# Hydrogen vs. Batteries: Comparative Safety Assessments for a High-Speed Passenger Ferry
## 5 Gas and Smoke Dispersion Analyses
### Hydrogen Version--Gas Dispersion Analysis  
The most severe hazard according to HAZID results is the leakage in the fuel cell room due to piping damage, which was analysed with a quantitative approach in this section using CFD software called PyroSim, provided by Thunderhead Engineering. These simulations aimed to gain insight and understanding of the hydrogen dispersion in cases of potential leakage due to pipe damage under certain vent and sensor arrangements in the room. Leakage probability was not assessed in this study. A mesh size of 7920 cells was used to perform the simulation in the fuel cell room. It was assumed for the analysis that the leakage due to the piping damage was at the inlet of the bottom stacks, which was 0.2 m above ground, but for simplicity of the simulations, the leakage position was assumed at a ground level. Fuel cell stacks are depicted with grey colour in Figure 5. Each of the six stacks provided a net power of 100 kW. This modular arrangement provides easy access and sufficient space for repairs and maintenance. There were three natural (inlet) vents, depicted with light blue colour in Figure 5a, for the supply of air in the room from the lower part as it is required by regulations for gases lighter than air. These are ducts that were routed upwards to the upper deck, and not just holes (openings), to reduce the possibility of flooding under excessive trim by stern since the fuel cell room was at the aft of the main deck. There were also four mechanically driven exhaust air fans at the ceiling of the room (Figure 5b), consi  
Figure 5: Vent and sensor arrangement in the fuel cell room: (**a**) inlet vents; (**b**) exhaust vents and hydrogen sensors in the ceiling. nature of hydrogen gas. Backflow of gas from the vent outlet to the vent inlet should be prevented by providing sufficient distance between them. For an ESD, protected machinery space at least 30 ACH ventilation rate is required [31]. There were also three sensors at the ceiling to detect hydrogen concentrations above 0.4% and trigger the automatic valves to stop the flow of hydrogen in the room within 1-2 s.  
The double block and bleed valves in the fuel cell room stop the supply of hydrogen in the fuel cells and bleed any remaining hydrogen in the fuel pipes to the vent mast at the upper deck through vent pipes. Hydrogen supply in the room where leakage or ventilation failure has been detected should be stopped by the master-gas valves outside the fuel space enclosure. The hydrogen release rate (m\({}^{3}\)/s) was calculated on the basis of [39] and [40], as shown in Equation (2). \[\mathrm{Q_{gas}}=\frac{\mathrm{C_{d}}\times\mathrm{A}\times\sqrt{\rho\times \mathrm{P}\times\gamma\times\left(\frac{2}{\gamma+1}\right)^{\left(\frac{ \gamma+1}{\gamma-1}\right)}}}{\rho}\mathrm{m}^{3}/\mathrm{s} \tag{2}\]  
where Cd is the discharge coefficient for circular bore which is equal to 1 [17]. A is the hole area, which is \(\mathrm{\pi d^{2}/4}\), where d is the leakage diameter (meters). In these simulations, leakage diameters considered were 3, 16, and 33 mm. The ratio of specific heats (\(\gamma\)) was assumed to be constant and equal to 1.4  
The density (\(\rho\)) of hydrogen can be calculated as shown in Equation (3):  
\[\rho=\mathrm{P}/\mathrm{RT~{}kg/m^{3}} \tag{3}\]  
where P is the pressure at the inlet of the stack and was \(8\times 10^{5}\) N/m\({}^{2}\) and temperature (T) at the same position was 10 degrees Celsius so 283.15 K, according to the Powercell MS-100 data sheet [37]. The gas constant of hydrogen (R) was 4124 J/kgK, considering universal gas constant and hydrogen's molecular weight. On the basis of these data and Equation (3), the density was obtained: \(\rho=0.68\) kg/m\({}^{3}\). Only the area of leakage varied for the different simulations since the pressure and temperature were the same at the inlets of fuel cell stacks, regardless of position.