How flow batteries work and why they could be the future of renewable energy storage

In the coming decades, solar and wind power are expected to dominate the traditional power grid as renewable energy sources. However, these sources only generate electricity when there is sunlight or wind, necessitating the need for reliable energy storage to ensure a continuous power supply. One promising technology for this purpose is the flow battery, an electrochemical device capable of storing a large amount of electricity for an extended period.

Flow batteries have the capacity to store hundreds of megawatt-hours of energy, which can sustain thousands of homes for several hours on a single charge. Their unique design allows for long lifetimes and cost-effectiveness. Unlike typical batteries found in phones and electric vehicles that use solid-state charge-storage materials, flow batteries dissolve these materials in electrolyte solutions and circulate them through the electrodes.

Fikile Brushett, an associate professor of chemical engineering at MIT, explains that this design offers numerous advantages but also presents certain challenges.

Flow batteries: Design and operation

A flow battery consists of two substances that engage in electrochemical reactions involving the transfer of electrons. During charging, the transfer of electrons causes the substances to enter a less energetically favorable state as they store excess energy, similar to pushing a ball up a hill. On the other hand, during discharge, the transfer of electrons shifts the substances into a more energetically favorable state, releasing the stored energy, much like allowing the ball to roll down the hill.

At the heart of a flow battery are two large tanks that hold liquid electrolytes, one positive and the other negative. Each electrolyte contains dissolved “active species” – atoms or molecules that undergo electrochemical reactions to release or store electrons. The electrolytes are circulated through separate electrodes using pumps. These electrodes are made of porous materials that provide ample surfaces for the active species to react. A thin membrane between the adjacent electrodes prevents the electrolytes from directly contacting each other and potentially reacting, which would result in heat release and energy wastage that could otherwise be used in the grid.

During discharge, the active species on the negative side oxidize, releasing electrons that flow through an external circuit to the positive side, where the species undergo reduction. This electron flow through the external circuit can power the grid. Additionally, supporting ions, other charged species present in the electrolyte, pass through the membrane to facilitate the reaction and maintain electrical neutrality in the system.

Once all the species have reacted, and the battery is fully discharged, it can be recharged. In this process, electricity generated from sources such as wind turbines and solar farms drives the reverse reactions. The active species on the positive side oxidize, releasing electrons back through the wires to the negative side, where they reunite with their original active species. The battery is now reset and ready to supply electricity when needed. This cycling process can be repeated for years without significant degradation, making flow batteries durable and long-lasting.

Benefits and challenges

One significant advantage of the flow battery system design is the separation of energy storage (tanks) from the electrochemical reactions (reactor with electrodes and membrane). This separation allows for independent adjustment of the battery’s capacity (energy storage) and power (charge/discharge rate). By increasing the size of the tanks, the capacity can be expanded, while increasing the size of the reactor enhances the power. This flexibility enables the customization of flow batteries for specific applications and allows for modifications to meet future needs.

However, flow batteries face challenges related to electrolyte degradation over time. While all batteries experience electrolyte degradation, flow batteries are particularly prone to a form of degradation called “crossover,” which occurs at a relatively faster rate. Despite the membrane’s selective design to allow only small ions to pass through while blocking larger active species, some active species can still “cross over” and mix with the electrolyte in the other tank. This crossover can lead to chemical reactions that discharge the battery or result in a reduced capacity as the active species are no longer in their designated tanks.

Restoring the lost capacity due to crossover requires remedial actions such as replacing the electrolyte in one or both tanks or finding a method to reestablish the oxidation states of the active species in the tanks. Flow batteries offer advantages in executing such remediation since all the components are more easily accessible compared to conventional batteries. This accessibility contributes to cost-effectiveness in maintaining and restoring flow battery performance.

The state of the art: Vanadium

A crucial aspect of flow battery design revolves around the choice of chemistry. While different chemicals can be used in the two electrolytes, the most widely adopted configuration today involves vanadium in different oxidation states on each side. This setup effectively addresses two major challenges in flow batteries.

