Comparative Analysis on AC and DC Distribution Systems for Electric Propulsion Ship

: Decarbonization is an ongoing issue in the shipping industry, and electric propulsion systems are being proposed as alternative solutions to satisfy this requirement. To answer the fundamental questions of “Are electric propulsion systems the green solution?” and “Can DC distribution systems contribute to the decarbonization in shipping?”, this paper analyzed DC distribution system electrical characteristics, economic feasibility, and environmental evaluation for a full-scale AC-DC hybrid distribution electric propulsion system facility. By applying the actual ship’s operating proﬁle as inputs, a DC distribution system with an active front end rectiﬁer and a variable speed generator engine was proven to guarantee the same electric stability as the conventional AC distribution system. The life cycle assessment results achieved economic and life cycle environmental beneﬁts of about 10% (8.9% for Case 1 and 12.4% for Case 2). These research ﬁndings offer meaningful insights into the DC distribution system to minimize fuel consumption and emissions toward cleaner shipping.


Background
With growing concerns about accelerated climate change, the United Nations (UN) has made a series of international agreements to curb greenhouse gases over the last decades. The United Nations Framework Convention on Climate Change (UNFCCC) adopted the Kyoto Protocol in 1997, starting with the United Nations Framework Convention on Climate Change in May 1992, followed by the Paris Agreement in December 2015 and prepared detailed implementation rule [1].
According to the 'Fourth IMO Greenhouse Gas Study' report [2] of the International Maritime Organization (IMO) under the United Nations in 2020, greenhouse gas (GHG) emissions (including carbon dioxide, methane, and nitrous oxide) from shipping activities increased from 2.76% in 2012 to 2.89% in 2018. IMO adopted an ambitious resolution to curb GHG emissions from all ships by 50% by 2050, compared to the 2008 level in 2018 [3].
The Republic of Korea enacted the 'Act on Promotion of the Development and Distribution of Environmentally Friendly Ships' that promotes comprehensive measures and policies to enhance cleaner shipping, which came into force on 1 January 2020 [4]. In addition, the 'Green Public Vessel Conversion Project' was also proposed to replace government-own ships aged 20 years and over with greener ones. Similarly, the 'Green Ship Certification System' was designed to encourage private shipping sectors to take part in cleaner shipping with national support.
In the Korean government's green ship project, technologies that can improve the efficiency of conventional ships and reduce emissions were categorized as 'green technologies'. Electric propulsions, hybrid systems, and fuel cells are good examples. Siemens and Bellona in 2019 [7] indicated that 70% of 180 Norwegian ferries have adopted battery hybrid-electric propulsion systems. Several examples support the drastic movement to convert the conventional mechanical propulsion system to electric or hybrid propulsion.
In January 2019, Wartsila signed a contract with Hagland Shipping AS to convert the propulsion system of its cargo ship from a diesel-based mechanical propulsion system to an electric propulsion system with a battery [8]. Additionally, in 2020, it can be confirmed that cargo ships with fully electric propulsion systems are being built and operated in China [9]. As such, it can be seen that the paradigm of ship propulsion systems worldwide is rapidly changing from the existing mechanical propulsion systems to electric propulsion systems [10].
Despite the growing attention to electric-powered ships, there is still a lack of information on whether electric-powered vessels can contribute to reducing the environmental impacts overall. This can be answered if the performance of the entire electric propulsion system, such as the generator, switchboard, power conversion device, propulsion motor, and load bank, is measured simultaneously. Those systems have been applied mainly to coast sailing-small ships rather than medium-large ocean-going ships due to their technical hindrances, such as low energy density.
In addition, most hybrid-electric propulsion ships have currently been operated based on AC power-distribution systems. However, the remarkable advancement in semiconductor devices for power with improved capacity and switching speed has improved power control, power converter efficiencies, and DC circuit breakers. As a result, the shipbuilding sector has started to draw attention to DC power-distribution systems onboard. Nevertheless, their applications, as well as impact assessments, are still limited.
This paper was motivated to investigate the benefits of using DC distribution systems from economic and environmental perspectives and to offer insights into whether DC distribution systems can be a new standard for electric propulsion ships. Figure 2 compares the key differences between AC and DC grid systems. Siemens and Bellona in 2019 [7] indicated that 70% of 180 Norwegian ferries have adopted battery hybrid-electric propulsion systems. Several examples support the drastic movement to convert the conventional mechanical propulsion system to electric or hybrid propulsion.
In January 2019, Wartsila signed a contract with Hagland Shipping AS to convert the propulsion system of its cargo ship from a diesel-based mechanical propulsion system to an electric propulsion system with a battery [8]. Additionally, in 2020, it can be confirmed that cargo ships with fully electric propulsion systems are being built and operated in China [9]. As such, it can be seen that the paradigm of ship propulsion systems worldwide is rapidly changing from the existing mechanical propulsion systems to electric propulsion systems [10].
Despite the growing attention to electric-powered ships, there is still a lack of information on whether electric-powered vessels can contribute to reducing the environmental impacts overall. This can be answered if the performance of the entire electric propulsion system, such as the generator, switchboard, power conversion device, propulsion motor, and load bank, is measured simultaneously. Those systems have been applied mainly to coast sailing-small ships rather than medium-large ocean-going ships due to their technical hindrances, such as low energy density.
In addition, most hybrid-electric propulsion ships have currently been operated based on AC power-distribution systems. However, the remarkable advancement in semiconductor devices for power with improved capacity and switching speed has improved power control, power converter efficiencies, and DC circuit breakers. As a result, the shipbuilding sector has started to draw attention to DC power-distribution systems onboard. Nevertheless, their applications, as well as impact assessments, are still limited.
This paper was motivated to investigate the benefits of using DC distribution systems from economic and environmental perspectives and to offer insights into whether DC distribution systems can be a new standard for electric propulsion ships. Figure 2 compares the key differences between AC and DC grid systems. As shown in Figure 2a, the constant speed power generation system uses AC distribution, and the converted voltage (220V, 460V, or 3300 V) via a transformer is supplied to loads. To provide DC power, this voltage goes through an additional AC-DC converting process. In the case of the conventional AC power-distribution system, since the rotational speed of the power source is constant, the frequency is also constant with stable AC voltage, and the load also uses the AC source. However, in the case of a variable speed power generation system, the voltage is irregularly changed to increase the efficiency of the prime mover at a low load with different rpm. In the variable speed power generation system of Figure 2b, the rectification system is required for voltage and frequency controls in response to various changes in engine speed.
The rectification system constant voltage is supplied to DC distribution by the rectification system, mainly using the AFE (Active Front End) rectifier. After that, it is converted to AC voltage according to the load required, and then power is supplied through the step-down and step-up process via a transformer, and DC power is supplied in the same way as in the AC distribution system [11]. A significant difference between the constant speed and the variable speed power generation systems is the configuration of the prime mover and distribution system with rectification control [12]. A power generation system applies a stable DC power in the distribution system according to the frequency change. The variable speed generator and the constant speed generator are the same; however, a key difference is placed in their mechanism on voltage and speed controls [12,13]. Table 1 shows the advent of ships using DC power-distribution systems. It can be observed that there is an increasing number of cases of applying the DC distribution system to replace the AC distribution system, centering on small-and medium-sized power ships [10,14]. As shown in Figure 2a, the constant speed power generation system uses AC distribution, and the converted voltage (220V, 460V, or 3300 V) via a transformer is supplied to loads. To provide DC power, this voltage goes through an additional AC-DC converting process. In the case of the conventional AC power-distribution system, since the rotational speed of the power source is constant, the frequency is also constant with stable AC voltage, and the load also uses the AC source. However, in the case of a variable speed power generation system, the voltage is irregularly changed to increase the efficiency of the prime mover at a low load with different rpm. In the variable speed power generation system of Figure 2b, the rectification system is required for voltage and frequency controls in response to various changes in engine speed.
The rectification system constant voltage is supplied to DC distribution by the rectification system, mainly using the AFE (Active Front End) rectifier. After that, it is converted to AC voltage according to the load required, and then power is supplied through the step-down and step-up process via a transformer, and DC power is supplied in the same way as in the AC distribution system [11]. A significant difference between the constant speed and the variable speed power generation systems is the configuration of the prime mover and distribution system with rectification control [12]. A power generation system applies a stable DC power in the distribution system according to the frequency change. The variable speed generator and the constant speed generator are the same; however, a key difference is placed in their mechanism on voltage and speed controls [12,13]. Table 1 shows the advent of ships using DC power-distribution systems. It can be observed that there is an increasing number of cases of applying the DC distribution system to replace the AC distribution system, centering on small-and medium-sized power ships [10,14].

