Playing with Fire? - An analysis of BESS failures
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Now onto our regular programming… Kush Sutaria explores the data behind Battery Energy Storage System (BESS) failures. With all the buzz around battery fires, it’s easy to think they’re a frequent issue—but the numbers tell a different story. Let’s take a closer look at the rates, causes, and safety systems in place to minimize both the frequency and severity of battery fires.
Media Attention
Recent BESS fires, like the ones in Escondido and LA, have once again brought media attention towards the issue of stationary storage battery fires. So much so that the city of Escondido in California recently passed a 45 day moratorium on new BESS installations and is considering a 10 month moratorium due to a recent fire at a BESS site.
Whilst incidents such as these prompt very valid questions around battery safety and must be understood in order to prevent repeats, I think it’s helpful to put BESS fire numbers in context.
Around this topic I think there a few key questions to ask:
Are batteries inherently unsafe?
What if a fire does occur?
How often do incidents occur?
Note in this article I’m limiting my analysis to BESS installations. EV fires, battery factories and residential systems require their own analysis and would be too much to cover here. EVs are exposed to a wider variety of abuse conditions, as they are driven at high speeds and can be involved in crashes, making them more likely to suffer damage to their batteries.
The BESS Failure Incident Database
Before digging into the topic, a resource I’ve used to help is the EPRI BESS Failure incident database which tracks BESS failure incidents. Utility and Commercial/Industrial battery systems are tracked in the database but residential battery systems are currently not. The focus of the database is on incidents that had a wider public health and safety impact, rather than on purely operational failures.
FYI if you’re in the energy storage field like me and want to be notified when an incident is added to the database, you can email Storage-Safety@epri.com to be added to the email list.
Are batteries inherently unsafe?
Most BESS on the market are using Li-ion at the moment, and it’s usually LFP. A fundamental characteristic of Li-ion technology as it stands is that, in the wrong conditions, thermal runaway can occur. This is most commonly as a result of misuse of the battery, specifically overcurrent, overcharge, excessive temperature due to thermal mismanagement, attempted charge below 0℃, mechanical puncture of the cell or an external short. One cause of thermal runaway which does not originate from abuse is that of an internal short caused by a manufacturing defect, such as metallic debris inside the cell due to poor quality control at the factory.
With this in mind, we could probably say that good quality cells only really present a thermal runaway concern when subjected to abusive or extreme conditions.
The BMS in the BESS will function to prevent electrical abuse of the battery by opening contactors to prevent further charge/discharge of the battery if one of these unsafe conditions arises. Liquid cooling systems also help to maintain cell temperatures in safe operating ranges.
But protection systems do have the potential to fail and unforeseen circumstances could arise, so what happens if a fire does break out?
What if a fire does occur?
BESS systems are typically arranged as large containers (about the size of shipping containers), each containing a series of modules filled with cells. There is a testing standard called UL9540A that aims to verify the thermal runaway performance of the system. The cell level testing aims to verify if a cell can be driven into thermal runaway (which almost all can) and what the behavior of the cell is under thermal runaway. Module level testing aims to force propagation between cells in a module and verify if there are any flames and smoke ejected from the module as a result of this. Unit level testing is performed on the racks that are within the container (or the entire container if there are no separate racks) and aims to see if any flames or smoke are ejected from the unit and whether the recommended spacing provided by the manufacturer is sufficient to ensure the thermal runaway doesn’t spread to surrounding units.
BESS containers are equipped with gas, smoke, and heat sensors to detect thermal runaway, which are linked to a station-level fire control panel accessible by the fire department. Depending on whether a fire is detected, appropriate action is then taken—or is it?
Currently, the approach most commonly used by energy storage project developers, is that if a fire does arise, simply let the container burn itself out and ensure that the fire does not spread to neighboring containers. This can be done by spreading containers out and conducting tests to show that a fire in one container does not spread to the next (containers can be placed back to back if testing shows that fires don’t propagate). Rather than trying to fight the battery fire (very difficult to extinguish), the idea is to let the fire consume the batteries so that the event is over as quickly as possible with as little damage as possible.
UL9540A certification assesses fire propagation when the trigger method is a cell thermal runaway. But BESS manufacturers are now also conducting burn tests where the fire is started from other parts of the BESS, for example faulty wiring or an overheating component. Now, the cells (although not the cause of the fire) are a fuel source. Usually the tests will initiate the fire using a blowtorch. The aim is to get a container fully alight, and then assess if the fire spreads from container to container when spaced out as per the manufacturer’s recommendation.
Explosion control is another topic. The concern here is that a build up of flammable gas from a thermal runaway in the container could lead to an explosion. The widely adopted standard here is NFPA69 which uses fans to bring in clean air if the gas level in the container reaches 10% of the lower explosive limit. Some BESS use a sparker system as a form of explosion control. This is essentially a spark plug which constantly sparks in the container such that if any gas is released by the cells, the sparker ignites the gas in small quantities, preventing a large build up of gas and a bigger explosion.
Some BESS allow you to connect water supplies up to the container to flood it in case a fire breaks out, or use aerosols which suppress fires, however, these are becoming less popular as Li-ion fires are very difficult to put out and so these systems are just prolonging the fire event and often the fire suppression systems introduce new points of failure which can cause a fire. For example, a leak in the water system shorting a module.
How often do incidents occur?
The chart above, taken from the BESS failure database, shows that the number of BESS incidents globally has remained in the teens annually, despite a 25x increase in deployed GW, indicating that safety improvements are effective. In 2023, the incident rate was under 0.5 per GW deployed.
While direct comparisons with other industries are difficult, one study reported an annual incident rate of 28.9 fires per GW for rooftop solar panel installations, using data from Australia, Germany, Italy, the UK, and the USA. Based on this, it appears that BESS fires are significantly less common than rooftop solar fires, although solar rooftop installations are more prevalent in residential areas.
So what does this all mean?
BESS safety has significantly improved in recent years and incident rates remain exceptionally low. While some failures are inevitable due to the scale of battery deployments, it's important to recognize how rare these events are.
Even in the event of a fire, industry standards now ensure that they are contained within individual battery containers, preventing the spread and allowing the fire to burn itself out. Although no system is completely failure-proof, advancements in BESS safety protocols have minimized risks, ensuring that any incidents are effectively managed to prevent widespread harm.
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