Flow battery-structure, working principle and technical route introduction

In China, by 2025, the new energy storage will have entered the large-scale development stage from the initial stage of commercialization, with the conditions for large-scale commercial application, the innovation ability of new energy storage technologies will be significantly improved, and the level of independent and controllable core technology and equipment will be significantly improved.

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By 2030, new energy storage will usher in comprehensive market development. It is worth noting that although China’s relevant policies propose to promote the development of diversified energy storage technologies and carry out key core technologies such as sodium ion battery, new lithium ion battery and flow battery, large-scale lithium-ion battery storage power stations have frequent fire and explosion accidents.

In this regard, China Energy Resources Bureau proposed that ternary lithium batteries and sodium sulfur batteries should not be used for medium and large electrochemical energy storage power stations.

Therefore, the relatively stable and safe flow battery has attracted more and more attention. This article will introduce the structure, principle and technical route of the flow battery, and focus on the comparison between all vanadium redox flow battery and iron chromium flow battery.

What is flow battery

What’s the structure of flow battery

The flow battery is mainly composed of the following parts:
● Battery stack (including ion exchange membrane, bipolar plate, etc.)
● Electrolyte
● Liquid storage tank
● Circulating pump

The cathode electrolyte and anode electrolyte are isolated by an ion exchange membrane inside the battery stack, and two reaction chambers are formed. When the battery is running, the circulating pump drives the electrolyte to circulate in their respective reaction chambers, and the redox reaction takes place near their respective electrodes, forming current through the bipolar plates connected to the outside.

How does a flow battery work

The principle of flow battery is to store electric energy with chemical energy. When the battery is charged, the electric energy is converted into chemical energy and stored in the electrolyte of the liquid storage tank; When the battery is discharged, the chemical energy in the electrolyte is converted into electrical energy.

Technical route of flow battery

There are three technical routes of flow battery:

● All vanadium redox flow battery
It is the battery technology with the largest scale of demonstration projects in China, and the electrolyte is sulfuric acid solution of vanadium ions with different valence.

● Iron chromium flow battery
The electrolyte is a hydrochloric acid solution of iron ions and chromium ions with different valence states.

● Zinc bromide flow battery
Due to the short cycle life of zinc bromide battery, low battery efficiency, and the metal corrosivity of bromine in zinc bromide electrolyte, zinc bromide battery may be eliminated at present.

What is an all vanadium redox flow battery

Vanadium redox battery (VRB) is an redox battery in which active substances are circulating liquid. The mutual conversion of electrical energy and chemical energy is realized by exchanging electrons between two vanadium ions with different valence separated by a diaphragm.

After the vanadium battery is charged, the positive pole is V5+and the negative pole is V2+; After discharge, the positive and negative electrodes are V4+and V3+solutions respectively. The positive pole and negative pole are separated by a diaphragm, which only allows H+to pass through, and H+also plays the role of conducting electricity inside the battery.

With the drive of relevant demonstration projects, the cost of all vanadium redox flow battery is expected to be reduced, so as to promote its commercialization in the product technology side. In May 2022, the world’s largest 100 MW/400 MWh all vanadium redox flow battery energy storage power station will be formally connected to the grid, which will accelerate the commercialization of all vanadium redox flow battery.

What is an iron chromium flow battery

Fe Cr flow battery uses Fe and Cr ions which are cheap and rich in raw materials as active materials. In the 1970s, NASA put forward the concept of flow battery. Japan also developed a kilowatt prototype ten years later. However, due to a series of technical problems such as low chromium ion activity and fast battery capacity decay, the industrialization process is slow.

All vanadium redox flow battery vs iron chromium flow battery

It is understood that among the technological routes of flow battery, vanadium flow battery and iron chromium flow battery are leading in industrialization.

Comparison of technical routes

At present, vanadium flow battery and iron chromium flow battery are the two mainstream flow battery technology routes. The following is a comparative analysis of the two.

Upstream resources: vanadium vs chromium:

According to data, in 2021, the global vanadium ore reserves was 24 million tons, and China’s reserves will be 9.5 million tons, accounting for 40%. China is the country with the highest vanadium reserves in the world.

The global average annual vanadium demand is about 120000 tons, and 90% of the downstream is used in the iron and steel metallurgy industry. In addition, in 2021, the world’s chromite reserves was 575 million tons, and chromium resources was abundant.

In addition, China’s dependence on foreign chrome ore exceeds 90%, making it the world’s largest importer. China’s chromium reserves are only 4.07 million tons, accounting for less than 1% of the global reserves. The annual import of chromium ore in China is about 14 million tons, mainly from South Africa. 90% of the downstream applications are used in the iron and steel metallurgy industry.

Price difference of key materials: vanadium pentoxide vs ferrochrome:

The key raw materials of electrolyte for all vanadium redox flow battery and iron chromium flow battery are vanadium pentoxide and iron chromium alloy respectively. The overall price of ferrochrome is cheaper due to the rich supply of elements. The recent average price of vanadium pentoxide is about 100000 RMB/ton, while the average price of iron chromium alloy is only 8000 RMB/ton, far lower than vanadium.

The vanadium price is more likely to fluctuate. Considering the source of raw materials, at present, 75-85% of vanadium comes from the extraction of vanadium slag obtained from iron and steel metallurgical processing. The supply of vanadium is limited by steel production capacity, which is likely to lead to greater price fluctuations. Chromium can be directly extracted from chromium ores, without similar problems.

