Volts and vulnerabilities: Exploring the hazards of battery energy storage systems

The transition towards renewable energy sources, notably wind and solar, is essential for a low carbon energy future. However, these sources are intermittent – the wind does not always blow, and the sun does not always shine. This creates gaps in power generation that must be filled to maintain a stable electrical grid. The Battery Energy Storage System (BESS) has emerged as an adaptable and scalable solution to this challenge.

Recent BESS-related fires and explosions have highlighted the potential harm to people and the environment. With energy storage capacity growing rapidly, it is crucial to understand BESS hazards and effectively manage the associated risks to ensure the safe expansion of this critical component of future energy networks.

Introduction

Electricity grids require the right power at the right time to maintain stability, which does not align with the intermittent nature of renewable sources. We unfortunately cannot turn up the wind or the sun to meet demand.

In the UK, peak sunlight hours, when solar generation is at its highest (typically between 9 AM and 3 PM), coincide with when demand on the electricity grid is in a minor lull, as shown in Figure 1. Electricity demand peaks when people return home in the evening, often after the sun has set. This highlights a clear mismatch between the timing of solar power generation and periods of highest electricity consumption.

Figure 1: Graph showing the disparity between solar power generation and peak electricity demand in the UK. Data from National Grid ESO for 15^th January 2024 (Ref. 1, Ref. 2).

As more solar assets have come online in recent years, the disparity has become more pronounced. Renewable energy assets are sometimes temporarily disconnected from the grid when generation exceeds grid capacity, leading to lost production. It has also increased the strain on the electrical grid’s ability to maintain power quality and deliver consistent power.

The net load (demand minus renewable supply) is currently met by conventional, fossil fuel power plants. This poses challenges in the short-term. Ramping up quickly to meet the steep evening demand can be technically challenging and expensive. Furthermore, plants may need to shut down during peak solar and wind hours, reducing their operational efficiency. In the long-term, conventional plants are not a sustainable solution for achieving net carbon zero. So, what can be done?

BESS: capacity for greatness?

An Energy Storage System (ESS) stores energy when generation exceeds demand and releases it to the grid when needed. It provides a flexible solution to balance supply and demand effectively.

At COP28 in December 2023 (Ref. 3), commitments made by governments to achieve net zero emissions by 2050 would require the total global energy storage capacity to rise to 1,500 GW by 2030 – a sixfold increase on 2023 capacity. BESS represents 1,200 GW of this capacity (Ref. 4), which is an enormous amount of energy storage capacity, equivalent to the power output of about 1,200 large nuclear reactors.

BESS, briefly

The most common battery type used for grid-scale BESS is lithium-ion. Working on the same principles as rechargeable batteries found in mobile phones and laptops, lithium-ion batteries store electricity as chemical energy during charging, which is then discharged via chemical reactions in the cell as electricity.

BESS facilities can consist of thousands of individual cells, connected into batteries and grouped in enclosures often resembling shipping containers. Grid-scale BESS can comprise tens to hundreds of containers, alongside substations, transformers and other infrastructure to interface with the grid.

Figure 2: Example Battery Energy Storage System (BESS)

What can go wrong?

Like all electrical systems operating at high voltage, a battery facility poses traditional hazards such as arc flashing, electrocution and electrical fires. These hazards are well-known, and the controls understood. However, the US-based National Fire Protection Association (NFPA) has highlighted four hazards specific to BESS (Ref. 5).

1. Thermal runaway

Thermal runaway occurs in a cell due to abuse (e.g. overcharging, overheating, water ingress). The abuse triggers exothermic reactions, producing gas and heat. Eventually, heat is produced faster than it can dissipate, the exothermic reactions become self-sustaining, and an uncontrolled positive thermal feedback loop is created –“thermal runaway”.

The affected cell can then heat up adjacent cells and initiate thermal runaway in them, a cascade effect called “thermal propagation”. The cells will vent toxic and flammable gases (“off-gassing”), entraining droplets of the cells’ organic electrolyte to form a flammable and toxic vapour cloud.

