## 4 Research Clusters: Content Analysis
### Cluster 2: Alternative Fuels and Low-Carbon Strategies for Maritime Transport
#### 4.2.2 Subcluster 2: Comparative and Decision-Based Evaluations of Alternative Fuels  
This subcluster is centered on detailed, comparative evaluations of specific alternative fuels. Two studies employ advanced modeling techniques--such as energy systems modeling and multi-criteria decision analysis--to assess the performance, sustainability, and viability of various fuel options. One article is a comparative study, including technology assessments and scenario analyses as central themes. [PERSON] et al. (2020) evaluate ammonia's prospects as a future marine fuel by comparing it with other marine fuels through both energy systems modeling and a multi-criteria decision analysis (MCDA) framework [42]. The results indicate that no single fuel option excels across all considered performance measures, such as cost, emissions, and reliable supply. However, when taking stakeholder preferences, the following should be taken into consideration. Stakeholder groups such as ship owners, fuel producers, and engine manufacturers place a high emphasis on economic factors and fuel costs, resulting in LNG and natural-gas-based methanol being ranked at the top, while ammonia options tend to rank lower for these groups. In contrast, governmental authorities, prioritizing greenhouse gas emissions and regulatory compliance, rank renewable hydrogen in fuel cells highest and renewable ammonia in fuel cells next, whereas natural-gas-based ammonia remains at the bottom of their ranking. For the combined stakeholder group, LNG is ranked highest, followed by hydrotreated vegetable oil used in ICEs and renewable hydrogen in fuel cells, with renewable ammonia options ranking fifth (when used in fuel cells) and eighth (when used in ICEs) and natural-gas-based ammonia ranking lowest. [PERSON] et al. (2020) develop a specific methodology to compare the safety and sustainability of alternative fuel systems for large cruise ships, providing essential guidance during the early design stages [35]. The methodology is employed in the following four respective schemes: (1) low-pressure dual-fuel (LNG-based); (2) high-pressure dual-fuel (LNG-based); (3) lean burn spark ignition (LNG-based); and (4) conventional marine fuel oil (MGO-based). The study demonstrates that LNG-based fuel systems markedly outperform conventional MGO systems in sustainability, with LNG alternatives achieving substantial environmental benefits and a lower overall sustainability impact. Among LNG-based options, the low-pressure dual-fuel system (Scheme 1) emerges as the most sustainable, delivering a 35% overall reduction in sustainability impact relative to MGO--primarily due to a 44% reduction in environmental impact and moderate cost reductions. In contrast, the high-pressure dual-fuel system (Scheme 2) is penalized by higher operating and maintenance costs, as well as a 62% increase in the inherent safety index compared to MGO, making it the least sustainable among the LNG alternatives. [PERSON] et al. (2021) adopt a bottom-up approach, directly comparing hydrogen, ammonia, and methanol for long-distance international shipping by calculating key technical requirements--including fuel storage volume and mass--and evaluating emissions, supply, safety, and storage aspects using real-world data [56]. The scholars find that hydrogen is the most promising candidate, as its production and supply chain can be decarbonized more easily than those of ammonia and methanol, which require energy-intensive processes such as the Haber-Bosch process. The production upscaling needed to meet global shipping demand is significantly lower for hydrogen (171%) compared to ammonia (391%) and methanol (859%), highlighting hydrogen's superior feasibility. While methanol requires less mass and volume for onboard storage than ammonia and is easier to store than hydrogen, the study shows that the cryogenic storage of hydrogen is still viable, capable of powering a large vessel using the equivalent of 85 containers.