The working principle, characteristics, advantages and disadvantages of super capacitor

As a new energy storage component, supercapacitors fill the gap between traditional electrostatic capacitors and chemical power supplies. This paper introduces the principle, characteristics, advantages and disadvantages of supercapacitors, analyzes the working principle of supercapacitors in composite electric vehicles, and summarizes the application research of supercapacitors in various fields at home and abroad.

With the advancement of science and technology, people now pay more and more attention to the application of ecological environment protection and green energy. As a new type of energy storage component, supercapacitor has attracted people's attention and attention. Supercapacitor is a new type of energy storage device developed between the battery and the traditional capacitor developed in the 1970s and 1980s. Its appearance fills the gap between the traditional electrostatic capacitor and the chemical power supply.

1. Principle and classification of super capacitor

The supercapacitor is a physical secondary power source with super power storage capability that provides powerful pulsating power. Supercapacitors are mainly classified into three types according to the energy storage mechanism [1]: 1 an electric double layer capacitor generated by charge separation at the carbon electrode and electrolyte interface; 2 using metal oxide as an electrode to oxidize on the electrode surface and bulk phase a Faraday capacitor that produces a reversible chemisorption by a reduction reaction; and a capacitor that undergoes a redox reaction by using a conductive polymer as an electrode. The double-layer supercapacitor is a new type of energy storage device that uses polarized electrolyte to store electrical energy. The structure is shown in Figure 1:

Since the charging and discharging of the electric double layer capacitor is purely a physical process, the cycle number is high and the charging process is fast, so it is suitable for application in an electric vehicle. The electric double layer supercapacitor is two inactive porous plates suspended in an electrolyte, and the voltage is applied to the two plates. The potential applied to the positive electrode plate attracts negative ions in the electrolyte, and the negative electrode plate attracts positive ions, thereby forming an electric double layer capacitor on the surface of the two electrodes. The capacitance of a super capacitor unit is as high as several to tens of thousands of methods [2] because this structure uses a special process, its equivalent resistance is very low, the capacitance is large, and the internal resistance is small, making the super capacitor very high. The peak current, so the super capacitor has a high specific power, its power density is 50 ~ 100 times that of the battery, can reach 10 × 103W / kg, this feature makes the super capacitor is very suitable for applications in short-term high power .

2. Characteristics of super capacitor

There is no chemical reaction during the use of supercapacitor, and there is no mechanical movement such as high-speed rotation; there is no pollution to the environment, and there is no noise; its structure is simple and small, it is an ideal energy storage device. Supercapacitor products have the following technical characteristics [3]:

(1) The charging speed is fast. It takes only 10 seconds to 10 minutes to fill more than 95% of its rated capacity;

(2) Long cycle life. The deep charge and discharge cycle can reach 10,000 to 500,000 times. For example, the HCC250F/2.7V supercapacitor produced by Beijing Hezhong Huineng Company and the series capacitors produced by Beijing Jixing Technology Co., Ltd. have a cycle life of more than 500,000 times;

(3) High energy conversion efficiency. High current energy cycle efficiency >90%;

(4) High power density. Up to 300W/kg - 50000W / kg, 5 to 10 times the battery;

(5) The raw material production, use, storage and dismantling process are non-polluting, and it is an ideal green environmental protection power source; it has high safety factor and is maintenance-free for long-term use;

(6) High charge and discharge efficiency. Since the internal resistance is small, the charge and discharge loss is also small, and the charging and discharging efficiency is high, and can reach more than 90%.

(7) The temperature range is wide Up to -40 ~ +70 ° C. The reaction rate of the supercapacitor electrode material is not affected by temperature;

(8) Convenient detection and control. The remaining power can be directly calculated by the formula E=CV2/2. The stored energy can be determined only by detecting the terminal voltage. The calculation of the state of charge (SOC) is simple and accurate, so it is easy to manage and control energy.

