**1. Introduction**

In 2015, the "Third IMO Greenhouse Gas Study," conducted by the International Maritime Organization (IMO) [1], reported that air pollutants emitted from ships in 2012 accounted for 13% of NOx, 12% of SOx, and 2.6% of CO2 in terms of global atmospheric pollutant emissions [2,3]. The International Council on Clean Transport (ICCT), an international environmental non-profit organization, has analyzed and forecasted the pollutant emissions from ships from 1990 to 2050, and reported that the NOx and SOx emitted from ships are expected to increase to 30% and 20%, respectively, of all global pollutant emissions [3–5]. These study results support the view that the long-term effects of atmospheric pollutants caused by ships are foreseen to become more severe, considering the trend of increasing global trade in the future. Clearly, there is a need to develop technology for reducing pollutant emissions from ships [6,7].

Currently, fuel cells that use hydrogen fuel to reduce pollutant emissions from ships are being studied [8–11]. Fuel cells take the chemical energy within the hydrogen that is used as fuel and convert it into electrical and thermal energy through an electrochemical reaction with the oxygen present in the air. They produce almost no pollutant emissions or noise when generating electricity, and they can use various fuels as sources of hydrogen. Fuel cells are an eco-friendly energy source with very high electrical efficiency. Fuel-cell-based power generation can reduce greenhouse gas emissions by 30% compared to existing power generation methods [12]. Therefore, most advanced countries throughout the world regard fuel cells as a next-generation technology and are actively developing them [13,14].

The research on a hybrid system combining a fuel cell system with a diesel engine, which is the main power source of a ship, has been conducted in Europe [15–18]. The report of the European Maritime Safety Agency (EMSA), 'Study on the Use of Fuel Cells in Shipping', applied fuel cells as the main power or auxiliary power of ships from the beginning of 2000 and 24 projects in Europe and the United States with the beginning of the 'US Ship Service Fuel Cell Program [US SSFC]' project [19]. Analysis of these studies shows that most of the methods are generally aimed at improving the performance of hybrid systems (fuel cells, diesel generators) or the configuration of their systems and that there is no experimental study on the reduction of CO2 emissions from the hybrid power generation systems [20,21].

Therefore, an empirical study was conducted through experiments on CO2 reduction that has not been carried out in previous projects so far. Molten carbonate fuel cells (MCFCs) were selected as fuel cell systems of a combined power source because the characteristics of MCFCs are suitable for application to ships [22,23]. Since MCFCs operate at high temperature, the reaction rate is fast, even when using low-cost catalysts, as compared with relatively different fuel cell systems. Even when the ship is sailing for a long time, the external reformer is not installed separately and natural gas or coal gas is directly used as fuel. It is appropriate to apply it as the main power source for the base load of the ship [24].

In this study, to reduce the emissions from ships, empirical experiments on the fuel consumption and carbon dioxide emission reduction effect of a combined power source (fuel cell + battery + diesel generator) instead of the diesel generator were conducted. The capacity of the combined power source was 100 kW for MCFCs, 30 kW for batteries, and 50 kW for diesel generators. In order to carry out the experiment on the test bed, the power amount for each operation mode was analyzed according to the type of the commercial vessel, and the scale was downsized according to the capacity of the test bed. The fuel consumption and carbon dioxide emissions of the ship were calculated, according to the load profile of the ship, within 180 kW of the configured system. It can be confirmed through the demonstration that carbon dioxide emission and fuel consumption was considerably reduced compared to the conventional diesel power source.

#### **2. Background**

The International Convention for the Prevention of Marine Pollution from Ships (MARPOL) agreed that emissions, such as NOx and SOx, from ships should be reduced by 20% or less of the current emission amounts from 2008 to 2015. Since 2016, the agreement has recommended an 80% reduction in pollutant emissions [25]. In addition, the IMO has introduced the energy efficiency design index (EEDI), which is an index of factors to be considered in ship design to contribute toward reducing CO2 emissions. The CO2 emission regulations based on the EEDI that were imposed by the IMO on all new ships built since 2013 are listed in Tables 1 and 2. Ships that do not meet the required EEDI levels are prohibited from entering ports [26]. In Table 2, the EEDI will be implemented in phases. Currently, it is in phase 1, which runs from year 2015 to 2019. Phase 2 will run from year 2020 to 2024 and phase 3 from year 2025 onwards [27].


**Table 1.** International Convention for the Prevention of Marine Pollution from Ships (MARPOL) 73/78—Annex VI Regulations for the Prevention of Air Pollution from Ships.

**Table 2.** The energy efficiency design index (EEDI)-based CO2 emission reduction goals.


National and international regulations are gradually being strengthened and require ocean pollutant emissions from ships to be reduced continuously. However, it is not possible to address this problem solely through modern engine technology without installing additional devices for preventing environmental pollution. Therefore, there is an increasing demand for high-efficiency power sources for ships with almost no pollutant emissions. Normal high-efficiency diesel engines have an energy efficiency of approximately 40%, and facilities equipped with CO2-capturing devices or pollutant-processing devices for emission gases have limited effectiveness owing to increases in the system volume and fuel energy consumption [28].

On the other hand, if fuel cells powered by hydrogen, which are eco-friendly high-efficiency power sources, could be an alternative solution, instead of diesel engines, to a propel ship, there could be almost no emissions (for example, NOx, SOx, CO2, or PM); the fuel cells would produce no noise or vibration and would have good power generation efficiency [14]. As such, fuel cells powered by hydrogen have considerable potential as a next-generation main power source for ships. In addition, they can be modularized to reduce complexities in terms of their construction and installation. Therefore, their capacity can be adjusted such that it is most effective for specific types or functions of ships. They have a very wide range of uses and are considered a technology that will play a leading role in ship propulsion systems in the future [8,29].