Past Research
To introduce and utilize past research results on our DC distribution system research, we performed a diverse literature review, as shown in Table 2, about the generator, rectification system, distribution system, control, and efficiency. It was confirmed that when DC-based systems, such as fuel cells and batteries are applied, energy efficiency is much more advantageous when DC power distribution is used than when AC power distribution is used.
Comparing AC and DC distribution, which have been used for over 100 years, proved that energy efficiency could be improved when DC distribution is applied depending on the energy source.
There is a limitation to simply comparing AC and DC distribution by applying a general theory and presenting the essential contents.
[15] The simulation proposes a method for controlling a DC power-distribution system's power converter and suggests that energy efficiency can be improved.
There is a limitation as results were only through simulation. [19]  The methodology is derived based on the primary control rule, and the methodology is verified through simulation.
There is a limitation as results were only through simulation. [25]

DC grid protection
Suggestion for semiconductor-based circuit breaker for protection of DC power-distribution system and control of current flow The simulation of the innovative multifunction integrated DCCB confirmed that it could protect the system while regulating the current value.
Performance verification of the proposed system that regulates grid current through simulation There is a limitation as results were only through simulation. [26] Converter Interest in DC circuit breaker is increasing, and circuit breaker with improved performance is proposed and verified through simulation.
There is a limit as results were only through simulation. [28] DC grid circuit breaker, modeling Introduces the basic principles related to the control and switching technology of circuit breaker, one of the disadvantages of the DC distribution system, and compares the concepts of various circuit breakers Provides a technical and economically feasible solution to the circuit breaker, one of the problems to expand the application of the DC power-distribution system Compare and review various circuit breakers' technical and economic aspects on the market.
A partial explanation of circuit breakers, in theory, principle content [29] DC grid circuit breaker modeling, control The proposal of a circuit breaker capable of blocking fault current within a few milliseconds of circuit breaking is the biggest significant advantage of the DC power-distribution system.
Confirm that the current cut-off time of the circuit is shortened and the current peak value is reduced through the hybrid circuit breaker Verification of the design principle through simulation for the proposed hybrid circuit breaker There is a limitation as results were only through simulation. [30] DC grid, Variable speed engine, fuel saving Improvement of fuel consumption of DC power-distribution system using variable speed engine generator applicable to DP ships and analysis of advantages of the dynamic performance of propulsion system When the DC power-distribution system for variable speed engine power is applied, fuel consumption is improved by more than 20%. Various advantages, such as advantageous aspects of maintenance are concluded.
Describe the advantages that can occur when applying the variable speed engine power generation DC distribution system to the OSV vessel to which the DP system is applied, using data from the developer Verification of contents using developer's data. The range is limited to the DP system, not the electric propulsion system. [31]

DC gird ECMS control
Analysis of potential benefits arising from the application of ECMS control in hybrid ships equipped with variable speed engine generators The simulation confirmed that the fuel consumption was reduced by 17.6% when the hybrid-electric propulsion system was applied.
Applied to ships based on the control system used in vehicles As a system for vehicles, it is not a ship electric propulsion system [32] Variable-speed engine, Control Presenting a new solution for variable speed engine power generation that can be applied to DP grade drilling equipment Identification of fuel savings potential through the study of operating profiles and actual loads on thrusters and generators and verification through simulation Research progress through simulation on variable speed engine generator applicable to drilling equipment of DP ship Verification of contents limited to the DP system, not the electric propulsion system, through simulation [33] Hybrid power systems, DC grid, Control systems A study to analyze energy efficiency and environmental benefits of the power system of hybrid-electric propulsion vessels Output, the SFOC result for applied to the hybrid-electric propulsion ship through the modeled simulation and comp it with the case with the battery.

Model the power system of a hybrid-electric propulsion ship through simulation
There is a limitation as results were only through simulation. [34] Variable-speed engine control Verification of the performance of the entire system through speed control of variable speed engine machines Check the results of performance improvement, fuel consumption reduction, and exhaust gas reduction in the system to which the variable speed engine is applied.
Experimental results are derived through a small-scale device configured in the laboratory.
There is a limitation as a result through only simulation. [35] Speed control, variable speed engine Analysis of the characteristics of the power system of an electric propulsion ship Modeling the PID speed control system of a diesel engine and the AVR and excitation control system of a synchronous generator to verify stability, speed, and robustness

Validation of results through simulation modeling
There is a limitation as a result through only simulation. [36] Energy efficiency Presenting the latest technological trends of variable-speed engine generators and comparing their performance in terms of fuel consumption reduction, engine life extension, and greenhouse gas emission reduction Confirmation of reduced fuel consumption and stable output production by applying the proposed system

