Image source@Visual China
Text | Tianfeng Tianrui Investment
Under the guidance of the "dual carbon" goal, the mainland's new energy industry has ushered in a new era of rapid development. Renewable energy, represented by photovoltaic and wind power, is rapidly changing our energy landscape. With the rapid increase in the proportion of renewable energy power generation connected to the grid, the "triple double" characteristics of "double-sided randomness" and "double-peak and double-high" of the power system are becoming increasingly significant, and the power grid is facing the challenges of absorption pressure and operation safety.
In order to ensure the stability of the power system, economic dispatching and the high quality of electric energy, and further form a controllable and dispatchable power grid, the power system needs to be equipped with energy storage to make renewable energy become a more friendly high-quality new energy source.
With the gradual increase in the penetration rate of renewable energy, the "time + space" mismatch of renewable energy power generation such as photovoltaic and wind power has gradually become prominent, further giving rise to the demand for long-term energy storage dispatch in the power system. Long-term energy storage (energy storage technology with a continuous discharge time of more than 4 hours) can improve the power generation and consumption capacity of renewable energy, and effectively reduce the consumption pressure and operating cost of the power system. The all-vanadium flow battery (hereinafter referred to as "vanadium battery"), which has the advantages of high material intrinsic safety, long cycle life, recyclable electrolyte, high cost performance in life cycle, and environmental friendliness, may stand out in the field of long-term energy storage.
Energy storage is indispensable in the wave of new energy
In recent years, the need for renewable energy has become more urgent. Renewable energy sources such as photovoltaic and wind power are leading the pace of the future energy revolution with their unique advantages and huge potential. As the proportion of renewable energy in the grid continues to increase, battery energy storage systems (BESSs) will play an increasingly critical role in the balance between renewable energy supply and grid load, and become an indispensable technology to accelerate the replacement of traditional fossil fuels with renewable energy.
According to the different principles and technologies of energy storage, energy storage technologies can be divided into three categories: electric energy storage, thermal energy storage and hydrogen energy storage. Among them, the energy storage technology industry other than pumped storage is collectively referred to as new energy storage, mainly including lithium-ion batteries, flow batteries, compressed air energy storage, flywheel energy storage, hydrogen (ammonia) energy storage, etc. Compared with pumped hydro storage, new energy storage generally has the advantages of short construction period, flexible site selection, fast response and strong adjustment ability, which can provide more time-scale regulation and control capabilities for the power system.
Figure 1: Classification of energy storage technology Source: Shanghai Peneng Energy Technology Co., Ltd. prospectus, public information collation, Tianfeng Tianrui mapping
The output of renewable energy sources, such as wind and solar, fluctuates greatly and is accompanied by uncertainty.
From the perspective of wind and solar power output and grid load peaks and valleys, wind power generally has low output during the day and high output at night, photovoltaic power generation has high output at noon and no longer outputs at night, while the daily electricity load presents two peaks in the morning and evening, the net load peak during the peak wind and solar output period is significantly reduced, the net load shows a significant "duck curve" characteristics, and the load volatility increases significantly.
From the perspective of seasonal wind and solar output and grid load peaks and valleys, the peak wind power output is spring and autumn, and the peak of photovoltaic power generation is summer and autumn (daytime), and the load power is high in winter and summer, especially at night, while the renewable energy generation is low, and the grid load is "winter and summer" double peak characteristics, which is difficult to match the seasonality of renewable energy power.
From the perspective of the geographical distribution of renewable energy output and grid load, domestic photovoltaic resources and wind energy are mainly distributed in Northeast China, North China and Northwest China, but the areas with high electricity load are mainly in the eastern region, and long-distance transmission puts forward higher requirements for grid stability and peak regulation capacity.
图2：最佳储能时长与风光渗透率关系 图源：Albertus P , Manser J S , Litzelman S . Long-Duration Electricity Storage Applications, Economics, and Technologies[J]. Joule, 2020, 4(1):21-32.DOI:10.1016/j.joule.2019.11.009.
