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Introduction

Flow batteries are a type of rechargeable battery that store and release energy through chemical reactions involving liquid electrolytes. Unlike conventional batteries, flow batteries separate the energy storage medium (the electrolytes) and the energy conversion device (the electrochemical cell). This allows for independent scaling of power and energy, as well as long cycle and calendar life. Flow batteries are ideal for applications that require large-scale energy storage, such as renewable energy integration, grid stabilization, and peak shaving.

In this article, we will explore the basic principles, advantages, disadvantages, and examples of flow batteries. We will also compare them with other types of batteries and discuss their role in the future of energy systems.

How Flow Batteries Work

A typical flow battery consists of two tanks of liquids, called the positive electrolyte and the negative electrolyte, which are pumped through a membrane held between two electrodes. The membrane allows ions to pass through, but prevents the electrolytes from mixing. The electrodes are connected to an external circuit that delivers or receives electric current.

During discharge, the negative electrolyte undergoes an oxidation reaction at the negative electrode, releasing electrons to the external circuit. The positive electrolyte undergoes a reduction reaction at the positive electrode, accepting electrons from the external circuit. The electrons flow from the negative electrode to the positive electrode, powering the connected device. The ions flow from the negative electrolyte to the positive electrolyte, balancing the charge.

During charge, the opposite process occurs. An external power source drives the electrons from the positive electrode to the negative electrode, reversing the chemical reactions. The ions flow from the positive electrolyte to the negative electrolyte, restoring the original state of the electrolytes.

The amount of energy stored in a flow battery depends on the volume and concentration of the electrolytes, while the power output depends on the size and number of the electrochemical cells. By increasing the tank size, the energy capacity can be increased without affecting the power. By increasing the cell area or stacking more cells in series, the power can be increased without affecting the energy.

Types of Flow Batteries

There are various types of flow batteries, depending on the chemical composition of the electrolytes and the membrane. Some of the most common types are:

• Vanadium redox flow battery (VRFB): This is the most widely used type of flow battery, which uses vanadium ions in different oxidation states as the active species in both electrolytes. The advantage of using the same element in both electrolytes is that it eliminates the risk of cross-contamination and improves the stability and durability of the battery. The VRFB has a cell voltage of about 1.4 volts and a specific energy of about 25 Wh/kg. ¹
• Zinc-bromine flow battery (ZBFB): This type of flow battery uses zinc and bromine as the active species in the negative and positive electrolytes, respectively. The ZBFB has a higher cell voltage of about 1.8 volts and a higher specific energy of about 70 Wh/kg than the VRFB. However, it also has some drawbacks, such as the need for periodic electrode maintenance, the formation of toxic bromine gas, and the risk of zinc dendrite growth that can short-circuit the cell. ²
• Iron-chromium flow battery (ICFB): This type of flow battery uses iron and chromium as the active species in the negative and positive electrolytes, respectively. The ICFB has a low cell voltage of about 1.2 volts and a low specific energy of about 15 Wh/kg, but it has the advantages of using abundant and inexpensive materials, being environmentally benign, and having a long shelf life. ³
• Organic flow battery (OFB): This is a new type of flow battery that uses organic molecules, such as quinones or ferrocenes, as the active species in the electrolytes. The OFB has the potential to offer higher energy density, lower cost, and greater flexibility than conventional flow batteries, as the organic molecules can be tailored to achieve desired properties. However, the OFB is still in the early stages of development and faces challenges such as low solubility, low stability, and low conductivity of the organic electrolytes. ⁴

Advantages and Disadvantages of Flow Batteries

Flow batteries have several advantages over conventional batteries, such as:

• Scalability: Flow batteries can easily scale up or down their power and energy by changing the size of the tanks and the cells, respectively. This allows for optimal design and customization for different applications and scenarios.
• Longevity: Flow batteries have a long cycle and calendar life, as they do not suffer from capacity degradation or memory effects that affect conventional batteries. They can also operate at a wide range of temperatures and state of charge, without compromising their performance or safety.
• Safety: Flow batteries are generally safer than conventional batteries, as they do not use flammable or explosive materials, and do not generate heat during operation. They also have a low risk of thermal runaway or fire, as the electrolytes are stored separately from the cells and can be isolated in case of emergency.
• Sustainability: Flow batteries can use renewable or recycled materials, such as vanadium, iron, or organic molecules, as the electrolytes, reducing the environmental impact and the dependence on scarce or expensive resources. They can also support the integration of renewable energy sources, such as solar or wind, by providing reliable and flexible energy storage and grid services.

