CFD analysis of performance-based explosion protection design for battery energy storage systems (BESS)

Abstract

This study evaluates three explosion protection designs for a Battery Energy Storage System (BESS) unit as part of a Hazard Mitigation Analysis (HMA). This is done in accordance with the requirements for explosion protection in NFPA 855, Standard for the Installation of Stationary Energy Storage Systems. The BESS unit is a lithium-ion-based stationary energy storage system with nominal internal dimensions of 3.1 m (L) x 2.1 m (W) x 2.4 m (H) and a free air volume of 6.1 m³. It has four racks composed of eight modules each. Two commercially available cells—EVE and CATL—are used in the analysis to highlight the differences between cell compositions and the implications for explosion pressure and flame propagation. The analysis is performed using the FLACS (Flame Acceleration Simulator) computational fluid dynamics (CFD) tool developed by Gexcon.

The three designs considered are natural ventilation, combustible concentration reduction, and standard deflagration venting. For the natural ventilation method, the installed ventilation panel is designed to open at 60^o (to the horizontal plane) on activation by a gas sensor located in the BESS unit. The sensor triggers the ventilation panel actuator when the concentration of the released gas inside the unit has reached a pre-determined level. The analysis determines whether the natural ventilation provided by the vent opening is sufficient to maintain the gas concentration within the unit at or below 25 % of the lower flammability limit (LFL), thereby preventing an explosion in the unit. The combustible concentration reduction method is one of the standard methods of deflagration prevention for equipment handling combustible materials discussed in NFPA 69, Standard on Explosion Prevention System. NFPA 69 requires that the mechanical ventilation provided for the unit should be sufficient to maintain the gas concentration within it at or below 25 % of the LFL. The third and final design is standard deflagration venting as specified in NFPA 68, Standard Explosion Protection by Deflagration Venting. A single vent panel is provided to relieve explosion pressure in the unit. It is designed to activate at a static pressure (P_stat) of 0.05 bar-g. The analysis determines whether the vent size is adequate to safely vent the unit and prevent its structural failure in the event of a deflagration. Results of large-scale testing show that for typical BESS units, panels, fasteners, and other components may begin to fail at about 0.07–0.14 bar-g. Thus, this pressure range is used as the performance criterion for this analysis.

The results of this analysis show that the second design option (the combustible concentration reduction method) provides the best outcome for explosion protection of the BESS unit. The other designs provide modest degrees of pressure relief, depending on several factors. Consequently, these design approaches can be considered individually or combined as an innovative performance-based design approach to protect BESS installations.