3. Problems with super capacitors

In the case of supercapacitor applications where the energy density is high and the duty cycle is long, the main disadvantages are as follows:

(1) Low specific energy. The energy density of a supercapacitor is about 20% of that of a lead-acid battery; if the same energy is stored, the volume and weight of a supercapacitor are much larger than that of a battery.

(2) Low pressure resistance. The current supercapacitor withstand voltage is much lower than the ordinary capacitor, and the voltage is about 1-3V. If the series method is used to drive, the energy storage system is relatively bulky, which is not conducive to driving high-power equipment.

(3) The terminal voltage fluctuates severely. When using a supercapacitor, its terminal voltage is exponentially changed. When the supercapacitor releases 3/4D of energy, its terminal voltage will drop to 1/2 of the original voltage.

(4) Voltage equalization problem in series. In the manufacturing process of supercapacitors, there are problems of uneven process and material. There are some differences in parameters such as internal resistance and capacity of the same batch of capacitors of the same specification. Therefore, the supercapacitor assembly needs to be equipped with a series equalizing device to improve the energy efficiency and safety of the component.

4. Application of super capacitor

4.1 Application on electric vehicles

The main factors restricting the development of modern industry are environmental pollution problems and the shortage of energy. Nowadays, the automobile industry has occupied most of the energy consumption of modern industry. Therefore, the energy-saving technology of automobiles has become a key technology that must be solved in the development of the automobile industry. Due to the superior performance of supercapacitors, countries around the world are scrambling to research and increasingly apply them to electric vehicles. The tens of thousands of Farad-class supercapacitors can be used as a short-time driving power source for electric vehicles. Under the short-time and high-power working conditions such as starting, accelerating and braking energy recovery of vehicles, the power and economy of electric vehicles can be significantly improved. Sexuality and can effectively improve the performance of the battery. Supercapacitors have become a new trend in the development of electric vehicle power, and the composite power system composed of super capacitors and batteries is considered to be one of the best ways to solve the problem of electric vehicle power in the future.

Japan's Honda FCX fuel cell-supercapacitor hybrid car is the world's first commercial fuel cell car. It was launched in 2002 in Japan and California. Japan's Nissan also introduced a natural gas-supercapacitor hybrid bus, as shown in Figure 2. The car's fuel economy is 214 times that of the original natural gas car.

The PSI Institute in Switzerland installed a 360Wh supercapacitor bank on a 48kW fuel cell vehicle. The supercapacitor assisted the fuel cell with a 15kW rated pulse power of 50kW, taking full advantage of the drive system deceleration and starting state power. In 1996, Eltran of Russia developed an electric vehicle with a supercapacitor as a power source. It uses 300 capacitors in series and can drive for 12km at a speed of 25km/h. The hybrid bus developed by the NASALewis Research Center in the United States uses supercapacitors as the main energy storage system; the American Electric Fuel Company (EFC) designed and developed the zinc-air fuel cell electric vehicle, which also uses supercapacitors as auxiliary energy, and installs super capacitors. It has increased its mileage by nearly 25%.

After the launch of China's "Eleventh Five-Year" and "863" electric vehicle projects, domestic companies that develop supercapacitors have also increased their development efforts. China's first "capacitor energy storage variable frequency drive trolleybus" was put into trial operation in Zhangjiang, Shanghai in July 2004. When the tram is at the stop, it can be quickly charged in 30S, with a speed of 44km/h, and sustainable power supply. . This trolleybus takes full advantage of the advantages of supercapacitor power and fixed-point parking in public transportation. The super capacitor electric bus developed by Harbin Institute of Technology and Jurong Group has a top speed of 20km/h and can accommodate up to 50 passengers. In 2010 Shanghai World Expo, a total of 1,147 energy-saving and new-energy vehicles were invested: super-capacitor vehicles, fuel-cell vehicles, pure electric vehicles and hybrid vehicles. The new energy vehicles in the park took up 66% of the capacity in the park during the actual operation. It has achieved zero discharge of public transportation in the park and low emission of public transportation around the park. In addition, Shanghai Aowei Company, Beijing Jixing Company, Jinzhou Baina Electric Company and Harbin Jurong Company have launched their own supercapacitor products for HEV (Hybrid Power Unit) or EV (Electric Vehicle). However, the current design and control of supercapacitor and battery composite power electric vehicles are basically in their infancy, and there is still a gap between the super capacitors produced by domestic companies and foreign products.

