Revealing the contribution of flame spread to vertical thermal runaway propagation for energy storage systems

The rapidly growing energy storage systems necessitate more high-capacity lithium iron phosphate batteries but pose significant safety concerns. In multi-layer battery clusters, if thermal runaway propagation occurs between modules, particularly in the vertical direction, the ensuing fire spread can further result in the accelerated propagation of battery and even an irrevocable catastrophe. Clarifying the contribution of flame spread to vertical thermal runaway propagation is the goal of this investigation. The unexpected propagation characteristics between the upper and lower modules are explored. Further, the critical heat and the percentage of heat that leads to thermal runaway in the upper battery are determined using the equivalent replacement battery. The flame heat transfer is finally decoupled using the thermal radiation model. And the mechanism of vertical thermal runaway propagation induced flame between modules is analyzed. The findings reveal that the higher module's thermal runaway and venting sequence differs from the lower module's, suggesting that flame spread dominated the thermal runaway propagation paths. The critical triggering energy of the upper battery is 1193.6 kJ, comprising 279 kJ of conductive heat, 750 kJ of flame heat, and 164.6 kJ of self-generation heat, with the flame heat accounting for approximately 1.18 % of the total heat released from the battery fire. In contrast to horizontal thermal runaway propagation, where thermal conduction is predominant, the convection heat from battery fire serves as the main heat source for vertical propagation. The findings serve as a foundation for both emergency response to fire incidents and the safe design of battery modules in existing energy storage systems.