This is the third of three blog posts on Flow battery energy system (FBES) safety including the current code landscape, the relevance of and gaps in the current codes and recommendations on bridging those gaps.
Addressing the gaps in the North American safety standards
The noted gaps in the North American safety standards, with regards to redox flow battery (RFB) testing (see blog post 2/3: Towards an improved scope for flow battery testing in North American safety standards), can be bridged in two main ways:
- Integrating RFB cell stack testing and electrolyte toxicity characterization into UL 9540A and UL 1973, respectively
- Leveraging the currently available, more comprehensive European and Chinese RFB standards
Integration of RFB cell stack testing and electrolyte toxicity characterization
The quantification of the fire risk of the other components of the RFB is not done in UL 9540A tests. RFB cell stack test would facilitate the characterization of fire risks for two scenarios: fire risk due to external factors and fire risk of a fully drained RFB cell stack (system in off mode). Component-level analysis of RFB fire risks can be considered a proxy for cell stack tests. Herein, the results of the component-level fire risk investigation performed by Chen et al have been reviewed. The thermal behavior, thus the fire risk, of vanadium redox flow battery (VRFB) components was determined through C80 and cone calorimetry. C80 calorimetry is used to determine thermal stability through the quantification of the amount of heat generated by a burning component while cone calorimetry is a combustion test based on the rate at which a component burns[1].
Table 1-1 below shows the fire risk of each of the components of a cell, based on C80 and cone calorimetry.
| Component | Material | Total heat release (MJ/m2) | Thermal behaviour (J/g) | Rate of combustion ranking | Thermal stability ranking |
| Bipolar plate | Poly-phenylene, graphite | 16.85 | -42.8 | 3 | 3 |
| Electrode frame | Poly-phenylene | 93.51 | -95.9 | 1 | 2 |
| Electrode | PAN - carbon | 0 | 0 | 4 | 1 |
| Membrane | Nafion 117 | - | 59.6 | 2 | 4 |
In Table 1-1, 1 represents the highest fire risk while 4 represents the lowest fire risk in the rate of combustion ranking. The reverse is true for thermal stability ranking. As such, a ranking combination of 4 in rate of combustion and 1 in thermal stability implies the lowest fire risk. The electrode frame demonstrates the highest fire risk while the membrane has the highest susceptibility to losing its structural integrity due to heat. The electrode poses extremely low fire risks. Based on this interpretation, fire risk mitigation tests for RFBs should focus on the electrode frame. Overall, UL 9540A tests should promote the design of RFB membranes and electrode frames with higher fire resistance.
Component-level tests provide useful data but disassembling fully built stacks would not be economically sound. Full cell stack tests that utilize the component level test results, as guidance for result interpretation, should be adopted into UL 9540A.
Electrolyte toxicity has been identified as the leading safety concern in most RFB chemistries. The significance of incorporating exhaustive electrolyte toxicity and corrosivity tests into UL 1973 is therefore clear. The implementation of this recommendation could be as simple as requiring a specific safety data sheet (SDS) for each unique electrolyte as a prerequisite to UL 1973 certification.
Leveraging European and Chinese flow battery standards
Europe currently leads North America in the development of RFB safety codes. The establishment of the International Flow Battery Forum (IFBF) in 2010 provided a major boost to the advancement of RFB safety codes in Europe. At the inception of IFBF, a CENELEC workshop was set up to aid the development of RFB codes[2]. Table 1-2 summarizes the codes that have been published with either the inspiration and/or direct involvement of IFBF.
Table 1-2 European RFB codes[3] [4] [5] [6]
| Standard | Full title | Description |
| IEC 62932-1:2020 | Flow battery energy storage systems for stationary applications – Part 1: Terminology and general aspects | Definition of terms used to express unit parameters, test methods, safety and environmental issues. Terms specific to RFB operation such as cold standby, hot standby and stopped state are covered. Overview of a flow battery energy system (FBES), including RFB types and chemistries. Exclusive to installations not exceeding 1500 Vdc. |
| IEC 62932-2-1:2020 | Flow battery energy storage systems for stationary applications – Part 2-1: Performance, general requirements and test methods | General testing conditions: accuracy of measuring instruments, ambient temperature, test object selection, point of connection (POC) and point of measurement (POM). Test procedures for: energy at constant power, maximum power output, maximum power input, energy efficiency at constant power, cycle life and variation of discharge power with energy or energy efficiency. |
| IEC 62932-2-2:2020 | Flow battery energy storage systems for stationary applications – Part 2-2: Safety requirements Applicable to both indoor and outdoor installations in unclassified areas. | Hazards addressed: electrical, gaseous emissions, electrolyte leakage, mechanical, operational (auxiliary power failure), transportation, storage and disposal. Tests and acceptance criteria for protective measures: dielectric strength of the parts in contact with the fluid, operational sequence, emergency stop, and stacks (external short circuits, heat shock strength and leakage). Recommended user manual content: product description, site requirements, operations, alarms and fault finding, maintenance and contact information. |
| CENELEC CWA 50611:2013 | Flow batteries – Guidance on the specification, installation and operation | Stipulates considerations for technology selection (life cycle cost methodology, system costs and battery sizing), test procedures at system level, user manual content and installation procedures (pre-commissioning, commissioning, operation and decommissioning) |
The IEC standards are RFB-centric thus address the unique requirements of RFBs with regards to testing, transportation, installation and operation.