With the increase in the penetration rate of renewable energy, the "time + space" mismatch of renewable energy power generation such as wind and solar has become increasingly prominent, driving the demand for energy storage during growth.
The higher the penetration rate of renewable energy sources such as wind and solar, the higher the demand for long-term energy storage. In addition, compared with short-term energy storage, long-term energy storage has the role of fast response regulation and long-term output to balance the load of the grid.
In January 2022, the National Development and Reform Commission (NDRC) and the National Energy Administration (NEA) issued the "14th Five-Year Plan for the Development of New Energy Storage", proposing to achieve breakthroughs in long-term energy storage technologies such as hydrogen energy storage and hot (cold) energy storage by 2025, and to increase the research and development of key technologies and equipment such as flow batteries and sodium-ion batteries. According to the data of the national energy information platform, as of the end of November 2023, more than 20 provinces, cities and regions in China have clarified the requirements for new energy distribution and storage, and the average distribution and storage time has exceeded 2 hours, of which Shanghai, Tibet, Fujian, Inner Mongolia, Hebei, Gansu and Hexi have exceeded 4 hours, and more than 20 domestic energy storage projects have been signed for more than 4 hours, including compressed air energy storage, flow batteries, gravity energy storage and other technical routes.
Overseas, at the end of 2022, the U.S. Department of Energy (DOE) announced that long-term energy storage system demonstration projects with energy storage duration of 10-24 hours will be eligible for a total of $349 million in funding to support the construction of low-cost, reliable, carbon-free modern power grids in the United States. In February 2022, in order to support innovative long-term energy storage technology projects in the UK, the UK Department for Business, Energy and Industrial Strategy (BEIS) announced a grant of 39.6 million pounds, and the first batch of 24 selected projects cover green hydrogen electrolyzers, gravity energy storage, all-vanadium flow batteries, compressed air energy storage, seawater + compressed air combined energy storage and other technical routes.
With the successive implementation of long-term energy storage policies and projects at home and abroad, the global new energy storage market continues to grow. According to data from energy storage research platform CNESA (China Energy Storage Alliance, referred to as "CNESA"), as of the end of June 2023, the cumulative installed capacity of domestic energy storage projects reached 70.2GW, a year-on-year increase of 44%, of which the installed capacity of new energy storage was 21.06GW, accounting for 30.9%, and the installed capacity of pumped storage was 48.51GW, accounting for 69.1%, a year-on-year decrease of about 10%. Among the new energy storage installations, lithium batteries occupy the mainstream, accounting for 95.9%, and flow batteries account for 0.8%. At present, the penetration rate of flow batteries in the energy storage market is still low, and with the increasing attention to energy storage safety and the long-term energy storage planning, vanadium batteries may accelerate their penetration in the new energy storage market.
Figure 3: Cumulative domestic energy storage installation unit results in June 2023 Source: CNESA, Tianfeng Tianrui mapping
Vanadium batteries, the winner of flow batteries
Flow battery technology can be traced back to 1884, when French engineer Charles Renard developed the original zinc-chlorine battery for military airships, which was not yet equipped with the fluid drive system, the core component of modern flow batteries. With the advent of separators in the 1950s, flow battery technology began to take off. Its evolutionary process can be broadly divided into three stages:
■ Embryonic stage
In 1974, NASA scientist L.H. Thaller tried to explore the method of storing solar energy on the lunar base, and proposed to use ferric chloride (FeCl2) and chromium trichloride (CrCl3) as the electrochemically active substances of flow batteries, using hydrochloric acid as the matrix and the negative diaphragm as the separator, designed the first Fe-Cr double flow battery, and developed a prototype with a power of 1kW. However, due to the cross-contamination of the active substances in the electrolyte of the positive electrode and the negative electrode during operation, the voltage is unstable, the battery capacity is attenuated and cannot be operated for a long time, which greatly reduces the actual service life of the battery.
