LR6 alkaline battery.Overview of research on the application of supercapacitor and lithium battery technology in ship electric propulsion systems?

　　Overview of research on the application of supercapacitor and lithium battery technology in ship electric propulsion systems?

　　In most cases, the ship's electric propulsion system uses an internal combustion engine to drive a generator set to provide power to the system. Due to the complex and changeable marine environment, the load changes. When the load deviates from the optimal load point, the fuel will not be fully burned, the fuel utilization rate will drop significantly, and a large amount of nitrogen oxides and sulfur will be produced. oxides, causing pollution to the environment. Energy storage technology is one solution to this problem.

　　The energy storage unit is used to store excess energy when the system is lightly loaded to prevent the impact of this energy on the power grid. When the system is overloaded, the energy storage unit releases energy to meet the load's needs. Energy storage technology is already well used in the electric vehicle industry. The development of large-capacity energy storage technology has made it possible for energy storage units to be used in ship electric propulsion systems. Using energy storage units to overcome the impact of power fluctuations on ship electric propulsion systems will be a new direction for the development of ship propulsion technology in the future.

　　1Supercapacitor technology

　　1.1 Structure of supercapacitor

　　Supercapacitors, sometimes called electric double-layer capacitors, or double-layer capacitors, are electrochemical capacitors with high energy density. A standard battery-sized electrolytic capacitor has a capacitance of tens of microfarads, but a supercapacitor of the same size can Up to several farads, the difference can be five orders of magnitude. A capacitor stores energy through the capacitance effect. Its capacity is directly proportional to the surface area of the electrodes and the dielectric constant of the electrolyte, and inversely proportional to the distance between the electrodes. Supercapacitors use high dielectric constant electrolytes, and their electrodes use porous activated carbon materials that can greatly increase their surface area. This is the main reason why supercapacitors can store huge amounts of energy. Typically, the two electrodes are separated by a porous activated carbon membrane in the middle, with water or organic electrolyte on both sides. Figure 1 shows the structural principles of double-layer capacitors and ordinary capacitors.

　　1.2 Advantages and Disadvantages of Supercapacitors

　　Advantages of supercapacitors:

　　1) High current capacity. Supercapacitors are designed with a low equivalent series resistance, so the capacitor can send and sink very high currents. The low equivalent series resistance of supercapacitors allows the capacitors to charge quickly. The characteristics of the capacitors themselves allow the capacitors to charge and discharge at the same speed, which is not possible with batteries.

　　2) Long service life. The energy storage mechanism of supercapacitor is a highly reversible process. This process only moves charges and ions without creating or destroying chemical bonds, so the number of charge and discharge cycles can reach hundreds of thousands of times. Neither small cycle charge and discharge nor deep cycle charge and discharge will damage the performance of supercapacitors, making the use of supercapacitors more flexible. In addition, supercapacitors are easy to store and their performance will not be affected by long-term placement.

　　3) Wide temperature range. Because supercapacitors work without resorting to chemical reactions, they can operate over a wide range of temperatures. Supercapacitors can operate normally within the temperature range of -40℃~+65℃.

　　4) Environmentally friendly. The materials used in supercapacitors are non-environmentally polluting, and no toxic or harmful substances are produced during operation.

　　5) Easy maintenance. Supercapacitors basically require no maintenance, have no storage performance, no excessive discharge, and can work at any rated voltage or lower than the rated voltage.

　　6) Status monitoring is easy. Since the energy stored in a capacitor is only a function of capacitance and voltage, and the capacitance is relatively constant, a single open circuit voltage measurement can determine the state of charge.

　　7) Extend the service life of other energy sources. Energy sources like batteries, specialized engines, and fuel cells do not perform well under transient conditions. For some components, transient processes can significantly shorten the service life of the component. Combined with the use of supercapacitors and these energy sources, many of these transients can be unloaded from the primary energy source.

　　Disadvantages of supercapacitors:

　　1) Low energy density. The energy density of supercapacitors is 1~10Wh/kg, which is 1/10 of lithium batteries.

　　2) The terminal voltage changes greatly. The terminal voltage of the supercapacitor changes continuously during the charging and discharging process, so a voltage regulating device needs to be installed between the energy storage component and the load to maintain load-side voltage stability, which increases the cost of the energy storage system.

　　3) Expensive. The price of a supercapacitor cell is dozens of times that of a lithium battery.

　　2Lithium battery technology

　　2.1 Structure of lithium battery

　　Lithium battery is a rechargeable battery that mainly relies on Li+ ions to move between the positive and negative electrodes. During the charge and discharge process, Li+ intercalates and deintercalates back and forth between the two electrodes: During charging, Li+ deintercalates from the positive electrode and embeds into the negative electrode through the electrolyte, and the negative electrode is in a lithium-rich state; during discharge, the opposite is true. The schematic diagram is shown in Figure 2.

　　2.2 Analysis of the advantages and disadvantages of lithium batteries

　　Advantages of lithium batteries:

　　1) High energy density. Its volumetric energy density and mass energy density can reach 450Wh/dm^3 and 150Wh/kg respectively, and are still improving.

　　2) The cell voltage is high, about 3.6V, the voltage when fully charged is generally 4.2V, and the termination discharge voltage is not less than 2.5V.

　　3) The self-discharge rate is small, 5% to 10% per month, less than half of nickel-cadmium batteries and nickel-metal hydride batteries, and has no memory effect and excellent cycle performance.

　　4) High output power.

　　5) Wide operating temperature range. It can work normally between -20℃～60℃.

　　Disadvantages of lithium batteries:

　　1) The cost is high. The main reason is that the price of cathode material LiCoO2 is high. With the continuous development of cathode material technology, LiMn2O4, LiFePO4, etc. can be used as cathodes, which is expected to greatly reduce the cost of lithium batteries.

　　2) There must be a special protection circuit to prevent overcharge or overdischarge.

　　3) Poor compatibility with ordinary batteries, because generally only 3 ordinary batteries (3.6V) can be used to replace them with lithium-ion batteries.

　　3 Typical applications of energy storage systems

　　For ship electric propulsion systems, how to reduce fuel consumption, system maintenance costs, harmful gas emissions and increase the stability and reliability of the entire ship has always been the focus of researchers. Among them, Corvus is ahead of other companies in technology and has developed the widely used AT6500 energy storage system. Figure 3 shows the system structure diagram of the energy storage system applied to offshore support ships. The energy storage unit shares a common DC bus, and the DC bus is connected to the AC bus through a DC/AC conversion link to complete energy transmission. The entire energy storage system is composed of multiple AT6500 modules, namely lithium battery packs, which can continuously provide high power output, providing a better choice for ship system integration and design, thus reducing system complexity and unnecessary expenses.

　　4 Conclusion

　　Although energy storage units are currently expensive, energy storage units can smooth out ship power grid power fluctuations, improve system stability and reliability, improve fuel utilization, save fuel, and reduce harmful gas emissions. This is beneficial both from an economic and environmental perspective. In addition, with the development of large-capacity lithium battery and supercapacitor technology, the price of energy storage devices will inevitably decrease. The prospect of energy storage units being used in ship electric propulsion systems is worth looking forward to.

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