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Energy storage technology is widely used for regulating power supply fluctuations in the power grid and as an emergency energy source. On the basis of effectively solving the problems of intermittency, volatility, and randomness in renewable energy generation, it can provide fast response capacity for the power grid, alleviate the problem of wind and solar power abandonment, and become one of the most feasible solutions for a large number of renewable clean energy to be integrated into the power grid. With the continuous development of new energy distributed generation technology and the vast prospects of new energy vehicles, the energy storage industry will also usher in a development peak. Among them, the mixed energy storage of all vanadium flow batteries and lithium batteries has considerable advantages and rapid development.



1、 Currently, the main energy storage methods

At present, the main energy storage methods can be divided into mechanical energy storage (pumped storage, flywheel storage, compressed air storage), electromagnetic energy storage (capacitor storage, superconducting storage), and electrochemical energy storage (lithium battery storage, flow battery storage). Pumped water storage and compressed air storage have the characteristics of large capacity and low cost, but their special installation environment limits their layout in cities; Flywheel energy storage has high power density, but low energy density and high noise; Capacitor energy storage and superconducting energy storage have fast response speed, but their low capacity density and high cost limit their development.

Electrochemical energy storage utilizes the mutual conversion of chemical and electrical energy for long-term energy storage through battery charging and discharging, and can be configured with storage capacity according to electricity demand. At present, electrochemical energy storage has certain advantages in energy capacity and lifespan, and the technology is mature. The equipment has a small size, is not limited by geographical conditions, and is easy to install. It is undoubtedly the first choice for energy storage systems equipped with fast charging stations in the future. The following table shows the main performance and cost comparison of different energy storage methods [1].




Table 1 Main Performance Comparison of Different Energy Storage Methods [1]




Table 2 Cost Comparison of Different Energy Storage Methods [1]



2、 Advantages of All Vanadium Flow Battery and Lithium Battery Energy Storage and Hybrid Energy Storage

Lithium ion battery energy storage technology is currently the best choice for electric vehicle energy storage systems due to its excellent specific capacity, high specific power, low pollution, long cycle life, and small self discharge. However, its high magnification and frequent charging and discharging can cause irreversible capacity degradation of the battery, leading to accelerated aging of the battery. This is a challenge and difficulty that lithium batteries are currently researching and overcoming. The outstanding characteristics of all vanadium flow batteries (VRB) are high power, large capacity, long lifespan, high safety, and stable charging and discharging performance. Its energy storage efficiency can reach up to 90%, and it can be combined with distributed power sources to achieve energy storage for discontinuous energy generation. Unlike conventional battery energy storage, the active substances in flow batteries are distributed in different storage tanks, which can break through the capacity limitations of conventional batteries and give them unique advantages in large-scale integration into the power system.

The international research institution Navigant defines hybrid systems as stable energy storage systems that combine two or more complementary operational characteristics of energy storage technologies. In both vanadium flow batteries and lithium batteries, the two energy storage technologies have their respective advantages, namely the high specific power of lithium batteries and the high capacity of vanadium flow batteries. Combined with factors such as cost, technological maturity, efficiency, and lifespan, the mixed energy storage of vanadium flow batteries and lithium batteries can comprehensively meet the requirements of the power system for energy storage systems, making the energy storage system have both high power and high energy efficiency. Of course, there are also many research attempts to combine battery energy storage with supercapacitor energy storage as a hybrid energy storage method. Currently, it has entered the early stage of demonstration station construction and has great development space in the future. The comparison of different battery energy storage technologies is shown in the following table [2].




Table 3 Comparison of Different Battery Energy Storage Technologies [2]



3、 All vanadium flow battery and lithium battery hybrid energy storage station

At present, significant progress has been made in the construction of mixed energy storage stations for all vanadium flow batteries and lithium batteries, and they are currently in the stage of demonstration station construction. In December 2021, the national power investment Haiyang energy storage demonstration project in China was connected to the grid and put into trial operation. The project covers an area of 46 acres, with a total investment of about 440 million yuan and a designed total capacity of 101 MW/202 MWh, including 100MW/200MWh of lithium iron phosphate battery energy storage and 1MW/2MWh of all vanadium flow battery energy storage. At the same time, the Shanxi Provincial Development and Reform Commission also made public the first batch of "new energy+energy storage" pilot demonstration projects, among which there were three mixed energy storage station construction projects of "lithium iron phosphate+all vanadium flow battery", accounting for about 1/5 of all projects.




State Power Investment Haiyang Energy Storage Demonstration Project Power Station



On the foreign side, the Oxford Superenergy Hub Project (ESO) in the UK entered the live commissioning phase in December 2021. ESO combines a 50MW/50MWh lithium-ion battery system with a 2MW/5MWh all vanadium flow battery system, making it the largest all vanadium flow battery and lithium battery hybrid energy storage station to date, and will soon begin trading in the UK market.


Oxford Superenergy Hub Project (ESO) in the UK



References

[1] Faye Wong Application and Economic Evaluation of Hybrid Energy Storage Systems in Fast Charging Stations [D]. North China Electric Power University, 2018

[2] Zou Wanjia Economic Analysis of Battery Energy Storage Technology under Different Business Investment Models [D]. China University of Petroleum (Beijing), 2019

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