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

What happened

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.

Contributing factors

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.

Lessons learned

1. Thermal barriers between cells, modules and racks must stop propagation, as aluminium separators sagged and allowed the fire to spread.
2. Gaseous clean agent suppression cannot stop cascading thermal runaway in lithium ion systems, so add ventilation and cooling.
3. Entry into a sealed container after thermal runaway must be treated as high hazard until ventilated and gas levels checked.

More lessons on spacing, layout and fire spread, fire suppression and firefighting, from every incident on the register.

Not yet known

• Exact cell model and age
• Duration of site outage
• Cost of damage and repairs

This entry is updated when these are reported. Get an alert for the next one like it.

Source

aps.com