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This paper was motivated to review and discuss the current issues and challenges for decarbonisation in the international shipping sector. It was mainly focused on international and local efforts to reduce greenhouse gas (GHG) emissions from shipping activities while introducing new strategies, agreements, and regulations to meet the target on GHG reduction levels. It firstly reviewed the impact of the Clydebank Declaration made at 26 th UN Climate Change Conference in Glasgow, 2021. Then, the review was moved to UK and EU policies and activities. Findings from this research offer some insights into local governmental authorities and shipping companies in terms of how to respond to gradually stringent environmental regulations as well as their roles to make the shipping sector cleaner.

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In Korea, fishing boats account for 65,452 out of 71,174 coastal ships, and aging ships are a leading cause of environmental problems and economic loss. To solve this problem, the government has enacted a policy to promote the development and distribution of environmental vessels. Electric propulsion systems and hybrid systems are suggested as alternatives. However, to study whether this hybrid system is environmentally friendly, a system for measuring the performance of conventional diesel-powered ships was developed and measured. A case study applying the battery hybrid system was analyzed through a Matlab/Simulink simulation, and the effect of reducing the carbon dioxide emissions by 7.6% was verified. In addition, comparing with a case where the load-sharing optimal control was introduced, we proved that the carbon dioxide reduction with the controller was 10.7%. By applying the results of this study to various countries with different fuel production processes through the LCA technique, it was confirmed that the battery hybrid system is eco-friendly from the energy production stage, rather than unconditionally, according to the national power production method. Additional research is needed to reflect the aging trend of the entire fleet and to create a detailed category classification through future research.

The pathway to zero carbon emissions passing through carbon emissions reduction is mandatory in the shipping industry. Regarding the various methodologies and technologies reviewed for this purpose, Life Cycle Cost Analysis (LCCA) has been used as an excellent tool to determine economic feasibility and sustainability and to present directions. However, insufficient commercial applications cause a conflict of opinion on which fuel is the key to decarbonisation. Many LCCA comparison studies about eco-friendly ship propulsion claim different results. In order to overcome this and discover the key factors that affect the overall comparative analysis and results in the maritime field, it is necessary to conduct the comparative analysis considering more diverse case ships, case routes, and various types that combine each system. This study aims to analyse which greener fuels are most economically beneficial for the shipping sector and prove the factors influencing different results in LCCA. This study was conducted on hydrogen, ammonia, and electric energy, which are carbon-free fuels among various alternative fuels that are currently in the limelight. As the power source, a PEMFC and battery were used as the main power source, and a solar PV system was installed as an auxiliary power source to compare economic feasibility. Several cost data for LCCA were selected from various feasible case studies. As the difficulty caused by the storage and transportation of hydrogen and ammonia should not be underestimated, in this study, the LCCA considers not only the CapEx and OpEx but also fuel transport costs. As a result, fuel cell propulsion systems with hydrogen as fuel proved financial effectiveness for short-distance ferries as they are more inexpensive than ammonia-fuelled PEMFCs and batteries. The fuel cost takes around half of the total life-cycle cost during the life span.