A 15-month investigation into an explosion at a lithium-ion battery storage facility operated by the Arizona Public Service (APS) utility in the US has found the cause was a cascading thermal runaway.
The probe into the fire found the explosion on 19 April, 2019, that seriously injured four fire fighters, began with an internal cell failure within one of the battery cells.
An abnormal lithium metal deposition and dendritic growth within a cell caused the failure, which led to thermal runaway moving through every cell and module in the ESS’ rack 15 via heat transfer.
Contributing factors into the explosion included: lack of thermal barriers between cells; no ventilation for flammable ‘off-gasses’; lack of extinguishing, ventilating and entry procedure for emergency services.
Analysis and modelling confirmed gases created by the thermal runaway led to a flammable atmosphere within the container, which then ignited when fire fighters opened it around three hours after the event began.
A flooding clean agent fire suppression system was activated, but the investigation found it was not able to stop cascading thermal runaway in an ESS.
The batteries for the 2.16MWh system were supplied by South Korean battery maker LG Chem.
In a statement, LG Chem refuted claims the thermal runaway was caused by dendritic growth. It said: "The independent experts retained by LG Chem believe that the evidence rules out the theory regarding the cause of the initial thermal runaway event."
APS coordinated the investigation, which included representatives, experts, and consultants from AES, Fluence, and LG Chem.
The utility has made a number of recommendations into dealing with grid-scale lithium-ion ESS to prevent similar incidents from occurring:
- Address vulnerabilities to thermal runaway cascading, ventilation, and suppression in existing and operational systems.
- Update standards and codes to directly address cascading thermal runaway in future ESSs. Merely acknowledging cascading thermal runaway in the annex or appendix of the standard is insufficient to warn the industry of the hazard and falls short of requiring prevention.
- Implement ventilation and extinguishing or cooling systems to manage thermal runaway in future energy storage facilities.
- Implement battery and battery storage system designs that aim to slow or halt cascading or propagation of battery cells and modules during thermal runaway.
- Implement education, training, and emergency response procedures that account for the risks and hazards of cascading thermal runaway—including flammable gases—and how to enter systems after a failure.
The APS report titled ‘McMicken Battery Energy Storage System Event- Technical Analysis and Recommendations’ was published on July 18, 2020.
The APS report noted: “While today’s energy storage safety codes and standards acknowledge cascading thermal runaway as a risk, they stop short of prohibiting it, and fail to address the risk of non-flaming heat transfer to neighbouring cells, modules, and racks.
“Standards today focus on the means to manage a fire, but have so far avoided prescribing solutions that restrict or slow cell-to-cell and module-to-module thermal runaway propagation (likely due to a reticence to prescribe anything that may be perceived as prohibitively expensive or non-commercial).”
A separate, first-of-its-kind report by the UL Firefighter Safety Research Institute (UL FSRI) was conducted.
UL FSRI vice president of Research Steve Kerber, said: “The ability to study lithium-ion battery-related fires on this scale with first-person accounts from the responding firefighters is critically important to protecting the lives of first responders in similar situations.
“We’re dealing with new technology, which brings about new fire-related hazards. We have an opportunity to learn from this incident and improve future outcomes by sharing resources and enhancing training and safety protocols.”