Designing BESS Explosion Prevention Systems Using CFD-Based Methodology

Jens Conzen

Since 2019, BESS energy capacity has grown over 200%, with developers expected to add at least 9 GW of capacity in 2022

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Lithium-ion based energy storage is one of the leading storage technologies that enables sustainable and emission-free energy. In recent years, due to their power density, performance, and economic advantages, lithium-ion battery energy storage systems (BESS) have seen an increase in use for peak shaving and grid support in residential, commercial, industrial, and utility applications. Since 2019, BESS energy capacity has grown over 200% in the U.S., with developers and operators expected to add at least 9 GW of capacity in 2022.

Explosion Risks Associated with Lithium-Ion BESS

Unfortunately, as the use of lithium-ion battery energy storage systems expands, field failures resulting in fires, explosions and toxic exposure have become more prevalent. Although the technology is considered safe and continuously improving, lithium-ion batteries contain flammable electrolytes that can create unique hazards when battery cells become compromised. Overcharging, overheating, and thermal, mechanical, and electrical abuse can cause a short circuit leading to thermal runaway and the sudden release of thermal and electrochemical energy. During the exothermic reaction process, large amounts of flammable and potentially toxic battery gas is generated and liberated into the BESS enclosure. The released gas largely contains hydrogen, which is highly flammable under a wide range of conditions. This may create an explosive atmosphere in the battery room or storage container.

NFPA 855/69 Requirements for Explosion Control in BESS

To address the safety issues associated with lithium-ion energy storage, NFPA 855 and several other fire codes require any BESS the size of a small ISO container or larger to be provided with some form of explosion control. This includes walk-in units, cabinet style BESS and buildings. One way to achieve this is by outfitting the BESS with an explosion prevention system that meets NFPA 69 requirements.

NFPA 69 requires the combustible concentration within the BESS enclosure to be maintained at or below 25% of the lower flammable limit (LFL) for all foreseeable variations in operating conditions and material loadings. To achieve this criterion, a mechanical exhaust ventilation/purge system is typically used to remove flammable battery gas from the ESS space and replace it with clean air through intake louvers. Simulations are often preferred to determine if an explosion prevention system can effectively mitigate gas concentrations according to NFPA 69 standards.

Designing BESS Explosion Prevention Systems Using Computational Fluid Dynamics (CFD)

CFD-based methodology can assist with the performance-based design of explosion prevention systems containing exhaust systems. CFD is a simulation tool that produces predictions of fluid-flow phenomena based on the laws governing fluid motion (i.e., mass, momentum, and energy). Frequently used for simulating the accidental release of flammable gas, CFD simulations can help demonstrate the evolution of gas release as a function of space and time. One advantage over analytical steady-state methods is that the CFD simulation can capture the initial transient of the event. This provides important insights into the required detection timing.

A variety of metrics can be used to quantify the global parameters in CFD simulations, such as volume fraction and mass within an enclosure. Additionally, displaying the gas cloud between the LFL and upper flammability limit (UFL) can help quantify the size of the flammable cloud. This detailed information is useful in understanding the consequence of a scenario and designing mitigation measures, such as gas detection and explosion prevention systems.

Process-safety industries dealing with flammable fluids and explosible dust will often employ CFD for designing explosion prevention systems. Different scenarios involving spills, buoyancy-driven leaks, momentum-driven leaks and a sudden loss of containment can be prescribed using a source term in the CFD model. These different leak scenarios require a deep understanding of the flammable fluid, storage and operating conditions, and the associated hazards. The critical challenge in designing an explosion prevention system for a BESS is to quantify the source term that can describe the release of battery gas during a thermal runaway event. Hence, full-scale fire test data such as from UL 9540A testing are important inputs for the gas release model.

CFD-based methodology can be extended to design an explosion prevention system for any ESS enclosure. Results can also provide the controlled release rate of flammable and toxic materials which is useful information for first responders and to assess environmental impacts. Click here to learn more about our how our experts can assist with energy storage systems design.

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