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.
