On the clear warm evening of April 19, 2019, the Fire-Medical Department of Surprise, Ariz., received a call reporting smoke near an electrical substation. Units were dispatched; once on scene, they elevated the incident to a hazardous materials (hazmat) operation. The event involved gases from a utility-operated, state-of-the-art, 2.16 MWh lithium-ion battery energy storage system (ESS) that supported the electrical grid.¹

Working with the facility-site personnel, the hazmat firefighters monitored the event from a distance. The unit was isolated in the desert and without nearby exposures, allowing them to monitor from the exterior. As they approached the on-scene-two-hour mark, and with conditions holding seemingly steady, a decision was made to open a door into the unit to obtain better readings. 

Conditions changed relatively abruptly once the door was opened. Within minutes, a significant deflagration event occurred with destructive over-pressures. Four firefighters on the exterior were seriously injured as a result. 

A Wake-Up Call

Fortunately, no fatalities occurred in the Arizona desert, but it was indeed a close call. The Surprise event was a sharp wake-up call for the energy storage system industry, which is rapidly expanding today.² As an emerging technology that holds significant promise for an energy-hungry world, it seeks to mature on multiple levels as its marketplace comes of age. One of these levels is safety.

What happened at Surprise? Symbolic of the significant implications, three independent, credible reports were generated.^1,3,4 Interestingly, they were all released within days of each other more than a year later. Not surprisingly, they were not entirely consistent in their respective conclusions. Among the big takeaways for all is the overall challenge we face to predict the nature of future failure events realistically.

The electrical batteries in Surprise experienced a thermal runaway event, which describes a phenomenon involving destructive overheating of battery components. Thermal runaway results from some manner of internal defect or external assault on the components (e.g., mechanical, electrical, heat, radiation, etc.). Thermal runaway within battery cells can proceed slowly (e.g., over weeks) or rapidly (e.g., minutes).⁵

Over time, a thermal runaway event will escalate and generate off-gases. Eventually, the unit will noticeably and openly burn; in some cases, confined off-gases may ignite and create a destructive over-pressure event. 

Re-creating and defining these events for the sake of standardization is a priority. Addressing a consistent approach to mitigating these events also is a priority; key industry standards have surfaced to provide valuable guidance. For example, NFPA 855, Standard for the Installation of Stationary Energy Storage Systems provides important requirements for the basic design and installation of these systems.⁶ 

Documents such as NFPA 855 are rooted in research to support the body of knowledge and test methods that can be repeatably reproduced to assure safety. In this regard, another example is UL 9540A, Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems. UL 9540A has surfaced as a relative mainstay to evaluate lithium-ion battery ESS technology in the last several years.⁷

As we continue to address knowledge gaps and provide practical safety protections, these documents and other approaches are helping to better define this elusive hazard. 

How We Store Energy

Battery energy storage systems or electrochemical storage is not the only way to store electricity. Neither is electricity the only type of energy being stored. There are other ways to contain energy using different technologies, approaches and methods. 

Classifications of ESS are generally according to the form of energy. The primary classes are: mechanical, electrochemical, chemical, electrical and thermal.⁸ Some of these classes have existed for a very long time, though their applications are not necessarily widespread for various reasons such as cost-effectiveness, efficiency or durability. 

Specific subclasses evolved in each of these categories. For example, mechanical has subclass applications using flywheel energy storage, compressed-air energy storage and pumped hydro storage. As a further example, with electrical battery energy storage, there are multiple types of batteries, such as lead-acid, lithium-ion, nickel-cadmium and sodium sulfur-based technologies. 

Among this flurry of different approaches, electrochemical using batteries is the front runner in overall popularity. Further among the different battery approaches, lithium-ion batteries are far and away the popular front runner in terms of installations. Today, lithium-ion batteries control more than 90 percent of the global grid battery storage market because of their high energy density and lightweight characteristics.⁹

These technologies are in wide use. More than 1,600 grid-connected energy storage projects located around the world are tracked by the U.S. Department of Energy’s Global Energy Database.¹⁰ Today, these storage applications are an inherent part of the energy sector.

Where We Are Heading

Our societal appetite today for energy is ferocious. The modern world, for example, has evolved an apparent high dependency on electrical and electronic items. The need for energy storage applications ranges from huge grid-connected systems to small individual household units. On top of this, the upkeep of the aging electrical infrastructure encourages alternative approaches dependent on ESS, such as with local de-centralized power generation.

There is little question on the essential nature and need for electrical energy storage. Based on one industry analysis, the global battery ESS market size is predicted to grow at a compound annual growth rate of 32.8 percent from $2.9 billion in 2020 to $12.1 billion by 2025 (in USD).¹¹ One big driver for this is the “peak-shaving” that utilities often pursue. This involves storing electrical energy during daily time slots of low-generation expense and using it when generation expense is high (i.e., high demand periods).

Electrical energy storage has proven its value for all energy generation types, but it has special value for renewable energy sources. Certain types of renewable energy generation involve variable environmental conditions, such as solar only when sunny or wind only when windy. Renewable energy and energy storage are proverbially tied at the hip.

As renewable energy generation (e.g., solar, wind, geothermal, etc.) grows, so too will electrical energy storage. Over the next three decades, the long-term growth of renewable energy is predicted to grow by an estimated 4 percent.¹² Other energy production methods are anticipated to remain steady or grow or drop only slightly.

Fire Protection for Battery ESS

For our mainstream focus on lithium-ion battery ESS, it is crucial to understand the hazards involved. These include not only the potential for fires and explosions but also electrical, chemical and physical hazards.⁶

Consideration of these hazards will be different under normal operating conditions versus abnormal or emergency conditions. Other features can significantly influence these hazards, such as the oft-changing battery state of charge (from 0 to 100 percent). A fully charged battery can have dramatically different burning characteristics in a fire than the same type of battery with a low state of charge.

