Thermal runaway and explosion at McMicken battery storage site, Surprise, Arizona

In April 2019 the McMicken Battery Energy Storage System, a small battery energy storage system of approximately 2 MW and 2 MWh installed at a distribution substation near the APS McMicken substation in Surprise, Arizona, and in service since 2017, suffered a thermal runaway event followed by an explosion. The system was owned and operated by Arizona Public Service (APS) and provided grid services and supported the integration of distributed solar generation. In the late afternoon a report of smoke and odour at the site was received and fire crews arrived within about ten minutes. The response was elevated to a hazardous materials incident once responders identified an energised lithium ion battery installation. After approximately two hours of discussion between fire personnel and utility representatives, responders approached the container door. Shortly after the door was opened an explosion occurred, ejecting the side and rear doors of the container together with other debris. Eight firefighters and one police officer were taken to hospital, and four of the firefighters were seriously injured. The battery system was destroyed. The installation used nickel manganese cobalt lithium ion cells supplied by LG Chem, with the system supplied by Fluence. A technical investigation lasting around fifteen months followed.

According to the independent technical analysis report commissioned by Arizona Public Service, the most likely root cause was an internal failure in a single cell, attributed to abnormal lithium metal deposits creating an internal short circuit. That cell failure initiated thermal runaway which cascaded through neighbouring cells, modules and racks. The report identified several factors that allowed the initial failure to escalate. First, there were no effective thermal barriers between cells, modules and racks, and the aluminium separators that were fitted sagged and permitted propagation. Second, the installed gaseous clean agent fire suppression system was not capable of arresting cascading thermal runaway once it had begun. Third, off gases released during the runaway accumulated to form a flammable atmosphere inside the sealed container. Fourth, the absence of pre incident emergency response planning and coordination meant that first responders had limited information available when deciding to open the container door. The cell supplier subsequently disputed the internal cell failure finding and put forward an alternative external arcing mechanism. That difference of view was recorded in regulatory filings, and the original technical report remains the primary public reference. The off gas and flammable atmosphere mechanism was not disputed.

1. Provide thermal barriers between cells, modules and racks, because enclosure design alone is not sufficient to prevent cascading thermal runaway.
2. Do not rely on gaseous clean agent suppression alone for lithium ion systems, and combine any suppression with a strategy for controlled ventilation and cooling.
3. Design ventilation and deflagration protection to address the accumulation of flammable off gases, and treat entry into a sealed enclosure after a thermal runaway event, before it has been ventilated and monitored, as a high hazard activity.
4. Document emergency response procedures, keep a copy accessible from outside the enclosure or building, and rehearse them with responders in advance.
5. Engage local fire and rescue services early and repeatedly, with site familiarisation visits, agreed response protocols and shared information on chemistry, isolation points and gas hazards.
6. Review existing installations against current battery energy storage standards and codes, including requirements for barriers, gas detection, ventilation and explosion control, and record where an older design does not meet them.

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