In order to avoid cross-contamination of positive and negative active materials, the solution is to construct all positive and negative active materials with compounds of different valence ions of the same element, and to continue to improve the separator. Vanadium metal compounds are particularly compelling because of their wide range of valence states and high safety.
■ R&D period
After more than ten years of exploration, the all-vanadium flow battery technology has made great progress due to the lack of cross-contamination of the positive and negative electrolytes of vanadium batteries, and its high safety and reliability.
In 1986, Professor Maria Skyllas-Kazacos of the University of New South Wales (UNSW) in Australia applied for the first international patent for an all-vanadium flow battery and built a 1kW test stack with an energy efficiency of 72-88%. The battery uses vanadium ions of different valence states to form redox pairs, graphite felt as the electrode, graphite-plastic grid as the current collector, proton conduction film as the battery separator, positive and negative electrolytes flow through the electrode surface during charging and discharging to undergo electrochemical reaction, and can operate in the temperature range of 5-45°C for a long time.
UNSW's research results are a milestone in the history of all-vanadium flow batteries, which marks the beginning of the technology from the laboratory to industrialization.
■ Early stage of commercialization
After years of exploration and accumulation, the technology of all-vanadium flow battery has been fully feasible. After entering the 21st century, all-vanadium flow batteries began to be truly commercialized.
In 1997, UNSW sold its vanadium battery patents to Pinnacle, in 2001, Vanteck acquired Pinnacle and acquired core patent rights, in 2002, Vanteck changed its name to VRB Power Systems, and in 2004 acquired Reliable Power, thus controlling the all-vanadium flow battery market in the entire North American region, becoming the world's largest all-vanadium flow battery company at the time.
The basic research of all-vanadium flow batteries in China started early, starting in the late 80s of the 20th century. The Institute of Electronic Engineering of the China Academy of Engineering Physics took the lead in building a 500W, 1kW all-vanadium flow battery prototype in 1995. In 2006, the Dalian Institute of Chemical Physics, Chinese Academy of Sciences built a 10kW all-vanadium flow battery test stack. In 2009, Beijing Puneng Century Technology Co., Ltd. acquired VRB Power System at a low price and obtained its various technologies, patents, trademarks, equipment and core technical team. At the same time, Dalian Rongke Energy Storage Technology Development Co., Ltd. (hereinafter referred to as "Rongke Energy Storage") was established in Dalian High-tech Industrial Park in October 2008, which was jointly built by Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Dalian Borong Holding Group.
Thanks to the accumulation of early experience and external technology, the R&D and industrialization process of China's all-vanadium flow battery technology has been greatly accelerated, and it has now become the global main force in this field.
All-vanadium flow battery is the preferred technology route for flow batteries with more mature technology, more controllable raw materials and leading commercialization process.
From the perspective of the industrialization degree of the technical route - at present, the industrialization degree of the two technical routes of all-vanadium flow battery and iron-chromium flow battery is leading. Due to a series of technical problems such as low chromium ion activity, fast battery capacity attenuation, low voltage level and energy density, the industrialization process of iron-chromium flow batteries has been slow. Zinc-bromide flow batteries may now be phased out due to their short cycle life, low cell efficiency, and the metallic corrosiveness of bromine in zinc bromide electrolytes. At present, there are relatively few companies doing zinc bromine liquid flow in China.
From the perspective of key raw material reserves, the total amount of vanadium resources in the earth's crust is small, but the continent is the country with the highest known vanadium reserves in the world. According to the U.S. Geological Survey, as of 2022, the total global vanadium resource reserves exceeded 63 million tons (vanadium metal), of which about 25.57 million tons of vanadium resources have been identified to meet current mining and production requirements. Specifically, China, Australia, Russia and South Africa have reserves of 950, 740, 500 and 3.5 million tons respectively, accounting for 37.2%, 28.9%, 19.6% and 13.7% respectively.
