Principle and application of supercapacitors in electric vehicle drive systems

Supercapacitor is an electrochemical device, a transitional component between batteries and ordinary capacitors. Its charge and discharge process is highly reversible, and it can be charged and discharged quickly (in seconds) with high efficiency (0.85-0.98). Its advantages also include high specific power, long cycle life, and maintenance-free.

In the past, supercapacitors were difficult to apply to the automotive field because of their low specific energy and short discharge time. With the rapid development of supercapacitor technology, it has become a new hot spot for research and application in the automotive field. Supercapacitors are not only suitable for use as auxiliary energy for subsystems such as automobile engine starting and power steering, but can also be combined with batteries, fuel cells, etc. as auxiliary energy for electric vehicles, thereby increasing battery life, making up for the lack of specific power of fuel cells, and maximizing the recovery of braking energy. In short, it has a very broad application prospect in the automotive field.

Principle and classification of supercapacitors

To be precise, supercapacitors should be called electrochemical capacitors. They can provide higher specific energy than electrolytic capacitors, higher specific power than batteries, and longer life.

Supercapacitors can be divided into three categories according to the different electrode materials used:

1. Double Layer Capacitor (DLC) using carbon electrodes As shown in Figure 1, the double layer supercapacitor can be regarded as two inactive porous plates suspended in the electrolyte, and the voltage is applied to the two plates. The potential applied to the positive plate attracts negative ions in the electrolyte, and the negative plate attracts positive ions. Thus, a double layer capacitor is formed on the surface of the two electrodes.

Figure 1 Double-layer supercapacitor

DLC is essentially an electrostatic energy storage method. Therefore, the size of the double-layer capacitor is related to the electrode potential and the size of the specific surface area. Therefore, activated carbon with a high specific surface area is often used as the electrode material of the double-layer capacitor to increase the capacitance. For example, after a specific chemical treatment, the surface area of ​​activated carbon can reach 1000m2/g, so that the capacitance per unit weight can reach 100F/g, and the internal resistance of the capacitor can be maintained at a very low level. Carbon materials also have the advantages of low cost and mature technology. This type of supercapacitor is also most widely used in automobiles.

2. Supercapacitors using metal oxide electrodes originally refer to capacitors with precious metal oxides RuO2 and IrO2 as electrodes. Through a reversible oxidation/reduction reaction, the charge is transferred between the two electrodes and adsorption capacitance is generated. It is different from the mechanism of double-layer capacitors and is called Faradaic pseudocapacitance. Compared with the electrostatic capacitance of double-layer capacitors, the capacity of supercapacitors is 10 to 100 times larger under the same surface area, so capacitors with very small volume and large capacity can be made. However, due to the high price of precious metals, they are mainly used in the military field.

3. Capacitors using organic polymer electrodes. Currently, the technology is not very mature, the price is relatively high, and it is still in the laboratory research stage.

Research progress of supercapacitors for automobiles

At present, the United States, Europe and Japan are actively carrying out research and development of supercapacitors for electric vehicles. Since 1992, the U.S. Department of Energy and USABC have organized national laboratories (Lawrence Livermore, Los Alamos, etc.) and the industry (Maxwell, GE, etc.) to jointly develop double-layer supercapacitors using carbon materials. The initial goal of the research is to increase the energy density of supercapacitors to 5Wh/kg while maintaining a power density of 1kW/kg. This goal has been basically achieved, but the goal set by PNGV has not yet been completed on schedule. Relevant data show that if the specific energy of supercapacitors reaches 20Wh/kg, it will be ideal for hybrid vehicles.

In 1996, the European Community formulated a development plan for supercapacitors for electric vehicles. Led by SAFT, its members include Alcatel-Asthom, Fiat, etc. The goal is to achieve a specific energy of 6Wh/kg, a specific power of 1500W/kg, a cycle life of more than 100,000 times, and meet the requirements of electrochemical batteries and fuel cell electric vehicles.