Comparison of results through simulation modeling
There is a limitation as a result through only simulation.
[37] It is not easy to objectively prove facts by conducting research through experts in each field rather than researching energy efficiency improvement and CO 2 emission reduction by constructing actual simulations or experimental devices. [39] Energy efficiency, Emissions reduction Confirmation of energy efficiency improvement and emission reduction effect of hybrid-electric propulsion system application Developed a control system for the hybrid-electric propulsion system and confirmed the effect of reducing fuel consumption and reducing emissions compared to the conventional propulsion system through system modeling, simulation, and experiments Model the electric propulsion system using the conventional diesel generator and solar power and battery system and conduct comparative research with the existing system after establishing the experimental equipment Build simulation models and configure small experimental equipment. However, there is a limit as it is a small capacity that is not a scale that applies to ships. [40] Energy management system, Hybrid electric propulsion Proposal of energy efficiency optimization method for energy saving and efficiency improvement Modeling by proposing a control method to optimize energy efficiency through a genetic algorithm Comparison between modeling results and actual ship's power system data.
Limitations due to the contrast of computer analysis results and real data for the system proposed in this study. There is no test result by applying the proposed method to a real testbed or ship. [41] Environmental, economic assessment, Electric propulsion system The suggestion of advantages and disadvantages of battery-electric powered ships through LCA and LCCA techniques and estimation of ship lifespan and cost It was confirmed that GHG was reduced by 30% and cost by 15% when battery-powered ships use shore charging.
Objectively present the effect of approaching from the LCA point of view of the conventional electric-powered ship power system and battery electric powered ship.
Limitation of results calculated from data through modeling rather than measured data from actual ships [42]  The main components of the DC distribution system were introduced in a conceptual study on the improvement of ship energy efficiency [14]. Previous research also compares the advantages and disadvantages of the AC power-distribution system and the DC powerdistribution system [17,31] with some example cases where the basic concept of the overall system has been studied. The DC power-distribution system for electric propulsion ships converts the output of the AC generator into DC using a power converter by using a power conversion device installed at the rear end of the generator. Parallel operation between grids is possible with the DC voltage as their bus terminal remains the same [24,25].
The DC switchboard has no particular internal configuration except for the control of the circuit breaker. It connects the output generated from the generator to the load stage through the on/off of the installed circuit breaker. This plays a role in transmitting to the monitoring system in real-time. In other words, the parallel operation with an AC distribution system requires a much-complicated process. A study was conducted to verify the control method through simulation and to analyze the results for each situation according to circuit applications through various system designs of DC power-distribution systems.
There has been a series of research investigating the performance of DC power systems: a study related to the fuel-saving effects in a DC power-distribution system applying a variable speed engine generator [34,35] through simulation [19]; a study on the reference voltage control algorithm that occurs when the speed of a variable speed engine synchronous generator changes [12]; optimal power control algorithm design for electricpowered ships applying DC distribution [46]; power quality improvement simulation studies of electric-powered ships using DC distribution system [47]; equivalent consumption minimization strategy compared to conventional control methods [32]; and a study on the fuel efficiency improvement simulation of the DC distribution system of an electric-powered ship applying the and the design and control of circuit breaker, which is the current task of the DC distribution system [28].
Most of the past research was focused on conceptual and laboratory-level simulations. Their focus is on the evaluation of variable-speed engine generators, power conversion devices, and power-distribution systems. In this regard, their studies were overly laden with theoretical verification with model simulation or experimental tests. However, it was found that there is a significant lack of research for evaluating the overall performance of the DC distribution electric propulsion system to which the variable speed engine generator is applied in consideration of actual ship-operating conditions.
Many studies have been conducted on motor control, rectifier control, load sharing control, energy management control, and motor and switching element improvement. There was no case demonstrating how much the fuel consumption is reduced with the same electric propulsion system and how it affects the ship's environmental aspect. Research on the electric propulsion system's control algorithm has focused on stability and robustness without considering the economic aspects of the environmental impact analysis.
The application of the DC power-distribution system with the variable speed generator through an AFE rectifier can improve the efficiency of the entire electric propulsion system. Only the electrical characteristics are considered in the past research without establishing a full-scale electric propulsion system in terms of the controller's performance applied to the electric propulsion system. While green technology is highly demanded, it is not easy to find research on overall system efficiency improvement and the environmental impact of the controller.
This study can answer the fundamental question of whether the DC distribution system's improvement can affect decarbonization in the shipping industry. This research is motivated by the research gap that brings out the need to investigate further the performance of integrated systems: DC power-distribution system with various speed engine generator sets, power conversion devices, and the propulsion motor. In this context, this research was proposed to evaluate the holistic DC propulsion systems using an onshore testbed by implementing actual electric-ship-operating conditions by applying an AFE rectifier. Thus, we provide meaningful insight into how the DC distribution system can contribute to reducing environmental impacts from electric ships.

Methodology
To answer the fundamental question of how much the DC distribution system with AFE rectifier and variable speed engine can improve energy-savings and reduce the environmental impacts of electric ships, its effectiveness is demonstrated in comparison with a conventional system. Figure 3 presents the step-by-step approach through the experiment.
Therefore, this paper evaluates the overall efficiency of the DC distribution system under the integrated systems. The data captured was analyzed to identify the stability and dynamic characteristics. This process enables the comparison of the performances of two different concepts. Finally, Step 6 is to quantify the energy-saving and environmental impact reduction for the proposed operation mode.

Step I: Select Reference Ship
To apply the same load environment for comparison between different systems, two case ships, a 5500 TEU reefer container and a 13K TEU container ship currently in operation were selected, and their propulsion load variation data over time was compiled.

Step II: Data Modification
The data from the ships were fed into the experiment as key inputs to implement simulations under the actual shipload profiles in Step 2. Under the test-bed experiment, this step confirms the electrical output characteristics, fuel consumption reduction ratio, and carbon dioxide emission reduction ratio. Two load scenarios were developed and applied for the testbed simulation based on the linear interpolation method generally used in shipyards and related research institutes [48]. Figure 4 shows the load profile scenario used in this experiment. The container ships have operation modes of Normal seagoing, Port in/out (w/o thruster), Port in/out (w/ thruster), Load/Unload, and Harboring during operation. The actual ship's load value was scaled-up, and the operating trend was converted to the specific time and applied as a load to the test generator. To answer the fundamental question of how much the DC distribution system with AFE rectifier and variable speed engine can improve energy-savings and reduce the environmental impacts of electric ships, its effectiveness is demonstrated in comparison with a conventional system. Figure 3 presents the step-by-step approach through the experiment. Therefore, this paper evaluates the overall efficiency of the DC distribution system under the integrated systems. The data captured was analyzed to identify the stability and dynamic characteristics. This process enables the comparison of the performances of two different concepts. Finally, Step 6 is to quantify the energy-saving and environmental impact reduction for the proposed operation mode.

Step I: Select Reference Ship
To apply the same load environment for comparison between different systems, two case ships, a 5500 TEU reefer container and a 13K TEU container ship currently in operation were selected, and their propulsion load variation data over time was compiled.

