Report: ‘Thermal runaway’ likely caused APS explosion that injured 4 firefighters in Surprise 

PHOENIX (3TV/CBS 5) – A newly released report details what likely led up to the explosion at an APS battery energy storage facility that injured four Peoria firefighters more than a year ago. It happened the night of Friday, April 19, 2019, near Deer Valley Road and Grand Avenue in Surprise.

The facility, which was part of the utility’s clean energy project, was designed to store unused power generated by homes with solar panels allowing the solar energy to be used after the sun goes down. The battery energy storage system, or BESS, was built in early 2017. The firefighters who were hurt in the explosion were there to check on a reported fire.

According to the report, “an internal cell failure within one battery cell,” likely caused “by an internal cell defect” sparked the fire. After that, there was an “an extensive cascading thermal runaway event.” The report says the fire suppression system kicked on like it was supposed to, but it was only designed to handle an ordinary fire. “Such systems are not capable of preventing or stopping cascading thermal runaway …”

The heat spread from the single cell to every cell in the rack because there were no thermal barriers between them, the report explains. The “uncontrolled cascading thermal runaway” caused flammable gases to build up in the facility because there was no ventilation. That went on for about three hours.

According to the report the Surprise Fire Department released four months after the explosion, the temperatures and gas meter readings were lower than when the team – Peoria Firefighter Jake Ciulla, Fire Engineer Justin Lopez, Firefighter Matt Cottini and Cpt. Hunter Clare – first arrived. That’s why they decided to open the door. When they did that, the flammable gases that had built up were agitated and exposed to a spark or heat source. The explosion happened 2 minutes later.

The 69-page APS report, which was completed by a third-party company earlier this month and sent to the Arizona Corporation Commission on Monday, boiled it down to “five main contributing factors.”

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 pal did not have extinguishing, ventilation, and entry procedure”

The report suggests that a similar explosion could happen again. “While today’s energy storage safety codes and standard 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 neighboring cells, modules, and racks,” explains the executive summary. “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 ….” The report goes on to say that there are “commercially available technologies and design methods” meant to deal with the potential for thermal runaway.

“In addition, better practices for ventilation, extinguishing, and cooling thermal runaway scenarios exist today and should be implemented in future energy systems,” the reports says. While fire suppression systems like the one at the BESS that exploded are good for handling fires in their early stages, they “should not be the only barrier against thermal runaway.”

APS has similar BESS facilities in Buckeye and near Payson and planned to build more to go with its existing solar power plants.

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