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Causes of 2019 solar facility explosion which injured nine revealed in new report

04 August 2020

A final investigative report into the April 2019 explosion at utility company Arizona Public Service's (APS) solar battery facility in Surprise, Arizona was published on July 27. The report into the incident, which injured nine first responders, explains the reasons behind the explosion and offers several recommendations to prevent similar incidents occurring at other battery energy storage systems.

Image: APS
Image: APS

The incident on April 19, 2019 started when there were reports at around 17:00 of smoke from the building housing the battery energy storage system (BESS) at APS’s McMicken site in Surprise. Hazardous material units and first responders arrived at the scene to secure the area. A few hours later, at approximately 20:04, an explosion occurred from inside the BESS. Nine people were injured and taken to local hospitals, including one firefighter who was in a critical condition and two others who were in serious conditions. 

APS ordered an investigation into the incident to determine the cause of the incident and identify lessons that can be applied to future battery energy storage systems. Once the investigative work was completed, APS chose DNV GL to combine various forensic and expert inputs into the single, consolidated report which was published on July 27.

The report explains how the BESS in Surprise was commissioned and integrated by AES, on behalf of APS and was assembled with Lithium ion (Li-ion) batteries manufactured by LG Chem. The factual conclusions reached by the investigation into the incident were:

- The suspected fire was actually an extensive cascading thermal runaway event, initiated by an internal cell failure within one battery cell in the BESS: cell pair 7, module 2, rack 15 (battery 7-2).

- It is believed to a reasonable degree of scientific certainty that this internal failure was caused by an internal cell defect, specifically abnormal Lithium metal deposition and dendritic growth within the cell.

- The total flooding clean agent fire suppression system installed in the BESS operated early in the incident and in accordance with its design. However, clean agent fire suppression systems are designed to extinguish incipient fires in ordinary combustibles. Such systems are not capable of preventing or stopping cascading thermal runaway in a BESS.

- As a result, thermal runaway cascaded and propagated from cell 7-2 through every cell and module in Rack 15, via heat transfer. This propagation was facilitated by the absence of adequate thermal barrier protections between battery cells, which may have stopped or slowed the propagation of thermal runaway.

- 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. Analysis and modelling from experts in this investigation confirmed that these gases were sufficient to create a flammable atmosphere within the BESS container.

- Approximately three hours after thermal runaway 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 report lists the following five main contributing factors that led to the explosion: internal failure in a battery cell initiated thermal runaway, the fire suppression system was incapable of stopping thermal runaway, lack of thermal barriers between cells led to cascading thermal runaway, flammable off-gases concentrated without a means to ventilate, and emergency response plan did not have an extinguishing, ventilation, and entry procedure.

DNV GL’s report concludes that today’s standards better address hazard assessment and training for first responders, although the industry expectation should go even further and require that hazard assessments and training take place before and during the commissioning of energy storage systems. In today’s practice, the systems integrator and EPC contractor typically coordinate safety response plans on behalf of the owner, and then train the operations and maintenance (O&M) personnel to execute them.

While today’s energy storage safety codes and standards 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 neighbouring cells, modules, and racks, the report says. 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 (likely due to a reticence to prescribe anything that may be perceived as prohibitively expensive or non-commercial). 

The report adds that standards today therefore also fall short in addressing the issue and risks associated with off-gassing. However, it says there are commercially available technologies and design methods available that can address thermal runaway propagation, and the standards should be appropriately updated to acknowledge these methods and technologies. The main codes examined in the report are National Fire Protection Agency (NFPA) 855 and past and current versions of the International Fire Code, along with Underwriters’ Laboratories (UL) 1973, UL 9540, and the UL 9540A test method.

In addition, the report says that better practices for ventilation, extinguishing, and cooling thermal runaway scenarios exist today and should be implemented in future energy storage systems. Finally, clean agent systems may still be appropriate for use in energy storage facilities to manage incipient fires, but they must be used in conjunction with additional practices – i.e., ventilation, extinguishing, and cooling – to manage thermal runaway scenarios. Clean agent or aerosol extinguishing methods should not be the only barrier against thermal runaway.

Read the full report by DNV GL by clicking here.


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