As the world shifts towards renewable energy, the reliance on BESS and their deployment in urban locations is increasing rapidly. However, the proliferation of such systems mainly powered by lithium-ion (Li-ion) batteries, introduces significant safety challenges – particularly the threat of ‘thermal runaway’.

A BESS may contain hundreds or even thousands of Li-ion battery cells, any of which can unpredictably malfunction, leading to a rapid increase in temperature and the generation of flammable gases. This phenomenon, known as thermal runaway, can quickly escalate causing cascading failures across adjacent battery cells and resulting in large-scale fires or explosions.

The stages of cascading thermal runaway include: 

• Exothermic reaction: Overcharging, short-circuiting, mechanical damage or exposure to high temperatures causes chemical chain reactions within the battery cell to accelerate, leading to the release of heat. The heat generated rapidly increases the battery temperature. 
• Gas generation: The high temperatures cause the decomposition of electrolyte components, leading to the release of flammable gases such as hydrogen and carbon monoxide. The gas generation contributes to an increase in pressure within the cell. 
• Venting and fire: At a certain temperature and pressure threshold, the cell may rupture or vent to release gases. If the released offgas comes into contact with an ignition source, such as a nearby flame or spark, or a flame from the battery itself, it can ignite and result in an even greater fire or explosion. 

NFPA 855 (US National Fire Protection Association), last updated in 2023, provides guidelines for protecting BESS and nearby people from thermal runaway. However, there is ambiguity and differing opinions persist about the effectiveness of certain solutions.

For example, NFPA 855 A.4.11.1 states: ‘While non-water-based fire suppression has been shown to be effective at suppressing Class B and Class C fires in BESS enclosures, current suppression agents, both water-based and non-water-based, are probably not going to be able to stop thermal runaway. No published case studies, test reports or data generated to date indicate otherwise.’

First responders are often unprepared for the unique challenges of thermal runaway events. Therefore, it is important for BESS to include a protection strategy if thermal runaway were to occur. Traditional fire suppression methods, like water and chemical agents, effectively extinguish fires but fail to tackle the root cause of thermal runaway, which is a thermal management issue. Neither water nor chemical agents have the ability to decrease effectively the intense heat produced from the battery cells undergoing thermal runaway. Therefore, even if the fire is suppressed, a high chance of reignition remains. This is obviously extremely dangerous for first responders.

To address the challenge of thermal runaway, in 2022 Fike developed, tested and validated an immersible solution with a boiling point much greater than water, capable of absorbing the exothermic heat and cooling the battery cells. Named Fike Blue, this solution not only slows down the hazard but stops cascading thermal runaway from propagating further. (There is also a range of alternative systems on offer to tackle thermal runaway in BESS.)

First responders are often unprepared for the unique challenges of thermal runaway events. Therefore, it is important for BESS to include a protection strategy if thermal runaway were to occur.

The firefighter’s perspective
Matt Deadman, Director at Kent Fire and Rescue Service and National Fire Chiefs Council Alternative Fuels Lead Officer, explains that Li-ion fires challenge the traditional model of firefighting, which involves disrupting at least one the three main sources of energy for a fire: fuel, heat and oxygen.

He says: ‘The issues that we have with Li-ion is that they generate their own oxygen and heat, and effectively they have their own fuel as well. This means that traditional approach no longer applies, or not in the same way. It is also quite difficult practically speaking to apply water where it is needed, which is at the cell level, because of the configuration of the packs, depending on the application. The new challenge is to think about the way we extinguish fires in a different way.’

The Alternative Fuels Lead Officer is sceptical about the efficacy of new products coming to market. He says: ‘What’s clear is that there is no silver bullet to solve this issue. In fact, we are looking to what we already do. That is, effectively, to contain the fire, apply water to prevent the fire from spreading and further escalation. In the meantime, R&D (research and development) will continue, and it will probably be some time before anyone comes up with a solution that resolves the issue or proves more effective.’

