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Aggressive load-shifting could increase battery fire risk- investigators

06 February 2020 Jim Day

Raising serious concerns about the safety of battery storage, investigators into the alarming spate of lithium-ion battery fires in South Korea said one of the primary causes was the practice of using nearly the full charging and discharging battery capacity on a daily basis—a pattern that can roughly correspond with plans by U.S. operators to use storage to shift wind and solar generation to the evening periods of peak demand.

Last year, South Korean government officials largely halted deployment of new lithium-ion battery systems and urged operators to curtail operations of existing ones after 23 battery fires broke out over a year-and-a-half. Many owners continued operating, however, and in recent months another five battery fires have been reported in South Korea, which had represented the world's largest market for stationary battery storage before the fires cratered new deployments.

South Korea's Ministry of Trade, Industry and Energy completed a months-long investigation into the fires in June, generally concluding that they were caused by a range of issues including lack of protections against shocks, faulty installation practices and control systems that were incompatible with some components.

More broadly, though, investigators at DNV GL—a global engineering standards firm contracted to investigate the root cause of one of the fires—said the common practice in South Korea of cycling the lithium-ion batteries from close to 0 percent to 100 percent and then back down again on a daily basis has led to extreme wear-and-tear on the systems and was an underlying cause of failures in the battery cells that sparked the fires.

That hard-driving cycling pattern, which has been common in South Korean storage systems co-located with wind and solar farms to shift the output to periods of higher demand, is different from most batteries deployed to date in the United States, where they have been used primarily for fast-responding frequency regulation within a narrower and less-stressful band.

"If we start cycling those batteries as aggressively as we do in Korea, we will likely see similar failure rates," George Garabandic, DNV GL's energy storage leader for the Asia-Pacific region, told The Energy Daily. "It should be expected that a higher component stress will result in higher levels of random component failures. In other, more developed energy storage system (ESS) markets, the batteries are providing services similar to frequency regulation, and the component stress is relatively milder."

If accurate, that assessment could prove problematic for U.S. utility-scale battery developers, who are increasingly co-locating storage with solar generation and touting its ability to charge up during the day when the sun is shining and then discharging that power in the late afternoon and evening—a cycle that more closely resembles the pattern in South Korea where the dozens of fires have broken out.

In fact, most of the fires under investigation in Korea were at facilities co-located with renewable resources, according to Korean media accounts.

Such deep cycling to shift load has long been known to accelerate degradation of lithium-ion batteries, with researchers at the National Renewable Energy Laboratory (NREL) reporting in 2017 that batteries used daily for such utility-scale applications will wear out in seven years even if depth-of-discharge (DOD) is limited to 74 percent. Batteries will wear out in 10 years if DOD is further limited to just 54 percent, according to NREL's estimates.

Given that accelerated degradation and the potential to raise risks of fires that comes with deep cycling, U.S. battery operators often strictly limit the depth-of-discharge, but that can severely constrain the system's ability to significantly shift renewable energy to periods of high demand as advertised. Battery experts also stress that there are ways to limit the threat of fires even with more aggressive cycling, which the industry is rapidly developing and working to adopt.

The risk of battery fires gained prominence last April when an explosion at Arizona Public Service's McMicken battery facility near Phoenix sent several firefighters to the hospital. Arizona regulators were subsequently surprised to learn that a fire in 2014 also destroyed APS's Mt. Elden battery storage system.

The cause of the McMicken fire is still under investigation, but in the meantime APS has suspended plans to deploy 850 megawatts of battery storage—which had been the nation's largest battery storage initiative—until the utility can figure out what happened.

Lithium-ion batteries can present heightened risks of fire, in part due to a phenomenon called "thermal runaway," in which excessive heat in the battery can create more heat that can then cause cascading fires in adjacent battery cells, according to a battery risk assessment compiled by the insurance group AIG.

The burning batteries also release highly toxic hydrofluoric acid, cyanide and other gases that can explode upon ignition, which is what appears to have happened in the McMicken fire, according to a November 5 preliminary report from APS.

While the chances of an individual battery cell failing and starting a fire is extremely remote, Garabandic from DNV GL noted that large battery storage systems can have the equivalent of 10 million or more lithium-ion cells packed into a small space.

"One cell out of those 10 million will fail and if, in such circumstances, we don't implement adequate safety measures, a cascading effect will certainly lead to a large-scale fire," he said. "The lack of such safety measures has been the main reason why the accidents in Korea had such catastrophic outcomes; and consequently, the entire Korean ESS industry was brought to a halt."

To strengthen safety, battery operators must recognize that an individual cell is likely to fail at some point, and therefore use monitoring analytics to replace cells that present risks before they fail, Garabandic said.

Beyond that, developers must implement "all technical and operational measures that will prevent the cascading effect of failures and contain the damage to a reasonable level," he said.

Those measures include barriers between cells to prevent cascading fires, adequate control and monitoring systems and proper ventilation of potentially explosive gases, along with systems that can douse a burning battery cell quickly with water, which counterintuitively has been shown to be the best way to extinguish a battery fire.

Battery experts have reported that various technologies already exist that effectively reduce lithium-ion battery fire risks, and that safety will improve in the still-nascent storage industry as these are scaled up and integrated into manufacturing and installation processes.

Korean battery makers LG Chem and Samsung have recently attained compliance with newly developed international battery fire prevention standards, and the U.S. Energy Storage Association last year released guidelines for best practices to reduce fire risk in an effort to address what is emerging as a serious threat to the fast-growing industry.

Several battery makers also are touting different chemistries as inherently safer alternatives to traditional lithium-ion batteries. Among these are Sonnen, SimpliPhi and Chinese battery giant Contemporary Amperex Technology, which are marketing lithium iron phosphate batteries they say are less likely to catch fire and are less toxic if they do start to burn.

Reprinted from The Energy Daily. For more comprehensive daily coverage of US energy policy, regulatory, and business trends from IHS Markit, visit The Energy Daily website.

Jim Day is a Reporter for The Energy Daily.

Posted 6 February 2020.

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