Step II: Data Modification
The data from the ships were fed into the experiment as key inputs to implement simulations under the actual shipload profiles in Step 2. Under the test-bed experiment, this step confirms the electrical output characteristics, fuel consumption reduction ratio, and carbon dioxide emission reduction ratio. Two load scenarios were developed and applied for the testbed simulation based on the linear interpolation method generally used in shipyards and related research institutes [48]. Figure 4 shows the load profile scenario used in this experiment. The container ships have operation modes of Normal seagoing, Port in/out (w/o thruster), Port in/out (w/ thruster), Load/Unload, and Harboring during operation. The actual ship's load value was scaled-up, and the operating trend was converted to the specific time and applied as a load to the test generator.

Step III: Apply Data to the Testbed System
In Step 3, the same load data was applied to the electric propulsion system using the constant speed generator engine with the conventional AC distribution and the variable speed generator engine with the DC distribution system. Both experiments were applied in the testbed with the same capacity.

Test Facility
Before the testbed was built, the electric propulsion system was modeled according

Step III: Apply Data to the Testbed System
In Step 3, the same load data was applied to the electric propulsion system using the constant speed generator engine with the conventional AC distribution and the variable speed generator engine with the DC distribution system. Both experiments were applied in the testbed with the same capacity.

Test Facility
Before the testbed was built, the electric propulsion system was modeled according to the capacity of the equipment for the testbed using the power analysis program (PSIM). The power characteristics of the power generation source, AFE rectifier, inverter, and propulsion motor were analyzed through the modeled electric propulsion system. The system configuration was verified before the testbed was built in Figure 5.

Step III: Apply Data to the Testbed System
In Step 3, the same load data was applied to the electric propulsion system using the constant speed generator engine with the conventional AC distribution and the variable speed generator engine with the DC distribution system. Both experiments were applied in the testbed with the same capacity.

Test Facility
Before the testbed was built, the electric propulsion system was modeled according to the capacity of the equipment for the testbed using the power analysis program (PSIM). The power characteristics of the power generation source, AFE rectifier, inverter, and propulsion motor were analyzed through the modeled electric propulsion system. The system configuration was verified before the testbed was built in Figure 5.  Figure 6 shows the overall system of the AC-DC hybrid distribution electric propulsion system testbed, which has the following key elements: the power source of the actual  Figure 6 shows the overall system of the AC-DC hybrid distribution electric propulsion system testbed, which has the following key elements: the power source of the actual electric propulsion ship to the propulsion motor, a load bank, and a control and monitoring system.

Generator Engine
The generator installed on the testbed is shown in Figure 7 and Table 3. This is a generator that is generally installed in electric propulsion ships, and it is a brushless synchronous generator.
To implement the DC power distribution in the electric propulsion system, variable engine speeds ranging between 1100 and 1800 RPM were applied. To adjust the generator's output voltage according to the speed change of the variable speed engine, as shown in Figure 8, a lookup table that outputs paired output voltages for each speed according to the speed change of the generator was configured in advance. The control unit of the power management device is an automatic voltage adjusting device that outputs a reference voltage requested through a lookup table. It outputs a target resistance value of an external input through a proportional integral controller. The input current of the potentiometer for outputting the target resistance value is calculated through the proportional integral controller of the control unit and then input to the potentiometer. The control system was designed for the output voltage. electric propulsion ship to the propulsion motor, a load bank, and a control and monitoring system. Figure 6. Configuration of the AC-DC hybrid distribution electric propulsion system for the testbed.

Generator Engine
The generator installed on the testbed is shown in Figure 7 and Table 3. This is a generator that is generally installed in electric propulsion ships, and it is a brushless synchronous generator.   electric propulsion ship to the propulsion motor, a load bank, and a control and monitoring system.

Generator Engine
The generator installed on the testbed is shown in Figure 7 and Table 3. This is a generator that is generally installed in electric propulsion ships, and it is a brushless synchronous generator.  To implement the DC power distribution in the electric propulsion system, variable engine speeds ranging between 1100 and 1800 RPM were applied. To adjust the generator's output voltage according to the speed change of the variable speed engine, as shown in Figure 8, a lookup table that outputs paired output voltages for each speed according to the speed change of the generator was configured in advance. The control unit of the power management device is an automatic voltage adjusting device that outputs a reference voltage requested through a lookup table. It outputs a target resistance value of an external input through a proportional integral controller. The input current of the potentiometer for outputting the target resistance value is calculated through the proportional integral controller of the control unit and then input to the potentiometer. The control system was designed for the output voltage.

Distribution System
The DC switchgear is configured to receive AC power produced by generators, to convert AC to DC through a rectifier installed inside, and to supply the converted DC to the motor drive unit. The installed rectifier is an AFE (Active Front End) rectifier using an IGBT device so that it can control on/off by applying an IGBT element as shown in Figure  9.

Distribution System
The DC switchgear is configured to receive AC power produced by generators, to convert AC to DC through a rectifier installed inside, and to supply the converted DC to the motor drive unit. The installed rectifier is an AFE (Active Front End) rectifier using an IGBT device so that it can control on/off by applying an IGBT element as shown in Figure 9.  Figure 10 shows the configuration of AFE rectifier, which has three stages (pole) and six semiconductor switches. It receives the three-phase AC power output from the generator and outputs 750VDC through control.  Figure 10 shows the configuration of AFE rectifier, which has three stages (pole) and six semiconductor switches. It receives the three-phase AC power output from the generator and outputs 750VDC through control.

Control Algorithm of AFE Rectifier
The control system of the AFE rectifier consists of a voltage controller and a current controller. In the voltage controller, if the input power supplied from the power source is greater than the output power consumed by the load, the DC output voltage of the DC link terminal increases, and vice versa.  Figure 10 shows the configuration of AFE rectifier, which has three stages (pole) and six semiconductor switches. It receives the three-phase AC power output from the generator and outputs 750VDC through control. Figure 10. Configuration of the AFE rectifier system in the testbed.

