Advances in Engineering Advances in Engineering features breaking research judged by Advances in Engineering advisory team to be of key importance in the Engineering field. Papers are selected from over 10,000 published each week from most peer reviewed journals.
Significance
Vanadium flow batteries (VFBs) are a type of rechargeable battery that uses vanadium ions in two electrolyte solutions separated by a membrane to store energy. They are a type of redox flow battery, which means that the energy is stored and released by means of chemical reactions that occur between the vanadium ions contained in the electrolyte solutions. VFBs consist of two electrolyte tanks, one containing a positive electrolyte solution and the other containing a negative electrolyte solution. The two electrolytes flow in the cell electrodes that are separated by a membrane, which allows the anions or cations to move between the two solutions but prevents electron and vanadium ions crossing and the two electrolytes mixing. A potential difference established between the two electrolytes. When the battery is charged, the vanadium ions are oxidized in the positive electrolyte solution, and reduced in the negative electrolyte solution. When the battery is discharged, the reverse reaction occurs, with the vanadium ions being reduced in the positive electrolyte solution and oxidized in the negative electrolyte solution. This creates an ionic current between the two electrolytes in the membrane, which closes in an external circuit as electron current, to absorb electric power from a source or to release it to an electrical load. One of the advantages of VFBs is that they can be scaled up to store and release large amounts of energy. This is because the amount of energy that can be stored in vanadium flow batteries is determined by the size of the electrolyte in the tanks, rather than the size of the electrodes. Large vanadium flow batteries can be used for applications such as grid-scale energy storage, where large amounts of energy need to be stored and discharged over long periods of time. Another advantage of VFBs is that they have a long cycle life, with the ability to be charged and discharged tens of thousands of times without significant degradation in performance. Consequently, they are well-suited for applications where the battery is used frequently, such as in renewable energy systems.
However, vanadium flow batteries also have some disadvantages. One of these is that they have a relatively low energy density compared to other battery technologies, meaning that they require a larger physical footprint to store the same amount of energy. Additionally, VFBs are currently more expensive than other battery technologies in terms of stored energy [$/kWh] unless they are sized for long discharge durations (above 4-6 hours), although this cost is expected to decrease as the technology improves and becomes more widely adopted. However, they present a lower cost of managed energy {$/kWh/cycles]. The economic viability of VFBs has been analyzed in several studies, considering different assumptions. Another challenge characterizing the applicability of VFBs is their ability to undergo electrolyte oxidation from atmospheric hydrogen and/or oxygen evolution due to operations at extreme states of charge. This results in electrolyte imbalance which degrades the battery capacity and cycle life. In these cases, electrochemical rebalancing is needed though it cannot be achieved using simple mixing operations.
On this account, PhD candidate Nicola Poli, Dr. Andrea Trovò, Dr. Peter Fischer, Dr. Jens Noack, led by Professor Massimo Guarnieri from the University of Padua presented a sizing analysis of the electrochemical rebalancing process for VFBs. Specifically, they adopted an electrochemical method based on an electrolysis reactor consisting of stacks of several cells. The economic feasibility of this method was also assessed using a techno-economic model considering the major parameters affecting the reactor and process performance as well as investment and operating expenditure analysis. Their work is currently published in the peer-reviewed Journal of Energy Storage.
The investment and operative expenditures analysis showed that relatively frequent rebalancing operations are economically feasible. Performing a rebalancing process once a year proved suitable only with a slow regeneration process, lasting 5 – 10 h, and the power required by the process exceeded that of the VFB system. As regards the investment and operation expenditure, the results revealed that minimum total costs were achieved at a current density consistent with the sporadic use of stacks. A 70% cost reduction could be achieved by running a 10-hour process every three months instead of once a year. In order to successfully reduce further the operational costs, it is important to carry out the rebalancing procedure without missing the remunerative VFB commercial service.
In summary, Professor Massimo Guarnieri and colleagues reported a new ad-hoc balancing method for the electrochemical rebalancing of VFBs and an analysis of its operation from economic and technical perspectives. The power rating was defined in terms of rebalancing key parameters, including electrolyte imbalance rate, VFB energy rating, periodicity, active area, cell number, and rebalancing duration. With proper optimization, this electrochemical rebalancing could remarkably reduce the overall investment and operational cost more than chemical rebalancing processes. In a statement to Advances in Engineering, the lead and corresponding author, Professor Massimo Guarnieri stated that the findings would promote the use of VFBs for energy storage in different applications. Indeed, VFBs are a promising technology for large-scale long-duration energy storage applications, and ongoing research and development is likely to make them even more competitive in the future.
Reference
Poli, N., Trovò, A., Fischer, P., Noack, J., & Guarnieri, M. (2023). Electrochemical rebalancing process for vanadium flow batteries: Sizing and Economic Assessment. Journal of Energy Storage, 58, 106404.
