A flow battery is a type of rechargeable battery where the electrolyte solution, instead of being stored inside the cells, flows in external tanks. Flow batteries have several advantages over conventional batteries, including longer life and scalability, as they can be scaled up or down to meet energy demands. This makes them ideal for storing long-duration electricity on the grid.

The development of flow batteries for large-scale, long-duration energy storage has been hindered by the complexity of the system design. In response to this challenge, scientists from MIT have developed a modeling framework that can be used to speed up the development process.

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Design and operation of flow batteries

At the heart of flow batteries lie two big tanks, each holding liquid electrolytes, one negative and the other positive. These electrolytes contain dissolved atoms or molecules that undergo electrochemical reactions to either release or store electrons called "active species." During the charging process, one species oxidizes, releasing electrons, while the other reduces, gaining electrons. Conversely, during discharge, their roles are reversed. This intricate dance of oxidation and reduction powers the flow battery, enabling it to store and release energy as needed.

The electrolytes are circulated through an external circuit, flowing toward the connected electrodes. These electrodes, crafted from porous materials, offer ample surface area for the active species to engage in reactions. Separating the adjacent electrodes is a membrane that is quite thin, preventing direct contact between the electrolytes and potentially releasing heat and wasted energy. Instead, this energy can be harnessed efficiently within the grid.

During charging, electrons are transferred from one substance to the other, forcing them into a “less energetically favorable” state as it stores extra energy — analogous to a ball being pushed up to the top of a hill. When the battery is discharged, the transfer of electrons shifts the substances into a more energetically favorable state as the stored energy is released, similar to releasing the ball to roll down the hill.

Benefits of flow batteries for grid-scale energy storage

Flow batteries are increasingly favored for grid-scale energy storage due to their high cycle life, scalability and ability to store large amounts of energy. The system design offers significant advantages compared to conventional battery designs. It enables independent adjustment of the battery's capacity, determining the energy it can store, and the power it can produce, controlling the charging and discharging rate. This flexibility makes it possible to design flow batteries to suit specific applications and modify them if it needs to change.

Flow batteries also can store energy for extended periods without degrading, allowing them to provide a reliable, continuous energy source. In addition, their modular design makes them easy to install and maintain. The tanks can be modified to increase the battery’s capacity or power as needed, and they can also be easily replaced if damaged or worn out. This means that flow batteries are a good option for long-term energy storage solutions on the grid.

Finally, flow batteries are relatively inexpensive to operate compared to other forms of energy storage. The cost of operating a flow battery depends on the efficiency and lifetime of the components, as well as the cost of pumping electrolytes through the system. With proper maintenance, flow batteries can provide reliable, affordable energy storage for years to come.

However, flow batteries can have their drawbacks. The membrane separating the electrolytes is not perfectly selective, and some of the active species in one tank can “cross over” and mix with the electrolyte in the other tank. This can cause unwanted chemical reactions that reduce the battery’s capacity or even damage it beyond repair. In addition, recovery from electrolyte crossover can be difficult and costly. Therefore, proper flow battery maintenance and monitoring are essential for optimal performance.

Techno-economic modeling framework

The techno-economic modeling framework is an established way of understanding the economic viability of new energy technologies. This framework enables users to recognize the capital cost of a system and estimate its projected performance over time. This projection allows potential purchasers to compare different options on a “levelized cost of storage.” By considering different chemistries and battery forms, the techno-economic modeling framework is invaluable for evaluating new energy technology's economic feasibility.

The model includes a dynamic physical battery model, which tracks performance over time and considers any storage capacity changes. This allows for accurate calculation of operating costs over decades, including steps for remediation taken in response to species crossover and degradation.

To evaluate the options available and narrow down their analysis, the team at MIT focused on batteries whose active species dissolve in water, as these have the best chance to be successful. They also limited their analysis to asymmetric chemistries, where different materials are used in each tank. Ultimately, the possibilities were classified into two distinct categories. On one hand, there were species with a finite lifespan, gradually deteriorating over time. On the other hand, there were species with an infinite lifespan, remaining unaffected by the passage of time.

In conclusion, grid-scale energy storage is becoming increasingly important as societies shift away from fossil fuels and toward renewable energy sources. Flow batteries offer a unique approach to this problem that is more reliable than traditional batteries, and their potential for cost savings and efficiency makes them an attractive option for organizations across the world looking to increase the capacity of their energy storage systems.

As technology continues to develop and research into flow batteries increases, it is likely that they will become even more attractive and cost effective in the near-term future. Ultimately, flow batteries promise to be a strong competitor to traditional battery technologies in grid-scale energy storage applications, which would enable wider integration of renewable energy into our power networks.