# Hydrogen vs. Batteries: Comparative Safety Assessments for a High-Speed Passenger Ferry
## 6 Results and Discussion
### Hydrogen Version
#### 6.1.3 Scenario 3  
Full Pipe Rupture (33 mm) at the Inlet of Bottom Right Stack  
The hydrogen release rate (Q\({}_{\rm gas}\)) for a leakage of 33 mm was calculated on the basis of Equation (2): Q\({}_{\rm gas}\) = 0.635 m\({}^{3}\)/s.  
Full pipe rupture is the worst scenario, but it is also considered to be highly unlikely since pipes are double-walled, well-protected, away from the sides and of proven usage. Initially, a ventilation rate of 30 ACH was assumed. Leakage was detected at 1.3 s by the middle sensor (Figure 12a). Hydrogen concentration levels reached 15%, which was below the lower explosion limit (LEL) of 18% (Figure 12b-d). With 30 ACH, flammable masses (4-9%) were mostly concentrated near the ceiling, but they were not completely released outside, even after 40 s (Figure 12e). Hence, higher ventilation rates than 30 ACH are suggested for safety under the worst-case scenario of full pipe rupture.  
Figure 11: Leakage diameter 16 mm at the inlet of the bottom right stack (60 ACH).  
If 120 ACH ventilation rate is used (Figure 13), the total volume flow rate of air needs to be 0.64 m\({}^{3}\)/s. Hence, flow rate of each exhaust vent was set at 0.16 m\({}^{3}\)/s. With 120 ACH, flammable masses were released after 18 s, and at the end of the simulation (40 s), the room was completely emptied of hydrogen (100% air). Maximum concentrations remained at around 15% for the different ventilation rates considered, under full pipe rupture at the inlet of the bottom right stack. Unrealistically high ventilation rates (>200 ACH) would be required to avoid the flammability range completely during the entire simulation time for large leakages.  
If the flow of hydrogen was stopped 2 s after leakage was detected instead of 1 s as in all the previous scenarios, maximum concentrations reached 20%, which was within the explosion range (Figure 14). Flammable masses were released after 25 s, and thus 7 s later compared to the corresponding scenario where the leakage was stopped 1 s after being detected (Figure 13a). This indicates that the leakage must be stopped within 1 s after being detected.  
Figure 12: Full pipe rupture at the inlet of the bottom right stack with 30 ACH: (**a**) detection at 1.3 s; (**b**) concentrations at 2.8 s; (**c**) concentrations at 6 s; (**d**) concentrations at 28.1 s; (**e**) concentrations at 40 s.  
Figure 13: Full pipe rupture at the inlet of the bottom right stack with 120 ACH: (**a**) concentrations at 18 s; (**b**) concentrations at 40 s.  
A denser mesh with more cells included in the domain of the analysis resulted in higher accuracy. Hence, we attempted to increase the mesh from the original size of 7920 cells up to 16,000 cells to assess if the accuracy of simulations was enhanced. However, minor changes were observed in the maximum hydrogen concentrations and release time of flammable gases outside the fuel cell room. Hence, to save computational cost, without significant simulation time required to obtain the results, the mesh size remained unchanged (7920 cells). The cells of the computational domain are shown in Figure 5.