Control Algorithm of AFE Rectifier
The control system of the AFE rectifier consists of a voltage controller and a current controller. In the voltage controller, if the input power supplied from the power source is greater than the output power consumed by the load, the DC output voltage of the DC link terminal increases, and vice versa.
As the DC output voltage decreases, it controls the DC Link stage's by controlling the input power. In addition, the current controller using the value output from the voltage controller uses the three-phase input current output from the generator for control using the DC value obtained by converting the synchronous rotation coordinate system − axis. Since the DC controller designed with the value of the synchronous rotation coordinate system controls the DC current and DC output voltage, its performance can be excellent using the proportional integral controller. The voltage equation of the AFE rectifier is as follows. Figure 10. Configuration of the AFE rectifier system in the testbed.
As the DC output voltage decreases, it controls the DC Link stage's by controlling the input power. In addition, the current controller using the value output from the voltage controller uses the three-phase input current output from the generator for control using the DC value obtained by converting the synchronous rotation coordinate system d − q axis. Since the DC controller designed with the value of the synchronous rotation coordinate system controls the DC current and DC output voltage, its performance can be excellent using the proportional integral controller.
The voltage equation of the AFE rectifier is as follows.
e a = Ri a + L di a dt + V a (1) Here, e a , e b , and e c are power supply voltages; i a , i b , and i c are phase currents; and V a , V b , and V c are rectifier input voltages.
To convert the three-phase AC value, which changes in real-time with time, into two DC values that are convenient to control, the converted value of the synchronous rotational coordinate system is used to manage the AFE rectifier.
Here, e d and e q are the power voltage converted to the d − q axis of the synchronous rotation coordinate system; i d and i q are the current converted to the d − q axis of the synchronous rotation coordinate system; ω is the arbitrary angular velocity; and V d and V q are the rectifier input voltage converted to the d − q axis of the synchronous rotation coordinate system. Figure 10a shows the power supply voltage phase angle controller, (b) presents the current controller, and (c) is the voltage controller. The voltage controller (c) compares the DC output waveform of the DC-link with the command voltage value and outputs the error as the d-axis current through the proportional integral controller to control the DC output voltage.
i * d is the current command value converted to the d − q axis of the synchronous rotation coordinate system, Vdc * dclink is the DC voltage target value, Vdc dclink is the DC link actual measured voltage, K Pdclink is the PI controller proportional gain, and K idclink is the PI controller integral gain.

Inverter Drive and Propulsion Motor
The inverter drive unit outputs an AC with variable frequency and voltage using an IGBT element that can control on/off of the power supplied from the power-distribution system and converts the output voltage waveform by pulse width modulation (PWM). This drive generates a sine wave. The FOC (Field Oriented Control) method was applied to the inverter drive unit used for the simulation, and a system was configured to control the rotational speed of the propulsion motor as a load using the voltage and frequency converted by the FOC control method as shown in Figure 11. nous rotation coordinate system; and are the current converted to the − axis of the synchronous rotation coordinate system; ω is the arbitrary angular velocity; and and are the rectifier input voltage converted to the − axis of the synchronous rotation coordinate system. Figure 10 (a) shows the power supply voltage phase angle controller, (b) presents the current controller, and (c) is the voltage controller. The voltage controller (c) compares the DC output waveform of the DC-link with the command voltage value and outputs the error as the d-axis current through the proportional integral controller to control the DC output voltage. * = ( * − )( + ) (6) * is the current command value converted to the − axis of the synchronous rotation coordinate system, * is the DC voltage target value, is the DC link actual measured voltage, is the PI controller proportional gain, and is the PI controller integral gain.

Inverter Drive and Propulsion Motor
The inverter drive unit outputs an AC with variable frequency and voltage using an IGBT element that can control on/off of the power supplied from the power-distribution system and converts the output voltage waveform by pulse width modulation (PWM). This drive generates a sine wave. The FOC (Field Oriented Control) method was applied to the inverter drive unit used for the simulation, and a system was configured to control the rotational speed of the propulsion motor as a load using the voltage and frequency converted by the FOC control method as shown in Figure 11.  λ qr = ρλ qr = 0 (7) λ qr is the synchronous rotation coordinate system q-axis magnetic flux, and ρ is the differential operator.
The torque equation is as follows.
T e is the motor torque equation, P is the number of motor poles, L m is the mutual inductance, L r is the self inductance, λ dr is the synchronous rotation coordinate system d-axis magnetic flux, and i qs is the stator current converted to the q-axis of the synchronous rotation coordinate system.
When i ds is constant, the slip relation can be expressed as follows.
ω sl is the synchronous rotation coordinate system slip angular velocity, i qs is the stator current converted to the d-axis of the synchronous rotation coordinate system, i ds is the stator current converted to the d-axis of the synchronous rotation coordinate system, R r is the rotor resistance, and L r is the self-inductance.
Since the rotor flux position is the integral value of the sum of the motor speed and the slip command angular speed, the following equation is obtained. θ e = (ω r + ω sl )dt (10) θ e is the angular velocity of the synchronous rotation coordinate system, and ω r is the angular velocity of the motor rotor.
The propulsion motor is an induction motor widely applied to electric propulsion ships was selected, and the motor's capacity is 400 kW. Figure 12 presents the drive unit and the propulsion motor.
is the motor torque equation, is the number of motor poles, is the mutual inductance, is the self inductance, is the synchronous rotation coordinate system d-axis magnetic flux, and is the stator current converted to the q-axis of the synchronous rotation coordinate system. When is constant, the slip relation can be expressed as follows.
is the synchronous rotation coordinate system slip angular velocity, is the stator current converted to the d-axis of the synchronous rotation coordinate system, is the stator current converted to the d-axis of the synchronous rotation coordinate system, is the rotor resistance, and is the self-inductance Since the rotor flux position is the integral value of the sum of the motor speed and the slip command angular speed, the following equation is obtained.
is the angular velocity of the synchronous rotation coordinate system, and is the angular velocity of the motor rotor The propulsion motor is an induction motor widely applied to electric propulsion ships was selected, and the motor's capacity is 400 kW. Figure 12 presents the drive unit and the propulsion motor.

Load Bank
A load bank is a system designed to consume the power generated by the generator built in the testbed as an electric load. The load bank used is shown in Figure 13 and Table  4, and can generate loads up to 1100 kW at 440 V and 60 Hz.

Load Bank
A load bank is a system designed to consume the power generated by the generator built in the testbed as an electric load. The load bank used is shown in Figure 13 and Table 4, and can generate loads up to 1100 kW at 440 V and 60 Hz.   The resistance part of the load bank was constructed using a helical-type element. The state of the load bank can be monitored and controlled by the controller system installed inside the load bank. Figure 14 shows the control and monitoring system. The power data of the equipment built in the testbed is monitored.

Step IV: Key Key-Value Acquisition
The voltage, current, output amount, and system status of each item of equipment are monitored in real-time so that the system can be operated stably. The control and monitoring system are configured so that it is possible to configure the I/O list for the data required for each device and to check the information on the current data using the Lab-VIEW-based 'NI cRIO'.
In addition, the system was constructed so that the generator's output could be varied by creating a load pattern to be applied during the load test and adjusting the load in real-time by increasing or decreasing the resistance of the load bank according to the load pattern. The testbed was configured to send and receive the device status and operation commands through the interface between each device's control and monitoring system. Communication is based on RS-485 and ethernet communication considering external environmental factors, such as noise and peripheral devices. In addition, it was designed to monitor the testbed system in operation where the internet is available using an external internet network. The resistance part of the load bank was constructed using a helical-type element. The state of the load bank can be monitored and controlled by the controller system installed inside the load bank.

