This is the first of three blog posts on redox flow battery (RFB) safety including the current code landscape, the relevance and gaps of the current codes and recommendations on bridging the gaps.
Redox flow battery (RFB) safety overview
Alternative battery technologies include mature products such as lead-acid and nickel-cadmium batteries as well as emerging technologies such pure RFBs, hybrid RFBs, non-flow zinc-bromine batteries, sodium-ion battery, sodium-sulphur battery, liquid metal batteries such as the calcium-antimony battery and metal-air batteries such as zinc-air and iron-air batteries. The listed technologies can offer long duration energy storage more cost effectively than LIBs. Long duration storage is defined as discharge periods exceeding 8 hours. Increased integration of renewable energy generation into the power grid has revealed the necessity for cost effective long duration energy storage.
Among battery technology alternatives, RFBs appear best suited for long duration energy storage stretching from 8 hours to seasonal storage. This technology was developed in the latter half of the 20th century but has become commercially viable in the last few decades. A redox flow battery consists of a stack of serially connected cells, a positive electrolyte (posolyte) reservoir and a negative electrolyte (negolyte) reservoir, plumbing that connects the electrolyte reservoir to the cell stack and a pump that shuttles the electrolyte back and forth between the cell stack and the reservoirs. The active materials in the posolyte and negolyte possess different redox potentials which results in potential difference across the battery terminals that yield current flow in the external circuit. The main component of a flow battery cell is the membrane that prevents the mixing of the posolyte and the negolyte but allows charge carriers to flow across to complete the circuit.[1] Like conventional lithium-ion battery energy storage systems (BESS), a flow battery energy system (FBES), depicted in Figure 2 1 below, consists of the flow battery, a power conversion system (PCS), a battery management system and a control system.
Figure 2-1 A Flow Battery Energy System[2]
The main appeal of flow batteries is the decoupling of power rating and energy capacity enabled by the spatial separation of the electrochemical stacks and the electrolyte tanks. Thus, unlike conventional enclosed battery types, such as LIBs, the power and energy capacity of flow batteries can be altered independent of each other. The power rating is dependent on the size of the electrochemical stack while energy capacity varies directly with the volume of the electrolyte tanks. Thus, at the rated power, discharge duration is dependent on the volume of the electrolyte. This feature has a big influence on storage cost as the electrolyte tends to be cheaper than the electrochemical stack. There are various RFB chemistries classified by the composition of their electrolytes. The most mature flow battery technology is the vanadium redox flow battery (VRFB). The largest flow battery installation to date is the 200 MW/800 MWh Dalian Vanadium Redox Flow Battery project developed by Rongke Power in China.[1] The first phase of the project, 100 MW/400 MWh, was commissioned in July 2022.[3]
Though mostly viewed as a cost-saving feature, the decoupling of power rating and energy capacity in RFBs has significant fire safety benefits too. When a flow battery is in operation, typically only 1% of the electrolyte volume is contained in the cell stack while the rest resides in the bulk electrolyte tanks. The cell stacks are the sites for electrochemical reactions that facilitate the chemical to electrical energy conversion and vice-versa. Heat dissipation in an RFB cell can be caused by electrical short circuit, electrolyte short circuit, overcharge and over discharge. An electrical short circuit occurs when the positive and negative electrode come into contact while electrolyte short circuits occur when the positive and the negative electrolytes mix. Both destructive events are mitigated by the presence of a separator in the cell. In the event of heat generation in an RFB cell, a very small proportion of electrolyte volume is affected. The bulk electrolyte in the tanks effectively serve as a cooling reservoir due to its high thermal mass. A high thermal mass means that the magnitude of change in the electrolyte’s temperature is comparatively lower for a given amount of heat absorbed. This limits temperature rise in the electrolyte in the event of heat generating faults. In addition to heat dissipation due to high thermal mass, aqueous electrolytes reduce fire risks even more by virtue of being non-flammable.[4]
Despite great performance in the fire safety arena, the main safety concern in RFB technologies is electrolyte leakage and spills given the extensive plumbing. Some RFB chemistries utilize highly corrosive acidic and alkaline electrolytes such as concentrated sulfuric acid and potassium hydroxide solutions respectively. Additionally, some of the electrolytes are toxic. As a result, leakage presents a high risk of contamination for water sources, air and soils. The effects of leakages are exacerbated by the large volumes of electrolyte, which might be as much as thousands of cubic meters in utility scale FBES. Skin contact with electrolyte spills may cause rashes and chemical burns while eye contact can cause irritation, corneal burns and partial or complete blindness.[5] Solvents may also enter the body’s blood circulation system through the skin and cause damage to internal organs such as the heart and the blood vessels.[6] Electrolyte spills also have ecological effects such as the formation of localized gas clouds and threat to aquatic life.[5] However, it is important to note that the concentration of acids and bases in flow batteries is lower than in most other battery types such as lead-acid batteries.[4] This makes the RFB electrolytes less harmful from a corrosion perspective.