Japan also established the "New Capacitor Research Association" and the NEW SUNSHINE Development Organization.

At present, the countries that are leading in this technology are Russia, Japan, Germany and the United States. Russia has focused on the research of capacitor vehicle technology and electric vehicle braking energy recovery and has made remarkable progress. Its starting supercapacitor has a specific power of 3000W/kg and a cycle life of more than 100,000 times, leading other countries. In Russia, a 950kg supercapacitor was used to drive an electric bus with 50 passengers. Although its driving range was only 8 to 10km, its charging time was only 15 minutes.

Maxwell predicts that the price of its PowerCacheTM product will reach $30/cell in 2003. By 2003, the demand for supercapacitors in the automotive market will reach one million units, and will rapidly increase to 100 million units in 2008. Currently, Full Power Technologies in the United States is developing low-cost supercapacitors.

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my country has been developing super double-layer capacitors since the 1990s, but there is still a certain gap with the advanced level abroad. According to relevant data, some domestic units have developed high-energy supercapacitor samples with a specific energy of 10Wh/kg and a specific power of 600W/kg, and high-power supercapacitor samples with a specific energy of 5Wh/kg and a specific power of 2500W/kg, which can be recycled more than 50,000 times. The performance indicators have reached the international advanced level, and the cost has dropped significantly compared with the international average price. It has initially reached the application level.

Application of supercapacitors in automobiles

1. Auxiliary power of electric vehicles

Frequent starting, climbing and braking of cars cause their power demand curve to change greatly, especially in urban conditions. The ratio of peak power to average power of a high-performance electric car can reach 16:1. However, the characteristics of these peak powers are generally short-lived and the energy required is not high.

For pure electric, fuel cell and series hybrid vehicles, this means that either the vehicle is not powerful enough or the voltage bus is often subjected to large peak currents, which will undoubtedly greatly damage the life of the battery, fuel cell or other APU.

However, if a supercapacitor with a larger specific power is used, when the instantaneous power demand is large, the supercapacitor will provide peak power, and absorb the peak power during braking feedback, so the pressure on the auxiliary battery, fuel cell or other APU can be reduced. This can greatly increase the power output of the system during starting and acceleration, and can efficiently recover high-power braking energy. Doing so can also increase the service life of the battery (fuel cell) and improve its discharge performance.

FIG2 shows the starting process of a fuel cell vehicle. Since the supercapacitor provides instantaneous high power when the vehicle starts, the starting process of the vehicle is greatly accelerated.

Figure 2 Comparison of starting acceleration performance between FC+C and FC vehicles

In addition, the use of supercapacitors can also focus on issues such as specific energy and cost when designing (selecting) power components such as batteries, without having to consider their specific power too much. By leveraging strengths and avoiding weaknesses, the optimal matching of power sources can be achieved.

2. Typical drive structure

The driving structure of electric vehicles with supercapacitors as the only power source is relatively simple, and the technology is not yet mature. Therefore, supercapacitors are generally used as auxiliary power sources to form a multi-energy powertrain with batteries, fuel cells or other APU systems to drive the vehicle. Common structural combinations include: B+C, FC+C, FC+B+C, ICE/G+C, etc. (where B represents battery, C represents supercapacitor, FC represents fuel cell, ICE represents internal combustion engine, and G represents generator). These structures all belong to series hybrid drive structures.

Figure 3 shows the typical structure of supercapacitors used in electric vehicles.

Figure 3 Typical drive structure of supercapacitors for electric vehicles

UCMS (Ultracapacitor Management System) realizes the packaging of supercapacitors. Its main function is to manage the current of each monomer, prevent the voltage from exceeding the decomposition voltage of the electrolyte and causing damage, and limit the impact of monomer non-uniformity. This makes the supercapacitor group work stably and reliably, and improves the overall efficiency and life of the supercapacitor group.