According to the 2023 report of the United States Geological Survey (USGS) Minerals, global chrome ore production in 2022 was about 41 million tons. The distribution of global chrome ore production is roughly as follows: South Africa accounts for 44%, Turkey 17%, Kazakhstan 16%, India 10%, Finland 5%, and other 8%. The mainland's chrome ore reserves are only 4.07 million tons, accounting for less than 1% of the world's reserves, and its dependence on foreign countries exceeds 90%, making it the world's largest importer.
Unlike lithium batteries and iron-chromium flow batteries, which have serious resource bottlenecks or import dependence, vanadium battery raw materials are highly self-sufficient and controllable, and the upstream price is relatively stable, which is conducive to the energy security of the mainland.
Compared with other long-term energy storage technologies, all-vanadium flow batteries not only stand out among many technical routes in the field of flow batteries, but also have better site selection flexibility and significant cost advantages compared with other long-term energy storage technologies.
At this stage, the long-term energy storage technology routes are mainly pumped hydro storage, molten salt heat storage, flow battery energy storage, compressed air energy storage, hydrogen energy storage and lithium battery energy storage with a mature industrial chain. Among them, pumped hydro storage, as a traditional energy storage method, has the highest cumulative installed capacity and the best economy in the market, but the proportion is gradually decreasing due to the limitation of site selection conditions; compressed air energy storage is still limited by site selection to a certain extent; molten salt heat storage and hydrogen energy storage industry chain is still immature, the initial investment cost is high, the conversion efficiency is low, and the cost of electricity is high; the lithium battery energy storage industry chain is mature, the cost is low, but the safety problems are prominent; compared with other long-term energy storage technologies, vanadium batteries are in the application scenarios, The comprehensive advantages of energy storage in terms of time scale and cost are outstanding.
Table 1: Performance comparison of long-term energy storage routes Source: CNESA, public information, Tianfeng Tianrui tabulation
Compared with mainstream lithium batteries, all-vanadium flow batteries have the advantages of good safety, long cycle life, and detachable power and capacity modules.
According to incomplete statistics, there have been more than 50 electrochemical energy storage explosion accidents in the past decade. Among them, ternary lithium accounted for 63.16% of the total number of accidents, mainly due to thermal runaway. The advantages of lithium batteries are high energy density, low loss, and fast response speed, but their own cycle life is not as good as that of flow batteries, and the flammability caused by solid electrodes poses great challenges to the commercial application of lithium battery energy storage.
Figure 5: Statistics on the technical path of explosion accidents in energy storage power stations in the past ten years Source: Planet Energy Storage Institute, public data collation, Tianfeng Tianrui mapping
All-vanadium flow batteries are intrinsically safe. From the perspective of raw material properties, the electrolyte of lithium batteries is a mixed carbonate solution of lithium hexafluorophosphate (LiPF6), which is a flammable substance, while vanadium batteries use water-based electrolyte, which has no risk of fire and explosion. From the perspective of battery structure, the positive and negative electrodes and electrolytes of lithium batteries coexist in a system, and when the battery is overcharged or in a low-temperature environment, lithium precipitation will occur, forming lithium dendrites, which is easy to cause short circuit and bring the risk of thermal runaway, while the electrolyte of vanadium batteries is stored independently in the electrolytic tank, and the reactants can be quickly extracted from the electrode surface through the circulating pump during charging and discharging, which can effectively avoid concentration polarization and heat accumulation effect, and there is no risk of thermal runaway.
Vanadium batteries are also more economical. The power and capacity of vanadium batteries are independent of each other, with strong scalability, and long-term energy storage can reduce marginal costs. The stack of the vanadium battery, as the place where the reaction occurs, is separated from the storage tank where the electrolyte is stored, which fundamentally overcomes the self-discharge phenomenon of the traditional battery and reduces its own loss. The power of the vanadium battery only depends on the size of the stack, and the capacity of the battery only depends on the electrolyte storage and concentration, and when the power is constant, if the energy storage capacity is increased, it is only necessary to increase the volume of the electrolyte storage tank or increase the volume or concentration of the electrolyte without changing the stack size, which increases the flexibility of the design. If the scale of the battery is expanded, the power can be increased by increasing the stack power and the number of stacks, and the storage capacity can be increased by increasing the electrolyte, which can be applied to the construction of kilowatt to 100 megawatt energy storage power stations, which has stronger adaptability.