Step V: Comparative Analysis of the Electrical Characteristic for the AC-DC Distribution System
To verify the operation characteristics of the proposed DC distribution electric propulsion system, various data for electric characteristics of the system were monitored and stored in real-time every second as shown in Table 5.  The voltage, current, output amount, and system status of each item of equipment are monitored in real-time so that the system can be operated stably. The control and monitoring system are configured so that it is possible to configure the I/O list for the data required for each device and to check the information on the current data using the LabVIEW-based 'NI cRIO'.
In addition, the system was constructed so that the generator's output could be varied by creating a load pattern to be applied during the load test and adjusting the load in real-time by increasing or decreasing the resistance of the load bank according to the load pattern. The testbed was configured to send and receive the device status and operation commands through the interface between each device's control and monitoring system. Communication is based on RS-485 and ethernet communication considering external environmental factors, such as noise and peripheral devices. In addition, it was designed to monitor the testbed system in operation where the internet is available using an external internet network.

Step V: Comparative Analysis of the Electrical Characteristic for the AC-DC Distribution System
To verify the operation characteristics of the proposed DC distribution electric propulsion system, various data for electric characteristics of the system were monitored and stored in real-time every second as shown in Table 5.

Step VI: Economic Feasibility and Environmental Evaluation
To compare and verify the economic feasibility and environmental characteristics of the DC distribution system in an electric propulsion ship, we conducted comparative analysis and evaluation. It is possible to calculate the fuel consumption per kW output, carbon dioxide emissions per 1 kW output, and cost per kW. This can verify the total fuel consumption with the total fuel cost and the estimated total carbon dioxide emissions.

Method to Calculate Total Fuel Consumption
This research measured the fuel oil consumption through a flowmeter having a correction factor according to temperature. The data received in kg/s per second is converted into g/s, and the integrated value is calculated as the total fuel oil consumption.
Total f uel oil consumption(g) = f uel oil f low(Q) * temperature f actor(c) t n is the time [sec], and g is the fuel oil consumption calculation result by flow meter measurement at t n considering the temperature factor. Fuel cost(KRW) = f uel oil price(KRW/g) * total f uel oil consumption(g)

Method to Calculate CO 2 Emission
The total carbon dioxide emissions can be calculated using the following formula based on the fuel oil consumption.
Total CO 2 Emission (ton) = Total f uel oil consumption(ton) * C F (14) where the C F value represents the carbon dioxide emission coefficient according to the fuel consumption according to the type of oil as shown in Table 6. The Diesel/Gas Oil used in this experiment is ISO 8217 Grades and has a C F value of 3.206, which was applied to the conversion of carbon dioxide emissions.

Step VII: Lifecycle Assessment
Life cycle assessment (LCA) is a methodology for evaluating the environmental impacts associated with all phases of the life cycle of a commercial product, process, or service. For marine fuels, for example, the environmental impact is assessed, from the extraction of the primary energy sources, refinery, and supply to onboard usage. Recently LCA has been drawing more attention to the marine industry to examine the holistic environmental benefits/harms of conventional marine fuels and alternative ones.
To quantify the environmental benefits obtainable from the DC distribution system, this paper also adopted the LCA method in aid of GaBi LCA, the most common commercial LCA software (Sphera, Chicago, IL, USA) [49].
The International Maritime Organization also recognized the limitations of the existing environmental assessment methods for ships. LCA guidelines applicable to marine vessels are presently under development and are highly expected to become a new standard for maritime environmental impact assessment. This paper was motivated by the current maritime trend so that it applies LCA for the proposed cases.

Results
This section presents the results of the comparative analysis between the DC distribution system with a variable speed power generation system and the conventional AC distribution system with a constant speed power generation system.

Electric Characteristic
Comparative Analysis in Voltage, Current, Power 3.1.1. Container 5500 TEU Case Ship Figure 15 shows the power characteristics according to the 5500 TEU load profile scenario.
Both systems use constant DC voltage and display 730~751 V, indicating stable operation in voltage control. It can be confirmed that there is no difference in propulsion performance with the same power characteristics as the changes in current and power according to a given load variation.
In addition, even in the section where the load changes, the current is stably controlled without a large ripple and shows a desirable output even in the section of the sudden rise and fall.
bution system with a variable speed power generation system and the conventional AC distribution system with a constant speed power generation system. Both systems use constant DC voltage and display 730~751 V, indicating stable operation in voltage control. It can be confirmed that there is no difference in propulsion performance with the same power characteristics as the changes in current and power according to a given load variation.
In addition, even in the section where the load changes, the current is stably controlled without a large ripple and shows a desirable output even in the section of the sudden rise and fall.

Container 13K TEU Case Ship
The second scenario was applied to compare the electric dynamic characteristics of two different modes in the AC-DC hybrid distribution electric propulsion system. Figure  16 shows the same power characteristics according to the given 13K TEU load profile scenario.

Container 13K TEU Case Ship
The second scenario was applied to compare the electric dynamic characteristics of two different modes in the AC-DC hybrid distribution electric propulsion system. Figure 16 shows the same power characteristics according to the given 13K TEU load profile scenario. Both systems use constant DC voltage and display 730~751 V, indicating stable operation in voltage control. It can be confirmed that there is no difference in propulsion performance with the same power characteristics as the changes in current and power according to a given load variation.
In addition, even in the section where the load changes, the current is stably controlled without a large ripple and shows a desirable output even in the section of the sudden rise and fall.

Container 13K TEU Case Ship
The second scenario was applied to compare the electric dynamic characteristics of two different modes in the AC-DC hybrid distribution electric propulsion system. Figure  16 shows the same power characteristics according to the given 13K TEU load profile scenario.   Figure 17a is an SFOC graph. The actual measured data was obtained by dividing the fuel consumption per hour and kW of about 5000 extracted from the experimental engine.

Characteristics of Fuel Consumption and Carbon
The SFOC data is greatly affected by the generator load. In the low-load region, where the turbocharger efficiency is low, it can be confirmed that the fuel consumption per hour and kW is higher than in the low-load region due to the decrease in engine efficiency. It can be seen that the SFOC is consistently low in the load region of 75% or more.
The carbon dioxide emission per-hourly horsepower is shown in Figure 17b, and a curve proportional to SFOC is demonstrated.
grid system (Constant speed mode), (b) DC grid system (Variable speed mode). The SFOC data is greatly affected by the generator load. In the low-load region, where the turbocharger efficiency is low, it can be confirmed that the fuel consumption per hour and kW is higher than in the low-load region due to the decrease in engine efficiency. It can be seen that the SFOC is consistently low in the load region of 75% or more.

