## 4 Results and Discussion  
In the below results, D denotes diesel, MeOH is methanol, AES denotes all-electric ship, A-FC and A-ICE refer to the use of ammonia in the fuel cell and ICE, and H-FC denotes the use of hydrogen in the fuel cell, where hydrogen can be grey or green. Other fuel abbreviations are already explained with the first mention in the paper (LNG, LPG, DME, LBG, B100, B20). The LCA investigated life-cycle emissions related to ship power systems. The impact categories of climate change, human toxicity, and acidification were selected for analysis, and the results are presented in Figures 18-20. According to Figure 18, fossil fuels (diesel and low-carbon fuels) greatly contribute to climate change with a high amount of released GHGs. A major part of these emissions are emissions from TTW (a result of fuel combustion in marine engines). The LCA highlighted green hydrogen, B100, methanol, and electricity as potential alternative fuels whose use instead of diesel would reduce the carbon footprint of the investigated ship. The alternatives that resulted in the highest GHGs are LNG, ammonia (used in fuel cells and ICE) and grey hydrogen (used in a fuel cell). Figure 19: LCA comparison of different power systems (impact category: acidification). Figure 20: LCA comparison of different power systems (impact category: human toxicity). Regarding acidification (Figure 19), the diesel-powered ship is a major contributor due to the high sulphur content compared to investigated alternatives. Biodiesel-diesel blend (B20) and B100 have similar _AP_ values as the diesel-powered ship, while green hydrogen and LPG have the lowest impact. Similar results are presented in Figure 20, where life-cycle emissions are used to calculate the impact on human toxicity. Diesel, B100, and B20 are the greatest contributors, while green hydrogen and LPG result in the lowest emissions. To gain an insight into their cost-effectiveness, alternative fuels are investigated through LCCA, in which costs related to ship power systems are summed. Besides investment, fuel, and maintenance costs, the carbon tax is considered, taking the NZES from Figure 3. The LCCA results are compared in Figure 21.  
The diesel-powered ship represents a baseline scenario, in which 60% of total costs refers to fuel cost. The most cost-efficient alternatives to replace diesel are DME, methanol, and ammonia used in dual-fuel engines. The most expensive alternatives are powering options that offer zero-emission shipping. High fuel costs and high costs related to the purchase of the main element of the system are the reasons why these alternative solutions are not currently profitable. The LCA and LCCA comparison highlighted particular fuels as the most appropriate alternatives. They are presented in Table 6, where their reduction/increase in emissions and costs compared to the diesel-powered ship is indicated. \begin{table}
\begin{tabular}{c c c c c} \hline  & _GWP_ & _AP_ & _AFP_ & **Costs** \\ \hline H-FC-green & \(-\)88.4\% & \(-\)84.5\% & \(-\)90.5\% & +29.7 \\ \hline A-ICE & +27.1\% & \(-\)79.5\% & \(-\)80.1\% & \(-\)25.2\% \\ \hline DME & \(-\)2.3\% & \(-\)79.3\% & \(-\)79.3\% & \(-\)22.0\% \\ \hline AES & \(-\)33.1\% & \(-\)87.2\% & \(-\)92.0\% & +18.2\% \\ \hline B100 & \(-\)87.1\% & \(-\)0.3\% & \(-\)0.6\% & +2.8\% \\ \hline LPG & \(-\)24.4\% & \(-\)90.7\% & \(-\)90.7\% & \(-\)15.0\% \\ \hline MeOH & \(-\)23.3\% & \(-\)73.8\% & \(-\)73.4\% & \(-\)22.4\% \\ \hline \end{tabular}
\end{table} 
Table 6: Results of the replacement of the diesel-power system with highlighted alternatives. Figure 21: LCCA results. According to the results presented in Table 6, green hydrogen in a fuel cell significantly reduces the environmental impact of the ship, but it increases its lifetime costs by 30%. Moreover, by omitting the carbon tax, the costs increase by 53%. Further development of fuel cell technology would lead to decreasing their costs, which would make fuel-cell-powered purse seiners a feasible powering solution.