Vistra said its Moss Landing Energy Storage Facility in California has restarted with more than 98% of its 400 MW back in service.

The company said it began bringing the facility online during May and June after implementing corrective actions, including modifications related to connectors in the facility’s water-based heat suppression system.

The remaining 7 MW is expected be restored later this summer after replacement batteries and electrical components are received.

The energy storage facility was knocked out of service in September after a water-based fire suppression system activated and doused battery racks.

Vistra’s Moss Landing Energy Storage Facility consists of two separate buildings – one housing the 300 MW Phase I system and the other housing the 100 MW Phase II system. Construction is underway on the 350 MW Phase III expansion, which is set to be commercially operational next summer. A potential fourth phase could expand the site’s capacity to 1,500 MW.

A second, nearly identical incident involving the early detection safety system occurred in mid-February in the 100 MW Phase II building. Vistra said that in that incident, the water-based suppression system released water that contacted some batteries. It said early evidence pointed to water hose leaks that caused some batteries to short and that led to smoke filling the building. Vistra said the February incident was “similar” to what occurred in the September incident at the next-door 300 MW Phase I facility.

The 300 MW facility that was knocked offline last fall includes three 100 MW arrays. Each array consists of 32-33 cores, each made up of 47-48 racks containing 22 battery modules. Altogether, the facility has a total of 99,858 modules in 4,539 racks making up 98 cores.

As designed, the building housing the Phase I battery system has a fire suppression system, including sprinkler pipes. A separate water-based heat suppression system protects against thermal runaway in individual battery modules. This system includes 25 “preaction zones,” each of which services 3-4 cores. The preaction zones consist of carbon steel header pipes that are connected by flexible hoses to piping on each rack. The rack piping includes sprinkler nozzles that are inserted into each battery module.

The design calls for release of water to the header pipes in a particular zone if a certain level of smoke is detected by the Very Early Smoke Detection Apparatus (VESDA). After release to the header pipes, water is injected into a battery module if the temperature in that module rises high enough to activate the nozzle in that module.

An investigation into the September incident found that in response to low levels of smoke in one area of the facility, the water-based heat suppression system armed. Then, because of failures of what were said to be a small number of couplings on flexible hoses and pipes, the system improperly sprayed water on battery racks. 

The smoke was believed to have originated from a failed bearing in a separate air handling unit. The water damaged the batteries and caused some to overheat, creating more smoke which, in turn, resulted in the release of more water, damaging additional batteries. In total, around 7% of the facility’s battery modules were damaged, the report said.

It said that at the time water was first released by the heat suppression system, all battery module temperatures were recorded as “within established temperature limits.” The investigation found that the batteries “were not the initial source of smoke or a cause of the incident.”

Vistra made several modifications prior to restarting Phase I, including pressure testing the entire heat suppression system and repairing any leaks that were found. In addition, the company installed an air supervision system to monitor for leaks in the heat suppression system. Smoke detectors also were installed in all air handling units and gaps in the facility’s upper floor were sealed.

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