A malfunctioning heat suppression system caused the incident that damaged Vistra Corp.’s Moss Landing Energy Storage Facility in California, according to investigative findings released by the company.
Moss Landing Phase I (300 MW/1,200 MWh) has been offline since the September 4, 2021 incident. Responding emergency crews that day found scorched battery racks and melted wires.
The facility uses a heat suppression system that protects against thermal runaway in individual battery modules. That system’s preaction zones consist of carbon steel header pipes that are connected by flexible hoses to rack piping, which has sprinkler nozzles that are inserted into each battery module.
The design calls for the release of water in a particular zone upon detection of certain levels of smoke by the Very Early Smoke Detection Apparatus (VESDA). After release, water is then be injected into a battery module if the temperature in that module becomes too high.
The investigation found that in response to detection of very low levels of smoke in one area of the facility, the heat suppression system activated.
“Due to an apparent programming error in the VESDA, these actions occurred at detected smoke levels below the specified design level at which water was intended to be released and E-Stop was intended to be initiated,” said Vistra in a two-page findings report.
Additionally, Vistra said a small number of couplings on the system’s hoses and pipes failed, causing water to spray on the battery racks. This in turn resulted in short circuiting and arcing, which caused battery damage and more smoke. This additional smoke was detected by other VESDA units, resulting in the release of water to other preaction zones, followed by hose and pipe failures, release of more water and more damage and smoke.
The findings report said some of the water was sprayed directly onto the battery racks, and some of it leaked through gaps in the upper floor onto racks located on the lower floor. In total, roughly 7% of the battery modules and other facility systems were damaged.
According to the report, the only identified potential source of smoke was an air handling unit, which supplied air to the area where smoke was detected and experienced a failed bearing around the time smoke was first detected.
Shortly after the incident, Vistra officials said they didn’t believe the system’s lithium-ion batteries, supplied by LG Energy Solution, were the cause. The company said when water was first released to the heat suppression system, all battery module temperatures were recorded as “within established temperature limits and well below the level that would indicate a thermal runaway.”
The investigation also revealed that flexible hoses, where most failures occurred, were not pressure tested.
“Before they were connected to each other, the preaction header piping and the piping on each battery rack underwent separate pressure testing,” the findings report said. “However, Vistra has been unable to confirm that the contractor pressure tested the complete heat suppression system after the racks were connected to the header pipes.”
Now that the five-month investigation has concluded, Vistra said it will complete the following corrective actions prior to restarting Moss Landing Phase I:
-The complete heat suppression system will be pressure tested, and any identified leaks will be addressed.
-An air supervision system will be installed to continuously monitor for leaks in the heat suppression system.
-The VESDA system will be reviewed to ensure it is programmed in accordance with the specifications.
-Smoke detectors will be installed in all air handling units.
-Gaps in the upper floor will be sealed.
“Throughout this process, Vistra committed to share its learnings from the incident in order to support the battery storage energy industry and the shared goal of decarbonizing the electric system,” said Vistra officials in the report.
Vistra said it is working with regulators to return the Phase I facility to service. The company expects 100 MW to be back online by the end of the first quarter in 2022 and the remaining 200 MW returned to service in the second quarter.
“Our priority was to conduct a safe and thorough investigation as quickly as possible, and implement any necessary corrective actions,” said Curt Morgan, chief executive officer at Vistra.
Phase II (100 MW/400 MWh) of the Moss Landing facility has remained online throughout the incident and investigation.