Mitigating Lithium-Ion Battery Energy Storage Systems (BESS) Hazards

Jens Conzen + Michael Townsend

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Dec 8, 2023

Battery energy storage systems (BESS) use an arrangement of batteries and other electrical equipment to store electrical energy. Increasingly used in residential, commercial, industrial, and utility applications for peak shaving or grid support these installations vary from large-scale outdoor and indoor sites (e.g., warehouse-type buildings) to modular systems. Containerized systems, a form of modular design, have become a popular means of efficiently integrating BESS projects.

Due to the fast response time, lithium-ion BESS can be used to stabilize the power grid, modulate grid frequency, and provide emergency power or industrial-scale peak shaving services, reducing the cost of electricity for the end user. However, high-powered and rapid charge cycles can put stress on the batteries resulting in degradation over time, which is not beneficial to safety.

In the past four years, more than thirty large-scale BESS around the world experienced failures that resulted in fires and, in some cases, explosions. Given these concerns, professionals and authorities need to develop and implement strategies to prevent and mitigate BESS fire and explosion hazards. The guidelines provided in NFPA 855 (Standard for the Installation of Energy Storage Systems) and Chapter 1207 (Electrical Energy Storage Systems) of the International Fire Code are the first steps.

Thermal Runaway

Prevention and mitigation measures should be directed at thermal runaway, which is by far the most severe BESS failure mode. If thermal runaway cannot be stopped, fire and explosion are the most severe consequences.

Thermal runaway of lithium-ion battery cells is essentially the primary cause of lithium-ion BESS fires or explosions. Under a variety of scenarios that cause a short circuit, batteries can undergo thermal runaway where the stored chemical energy is converted to thermal energy. If the process cannot be adequately cooled, an escalation in temperature will occur fueling the reaction, which can result in cell rupture and release of flammable and toxic gases. The most common initiating events for thermal runaway include:

  • Manufacturing defects in the cells
  • Overcharging (e.g., inverter failure)
  • Overheating (e.g., poor cooling capacity or cooling system failure)
  • Mechanical abuse (e.g., seismic event or impact)

Battery Management System as a Barrier to Thermal Runaway

In battery energy storage systems, one of the most important barriers is the battery management system (BMS), which provides primary thermal runaway protection by assuring that the battery system operates within a safe range of parameters (e.g., state of charge, temperature). In a UL 9540 listed BESS, the BMS monitors, controls and optimizes the performance of battery modules and disconnects them from the system in the event of abnormal conditions. The BMS also provides charge and discharge management of the batteries.

In case of undervoltage or overvoltage, over-temperature, or overcurrent conditions, the BMS will alarm and then limit the charge and discharge current or power. Under emergency conditions, the BMS will cease operations and electrically disconnect each battery enclosure. This is assuming that the BMS is not damaged and operational. However, if an internal cell breakdown has occurred, the BMS will not stop the thermal runaway.

Explosion Control

A thermal runaway with fire or explosion as the consequence is the most severe hazard to prevent or mitigate. While there has been some guidance on fire control and suppression, many BESS manufacturers, integrators and end-users struggle with the explosion control requirement. Explosion control can be achieved by providing one of the following:

  • Explosion prevention systems are designed, installed, operated, maintained and tested by NFPA 69 (Standard on Explosion Prevention Systems)
  • Deflagration venting installed and maintained following NFPA 68 (Standard on Explosion Protection by Deflagration Venting)

If implementing an explosion prevention system according to NFPA 69, the combustible concentration shall be maintained at or below 25 percent of LFL for all foreseeable variations in operating conditions and material loadings. One option for achieving these requirements is ventilation or air dilution. This can be accomplished by installing a forced ventilation system, which can be automatically actuated by a gas-detection system when gas concentration levels exceed a pre-determined set point.

Moreover, deflagration venting creates a pathway for rapidly expanding gases to exit the enclosure in the event of a deflagration. It can be challenging to protect BESS enclosures with little free air volume and a high degree of internal obstruction. Performance-based engineering methods, such as Computational Fluid Dynamics (CFD), may be required in this case.

Best Practices for Mitigating BESS Hazards

Compliance with NFPA 855 is increasingly required to permit a BESS, and the International Fire Code (IFC) has influenced local fire code requirements concerning these systems. Hence, NFPA 855 and the IFC are used to guide best practices along with industry experience (i.e., lessons learned from failure events).

The following are best practices for BESS with an energy capacity greater than 600 kWh. These are for the BESS product level and do not have general applicability to various installation sites. Depending on the installation location of the BESS, additional local requirements and preferences may need to be considered. Furthermore, all features shall meet applicable local codes and standards, including the use of listed equipment.

  • Hazard Mitigation Analysis (HMA). HMA aids in identifying and mitigating hazards created with the BESS technology. At a minimum, the HMA should address the failure modes identified in NFPA 855 and the IFC. The HMA can be used to analyze the effectiveness of installed safety measures.
  • Smoke and fire detection. Smoke and fire detection equipment is required to be installed in large BESS enclosures that are not remote or indoor applications. The IFC requires smoke detection and automatic sprinkler systems for “rooms” containing stationary battery energy storage systems.
  • Fire control and suppression. Fire control and suppression is prescriptively required by NFPA 855 but may be omitted if approved by both the authority and the owner if the project site is remote and outdoors. The IFC requires automatic sprinkler systems for “rooms” containing stationary battery energy storage systems. In case of thermal runaway with the resulting fire, water is the preferred agent for suppression. While incapable of stopping thermal runaway in the cells where that process has already started, fire sprinklers are capable of controlling fire spread and reducing the hazard of a lithium-ion battery fire.
  • Explosion control. NFPA 855 requires explosion control measures in the form of deflagration venting (NFPA 68) or explosion prevention (NFPA 69), including cabinet-style BESS enclosures.
  • Gas detection. Gas detection may be used as part of an NFPA 69 explosion control solution.
  • Thermal runaway protection. Thermal runaway protection is required and can generally be achieved by using a battery management system that is UL 1973 certified.
  • Size and separation requirements. Separation distances between each BESS container and adjacent structures should be maintained to reduce fire spread. There are prescriptive distances that can be shortened under the consideration of full-scale fire test data, performance-based methods or by using engineered fire barriers.
  • Water supply. Since water is the preferred agent for suppressing lithium-ion battery fires, a permanent source of water is recommended.

Address the Fire Safety Challenges of Lithium-Ion Battery Storage

BESS is an important element in reducing carbon emissions and enabling renewable power generation technologies. In a time of increased development and deployment of BESS installations, it is important to make sure that it is done safely. Jensen Hughes can help you address the unique fire safety challenges associated with lithium-ion battery storage and handling and ensure that building and fire code requirements are met.

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About the author

Jens Conzen
Jens supports research related to lithium-ion battery safety as well as fire and explosion safety for energy storage systems (ESS) and is extensively involved with the development of chemical reactor safety systems.