By the time visible smoke is detected in a Battery Energy Storage System (BESS), a battery fault may already have progressed significantly.
Introduction
Battery Energy Storage Systems present a range of fire and explosion hazards, particularly when lithium-ion cells begin to fail. While conventional smoke and heat detection remain important parts of a fire protection strategy, relying on these technologies alone may mean that a developing battery fault is not identified as early as it could be.
The key issue is that a battery does not necessarily begin producing visible smoke or flames at the start of a failure. Before combustion occurs, a damaged or failing cell can begin to heat up and release gases as its internal materials and chemistry begin to break down.
This is why BESS detection should be based around how batteries can fail, rather than simply relying on the minimum level of detection required by a particular project, standard or design approach.
Understanding Thermal Runaway in Battery Energy Storage Systems
Thermal runaway is a rapid, self-heating process within a battery cell that can occur when the cell is subjected to conditions such as internal short circuits, overcharging, physical damage, manufacturing defects or excessive temperatures.
The exact progression of a battery failure can vary depending on factors such as cell chemistry, cell and battery design, state of charge and the conditions surrounding the failure. However, the process can involve increasing temperature, internal pressure, electrolyte and other material decomposition, gas release and, in some circumstances, fire.
The following stages provide a simplified illustration of how a battery failure can develop. The actual sequence and timing can vary depending on the battery technology and the nature of the initiating fault.
Stage 1: An Abnormal Condition Develops
A battery cell can experience an abnormal condition such as an internal short circuit, overcharging, physical damage, manufacturing defect or excessive heat.
At this point, the cell temperature may begin to increase without producing visible smoke or flame.
This is an important stage because the battery may already be developing a fault before conventional fire detection systems have a clear combustion product to detect.
Stage 2: Off-Gassing Begins
As the temperature inside the cell increases, components such as the separator and electrolyte can begin to break down.
This can result in the release of gases and vapours from the cell. Depending on the battery chemistry and failure conditions, these can include hydrogen, carbon monoxide, carbon dioxide and a range of hydrocarbon and other volatile compounds.
These gases and vapours may be released before enough smoke or combustion products are present to activate a conventional smoke detector.
This is where gas detection can provide an additional layer of early warning.
Stage 3: Thermal Runaway Develops
If the developing fault is not controlled, the cell may enter thermal runaway.
The reactions occurring inside the cell can generate significant amounts of heat and accelerate the breakdown of battery materials. Gas and vapour production can increase, while the risk of the failure spreading to surrounding cells may also become greater.
At this point, the event is no longer simply an isolated temperature increase. The battery is undergoing a rapidly developing failure that can lead to fire and further thermal propagation.
Stage 4: Fire and Propagation
If thermal runaway progresses to combustion, the gases and vapours released from the battery can ignite, creating a fire and significant heat release.
The heat generated can contribute to thermal propagation between cells, modules and racks, depending on the battery system and its design.
This is why identifying developing battery faults as early as possible is an important part of a BESS fire safety strategy.
Why Is Off-Gassing Important for BESS Detection?
One of the key challenges with BESS fire detection is that gases and vapours associated with battery failure can be released before visible smoke or flames are present.
Thermal runaway can produce a mixture of gases, with the exact composition depending on factors such as battery chemistry, cell design, state of charge and the conditions that caused the failure.
Flammable and Combustible Gases
Gases and vapours produced during battery failure can include hydrogen and a range of hydrocarbons and other combustible compounds.
Some of these gases are highly flammable, meaning that their accumulation within an enclosed space can increase the potential for fire or explosion if an ignition source is present.
The concentration and behaviour of these gases will depend on the battery technology, enclosure and ventilation arrangements.
Toxic Gases
Battery failure can also produce toxic gases.
Carbon monoxide can be produced during thermal runaway, while hydrogen fluoride may be generated from the breakdown of fluorinated electrolyte components in some lithium-ion batteries.
These gases can present serious risks to personnel as well as creating additional challenges for emergency response.
Asphyxiant Gases
Carbon dioxide can also be released during battery failure.
In an enclosed environment, elevated concentrations of gases can contribute to oxygen displacement and create additional hazards for anyone entering the affected area.
The potential hazards therefore need to be considered as part of the overall fire and gas safety strategy.
Why Early Gas Detection Matters
The important point is that gas detection looks for a different indicator of battery failure than conventional smoke detection.
A smoke detector is designed to identify airborne particles associated with smoke, while a gas detection system can monitor for specific gases or compounds associated with developing battery failure.
This means that, in the right application, gas detection may provide an earlier indication of a developing battery failure before conventional smoke or flame detection is activated.
The amount of warning available will depend on factors such as the battery technology, enclosure design, ventilation, detector selection, sensor response and detector location. There is no universal detection window for every BESS.
