my country's energy structure transformation has brought the greatest opportunity to energy storage to date, and market expansion continues to accelerate.
The rapid development of energy storage has inevitably led to some problems.
For example, with the large-scale grid connection of renewable energy sources such as wind farms, photovoltaic power plants, and hydropower, my country's power grid has begun to show its inadequacy in accepting them. In response to this problem, the market has proposed large-scale energy storage technology as a solution.
Among numerous large-scale energy storage technologies, pumped hydro storage is limited by the environment, lithium battery energy storage has low safety, while iron-chromium redox flow batteries have advantages such as high safety, low cost, and high performance, making them one of the strong competitors in the future large-scale energy storage market.
The Four Kings of Flow Batteries
Flow batteries consist of positive and negative electrode electrolyte storage tanks, positive and negative electrode electrolytes, stack system, and management and control system. They generally have advantages such as high safety, long life, low toxicity, low corrosiveness, low self-discharge, and reusable electrolyte.
Currently, the most popular flow batteries on the market are mainly divided into four categories: iron-chromium flow batteries, vanadium redox flow batteries, lead-acid flow batteries, and lithium-ion flow batteries.
First, iron-chromium redox flow batteries have a long lifespan, low cost, mature technology, abundant raw materials, and a wide operating temperature range, making them a promising candidate for large-scale, long-term energy storage.
Second, vanadium redox flow batteries are the most technologically mature and commercially viable, but their disadvantage is the higher cost of the electrolyte.
On the 30th of last month, the Dalian Flow Battery Energy Storage Power Station, which boasts the world's largest capacity and power output, successfully connected to the grid and began generating electricity.
Third, lead-acid flow batteries have low initial design costs, but their disadvantages include short lifespan and high maintenance costs, which has prevented their widespread use.
Fourth, lithium-ion flow batteries do not have obvious market advantages. In terms of large and medium-sized energy storage, they are not as good as iron-chromium flow batteries and vanadium redox flow batteries. In the future, they may be used in small-scale energy storage and short-term energy storage.
Iron-chromium flow battery VS Vanadium redox flow battery
The mainstream commercial flow batteries are iron-chromium flow batteries and vanadium redox flow batteries.
Compared to the former, the iron-chromium redox flow battery has significant advantages in terms of operating temperature and raw material cost.
First, in terms of operating temperature, the operating temperature of the iron-chromium flow battery is -20~70℃, while that of the vanadium redox flow battery is 5~50℃, indicating that the iron-chromium battery has a significantly wider operating temperature range.
Second, in terms of raw material costs, the key raw materials for iron-chromium flow batteries and vanadium-based flow batteries are chromite and vanadium ore, respectively.
According to Shanghai price data on November 10, 2022, the price of 50-based ferrovanadium is 15.6 times that of 50-based ferrochrome, making the key raw material for ferrochrome flow batteries cheaper.
Furthermore, chromium accounts for only 0.01% of the Earth's crust, ranking 17th. Globally, chromite resources are abundant, with global chromite resources amounting to approximately 9 billion tons in 2020 and global chromite production reaching 34.8 million tons in 2021. Within China, the chromium industry chain is well-developed, and related technological reserves are comprehensive.
However, in terms of technological maturity and the proportion of self-sufficiency in raw materials, vanadium redox flow batteries have an advantage.
First, in terms of technological maturity, vanadium redox flow batteries are more mature than iron-chromium redox flow batteries.
Currently, the power output of a single vanadium redox flow battery can reach 400V, while that of a single iron-chromium redox flow battery is only 30-40V, a significant difference.
Industry experts predict that with the development of related technologies, by the end of 2025, the power of a single iron-chromium redox flow battery stack could reach 400V.
Second, in terms of the self-sufficiency rate of raw materials, my country has the world's largest reserves of vanadium ore, but it is heavily dependent on imports of chromium ore.
China's chromium ore reserves are only 4.07 million tons, accounting for less than 1% of the global total, while demand is huge and self-sufficiency is less than 10%, making it heavily reliant on imports.
In comparison, according to data from the USUG, my country's vanadium ore reserves in 2021 were 9.5 million tons, accounting for 40% of global reserves, making it the country with the largest vanadium ore reserves in the world.
Large-scale long-term energy storage adaptable king
Iron-chromium redox flow batteries have advantages such as long lifespan, low cost, high safety, high performance, large capacity, low toxicity, and low corrosivity.
In terms of long lifespan, the iron-chromium redox flow battery has a cycle life of more than 20,000 cycles and can be used for 20 years.
In terms of low cost, the cost per kilowatt-hour of iron-chromium redox flow batteries is only 0.4 yuan, which is lower than the 0.8 to 1.3 yuan per kilowatt-hour of all-vanadium redox flow batteries.
In terms of safety, the positive and negative electrolytes of the iron-chromium redox flow battery are both aqueous solutions, which are packaged in two separate storage tanks. The stack and the storage tanks are separated, eliminating the risk of explosion.
In terms of performance, the iron-chromium redox flow battery has a wide normal operating temperature range of -20 to 70°C, and an energy density of 10 to 20 Wh/L.
In terms of large capacity, my country has already put into operation several iron-chromium redox flow battery energy storage projects, including the Zhanshigou photovoltaic power station iron-chromium redox flow battery energy storage demonstration project of State Power Investment Corporation Hebei Electric Power Co., Ltd. Zhangjiakou Branch, with a capacity of 250kW/1.5MWh; and the iron-chromium redox flow battery long-term energy storage project in Mianyang Science and Technology City New Area, Sichuan Province, with a capacity of 1MW/4MWh.
Regarding weak toxicity and weak corrosivity, since the electrolyte solution of the iron-chromium flow battery is mainly a dilute hydrochloric acid solution containing iron and chromium salts, its toxicity and corrosivity are relatively weak.
As for the drawbacks, the main limiting factor for the commercialization of iron-chromium flow batteries is that the power of a single cell stack is still relatively low, only 30~40V.
Considering the advantages and disadvantages of iron-chromium redox flow batteries, they are poised to become the best fit for large-scale, long-duration energy storage applications such as wind, solar, and solar power.
Bright prospects
In recent years, thanks to the efforts of relevant research institutions, three problems that have long plagued iron-chromium flow batteries have been solved: electrolyte mixing, hydrogen evolution at the cathode, and electrolyte degradation.
Among these methods, the problem of electrolyte mixing was solved by using a mixed solution of iron and chromium ions; the problem of hydrogen evolution at the cathode was solved by using a catalyst; and the problem of electrolyte degradation was solved by designing and installing a rebalancing system.
With further attention from relevant investment institutions, the iron-chromium redox flow battery technology is bound to improve rapidly. If the current technical shortcomings, such as low power output of individual cells and stacks, are resolved, the iron-chromium redox flow battery will surely usher in a bright future.
