Energy Storage Systems and the New Demands on Fire Protection Engineering

Energy storage systems (ESS) are expanding rapidly to support renewable energy and strengthen the grid. Along with this growth come new fire and life-safety challenges. Unlike traditional electrical infrastructure, ESS units pack large amounts of energy into compact spaces. When failures occur, they can escalate within seconds, producing heat, smoke, toxic gases, and even explosions.

Preventing this requires specialized fire protection engineering that goes beyond basic code compliance. A pivotal example is the 2019 Arizona ESS incident, which revealed how quickly risks can escalate when hazards are underestimated.

A Real-World Lesson: The 2019 Arizona Explosion

On April 19, 2019, a 2.16 MWh lithium-ion energy storage facility in Surprise, Arizona, experienced a catastrophic failure. What began as thermal runaway in a single cell quickly propagated, releasing heat, smoke, and flammable gases. According to the investigation report by Arizona Public Service and third-party experts, the clean-agent system extinguished visible flames but could not stop the thermal runaway reaction. Gas continued to accumulate inside the sealed container. Hours later, when firefighters opened the door, the vapor ignited in a violent deflagration, injuring four responders and destroying the system.

This event underscores a key point: extinguishing flames alone might not eliminate the hazard in an ESS facility. Without proper thermal barriers, gas management, and responder protocols, the danger can persist long after the fire appears to be out.

Why Are ESS Containers High-Risk?

Modern ESS technology allows excess electricity from sources like solar or wind to be stored and delivered later when needed. This ability makes them vital for balancing the grid and building a more sustainable energy future. However, the same dense concentration of energy that makes them efficient also creates significant fire protection risks, particularly when these systems are housed in containerized units.

The primary hazards include:

• Thermal Runaway: When a single battery cell cell produces more heat then it can dissipate, it can trigger a chain reaction, causing nearby cells to overheat as well. This can escalate into a fire or explosion.

• Stranded Energy: Even after a fire, residual charge remains. Damaged terminals make it difficult to safely discharge, creating a shock and reignition hazard hours or even days later.

• Toxic and Flammable Gases: During thermal runaway, batteries release toxic and flammable gases, which can build to explosive concentrations if not vented properly.

• Deep-Seated Fires: ESS units are housed within protective metal or plastic casings. These casings are designed to protect the batteries but also make fires deep-seated and difficult to extinguish.

For owners and operators, these hazards mean that ESS safety is not only about protecting equipment but also about ensuring business continuity and responder safety. These threats underscore the complexity of ESS protection. Among these risks, thermal runaway is widely recognized as the most critical because it can trigger multiple secondary hazards.

Thermal Runaway: More Than an Electrical Fire

It’s important to distinguish between two fire types in ESS:

• Class C Fires are electrical fires that occur when systems are energized. These can result from wiring faults, damaged connections, or short circuits inside battery modules. Chemical suppression systems are generally effective at controlling these events because once power is cut and flames are extinguished, the hazard is usually neutralized.

• Class B Fires are caused by thermal runaway, which behaves very differently. Thermal runaway is a chain reaction of heat transfer rather than an electrical fault. When one cell overheats, the intense heat and flammable gases it produces can ignite neighboring cells. This reaction can continue even after the power supply is disconnected. In many documented cases, cells that appeared stable reignited hours or even days after the initial event, making reflash a serious concern. While chemical suppression may knock down flames or reduce smoke, it might not halt the underlying thermal propagation.

This distinction is critical: treating thermal runaway like a conventional electrical fire is a recipe for failure. Effective protection must address both electrical faults and self-sustaining chemical reactions.

Detection and Suppression: Tailoring to the Hazard

While some ESS projects may share similarities, each has unique characteristics. Detection and suppression systems must be customized for the specific application, battery chemistry, and operational context. Detector placement, battery management systems, technology selection, and alarm integration determine how quickly an event is identified, while suppression methods dictate how effectively it is contained. No single technology works for every ESS application, which is why design evaluation and coordination with AHJs are critical. Collaboration between engineers, inspectors, and building owners is essential to mitigate hazards in ESS facilities. Effective protection begins with detection, the earliest opportunity to intervene before a small problem escalates.

