Those in the clean-energy industry will be familiar with the 16 January fire at the Moss Landing battery energy storage system (BESS) in Monterey County, California, where at least 80% of a 300 MW, 1.2 GWh battery system was destroyed. The nickel-manganese-cobalt (NMC) lithium-ion batteries (supplied by LG Energy Solution) were installed indoors in a former turbine hall and commissioned in December 2020. The cause of the event is not yet known, but it is clear that the integrated sprinkler system was ineffective. Indeed, at least one of the two previous battery overheating incidents at the same site was caused by accidental sprinkler operation.
Fortunately, there were no injuries, air quality in the area remained at non-hazardous levels, and there was no contamination of drinking water. It appears that the sheer volume of battery energy involved has resulted in increases in heavy metals in the nearby environmentally sensitive Elkhorn Slough area, although soil sampling is ongoing and results need to be compared with baseline levels.
This event comes on the heels of the May 2024 Otay Mesa 50 MWh battery building fire in the San Diego area, where Cal Fire estimates that 8 million gallons of water were pumped through the building’s sprinkler system and by fire trucks over the 11-day period of the event. Water was a little more successful in the May 2023 East Hampton, New York event, where smoke was emitted from the building, but there was no major fire. Even so, the sprinklers were left running for 30 hours to ensure the event was controlled, ruining the entire 48 MWh battery.
In contrast, the September 2022 Tesla Megapack fire at the PG&E-owned Elkhorn BESS adjacent to the Moss Landing site was confined to just one of the 256 enclosures. The fire burned out quickly and the facility was back in operation three months later, after an investigation, corrective actions, and extensive tests were performed.
The Elkhorn incident represents the latest thinking about how BESS facilities should be designed and how battery fires should be handled. Using outdoor enclosures limits the amount of battery energy involved in a fire — much of the industry is standardising around 5 MWh in a 20-foot container; less than one half of one percent of the Moss Landing building. Recognising that explosion constitutes a greater hazard to first responders than fire, system designers are prioritising explosion prevention over fire suppression. Allowing a battery fire to consume itself eliminates stranded energy and the possibility of reignition. Firefighters use defensive measures only as needed to protect adjacent equipment, which avoids the possibility of contaminated run-off.
Part of the problem with indoor systems is that safety studies are largely based on fire and explosion testing to UL9540A, which is written around the implied concept that all battery thermal events are the result of internal cell failures, and that if such failures can be prevented from propagating, all is well with the world. In contrast, the NFPA 855 technical committee has recognised that many failures occur through external causes — such as accidental sprinkler operation — and is instituting a requirement for large-scale fire testing (LSFT) in the 2026 edition, in which a complete enclosure must be burned to verify that there is no propagation of thermal runaway to adjacent units.
Fire codes require BESS in buildings to be equipped with sprinklers, despite the fact that sprinklers at best are marginally effective and at worst can cause a battery thermal event. Adapting LSFT for indoor systems, while not impossible, would be difficult and extremely costly to implement. Assuming a planned BESS building is compartmentalised into fire areas (Moss Landing was not), the implication is that a fire area with its full complement of batteries would have to be constructed, and the battery units set on fire. Sprinklers would be activated, and either the sprinklers would extinguish the fire, or, if ineffective, the area’s fire barriers as designed would have to contain the fire until the event is over, while temperatures in adjacent fire areas are maintained at safe levels. While LSFT procedures for outdoor enclosures are relatively simple, in that the battery can be set on fire with a blowtorch and just allowed to burn, adding sprinklers to an indoor test injects a level of variability that would make it difficult to know how representative the test is of a real-world event.
The Moss Landing owner, Vistra, has a plan for a 600 MW, 2.4 GWh BESS in two buildings (equivalent to two Moss Landings) at another decommissioned generating station in California’s Morro Bay (the plans actually go back to 2018 or earlier). In addition, Flatiron Energy has proposed a 300 MW, 1.2 GWh BESS in a two-storied building in an area of Boston with a ‘dense urban feel.’ What could possibly go wrong? We should hope that these projects never get a green light and that indoor BESS installations are seen as a bad idea that should never be repeated.
Reignition, delinking and demolition
As of the end of February, all battery facilities at the Moss Landing site remained offline, while the natural gas fuelled power plant was operational, Vistra reported.
On 18 February, emergency, health and safety agencies responded to a reignition of the Moss 300 fire. However, the flare-up was confined to a previously burned structure from the initial 16 January incident and by 3:00 am on 19 February began to burn out.
Air monitoring has remained in place at the Moss Landing site and was active when the reignition occurred. The US EPA is overseeing the air quality monitoring at the site and in the surrounding community by CTEH, Vistra’s environmental consultant.
Vistra notes that “instances of smoke and flare-ups are a possibility given the nature of this situation and the damage to the batteries.” Since the 16 January fire, it has brought in a private professional fire brigade that is onsite 24/7 to monitor the Moss 300 building. Additionally, CTEH has permanent air-quality monitors around the plant site’s perimeter along with other continuously operating monitors at locations in the surrounding community.
Under the direction of the US EPA, Vistra has been cleared to enter a portion of the Moss 300 building with a view to starting work on isolating the batteries and reducing the risk of reignition ahead of demolition work. The EPA is overseeing all work at the site and has approved the beginning of these “de-linking” activities, which involve unbolting the metal plates that connect the batteries. This is the first step in what is expected to be a lengthy process culminating in removal of the batteries and demolition of the building.
Together, EPA and Vistra have developed specific procedures to perform these activities, and workers have completed special training to enter the building and begin delinking the batteries.
Round-the-clock fire watch and air monitoring are ongoing and will continue throughout the demolition process.
As to the cause of the fire, Vistra says it is “conducting a thorough, methodical investigation” and has “retained multiple outside experts who are assisting in this effort”, noting that “investigations like these take time.”
Vistra’s battery facility in Monterey County, California, located on an existing power plant site, was built in three phases: Moss 300 – Phase 1 (location of the 16 January fire); Moss 100 – Phase 2; and Moss 350 – Phase 3. All phases employ nickel-manganesecobalt (NMC) lithium-ion batteries supplied by LG Energy Solution.
While two previous incidents at Vistra’s Moss Landing site “have been described by some as fires,” says Vistra, “they were not.” Both incidents, one in Moss 300 in September 2021 and one in Moss 100 in February 2022, “involved a small number of leaks in the water-based heat suppression system that sprayed water on batteries, causing overheating that led to smoke,” however, “neither occurrence led to fire,” Vistra notes.
This article first appeared in Modern Power Systems magazine.
