- Comparison of supercapacitors and power batteries
Compared with traditional capacitors of the same size, the energy that supercapacitors can store is much greater than that of traditional capacitors, but compared with batteries of the same size, the energy that supercapacitors can store is smaller than that of batteries, but the power performance of supercapacitors is superior. For batteries, because supercapacitors can discharge at high rates, the peak current is only limited by the internal resistance and the size of the supercapacitor. Therefore, when the size of the energy storage device is determined by the power, the use of supercapacitors may be a better solution. Generally speaking, combining the excellent power performance of supercapacitors with the good energy storage performance of batteries may be the best solution. (Join us at tycorun.com to discuss more about capacitive battery energy storage.)
The supercapacitor can be charged to any voltage value within its rated voltage range, and can release all the stored power when discharging, but the power battery can only work within a very narrow voltage range, and overdischarge will cause the power battery to be permanently damaged. sexual damage; supercapacitors can discharge energy pulses frequently without harmful consequences, but power batteries can reduce lifespan if they discharge energy pulses frequently; supercapacitors can be charged extremely quickly, while power batteries can be damaged if charged quickly ; Supercapacitors can have hundreds of thousands of charge-discharge cycles, while power batteries have only a few hundred or thousands of cycles. The performance comparison of lead-acid batteries, supercapacitors and ordinary capacitors is shown in Table 1.
- Voltage balance of supercapacitor banks
The voltage of a supercapacitor bank is determined by the number of capacitors connected in series, while the power is determined by the number of capacitors connected in parallel. Supercapacitors are similar to power batteries, and the voltage range of each supercapacitor cell is 1~3.0V (related to the type of capacitor).
Therefore, it is necessary to use supercapacitors in series to obtain the required voltage. In an ideal state, the performance of each supercapacitor cell should be consistent, that is, the voltage of each supercapacitor cell should be the same. However, due to factors such as manufacturing error, self-discharge rate, etc., the voltage between capacitor cells is different. During manufacture and throughout the product life cycle, changes in capacitance value and leakage current affect the distribution of capacitor voltage. Therefore, it is most effective to use a supercapacitor cell management circuit to improve the performance and life of supercapacitor cells used in series. The method of managing the supercapacitor cell (another management method is to discharge the overvoltage cell to protect the supercapacitor, but it also produces other problems). A good equalization circuit can respond quickly to abnormal cells. There are two methods of supercapacitor cell balance, namely passive equalization (Figure 1) and active equalization (Figure 2):
2.1 Passive equalization circuit
(1) The structure in which the resistor is directly connected in parallel with the super capacitor
In this way, as shown in Figure 1(a), a resistor is connected in parallel with each supercapacitor cell to suppress leakage current. In fact, a resistor with a small tolerance is used to force the voltage of a single module to be consistent.
During the charging process of the supercapacitor, the internal resistance determines the size of the charging current and the final voltage. After the supercapacitor is charged, the self-discharge internal resistance is an important parameter, and the voltage balance between the supercapacitor cells can be achieved with a small resistance. The resistance value of the resistor should be much larger than the internal resistance of the supercapacitor, but smaller than the self-discharge resistance. Depending on the resistance value, the voltage balancing process may take minutes to hours.
This method is most suitable for low-load operating conditions, such as UPS power supply, where the charging current is not large and the charging time is long, which can prolong the service life of the supercapacitor. This method has the advantages of simple structure and low cost, and the biggest disadvantage is that a large power loss is generated on the external resistance, and this loss is related to the resistance value and the magnitude of the current. If the charging time is long enough to complete the equalization process, it can also be used in automobiles, but charging with peak power may cause overvoltage, and this circuit does nothing to prevent overvoltage.
(2) The structure of switch-controlled resistors in parallel
In this way, as shown in Figure 1(b), a switch is connected in series with the resistance of the previous structure. When the cell voltage is higher than the preset voltage value, the switch is turned on; when the cell voltage is lower than the preset voltage value, the switch is turned on. When the specified voltage value is reached, the switch is closed. This structure needs to measure the cell voltage, which will increase the cost.
(3) Structure using DC/DC converter
In this way, as shown in Figure 1(c), a DC/DC converter is connected between adjacent cells to balance the voltage of the cells. In addition to the loss of the converter, there is no other loss, and the efficiency is higher than the above two balance methods. However, due to the high cost of hardware implementation and control, this structure has not attracted much interest.
(4) Structure using Zener diode
In this way, as shown in Figure 1(d), a Zener diode is connected in parallel with the cell, and as long as the working voltage of the Zener diode is reached, the cell voltage remains unchanged. The main disadvantage of this structure is that the power loss of the diode is very large, and the voltage of the diode itself has a great relationship with the temperature, so it cannot be used in large quantities.