4.2 Composite power supply electric vehicle working principle diagram

Pure super capacitor electric vehicle is the only energy source for electric vehicles. This method is simple in structure, low in cost, practical and zero emission, so it is suitable for short-distance, fixed-line areas, such as schools and kindergartens. The delivery cart can be used, the tractor at the train station or the airport can be used, the park tour bus and the electric bus can also be used. The electric vehicle application of the super capacitor and the battery or the fuel cell to form a composite power system is more flexible and has a wider application space.

Supercapacitor-battery composite power system The working principle of electric car is shown in Figure 3. First, the electric vehicle supplies energy to the electric motor through a power converter under normal driving conditions. Under light vehicle driving conditions, the battery charges the super capacitor, so that the super capacitor has high power output capability; when the vehicle accelerates or climbs, the super capacitor and the battery simultaneously provide energy to the motor; when the vehicle brakes or goes downhill When the motor is in the generator mode, the regenerative energy is charged by the power converter for the super capacitor. If the super capacitor cannot accept all the regenerative energy, the remaining part is absorbed by the battery.

4.3 Application in UPS System

Most of the current USP (uninterruptible power supply) systems use lead storage batteries as electrical energy storage devices. If used in the event of frequent power outages, the batteries will be sulfated due to insufficient long-term charging, and the service life will be greatly shortened. Supercapacitors are not affected by frequent power outages, and they can be fully charged in a short period of time. Supercapacitors make it a good emergency power source due to its high power density output characteristics. For example, in the blast furnace cooling water process of the steel mill is not allowed to be interrupted. Once the power is cut off, the supercapacitor can immediately provide a high output power to start the diesel generator set, and supply power to the blast furnace and the pump to ensure the safe production of the blast furnace.

4.4 Application in military systems

The US military applies supercapacitors to armored personnel carriers, heavy trucks and tanks. The power used by Oshkosh Motors to manufacture the HEMTTLMS concept car for the US military is Maxwell's ProPulse hybrid electric propulsion system, which uses PowerCache supercapacitors. PEMFC power generation technology has extremely broad application prospects in civil air defense command engineering because of its advantages of high efficiency, cleanness, light weight, small size and low operating temperature. However, no matter which power supply method is adopted, the unstable DC power generated by the PEMFC generator must be converted into a stable DC power to supply the load or the inverter. The dynamic characteristics of the PEMFC generator show a significant voltage transient drop in the event of a sudden load increase, which protects the subsequent DC/DC and DC/AC (straight/AC conversion) from functioning properly. The use of supercapacitors to compensate for the dynamic characteristics of the PEMFC generator eliminates the voltage drop spikes during sudden load increases, thereby improving the dynamic output performance of the generator and providing a stable DC voltage for subsequent DC loads and DC/AC.

4.5 Application on low-power electrical equipment

Traditional battery flashlights have a limited life span. Even with modern LED flashlights, it takes hours to fully charge and the battery has a very short cycle life. The use of supercapacitors as flashlights for energy storage components requires only 90 seconds to charge and a cycle life of 500,000 cycles. If it is charged and discharged once a day, it can be used for about 135 years. This tactical flashlight was developed by thespecial and the military. The use of supercapacitors as energy storage components ensures that emergency lights have the characteristics of power saving, high brightness, uninterrupted and long life.

5. Terminology

As a kind of energy storage system with large energy storage, fast charging and discharging speed, wide operating temperature range, safe and reliable operation, and no maintenance, with supercapacitor technology, it will gradually replace the battery, and the application field will be widened. It will certainly promote technological progress and achieve greater economic and social benefits.

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