The supercapacitor is coupled to the battery pack through a bidirectional high-frequency DC/DC. In order to connect fewer supercapacitor cells in series, the DC/DC is generally a current-type boost converter, which controls the output power by controlling the output current of the DC/DC.

Since the energy stored in a supercapacitor is proportional to the square of the voltage, the terminal voltage of the supercapacitor, which is determined by the state of charge, will vary over a wide range. For example, if the supercapacitor is discharged by 75%, the terminal voltage of the capacitor will be reduced to 50% of the initial voltage. In order to control the energy input and output of the capacitor and coordinate the supercapacitor voltage and the battery voltage, a DC-DC converter must be used.

3. Control method

For B+C electric vehicles, the main thing is to control the current of the supercapacitor to achieve power distribution between the battery and the supercapacitor as the main power source. The following aspects should be considered: the battery power output should be kept constant or smooth as much as possible; the supercapacitor mainly plays a role in power peak regulation, providing the remaining power after deducting the battery power from the road demand power, and recovering braking energy; it must be ensured that the battery and the supercapacitor are working within their respective safe voltage ranges; the overall efficiency of the system should be maximized as much as possible. In addition to taking the supercapacitor current as the control target, the capacitor voltage can also be used as the control target.

4. Demonstration car

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With the support of the Bavarian government, MAN, Siemens and EPSOS have jointly built the first city bus in Europe that uses diesel-electric drive and double-layer capacitors as high-power energy storage devices. Compared with conventional diesel-driven vehicles, fuel consumption is reduced by 10-15%, and comfort is improved, noise and pollution are reduced. In the future, this research project will use supercapacitors in the drive system of fuel cell vehicles.

Figure 4 “CNG+C” 15-ton series hybrid bus

Since 1992, the Swiss Central University of Applied Sciences (HTA-Luzerne) has developed an energy storage system suitable for vehicle use - SAM (Super Accumulator Module), which is based on supercapacitors and batteries. In 1997, the "Blue Angel" mild hybrid vehicle developed only used supercapacitors to tow the 80-ton locomotive of the Swiss Federal Railways. This project also realized a 16-seat 4-ton minibus whose energy storage system is completely composed of supercapacitors.

Nissan Diesel has developed a 15-ton "CNG+C" series hybrid bus as shown in Figure 4, which has a driving range 2.4 times higher than that of a conventional CNG bus. The total weight of the supercapacitor is 200 kg, and the CNG engine drives a 75 kW generator at the optimal efficiency point.

In addition, Honda's fuel cell sedan FCX-V3 also adopts the "FC+C" drive structure.

Roma Tre University in Italy is carrying out "FC+B+C" research with government funding.

In January 2001, GM announced that it would use Maxwell's PowerCacheTM ultracapacitors as part of its Allison Electric DrivesTM hybrid drive solution for trucks and buses.

5. Auxiliary energy for automobile parts

In addition to being used in power drive systems, supercapacitors are also widely used in the field of automotive parts. For example, the 42V electrical system (steering, braking, air conditioning, high-fidelity audio, electric seats, etc.) used in future automotive designs can greatly improve the performance of subsystems with frequently changing power requirements if long-life supercapacitors are used. In addition, the wiring for subsystems such as electric braking and electric steering in the car can be reduced. Moreover, if supercapacitors are used to provide the large current required for engine starting, it can not only protect the battery, but also enable smooth starting even in low temperature environments and under conditions of insufficient battery performance.

in conclusion

Supercapacitors can provide/absorb large power in a short time, and have high efficiency, long cycle life, wide operating temperature range, and the basic materials used are also very cheap. Although supercapacitors still have disadvantages such as high price and need to further improve specific energy, as their technology becomes more mature and in-vehicle demonstration operation continues to deepen, supercapacitors will quickly enter the automotive market, increasing production and reducing prices. In short, supercapacitors have broad application prospects in the automotive field.