What is a super capacitor
Super capacitors, also known as electrical double layer capacitors (Electrical Double-Layer Capacitor), gold capacitors, farad capacitors. It is a new type of energy storage component between traditional capacitors and rechargeable batteries. Its capacity can reach hundreds to tens of thousands of laws. The power is more than 10 times that of the battery, the storage capacity is higher than that of ordinary capacitors, and 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. The supercapacitor is mostly an electric double layer structure, and the space between the activated carbon electrode and the electrolyte is a spatially distributed structure, and the characteristics of the supercapacitor can be described by series and parallel connection of a plurality of capacitors.
During the charging and discharging process of the ultracapacitor group, the terminal voltage range changes greatly, and it is usually necessary to use a DC/DC converter as an interface circuit to regulate the energy storage and release energy of the supercapacitor. The DC/AC converter can be 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.
Supercapacitor energy storage systems use multiple sets of supercapacitors to store energy in the form of electric field energy, when energy is urgently lacking or needed. The stored energy is released through the control unit to accurately and quickly compensate the active and reactive power required by the system, thereby achieving balance and stable control of the electrical energy. The advantages of the supercapacitor itself make it a winner in competing with other energy storage methods when applied to distributed generation.
Supercapacitor classification introduction
Supercapacitors are generally considered to include two types of electric double layer capacitors and electrochemical capacitors.
(1) Electric double layer capacitor
An electric double layer capacitor is a new type of component that stores energy through an interface double layer formed between an electrode and an electrolyte. When the electrode is in contact with the electrolyte, the solid-liquid interface is caused by Coulomb force, intermolecular force, and interatomic force. A stable, oppositely signed double layer charge is known as the interface 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 size 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 not the higher the porosity, the larger the capacity of the capacitor. Keeping the pore size of the electrode material between 2 and 50 nm increases the porosity to increase the effective specific surface area of ​​the material, thereby increasing the capacitance.
(2) The principle of tantalum capacitor
Tantalum capacitors, also known as Faraday quasi-capacitors, are in the two-dimensional or quasi-two-dimensional space of the surface or bulk of the electrode material, and the electroactive material is subjected to underpotential deposition, resulting in highly reversible chemisorption/desorption or oxidation/reduction reactions. A capacitance is generated which is related to the electrode charging potential. Since the reaction proceeds throughout the bulk phase, the maximum capacitance that can be achieved with this system is relatively large, such as an adsorption type quasi-capacitance of 2 000 & TImes; 10-6 F/cm 2 . For redox capacitors, the maximum achievable capacity is very large, and the specific volume of carbon material is generally considered to be 20 & TImes; 10-6 F/cm2, so tantalum capacitors in the same volume or weight The capacity is 10-100 times the capacity of the electric double layer capacitor.
At present, tantalum capacitor electrode materials are mainly metal oxides and conductive polymers. The electrode materials used in metal oxide supercapacitors are mainly transition metal oxides such as MnO2, V2O5, RuO2, IrO2, NiO, WO3, PbO2 and Co3O4 as the supercapacitor electrode materials. The most successful research is RuO2. The specific volume of the H2SO4 electrolyte reaches 700-760 F/g. But RuO2's rare resources and high prices limit its application. Researchers hope to find electrode materials with superior electrochemical properties from metal oxides such as MnO2 and NiO instead of RuO2.
The use of conductive polymers as electrode materials for supercapacitors has been developed in recent years. Polymer products have good electronic conductivity, with typical values ​​ranging from 1 to 100 S/cm. The conductivity of a conjugated polymer is generally compared to a doped semiconductor, and the terms "p-doped" and "n-doped" are used to describe the results of electrochemical oxidation and reduction, respectively. The conductive polymer introduces positive and negative charge centers on the electron conjugated polymer chain by means of electrochemical oxidation and reduction reactions, and the degree of charge 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 a limited number of conductive polymers can stably perform electrochemical n-type doping at a higher reduction potential, such as polyacetylene, polypyrrole, polyaniline, polythiophene and the like. At present, the research work mainly focuses 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 supercapacitors
Common supercapacitors are available in three ways: series, parallel, and serial-to-parallel. Tandem type supercapacitor assembly: Since the single capacitor operating voltage of the supercapacitor is not high enough to cover the voltage demand range of the application condition, multiple cells need to be connected in series to meet the voltage requirements of the application condition, but due to the single capacitor The inherent difference between the total voltages acting on the series components is not evenly distributed to different capacitors, which leads to asymmetry in voltage distribution.
