The growth of the global shipping industry has increased the interest in the environmental impact of this sector. The International Maritime Organization adopted the initial Greenhouse Gas strategy for reducing GHG emissions from ships at the 72nd Marine Environment Protection Committee in April 2018. In this study, we carried out a life cycle assessment of nine production pathways of alternative fuels, including LNG, ammonia, methanol, and biofuels, and conducted an economic analysis considering the life cycle carbon pricing of each fuel pathway. Our results indicate that biomass-based FT-diesel, e-methanol, and e-ammonia are the most environmentally friendly, with GHG reductions of 92%, 88.2%, and 86.6%, respectively. However, our net present value analysis of ship life cycle cost considering carbon price indicated that using those fuels would not be cost-effective during the target period of study. Sensitivity analysis was performed by changing the life cycle carbon pricing from the baseline scenario, and we investigated the approximate years for when these alternative fuels will become more cost-effective compared to conventional fossil fuels. Further, to provide practical implications for shipping stakeholders, we analysed the effect of blending the same kinds of fuels with different production pathways.

As the demand for alternative fuels to solve environmental problems increases worldwide due to the greenhouse gas problem, this study predicted the demand for liquid hydrogen fuel in aviation to achieve ‘zero-emission flight’. The liquid hydrogen fuel models of an aircraft and all aviation sectors were produced based on the prediction of aviation fleet growth through the classification of currently operated aircraft. Using these models, the required amount of liquid hydrogen fuel and the total cost of liquid hydrogen were also calculated when various environmental regulations were satisfied. As a result, it was found to be necessary to convert approximately 66% to 100% of all aircraft from existing aircraft to liquid hydrogen aircraft in 2050, according to regulations. The annual liquid hydrogen cost was 4.7–5.2 times higher in the beginning due to the high production cost, but after 2030, it will be maintained at almost the same price, and it was found that the cost was rather low compared to jet fuel.

This study performed a technical–economic analysis for ship-based ammonia transportation to investigate the feasibility of international ammonia transportation. Ammonia is considered to be a vital hydrogen carrier, so the international trade in ammonia by ship will considerably increase in the future. This study proposed three scenarios for transporting ammonia from the USA, Saudi Arabia, and Australia to South Korea and employed an 84,000 m³ class ammonia carrier. Not only traditional very low sulfur fuel oil (VLSFO)/marine diesel oil (MDO) but also LNG and ammonia fuels were considered as propulsion and power generation fuels in the carrier. A life-cycle cost (LCC) model consisting of capital expenditure (CAPEX) and operational expenditure (OPEX) was employed for the cost estimation. The results showed that the transportation costs depend on the distance. The unit transportation cost from the USA to South Korea was approximately three times higher than that of Australia to South Korea. Ammonia fuel yielded the highest costs among the fuels investigated (VLSFO/MGO, LNG, and ammonia). When using ammonia fuel, the unit transportation cost was approximately twice that when using VLSFO/MDO. The fuel costs occupied the largest portion of the LCC. The unit transportation costs from Australia to South Korea were 23.6 USD/ton-NH3 for the LVSFO/MDO fuel case, 31.6 USD/ton-NH3 for the LNG fuel case, and 42.9 USD/ton-NH3 for the ammonia fuel case. This study also conducted a sensitivity analysis to investigate the influence of assumptions, including assumed parameters.

This study proposed the integrated design of an NH₃ fuel supply system and a re-liquefaction system for an ocean-going NH₃-fueled ship. The target ship was a 14,000 TEU large container ship traveling from Asia to Europe. The NH₃ fuel supply system was developed to feed the liquid fuel at 40 °C and 80 bar and cope with the re-circulated fuel with the sealing oil. Its power consumptions and SECs ranged from 56.4 to 157.5 kW and from 0.0063 to 0.009 kWh/kg, respectively. An onboard re-liquefaction system with a vapor compression refrigeration cycle was also designed to liquefy the BOG from the fuel tank. The re-liquefaction system’s exergy efficiency and SEC were 34.71% and 0.224 kWh/kg, respectively. The equipment with the most exergy destruction was the heat exchangers, accounting for 60% of the total exergy destruction. NPV analysis found that it is recommended to introduce the re-liquefaction system to the target ship. At the NH₃ price of USD 250/ton, the reasonable cost of the re-liquefaction system is less than USD 1 million. According to LCC, NH₃ fuel is economically feasible if the carbon tax is more than USD 80/ton and the NH₃ price is around USD 250/ton.

This study analysed a novel re-liquefaction system integrated with a fuel supply system (FSS) for an LPG carrier to conventional systems. The re-liquefaction system and FSS were installed independently in a conventional LPG carrier, while those systems were combined in the novel system. The condensed LPG in the re-liquefaction system was directly transferred to the FSS without the cooling and expansion process in the novel system. 84,000 m³ LPG carrier equipped with a 10 MW engine at normal continuous rating (NCR) was selected as a target ship. Aspen HYSYS ver.12.1 was employed for process simulation. The results showed that the energy consumption for the novel system was reduced by 38%. The energy for re-liquefaction was decreased because the flow rate recirculated was decreased, and the energy for FSS was reduced as the temperature of the stream supplied to the FSS was relatively high in the novel system. A sensitivity analysis was conducted to investigate the effect of the parameters on the results. The investigated parameters were LPG compositions, seawater temperature, compressor efficiency, and pump efficiency. The energy consumption for the system was significantly different depending on the LPG composition, and the energy consumption was changed by 2.5% for conventional systems and 0.9% for the novel systems with the variation of 4 °C seawater temperature. The energy for the novel system was reduced by 2.8% for conventional systems and 2.3% for the novel systems with the 5% increment of compressor efficiency, whereas pump efficiency had little effect on the results.