1. Introduction
Maritime transportation is the primary means of transport and is responsible for over 80% of the world’s trade by volume [
1]. Thus, it is also a major source of climate change and air pollution, which negatively impacts health and the environment [
2]. In response to this issue, the International Maritime Organization (IMO) has adopted various regulations and mandatory measures to avoid further negative impacts on the environment, control airborne emissions, and decrease greenhouse gases (GHGs). The IMO 2020 lowered the limit of the sulfur content of ship fuel from 3.5% in 2015 to 0.5% [
3], and the IMO GHG initial strategy also agreed to reduce carbon dioxide (CO
2) emissions per transportation by at least 40% by 2030 and total yearly GHG emissions by no less than 50% by 2050 in comparison with 2008 [
4,
5]. These strategies and targets have driven global maritime shipping to new technologies, renewable energy, and alternative fuel sources with low- or decarbonization. Recently, ammonia has emerged as a promising and potential marine fuel for avoiding CO
2 emissions, resulting in low climate impacts and a clean future [
6].
Ammonia (NH
3) is a carbon-free compound and one of the intermediate products in various industrial applications [
7,
8]. NH
3 can be widely produced by well-established methods and infrastructures such as the Haber–Bosch process, the electrochemical process, and power-to-gas technologies or produced from algae through a hydrothermal gasification combined system [
9]. This fuel can be easily stored, either refrigerated at −33 °C and atmospheric pressure or approximately 29.5 °C and pressurized to 800–1000 kPa [
6,
7,
10,
11]. In addition, since NH
3 contains a 45% higher volumetric hydrogen density than liquid hydrogen, it is also a potentially attractive medium for hydrogen storage. When the end-user requires hydrogen as a fuel for a specific application, ammonia can be converted to hydrogen using a very low amount of energy for the reforming process [
7]. Thus, ammonia can be utilized for energy purposes in several maritime applications, such as fuel cells and internal combustion engines [
7]. Aziz et al. [
12] performed the comprehensive review on ammonia in terms of production, storage and applications, and proved that ammonia is a strong candidate for use as a hydrogen carrier in the future and direct ammonia fuel cells are promising in terms of total energy efficiency. Ammonia linked to fuel cells is currently being studied by researchers and manufacturers owing to its advantages over internal combustion engines, as they provide high efficiency, less noise, lower emissions of air pollutants, and higher thermal efficiencies. Depending on the working temperature of the fuel cells, ammonia can be used directly or indirectly by a reformer (that splits ammonia into H
2 and N
2 before supplying to the fuel cells). The two typical types of fuel cells that can be used with ammonia systems are solid oxide fuel cells (SOFCs) and proton-exchange membrane fuel cells (PEMFCs). Other types of fuel cells, such as alkaline and alkaline membranes, are under development, and few experimental reports are available [
5]. SOFCs use ammonia directly [
13,
14], whereas PEMFCs require purified hydrogen (indirectly using ammonia by reforming). Focusing on the future of electric propulsion vessels, SOFCs, which consume ammonia directly for electric power and obtain reasonable energy efficiencies, possess great potential [
15,
16,
17].
