The growth of containerised battery energy storage systems (BESS) has intensified the focus on safe design and operation, particularly given lithium-ion battery hazards including thermal runaway, flammable gas generation and fire propagation. Modern BESS safety therefore uses layers of protection, combining measures such as monitoring, gas and fire detection, ventilation, containment, suppression and explosion protection. Industrial safety expert BS&B Safety Systems explains further.
Standardised containers have become a common architecture for large-scale battery storage, allowing batteries, cooling systems, power electronics and safety equipment to be packaged in modular units. Utility-scale projects may deploy dozens or hundreds of these containers in arrays connected through power conversion systems and transformers.
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Hydrogen accumulation
According to Geof Brazier, Managing Director of BS&B Safety Systems Explosion Protection Division, battery system hazards and protection strategies include hazard-mitigation analysis, testing expectations, and attention to fire and explosion risk management in large battery installations.
“One of the recognised safety concerns was the build-up of hydrogen and other combustible gases in containerised BESS, because hydrogen is highly flammable and can accumulate to a combustible concentration in enclosed spaces if not properly ventilated or monitored and controlled,” says Brazier.
In BESS installations that use lithium-ion batteries, hydrogen and other combustible gases can be generated during thermal runaway or internal battery damage. When lithium-ion cells overheat or fail, chemical decomposition of the electrolyte and other cell components can produce a mixture of gases that may include hydrogen, carbon monoxide, methane, and other flammable compounds.
The danger arises when hydrogen or other flammable gases accumulate in an enclosed space and are then ignited by electrical equipment, static discharge, or other ignition sources.
Hydrogen has a very wide flammability range and a low minimum ignition energy. In air under typical conditions, it is flammable at concentrations of approximately 4% to 75% by volume; the lower end of this range, about 4%, is known as the lower flammability limit. Because it is lighter than air, hydrogen tends to accumulate near the ceiling or the upper portions of a container if ventilation is inadequate. This can further increase hydrogen concentrations in those upper areas.
Ignition at lower hydrogen concentrations can cause a deflagration which, although slower-moving than a detonation, can generate damaging pressures within a confined enclosure. At higher concentrations, the risk can progress towards detonation, producing a supersonic shock wave.
“When you get into the higher percentages, you are dealing with explosions that can transition to an unprotectable detonation, so it is important to do the utmost to reduce the level of hydrogen accumulation in the container so the conditions for an explosion do not arise,” says Brazier.
The resulting deflagration or explosion may not only damage the container but may propagate fire-driven overheating to adjacent BESS modules.
Because of these risks, Brazier says modern BESS designs emphasise early detection and layered protection strategies. These include monitoring battery temperature and voltage to detect failures early, detecting flammable gases before they reach hazardous concentrations, and providing controlled ventilation or explosion relief to prevent pressure build-up.
BS&B Safety Systems’ VSP Actuated Ventilation System is an explosion prevention device designed to protect BESS enclosures by actively releasing combustible hydrogen and other accumulated gases before an explosive concentration arises.
Sensors monitor combustible gas concentrations and trigger an actuator to open the vent when elevated levels are detected, allowing gases to escape before closing once concentrations return to acceptable levels.
“An explosion prevention device doesn’t necessarily have to respond to an explosion,” explains Brazier. “In this case, it responds before an explosion would occur to let the hydrogen out before it builds up into a combustible range.”
Explosion venting
Containerised BESS are also increasingly fitted with explosion vents to control the pressure spikes and direct flame and gas when a thermal-runaway event causes a flammable atmosphere to ignite and a low concentration of combustible gas results in a deflagration.
BS&B’s BESS-Saf range uses low-burst-pressure explosion vent panels mounted on container roofs or upper exterior walls to provide controlled pressure relief, with discharge directed away from egress routes.
Flame-Free versions incorporate a flame arrester rated for hydrogen and other gas deflagration conditions with an explosion vent. This combination provides a reliable layer of protection for enclosures exposed to deflagration and overpressure risks.
“If hydrogen or other gases accumulate and a deflagration arises, the explosion vent opens to relieve overpressure while the integrated flame arrester quenches the flame front to mitigate the release of flame to the atmosphere,” says Brazier.
Pressure relief vents of this kind are often combined with gas detection and forced ventilation systems to keep concentrations below the lower flammable limit.
“Explosion venting is not mandatory in every installation, but it is one of the permitted methods for achieving explosion control,” explains Brazier. “Because venting is often a comparatively economical solution, it receives significant attention and is frequently viewed as the preferred cost-effective approach.”
According to Brazier, vent selection is determined through an evaluation of the enclosure’s size and structural capacity, the design strength, and the total vent area necessary to maintain internal forces within allowable limits.
