Detection Design Must Adapt to Challenges of Li-ion Batteries
Lithium-ion batteries are a hot topic in fire safety circles, bringing the spectre of savagely hot, difficult to extinguish fires into all kinds of public buildings and spaces. While the debate rages as to whether electric cars in particular are likely to be the cause of a blaze, their high thermal loads undoubtedly require adaptation in detection design and response planning, with regulations yet to catch up and provide a standard solution.
The extraordinary energy density of lithium-ion batteries means the technology has been adopted across many sectors: the largest such batteries sit in Battery Energy Storage Systems (BESS), where fire has been an issue, particularly due to the toxic smoke produced. However, even though typical BESS sites contain many large batteries side by side, the known dynamics of the situation mean that fire containment has been successfully designed and implemented at these locations. Fire safety planners dealing with more general building types and occupancies can learn some lessons from this approach, but they still face a headache when it comes to ubiquitous, mobile batteries. In our latest Securiton case study, we argue for a performance-based design (PBD) approach incorporating early and localized detection.
In principle, the spontaneous combustion of an electric vehicle without external influence is extremely rare – although there is evidence that damaged batteries can auto-ignite. However, if an electric car does catch fire, it poses a special challenge in fighting it.
In a car with an internal combustion engine it usually takes five to ten minutes for a fire to develop from a smouldering but detectable incident into a full blaze. With an electric vehicle, a ‘thermal runaway’ between the battery’s cells means the fire can spread explosively in just a few seconds. Temperatures of up to 800°C (1,472°F) are generated while toxic clouds of smoke are produced, which make extinguishing the fire even more difficult. There have been a number of highly publicised cases of EV fires which have spread and caused widespread damage in car parks and even on cargo vessels at sea.
Part of the fire detection solution for EVs lies in monitoring the batteries closely with the vehicle’s own systems, but this is a challenge for the manufacturer and lies beyond the scope of fire safety planners and detection system designers. Instead, they must rely on additional, independent protection for areas where EVs are likely to be present. For the purposes of fire prevention and detection, these areas can be divided into two types: Those designed exclusively for electric vehicles; and mixed-use areas where they may be present alongside other vehicles. Charging points are an example of the first category, as are depots for electric lorries and buses.
Charging points not only attract a cluster of electric cars, they often include a substantial lithium- ion battery themselves. This is particularly so for chargers that are installed to run directly off solar panels placed over the parking area, as is increasingly common in some countries.
Additionally, many bus and lorry fleets are going electric, especially in urban areas where electric offers distinct advantages without concerns for range. Depots, loading areas and charging infrastructure for these large EV fleets are a fire risk where considerable value of assets are concentrated. Should a fleet be lost to fire, considerable business interruption would follow.
In all these cases, elements of secure design would be desirable: essentially, vehicles should either be separated by a reasonable spacing, or by fire retardant dividers. Charging stations and depots are therefore a crossover area between mobile and static battery fire risk areas, but there are differences. For a start, adjacent buses can be moved should one vehicle in the fleet catch fire – therefore, firewalls need not be engineered to the exacting standards of a BESS site, provided that suitable detection and manpower is available for a staged response. On the other hand, it is not always possible to incorporate dividers or suitable spacing in multi-storey car parks where space is at a premium.
A further difference between EV-only areas and those used by internal combustion engine cars is the possibility of using aspirating smoke detectors (ASDs). In areas where no smoke should be present, ASDs can use their extraordinary sensitivity to provide very early warning of a smoldering element. ASD pipes can be run next to or even inside charging columns, and closely above charging areas or parking points at a depot. Unfortunately, exhaust fumes from internal combustion vehicles tend to trigger false alarms in smoke detectors. Although advanced ASD systems are adept at avoiding being triggered by pollutants such as dust or fog, exhaust fumes are far harder to filter out as they are essentially smoke.
For mixed areas such as underground car parks, therefore, linear heat detection methods provide the best combination of reliability and early warning of a developing fire. In particular, linear heat detectors with precise localisation are extremely valuable to situation management and firefighting because in a smoke-filled garage it can otherwise be very difficult to ascertain the source of the fire. Point detectors are an alternative but the maintenance and testing regime ultimately makes them a false economy, especially as linear heat cables are far more robust and long-lasting. The system should be designed so that each sensor covers a particular area of parking spaces that can clearly be identified from a remote command centre. With SecuriHeat d-LIST, for example, the manufacturer recommendation for underground and multi storey car parks is to use 3 m or 4 m sensor spacings. This allows a layout that maximises coverage from each device and cable while each detection point equates to a maximum of four parking spaces. SecuriHeat d-LIST offers the option of alarming both at a pre-set maximum temperature and/or a differential alarm with reference to a monitored normal temperature range. This combination offers both reliability and an earlier warning than relying on a single approach, especially as a cable with up to 100 sensors can be sensitive to small anomalies in any single sensor while filtering out changes in ambient temperature, even when these are rapid.
An early alarm can facilitate the fire services response as well as allowing time for egress before the situation develops; it means fires in internal combustion vehicles can be tackled promptly, reducing the chance of a spread to nearby EVs; and a suitable heat detection system can also be used to operate the smoke management system and actuate pre-action and co incidence (or interlock, double interlock) suppression systems.
Crucially, localization means that teams tackling a blaze know what they are up against. If the fire can be identified as involving a lithium-ion battery from an early stage as part of a staged response, suitable fire-fighting and containment equipment can arrive on the scene early enough to prevent the kind of structural damage which prolonged exposure to 800°C fire can wreak on even the most robust of buildings.
As more and more devices and vehicles incorporating sizeable Li-ion batteries circulate into general public spaces, the approaches learned from tackling the fire safety challenges at BESS sites, EV charging areas and car parks can be further adapted to ensure public safety and protect property. Invariably, gaining time to respond through early detection, and preparing that response meticulously, will be the key to successful fire safety planning.
For more information, visit www.securiton.com.
