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At the end of 2021, many provinces and autonomous regions released development plans for new types of wind and solar energy storage, with a requirement of more than 4 hours for energy storage duration.
Notice of the National Development and Reform Commission on Increasing the Scale of Grid Connection: For the scale beyond the guaranteed grid connection of power grid enterprises, the peak shaving capacity shall be built at a power linkage ratio of 15% (for a duration of more than 4 hours) in the initial stage;

Liaoning Development and Reform Commission's Supplementary Plan for the Scale of Wind Power Construction in the Province: Encourage the construction of new energy storage facilities that are no less than 15% of the installed wind power capacity (with a duration of more than 4 hours);

The Implementation Opinions of the Inner Mongolia Regional Government on Accelerating the Development of New Energy Storage: The power of shared energy storage power stations shall not be less than 50000 kilowatts and the duration shall not be less than 4 hours;

Hebei Development and Reform Commission's Notice on Marketization and Grid Connection of Wind and Photovoltaic Power: It is required that wind and solar power stations connected to the grid in 2021 have a storage capacity of 10% -20%, with a duration of 4 hours;

Under the continuous demand for energy storage time, flow batteries in new energy storage technologies have shown unique advantages. As a new type of secondary battery, liquid flow battery achieves the charge and discharge of the battery through reversible changes in the valence state of chemical active substances, thereby achieving the goal of mutual conversion between chemical energy and electrical energy.
As is well known, renewable energy generation such as solar and wind energy has the characteristics of instability, discontinuity, and uncontrollability. Large scale grid connection will have an impact on the safe and stable operation of the power grid, resulting in a large number of wind and solar power abandonment issues. Developing supporting energy storage battery systems and achieving grid connection of electricity is currently an important issue for the utilization of new energy generation. Traditional lead-acid batteries have problems such as rapid capacity decline, short service life, and environmental pollution. Many lead-acid batteries experience a capacity reduction of more than half after 1-4 years of use. This type of inefficient battery no longer meets the needs of the times and its proportion is decreasing year by year. Lithium batteries, as a representative of new types of batteries, have fully entered the stage of industrial development. Due to their excellent specific capacity, high specific power, low pollution, long cycle life, and small self discharge, lithium battery energy storage is currently the best choice for electric vehicle energy storage systems. However, in renewable energy scenarios, its advantages are not obvious, and safety issues cannot be ignored. In contrast, the advantages of flow batteries are very obvious, and the advantages of flow batteries can be summarized as follows:
(1) Large energy storage capacity: This feature mainly depends on the fact that the active substance of the electrolyte in the flow battery is not encapsulated inside the battery. Its positive and negative electrolytes are stored in different external storage tanks, so it can break through the conventional battery capacity limit and design the storage tank capacity and electrolyte concentration according to the load demand in different scales, making it have the unique advantage of large-scale integration into the power system and suitable for different application ranges from hundreds of kilowatts to hundreds of megawatts;


Overall structure diagram of liquid flow battery [1]

