The core component of Super Capacitor Energy Storage (SCES) is the super capacitor. It is a new type of energy storage element that has been mass-produced in recent years. Its energy density is very high, it has both the advantages of high-efficiency discharge power of electrostatic capacitors and the large charge storage capacity of batteries, and its monomer capacity has reached ten thousand farads. Supercapacitors have a short charging time and reversible energy storage process, so they can be repeatedly charged and discharged hundreds of thousands of times, and have a long cycle life. At the same time, they also have the advantages of good temperature characteristics, no pollution in the environment, and wide range of uses.
The structure of a super capacitor is similar to that of a rechargeable battery. It is a two-terminal element, which is mainly composed of electrode plates, separators, electrolyte, and casing. Among them, the electrodes and electrolyte are made of special materials and are developed based on the electrochemical double-layer theory. The principle is the same as other types of electric double-layer capacitors, which use the electric double-layer structure composed of activated carbon porous electrodes and electrolyte to obtain super large capacity, as shown in Figure 1.
The working principle of super capacitor
Apply a voltage to the two plates of the supercapacitor. Like ordinary capacitors, the positive plate stores positive charges and the negative plate stores negative charges. The electric field is generated by the electric charges on the two electrode plates, and the interface between the electrolyte and the electrodes forms opposite electric charges under the action of the electric field to balance the internal electric field. This positive charge and negative charge are on two different contact surfaces, the gap between the two is very short and they are arranged in opposite positions. This charge distribution layer is called an electric double layer, and its capacitance is very large. When the potential between the two plates is lower than the oxidation-reduction potential of the electrolyte, the charge on the electrolyte interface will not leave the electrolyte, and the supercapacitor will work in a normal working state (usually below 3V). If the applied voltage across the capacitor is higher than the redox electrode potential of the electrolyte, the electrolyte will decompose. As the supercapacitor discharges, the charge on the positive and negative plates will be discharged through the external circuit, and the charge on the electrolyte interface will decrease accordingly. It can be seen that there is no chemical reaction in the charging and discharging process of the supercapacitor, and it is a physical process, so its performance is stable, which is completely different from the principle of a battery that stores energy through a chemical reaction.
Because the super capacitor adopts a special electrode structure, its electrode surface area has increased tens of thousands of times, and its charge layer spacing is very small (generally less than 0.5mm), so it can provide powerful pulse power and a large storage capacity. However, due to the low dielectric withstand voltage, leakage current, storage energy and holding time are limited, it must be used in series to increase the charge and discharge control loop and system volume.
Working characteristics of supercapacitors
The working characteristics of supercapacitors are as follows:
(1) Fast charging speed. Charging for 10s~10min can reach more than 95% of its rated capacity.
(2) Large storage capacity. The storage capacity of supercapacitors is 20~1000 times that of ordinary capacitors. At present, the maximum capacity of single supercapacitors can reach 5000F.
(3) There are many charging and discharging times, and the cycle life is long, up to 500,000 times, while the battery is only about 1,000 times. If the super capacitor is charged and discharged 20 times a day, it can be used continuously for 68 years.
(4) Environmental protection and pollution-free. The raw materials, production, use, storage and disassembly of super capacitor products are pollution-free, safe, non-toxic, and environmentally friendly. Lead-acid batteries, etc., will pollute the environment.
(5) High-current discharge capacity is super strong, energy conversion efficiency is high, and process loss is small. The high current energy cycle efficiency can reach more than 90%. For example, the rated discharge current of a 2700F super capacitor is not less than 950A, and the peak discharge current can reach 1680A. Generally, batteries cannot have such a high discharge current. Excessive discharge current will damage the battery.
(6) The power density is high, up to 300~5000W/kg, which is equivalent to 5~10 times of the storage battery.
(7) Good ultra-low temperature characteristics, and can work normally in the temperature range of -40~70℃. Generally, the temperature of a battery is -20~60℃.
(8) The detection is convenient, and the remaining power can be read directly.
(9) The charging and discharging circuit is simple, the safety factor is high, and it is maintenance-free for long-term use.
Due to the characteristics of high power density, easy maintenance, and no moving parts during operation, super capacitors have great advantages when used in distributed power generation systems. Combining super capacitors with battery packs can achieve complementary performance and better improve the efficiency and economy of the energy storage system. Take the variable pitch control system of wind power generation as an example. Every time the blades of the wind turbine stop, the internal turbine will automatically adjust the blades to the specified position. The electrical energy required for this operation is provided by the hydraulic system or batteries. For batteries, intermittent charging and discharging work intensity is high, which will affect battery life. Therefore, it is necessary to carry out “high-altitude operations” on wind turbines every few years to repair and replace batteries, which also increases maintenance costs. High-power supercapacitors have fast charging and discharging speeds and long cycle life, which can replace ordinary batteries for this job, saving costs and reducing labor intensity. Therefore, super capacitors play a great role in the development of new energy power generation and distributed power generation systems.