The design location of the ESS is paramount. An ESS located within another building or facility raises immediate fire exposure concerns compared to an ESS located by itself away from other possible exposures. With the former, the fire service will be pressed into an offensive fire attack due to the threat upon the bigger building encompassing the ESS. Eliminating all possible external exposure concerns based on the design location is highly preferred.

A fire event involving a battery ESS has some unique characteristics. From a firefighting perspective, a significant difference for an ESS fire as compared to other fires is that the event could involve a very long duration. Energized electrical equipment often has limited manual access to attack a fire readily, and fires could burn for hours or days.¹³

Battery components experiencing thermal runaway will tend to resist the suppression characteristics of water-based and non-water-based suppression agents. Strategies to control the fire include preventing ignition, early detection, separation and isolation to prevent propagation, confinement, suppression as able, and similar strategies.⁶

Hazardous products of combustion from a fire, as well as liquid run-off (e.g., sprinklers or hose streams), could be very protracted. A properly designed water supply for the local fire service will be essential. These details need to be considered in the up-front design with robust HVAC, drainage, explosion venting and similar features. 

These concerns are greatly magnified if the ESS is located with direct fire exposures such as, for example, on the upper floor of a high-rise building. A stand-alone external ESS with no threatened exposures greatly relieves these fire concerns.

Stranded electrical energy in damaged batteries within the ESS is a special problem. Damaged batteries have been known to re-engage in thermal runaway, sometimes hours or even days following their supposed extinguishment. Traditional fire service overhaul takes on a different dimension and may ultimately be measured in days or even weeks. 

Case in point, the dismantling of the ESS in the Arizona incident mentioned previously took approximately eight weeks to fully disassemble the unit to safely handle the damaged batteries containing stranded electrical energy. Future dismantling and overhaul are expected to become more efficient as the lessons learned from events such as this are embraced.

The storage of energy is important in today’s world. We need ESS; collectively, we want to see this technology proliferate. But the setbacks caused by a highly publicized disaster will be a collective burden on the entire industry. If the ESS industry wants to see successful broad-scale proliferation, it needs to work with safety professionals to assure future success.¹⁴

Likewise, the safety infrastructure has a big stake in the roll-out of these emerging technologies. It is essential that we work closely with technology providers. Energy storage systems are providing for a better world; we must safeguard these applications to allow them to proliferate for their intended purpose

References

¹ McKinnon M., DeCrane S., Kerber S., “Four Firefighters Injured in Lithium-Ion Battery Energy Storage System Explosion — Arizona,” UL Fire Safety Research Institute, Report dated: July 28, 2020, https://bit.ly/2X0iTXn, Cited: Nov. 20, 2020.

² Grant C., “Wake-Up Call,” NFPA Journal, July/August 2019, page 24.

³ Hill D, “McMicken Battery Energy Storage System Event Technical Analysis and Recommendations,” DNV-GL on behalf of Arizona Public Service, Doc #10209302-HOU-R-01, Report dated: July 18, 2020, https://bit.ly/2KNJPaz, Cited: Nov. 25, 2020.

⁴ Swart J., White K., Cundy M., “APS McMicken Progress Report,” Exponent on behalf of LG Chem Ltd., Report dated: July 20, 2020, https://bit.ly/35au0lp, Cited: Nov. 25, 2020.

⁵ Long R.T., Blum A., “Lithium-Ion Batteries Hazard and Use Assessment - Phase III,” Fire Protection Research Foundation, Nov. 2016, https://bit.ly/38EX996, Cited: Nov. 25, 2020. 

⁶ NFPA 855, Standard for the Installation of Stationary Energy Storage Systems, National Fire Protection Association, 2020 edition, www.nfpa.org/855, Cited: Nov. 24, 2020.

⁷ UL 9540A, Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems, Underwriters Laboratories, www.ul.com/services/ul-9540a-test-method, Cited: Dec. 10, 2020.

⁸ “Electrical Energy Storage,” International Electrotechnical Commission, Geneva, Switzerland, white paper, Dec. 2011, pages 17-34.

⁹ “Fact Sheet: Energy Storage (2019)”, Environmental and Energy Study Institute, Feb. 22, 2019, http://bit.ly/2L3UYUm, Cited: Nov. 26, 2020.

¹⁰ DOE OE Global Energy Storage Database, U.S. Department of Energy, Energy Storage Systems Program, https://bit.ly/2MjzxPV, Cited: Nov. 26, 2020.

¹¹ “Global Battery Energy Storage System Market with COVID-19 Impact Analysis by Element (Battery, Others), Battery Type (Lithium-Ion, Flow Batteries), Connection Type (On-Grid, Off-Grid), Ownership, Energy Capacity, Application, and Geography - Forecast to 2025,” Research and Markets, http://bit.ly/3nXIOLv, Cited: Dec. 5, 2020.

¹² US Energy Information Administration, Department of Energy, “Annual Energy Outlook 2020, Annual Projections to 2050, Table 1: Total Energy Supply, Disposition, and Price Summary,” http://bit.ly/3829aXf, Cited: Dec. 10, 2020.

¹³ Long R.T., Blum A., Bress T., Cotts B., “Emergency Response to Incident Involving Electric Vehicle Battery Hazards,” Fire Protection Research Foundation, July 2013, http://bit.ly/34VG0qB, Cited: Dec. 10, 2020.

¹⁴ Colthorpe A., “’Tough Love’ Approach Needed to Fix Energy Storage Industry’s Safety Shortcomings,” Energy Storage News, Nov. 24, 2020, https://bit.ly/3n6oTc5, Cited: Dec. 11, 2020.