## 4 Research Clusters: Content Analysis
### Cluster 2: Alternative Fuels and Low-Carbon Strategies for Maritime Transport
#### 4.2.1 Subcluster 1: Strategic Pathways to Clean Fuel Strategies  
This subcluster covers articles that adopt a forward-looking, strategic perspective on achieving low-carbon maritime transportation. It includes two works that articulate long-term pathways and identify comprehensive fuel-saving or clean fuel strategies. The emphasis is on mapping out transitions over time (e.g., toward 2050) and on understanding the systemic approaches required for a green maritime future. [PERSON] et al. (2021) conduct a comprehensive technological review to assess alternative marine fuels, aiming to achieve low-carbon maritime transportation by 2050 while simultaneously reducing the emissions of sulfur oxides, nitrogen oxides, and carbon dioxide ([PERSON] et al., 2021). Employing a multi-dimensional decision-making framework, the study qualitatively ranks various marine fuels and identifies zero carbon synthetic fuels--particularly hydrogen and ammonia--as vital for domestic and short-sea shipping, despite current cost and infrastructure challenges. The scholars conclude that ships require fuels with a high energy density--both by mass and by volume--to maximize the available space and weight capacity for payloads, making energy density a critical factor in fuel selection. Although hydrogen offers a high energy density by mass and dramatic CO\({}_{2}\) savings, its low volumetric energy density restricts its use on ships where reduced fuel storage capacity is not a critical issue, such as high-density cargo vessels, Ro-Pax, ferries, and short-sea shipping vessels. Ammonia emerges as a promising alternative because it is a carbon-free fuel that provides a more balanced compromise between mass and volumetric energy density, despite challenges related to the technical maturity of onboard equipment and upstream feedstock availability. Other marine fuels such as LNG, CNG, LPG, methanol, ethanol, biodiesel, and DME are considered as acceptable alternatives from an energy density perspective, as they maintain a sufficient energy output without significantly compromising vessel range or payload. To accommodate the distinct combustion characteristics of fuels like ammonia--which requires a secondary ignition fuel in internal combustion engines--and methanol, ship engines often necessitate significant modifications, including tailored ignition systems and optimized fuel injection mechanisms. Ultimately, the strategic evaluation of alternative marine fuels for achieving low-carbon maritime transportation prioritizes energy density alongside environmental benefits, ensuring that the selected fuels do not unduly limit the operational performance of ships while contributing to emission reductions. [PERSON] et al. (2023) state that achieving the goal of low-carbon shipping by 2050 necessitates a holistic approach that incorporates alternative fuels alongside various technological and operational initiatives [50]. The authors evaluate recent clean fuels and innovative technologies for vessels, categorizing alternative fuels into low-carbon, carbon-free, and carbon-neutral types based on their distinct properties. The article reviews comprehensive strategies for reducing CO\({}_{2}\) emissions from ships by evaluating a wide range of low-carbon fuels, clean renewable energy sources, and supportive regulatory frameworks, while emphasizing the role of intelligent energy management systems and battery storage in promoting energy savings. It highlights that robust operational and control practices--including advanced electricity control, energy conversion, and consumption monitoring--are essential for achieving effective energy efficiency, emissions mitigation, and cost savings in maritime operations. Biofuels, though commercially attractive when used in combination with other fuels, face significant challenges such as high production costs, feedstock limitations, and issues with cold flow, material compatibility, and fuel stability, with emerging research into novel feedstocks like algae and waste oil offering potential cost reductions.