2. Off-gassing

Off-gassing can occur independently of thermal runaway but is very often a precursor to it following cell abuse (e.g. overcharging). The composition of the off-gas released varies depending on the cell chemistry, but typically includes hydrogen, hydrogen fluoride, carbon monoxide, methane, and ethylene (Ref. 6). Off-gassing can lead to:

Fire. Ignition of the off-gas (e.g. by electrical sparks) can ignite the cells and accelerate thermal propagation. Fires can propagate throughout a battery container, potentially spreading to adjacent batteries, causing catastrophic asset damage and harm to people and the environment.
Explosion. Battery containers are confined spaces. If off-gassing is not detected or there is insufficient ventilation during thermal runaway, explosive volumes of off-gas can form, generating significant overpressures if ignited.
Gaseous releases. Off-gases can be toxic due to components such as hydrogen fluoride and carbon monoxide, posing a threat to people in the vicinity. Some components can also react with the water used to extinguish fires (e.g. hydrogen fluoride, which forms hydrofluoric acid), causing harm to people and the environment if not treated and disposed of properly.

3. Deep-seated fire

A deep-seated fire is a particularly challenging type of fire that occurs within the internal structure of the battery system. The fire originates and persists deep within the BESS structure, is difficult to access and extinguish, and can lead to rapid thermal propagation.

Battery cells are grouped in modules housed in metal or plastic casings. The modules are then stacked into units which are tightly packed within an enclosure. Due to this compact structure, water-based fire suppression methods often struggle to reach all cells effectively, making it challenging to remove sufficient heat to inhibit thermal reactions in the innermost cells.

Other fire suppressants, such as aerosols and inert gases, are effective in extinguishing fires but not in removing heat from cells and stopping thermal runaway.

Moreover, aerosols and inert gases require a closed environment to extinguish fires, as they work partly by removing oxygen. However, unlike fire, thermal runaway can occur in the absence of oxygen. If the fire is extinguished but thermal runaway and off-gassing continues, there is increased risk of an explosion occurring if oxygen re-enters the enclosure. Such a scenario was seen during an incident in Surprise, Arizona (Ref. 7). Additionally, aerosols are released with an explosive charge, so may act as an ignition source themselves.

Overall, it is very difficult to remove enough heat to entirely extinguish a battery fire, inhibit thermal runaway, and prevent reignition. This challenge must be considered when designing the layout of BESS facilities and developing the firefighting strategy.

Rather than attempting to extinguish a BESS fire, the firefighting strategy currently adopted by most operators is to let the affected battery consume itself – a “controlled burn”. The design focus is on preventing the spread of fire through sufficient separation of battery units, fire barriers, and cooling adjacent assets with water. This approach was taken during the Victorian Big Battery Fire incident in Australia (Ref. 8) – no people were harmed, and the fire only spread to the battery directly adjacent to the initiating unit.

Figure 3: Victorian Big Battery (VBB) fire, Australia. Source: Solarquotes

4. Stranded energy

Stranded energy refers to residual electrical charge remaining within a battery cell when normal discharge methods are no longer safe or feasible due to damage or degradation of the cell, such as from a fire. It poses a risk of serious electrical shock from a seemingly inert cell. Stranded energy can act as an ignition source, with the potential to reignite fires days, or even weeks, after the initial incident.

How are the hazards managed?

The best practice in managing BESS hazards is constantly evolving, reflecting the relative infancy of the technology. Current standards, guidance, and recommendations include:

IEC 62619 (Ref. 9) and NFPA 855 (Ref. 10), which provide guidance for the design of battery units and BESS facilities.
UL standard UL 9540A (Ref. 11), which outlines a test method for manufacturers to demonstrate their product’s resistance to thermal runaway, thermal propagation, and fire.
In the UK, the National Fire Chiefs Council (NFCC) has published guidance (Ref. 12), which helps developers design BESS facilities to minimise the risks of thermal runaway and fire, and ensure that sites are safe for first responders attending an incident.

Certification of battery products against standards does not mean that they are “safe by design”, however (Ref. 13). Recent criticism of the UL 9540A standard has highlighted that at least 50% of the recorded lithium-ion BESS failure events in 2022 and 2023 were UL 9540A certified BESS (Ref. 14).

This highlights the importance of product selection, ensuring that batteries are assessed against strict safety criteria and developers and owner-operators are aware of the inherited risk that comes with off-the-shelf products (Ref. 15). They must accept the residual risk and, in the UK, show how this will be controlled to reduce the risk As Low As Reasonably Practicable (ALARP).

