| Abstract | This report summarizes the literature review examining the fire safety of enclosed vehicular facilities (EVF) for battery electric buses (BEBs), focusing on key aspects such as BEB fire size, potential for reignition and explosion, toxic emissions and environmental impacts and challenging suppression. The report also discusses the fire protection systems for EVFs, in particular: fire detection, suppression, ventilation and smoke control, separation distances and fire barriers, quarantine zones and first responders’ challenges. Due to the limited research on BEB fire safety, previous studies involving passenger electric vehicles (EVs), internal combustion engine vehicles (ICEVs), and conventional internal combustion engine buses (ICEBs) are also reviewed to draw insights and establish background knowledge.
This review identifies significant knowledge gaps regarding BEB fire safety. Full-scale fire test data for BEBs are scarce, limiting the understanding of peak heat release rates (PHRR) and associated hazards unique to BEBs, since BEBs are larger than passenger EVs. While some modeling research extrapolates similar behaviors observed in passenger EVs and ICEVs to BEBs, scalability assumptions lack experimental validation, particularly for high-performance EVs (>100 kWh) and heavy-duty EVs (up to 1000 kWh).
Early battery fault and thermal runaway detection systems, including the role of battery management systems (BMS) and advanced techniques such as electrochemical impedance spectroscopy and machine learning, are discussed, but building-level fire detection and suppression systems remain relatively underexplored.
This review examines the effectiveness of various fire suppressants and suppression systems in addressing the fire protection challenges in EVF, highlighting critical gaps in research and practice. Water remains the most practical and widely used suppressant for EV fires, with enhanced flow rate density requirements for parking structures now reflected in the updated NFPA 13 [1] provisions for modern vehicles. Other suppressants, including gaseous agents (e.g., C6F12O, HFC-227ea) and solid agents (e.g., ABC powder), show limited efficacy in preventing reignition or thermal runaway, often requiring confined spaces and careful consideration due to potential chemical by-products like hydrogen fluoride (HF). The review finds that further research is needed to explore innovative suppressants and effective suppression systems since the conventional ceiling-mounted sprinkler systems are reported to be inefficient at suppressing deep-seated battery fires in EVs without continuously providing a high volume of water.
In response to evolving risks, new active and passive fire protection requirements from the NFPA and local jurisdictions (e.g., San Francisco Fire Department) mandate higher water spray rates and extended spray durations, as well as separation barriers for areas with EV charging stations (either sprinklered or not); however, many questions remain regarding the heightened fire hazards associated with BEBs in parking structures and EVFs. In this regard, active research is ongoing in various research institutions around the world including SFPE, NFPA and UL. Gaining a deeper understanding of the hazards and developing improved mitigation technologies are essential for developing robust safety standards, enhancing fire mitigation strategies and facilitating the broader adoption of BEBs in various operational settings.
Advancing research in these areas will improve understanding of the unique fire safety challenges associated with BEBs and support the development of effective mitigation strategies, thereby promoting the safer deployment and integration of electric buses. |
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