Siddiqui et al. [
18] assessed the working performance of direct ammonia fuel cells and showed that an increase in electrolyte thickness decreases cell performance significantly. Thus, high-performance SOFCs can be obtained using low thickness, high temperatures, and electrolytes. Ezzat et al. [
19] developed a combined system of SOFCs and gas turbines using ammonia as fuel and evaluated the proposed system using the first and second laws of thermodynamics. They recovered the waste heat of the system through Rankine cycles and produced more power by its expanders. The obtained energy and exergy efficiencies of the suggested integrated system were 58.78% and 50.66%, respectively. Grasham et al. [
20] proposed a system of combined ammonia recovery and SOFCs for wastewater treatment plants and modeled it using Aspen Plus V.8.8. The results showed an increase of 45% in renewable electricity production, thereby achieving a 6% reduction in consumption and an annual reduction in GHG emissions of 3.4 kg CO
2 per person. Perna et al. [
21] designed a combined heat, hydrogen, and power system using ammonia as fuel for SOFCs to generate hydrogen at 100 kg/day via two different concepts. An analysis of the trigeneration efficiencies demonstrated that the two designed concepts achieved 81% and 71%. Eveloy et al. [
22] investigated a SOFC–Brayton cycle–Rankine cycle combined system to improve the efficiency and power generation of the system. The six working fluids used for the ORC were R123, R245fa, benzene, cyclohexane, toluene, and cyclopentane. The achieved energy and exergy efficiencies were 64% and 62%, respectively, and the efficiency improved by approximately 34% compared to the base gas turbine cycle. Baniasadi and Dincer [
17] analyzed the energy and exergy efficiencies of an SOFC system using ammonia as fuel. The total exergy efficiency of the system, as a function of the current density, was 60–90%, while the energy efficiency varied from 60% to 40%.
A survey of the existing literature and research shows that there is a lack of comprehensive studies and analyses on the integration of the SOFC and gas turbine (GT) bottoming cycle employing ammonia as fuel for marine applications. To fill this knowledge gap in the literature, the precise objectives of this study are as follows:
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To develop the novel integrated SOFC–GT–Rankine cycles–KC system using ammonia as a fuel.
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To analyze the energy and exergy efficiencies of the proposed system from a thermodynamic point of view.
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To conduct parametric studies to determine the reaction of this system to a variety of different circumstances.
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The novel aspects of this study are as follows:
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Usage of ammonia as ship fuel.
- -
Application of an integrated SOFC-based system for shipping vessels. The influence of the current density on the efficiency of SOFC and the integrated system.
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Usage of waste heat from an SOFC–GT hybrid system increases the thermal efficiency of the system. The estimation of energy and exergies efficiencies, the exergy destruction of main components of proposed system.
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Parallel integration of GT with cascaded Rankine cycles and KC to generate additional power for accommodation, lighting, auxiliary machinery and other purposes. The selection method for most suitable working fluid for optimize operation of the organic Rankine cycle.
The next structure of the current study is organized as follows.
Section 2 explains the background of ammonia-fueled SOFCs and energy systems for power generation, and introduces the proposed combined heat and power (CHP) system.
Section 3 evaluates each system section from a thermodynamic viewpoint and builds up the energy and exergy calculation model for each subsystem.
Section 4 outlines the assumptions, methodology, and procedures used in this study. The model is verified in
Section 5. The numerical modeling approach, results of this study in terms of energy and mass balances, and overall CHP performance, are presented in
Section 6. The conclusions are presented in
Section 7.
2. Materials and Methods: System Description
A general cargo ship with ammonia as fuel was chosen as the target of the proposed system with a total propulsion power of 3800 kW. The specifications of the modeling ship are listed in
Table 1. The target ship utilizes a type of electric propulsion powered by an ammonia supply system. A schematic of the overall configuration of the system is shown in
Figure 1. The general idea underlying the concept integrated system is the utilization of waste heat from SOFCs to generate useful work (electricity). As shown, ammonia is supplied to the SOFC before the working temperature is obtained from the regenerative heat exchanger. The steam Rankine cycle (SRC) absorbs waste heat from the SOFC and releases it into the working fluids. These processes generate electric power through their expander devices.
