With technological advancements and the expansion of application scenarios, the potential of hybrid energy storage applications is becoming increasingly prominent, gradually becoming one of the important development directions in the field of energy storage. Recently, the Ministry of Industry and Information Technology released the "Action Plan for High-Quality Development of New Type Energy Storage Manufacturing Industry (Draft for Comments)", proposing to encourage the exploration and development of various types of hybrid energy storage technologies in combination with application needs.

Hybrid energy storage systems improve overall system performance by adopting combinations of two or more energy storage technologies with different performance characteristics. The background for the emergence of hybrid energy storage systems is that there is currently no single energy storage solution that can dominate the market. Hybrid energy storage, with its strong complementary performance, multiple functions, risk diversification, and high comprehensive efficiency, can achieve a "1+1>2" effect, thus attracting attention in the industry. In 2022, the National Development and Reform Commission and the National Energy Administration issued the "14th Five-Year Plan for the Development of New Type Energy Storage", which also mentioned promoting the joint application of various energy storage technologies in line with system needs and carrying out pilot demonstrations of composite energy storage.

At present, electrochemical energy storage represented by lithium batteries has a short storage time and a relatively small capacity scale, making it difficult to address the imbalance of electricity between new energy output and load demand on a long-term scale (weeks, months, years). There is a need for large-capacity, long-cycle energy storage technologies to enhance the system's ability to absorb new energy. By applying long and short cycle energy storage in conjunction, it is possible to achieve multi-time scale balance of electricity and electricity quantity in the power system, promote the efficient absorption of new energy, and has the potential to become a core regulatory resource supporting the low-carbon development and safe and reliable operation of new power systems. Therefore, it is urgent to carry out research on the optimized configuration, coordinated control, and energy management technologies of long and short cycle hybrid energy storage oriented towards new energy absorption, in order to fully leverage the complementary advantages between long and short cycle hybrid energy storage and meet the future power system's needs for cleanliness, safety, and efficiency.

Hybrid Energy Storage Technology: Key Technologies in Energy System Applications

(1) Optimization Configuration Technology

Reasonable optimization configuration is the premise for the optimized control and operation of long and short cycle hybrid energy storage, which can enhance the flexibility and stability of the power system, optimize the energy structure, improve economic benefits, and promote sustainable energy development. However, there is a wide variety of energy storage devices with significant differences in performance parameters, cost-effectiveness, environmental impact, market demand, and technological maturity. It is necessary to select appropriate energy storage devices based on different scenario requirements and comprehensively consider the above differences. The same type of energy storage device also has different power/capacity scales to choose from. In addition, the voltage level, electrical position, and geographical distribution of new energy接入 into the power system also pose higher requirements for the siting of energy storage systems. Therefore, the optimization configuration of long and short cycle hybrid energy storage systems mainly involves three aspects: selection, siting, and sizing.

Regarding the development of software and platforms for energy storage system optimization configuration, several domestic and international institutions have made attempts and achieved certain results. However, there are still issues such as single-function and complex operation, insufficient universality and adaptability of models, and the need to enhance technological integration and innovation capabilities. Future development of energy storage optimization configuration platforms should focus on improving the comprehensiveness and adaptability of functions, strengthening technological integration and innovation capabilities, providing more accurate, efficient, and convenient energy storage system optimization configuration services, and promoting the development and application of energy storage technology.

(2) Coordinated Control Technology

Due to the different operational control characteristics of long and short cycle energy storage, an efficient and reliable coordinated control strategy is a key prerequisite for ensuring the efficient management of energy in long and short cycle energy storage systems and the realization of optimization objectives. The coordinated control of long and short cycle hybrid energy storage generally receives control signals from the upper layer and carries out differentiated control of the underlying converters through control algorithms to ensure the coordinated operation of energy storage units and inverters in the system. Coordinated control technology can be divided into classical strategies and intelligent strategies. The main difference between the two is that classical strategies usually require accurate system models and are sensitive to parameter changes, while intelligent strategies do not require accurate models and are robust to parameter changes, including nonlinear control, model predictive control, fuzzy logic control, and artificial intelligence control strategies.

