Incidents involving lithium-ion batteries have hit the headlines in recent years – think spontaneously exploding mobile phones and laptops on planes and electric vehicle fires after an impact or crash. Adding to this volatile mix is a relatively new technology, Battery Energy Storage Systems (‘BESS’), an asset key to the renewable energy transition, with all forecasts pointing to exponential growth. BESS fires have already been reported worldwide, from South Korea to the US to the UK, with fire regulations and standards seeing numerous revisions to keep pace with a rapidly evolving industry.
Although an energy asset, Battery Energy Storage Systems are not the preserve of traditional power and utility companies accustomed to dealing with the specialised operational demands. BESS developers and end-use customers are as likely to be financial investors, property developers, industrial parks, factories or councils with limited understanding of the inherent risks and dangers.
Furthermore, as BESS is a relatively nascent industry, many firefighters and other emergency services have little or no experience of this type of hazard, which presents risk of fire, explosion, high voltage and fume toxicity, with the use of water potentially perpetuating the battery fire by additional cell shorting. It also goes without saying that water damage and fire water run-off could result in the total loss of assets, which at scale have a capital cost in the millions with attendant insurance issues. As such, it is critical to work with BESS owners, contractors, integrators and other stakeholders at the initial design stage to fully understand all aspects of fire risk and associated hazards specific to the site.
Regulations and standards
UL is the underlying standard on which many international and national organisations base their regulations and fire codes. In addition, UL 9540A was drawn up in November 2017 to specifically address ‘Thermal Runaway Fire Propagation in Battery Energy Storage Systems’. Three further iterations of the standard have been published in the intervening period and the regulatory environment is unlikely to stand still.
Furthermore, more recently the National Fire Protection Association of the US published its own standard for the ‘Installation of Stationary Energy Storage Systems’, NFPA 855, which specifically references UL 9540A. The International Fire Code (IFC) has also published more robust ESS safety requirements in its most recent 2021 edition.
That being said, in the UK there are no laws or mandatory regulations governing BESS fire protection. For many BESS projects, the driving force behind implementation of fire-protection measures that adhere to a recognised standard is likely to come from insurers, who on the whole prefer a ‘belts-and-braces’ approach.
Lithium-ion batteries: the risks
The most dominant battery type installed in a BESS is lithium-ion, which brings with it particular fire risks including ‘thermal runaway’. Thermal runaway is a self-perpetuating chain reaction in which excessive heat keeps creating more heat, potentially spreading from one battery cell to the next and causing widespread damage. During thermal runaway, oxygen is believed to be self-generated during cathode consumption, plus there are multiple internal sources of fuel in a lithium-ion battery (metals, plastic, electrical, flammable gases and liquids).
Also, lithium-ion battery fires are ‘deep-seated’ in nature, as the materials involved in the ignition and propagation of the fire are tightly integrated into a cell, making firefighting a challenge. To add to this equation, lithium-ion battery fires are at risk of ‘re-flash’, hours or even days later having seemingly been controlled and extinguished. Lithium-ion batteries must be handled with care, in transport and during installation, as they are sensitive to mechanical damage (such as crush or puncture) and electrical surges, which can result in short circuits leading to internal battery heating, battery explosions and fires.
Furthermore, battery management control systems can be faulty or fail, leading to an inability to monitor the operating environment, such as temperature or cell voltage, with the potential for overcharging.
Understanding lithium-ion battery failure
In terms of timeline there are four main phases of lithium-ion battery failure: initial battery ‘abuse’, the cause of cell damage being thermal, electrical or mechanical, followed by so-called ‘off-gassing’, in which minute quantities of gas (for example, hydrogen) and other cell vapours are generated, resulting in heat release. If the battery temperature continues to increase, the next phase is a ‘smoke’ condition with the level of heat likely to result in ignition and thermal runaway. Catastrophic failure is imminent, ultimately resulting in a ‘fire’ with the potential for propagation and even an explosive event.
Given our understanding of lithium-ion battery failure, there are two main windows of opportunity to implement fire-protection measures – a ‘prevention’ window and a ‘containment’ window. Off-gas generation in a lithium-ion battery should be considered as the trigger to take action to prevent thermal runaway. Results from independent testing suggest an average of 11–12 minutes between detection of off-gas and thermal runaway. However, if preventative measures are unsuccessful and a damaged lithium-ion battery ignites, measures must be put in place to contain the resulting fire and minimise the potential for propagation to other battery cells.
A highly sensitive monitoring and detection system such as Li-on Tamer is the ideal prevention solution. Li-on Tamer is designed specifically to detect the very beginnings of off-gassing in a faulty lithium-ion battery of all chemistries, with an ultra-rapid response time to provide an early warning to BESS system controls. Conventional gas detection devices are not sensitive enough or honed to this environment to create the speed of response needed in such a dynamic and critical location. The primary course of action is to send a signal to the battery management system to shut off power to batteries, with the aim of preventing any further increase in battery cell temperature; that is, lower than the point of thermal runaway. Also ventilation activation to remove flammable gas accumulation, if required.