The schematic in
Figure 2 depicts the proposed combined system using ammonia as fuel, which comprises a fuel gas supply system, SOFC–GT, SRC, ORC and KC. The air and ammonia are supplied to the SOFC through the fuel gas supply system (FGSS). After reaction in the SOFC, the exhaust gas is led to the afterburner for completing combustion. This generates a considerable amount of heat and enlarges the temperature of the exhaust gas. Subsequently, the exhaust gas is utilized in the gas turbine (GT), heat exchangers, and bottom cycles to generate additional power. Thus, the cascade waste heat is utilized. The main components and working principles of the cycles are described below:
- ①
SOFC–GT: First, compressed air and ammonia are preheated in turn by the exhaust gas of the SOFC using two heat exchangers. Thus, the ammonia and compressed air can reach the required inlet temperature for the SOFCs. After preheating, the reforming and electrochemical reactions occur in the SOFC. These reactions generate electrical energy (by converting chemical energy to electrical energy) and produce a large amount of heat. Subsequently, the generated electricity (DC) is converted to an alternating current (AC) before being supplied to the ship propulsion system. There are two main exhaust streams from solid oxide fuel cells. There is exhaust of a nitrogen-rich stream from the cathode (stream 4–1) and a mixed water and nitrogen stream from the anode (stream 6–1) and main exhaust stream 6 to provide to the afterburner.
- ②
SRC: This operates mainly using the heat received from the exhaust gas of the heat exchanger (E-102). Water in the SRC is first pumped by a pump (P-100) to high pressure. Then, it proceeds to the heat exchanger (E-102) to become a superheated fluid. Next, the high-pressure steam (17) is depressurized in the expander (K-102) to drive the reversible heat pump and produce extra electric power. The saturated water mixture (19) is condensed in the heat exchanger (E-104) and releases heat to the fresh cooling water. This cooling water (21) after the heat exchanger (E-104) at 70.98 °C is used for the seafarers aboard ships.
- ③
KC: The evaporator heats the ammonia—water using the exhaust gas from the SOFC. The gas then powers the expander (K-104), which produces mechanical work. The liquid blends with the gas after the throttling valve. Subsequently, the working fluid is condensed in the condenser (E-106) before being pressurized by the pump (P-102). Finally, the working fluid is cooled to the required temperature before entering the heat exchanger (E-103) to complete the cycle.
- ④
ORC: The energy required by the ORC subsystem is supplied by heat exchange (E-108). 1,1-Difluoroethane (R152a) was chosen as the working fluid in this subsystem process. In addition, the working fluid is charged by heat transfer in the heat exchanger (E-108) and transferred to the ORC turbine with flow number 35. Power is produced by expanding the working fluid between flows 35 and 36. The working fluid from the ORC turbine is transferred to the heat exchanger (E-105) with a flow number of 37. In E-105, energy transfer occurs between the working fluid in the ORC and fresh cooling water, thereby providing hot water (39) for the accommodation of seafarers. The working fluid in the ORC subsystem from E-105 is transferred to the pump (P-101) with flow 37 to increase the pressure, and then to E-108 with flow 34 for energy transfer.
The subsystem coupling connection must be compatible with the exhaust gas cascade. The SRC recompression cycle works at high temperatures, but KC and ORC recover waste heat at lower temperatures, improving the thermal efficiency.
5. Materials and Methods: Modeling Verification
The values calculated using the integrated model employing ammonia as fuel, which was proposed in this study, and the corresponding values listed in the literature are presented in
Table 6. The estimated values are consistent with the data in the literature, and the discrepancy between the current data and the literature data is maintained within a tolerable range.
The proposed system has the potential to simultaneously supply power for the propulsion plant and other electric equipment, and provide hot water for the seafarers on board ships.
Since the subsystem is responsible for 26.59% of the total power production of the integrated system, its verification is critical. Unfortunately, there are no experimental installations of the SOFC–GT subsystem with direct ammonia injection accessible [
9]; however, there are experimentally validated models in the literature. Chitgar et al. [
32] reported a multigeneration system, SOFC–GT, that generated a total power of 4910.4 kW (0.07% difference) with energy and exergy efficiencies of 56.9% and 54.7%, respectively. The present model has relative errors of 3.5% and 2.6% for the SOFC–GT energy and exergy efficiencies, respectively. These error values demonstrate that the proposed model yields reasonable results.