Electric Vehicles

This is a transcript of my presentation ‘Fire Risk of Electric Vehicles in Buildings’ at the FireNZ conference in Wellington, NZ, in October 2024.

Introduction

The number of electric vehicles (EV) has been rapidly increasing worldwide since their inception in 2010. Currently, there are more than 40 million electric-powered cars in the world. Nearly 20% of all car sales are now electric, and this is projected to reach 50% by 2030.

Electric cars are powered by a traction battery pack. Each battery pack consists of a series of battery modules, each module containing battery cells. Battery cells come in different shapes and sizes, with Tesla using the cylindrical type cell. For example, a Tesla Model S has over 7,000 cylindrical cells within a battery pack.

Lithium Ion Batteries

The lithium-ion battery consists of an anode and a cathode made of differing metallic compounds, along with an electrolyte solution and a porous separator.

The electrolyte is lithium salt dissolved in an organic solvent, which in liquid state is a Class 3 flammable liquid. The separator is polyethylene, which is a combustible plastic. Some battery packs also use glycol to cool the battery pack, which is a combustible liquid.

The combustible content of a battery does not present a hazard until the battery is abused. There are three main types of battery abuse:

• Thermal Abuse – Ambient temperature is outside the normal operating battery temperature
• Electrical Abuse – Battery is either over-charged or is short circuited internally
• Mechanical Abuse – Battery is physically damaged due to impact or excessive force.

Any abuse of the battery can cause abnormal chemical reactions to occur within the cell, causing internal heating that leads to a phenomenon called ‘thermal runaway’. Thermal runaway is a chain reaction of self-heating within the cell that leads to a rapid rise in internal cell temperature. This heating quickly transfers to the other cells, which cannot be stopped until the cells are cooled back to normal operating temperature.

Thermal runaway causes the electrolyte to decompose into a gas. The gas builds up pressure within the cell, causing the safety valve to open. The flammable gas mixes with air, creating a vapor cloud that can ignite into a fire.

Hazard Assessment

The likelihood of electric vehicle fires is currently very low. According to EV FireSafe only a total of 511 fires have occurred since 2010. While this presents a very low probability, the number of fires is expected to increase as EV’s get older and more mechanical wear and number of charge cycles.

The consequence of car park fires has been known to be significant. Within the last 10 years, there have been several serious car park fires that have caused significant strucutural damage and harm to occupants:

• Stavanger Airport Sola, Norway, 2020 – Involved 300 cars, causing partial structural collapse.
• King’s Dock Car Park Liverpool 2017 – A total of 1,150 cars lost. Structure to be demolished.
• Apartment Car Park, South Korea, 2024 – 20 people hospitalized due to smoke inhalation.

Electric vehicles have two main hazards: vapor clouds and jet fires.

• Vapor cloud is the off-gassing of the battery electrolyte in thermal runaway, which produces a very toxic gas. The most toxic gas compound is hydrogen fluoride (HF), which is 20 times more toxic than carbon monoxide and is fatal at low doses. The vapor cloud also has metal particulates such as Li, Mg, Co, and Ni that are also very toxic.
• Jet fire is electrolyte vapour burning at a high velocity from the battery compartment. These jet fires have been measure at temperatures of 1400°C and velocities of up to 200 m/s. This is a very hot fierce fire that will instantly burn any surrounding combustibles, people, or structures. Jet fires can be up to 2.5m long projecting horizontally from underneath the vehicle. This is a hazard in a car parking building where cars are parked close together.

Once the electric vehicle is alight, the fire growth behavior and fire size have been shown to be similar to internal combustion vehicle fires (ICEV). However, this research has highlighted that the fire severity of all vehicles has increased in the last 10 years as follows:

• Fire growth rates – The growth rate for all vehicles has been increasing since the introduction of foam plastics. Cars originally had a slow growth rate; however, new vehicles have been shown to have fast to medium growth rates. It is recommended to design for fast fire growth.
• Fire load – Vehicles are increasing in size and weight, which leads to a larger fire load. The fire load of a passenger vehicle has increased from 7 GJ in 2013 to 10 GJ in 2023. This equates to 500 MJ/m² within car parking buildings, which has increased from 270 MJ/m² in 2015.
• Heat of Combustion – The increased use of plastics causes an increase in the heat of combustion to 30 MJ/m², which is above the standard value of 20 MJ/m² used in car park design.

