No cause has been named for the fire at an Arizona battery facility that sent four firefighters to the hospital Friday.

Peoria Fire Department spokesperson Michael Selmer described "an explosion" when the responders tried to check on the battery enclosure, raising questions about whether the batteries or some other equipment triggered that event. Lithium-ion batteries have been involved in fires before, but only a handful of times in the U.S. grid infrastructure context.

Utility Arizona Public Service, which owns the site, and Fluence, which provided the battery system, have been investigating the cause of the conflagration since Saturday. 

Four members of the Peoria Fire Department were hospitalized with chemical and chemical-inhalation burns, according to the department. The responders' conditions had stabilized by Saturday evening, local outlet AZ Central reported.

This calamity comes at a sensitive time for the fledgling storage industry, which is poised to double deployments this year and triple them in 2020, according to data from Wood Mackenzie Power & Renewables.

Arizona Public Service adopted the technology more quickly than almost all its peers, and in February announced plans to install 850 megawatts more by 2025, the largest procurement announced by a U.S. utility so far. The company wants to use batteries to shift solar power into the nighttime, when it becomes more valuable for the grid. This strategy offers a cleaner and cheaper alternative to relying on natural-gas peaker plants.

The outcome of the investigation, then, holds a great deal of import for whether Arizona's battery expansion proceeds as planned. It could influence states like California, New York and Massachusetts, which have made storage central to their grid planning. The findings also will matter for the other jurisdictions that want to use batteries as a tool for decarbonizing their electrical systems.

Here are the big questions the industry will be asking as the investigation unfolds. GTM will provide updates as the story evolves.

Who are the key players?

Storage industry insiders sometimes worry that novice integrators and operators could cause a battery meltdown and give storage a bad name. That concern is not at play in Arizona, because both participants are seasoned hands in the battery deployment arena.

Arizona Public Service is the largest utility in the state and an enthusiastic adopter of battery technology as a grid asset. The company has incorporated batteries into its macro-level grid planning at a time when many utilities have yet to seriously grapple with the technology.

APS installed the facility in question in 2017 as part of the Solar Partner Program, whereby the utility uses two grid-scale batteries to balance out the effects of numerous rooftop solar systems on a particular distribution feeder. Based on what APS learned from that project, it opted for a battery solution at Punkin Center, offsetting a more expensive wires upgrade to maintain reliability at that remote town.

Since then, APS has scaled up its battery ambitions, making them a centerpiece of its strategy to shift abundant solar power into the evening peaks, rather than relying on expensive gas peaker plants. A 50-megawatt battery charged by solar power won a solicitation in 2018. APS announced in February that it would build another 850 megawatts by 2025.

Fluence, which supplied the system to APS, has a decade-long track record building and operating large-scale storage.

The company started as an internal unit within the power producer AES tasked with finding storage business opportunities within that company's portfolio. After years of developing projects, AES spun out its storage business as a joint venture with German industrial giant Siemens. Fluence launched in early 2018 with a focus on supplying its utility-scale Advancion platform to storage projects around the world.

Fluence claims 760 megawatts deployed across 80 sites in 17 countries, placing it firmly in the upper echelons of storage providers. It achieved this with a clean track record.

"Up until the recent incident, the Fluence fleet had delivered more than 6.3 million megawatt-hours of grid services with no major safety incidents or fires," a company spokesperson told GTM.

Where did the fire originate?

We still don't know the answer to this crucial question.

"What we know at this point is that we had an equipment failure," APS spokesperson Anne DeGraw told GTM in an email Monday. "A thorough investigation will help us determine what exactly failed and why."

That investigation is underway, with personnel from APS and Fluence onsite. The outstanding question is whether the fire originated with the battery system itself, or in a different part of the substation that occupies the same site.

Grid battery fires have been extremely rare in the U.S. (more on this later), but catastrophic failure is a non-zero risk of any high-powered grid equipment. 

"Fires and explosions are not uncommon within power systems," said Ben Kellison, director of grid research at Wood Mackenzie.

Transformer explosions, for instance, happen often enough — sometimes multiple times a year — that utilities use specialized walls to separate large power transformers from each other to prevent cascading failures, he noted.

It will be up to investigators to trace the source of the fire among the various types of grid equipment present at the substation.

