New reports detail cause of APS battery explosion that left 8 injured


New details have come out surrounding the Arizona Public Service (APS) battery failure and corresponding explosion that left eight firefighters and one police officer hospitalized in Surprise, Arizona in April of 2019.

All of the new details regarding the incident are put forth in two reports, one released by APS, the other by the Underwriters Laboratory Firefighter Safety Research Institute.

According to the APS report, developed by DNV GL Energy Insights, “The suspected fire was actually an extensive cascading thermal runaway event, initiated by an internal cell failure within one battery cell in the BESS: cell pair 7, module 2, rack 15.” The cascading thermal runaway is believed to have been caused by an internal cell defect, specifically abnormal Lithium metal deposition and dendritic growth within the cell.

And while the system’s clean agent fire suppression system began operating to contain the event, the system is designed to extinguish developing fires in ordinary combustibles, rendering it entirely ineffective against cascading thermal runaway. From there, the event spread through every cell and module in rack 15 of the system, via heat transfer, as the system did not have adequate thermal barrier protections between battery cells. These thermal barriers could have significantly deterred the spread.

This expansive thermal runaway led to the production of a large quantity of gases, which created a flammable atmosphere within the system. Three hours after the event began, firefighters opened the system’s door, agitating the gasses and allowing them to make contact with a heat source or spark, triggering the explosion.

In short, the two reports find five root causes of the explosion:

  1. Internal failure in a battery cell initiated thermal runaway
  2. The fire suppression system was incapable of stopping thermal runaway
  3. Lack of thermal barriers between cells led to cascading thermal runaway
  4. Flammable off-gases concentrated without a means to ventilate
  5. Emergency response plan did not have an extinguishing, ventilation, and entry procedure

According to APS, existing battery storage system safety standards and procedures only acknowledge cascading thermal runaway as a risk. These standards do little to prohibit thermal runaway, and fail entirely to address the risk of non-flaming heat transfer to neighboring cells, modules and racking. According to the utility, those same standards focus on the means to manage a fire, but provide no solutions to restrict or slow cell-to-cell and module-to-module thermal runaway.

And while the Firefighter Safety Research Institute report came to the same conclusions regarding what led to the event, it also outlines steps that can be taken in order to mitigate the likelihood of a failure to this degree in the future.

  • Basic Firefighter, Officer, and HAZMAT training should emphasize ESS safety
  • Research and full-scale testing should be conducted to understand the most effective and safest tactics for response to lithium-ion battery incidents
  • Fire service personnel should define a conservative potential blast radius and remain outside of it while treating the event, until definitive tactics and guidance can be established
  • Lithium-ion battery systems should incorporate gas monitoring that can be accessed remotely
  • Additional plans for increased monitoring, safety standard development and communication between the battery system, operators and first responders

While these standards will take some time to develop, there is a pressing need for them sooner rather than later. APS has plans in place to install at least 850 MW of nearly-identical batteries across Arizona in the near future, not to mention that the United States is on track install as much as 2,500 MW of battery storage by 2023, according to data from the U.S. Department of Energy’s Energy Information Administration.

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