1. Introduction
Global industry is expected to bring about technological change to prevent environmental pollution and global warming crises, as well as to address fluctuating fuel prices [
1]. For the international transportation industry, there is no other more dominant mode than shipping, as almost 80–90% of all international cargo is transmitted by sea [
2]. The greenhouse gas (GHG) emissions from the shipping industry account for more than 4.3% of global emissions [
3]. On the other hand, to mitigate the impact of shipping on the environment, the International Maritime Organization (IMO) has proposed the Energy Efficiency Design Index (EEDI) to demand fuel efficiency on new and existing vessels [
4]. Therefore, marine energy conservation and emissions reduction are serious issues that must be addressed sooner rather than later.
Marine diesel engines have been widely applied on vessels as the prime movers for propulsion and auxiliary genset [
5]. The use of alternative fuels, such as methanol [
6], ethanol [
7], and hydrogen, in marine engines is an essential option to reduce shipboard emissions. However, due to safety concerns and cost effectiveness problems, they do not seem to be preferable currently [
8]. Liquefied natural gas (LNG), as a relatively clean fossil fuel, is becoming the transition fuel on the path to zero-carbon ships [
9]. Compared with traditional marine fuels, it is an effective way to save fuel consumption and mitigate environmental pollution. It hardly produces any SO
X emissions and significantly reduces NO
X emissions, and there is also an elimination of around 25% in carbon dioxide (CO
2) emissions [
8]. Therefore, LNG-fueled ships equipped with marine dual fuel engines have developed rapidly in recent years [
10]. However, there is no single route to fully decarbonizing the maritime industry. Because marine engines with LNG as fuel cannot meet the 50% CO
2 emissions reduction target without further improvement, a multifaceted response is required [
11].
For a heavy-duty marine engine, about half of the fuel energy input is discharged into the environment from waste heat (e.g., exhaust gas, charge air, and jacket water) [
12]. Accordingly, waste heat recovery (WHR) technology has been recognized as one of the most efficient ways to improve an engine’s fuel efficiency and reduce CO
2 emissions for vessels [
13]. On the other hand, compared to automotive applications, marine engines have a more stable operating profile. Thus, the relatively stable waste heat is easier to recover [
14]. Under these circumstances, the thermodynamic cycle, as the bottoming cycle of marine engines, has been studied extensively [
15]. The single-pressure steam Rankine cycle (SRC) has been studied to replace the exhaust steam boiler for harvesting the exhaust gas heat of marine engines. Uusitalo A et al. [
16] adopted the SRC with four stages of a 1 MW-scale radial outflow turbine for exhaust gas heat recovery in cruise ships. Liang et al. [
17] considered using the engine jacket water to preheat the pump outlet working fluid of an SRC to increase its output power. Liu et al. [
18] utilized an SRC with the engine jacket water as its working fluid to reclaim the exhaust gas heat, and they found that the fuel efficiency of the marine engine can be boosted by more than 2.5% at 100% engine load.
As the space of a ship’s cabin is limited and the exhaust gas temperature of the low-speed turbocharged marine diesel engine is relatively low, some scholars consider the organic Rankine cycle for its waste heat recovery. Song et al. [
19] compared two separated ORC with a single ORC for the simultaneous recovery of the exhaust gas and jacket water waste heat of a marine engine. Their conclusion showed that the output work of the dual-loop ORC is 1.4% higher than the single cycle. Zhu et al. [
20] carried out a thermoeconomic analysis of a combined system integrated with a turbocharged marine diesel engine and an ORC. Sung T and Kim K C [
21] designed a dual-loop ORC for recovering the energy of the engine exhaust gas, jacket water, and LNG cold, and they concluded that the system could improve the power output of the marine dual fuel engine by 5.17%. An integrated dual-loop ORC for extracting a marine engine exhaust gas heat was proposed by Civgin M G and Deniz C [
22]. The results showed that when the working fluids of the dual loop are cyclohexane and R1234ze, the output performance of the WHR system is best.