Did the batteries themselves explode?

Selmer said that "an explosion occurred" when the firefighters entered the battery facility (AZ Central has video of the statement). He also noted that the cause of the explosion was unknown.

Lithium-ion batteries are susceptible to catching fire under certain conditions. If they short-circuit, cells can enter a state known as "thermal runaway," in which they continue heating up to a point that can eventually ignite.

That said, battery fires tend to emerge as flames, not as explosions. 

Again, we don't know the cause of the explosion, but the fire department's description from the scene calls to mind a passage from a 2017 battery fire safety study that DNV GL conducted on behalf of New York stakeholders. The authors address the exact question of whether lithium-ion batteries explode:

"In this program DNV GL tested dozens of lithium-ion batteries and could not conclusively say that any of them 'exploded,'" the report notes. "DNV GL has conducted hundreds of abuse tests on cells in other programs and has not conclusively observed an event where a battery exploded or was the source of a rapid energy event."

The company did observe a "highly repeatable condition," however, in which battery cells exposed to heat released flammable gases. 

"The explosion hazard is not the battery itself, but the gases it may generate," the report notes.

Depending on the ratio of gases present in the enclosure before the responders entered, the influx of outside air could have created conditions for combustion that were not present before.

What safety systems did the battery plant have?

GTM toured McMicken's sister site, the Festival Ranch battery system, in 2017; the two are identical except McMicken sits at a substation.

The rectangular enclosure holds two rows of battery cell racks, behind mesh doors, on either side of a central walkway. It resembles a small data center more than a power plant, but the stacks of LG Chem batteries add up to 2 megawatts and 2 megawatt-hours of storage capacity.

The interior was climate-controlled to keep the batteries in a safe operating range despite whatever heat the Arizona summer may throw at it. A red tank of fire suppressant stood as wide and nearly as tall as a grown human, connected to an array of sensors designed to spot signs of trouble before danger arose.

"The safety systems on these battery units include a built-in alarm and a self-activating fire-suppression system, as well as a container designed to withstand some significant heat and pressure parameters," DeGraw said.

Did the battery safety systems go off?

DeGraw did not confirm whether the safety systems were triggered on Friday.

What does this mean for APS' operations?

The utility has grounded its energy storage operations while the investigation continues.

"We have no reason to believe there are any safety issues with similar energy storage facilities, but out of an abundance of caution, we have temporarily taken our other battery systems offline," DeGraw said. "This is a typical step for a safety-focused business like ours."

Other systems include the identical Festival Ranch battery and the Punkin Center system, both supplied by Fluence.

How frequent are grid battery fires?

In the U.S., only a handful of fires have been reported at grid-scale battery facilities.

APS experienced its first fire in 2012. A 1.5-megawatt system it owned and operated in Flagstaff, supplied by Electrovaya, caught fire

Also in 2012, a 15-megawatt plant burned on the north shore of Oahu, Hawaii. That plant was supplied by VC-backed Xtreme Power, which used advanced lead acid batteries, rather than lithium-ion technology. Xtreme Power went bankrupt two years later.

S&C Electric Company experienced a fire at its internal battery testing facility in 2016, and released a detailed report about how it resolved the situation. That facility, though, was not operating in the field as a grid asset, so it belongs in a different category.

WoodMac's energy storage database tracks a total of 200 utility-scale battery systems installed in the U.S. That puts the ratio of fires to batteries at two or three out of 200, depending on whether the latest incident is traced back to the storage system or something else.

U.S. grid battery fires may be few in absolute terms, but the rate of incidence is nontrivial in relative terms, especially when the industry plans to expand massively in the coming years.

"The ratio has to go down at least an order of magnitude, if not more," said Ravi Manghani, energy storage research director at WoodMac. "The industry needs to do a better job of manufacturing safer sales, designing systems that have sufficient levels of redundancy, and having real-time monitoring that engages predictive analytics."

Will this affect future deployments of batteries?

This will all come down to the cause of the fire. If it turns out to be a different piece of substation equipment, the inquiry may turn to how to properly insulate the battery enclosure from external threats.

If the investigation implicates the battery system itself, that would raise a series of tough questions for the parties involved, and could force reconsideration of APS' plans for a dramatic ramp-up in battery installations.