Advantages and disadvantages of all vanadium redox flow battery

Advantage:

● Voltage level and energy density are slightly higher than iron chromium flow

The reaction activity of vanadium ion is higher than that of iron chromium ion, and the battery power is relatively high. According to the data, the open circuit voltage level of the all vanadium redox flow battery is 1.26V, and the iron chromium flow is 1.18V.

Therefore, the voltage level and energy density of the all vanadium redox flow battery are slightly higher than those of the iron chromium battery, but there is still a big gap between the two as flow batteries and lithium ion battery.

● Single stack power and commercialization process are more advanced

In the field of flow battery, the commercialization of all vanadium battery is the fastest. At present, the total number of projects and the scale of single project of all vanadium battery are ahead of that of iron chromium battery, and the technical maturity and commercial maturity are better.

● The all vanadium battery single stack has higher power and more mature technology

The power of single stack can be used as a key auxiliary indicator to judge the maturity of technology. At present, the power of single stack of all vanadium redox flow battery project has reached 400V, while the power of single stack of iron chromium battery is 30-40V, which still needs to be further pursued.

It is inferred that by 2025, the power of single iron chromium battery stack may reach the current level of all vanadium batteries.

Disadvantages:
● The cost of energy storage technology is too high, and it is difficult to apply it on a large scale
● The technical production technology has not been stabilized, and the leakage technology has not been overcome

Advantages and disadvantages of iron chromium flow battery

Advantage:

The biggest advantage of iron chromium battery over all vanadium battery is that the cost of electrolyte is lower. Although the battery structure is similar, due to the higher price of vanadium, the cost of all vanadium battery electrolyte can account for 53% of the total battery cost, compared with only 10% of the cost of iron chromium battery electrolyte.

According to the calculation, the cost per watt hour of iron chromium electrolyte is only 11% of that of vanadium electrolyte, with significant cost advantage.

Under the trend of long-term energy storage, with the increase of system storage time, the proportion of electrolyte cost in system cost is expected to continue to rise, and the advantages of low-cost electrolyte will become more obvious in the future.

Disadvantages:

● Low energy density

The energy density of lithium batteries on the market can basically reach 400Wh/L, while the maximum energy density of iron chromium flow batteries can only reach 20Wh/L, which is far from the same. It is precisely because the energy density of iron chromium flow batteries is low, so the volume is much larger than other batteries.

● Low energy conversion rate

At present, the energy conversion rate of power battery on new energy vehicles can reach more than 80%, while the energy conversion rate of iron chromium flow battery is far lower than 80%. After all, it is a newly introduced battery technology, and there is still much room for progress in this regard.

Summary

Compared with vanadium flow battery technology, iron chromium flow battery has more obvious advantages, mainly reflected in:

● Large global chromium reserves and low electricity cost

The proven reserves of chromium in the world have reached 685 million tons, while vanadium is only 22 million tons. This is the key to the low cost of iron chromium flow battery, and there is room for a substantial reduction.

● Wider temperature adaptation range

The iron chromium flow battery can adapt to the environment of minus 20 ℃, and the operating temperature range is – 20C-70C. The operating temperature of vanadium flow battery is 5C-50C.

Iron chromium flow battery has strong competitive advantages, and is expected to be widely used in solar, wind and other power generation side, as well as smart micro grid, user side and other fields in the future.

Iron chromium flow battery is favored by the market, which further enables the chromium industry chain to have a good prospect. It is predicted that in 2022, the global shipment of iron chromium flow battery is 0.42 GW, driving the demand for sodium dichromate by 11700 tons; in 2025, the global shipment of iron chromium flow battery will be 12.01 GW, driving the demand for sodium dichromate by 334700 tons.

Future development trend of flow battery

The long-term trend of long term energy storage is clear, but due to immature technology, high cost, insufficient urgency of demand and other reasons, there is still no large-scale application in the short term. In China, the installed capacity of energy storage is still dominated by the policy of mandatory short-term energy storage of 2-4 hours.

In the future, with the increase of storage time, the cost per watt hour of flow battery will decrease. The power and capacity of the flow battery are designed independently. The most increased cost after improving the energy storage capacity is the electrolyte and the liquid storage tank. The cost of the stack is equivalent to the fixed cost under the condition of constant power.

As a result, the longer the energy storage time is, the lower the cost of the stack is, and the lower the cost of the overall battery system is. At present, the flow battery project lasts for 2-4 hours. If the energy storage time is increased to 8-10 hours or more in the future, the economy of the flow battery will be further reflected.

In the field of flow battery, the development of all vanadium battery is relatively mature, but the long-term cost advantage of iron chromium battery is more obvious.

In addition to the iron chromium flow battery, China has also made deep efforts in flywheel energy storage, water heat storage, molten salt heat storage, solid heat storage, phase change heat storage and other technologies. In the future, it is still uncertain whether the all vanadium flow or the iron chromium flow will dominate the flow battery. 

For the flow battery itself, the cost of lithium battery may decline rapidly in the future, or the development speed of hydrogen energy storage will be far faster than expected, and even newer and better technologies will replace the existing technical route in the future. Everything is possible, because science and technology continue to progress in twists and turns.