Peoria firefighters Matt Cottini, Justin Lopez, Jake Ciulla and Capt. Hunter Clare

An explosion at an APS solar battery facility caused by a “cascading thermal runaway” chain reaction injured Peoria firefighters Matt Cottini, Justin Lopez, Jake Ciulla and Capt. Hunter Clare.

A chain reaction “cascading thermal runaway” led to an explosion that injured four Peoria firefighters, according to an Arizona Public Service (APS) report. 

The report, released Monday, July 27, lists “five main contributing factors that led to the explosion.”

Among the five factors:

•“The fire suppression system was incapable of stopping thermal runaway.”

•“Emergency response plan did not have an extinguishing, ventilation and entry procedure.” 

Mario Bravo, a Peoria Fire-Medical spokesman, directed queries on the report to the city of Peoria. 

“With this horrific event behind us, we are left with lessons learned and the personal stories of strength, bravery and healing of our firefighting heroes,” said Peoria Mayor Cathy Carlat.

 The 78-page APS report is filled with engineering and chemical details regarding the April 19, 2019, explosion at the McMicken Battery Energy Storage System (BESS) in Surprise.

The report described how intense heat spread from one lithium-ion battery cell to another, filling the unmanned facility with smoke and gas.

“On the day of the event, the BESS was performing a solar smoothing function, which involved charging during daytime (absorbing solar energy produced from rooftop solar on the circuit) and discharging through the evening,” the report states.

At 4:54 p.m., a battery cell “experienced a sudden drop in voltage,” quickly followed by a “thermal runaway” that generated gas and smoke. “The initiating cell’s thermal runaway cascaded into thermal runaway of neighboring cells … and subsequently into the batteries contained within neighboring modules.”

The melting battery cells “created a flammable atmosphere within the BESS,” according to the report. 

When the BESS fire suppression system was activated, APS was automatically notified and in turn contacted a contractor, which sent an engineer to investigate, according to the report.

The engineer saw smoke and called 911 at 5:40 p.m.—nearly an hour after the fire suppression system was automatically activated.

According to the report, firefighters were on scene within 10 minutes of the 911 call.

Members of the Peoria Fire-Medical Hazardous Materials Response (“hazmat”) team were called to assist at the facility near Deer Valley Road and Grand Avenue.

At 8:02 p.m., the Peoria firefighters opened a door and entered the battery facility, which is about the size of a shipping container.

Two minutes later, an explosion injured Capt. Hunter Clare, Engineer Justin Lopez and firefighters Matt Cottini and Jake Ciulla. All four were hospitalized.

Lopez sustained nose, skull, rib, right-leg tibia and fibula fractures as well as a collapsed lung. Clare suffered bilateral ankle and thoracic fractures and multiple burns and lacerations.

Cottini suffered head and jaw lacerations and burns as well as a knee injury. Ciulla received minor burns and lacerations.

Four Surprise firefighters also were injured.


Chain reaction

According to the report, “The suspected fire was actually an extensive cascading thermal runaway event. It is believed to a reasonable degree of scientific certainty that this internal failure was caused by an internal cell defect. 

“The uncontrolled cascading of thermal runaway from cell-to-cell and then module-to-module in Rack 15 led to the production of a large quantity of flammable gases within the BESS.”

The report states that, three hours after the chain reaction began, “the BESS door was opened by firefighters, agitating the remaining flammable gases and allowing the gases to make contact with a heat source or spark.”

The facility was equipped with a “total flooding” clean agent fire suppression system.

When the facility went into use in 2017, “clean agent systems were a common method for managing fire suppression in energy storage containers,” according to the APS report.

“In an August 2017 letter to the NFPA clean agent manufacturer 3M stated that clean agents could not prevent or suppress cascading thermal runaway in Lithium-ion battery systems,” the report states.

“While this statement was made more than a year after the design of the McMicken BESS, it was not widely circulated.”

According to APS, the BESS site uses lithium-ion batteries with a function of “integrating solar energy resources in an area with high rooftop solar penetration and grid services including voltage regulation and power quality.”