Firstly, vanadium is highly durable and does not degrade over time. As long as there is no physical leakage, if 100 grams of vanadium is initially placed in the battery, it should still be possible to recover 100 grams of vanadium even after 100 years, according to Brushett.

Secondly, if some vanadium from one tank permeates through the membrane to the other side, it does not result in permanent cross-contamination of the electrolytes. Instead, it leads to a shift in oxidation states, which can be easily remedied by rebalancing the volumes of electrolyte and restoring the oxidation state through a minor charging step. Many commercial flow battery systems today incorporate a pipe that connects the two vanadium tanks, automatically transferring electrolyte between them when an imbalance occurs.

However, as renewable energy increasingly dominates the grid and greater numbers of flow batteries are required for long-duration storage, the demand for vanadium will surge, posing a problem. Vanadium is found in limited quantities around the world, and its extraction is challenging. The majority of production is concentrated in Russia, China, and South Africa, resulting in an unreliable supply chain. Consequently, vanadium prices are high and extremely volatile, presenting a hindrance to widespread deployment of vanadium flow batteries.

Beyond vanadium

The question of finding an alternative to vanadium in flow batteries is a significant focus of researchers worldwide. Many are exploring chemistries that utilize materials that are more abundant and cost-effective than vanadium. However, identifying suitable alternatives is not a straightforward task.

While other chemistries may offer lower initial capital costs, they may prove to be more expensive to operate over time. They might require periodic servicing or even the replacement of electrolytes, leading to recurring capital costs. Assessing the economics of different options is challenging due to the numerous interdependent variables involved in flow battery systems. Any modification to one component can have cascading effects on other aspects of the system.

Therefore, it is essential to compare these emerging chemistries with existing vanadium systems in a meaningful way. Additionally, comparisons need to be made between the different emerging chemistries themselves to determine their relative promise and potential drawbacks. By addressing these questions, researchers can make informed decisions about where to focus their research efforts and allocate development resources effectively.

By understanding the comparative performance and challenges of various chemistries, the path forward for flow battery technology can be better defined, guiding research and development investments to drive progress in the field.

Techno-economic modeling as a guide

Techno-economic modeling is a valuable approach for understanding and evaluating the economic feasibility of new and emerging energy technologies. By employing such models, it becomes possible to consider the capital costs of a defined system and project the operating costs over time, ultimately generating a discounted total cost over the system’s lifetime. This approach enables a comparative assessment of options based on the “levelized cost of storage.”

Kara Rodby developed a framework for estimating the levelized cost of flow batteries using techno-economic modeling. The framework incorporates a dynamic physical model that tracks the battery’s performance over time, accounting for changes in storage capacity and encompassing all necessary operating costs for decades of operation. This includes the remediation steps required to address species degradation and crossover.

Considering the vast number of potential chemistries, the researchers focused their analyses on specific classes. Firstly, they narrowed down the options to those with active species dissolved in water, as aqueous systems are more advanced and have a higher likelihood of commercial success. Secondly, they limited their analysis to “asymmetric” chemistries, where different materials are used in the two tanks. Vanadium, being an exception, typically requires the same parent material in both tanks. Lastly, they categorized the possibilities into two classes: species with finite lifetimes (that degrade over time) and species with infinite lifetimes (that do not degrade).

The results of their analyses do not identify a single chemistry that stands out as superior. However, they offer general guidelines for selecting and pursuing different options within the defined classes.

Finite-lifetime materials

While vanadium is a single element, the finite-lifetime materials used in flow batteries are typically organic molecules composed of multiple elements, including carbon. Organic molecules offer advantages such as being synthesizable in a lab and at an industrial scale, with the ability to modify their structure to suit specific functions. For example, their solubility can be enhanced to increase their presence in the electrolyte, thereby improving the energy density of the system. They can also be designed to be larger in size, preventing them from crossing the membrane and causing crossover.