2.2 Active equalization circuit
As shown in Fig. 2(a) active equalization circuit, the time required for active equalization is shorter than that required for passive equalization, the voltage distribution is exactly equal, and the parasitic loss is small. If the limit voltage is reached, the circuit is balanced by the bypass action of a low-power resistor connected in parallel with the supercapacitor. The function of this resistor is the same as that of passive equalization, but due to the large equalization current, the equalization process is very short. Below the limit voltage, the resistance does not work and the charging current can be very large. When the bypass part works, the current can be higher, but this is limited by the parallel resistance (generally the upper limit current is up to 1A). Therefore, this circuit cannot be applied on the vehicle, because when the vehicle is braking, the charging current generated by the brake feedback is much larger than 1A, which will damage the entire circuit.
Figure 2(b) shows the structure using auxiliary current sources, that is, two auxiliary current sources are used to adjust the charging and discharging current of the supercapacitor, and the equilibrium current is determined according to the voltage of the supercapacitor during charging and discharging.
- Application of super capacitors in vehicles
In recent years, supercapacitors have gradually become a research hotspot. Supercapacitors have been applied in hybrid power systems, low-temperature starting systems and vehicle 42V power supply systems. Major automakers around the world have applied supercapacitors to cars. On the power city bus, the super capacitor is used in the braking energy co-feeding system.
The German Institute of Electronic Technology (Elektrotechnisches Institute), Karlsruhe University (Karlsruhe University) and the Offenburg Institute of Technology have carried out a test of a supercapacitor as an auxiliary energy storage system for electric vehicles. Auxiliary energy storage device is installed on it, which consists of 105 DLCs (Panasonic’s POWERCAP type, 1500F/2.3V) and is equipped with a detection device to collect the parameters of the power flow in the dynamic test. At the same time, the ZOXY type high-energy zinc-air battery produced by ChenTEK is used as the main power source of the electric vehicle.
Volkswagen in Germany and some partners such as the Swiss Federal Institute of Science and Technology in Zurich have jointly developed a fuel cell hybrid project. Volkswagen has tried super capacitors in the BORA sedan. A prototype was shown to the public at the Geneva Motor Show in March 2002.
In its fuel cell hybrid vehicle FCX-V3 developed by Japan’s Honda, super capacitors are used as auxiliary energy storage elements to improve its dynamic response performance and dynamic performance. Mazda’s FCEV fuel cell hybrid electric vehicle prototype car , using supercapacitors as energy storage elements for auxiliary energy recovery systems. Nissan has developed a commercial hybrid diesel truck that uses supercapacitors to recover braking energy and assist the primary power source to work. Japan’s Oshkosh Truck Company has developed a hybrid-drive heavy-duty military truck that uses a diesel engine as the main power, and a 400kW generator and a supercapacitor bank as the auxiliary power.
Electro-Fuel Corporation (EFC) has used supercapacitor banks in the development of fuel cell buses, increasing the total driving range by nearly 25%. In addition to major automobile companies, research groups in many schools and scientific research institutions are also conducting research on the application of supercapacitors in vehicles. Featured program. The application of supercapacitors in vehicles is in the transition stage from concept cars to commercial production models.
The practical application prospects of supercapacitors in hybrid electric vehicles are very broad, but at present, supercapacitors have no price advantage, and it is necessary to further improve performance and reduce costs.
- Development direction of automotive super capacitors
In the National “Twelfth Five-Year Plan” (863 Plan) Key Technology and System Integration of Electric Vehicles in the Modern Transportation Technology Field (Phase I) Major Project Application Guide, the industrialization technology of supercapacitors is a key topic, which indicates the development direction of supercapacitors for vehicles.
(1) Research objectives
Improve the single technology level of power supercapacitors, develop standardized and modular hybrid vehicle power modules, break through the key technologies of industrialization, and break through the core technology of energy-based supercapacitors, while maintaining the high specific power, long life and fast charging characteristics of supercapacitors On the basis of , the specific energy is greatly improved.
(2) Research content
Power supercapacitors: study key material technologies such as carbon materials and electrolytes; study electrode technology, system packaging, uniformity, screening combination, electrical balance, thermal balance and system integration technology. Energy-based supercapacitors: Research on key material technologies such as advanced electrodes; research on advanced manufacturing processes and electrolyte preparation technologies; research on electrical performance design and structural design of single capacitors, module design, module balance and thermal management technologies; technology, etc.
3) Main assessment indicators
Power type super capacitor: power density ≥ 8000w/kg. Energy density ≥ 6w.h/kg, cycle life ≥ 500,000 times, and safety meets national standards or specifications. Energy-based supercapacitors: power density ≥3000W/kg, energy density ≥30W·h/kg, cycle life ≥10,000 times, and safety meets national standards or specifications.
Read more: Energy storage mechanism and classification of supercapacitors