UL 9540A recognizes and quantifies off-gas events as precursors to thermal runaway, while independent testing by DNV-GL has concluded that Li-on Tamer can prevent thermal runaway after a two-year battery failure testing program.
In the event of off-gassing, there is no guarantee a BESS battery management system will shut down power to a battery in time or that a damaged battery cell will not continue to increase in temperature to the point of thermal runaway. If it does and you end up past prevention point, you’re then in containment mode. This phase employs further automated systems, which take the form of active fire suppression. These elements contain agents such as condensed aerosol or chemical gases. Nobel recommends Stat-X, a condensed aerosol system, which is now the fire suppression system of choice of several lithium-ion battery OEMs and leading global BESS integrators, having undergone rigorous private and commercial testing in line with UL and NFPA standards.
DNV-GL testing has concluded that Stat-X can put out a lithium-ion battery fire, that residual Stat-X airborne aerosol in the hazard will provide additional extended protection against a re-flash of the fire, and that Stat-X can reduce oxygen in an enclosed environment during a battery fire.
Notwithstanding the implementation of best-in-class prevention and containment measures, the very nature of lithium-ion batteries means there is a certain element of randomness to how any given battery cell (or cells) reacts once damaged, be it the nature or extent of off-gassing, temperature increase, fire condition, or propagation from cell to cell. There is also the potential for explosion if left unchecked. As previously mentioned, lithium-ion battery fires are at risk of ‘re-flash’, minutes, hours or even days later having seemingly been put out. As such, Nobel recommends a back-up Cooling option, specifically a watermist system with deluge misting nozzles located internally within the BESS. The system can be linked directly to a water supply such as a dedicated tank, alternatively a fire brigade pumping-in breech can be installed externally on the BESS container.
BESS assets can be found at all scales, from in-cabinet to container to in-building. In addition to the principal prevention, containment and cooling measures outlined above, there is a suite of additional solutions to consider in monitoring, protecting and managing BESS fire risk, including control panel technology; other detection (heat, smoke, gas, etc); ventilation control; battery separation and containment; interface with customer house alarm and other systems; emergency procedures, including warning signs, sounders and manual release facility; communication with local fire brigades and other community stakeholders; maintenance, servicing and ongoing customer support; and installation protocol.
Addendum – other types of storage involving lithium-ion batteries
A powered-up BESS linked to a renewable energy asset (such as a wind turbine or solar array) or connected directly to the grid might represent the most challenging fire risk involving lithium-ion batteries; however, as part of the energy transition there is a general trend towards electrification, either directly or as back-up power. And this means lithium-ion batteries are being ‘stored’ in multiple industrial, commercial and even residential settings.
Examples include electric vehicle (‘EV’) manufacturing facilities, robotic assembly, mobile plant and the manufacture of many portable appliances such as laptops, tablets, mobile phones, medical devices, power tools, vacuum cleaners, lawnmowers and many more. In such cases, lithium-ion batteries are stored in varying degrees of charge simply as stock, on the assembly line or as they are being transported. At any point, there is the potential for a lithium-ion battery to fail and begin to off-gas.
In residential settings an electric or hybrid vehicle represents a potential fire risk, particularly if there is charging infrastructure on the drive. The cost of charge points has been driven down to the point of being very affordable, particularly with government subsidies. It is also worth mentioning there are now domestic BESS systems, typically linked to solar panels on a roof, such as Tesla’s ‘Powerwall’. This brings the threat of thermal runaway into the home and the use of such appliances is set for mass adoption over the coming years.
Nobel has installed fire suppression for a number of non-BESS projects involving lithium-ion batteries. For example, an EV manufacturing facility in the UK has a system in place to detect faulty lithium-ion batteries that have the potential for a fire condition, resulting in robots removing such batteries from the assembly line into a steel ‘quarantine’ bunker – rather than let the batteries burn out in the case of fire, the bunker is equipped with a Stat-X fire-suppression system.
Nobel has also installed Stat-X units in charging pods for an electric scooter rental company, the pods being located at various strategic locations within an urban area. The pods contain lithium-ion batteries in the process of being charged. When the charge on a scooter gets low, the user is directed towards the nearest pod where they can swap out the battery. Again, this brings the threat of lithium-ion battery fires to the public at large.
The implications of a BESS fire and explosion are likely to be the most profound for firefighters. Significant reference cases include incidents in Liverpool in the UK and Arizona in the US, the latter involving catastrophic injuries to firefighters. Lessons learned from these incidents from a ‘first responder’ perspective will be the subject of a future article.
For more information, go to www.nobel-fire-systems.com