However, flow batteries also have some disadvantages, such as:

• Complexity: Flow batteries require more components and infrastructure than conventional batteries, such as pumps, pipes, valves, sensors, and controllers, to circulate and manage the electrolytes. This adds to the complexity, cost, and maintenance of the system, and also introduces potential points of failure or leakage.
• Efficiency: Flow batteries have lower energy efficiency than conventional batteries, as they incur losses due to the resistance of the membrane, the pumping of the electrolytes, and the crossover of the active species. The typical round-trip efficiency of flow batteries ranges from 50% to 80%, compared to 80% to 95% for conventional batteries.
• Energy density: Flow batteries have lower energy density than conventional batteries, as they need a large volume of electrolytes to store a given amount of energy. The specific energy of flow batteries ranges from 10 to 100 Wh/kg, compared to 100 to 250 Wh/kg for conventional batteries. This limits the applicability of flow batteries for applications that require high power and energy density, such as electric vehicles or portable devices.

Examples of Flow Battery Projects

Flow batteries have been deployed in various projects around the world, demonstrating their capabilities and benefits for energy storage and grid services. Some of the notable examples are:

• Rongke Power VRFB project in China: This is the world’s largest flow battery project, which aims to install a 200 MW/800 MWh VRFB system in Dalian, China, by 2021. The project is expected to provide frequency regulation, peak shaving, and renewable energy integration for the Liaoning province, as well as reduce the reliance on coal-fired power plants. The project is funded by the Chinese government and developed by Rongke Power, a leading VRFB manufacturer.
• UniEnergy Technologies (UET) VRFB project in Washington, USA: This is the largest flow battery project in North America, which installed a 2 MW/8 MWh VRFB system in Pullman, Washington, in 2016. The project is part of a smart grid demonstration project, which aims to improve the reliability, efficiency, and resilience of the grid, as well as integrate renewable energy sources, such as wind and solar. The project is funded by the U.S. Department of Energy and developed by UET, a leading VRFB manufacturer.
• Redflow ZBFB project in Queensland, Australia: This is the world’s largest ZBFB project, which installed a 60 MWh ZBFB system in Queensland, Australia, in 2019. The project is part of a hybrid microgrid system, which combines solar, wind, diesel, and battery storage, to provide reliable and clean power to a remote mine site. The project is funded by the Australian Renewable Energy Agency and developed by Redflow, a leading ZBFB manufacturer.
• Harvard OFB project in Massachusetts, USA: This is the world’s first OFB project, which installed a 0.6 kWh OFB system in Cambridge, Massachusetts, in 2014. The project is part of a research project, which aims to develop and test novel organic molecules for flow battery applications. The project is funded by the U.S. Department of Energy and developed by Harvard University, a leading OFB research institution.

Conclusion

Flow batteries are a promising technology for large-scale energy storage and grid services, offering scalability, longevity, safety, and sustainability. They have various types, such as VRFB, ZBFB, ICFB, and OFB, each with its own advantages and disadvantages. Flow batteries have been deployed in various projects around the world, demonstrating their capabilities and benefits for different applications and scenarios. Flow batteries are expected to play a significant role in the future of energy systems, as they enable the transition to a more renewable, resilient, and efficient grid.

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¹: What is a Flow Battery: A Comprehensive Guide to Understanding and Implementing Flow Batteries
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Source: Conversation with Bing, 12/25/2023
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