Hydrodynamic Failure Modes and Mitigation Strategies in Battery Energy Storage Systems: A CFD Analysis of Active vs. Passive Ventilation

Thermal runaway in Battery Energy Storage Systems (BESS) poses a coupled hazard involving both the accumulation of flammable hydrogen gas and extreme pressure transients that can compromise enclosure integrity. In this work, hydrogen dispersion and pressure evolution within a representative BESS enclosure are examined using Large Eddy Simulation (LES) conducted in the Fire Dynamics Simulator (FDS). Three ventilation configurations are investigated: a fully sealed enclosure, an enclosure equipped with active mechanical exhaust only, and a hybrid mitigation strategy combining mechanical exhaust with a Pressure Relief Vent (PRV). Simulation results show that the sealed configuration rapidly exceeds the hydrogen Lower Flammability Limit, with volumetric concentrations surpassing 3.3%, thereby presenting a significant explosion risk. In contrast, the active exhaust–only case produces a severe imbalance in mass flow, leading to extreme negative pressurization. Internal pressures fall to −85.7 kPa, a level well beyond typical structural resistance thresholds and indicative of potential vacuum-induced structural collapse. The hybrid configuration, incorporating a 175 Pa PRV in conjunction with mechanical exhaust, effectively stabilizes enclosure pressure while promoting efficient gas removal. This arrangement maintains structural integrity and achieves a hydrogen removal efficiency of 99.74%. The results demonstrate that passive intake and pressure-relief pathways are a fundamental requirement for safe active ventilation in BESS enclosures. The study provides a validated computational basis for hydrogen mitigation design and offers quantitative guidance for the development of robust BESS safety standards.