Detection Strategies

A layered detection strategy is often the best approach. Smoke detection remains the most common, but there are several types of detection that can be used.

• Smoke detection: Senses airborne particulates for early warning, including smoldering or off-gassing; can be spot or aspirating (VESDA).
• Heat detection: Fixed-temperature or rate-of-rise sensors that flag abnormal temperature increases.
• Infrared/thermal imaging: Monitors for hotspots on cells or equipment before flames appear.
• Gas detection: Monitors the environment within an ESS container that can sense the rise in concentration of combustible gases indicating thermal runaway has occurred.
• Flame detection (UV, IR, UV/IR): Line-of-sight sensors that react quickly to open flame; effective near fuel-powered or solvent areas.
• Video analytics: Camera-based smoke/flame recognition that supplements other detectors and adds situational awareness.

Suppression Strategies

Early detection is only half the battle. The speed and effectiveness of suppression will determine whether the incident is contained or escalated. Because the root hazard is thermal runaway rather than open flame, suppression must focus on cooling and containment.

• Water-Based Systems: Despite concerns about electrical hazards, NFPA 855 recognizes water-based suppression, especially sprinklers, as the most practical and proven option for ESS fire mitigation. This is due to water’s unmatched cooling capacity, a finding established by research and fire testing:
  • A 2017 DNV-GL report comparing suppression agents found water to be the most effective suppressant on battery modules, attributing the success to its superior cooling capacity.
  • Subsequent large-scale fire tests by Factory Mutual confirmed this insight, demonstrating that sprinkler protection using water can successfully control or limit ESS fires in commercial settings. While water can’t directly extinguish a failing cell in thermal runaway, its application prevents heat transfer to adjacent cells and modules, effectively containing the incident.

• Condensed Aerosol: Condensed aerosol systems release fine particles that interrupt combustion chemistry and linger to reduce reignition risk. They are most effective in the earliest stages of an event, providing time for other systems to act. However, they cannot remove enough heat to stop runaway once it begins, making them best as a supplemental layer to cooling-based systems.

• Clean Agents: Clean agents such as FM-200 or Novec 1230 suppress fire by absorbing heat and disrupting combustion at the molecular level. They are highly effective for ancillary spaces like control rooms or PCS enclosures. Within battery racks, they provide meaningful protection as part of a combined approach, as they effectively reduce flames and smoke but might not control the underlying thermal runaway process.

Passive Fire Protection

Active detection and suppression form the first line of defense, but passive protection measures add an essential layer that limits consequences and prevents escalation. These passive measures are often the difference between a contained module-level event and a fire that engulfs an entire facility.

NFPA 855 requires:

• A minimum distance between individual battery modules and packs, which reduces heat transfer and slows the spread of fire.
• A minimum distance from other structural elements or combustible materials, helping to protect the wider facility.
• Physical barriers within packs, which compartmentalize failures and keep them from cascading.
• Ventilation requirements to manage flammable gas emissions and prevent explosion.

RAN’s Role in Fire Protection for ESS

At RAN Fire Protection Engineering, we bring deep expertise to the growing field of ESS projects. Our engineers design and implement tailored fire protection strategies that address complex hazards like thermal runaway. We work closely with Authorities Having Jurisdiction (AHJs) to ensure compliance with NFPA 855, NFPA 13, and NEC requirements, helping clients navigate the code environment. By bridging the gap between system designers, building owners, and fire services, we create facilities that are not only code-compliant but also safe for first responders during emergencies. Our role doesn’t end at installation. We support clients with long-term, code-compliant maintenance programs that keep systems effective throughout their lifecycle.

ESS challenges are complex, but with the right partner, they can be effectively managed.

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As ESS deployment accelerates, so does the need for specialized fire protection expertise. Contact RAN Fire Protection Engineering to learn how we can help safeguard your next project.

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