Electric buses are becoming increasingly popular as cities look for ways to reduce their carbon footprint and improve air quality. They are quieter, energy-efficient, produce no emissions at the point of use and have lower operating costs over their lifetime, compared to traditional diesel-fueled buses. These buses are typically powered by lithium-ion (Li-ion) batteries, which offer several advantages such as high energy density and fast charging. However, as with other technology, there are also risks involved with their use.
In recent years there have been several high-profile incidents involving Li-ion battery fires in electric buses, highlighting the need for improved safety measures and risk-management strategies. The risk of thermal runaway, a chain reaction that can occur when a single Li-ion battery cell is subjected to mechanical, electrical or thermal abuse, is especially high in electric buses where large numbers of Li-ion batteries are used in a relatively small enclosure. The excessive vibrations from normal bus operation are an additional stress factor for the Li-ion batteries, contributing to a potential short-circuit, heating up and release of gases that can lead to a fire or explosion.
During a thermal runaway event, the Li-ion battery’s electrolyte starts to decompose (~120°C), releasing a large volume of gases, which, due to pressure build-up, ruptures the metal or plastic pouch casing containing the electrolyte. These gasses normally burn (flash fire) at temperatures in excess of 800°C when in contact with the atmosphere (O2) producing a toxic, flammable or explosive mixture depending on the type of electrolyte used in the cell. Some examples of these gasses are hydrogen (H₂), carbon monoxide (CO), hydrogen fluoride (HF), hydrochloric acid (HCI), hydrogen cyanide (HCN), sulphur dioxide (SO2) and hydrogen sulphide (H2S). Furthermore, the excessive loss of lithium ions from the cathode material leads to a structural collapse of the cathode electrode, causing the release of oxygen (O2).
Main challenges of Li-ion battery fires
Heat output – In the scenario where the Li-ion batteries have entered an advanced thermal propagation stage, the battery generates its own heat, making it difficult to cool down.
Oxygen supply – The production of oxygen during cathode and electrolyte decomposition supports the chemical processes that occur during a fire.
Battery pack – Li-ion battery cells are densely stored in their packs making it hard for a fire suppression agent to reach the fire.
How FirePro suppresses Li-Ion battery fires
FirePro Condensed Aerosol technology suppresses fire by interrupting the chemical chain reactions that occur in the flame, rather than by cooling or depleting oxygen in the enclosure. Upon activation, the condensed aerosol-forming compound transforms from a solid state into a rapidly expanding two-phase fire-suppression agent consisting of potassium carbonate solid particles K2CO3 (the active part) suspended in a carrier gas. When the condensed aerosol reaches and reacts with the flame, the potassium radicals (K*) are formed mainly from the dissociation of K2CO3. The K*s bind to other flame free radicals (hydroxyls OH-) forming stable products such as KOH. KOH then further reacts in the presence of CO2 and forms stable K2CO3.In the event of a Li-ion battery fire, both the active agent K2CO3 and the intermediate product KOH react with the electrolyte’s decomposition products, such as hydrogen fluoride (HF), forming stable products such as potassium fluoride (KF) and potassium bifluoride (KHF2). Thus, preventing the formation of highly flammable gases such as hydrogen (H2). The resulting neutralizing action ultimately controls the fire and allows the temperature in the enclosure to drop below the threshold necessary (~120°C) for thermal runaway to sustain itself. Test results have shown that FirePro can protect the enclosure against re-ignition for as long as the minimum required fire-suppression density is maintained, allowing ample time for passengers to evacuate the bus and for emergency responders to begin post-fire management of the batteries.
FirePro solutions
FirePro cylindrical models are compact and provide a practical solution for applications with space limitations such as the electric bus battery compartments. They can be installed directly within the battery pack or within the battery compartment which houses the battery packs.
Each generator is an all-in-one system unit and is the equivalent of the agent storage tank, piping and nozzles of a pressurized gas system. They are activated automatically, either through electrical or mechanical means. A comprehensive fire-suppression system can be configured to include single or multiple generators, depending on the physical and technical parameters of the protected area, such as the class of fire, volume and height. In the case of a Li-ion battery hazard, the agent density used is the one proven to be effective in tests replicating real-life scenarios.
A wide range of control and monitoring equipment can be used with the generators. One of the simplest, most compact and innovative solutions developed by FirePro is the stand-alone Bulb Thermal Actuator (BTA). The BTA combines both detection and activation for a single FirePro condensed aerosol generator and operates without electricity. It can be easily installed, and its maintenance requirements are limited to visual inspection.
In case of a fire, once the temperature in the enclosure reaches the pre-selected detection rating (57°C, 68°C, 79°C, 93°C, 141°C, 182°C), the bulb bursts and mechanically activates the FirePro generator.
Alternatively, when multiple-generator systems are required, these activate electrically through a control module, selected based on the project requirements. Some of the main considerations when choosing a control module include the type of detection technology used (e.g. thermal sensors, smoke detectors), the activation method (e.g. automatic or manual), and the level of integration with other systems (e.g. vehicle control, alarm systems). The selection of the right control module and detection technology are part of the engineering of the system and are fundamental in meeting the expectations of the project’s fire strategy.
Key advantages of FirePro systems
- Efficiency and effectiveness for Li-Ion battery fires – FirePro is working closely with accredited laboratories and prominent Li-Ion battery manufacturers, performing nearly 100 tests over the past seven years, successfully proving its technology’s efficiency and effectiveness in suppressing and neutralizing Li-Ion battery fires.
- Space and weight saving – FirePro systems do not significantly affect the bus load capacity. They can be installed directly within the battery pack or the battery compartment.
- Easy installation and maintenance – FirePro systems can be easily installed in new or retrofit projects as the technology does not require a complex network of piping, nozzles or pressurized gas cylinders to operate. By extension they require minimum maintenance over their 15-year certified product life.
- Environmentally friendly – FirePro Condensed Aerosol is an environmentally friendly fire-suppression agent with a zero Global Warming Potential (GWP), zero Ozone Depletion Potential (ODP) and a negligible Atmospheric Lifetime (ALT).
Other benefits of FirePro systems:
– Certified, Listed and Approved Globally
– Non-pressurized
– Can tolerate small openings in the enclosure
– Operating temperature from -54°C to +100°C
– EPA approved for Normally Occupied Areas
Fire safety is of utmost importance when it comes to public transportation, especially buses. The potential for catastrophic fire and explosion consequences, threat of injury and loss of property is high, and as such, fire-suppression systems are crucial to protect passengers and minimize damage. FirePro, with its innovative solutions and partnerships with bus and chassis manufacturers, is making great strides in this area, ensuring the safety of passengers and drivers on the road.
About the Author
Pedro Mazzaro
Pedro Mazzaro is a Business Development Manager at FirePro in Brazil. He is a senior professional with extensive experience in the fire protection industry. Pedro's background is in mechanical engineering, having studied this discipline before earning his MBA.