Vanadium batteries also have a long cycle life, the highest of any electrochemical energy storage technology. The positive and negative electrodes of vanadium batteries are vanadium ions, which can avoid the capacity attenuation caused by cross-contamination of ions through the separator during charging and discharging. Vanadium batteries have a cycle life of up to 20,000+ times and a service life of up to 20 years, while lithium batteries generally have less than 10,000 cycles.
Compared with lithium batteries, the two extra circulating pumps of vanadium batteries will generate additional energy loss, so the energy conversion rate is lower than that of lithium batteries (80-90%), about 65%-75%, but considering that the cycle life of vanadium batteries is much higher than that of lithium batteries, the low energy conversion rate will not significantly reduce the economy of the whole life cycle of vanadium batteries.
Vanadium batteries have a narrow operating temperature range. The optimal operating temperature of vanadium batteries is 0-45°C, which is narrower than that of lithium batteries (-20-60°C). When the temperature is too low, the solidification of the electrolyte will affect the normal operation of the battery, and when the temperature is too high, the cathode pentavalent vanadium will precipitate into vanadium pentoxide, resulting in the blockage of the flow channel and the deterioration of the performance of the stack. In order to reduce the difficulty of thermal management, the electrolyte circulates during the charging and discharging of vanadium batteries, and the heat of the stack can be directly dissipated through the heat exchanger in the conveying pipe, and the temperature can be controlled by air cooling. The lithium battery energy storage system involves a large number of battery cells, which has higher requirements for thermal management, and the mainstream temperature control route is air cooling or liquid cooling: liquid cooling is the current mainstream trend, and its advantage is that the heat dissipation temperature difference is lower, while air cooling has the advantages of simple structure, low cost and easier maintenance.
The vanadium battery industry chain is gradually taking shape, and the investment value is beginning to appear
At present, the technical maturity of all-vanadium flow batteries is gradually improving, and the industrial chain is gradually taking shape, and it is in the transition stage from the introduction period to the growth stage. Its upstream mainly involves various raw materials, including vanadium pentoxide, perfluorosulfonic acid membrane, etc.; the midstream of the industry is electrolyte preparation, stack assembly, control system and other equipment, among which electrolyte configuration technology and stack manufacturing technology have the highest technical barriers; downstream energy storage is mainly used in power generation side, grid side and user side, mainly including wind and solar power generation distribution and storage, power grid peak regulation and frequency regulation, residential, industrial and commercial and independent energy storage and other subdivisions.
Figure 6: All-vanadium flow battery industry chain Source: public data collation, Tianfeng Tianrui mapping
An all-vanadium flow battery consists of an electrolyte, a stack, and other components. Among them, electrolyte and stack (separator, electrode, bipolar plate) are the core components of all-vanadium flow batteries, and their costs account for about 40% of the total cost.
Figure 7: Cost composition of all-vanadium flow batteries Source: iFinD, public information collation, Tianfeng Tianrui mapping
The main raw material of all-vanadium flow battery electrolyte is vanadium metal, and the domestic vanadium resource storage is abundant, and the vanadium battery industry chain has a good resource base, so it has good potential for large-scale commercial application.
At present, the development and investment opportunities of the vanadium battery industry chain in mainland China are more focused on the R&D and application of core materials and components such as electrolytes, stacks, separators, and electronic control systems.
■ Electrolyte preparation is the key to energy density
Due to the limitation of electrolyte concentration, the energy density of vanadium batteries is relatively low (12-40Wh/kg). This makes vanadium batteries more suitable for large-scale and long-term energy storage power station application scenarios that do not require high volume and mass energy density, but have higher safety requirements, that is, in the field of static energy storage, rather than in the fields of power and mobile power supply.