Structural diagram of liquid flow unit battery [2]
(2) Excellent charging and discharging characteristics: In current and successfully applied all vanadium flow batteries, due to the high reversibility and low polarization of vanadium ions in electrochemical reactions, their charging and discharging characteristics are excellent, and their response speed for charging and discharging switching is fast, with a small degree of performance degradation. At present, the switching time of charge and discharge for all vanadium flow batteries can reach 0.02s, and the start-up of the stack in a full electrolyte state can be controlled within 2 minutes;
(3) Adjustable output power: The output power of the liquid flow battery can be adjusted by increasing or decreasing the number of stacks and adjusting the size of the stack area, which also makes the energy output of the liquid flow battery flexible and adaptable to different power requirements.
(4) High safety performance and long lifespan: With the continuous emergence of news about lithium battery explosions, battery safety issues continue to receive widespread attention and research. In lithium batteries, the structural damage caused by lithium dendrite growth penetrating the separator and causing circuit short circuits increases the system temperature, and the electrolyte in the lithium battery system is a combustible low melting point organic lipid with a large amount of graphite negative electrode structure, which induces explosions. The energy storage medium of liquid flow batteries is aqueous solution, which is safer and more reliable, without the risk of explosion or fire; And the uniformity of the flow battery is good. Currently, the cycle life of all vanadium flow batteries can reach more than 15000 times, with a lifespan of more than 10 years, which is 3-6 times that of lithium batteries;
(5) Free site selection, environmentally friendly, and low operation and maintenance costs: Compared to the strong dependence of non liquid flow energy storage devices on site selection, the site selection of liquid flow batteries is relatively free and less restricted by geographical and environmental factors. The electrolyte of flow batteries can achieve repeated circulation and regeneration, thus saving mineral resources effectively. Moreover, the system can operate in a fully automatic and enclosed manner, resulting in minimal pollution and lower maintenance and operational costs.
Of course, flow batteries also have their own bottlenecks and challenges. Taking the most widely used all vanadium flow battery as an example:
(1) Low energy density: Due to the low energy density of liquid flow batteries, they require a certain amount of land area. However, due to their high safety characteristics, in scenarios with energy storage capacity of over 100 megawatts, the land area may even be better than that of lithium batteries.
(2) The working temperature range is relatively small: the optimal working temperature for all vanadium flow batteries is between 5 ℃ and 45 ℃, while the optimal working temperature for iron chromium flow batteries is between 40 ℃ and 60 ℃. Otherwise, other equipment needs to be adjusted, which increases equipment operating costs. Otherwise, once the temperature exceeds the optimal level, the precipitates in the positive electrode solution will block the flow channel, affecting the operation of the chimney and leading to the failure of the flow battery.
(3) High overall cost: For all vanadium flow batteries, their energy storage cost is 1-2 times that of lithium batteries, with the main cost being vanadium electrolyte and its key structure ion exchange membrane. The cost of vanadium electrolyte accounts for about 40% of the total cost. Due to the gap between the development of domestic ion exchange membranes and advanced foreign technologies, there is currently a strong dependence on imported membranes represented by DuPont Nafion, resulting in high costs. However, due to the gradual expansion of large-scale applications, the cost of liquid flow batteries is continuously decreasing, and it is expected to drop to 1.3 yuan/watt hour within 10 years.

Comparison of performance between all vanadium flow batteries and traditional lead-acid batteries [3]
At present, in addition to the most widely used all vanadium flow batteries, there are also iron chromium flow batteries, all iron flow batteries, zinc bromine flow batteries, zinc iron flow batteries, sodium polysulfide bromine flow batteries, zinc manganese flow batteries, and so on. At the end of 2020, the "Ronghe No.1" high-capacity battery stack successfully developed by State Power Investment Corporation was successfully applied in the Zhanshigou 250kW/1.5MWh demonstration project in Zhangjiakou, Hebei. State Power Investment Corporation has independent intellectual property rights to it, and the "Ronghe No.1" iron chromium flow battery stack production line was officially put into operation this year. Each production line can produce 5000 30kW "Ronghe No.1" battery stacks per year, marking the official commercialization stage of iron chromium flow batteries. It is expected that by the end of this year, the world's first megawatt level iron chromium flow battery energy storage demonstration project launched by State Power Investment Corporation in Huolinhe, Inner Mongolia will also enter the production stage. With the continuous development of iron chromium flow batteries, new possibilities will be provided in renewable new energy storage.

The world's largest 250KW iron chromium flow battery energy storage demonstration project located in Zhangjiakou
Summary: Liquid flow batteries have strong long-term energy storage advantages over traditional lead-acid batteries and new lithium batteries due to their large energy storage capacity, excellent charging and discharging properties, adjustable output power, high safety performance, long service life, free site selection, environmental friendliness, and low operation and maintenance costs when dealing with unstable, discontinuous, and uncontrollable new energy generation scenarios. At present, although liquid flow batteries still have certain limitations in operating temperature and comprehensive cost, with the industrial layout and accompanying technological improvement, liquid flow batteries will inevitably become the optimal solution for renewable energy storage devices.
Reference:
[1] Bao Wenjie. Analysis of the Current Status and Prospects of Zinc Dendrite Research in Zinc Iron Flow Battery [J]. Contemporary Chemical Research, 2021, (23): 14-16
[2] Bao Wenjie. Overview and prospects of typical liquid flow battery energy storage technology [J]. Science and Technology Information, 2021,19 (28): 33-39
[3] Zhang Yu, Wang Xiaoli, Zhao Honggui, Sun Min, Diao Yongfeng All Vanadium Liquid Flow Energy Storage Battery - A New Choice of Green Base Station Power Supply for New Energy [C]. Proceedings of the 2011 Communication Power Supply Academic Symposium at the Communication Power Supply New Technology Forum, 2011:286-290

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