Electrolyte leakage and spills are mitigated by employing secondary containment which creates a closed system thereby preventing electrolytes from escaping from the reservoirs. Tertiary containment is installed along pipe segments to mitigate leaks during system operation. Both secondary and tertiary containment should have sufficient capacity to hold the leaked electrolyte before the system is shutdown. Visual and electrical leak detection enable the isolation of the leaking portion for repair or replacement. Additionally, RFB systems utilizing toxic electrolytes are constructed far away from water sources and aquatic life. [5]
Effects of RFB chemistry on fire safety
Since the main differentiator among RFBs is electrolyte composition, it is important to evaluate the fire risk of different RFB chemistries based on the flammability of their electrolytes. Flow battery electrolytes are usually characterized as either aqueous, non-aqueous, acidic or alkaline depending on the supporting electrolyte used. A supporting electrolyte is a solvent in which the redox-active materials are dissolved to form a flow battery electrolyte. The redox-active materials can be either organic or inorganic. Aqueous electrolytes are comprised of redox-active materials dissolved in water or dilute acids and bases. On the other hand, redox-active materials dissolved in organic solvents such as acetonitrile and propylene carbonate form non-aqueous electrolytes. Non-aqueous electrolytes have attracted research interest as means to increase the cell potential of redox flow batteries beyond the 1.23 V threshold dictated by water splitting reactions in aqueous RFBs.
Organic solvents are carbon-based liquids that tend to be highly volatile and flammable.[6] [7] Solvents of this type possess higher fire risks than their aqueous-based counterparts due to their higher ease of combustibility. Efforts are being made to develop low-cost, low flammability organic solvents to preserve their cost benefit while improving their fire safety aspect.[7] Additionally, some organic solvents, such as benzene, are considered toxic and carcinogenic. The high volatility and toxicity of organic solvents and higher density of their vapors constrain their use to fume hoods.[6] As such, more stringent safety requirements, in line with personal protective equipment and solvent use and disposal, are applied to organic solvents compared to aqueous-based solvents. Given the cited concerns, the development of non-aqueous flow batteries lags that of their aqueous counterparts.
REFERENCES
[1] E. Sanchez-Diez, E. Ventosa, M. Guarnieri, A. Trovo, C. Flox, R. Marcilla, F. Soavi, P. Mazur, E. Aranzabe and R. Ferret, "Redox flow batteries: Status and perspective towards sustainable stationary energy storage," Journal of Power Sources, vol. 481, 2021.
[2] IEC, "Flow battery energy systems for stationary applications - Part 1: Terminology and general aspects," 2020.
[3] B. Santos, "PV Magazine," 29 September 2022. [Online]. Available: https://www.pv-magazine.com/2022/09/29/china-connects-worlds-largest-redox-flow-battery-system-to-grid/. [Accessed 7 November 2022].
[4] M. Paiss, "Energy Response Solutions," 11 August 2017. [Online]. Available: http://energyresponsesolutions.com/wp-content/uploads/VRB_SafetyReport.pdf. [Accessed 5 August 2022].
[5] D. Rosewater, J. Lamb, J. Hewson, V. Vilayanur, M. Paiss, D. Choi and A. Jaiswal, "Grid-scale energy storage hazard analysis and design objectives for safety systems," Sandia National Laboratories, Albaquerque, 2020.
[6] [Online]. Available: https://www.safety.fsu.edu/safety_manual/Organic%20Solvents.pdf. [Accessed 09 August 2022].
[7] D. Han, C. Cui, K. Zhang, Z. Wang, J. Gao, Y. Guo, Z. Zhang, S. Wu, L. Yin, Z. Weng, F. Kang and Q.-H. Yang, "A non-flammable hydrous organic electrolyte for sustainable zinc batteries," Nature Sustainability, pp. 205-213, 2021.
