An explosion that occurred at Arizona Public Service's McMicken Substation was most likely the result of a flawed battery cell that initiated a fire leading to a thermal runaway event, a 15-month investigation into the incident concluded.

APS led the investigation, which included first-responder representatives, system integrator AES, operations and maintenance contractor Fluence, battery manufacturer LG Chem and third-party engineering and safety experts. Davion Hill, energy storage leader for the global risk-management and quality-assurance firm DNV GL, issued a report July 18 that includes his analysis and a review of previous investigative technical reports on the incident.

A suspected fire was reported in the battery energy storage system at McMicken Substation in Surprise, Arizona, on April 19, 2019, prompting local emergency response (see CEM No. 1536). The AES battery energy storage system, assembled with lithium-ion batteries manufactured by LG Chem, was deployed at McMicken in March 2017, about 25 months prior to the incident. When first responders opened the door to the facility, an explosion injured several firefighters and "essentially destroyed the BESS and its container," according to Hill's report, made public by APS July 27.

Abnormal lithium deposits and "dendritic growth" within the problem cell caused the initial fire, which, due to the close packing of the battery cells within each module, "trigger[ed] thermal runaway in its neighbor, which spread to the next cell, and so on," the report says. The fire-suppression system, which activated as designed, was insufficient to prevent the cascading thermal runaway.

The Novec 1230 fire-suppression system filled the space with inert gases; however, as thermal runaway cascaded due to inadequate thermal barriers between cells, flammable gases from the affected modules concentrated inside the facility with no means of ventilation. The gases were agitated when the door was opened, causing them to come in contact with the modules—estimated to be as hot as 660 degrees Celsius—and resulting in the explosion, Hill hypothesized. The rack involved in the thermal runaway was completely destroyed, with all but one of its 14 modules melting into the others, but neighboring racks, including those on either side, were barely damaged in the fire.

McMicken Battery Disaster

All of the battery modules in McMicken's Rack 15 were damaged in the thermal runaway event that likely originated with a flaw in a single module, while heat damage to adjacent racks was minimal.

The batteries in the relatively unaffected racks were charged to 90 percent of their capacity, APS Senior Vice President of Operations Jacob Tetlow said in a telephone interview with California Energy Markets. The investigation provided experts with an opportunity to devise and practice a safe technique for discharging and disassembling the modules, Tetlow said. Pulling each of the charged batteries from the 26 racks not involved in the thermal runaway took about four months.

Tetlow said the incident has not compromised the utility's view that battery storage is the perfect complement to nondispatchable renewable resources, and that APS remains committed to its goal of 100-percent carbon-free generation by 2050 with an interim target of 45-percent renewably sourced generation by 2030. The report and investigative findings allow the company to see what is required to safely integrate more storage and provide assurance that battery storage can be done safely, he said.

The McMicken emergency response plan provided by AES fell short by not including procedures for responding to an explosion or entering the facility after the fire-suppression system had been deployed, the report says. The plan focused instead on electrical shutdown procedures and the roles and responsibilities of APS and Fluence, the operations and maintenance contractor for the system. The plan also addressed whether and when to notify the fire department, but did not address when or how to best enter the system under compromised circumstances.

"We can't undo the event, but we can certainly learn from it," Tetlow said, adding that findings from the investigation have applications for the entire industry in terms of determining safety requirements for battery energy storage systems. This was factored into APS' decision to make the report public and be transparent about the incident and investigation.

When the emergency response plan was developed, no BESS suppliers had communicated the possibility of a large, flammable-gas or module-to-module cascading thermal runaway event, the report says. Hill in the report recommends addressing this communication failure in revisions to National Fire Protection Association Standard 855 to prevent such occurrences in the future. NFPA 855 applies to the installation of stationary energy storage systems.

The report concludes that today's standards better address hazard assessment and first-responder training than those of 2017, when the McMicken system went on line. It recommends that such assessments and training take place prior to and during the commissioning of energy storage systems. Commercially available technologies and design methods can address thermal runaway propagation, the report says, "and the standards should be appropriately updated to acknowledge these methods and technologies."

Tetlow said APS' most recent integrated resource plan, submitted to the Arizona Corporation Commission June 26, takes into account the findings of the McMicken investigation and will not require any amendments. APS plans to move forward with new storage projects once revised safety requirements are in place. The company is working with various suppliers and vendors now "so that when we do award a new contract it will be compliant with the new requirements," Tetlow said. The company has not yet determined a timeline for the requirements.

Staff Writer

Abigail Sawyer grew up in northwestern New Mexico near two massive coal-fired power plants. She spent many hours gazing out the car window at transmission lines on family road trips across the Southwest and now reports on the region from San Francisco.