The UK’s electric bus fleet is set to become the largest in Europe by 2024, with numbers projected to grow by almost 180%.
Risks associated with fire safety
The primary technology fuelling our electric buses is the lithium-ion (Li-ion) battery, and, although much more sustainable, these batteries are bringing about fresh fire safety risks to the transport sector. If they become damaged in any way – through overcharging, mechanical failure, physical impact, or overvoltage, for example – the fire safety consequences can be severe.
Traditional suppression systems, designed for combustion engines, will only go so far in effectively preventing fire risks for electric buses and coaches. James Mountain, sales and marketing director, Fire Shield Systems, discusses the unique fire risks associated with EV bus fleets, and explores the practical steps managers, operators and OEMs can take to mitigate these.
The rising risk
Major cities like London, Coventry, and Oxford are pioneering the way for electric bus adoption. However, many cities across the UK are setting out clear measures through low-emission zones to improve public health and reduce air pollution, which is impacting the use of HGVs, buses, taxis, and private cars.
The ZEBRA scheme is making up to £120 million available to cities, which will deliver up to 500 zero-emission buses
The Zero-Emission Buses Regional Area (ZEBRA) scheme is making up to £120 million available to cities, which will deliver up to 500 zero-emission buses. This supports the UK government’s wider pledge to introduce 4,000 zero-emission buses.
As the number of EV buses across the UK – supported by these government-backed schemes – is set to continue to grow exponentially over the next few years, we must be thinking about the implications of these vehicles on traditional fire safety measures.
Li-ion batteries: A unique challenge
Most commonly powered by li-ion batteries, EV buses and coaches are largely considered as a safe and sustainable alternative to traditional combustion engine vehicles. However, Li-ion batteries carry their safety risks, which – if not addressed carefully – can lead to much more severe risks for bus fleets and passengers.
If these batteries are exposed to high temperatures, overcharging, and mechanical failure or subject to physical impact, they can be caused by to internally short circuit. In turn, the battery then produces excess heat, triggering a chemical reaction within its cells. This is referred to as ‘thermal runaway’, a state where excess heat in a battery produces more heat, leading to ignition, carbon monoxide, and other toxic gas emissions, and, in some scenarios, large explosions.
Electric battery fires
Although electric battery fires are much less common, when they do occur, they can be highly dangerous
When in thermal runaway, a battery can produce its source of oxygen from within its cells. This propels flames and makes traditional fire suppression methods less effective.
Although electric battery fires are much less common than traditional combustion engine fires, when they do occur, they can be highly dangerous. For example, there have been several fires in Germany and China this year, which have led to multiple electric buses being destroyed while charging.
Mitigating the risk
OEMs and bus operators can take several practical steps to manage the fire risks associated with electric fleets. These include:
1) Charging
Leaving buses to charge overnight brings about significant fire risks, as small battery components are building up and storing large amounts of energy. To minimize risk, OEMs and bus operators should put measures in place to monitor charging stations at all times while in use, and ensure fully-charged buses are immediately disconnected from charging points.
2) Storage
Additional risk assessments are also necessary for EV bus storage and parking locations. This includes evaluating proximity to other vehicles and combustible materials, increasing space where possible.
As thermal runaway can sometimes take a period to take hold from the initial battery damage, impact from the operating day can sometimes only initiate thermal runaway as the bus is stored overnight. As such, parking and storage areas should be monitored continuously to address and mitigate any risks as soon as they arise.
3) Suppression
As they pose unique fire risks, traditional fire suppression systems and techniques will only go so far as to prevent li-ion battery fires. Therefore effective protection requires a unique solution…
Need for a new suppression solution
The main goal for an electric bus suppression system should be to prevent thermal runaway
The main goal for an electric bus suppression system should be to prevent thermal runaway. In some situations, where this isn’t possible, the system should act to delay propagation, allowing ample time for passengers and drivers to evacuate, and the risk to be safely contained.
Following extensive testing and research, Dafo Vehicle Fire Protection and RISE (Research Institute of Sweden), as part of an EU-funded initiative, have developed a new suppression solution that directly addresses the unique fire risks associated with electric vehicles.
Early warning system offerings
While creating the new battery suppression system (Li-IonFireTM), the project partners explored all of the fire risks associated with battery spaces, including specific risks around charging, and processes for handling EVs and their batteries after impact, such as a crash.
This research revealed how, even with late deployment, the system can delay a battery from reaching thermal runaway, making the possibility of safe evacuation very high. The new suppression solution offers an early warning system, aided by spot cooling, to prevent thermal runaway from occurring, while containing and suppressing fire.
A safer future for the UK’s electric buses
As the adoption of electric buses and coaches grows throughout the UK and beyond, ensuring fire safety measures keep pace is crucial in protecting lives, property, infrastructure, vehicles, and other valuable assets.
Standards are slowly following change, but OEMs, vehicle maintenance teams, and operators all have a key role to play in ensuring the risks are managed effectively.
