and the use of supercapacitors only needs to store energy equivalent to peak loads.
3. Improve the power quality of the microgrid
The energy storage system plays a very important role in improving the power quality of the microgrid. Through the inverter control unit, the reactive power and active power provided by the super capacitor energy storage system to users and the network can be adjusted, so as to achieve the purpose of improving power quality. Because supercapacitors can quickly absorb and release high-power electric energy, they are very suitable for application in power quality adjustment devices of microgrids to solve some transient problems in the system, such as instantaneous power outages and voltage surges caused by system failure , Voltage sag, etc. At this time, super capacitors are used to provide fast power buffering, absorb or supplement electrical energy, and provide active power support for active or reactive compensation to stabilize and smooth the fluctuation of grid voltage.
4. Optimize the operation of micro power supply
Green energy, such as solar energy and wind energy, are often non-uniform, and the output of electrical energy is prone to change. This requires the use of a buffer to store energy. Because the electrical energy output generated by these energy sources may not meet the peak electrical energy demand of the microgrid, energy storage devices can be used to provide the peak electrical energy required in a short period of time until the amount of power generation increases and the demand decreases. A proper amount of energy storage can play a transition role when the DG unit cannot operate normally. For example, at night when solar power is used, wind power is generated in the absence of wind, or other types of DG units are under maintenance, then the energy storage in the system can play a transitional role.
In the case where the energy generation process is stable and the demand is constantly changing, energy storage devices are also required. By storing excess energy in the energy storage device, it is possible to provide the required peak energy through the energy storage device in a short time.
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