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The energy world is currently being pressured to move rapidly to renewable sources of energy. Two of the most commonly used renewable electrical energy sources, solar and wind power, are characterized by inconsistency. They often produce high amounts of energy when demand is low and vice versa. Many universities, academic establishments, and commercial companies are searching for the optimum energy storage solution. Batteries are the obvious choice, however, the challenge is to create a battery that can cost-effectively store an extensive amount of energy.
There are many types of batteries, some known and established, such as lithium-ion batteries powering many electronic devices and electric cars. Others, such as the lead-acid battery, have been commonly used in vehicles for many decades.
Redox flow batteries (RFBs) are probably less well known but have been around for decades. It is believed that the first RFB may have been deployed in a French airship in 1884. In 1949, W. Kangro was granted a patent for a redox flow cell battery. In 1995, Mitsubishi Chemicals / Kashima Kita Power Corporation collaborated with the University of New South Wales. It installed a 200 kW /800 kWh Vanadium Redox Flow Battery at Kashima Kita for a load-leveling application. The development of RFBs has continued ever since.
What is a Redox Flow Battery?
A redox flow battery stores electrical energy in an ionic electrolyte using reduction and oxidation characteristics.
A typical battery consists of two or more tanks/containers of electrolyte, a pump, and a cell containing electrodes separated by a dividing ion-selective membrane. The tanks can be of any size, meaning that the amount of energy stored is flexible. The voltage produced is typically low (one to six volts), but cells can be “stacked” to increase the voltage by having many cells in series. The number of cells in operation varies as they may all be fed from the same electrolyte storage.
Vanadium salts in sulphuric acid are a preferred electrolyte source as it has up to five different oxidation states. A vanadium flow battery (VFB) can store up to 38 Whr/liters of energy. This would require 26 tons of electrolyte per megawatt. This type of cell will typically run at between 10 degrees to 40 degrees Celsius. Precipitation could occur outside of this range.
Advantages of Redox Flow Batteries
Redox flow batteries have several features which could make them a preferred choice against more well-known types of battery, including:
- Flexible battery component layout
- Long life cycles, estimated at 30 years or more
- Most electrolytes do not rapidly degrade
- No harmful emissions are released
- Electrolytes are often 100% recyclable
- They are not typically flammable
- Little maintenance is required
- They are tolerant of overcharging and over-discharging
- They can operate at a wide range of temperatures
- Installation cost is similar to lithium-ion batteries, but storage cost is lower
- Redox flow batteries have a rapid response time, typically around 110 milliseconds
There are a few disadvantages, including:
- Redox flow batteries tend to have very low charge density and operate at low voltages
- They require relatively large electrodes as the charge/discharge rate is low
Uses of Redox Flow Batteries
Redox flow batteries have many possible uses:-
- Power storage, for example, storage of solar power generated during the day for use at night
- Load balancing where there are peaks and troughs in demand. The batteries can give short term boost to power supplies
- Uninterruptible power sources to supply continuous power to facilities such as data servers to ensure no data is ever lost when mains power fails
- Electric vehicles – rapid charging, allowing refueling times similar to those experienced by combustion engines
- Stand-alone power systems, for example, remote mobile phone masts, can be fueled by solar power and batteries alone
The Future of Redox Flow Batteries
In 2018, it was estimated that the current world market for redox flow batteries was around US$140 million and would rise to more than US$400 million by 2026. Other forecasts suggest a US$4.8 billion market by 2028.
Flow batteries will become more attractive for energy storage as power generation moves away from combustion-based technology. In 2021, lithium-ion batteries had more exposure than flow cell batteries through investors such as ElonMusk and his Tesla Corporation.
However, high-profile investors such as Bill Gates, Jeff Bezos, Micheal Bloomberg, Richard Branson, and Jack Ma have invested in flow cell battery company ESS Inc from Oregon USA, which specializes in iron redox flow cell batteries. Iron batteries utilize a globally abundant, low toxicity resource and can operate from -10 to 60 degrees Celsius.
Vanadium flow batteries are one of the most common flow batteries deployed at present. Companies such as Invinity Energy Systems have been working with utility companies such as Scottish Water to decarbonize production. They have also been working with a tidal power company in the Orkneys to produce green hydrogen.
An Invinity vanadium flow battery has been included in the innovative Oxford Energy Superhub in the UK which aims to remove 20,000 tons of carbon dioxide production in 2021 and 40,000 tons by 2032. This project demonstrates that hybrid battery installations can help achieve the ideal energy storage profile as it uses flow batteries and lithium-ion batteries.
Vanadium is a limited global resource with around 14 million tons of known reserves. A total of 97% of vanadium produced is mined in South Africa, Russia, and China. Some, like Imperial College London, are investigating the use of other inorganic electrolytes such as manganese that could reduce costs but also utilize a more globally abundant resource. The ESS Inc battery uses iron which is very plentiful on Earth.
Others are researching more redox-active inorganic chemistries and organic electrolytes for redox flow batteries. These can have higher charge densities but will negate the low toxicity of the vanadium flow battery. An advantage of using technologies other than vanadium could be that the raw materials will be more abundant and less costly, which will widen the appeal of RFBs.
RFBs are set to be a significant part of any future energy storage development. They have already reached utility-scale with San Diego Gas & Electric and Sumitomo installing a 2 MW/8 MWh flow battery (vanadium redox chemistry) in California, and Invinity installing an 8 MWhr battery for Yadlamalka Energy Trust in Australia. A 200 MW/400 MWh battery system is being planned in Dalian, China, by Rongke Energy.
The development of new inorganic chemistries and organic chemistries for redox flow will help to increase the size range and diversity of applications for redox flow batteries and make them even more benign for our environment.
References and Further Reading
W. Kangro. Verfahren zur Speicherung von elektrischer Energie. German Patent 914264, 1949
Javier Rubio-Garcia et al. (2019) Hydrogen/manganese hybrid redox flow battery. J. Phys. Energy. 1 015006. https://doi.org/10.1088/2515-7655/aaee17
Sánchez-Díez, E., et al. (2021) Redox flow batteries: Status and perspective towards sustainable stationary energy storage. Journal of Power Sources, 481, 228804-228826. https://doi.org/10.1016/j.jpowsour.2020.228804
Invinity Energy Systems [Online] Available at: https://invinity.com/ (Accessed on 22 April 2021).
ESS Inc. [Online] Available at: https://essinc.com/ (Accessed on 22 April 2021).
UPS Battery Cente [Online] Available at: https://www.upsbatterycenter.com/ (Accessed on 22 April 2021).
Energy Supehub Oxford [Online] Available at: https://energysuperhuboxford.org/ (Accessed on 22 April 2021).
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