Parallel Super Capacitors: Supercapacitor assemblies constructed in parallel can output or accept large currents. During the charging process, the voltage distribution between the cells is ensured by the series charging resistor, but the inherent charging resistance of the supercapacitor is a dynamic quantity with a certain degree of dispersion, making the control circuit for adjusting the resistance change extremely complicated and difficult to realize. Point-by-point control; in the discharge process, the discharge resistance is controlled to obtain a high output power, but in order to avoid excessive discharge current and ensure the permissible output power, the energy storage of the component should be properly controlled.
Serial and hybrid supercapacitor components: Combine the advantages of series and parallel mode to avoid the two methods are insufficient. Each capacitor is assigned a resistor to control the voltage during its charging process. Therefore, the new crane described in this article
In the hybrid system, the combination of the supercapacitors used is a combination of series and parallel connections.
Application of supercapacitor in microgrid
The microgrid consists of micro power supplies, loads, energy storage, and energy managers. The form of energy storage in the microgrid is: connected to the DC bus of the micro power supply, the feeder containing the important load or the AC bus of the microgrid. 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 network operation, the central energy storage starts immediately to compensate for the power shortage. The fluctuation of the load during the running of the microgrid or the fluctuation of the micro power supply can be balanced by the central energy storage or the distributed energy storage. Among them, there are two ways to balance the power fluctuation of the micro power supply, and the distributed energy storage and the micro power supply that needs energy storage are connected to a certain feeding line, or the energy storage is directly connected to the DC bus of the micro power supply.
Smart Grid Technology
1. Provide short-term power supply
There are two typical modes of operation in the microgrid: under normal circumstances, the microgrid and the conventional distribution network are connected to the grid, which is called the grid-connected operation mode; when the grid fault is detected or the power quality is not met, the microgrid will be timely The grid is disconnected and operated independently, called the isolated mode. Microgrid often needs to absorb some of the active power from the conventional distribution network. Therefore, when the microgrid is switched from the grid-connected mode to the isolated mode, there will be power shortage. Installing the energy storage device will help the smooth transition of the two modes.
2. Used as an energy buffer
Due to the small scale of the microgrid, the inertia of the system is not large, and the network and load often fluctuate very much, which has an impact on the stable operation of the entire microgrid. We always expect high-efficiency generators (such as fuel cells) in the microgrid to always operate at its rated capacity. However, the load on the microgrid does not remain constant throughout the day. Instead, it fluctuates as the weather changes. In order to meet the peak load supply, peak load adjustment must be carried out using a fuel-and-gas peaking power plant. Due to the high fuel price, the operation cost of this method is too expensive. The supercapacitor energy storage system can effectively solve this problem. It can store excess power of the power supply when the load is low, and feed back to the micro grid to adjust the power demand when the load is high. The high power density and high energy density of the supercapacitor make it the best choice for handling peak loads, and the use of supercapacitors requires only the storage of energy equivalent to the peak load.
3. Improve the power quality of the microgrid
The energy storage system plays an important role in improving the power quality of the microgrid. Through the inverter control unit, the reactive power and active power provided by the supercapacitor energy storage system to the user and the network can be adjusted, thereby achieving the purpose of improving the power quality. Because supercapacitors can quickly absorb and release high-power electric energy, it is very suitable to be applied to the power quality adjustment device of the micro-grid to solve some transient problems in the system, such as instantaneous power failure and voltage swell caused by system failure. Problems such as voltage dips, etc. At this time, supercapacitors are used to provide fast power buffering, absorbing or supplementing electric energy, and providing active power support for active or reactive power compensation to stabilize and smooth fluctuations in grid voltage.
4. Optimize the operation of the micro power supply
Green energy sources such as solar energy and wind energy tend to have unevenness, and the power output is subject to change. This requires the use of a buffer to store energy. Since the energy output generated by these energy sources may not meet the peak power requirement of the microgrid, the energy storage device can be used to provide the required peak power in a short time until the power generation is increased and the demand is reduced. A proper amount of energy storage can make a transition if the DG unit is not functioning properly. For example, during nighttime when solar power is used, wind power is in the absence of wind, or other types of DG units are in service, when the energy storage in the system can play a role.
In the case where the process of energy generation is stable and the demand is constantly changing, it is also necessary to use an energy storage device. 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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