Lithium-ion batteries have a regrettable habit of making news headlines because of safety issues, such as laptop battery recalls from several years ago or more recent incidents involving e-bike fires. Those events, while sometimes having tragic consequences, are nevertheless limited by the size of the batteries, with even the largest e-bike battery being a kilowatt-hour or so. Scaling up those batteries to tens, hundreds, or even thousands of megawatt-hours to support the electricity grid can cause outsized concerns on the part of people living near proposed facilities, as surfaced recently with the UK’s Cleve Hill PV+battery project near Graveney Marshes in Kent.
To understand how the industry is addressing these concerns, it is important to understand the nature of the hazards. Failures in lithium-ion batteries can lead to thermal runaway, during which cells can vent large amounts of flammable gas, including hydrogen, carbon monoxide, and volatile organic compounds from the electrolyte, potentially leading to fire or explosion. Failure events can start from a single cell, if there is an internal short-circuit, or from multiple cells, if there is a loss of isolation due to a coolant leak or stormwater infiltration, for example.
Early battery energy storage systems (BESS) universally relied on fire suppression systems to maintain safety. However, the problems with this approach became apparent in 2019, when the McMicken battery energy storage facility in Surprise, Arizona, experienced an explosion that injured four firefighters. The event began when the clean-agent fire suppression system discharged in response to a cell thermal runway. While the fire was suppressed, there was ongoing cell-to-cell thermal runaway propagation and cell venting, resulting in flammable gases accumulating above their upper flammable limit. This situation was stable until firefighters opened the door, allowing oxygen to enter and causing an explosion.
Since the McMicken event, BESS designers have recognised that explosion is a far greater hazard than fire. State-of-the-art BESS enclosures now incorporate explosion-prevention systems with emergency exhaust ventilation. Those ventilation systems would also exhaust clean-agent fire suppressant, rendering it ineffective, so these enclosures normally do not include fire suppression as standard. An important design consideration is that the enclosure walls are heavily insulated, so that even when closely spaced, complete combustion of one unit will not result in thermal runaway in adjacent units. Firefighting practices have evolved accordingly, with first responders being trained to allow a fire to burn itself out in a controlled manner while protecting nearby equipment as necessary. This strategy of fire containment avoids problems with reignition, stranded energy, and toxic runoff of firefighting water—an important consideration for environmentally sensitive areas like Graveney Marshes.
This holistic safety philosophy will be reflected in the next update to the industry-leading safety standard from the US National Fire Protection Association. NFPA 855, Standard for the installation of stationary energy storage systems. First published in 2020, this standard is updated on a three-year cycle, with task groups already hard at work on content for the 2026 edition. The requirements of NFPA 855 are incorporated into the US model fire codes that are adopted, in whole or in part, by states and other jurisdictions.
The 2023 edition of NFPA 855 includes requirements for maximum energy, spacing between units, and fire suppression, but allows the Authority Having Jurisdiction (AHJ) to waive these constraints for BESS facilities located more than 30 m from buildings, public ways, stored combustible materials, etc. The AHJ’s decision is based on the results of fire and explosion testing in accordance with UL9540A, Test method for evaluating thermal runaway fire propagation in battery energy storage systems, or equivalent.
On explosion hazards, the 2023 edition of NFPA 855 mandates explosion control either by explosion prevention in accordance with NFPA 69, Standard on explosion prevention systems, or deflagration venting in accordance with NFPA 68, Standard on explosion protection by deflagration venting. That said, it has been recognised by the BESS safety community that a system relying solely on NFPA 68 could potentially pose a McMicken-style hazard, and it is almost certain that such systems will be disallowed in the 2026 edition.
Among the documentation required to be submitted by BESS facility owners to the AHJ are a hazard mitigation analysis, an emergency response plan, and a commissioning plan. These documents respectively address the BESS design features to mitigate potential safety hazards, the plan for first responders and facility personnel to respond to safety events, and the procedures and tests to be conducted after installation to ensure that the system has been correctly installed and that all safety systems are functional.
Many energy storage developers have also recognised that they need to go beyond mandatory requirements by engaging early and often with first responders and local communities. Doing so is the best way to bring everyone involved to a higher level of comfort with the safety of these battery systems.
This article first appeared in Modern Power Systems magazine.