Risk reduction measures

The aim should always be to prevent incidents, before implementing means of mitigating the consequences. Examples of preventive and mitigative risk reduction measures include:

Preventive:

Temperature monitoring of cells to detect overheating, linked to a liquid cooling system, and alarm and shutdown system.
Managing the battery charging and discharging rates to prevent overheating and degradation of cells.
Ingress protection (IP) rating of BESS units to prevent water ingress into cells, which can cause damage that initiates thermal runaway.
Fault detection (e.g. over/under voltage, over/under temperature, over-current, communication faults, fuse faults).

Mitigative:

Gas, fire and smoke detection, linked to a forced ventilation system or automatically opening doors to prevent gas accumulation.
Active fire suppression system. However, given the issue of deep-seated fires, the suitability and benefit of many fire suppression systems are currently in question.
Deflagration panels, which provide a route to vent overpressure during an explosion.
Fire barriers and sufficient spacing between battery units to contain a battery fire within the initiating unit.
Firewater supply, emergency response plan and firefighting strategy developed in liaison with the local fire and rescue service.

Safety assurance process

A robust, proportionate, and pragmatic safety assurance process will ensure that such risk reduction measures are implemented and consider the interaction of the BESS with its surroundings. Project developers should produce an Outline Battery Safety Management Plan (OBSMP) in the early stages of development (pre-consent) which demonstrates that their proposed BESS facility can exist safely in its planned location and configuration, at a conceptual level. The OBSMP should demonstrate flexibility, accounting for potential changes in available BESS technology.

As the design is finalised and BESS technology is selected, a detailed Battery Safety Management Plan (BSMP) should then be produced which demonstrates how the risks posed by the BESS are reduced ALARP. The ALARP demonstration must show how all risks have been identified and mitigated, and that the developer has not simply relied on the certification of the procured batteries as its safety justification.

Conclusion

Like all emerging technologies, knowledge of BESS hazards and the means of managing them is constantly evolving. Developers, operators, regulators and all other stakeholders of BESS projects have a duty to keep abreast of the latest lessons learned. Whilst significant incidents involving BESS have been rare, the rapid increase in deployment of BESS heightens the likelihood of future incidents if the correct approach to risk management is not adopted.

BESS is a cornerstone of green energy grids. Understanding and addressing BESS hazards is vital for safe coexistence with this critical infrastructure. The design and development of BESS projects must follow a robust safety assurance process. This includes selecting safe products which are arranged safely and integrated to protect people and the surrounding environment.

References

National Grid ESO, Historic generation mix and carbon intensity. URL: https://www.nationalgrideso.com/data-portal/historic-generation-mix (accessed 3rd September 2024)
Elexon BSC Insights Solution, Demand outrun. URL: https://bmrs.elexon.co.uk/demand-outturn (accessed 3rd September 2024)
Climate Change Conference of the Parties 28th session (COP28), December 2023
International Energy Agency, Batteries and Secure Energy Transitions, World Energy Outlook Special Report, April 2024
National Fire Protection Association (NFPA), Energy Storage Systems Safety Fact Sheet, February 2024
Battery Hazards for Large Energy Storage Systems. Judith A. Jeevarajan, Tapesh Joshi, Mohammad Parhizi, Taina Rauhala, and Daniel Juarez-Robles. ACS Energy Letters 2022 7 (8), 2725-2733. DOI: 10.1021/acsenergylett.2c01400
UL Firefighter Safety Research Institute, Four Firefighters Injured in Lithium-Ion Battery Energy Storage System Explosion – Arizona, 28 July 2020
Fisher Engineering Inc., Victorian Big Battery Fire: July 30, 2021, Report of Technical Findings, 25 January 2022
IEC 62619:2022, Secondary cells and batteries containing alkaline or other non-acid electrolytes – Safety requirements for secondary lithium cells and batteries, for use in industrial applications
NFPA 855, Standard for the Installation of Stationary Energy Storage Systems – 2023 Edition. URL: https://www.nfpa.org/codes-and-standards/8/5/5/nfpa-855 (accessed 25 July 2024)
UL 9540A, Testing the fire safety hazards associated with propagating thermal runaway within battery systems
National Fire Chiefs Council, Grid Scale Battery Energy Storage System planning – Guidance for FRS, Version 1.0, November 2022
The pitfalls of certification dependency, Cairn Risk.
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Informed choices, safer outcomes – A process for off-the-shelf product selection, Cairn Risk.