The temperature of the dual fuel engine exhaust gas in the gas fuel mode is relatively higher than that in the liquid fuel mode, even after the turbocharger, due to the natural gas having a much higher flash point than diesel. The use of ORC to directly recover the exhaust gas heat has the risk of decomposing the working fluids [
23]. The supercritical CO
2 power cycle (SCPC) has the characteristics of thermal stability, high efficiency, compactness, and economy. In recent years, it has been widely employed for the waste heat recovery of internal combustion engines [
24]. Zhang et al. [
25] conducted a thermodynamic optimization of their own designed SCPC for utilizing engine exhaust heat. Li et al. [
26] analyzed the off-design performance of an SCPC for recovering the jacket water and exhaust gas heat based on the engine’s operational profile. For the efficient harvest of the waste heat of an internal combustion engine’s jacket water and exhaust gas, Song et al. [
23] upgraded a preheating SCBC and proved that it could improve the output work by 7.4%. Dedicated to saving the fuel consumption of an ocean-going vessel, Pan et al. [
27] employed an SCPC with a recompression layout for exhaust gas waste heat utilization. Through optimization, they stated that the SCPC could enhance the engine efficiency by 3.23%. However, when the heat source temperature was lower, the recuperative layout of an SCPC with a simple structure proved to have a better waste heat recovery performance than the recompression cycle.
To meet the multiple energy requirements of electricity and cooling capacity, some scholars considered the integrated cooling and power system driven by engine waste heat. Wu et al. [
28] performed a thermodynamic analysis of an engine cooling water heat-driven absorption refrigeration cycle (ARC) to enhance the performance of an SCPC driven by the exhaust gas heat, and they stated that the SCPC’s cycle efficiency can be raised by at least 9.8%. Xia et al. [
29] employed an SCPC and a thermally driven ejector refrigeration cycle (TERC) for engine exhaust gas waste heat cascade utilization. Huang et al. [
30] adopted an SCPC and an ORC combined cycle and a TERC to recover the energy carried by cooling water and flue gas, which are not utilized by an engine. Wu et al. [
31] designed an SCPC and an absorption refrigeration cycle combined system to harvest the engine exhaust gas waste heat. For marine applications, Wang et al. [
32] proposed four combined SCPC and ARC systems driven by marine diesel engine exhaust gas. They compared the performance of these systems and stated that the capital payback period of the best case is 6.85 years. Among these two thermal-driven refrigeration systems, due to the ARC having the disadvantages of high investment cost, large size, and corrosivity, the more inexpensive and compact TERC is recommended for shipboard waste heat recovery [
15].
From the perspective of the marine engine waste heat type, most of the studies mentioned above focus on exhaust gas and jacket water heat utilization. However, for large-scale marine engines, the waste heat of jacket water is inferior to the charge air heat in terms of energy quantity and quality.
Figure 1 displays the heat balance of a marine engine under full load [
12]. It can be seen that the exhaust gas and charge air heat together account for more than 80% of the engine waste heat and 42% of the fuel energy. These two types of waste heat have the most valuable recovery potential for the marine engine’s fuel efficiency improvement. On the other hand, from the viewpoint of energy output type, few studies simultaneously consider the shipboard electricity and cooling capacity requirements.
In this study, in order to meet the increasingly stringent emission requirements, a combined WHR system including an SCPC and a TERC is proposed for improving the overall energy efficiency of a marine dual fuel engine. In addition, the marine engine main waste heat of exhaust gas and charge air is set as the source of energy recovery, and the target needs of marine refrigeration and power are considered. Therefore, the WHR system is designed to harvest the main waste heat of the engine for shipboard power augmentation and refrigerating capacity production, which are needed the most. The motivation and purpose of this work are also demonstrated in
Figure 1. The detailed mathematical model of the proposed WHR system is built with the corresponding evaluation indicators and the optimization strategy. Firstly, the dual fuel engine and WHR combined system is compared with the standalone engine from the viewpoints of the thermodynamic, economic, and environmental aspects. The exergy destruction and exergetic efficiency are analyzed for the two cycles at the component level. Then, a detailed parametric study is performed. Finally, the system is optimized to maximize the overall system performance.