An investigation into the cause of an explosion at an Arizona energy storage facility that injured four firefighters confirms what engineers had feared: Packing lithium-ion batteries tightly together can spark a chain reaction that can cause catastrophic damage and endanger lives.
Arizona Public Service is holding off on its plan to aggressively expand battery storage capacity while its suppliers draw up new plans that will reduce the risk of similar accidents. APS has also taken two undamaged battery storage facilities offline until mitigation measures can be devised, said Scott Bordenkircher, director of technology innovation and integration for the utility.
Bordenkircher said APS, which serves 2.6 million customers, is letting its peers know that existing standards may not offer protection from catastrophic failures.
“We’re starting to spread the word,” he said. “There are still gaps in those standards across the industry.”
Bordenkircher said APS had been on the verge of signing two contracts to build additional energy storage, but has asked the vendors who submitted the winning bids to go back and review their designs to ensure there are adequate safeguards.
Knowledge about how to safely store energy is crucial as utilities invest heavily in renewable sources such as solar and wind, which cannot be cranked up or turned down to match consumer demand. APS plans to add 850 megawatts of electric storage capacity as it works toward a goal to produce 100% of its energy from renewable sources by 2050.
A law passed in California earlier this year requires the state’s utilities to obtain 50% of the electricity generated from renewable sources by 2025 and 100% by 2045. The state already has eight battery storage facilities in operation that are able to stow away 1,132 megawatts of electricity — enough to power about 1 million homes, according the state’s Independent System Operator, which runs the state’s electrical grid. Dozens of other projects are in the planning stages.
Until last year, APS was drawing power out of three electric storage facilities. But on April 19, 2019, the McMicken storage facility in Surprise, Ariz. started smoking. After consulting with the contractor who built the facility and other experts, firefighters opened a door to the steel structure that housed lithium-ion batteries to investigate the source of the smoke, according to a report by Underwriter Laboratories Firefighter Safety Research Institute. A few minutes later, a jet of flame shot out an estimated 75 feet from the door. Four firefighters were hospitalized, two of them with traumatic brain injuries.
APS commissioned DNV-GL to investigate the cause of the accident. Dr. Davion Hill, energy storage leader for the firm, concluded that the initial overheating was caused by a “thermal runaway” within one of the battery cells buried deep in an apparatus that held 27 separate racks containing a total of 10,584 lithium-ion battery cells.
A thermal runaway is a chemical reaction that produces heat, but no visible flames. Typically the reaction is caused by defects within lithium-ion batteries. Hill said his investigation confirmed that the battery cell that ignited the Surprise accident had lithium deposits within it, which indicate it was defective. He said he found similar defects in other batteries within APS’ electrical storage facilities. Because the batteries were stacked close together, the thermal runaway spread to others batteries, the report says. Hill called it a “cascading thermal runaway.” The Fire Protection Research Foundation warned that such chain reactions are possible in a 2016 paper.
The explosion at APS’ storage facility made apparent another hazard: When lithium-ion batteries heat up, they emit flammable gasses. Hill said the explosion was triggered when firefighters agitated the air inside the structure housing the batteries, causing that gas to combust.
Even though a fire-suppression system inside the structure went off as designed, the aerosol it released could not stop a chemical chain reaction, the report said. The design of the system did not allow any ventilation that could release the build-up of flammable gas.
LG Chem, the Korean company that manufactured the suspect batteries, refuted Hill’s conclusions in testimony to the Arizona Public Utilities Commission. The company submitted its own investigative report that concluded the explosion was possibly caused by a heat source such as external electrical arcing.
Lithium-ion batteries have long been known to be a potential fire hazard. The DNV-GL report provides a brief history. Dell laptops overheated in 2006 and 2007, leading to a product recall. Thermal runaways have also been observed in early models of the Chevrolet Volt, in Teslas that came in contact with road debris and in Boeing Dreamliner battery packs. In 2017 and 2018, several “fires” were reported at electrical storage facilities in South Korea caused by defective battery cells.
In a telephone interview, Hill said the danger of lithium-ion batteries its real but can be mitigated and will be as scientists learn more about the cause of thermal runaways. He said his paper points to some obvious first steps, such as ensuring proper ventilation and training firefighters on how to respond.
He said he doesn’t believe the technology presents an insurmountable risk to insurers.
“Risk equals probability times consequence,” Hill said. “Insurers should be aware of the risks and the consequences and plan accordingly.”
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