Therefore, the vanadium ion concentration of the electrolyte has become a key indicator to evaluate the performance of vanadium batteries, and the ways to increase the electrolyte concentration include replacing the electrolyte to improve the stability of vanadium ions under high concentration conditions, and using additives containing carboxyl and sulfonic acid groups to increase the stability of vanadium ions under high temperature conditions. There are many domestic enterprises in this field, including the world's largest vanadium electrolyte manufacturer with a global market share of 80%.
■ Diaphragm is the key to power density
The separator is a key factor in determining the power density of an all-vanadium flow battery. As one of the core materials of all-vanadium flow batteries, the performance and cost of separators directly determine the performance, reliability and system cost of batteries.
The separator separates the positive and negative electrolytes to prevent the mixing of vanadium ions from self-discharge, and selectively transmits ions to build a complete circuit in the battery structure.
The ideal all-vanadium flow battery separator needs to have the following characteristics: (1) high selective permeability, which reduces the self-discharge caused by the transmembrane transport of vanadium ions, (2) excellent chemical stability and high mechanical strength, so that the film can have a long life under acidic conditions, thereby increasing the battery life, (3) low resistivity, improving the battery rate performance, (4) low water flux, so that the cathode and anode electrolytes are balanced during the charging and discharging process, and (5) low processing and production costs, which is conducive to the wide application of separators.
At this stage, Nafion™ film produced by DuPont Company of the United States is widely used at home and abroad, which has the advantages of corrosion resistance and oxidation resistance, and its material synthesis difficulty is relatively small, but the key melt extrusion calendering molding technology has been monopolized by foreign countries for a long time, resulting in high costs. Domestic diaphragm production companies have achieved certain results in development, but there is still a certain gap in its mechanical strength compared with DuPont.
■ The electrode affects the operating efficiency and power
The electrodes in the flow battery do not participate in the redox reaction, but provide a place for the redox reaction, which affects the power of the all-vanadium flow battery. Good electrode materials will promote the charge-discharge reaction of flow batteries, increase the stability and service life of battery structures, and then improve the overall operating efficiency and output power of flow batteries.
A good electrode needs to meet the following properties: (1) excellent electrical conductivity, (2) outstanding mechanical properties, (3) good structural properties, (4) cost advantages and environmental friendliness.
Carbon electrodes are mainly used for vanadium battery electrodes at home and abroad. Carbon electrodes include carbon felt, graphite felt, glass carbon, carbon paper, etc. Among them, graphite felt and carbon felt are the mainstream materials for vanadium battery electrodes, mainly because of their advantages such as relatively low cost, good stability, outstanding conductivity and high specific surface area. At present, vanadium battery electrodes can basically be localized.
■ Bipolar plates support the stack structure
The bipolar plates are connected in series with the positive and negative electrodes of adjacent single cells, and the internal circuits are connected, the electrolytes on both sides are blocked, and the positive and negative electrodes are supported, which requires certain mechanical strength, good conductivity and corrosion resistance.
Vanadium battery bipolar plates are mainly divided into graphite bipolar plates, metal bipolar plates, carbon composite bipolar plates, etc. Graphite is brittle and metal is easy to corrode, so at present, carbon composite materials are mainly used for vanadium battery bipolar plates. The higher the carbon content of the carbon composite bipolar plate, the stronger the conductivity, but the toughness of the bipolar plate will deteriorate, which increases the difficulty of assembly and compression of the stack. At present, there are domestic enterprises to achieve the mass production of weldable carbon composite panels.
In the context of building a new power system with new energy as the main body, energy storage will become a basic element of the new power system. With the gradual increase in the proportion of new energy power generation, the importance of long-term energy storage to the new power system will become increasingly prominent, and the market demand will be gradually released.
All-vanadium flow batteries are becoming a strong competitor in the long-term energy storage market due to their high safety, long cycle life, recyclable electrolyte, cost-effective life cycle, and environmental friendliness. We believe that companies with core technology advantages in key links in the industrial chain, such as battery systems, electrolyte preparation, stack efficiency improvement, and separator production, will have the opportunity to achieve rapid development.