Characteristics of Fuel Consumption and Carbon Dioxide Emission
The carbon dioxide emission per-hourly horsepower is shown in Figure 17b, and a curve proportional to SFOC is demonstrated. Figure 18a shows the results of processing about the 5000 data extracted during operation in the variable speed mode. Unlike the constant-speed operation mode, since the engine's rpm is variable, the lower the speed compared to the same load, the lower the energy consumption required to obtain the rotational power. It can be seen that it is significantly lower than the constant speed operation mode. The reduced carbon dioxide emissions can be seen due to the reduction in fuel consumption in Figure 18. Therefore, the variable speed operation mode shows an excellent carbon dioxide emission reduction in the low load region compared to constant speed operation. The high load output shows the same carbon dioxide emission as the constant speed operation mode.  Figure 18a shows the results of processing about the 5000 data extracted during operation in the variable speed mode. Unlike the constant-speed operation mode, since the engine's rpm is variable, the lower the speed compared to the same load, the lower the energy consumption required to obtain the rotational power. It can be seen that it is significantly lower than the constant speed operation mode. The reduced carbon dioxide emissions can be seen due to the reduction in fuel consumption in Figure 18. Therefore, the variable speed operation mode shows an excellent carbon dioxide emission reduction in the low load region compared to constant speed operation. The high load output shows the same carbon dioxide emission as the constant speed operation mode.  Table 7 shows the results according to the load profile scenario of the 5,500 TEU vessel. The DC distribution system clearly showed higher efficiency (35.8%) compared with the AC distribution system (32.4%), with the observation that, where the load is variable, the constant speed generator consumes 73.6 kg of fuel, and the variable speed generator consumes 67.0 kg; an approximately 6.6 kg reduction. The cost difference was estimated at 8.9% based on the above fuel consumption. The research results are remarkable and convincing to demonstrate the excellence of the DC distribution system. Table 7 shows the amount of carbon dioxide emissions according to the fuel consumption. The DC distribution system emits 214.7 kg, and the AC distribution emits 235.9 kg, and thus it can be confirmed that 21.2 kg and 8.9% of the variable speed power generation system are lowered.  Table 7 shows the results according to the load profile scenario of the 5,500 TEU vessel. The DC distribution system clearly showed higher efficiency (35.8%) compared with the AC distribution system (32.4%), with the observation that, where the load is variable, the constant speed generator consumes 73.6 kg of fuel, and the variable speed generator consumes 67.0 kg; an approximately 6.6 kg reduction. The cost difference was estimated at 8.9% based on the above fuel consumption. The research results are remarkable and convincing to demonstrate the excellence of the DC distribution system. Table 7 shows the amount of carbon dioxide emissions according to the fuel consumption. The DC distribution system emits 214.7 kg, and the AC distribution emits 235.9 kg, and thus it can be confirmed that 21.2 kg and 8.9% of the variable speed power generation system are lowered. Table 7. The comparison results of the total output, total fuel oil consumption, system efficiency, total fuel oil cost, and the total CO 2 emissions for 5,500 TEU in the AC and DC distribution systems.

Item
Total  Table 8 shows the results according to the load profile scenarios of the 13K TEU vessel. In carbon dioxide emissions, the DC distribution system emitted 162.5 kg, and the AC distribution system emitted 185.7 kg, confirming that the emission rate of the DC distribution system was lowered by 12.4%.

Preliminary Lifecycle Assessment (LCA) in Response to Climate Change
Like other industries, the marine sector has been grappling with developing energysaving systems to reduce business costs and respond to climate change proactively. This research is in the same line with this common goal. Given this, there still needs to be some meaningful insight into the effectiveness of using the DC distribution system with a variable speed generator engine if applied for the case vessels from the lifecycle perspective. This paper further conducted a preliminary lifecycle assessment to evaluate the lifecycle reduction in emissions.
Marine fuels basically consist of two lifecycle stages: Well-to-Tank (WTT) and Tankto-Wake (TTW). The combination of the environmental impacts between WTT and TTS represents the lifecycle potentials contributed by the case ships. Figure 19 outlines the lifecycle stages of the marine fuels that also define the scope of preliminary LCA in this paper. The analysis was aided by GaBi LCA software version 2021. This commercial software enables users to develop lifecycle models for proposed products, energies, systems, etc.
This tool is coupled with an LCA database that stores massive information on energy consumption and emissions for thousands of life cycle activities. Using this tool under a user-friendly interface, all lifecycle models in the fuel production stages were developed, and key emission data were borrowed from the GaBi database. The WTT analysis was conducted based on the country and region. The models from the Gabi database consider the national fuel production methods from refinery methods and electricity production methods. meaningful insight into the effectiveness of using the DC distribution system with a vari-able speed generator engine if applied for the case vessels from the lifecycle perspective. This paper further conducted a preliminary lifecycle assessment to evaluate the lifecycle reduction in emissions.
Marine fuels basically consist of two lifecycle stages: Well-to-Tank (WTT) and Tankto-Wake (TTW). The combination of the environmental impacts between WTT and TTS represents the lifecycle potentials contributed by the case ships. Figure 19 outlines the lifecycle stages of the marine fuels that also define the scope of preliminary LCA in this paper. The analysis was aided by GaBi LCA software version 2021. This commercial software enables users to develop lifecycle models for proposed products, energies, systems, etc. This tool is coupled with an LCA database that stores massive information on energy consumption and emissions for thousands of life cycle activities. Using this tool under a user-friendly interface, all lifecycle models in the fuel production stages were developed, and key emission data were borrowed from the GaBi database. The WTT analysis was conducted based on the country and region. The models from the Gabi database consider the national fuel production methods from refinery methods and electricity production methods. Figure 20 outlines the overall LCA process conducted under the GaBi software process.  The proposed LCA model was developed in the GaBi user interface, which is directly linked to the GaBi LCA database. Using this database, the inputs (generally energies, resources, and materials) and the outputs (generally emissions) of the individual processes were determined. The quantification of those inputs and outputs of the holistic LCA process is represented as lifecycle inventory analysis. Then, the quantified emissions were then converted into global warming potential by means of CML 2001, which is a method to convert CO2, CO, and CH4 into global warming potential (GWP100 years) by proposing the normalization factors of 1 for CO2, 3 for CO, and 25 for CH4. Figure 21 shows the results of the WTT analysis across nine different countries. Since the case ships are engaged in international services, they are highly likely to receive bunker oil from various nations; thus, the analysis was extended to several countries to observe their environmental gaps. The proposed LCA model was developed in the GaBi user interface, which is directly linked to the GaBi LCA database. Using this database, the inputs (generally energies, resources, and materials) and the outputs (generally emissions) of the individual processes were determined. The quantification of those inputs and outputs of the holistic LCA process is represented as lifecycle inventory analysis. Then, the quantified emissions were then converted into global warming potential by means of CML 2001, which is a method to convert CO 2 , CO, and CH 4 into global warming potential (GWP100 years) by proposing the normalization factors of 1 for CO 2 , 3 for CO, and 25 for CH 4 . Figure 21 shows the results of the WTT analysis across nine different countries. Since the case ships are engaged in international services, they are highly likely to receive bunker oil from various nations; thus, the analysis was extended to several countries to observe their environmental gaps.
were determined. The quantification of those inputs and outputs of the holistic LCA process is represented as lifecycle inventory analysis. Then, the quantified emissions were then converted into global warming potential by means of CML 2001, which is a method to convert CO2, CO, and CH4 into global warming potential (GWP100 years) by proposing the normalization factors of 1 for CO2, 3 for CO, and 25 for CH4. Figure 21 shows the results of the WTT analysis across nine different countries. Since the case ships are engaged in international services, they are highly likely to receive bunker oil from various nations; thus, the analysis was extended to several countries to observe their environmental gaps.  For the TTW, given that the ship's lifespan is 30 years, the lifetime fuel consumption of the case ships was estimated based on their current operating profile and fuel records. The emissions from the fuel consumption are further estimated in Table 9.  Figure 22 shows the results of the comparative analysis between the AC distribution system and the DC distribution system.
The experimental results of 12.4% reductions in fuel consumption for 13K TEU and 8.9% reduction in fuel consumption for 5500 TEU ships were considered for this LCA comparison. The results suggest the importance of the distribution system in response to cost reduction and environmental protection, proving that a considerable amount of GWP emissions can be reduced by a DC distribution system with a variable speed generator engine.
On the other hand, it must be noted that this preliminary analysis was proposed to offer a brief idea of how much a proper system can reduce emissions; indeed, the results of this research must be considered indicative. Although this preliminary LCA study needs to be further elaborated with more accurate data and detailed analysis, the reduction level in emissions may be sufficiently significant so as to highly encourage the improvement of the DC distribution system in electric propulsion ships.