Another region that has been making great strides in both RFB standards development and deployment is China. China currently boasts the world’s largest FBES installation and plans to continue growing its grid connected VRFB capacity. China’s strong position in VRFB manufacturing and deployment is backed by the country’s huge vanadium reserves which mitigate potential supply chain issues. RFB standards development has kept pace with RFB deployment in China as shown in Table 1-3. Most of the Chinese RFB standards provide testing procedures for specific components of the VRFB.
Table 1-3 Chinese National RFB standards[2]
| Standard | Full title |
| GB/T 29840-2013 | Vanadium flow battery – terminology |
| GB/T 32509-2016 | General specification for vanadium flow battery |
| GB/T 33339-2016 | Vanadium flow battery system – test method |
| GB/T 34866-2017 | Vanadium flow battery – safety requirements |
| GB/T 37204-2018 | Electrolyte for vanadium flow battery |
| NB/T 42006-2013 | Electrolyte for vanadium flow battery – test method |
| NB/T 42007-2013 | Bipolar plate for vanadium flow battery – test method |
| NB/T 42080-2016 | Test method of the ion conducting membrane of [a] vanadium redox flow battery |
| NB/T 42081-2016 | Performance test method for single cell of [the] all – vanadium redox flow battery |
| NB/T 42082-2016 | Test method for [the] electrode of vanadium redox flow battery |
Despite the progress made by China’s standards organization, the RFB standards are not chemistry-agnostic. This is a significant concern since RFBs are primarily distinguished based on electrolyte(s) identity. However, this issue only presents a significant challenge when it comes to electrolyte testing since most RFBs leverage similar cell stack components. The prescribed electrolyte testing procedures could be modified to fit any new RFB electrolyte.
Another notable standard that originates in neither Europe nor North America is the Korean Standard: “KS C 8547: Redox flow battery for use in energy storage system – Performance and safety tests”. The author did not review this standard.
North American standards development organizations (SDOs) should consider referencing or adopting parts of the proven and tested RFB test methods from the European and Asian SDOs in developing their own testing procedures.
Conclusion
RFB technology continues to take greater strides towards bankability by showcasing superior fire safety capabilities in addition to the potential of lower cost. As such, there is a need to develop comprehensive safety standards for RFBs comparable to those of lithium-ion batteries (LIBs). Despite the increasing effort to incorporate RFBs into the most significant North American safety codes, more needs to be done. Table 1-4 summarizes the deficiencies in the most notable North American standards as well as the recommended remedies.
Table 1-4 Key North American standards and their inadequacies to RFBs
| Standard | Title | Inadequacy | Remedies |
| UL 1973 | Standard for safety: batteries for use in stationary and motive auxiliary power applications | Does not address corrosion of electrodes and membrane breakdown | Incorporate RFB component test and electrolyte toxicity characterization Full or partial adoption of IEC 62932:2020 which covers safety testing for RFBs |
| UL 9540 | Standard for safety: energy storage systems and equipment | Limitations on energy capacity and separation distances not modified for FBES | Seek guidance on energy capacity and separation distances from CENELEC CWA 50611:2013 which covers battery sizing) |
| UN 38.3 | Certification for lithium batteries | Exclusive to LIBs | Full or partial adoption of IEC 62932-2-2:2020 that covers hazards related to FBES transportation. |
To be effective and useful, codes need to focus on RFB technology’s weak spots instead of applying practices of low relevance borrowed from LIB testing. Thermal runaway, the core of the analysis of fire risk in LIBs, is of little concern in RFBs but toxicity and corrosiveness of the electrolyte are. A more RFB-oriented standard could therefore emphasize toxicity and corrosive tests over less relevant fire risk tests by adopting the test procedures stipulated in the European and Chinese standards. Additionally, characterization of the component with the highest fire risk, the electrode frame, should be adopted. A dedicated RFB standard would also aid advancements in RFB safety codification.
Future work
The author acknowledges the typical limitation of literature reviews on this kind of topics - updated information on advancements take a while to get published. As such, the author plans to pursue an in-depth investigation of the development in the testing and certification of commercial FBESs in North America through:
- Interviewing stakeholders including standard development organizations (SDOs) and RFB original equipment manufacturers (OEMs).
- Organizing workshops and webinars for key players to contribute to the discussion on RFB safety standards.
REFERENCES
[1] M. Chen, P. Liu, Y. H. Y. Li, Z. Hu and Q. Wang, "Preliminary study on fire risk of redox flow battery components," Journal of Thermal Analysis and Calorimetry, vol. 147, pp. 4131 - 4140, 2021.
[2] International Flow Battery Forum (IFBF), "International Flow Battery Forum (IFBF)," [Online]. Available: https://flowbatteryforum.com/standards-for-flow-batteries/. [Accessed November 2022].
[3] IEC, "Flow battery energy systems for stationary applications - Part 1: Terminology and general aspects," 2020.
[4] IEC, "Flow battery energy systems for stationary applications - Part 2-1: Performance general requirements and test methods," 2020.
[5] IEC, "Flow battery energy systems for stationary applications - Part 2-2: Safety requirements," 2020.
[6] CENELEC, "CENELEC Workshop Agreement CWA 50611:2013," 2013.
