# Hydrogen vs. Batteries: Comparative Safety Assessments for a High-Speed Passenger Ferry
## 2 Background  
Hydrogen is an abundant, non-toxic, zero-emission fuel from well to wake if it is produced by electrolysis, but with a highly flammable and explosive nature. It is more gravimetrically and volumetrically efficient to be stored in liquefied form (LH\({}_{2}\)) at \(-253\) degrees Celsius, compared to high-pressure compressed gas. The fuel parameters of liquefied hydrogen are presented in Table 1. Current experience with liquefied hydrogen is limited in the marine industry, and regulations are still under development. Regarding the hydrogen systems' applications on ships, [PERSON] and [PERSON] [11] presented a thermodynamic analysis for an integrated solid oxide fuel cell system onboard a liquefied hydrogen-fuelled ship to assess the overall energy and exergy efficiencies. [PERSON] et al. [12] presented a model-based dynamic analysis onboard a zero-emission ship focusing on the coupling of PEM fuel cells and metal hybrids (MH). MHs provided promising results in terms of fuel storage and supply of hydrogen to the fuel cells. [PERSON] et al. [13] investigated the environmental impact for auxiliary powered systems of a hydrogen fuel cell-powered chemical tanker until 2050, using the reference energy system concept. Diesel \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Storage temperature (\({}^{\circ}\)C) & \(-253\) \\ Storage pressure (bar) & 1 \\ Gas constant (J/kgK) & 4124 \\ Volumetric energy density (GJ/m\({}^{3}\)) & 8.5 \\ Autoignition temperature (\({}^{\circ}\)C) & 585 \\ Minimum ignition energy (mJ) & 0.019 \\ Energy density LHV (MJ/kg) & 119.96 \\ Flammability range in air (\%) & 4–75 \\ Explosive range in air (\%) & 18–59 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Liquefied hydrogen properties. generators were replaced with fuel cells in 2030, and this resulted in a reduction of around 7000 tonnes of CO\({}_{2}\) compared to 2029. Another alternative option to reduce gaseous emissions is by utilising batteries. They are energy storage systems but with high fire risk, which can negatively affect their applicability. Regarding the battery systems' applications on ships, [PERSON] et al. [14] presented a life cycle analysis, with a focus on environmental footprint, and cost assessment of a battery ferry, and compared the results with a conventional ferry. It was demonstrated that when grid mix electricity was supplied in 2019, a 30% reduction of GHG emissions could be achieved, along with a 15% reduction of lifecycle costs when battery-powered systems are used. [PERSON] et al. [15] studied the conversion of conventional offshore support vessels to hybrid by retrofitting of batteries, with the main focus on environmental and economic aspects. The results demonstrated a 40-45% reduction of annual global warming potential in Arctic regions when batteries and ICEs are combined. However, a significant payback period of 12.5 years was obtained, indicating that for existing old vessels, hybridisation with batteries will not be beneficial. [PERSON] et al. [16] discussed the various data-driven states of health modelling approaches that can be used to estimate the available energy stored in marine battery systems on the basis of sensor data during the operational phase. This is a crucial parameter to avoid loss of propulsion, which can lead to collision or grounding. Currently, in the literature, there are a few safety assessments performed for hydrogen-powered ships. [PERSON] et al. [17] used ANSYS Fluent software to analyse the overpressure and high-temperature damages induced by an explosion due to hydrogen leakage and ignition in the fuel cell room, control room, and passenger area. An explosion in the fuel cell room exhibited the greater brisance, imposing the most severe damages in ship structure. [PERSON] et al. [18] assessed the effect of fine water mist on suppressing jet fires around the hydrogen storage tank at the upper deck of a passenger ship.