Fire Fighting

One of the main challenges with electric vehicle fires is firefighting. There are several design features that present difficulty in applying water to a battery compartment of an electric vehicle:

• Battery Packs are typically located underneath the vehicle, restricting access for applying water to the fire source without upending the vehicle.
• Battery Armour protects against mechanical abuse and water damage, making it difficult to apply water within the compartment.
• Thermal Runaway can only be controlled once the whole battery is cooled to below its normal operating temperature. A single EV fire can use up to 10,000 liters of water and take up to an hour to extinguish.
• Flare-Ups can occur where cells have not been completely cooled or damaged cells are still holding an electrical charge that can cause re-ignition.

Charging Stations

There are four different modes of charging stations that range from Mode 1, which is a simple household plug for overnight charging, through to Mode 4, high-speed DC charging stations. Capacities range from 7 kWh for Mode 1 to 50 kWh for Mode 4.

There is no direct link between charging stations and the number of fires. However, there is an inherent risk with energized equipment within a car parking building, so a lower mode of station would be advised. All batteries have an internal resistance which produces heat while charging. If the battery is damaged, this resistance will increase and raise the risk of fire while the vehicle is charging.

Current Guidance

The current guidance from the UK is mainly from insurers and loss adjusters on the charging of electric vehicles in buildings. These requirements are overly conservative and don’t appear to be based on research or risk assessment. Requirements include 2-hour fire cells, charging stations within separate fire cells, sprinklers and smoke control, separation distances of 10 m from charging stations, and increased spacing between vehicles.

Guidance from Australia and New Zealand is very different and seeks to reduce the electrical risk by compliance with AS/NZS 3000, master isolation switches, using smart charging, RCD protection, collision protection, placards, and signage.

Risk Assessment

Risk is a combination of likelihood and consequence. The likelihood of an electric car fire is currently very low based on the number of fires compared to the number of cars worldwide. The consequence of an electric car fire in a car parking building is considered severe due to the hazards of toxic vapor clouds and battery jet fires.

The level of fire risk of electric vehicles in buildings is therefore considered to be Medium Risk. This means the risk is neither high nor acceptable. The risk needs to be treated in a measured way.

Risk Treatment

The following needs to be considered for all areas where vehicles are parked within a building:

• Detection System – Early warning of fire in a car park is important with the potential of jet fires and vapor clouds. This allows early warning of occupants in the remainder of the building and notification of Fire Services.
• Sprinkler Systems – Sprinklers not only control fire spread but also dilute the vapor cloud of toxic gases. Sprinklers allow water to penetrate the battery compartment to control thermal runaway.
• Smoke Control – Removing smoke from a car park has higher importance with the risk of vapor clouds. Natural and mechanical ventilation should be considered.
• Compartmentation – Higher fire ratings needs to be considered due to the increase in fire growth rates, fuel loads, and calorific values.
• Exit Points – Reducing travel distance is essential with the risk of vapour clouds that occupants can quickly evacuate, and also enables Fire Services to have ready access to the source of the fire if the fire involves an EV.

Conclusion

Electric vehicles have been rapidly increasing in number throughout the world without consideration of fire risk in car parking buildings. The combustible electrolyte of an Li-Ion battery powered vehicles has potential to produce toxic vapour cloud and jet fires when in thermal runaway. This presents a hazard to both occupants Fire Services due to the toxic gases and difficulty to extinguish. Vehicles in general are showing an increased trend in fire growth rates and fuel loads. Some significant car park building fires have occured in UK, Europe and Korea.

There is currently no consensus on the protection of electric vehicles in buildings with UK insurers applying very strict requirements unlike Australia and NZ that have no additional fire safety requirements.

The number of EV fires is currently very low, although this could increase as the cars age. The current risk level is low likelihood and high consequence. The hazard of EV’s in buildings needs to be addressed without being overly onerous on the level of protection. Additional fire protection to be considered includes detection, sprinklers, smoke control, and exit/access points.

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