(3) Energy Management Technology

Energy management also plays a crucial role in long and short cycle hybrid energy storage systems. Its core task is to fully consider the working characteristics, cycle life, and durability limits of various energy storage devices, based on following various operational constraints. It combines source-load forecasting and regulation needs to reasonably allocate the output power of each energy storage unit. Currently, according to the implementation method, energy management technology can be divided into three major categories: rule-based, optimization algorithm-based, and intelligent algorithm-based.

Under policy guidance, the construction of hybrid energy storage projects continues to accelerate, with multiple projects being connected to the grid and put into operation. Currently, the hybrid energy storage market is mainly dominated by the "lithium iron phosphate +" model. This year, "lithium iron phosphate + flow battery" and "lithium iron phosphate + flywheel" have shown an accelerating growth trend in the hybrid energy storage market. According to data from the CESA Energy Storage Application Branch Industry Database, in the hybrid energy storage installation projects from January to October, the operational power scale of lithium iron phosphate battery energy storage accounted for 76.22%, ranking first; flow battery power accounted for 18.79%, ranking second; and flywheel energy storage accounted for 3.57%, ranking third. This indicates that hybrid energy storage technology applications will increasingly highlight long-duration applications and power support.

As one of the long-duration energy storage technologies, flow batteries, compared to pumped hydro storage and compressed air energy storage, have the characteristics of flexible configuration, short construction periods, and higher system efficiency. Compared to the currently widely used lithium batteries, flow batteries have the advantages of large capacity, higher safety, and long-duration energy storage.

Furthermore, in the energy storage market within 2 hours, lithium battery technology is mature and has a lower cost. However, after more than 2 hours, the cost of lithium batteries gradually increases, and they are less cost-effective than flow batteries. Therefore, the combination of flow batteries and lithium batteries is thriving in the hybrid energy storage market.

In demonstration construction projects, the number of hybrid energy storage station construction projects with "lithium iron phosphate + vanadium flow battery" is the highest. In addition to vanadium flow batteries, projects such as lithium batteries + iron-chromium flow batteries, zinc-bromine flow batteries + lithium iron phosphate energy storage are also accelerating into the demonstration phase.

From April to May 2024, Inner Mongolia released two batches of independent new energy storage demonstration projects on the grid side, including 16 long-duration energy storage projects, 10 of which adopted hybrid energy storage technology, with 8 of them including flow battery energy storage.

Recently, the largest grid-forming energy storage project in China, and also the largest vanadium flow battery and lithium iron phosphate hybrid energy storage project - the Xinhua Wushi 500,000 kW/2,000,000 kWh grid-forming energy storage project, has made new progress. The first 220kV main transformer has completed testing and is ready, marking the imminent arrival of a critical moment for project equipment delivery. The project has a total installed capacity of 500MW/2GWh, including 250MW/1GWh lithium iron phosphate battery energy storage and 250MW/1GWh vanadium flow battery energy storage, with a storage duration of 4 hours.

Since the beginning of this year, under the impetus of multiple factors such as policy, capital, and technology, flow batteries have accelerated their penetration in the power grid frequency regulation market, combining with energy storage technologies such as lithium batteries, and rapidly landing in the hybrid energy storage market. Future energy storage applications will develop towards multi-scenarios, multi-technology routes, and diversification. Comprehensive optimization of energy storage costs and performance will be the key direction for current and future power system energy storage layout. By utilizing the strengths of different energy storage technologies through hybrid energy storage, a greater supportive role will be played in the construction of new types of power systems.

Partial data source: Zhao Bo, Lin Da, Chen Zhe, Du Kaijian, Zhang Leiqi, Liu Min, Zhang Xuesong (State Grid Zhejiang Electric Power Research Institute, Hangzhou 310004) DOI: 10.20121/j.2097-2784.ntps.240007, "China Energy News"

Product Series：

Vanadium Redox Flow Battery - Energy Storage System / BMS

Liquid Flow Battery - Non-Fluorinated Ion Exchange Membrane

LAB Series R&D Demonstration Equipment

NeLCOS® Energy Storage System Levelized Cost of Energy Calculator