CH4
0.0024 CO2 3.17 SOx 0.002 (=20 × (0.1)% S content PM 0.00095 Figure 22 shows the results of the comparative analysis between the AC distribution system and the DC distribution system. The experimental results of 12.4% reductions in fuel consumption for 13K TEU and 8.9% reduction in fuel consumption for 5500 TEU ships were considered for this LCA comparison. The results suggest the importance of the distribution system in response to cost reduction and environmental protection, proving that a considerable amount of GWP emissions can be reduced by a DC distribution system with a variable speed generator engine.
On the other hand, it must be noted that this preliminary analysis was proposed to offer a brief idea of how much a proper system can reduce emissions; indeed, the results of this research must be considered indicative. Although this preliminary LCA study needs to be further elaborated with more accurate data and detailed analysis, the reduction level in emissions may be sufficiently significant so as to highly encourage the improvement of the DC distribution system in electric propulsion ships.

Discussion
This paper attempted to remedy the limited understanding of the performance of the DC distribution systems through simulation-based investigation. We also presented the benefits of the DC distribution systems quantitatively by comparing the AC and DC distribution system's electrical characteristics and their differences in performance.
There has been a lack of empirical research to reveal the benefits of DC distribution systems under the same system and environment. In this regard, this study presents the results of system efficiency improvement by combining a DC distribution system with an AFE rectifier and a variable speed engine. Based on the results of this study through LCA analysis, the amounts of carbon dioxide emission reductions in nine representative countries (each country had different lifecycle environmental impacts of marine fuels) were analyzed. To this end, this study examined the electrical characteristics, economic feasibility, and environmental characteristics through an AC-DC hybrid distribution electric propulsion system that could provide the same environment and conditions.
The numerical results regarding engine efficiency and the carbon dioxide emission reduction rate with the DC distribution system were analyzed for valuable technical information in the maritime sector. These results provide empirical data that is helpful to shipyard designers, shipping company officials, and policymakers. The experimental results answer the fundamental questions presented in the study's introduction, "Are electric propulsion systems the green solution?" and "How much does the improvement of the DC distribution system with AFE rectifier and a variable speed engine affect the decarbonization in shipping?" In other words, an electric propulsion system can also reduce carbon dioxide emissions as an eco-friendly propulsion system through additional system improvement. In particular, through the LCA results, the electric propulsion system can reduce carbon dioxide in a country and contribute to the strategy of reducing carbon dioxide through the controller's progress.

Conclusions
In this study, we conducted a comparative analysis between DC and the AC distribution systems. The load scenario extracted from the merchant ship was applied to the full-scale AC-DC hybrid distribution electric propulsion system. As a result, the excellence of the DC distribution system was demonstrated. The electrical characteristics of the electric propulsion system and fuel consumption, carbon dioxide emissions, and fuel costs were reviewed by comparing the fuel consumption per horsepower-hour. Through this study, we drew the following conclusions.
(1) We found a higher efficiency of the DC distribution system with a variable speed operation mode, which has recently been in demand, over the AC distribution constant speed operation mode, in the low-load and medium-load range. In both scenarios, DC distribution systems with AFE rectifiers and a variable speed engine demonstrated about 10% to 15% improvements in the system efficiency. (2) In the case of an electric propulsion system, using a constant speed engine showed a relatively significant efficiency reduction in the low-load range that caused system efficiency problems. This can be supplemented by a variable speed engine and by controlling the AFE rectifier to make a constant voltage in DC power. Moreover, it was possible to obtain a stable voltage in the DC distribution system as well as in the AC distribution system. (3) The DC distribution system in variable speed operation mode with an AFE rectifier showed improved results and trends of 8.9~12. 4 [%] in carbon dioxide emissions and fuel consumption compared with the AC distribution system. When the conventional constant speed engine was replaced with a variable speed engine, the efficiency improved by about 10 [%] with a reduction in carbon dioxide emissions. (4) Preliminary LCA results suggest the significance of developing/applying a DC distribution system for electric propulsion. Indeed, it can be confidently said that a DC distribution system with a variable speed generator engine would reduce lifecycle emission levels as much as by about 10% as given in Figure 22. The DC distribution was also revealed to be effective in response to climate change and cost reductions for electric propulsion marine vessels.
In the case of a DC distribution electric propulsion system, an AFE rectifier is required due to the voltage change according to the speed change caused by using a variable speed generator. Compared to the AC distribution system, which does not reflect this system, the DC distribution system improved by about 10% emission reduction. Still, the result may vary depending on the load conditions and operating time. In addition, even if the system efficiency assumes energy conversion loss, a factor that considers the age of the ship and the aging of the engine, it remains in the 30% range instead of the 40-50% range.
Therefore, it is necessary to calculate the amount of carbon dioxide emission reduction with this aging factor in a country. In addition, among the approximately 70,000 coastal ships in Korea, small ships cannot install all-electric propulsion systems, and DC distribution system installations are also limited. It is necessary to investigate the specifications of entire vessels. As further study for applying electric propulsion systems in small-and medium-sized vessels, the study of hybrid systems linking the battery and the existing generator will also result in the development of low-carbon ships.