Once the fire is contained, then there is a decision to make about whether the next step will involve an active intervention, or to have a ‘controlled burn’ approach. In terms of the total balance of risks, that may prove more pragmatic, particularly for BESS, which are very powerful and often sited in a rural location. Not fighting the fire minimises not only the risk to firefighters but also the potential of environmental damage to the surrounding area from firefighting water run-off contaminated with battery chemicals.

But that does not mean giving up. ‘Having systems in place is hugely important. What we can’t do is have an attitude of, “If something happens, it happens”! That’s not cost-effective or risk-based,’ remarks Deadman.

In July 2024, the National Fire Chiefs Council updated its 2023 guidance for fire and rescue services on grid-scale energy storage system planning (of systems greater than 1 MWh). A consultation on that text closed at the end of August. The draft document summarises some of the issues which fire and rescue services may wish to clarify with the site developer at the pre-application/engagement phase. They include behaviour in a thermal event/deflagration event, site plans, water supply/suppression systems, BESS design, annunciation and environmental receptors (vulnerabilities).

Referring to the guidance, Deadman says: ‘We are trying to bring out the principles and push out guidance that’s fit-for-purpose and balances risks with our capabilities. We are very keen to have early engagement with developers to influence site design, to make sure we can have suitable strategies to prevent fires, as much as possible, and, when fires occur, making sure we have the facilities we need, and [physical] access and water supply to fight fires.’

There are signs of progress in BESS designs.

‘One pleasing development is a change in design, moving toward smaller, external-access designs, which is great news for fire safety, and shows the industry is developing and phasing out risks,’ observes Deadman.

He is referring to concerns about early BESS designs, which involved filling standard ISO curtainwall containers with batteries – where firefighters need to open the doors in response to a BESS fire. In a perilous incident in 2019 at a BESS fire in Arizona, four members of a HAZMAT team were seriously injured when an explosion occurred after the doors were opened. Subsequently, the incident was analysed in depth and a report funded by the US government was published.

In the UK, the live experience of battling a BESS fire is, thankfully, minimal. As Deadman explains: ‘This is partly because Li-ion batteries do not generally tend to go up in flames. And fortunately, fires are not that frequent, considering how many are now used in electric vehicles (EVs), consumer electronics and grid storage. Second, BESS systems in the UK are still new, and not many are installed yet.’

Therefore, UK fire services have been reaching out to global networks of other fire services to learn from them, also participating in UK working groups with representatives of government and academia.

How can we reduce the risk of fires? 
The Kent Fire and Rescue Service Director is also keen to help users try to get ahead of battery fires. Despite the technology differences, Li-ion fires in BESS are no different to any others, he points out, in that prevention is better than response.

Furthermore, the ‘in-built intelligence’ of BESS systems is claimed to give them an advantage. Deadman says: ‘There is a real opportunity with BESS to factor effective prevention into design. In terms of battery management systems, there’s an abundance of data to potentially get ahead of risk and identify risks occurring.’

He points out that: ‘All Li-ion batteries share two important characteristics in relation to fire risk, whether installed in a BESS, in a smartphone or in an EV – where a study found that where the battery was the cause of ignition, EVs were less likely to catch on fire than an internal combustion engine-powered vehicle.’

First, is speed. ‘The biggest risk I would point to is the speed with which fires develop when Li-ion batteries go into thermal runaway. A five-minute search on YouTube will give you videos that show how violent the reaction is. And the biggest issue that creates in a domestic space is that they can compromise escape routes.’

Second is to be alert during charging. ‘Our advice to the public is to make sure they are present and alert when Li-ion [batteries] are being charged. That’s not the only point where they may fail, but there is evidence that charging is one of the key risk factors.’

• Further reading: ‘Cybersecurity and BESS – battery energy storage systems’. Battery energy storage systems are becoming indispensable in modern power grids, integrating renewable energy sources, maintaining grid stability and providing backup power during emergencies. However, increasing digitalisation of energy systems and the inherent vulnerabilities of BESS to cyber threats pose significant risks to the stability of power grids and the safety of physical assets. 
• Find out more about how the growing adoption of electric vehicles is boosting demand for the critical minerals needed to build new batteries, which are then feeding through to an emerging market for second-life batteries with innovative applications.