Application of lithium iron phosphate battery in photovoltaic power generation
Abstract: High-performance energy storage batteries are critical to the development of the photovoltaic industry. Compared with lead-acid batteries, lithium iron phosphate batteries have the advantages of high specific energy, high energy storage efficiency, long cycle life, and low use cost. Using this type of lithium battery as an energy storage device can increase energy efficiency to about 90%, so it is very suitable for use as a new energy storage device. The lead-acid batteries and lithium iron phosphate batteries are compared in terms of conventional performance and energy storage characteristics. Based on the analysis of the feasibility of using lithium iron phosphate batteries as energy storage devices in photovoltaic systems, the corresponding photovoltaic energy storage systems are designed.
Keywords: lithium iron phosphate battery; photovoltaic power generation; storage efficiency
CLC number: TM 615 Document identification code: A Article number: 1002-087 X (2014) 08-1487-02
Among many renewable energy sources, solar energy has become the most potential energy form with its green, environmental protection, inexhaustible and inexhaustible characteristics, and has huge market prospects. China has very rich solar energy resources, and the development of the photovoltaic industry is of great significance to the improvement of electricity consumption in China's underdeveloped and remote areas. However, photovoltaic power generation has discontinuity and instability, and the power generation performance changes with changes in the external environment. Therefore, high-performance power storage links are crucial to the development of the photovoltaic industry.
At present, most photovoltaic systems use lead-acid batteries as energy storage devices. Compared with lead-acid batteries, lithium iron phosphate batteries have the advantages of high specific energy, high energy storage efficiency, long cycle life, and low use cost. This article compares lead-acid batteries and lithium iron phosphate batteries in terms of conventional performance and energy storage characteristics. Based on the analysis of the feasibility of using lithium iron phosphate batteries as energy storage devices in photovoltaic systems, the corresponding photovoltaic energy storage systems are designed.
1 The basic requirements of solar photovoltaic power generation system for energy storage
Solar power generation systems have high costs, low conversion efficiency, and strong variability with the environment, so they have higher requirements for energy storage. The service life of the solar photovoltaic power generation system is generally 20 years, and the energy storage link with it is required to have the characteristics of long service life, stable performance, high energy efficiency, and strong ability to adapt to the environment.
At present, the energy storage technologies used in photovoltaic power generation systems are mainly divided into electrochemical cells, flywheels, superconducting coils and so on. In comparison, flywheel energy storage and superconducting energy storage have unparalleled advantages in terms of performance of chemical batteries, and will inevitably become an inevitable trend in the development of energy storage devices. It is still the main method of solar photovoltaic power storage.
The use of chemical cells as energy storage links in photovoltaic power supply means that the energy released by the redox in the chemical reaction is directly converted into DC electrical energy for the load. At present, there are six main types of electrochemical media that can be used as solar energy storage units, as shown in Table 1 [1].
Table 1 Performance comparison of various batteries
Electrochemical medium cell voltage / V evaluation
Lead acid 2.0 lowest cost
Nickel cadmium 1.2 has a memory effect
Nickel metal cyanide 1.2 sensitive to temperature
Lithium ion 3.6 safe, no metallic lithium
Lithium polymer 3.0 contains metallic lithium
Zinc-air 1.2 requires good air management to limit the rate of self-discharge
Among the above six types of batteries, the energy density of lead-acid batteries is low, but their technology is mature and cost-effective. Therefore, the energy storage units of photovoltaic power generation systems currently mainly use valve-regulated sealed lead-acid batteries. However, with the further development of photovoltaic power generation, higher requirements are placed on the performance and safety of the battery. The weakness of the performance of the lead-acid battery itself has increasingly prominent adverse effects on photovoltaic power generation. Therefore, there is an urgent need for a high-efficiency chemical battery to serve as the corresponding energy storage link.
2 Performance analysis of lithium iron phosphate battery
Lithium iron phosphate battery is a new secondary power source developed in recent years. It has the characteristics of large input and output power, wide operating temperature range, no memory effect, maintenance-free, 2,000 charge and discharge service life, safety and green environmental protection. Is becoming the protagonist of chemical batteries [2].