For this reason, detector positioning and coverage are just as important as the detector technology itself.
Current UK fire and rescue guidance recognises the potential value of off-gas detection as an early-warning measure, while also emphasising that it should form part of a wider risk-management strategy rather than being relied upon as the sole means of predicting or preventing a thermal event.
Why BESS Fire Detection Is Also a Financial and Insurance Issue
BESS fire safety isn’t only about protecting people and equipment.
A major battery fire can result in significant asset damage, operational downtime, business interruption and complex recovery costs. For project developers, operators and insurers, the ability to demonstrate that appropriate hazards have been identified and mitigated is therefore becoming increasingly important.
Standards and guidance such as NFPA 855 address the installation and protection of stationary energy storage systems, including fire detection, explosion control, ventilation, gas detection and thermal runaway considerations.
However, meeting a standard or guidance document should not automatically be treated as the same thing as having a detection strategy that is appropriate for the specific risks of a site.
The applicable requirements will depend on the project, location, system design, battery technology and relevant legislation, standards and guidance. Insurers and other project stakeholders may also have additional requirements around detection, mitigation, testing and documentation.
This is why a risk-based approach to BESS fire and gas detection is important.
What Does a Good BESS Detection System Look Like?
There isn’t one detection technology that can provide the complete answer for every Battery Energy Storage System.
A strong BESS detection strategy should consider the different stages of a potential battery failure and use appropriate layers of detection and mitigation.
Continuous Gas Detection
Gas detection can be used to continuously monitor for gases associated with battery failure.
Depending on the application, sensors can monitor gases such as hydrogen and carbon monoxide, as well as other gases or compounds where appropriate.
The location and sensitivity of these detectors need to be considered alongside the enclosure design, ventilation, battery chemistry and expected gas dispersion.
Detector positioning should be based on the characteristics of the protected space and the behaviour of the gases being monitored rather than simply applying a standard detector layout.
Aspirating Smoke Detection
Aspirating smoke detection systems continuously draw air from the protected area and analyse it for very small concentrations of smoke particles.
This can provide very early warning of smoke compared with conventional point smoke detectors and can form another layer within a wider BESS detection strategy.
However, aspirating smoke detection and gas detection perform different functions. One should not automatically be treated as a replacement for the other.
Temperature Monitoring
Temperature monitoring can provide another important layer of detection.
Fibre-optic temperature sensing can be used to monitor temperature across areas of a BESS installation and identify abnormal temperature changes.
This can provide an additional indication of a developing thermal event, particularly when combined with other detection technologies and battery management system data.
Acoustic Gas Detection
Acoustic gas detection may also provide an additional detection method in suitable BESS applications.
When pressurised gas escapes from a battery cell or system component, it can create an ultrasonic acoustic signature. Acoustic detectors can identify these signals and provide an additional method of detecting gas release.
As with any detection technology, the effectiveness depends on the application, detector positioning and the characteristics of the protected area.
Emergency Response and Mitigation
Detection is only one part of the overall strategy.
Once a developing battery fault has been identified, the system may need to initiate an appropriate emergency response sequence.
Depending on the site design and risk assessment, this could include alarm activation, equipment isolation, ventilation controls or other engineered mitigation measures.
Where combustible gases can accumulate, explosion mitigation measures may also need to be considered. Depending on the enclosure, system design and applicable requirements, these can include engineered ventilation, gas concentration monitoring and purpose-designed deflagration or overpressure mitigation.
The objective is not simply to detect a problem. It is to detect it early enough for the appropriate response to take place.
How Here At Protex We Help
At Protex, we provide bespoke Fire & Gas engineering solutions designed around the specific requirements and hazards of each site.
We provide a range of detection technologies that can be considered as part of a layered BESS strategy, including point and open-path gas detection, acoustic gas detection, fibre-optic temperature sensing and aspirating smoke detection.
Overall
Smoke detection still has an important role to play in BESS fire safety. But smoke is not necessarily the first indication that a battery is beginning to fail.
Before visible smoke or flames appear, a battery cell can experience abnormal heating and begin releasing gases and vapours as its internal materials break down.
That off-gassing can provide an opportunity for earlier detection, particularly when gas detection is combined with other technologies such as aspirating smoke detection, temperature monitoring and acoustic detection.
The key is not simply installing more alarms. It is designing a detection strategy around how the batteries can actually fail.
For BESS developers and operators, that means considering the battery chemistry, enclosure design, ventilation, detector positioning, required response and applicable legislation, standards and guidance from the beginning of the project.
At Protex, we design Fire & Gas detection systems around the individual requirements of your site, using engineering analysis and 3D modelling to help establish appropriate detector coverage before construction.
If your BESS detection strategy hasn’t been reviewed since the site went live, it may be worth taking a second look.