Additionally, organic molecules can be derived from simple, abundant, and low-cost elements, potentially including waste streams from other industries. However, there are two primary concerns associated with these molecules. Firstly, their synthesis would likely require a chemical plant, and the cost of upgrading the low-cost precursors might be higher than desired. Secondly, organic molecules are complex structures that can be prone to degradation over time, introducing a new degradation mechanism in addition to crossover. Understanding and addressing the degradation processes of different organic molecules can be a significant challenge, requiring substantial effort and resources for the discovery and development of each new chemistry.

Currently, Rodby and Brushett find it challenging to make a compelling case for finite-lifetime chemistries, primarily due to their capital costs. Studies estimating the manufacturing costs of these materials suggest that current options are not sufficiently inexpensive to be economically viable, although they are cheaper than vanadium.

This highlights an important message for researchers working on new chemistries involving organic molecules: It is crucial to consider operational challenges early in the design process. Practical questions regarding the long-term operation of promising systems are often addressed late in the innovation pipeline. The MIT team recommends that understanding potential decay mechanisms and developing cost-effective methods for their reversal or remediation should be a fundamental design criterion from the outset.

Infinite-lifetime species

Among the infinite-lifetime species considered, metals like iron or manganese are potential candidates that are not prone to degradation and can be obtained at low costs. These metals offer a wider range of feasible options compared to vanadium. However, challenges remain. While these species do not degrade, they can trigger side reactions within the battery, such as hydrogen evolution, which reduces efficiency and leads to capacity loss. Finding a cost-effective solution to mitigate high rates of side reactions like hydrogen evolution is still necessary.

Crossover remains a problem even with infinite-lifetime species, requiring remediation strategies. The researchers evaluated two approaches for systems utilizing two types of infinite-lifetime species.

The first approach is known as the “spectator strategy,” where both tanks contain both active species. However, only one species is active at a time, while the other remains a spectator. Remediation methods similar to those used in vanadium flow batteries can be employed to address crossover. However, this approach comes with a drawback: Half of the active material in each tank becomes inaccessible for storing charge, resulting in wasted electrolyte and increased electrolyte costs per unit of energy.

The second method involves developing a perfectly selective membrane that allows only the necessary supporting ions to pass through, maintaining the electrical balance between the two sides. However, this approach increases cell resistance, impacting system efficiency. Additionally, producing such a specialized membrane using current methods and scales would be extremely expensive, likely requiring advanced materials like ceramic composites. Although research on these membranes is underway, their cost and performance metrics are far from being economically viable at present.

Overall, while infinite-lifetime species offer promising alternatives to vanadium, challenges related to side reactions and crossover persist and require further research and development efforts.

Time is of the essence

In the face of the urgent threat of climate change, grid-scale, long-duration storage systems are crucial. The researchers emphasize the need to identify solutions that can effectively compete with vanadium and be deployed and operated over the long term. With numerous chemistries under investigation, it is essential to narrow down the options and focus on viable alternatives.

The techno-economic framework developed by the researchers serves as a valuable tool in this process. By calculating the levelized cost of storage for specific designs, it enables comparisons with vanadium systems and among different chemistries. It helps identify knowledge gaps related to long-term operation and remediation, directing attention to necessary technology development and experimental investigations. Moreover, it assists in determining whether the trade-off between lower upfront costs and higher operating costs is reasonable for these next-generation chemistries.

A promising aspect highlighted by Rodby is the transferability of advances in flow battery research across different chemistries. Lessons learned from studying vanadium systems can often be applied to other flow battery configurations. This not only contributes to the growing understanding of flow battery principles but also enables the design of experiments that address common challenges, ultimately preparing the technology for its vital role in future grid-scale storage solutions.

Source: Massachusetts Institute of Technology

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