The main material of the lithium iron phosphate battery is carbon, the main material of the positive electrode is lithium ferrous phosphate LiFePO4, the electrolyte uses LiPF6 organic solvent, and the basic principle of charge and discharge is the insertion and extraction of lithium ions between the positive and negative electrodes.
The voltage platform of the lithium iron phosphate battery is 3.2V, and the specific energy is more than twice that of the lead acid battery, while the volume specific energy is 4 to 5 times that of the lead acid battery. If the lithium iron phosphate battery is used instead of the lead acid battery for photovoltaic power generation The system can greatly reduce the space occupied by the battery and reduce the maintenance workload. The conventional performance of the two batteries is shown in Table 2.
Figure 2 General performance table of lithium iron phosphate battery and lead-acid battery
Item Lithium iron phosphate battery Lead-acid battery
Nominal voltage / V 3.2 2.0
Mass specific energy / (Wh · kg-1) 90 40
Volume specific energy / (Wh · L-1) 190 70
Self-discharge (28 days) /% ≤10 ≤10
Operating temperature range / ℃ -20 ～ 55 -10 ～ 45
Best working temperature range / ℃ 5 ～ 50 ～ 25
Lithium iron phosphate battery can choose power type or energy type, which has a wide range of application, and its safety has been greatly improved compared with lead-acid battery. At the same time, the rate characteristic of the lithium iron phosphate battery is 5 C to 15 C, and the conversion efficiency is greater than 95%, while the rate characteristic of the lead acid battery is 0.1C to 1C, and the conversion efficiency is greater than 80%. Therefore, in terms of energy storage efficiency, lithium iron phosphate batteries have obvious advantages.
3 Development and design of lithium iron phosphate battery energy storage system
The basic architecture of the energy storage system of the photovoltaic power station is shown in Figure 1. The photovoltaic module array uses the photovoltaic effect of the solar panel to convert light energy into electrical energy, and charges the lithium iron battery through the photovoltaic controller, and then through the inverter. Directly supply power to electrical equipment or introduce electrical energy into the grid through grid-connected inverters. The battery management system BMS in Figure 1 plays a role in managing the storage battery. First, the data of the battery pack is collected, and the operating status of the battery is diagnosed based on the collected data, thereby monitoring the battery pack overvoltage and undervoltage. Pressure, over temperature, over charge, over discharge, over current, remaining capacity (SOC) and single cell health status (SOH).
Figure 1 Overall structure of photovoltaic power station energy storage system
Lithium battery packs play two roles in the system. One is energy storage and regulation, and the other is load balancing. It converts the electrical energy output by the photovoltaic power generation system into chemical energy and stores it for use when the power supply is insufficient. The lithium battery pack in this system uses a lithium iron phosphate battery as the basic structure shown in Figure 2.
Figure 2 Structure diagram of energy storage system
As can be seen from Figure 2, the energy storage subsystem is composed of DC link, lithium iron phosphate battery pack, battery management system BMS, local control center, remote control center and other links. The DC link links the solar current and sends it to the battery Stored, the storage battery is connected to the battery management system, and the information is communicated with the local monitoring center through short-distance communication, and the local monitoring center is connected to the remote monitoring center through the public communication network to achieve effective monitoring of the energy storage link purpose.
4 Conclusion
The rapid development of solar photovoltaic power generation systems has put forward higher requirements for energy storage systems. Lead-acid batteries, because of their own inability to withstand high temperatures, overcharge and overdischarge, large maintenance workload, and short life, impede the further development of photovoltaic power generation. The characteristics of lithium iron phosphate batteries, such as high energy storage efficiency, long life, and good charge and discharge performance, make it very suitable as a power source for new energy storage. In this paper, based on the analysis of the feasibility of using lithium iron phosphate batteries as photovoltaic energy storage devices, a photovoltaic power storage system based on lithium iron phosphate batteries is designed. Research shows that the system has high energy efficiency, long charge and discharge life, stable and reliable operation, and good application prospects.