1 Overview
A microgrid is a collection of loads and micro-energy sources. It operates in a networked mode under normal conditions and can operate independently under emergency conditions. In these two modes, as an important unit of the microgrid system, the high-performance energy storage system can store the excess energy and send it to the load when needed, making full use of various energy sources, which has a great impact on the entire microgrid. As an important role, lithium iron phosphate batteries are applied to microgrids with their superior performance, which can significantly improve the overall performance of the microgrid system and the utilization of various energy sources.
2 Functions and characteristics of microgrid
A microgrid is a form of the power grid that is different from traditional power grids. As countries in the world continue to study microgrids, although their definitions are different, their purposes are similar, such as improving the safety of the power system, ensuring the reliability of power supply, improving the quality of power, and improving the efficiency of various energy sources. Utilization rate. Based on these functional requirements, it is required that the microgrid can be connected to the grid for grid-connected operation, and can also be cut off from the grid for independent operation, which greatly improves the power supply reliability of the grid and the safety of the power system. For the power grid, the microgrid can not only absorb power from the power grid, but can also quickly switch its own state according to the operating state of the power grid to choose whether to cut off the connection with the power grid or absorb the energy of the power grid to supplement their own needs; for power users, Microgrid meets some of their specific needs, such as increasing the reliability of power supply, reducing the distance of power supply, and then reducing the loss of power transmission lines.
3 The need for energy storage systems
Electric energy storage technology is very important for realizing the basic functions of microgrids. Why the microgrid needs to store electrical energy is mainly due to the following four reasons: (1) To ensure the reliability of the power supply system; (2) To ensure the quality of power supply; (3) To improve the comprehensive utilization efficiency of electrical energy; ( 4) To improve the grid connection performance of various new energy sources. Therefore, a high-performance energy storage system is a powerful guarantee for the microgrid to realize its own functions.
3.1 Types of energy storage systems
Energy storage technology has been developed for quite a long time, and it has a variety of forms. In addition to lead-acid batteries and lithium batteries that can often be seen in daily life, there are also pumped energy storage, flywheel energy storage, and superconducting magnetic energy storage. , Superconducting capacitor energy storage, vanadium flow battery, sodium-sulfur battery, a nickel-hydrogen battery, etc. The international mainstream batteries include lead-acid batteries, lithium-ion batteries, nickel-hydrogen batteries, and so on. Flywheel energy storage, superconducting magnetic energy storage, and superconducting capacitor energy storage are still in the research stage and have no large-scale applications. Lead-acid batteries have been applied to a certain scale in microgrid energy storage systems, but due to their own characteristics, some shortcomings are slowly emerging.
3.2 Characteristics of traditional energy storage systems-lead-acid batteries
Lead-acid batteries have relatively high requirements for environmental temperature: the representative of traditional energy storage systems is lead-acid batteries. Lead-acid batteries have relatively high requirements for environmental temperature, which means that the environmental requirements of their use cases are improved.
Lead-acid batteries have high requirements for the power distribution room: the power distribution equipment of a system is mainly power equipment, and the area and weight of the battery room in the power equipment cannot be ignored.
The high-rate discharge performance of lead-acid batteries is poor: when the operation mode of the microgrid is switched from grid-connected to independent operation, the energy storage system will flow a large current in a short time, which requires the energy storage system to have high Rate discharge performance, while the performance of lead-acid batteries in traditional energy storage systems
is poor.
The monitoring of lead-acid batteries is inaccurate: Most of the monitoring systems of lead-acid batteries make judgments based on voltage, and the accuracy of this method is very limited, resulting in a large amount of electric energy stored in the energy storage system after long-term use. deviation.
Lead-acid batteries pollute the environment: Because lead-acid batteries contain lead that pollutes the environment, improper production and waste disposal will cause serious environmental pollution.
3.3 Application prospects of lithium iron phosphate batteries in microgrids
3.3.1 The electrochemical principle of lithium iron phosphate battery: the anode material of lithium iron phosphate battery is lithium iron phosphate (peroxy compound of lithium), the anode material is graphite or coke, and the lithium iron phosphate battery is a kind of lithium battery. Named as the cathode material for lithium iron phosphate batteries.
3.3.2 Characteristics of lithium iron phosphate battery:
High energy density: Energy density can indicate how much energy is stored per unit volume or weight. The energy density of an electrochemical battery refers to the electrical energy that the battery's unit volume or mass can provide to an external load. Under the same weight, the energy density of lithium iron phosphate batteries is 3 to 5 times that of lead-acid batteries. That is to say, when the rated capacity of the battery is the same, the lithium iron phosphate batteries will be much lighter than lead-acid batteries and relatively reduced. Requirements for the strength of the support.
Long service life: The life of a recyclable battery refers to the number of cycles that the battery can be charged and discharged normally. The life of a lead-acid battery is about 500 times, while the life of a lithium iron phosphate battery can reach 1600 times, and the capacity can be maintained at 80%. The life of iron-lithium batteries is significantly better than that of lead-acid batteries.
Safety: The safety of lithium batteries has always been the key reason hindering its development, and lithium iron phosphate batteries completely solve the instability factors of lithium compounds. Lithium cobalt oxide and lithium manganate will explode under certain conditions, such as collisions. And so on, and the lithium iron phosphate battery has been carefully designed and passed strict performance and safety tests. It will not explode under severe collisions, penetrations, etc.
No memory effect: a cyclic charge-discharge battery is fully charged for a long time without using, its capacity will be relatively lower than the rated capacity value, this is the memory effect. Lead-acid batteries have an obvious memory effect. After testing, lithium iron phosphate batteries basically have no memory effect.
3.3.3 The shortage of lithium iron phosphate batteries. The cost of the lithium iron phosphate battery remains high because its material is lithium metal and the maturity of the processing technology is not high. In addition, the capacity of the lithium iron phosphate battery needs to be further improved. Before the energy management system and battery, charge-discharge balance function of lithium iron phosphate battery is perfect, it is not suitable for large-scale application in the micro-grid system.
4 Conclusion
Like fuel cells, vanadium batteries, flywheel energy storage technologies are not yet mature, and the shortcomings of traditional lead-acid batteries are becoming more and more obvious, lithium iron phosphate batteries, as a representative of current electrochemical battery technology, have gradually matured and their applications Slowly gain popularity. National policy support has accelerated the development of lithium iron phosphate batteries. The price of lithium iron phosphate battery will gradually decrease in future development and popularization. It will become the mainstream battery in the future energy storage system with its superior performance, inject fresh blood into the microgrid energy storage system, and further enhance the microgrid The overall performance.
