1. Introduction
Due to the ever-increasing energy costs, the topic of energy efficiency is gaining more value each day. One way to improve energy efficiency is to recover waste heat in systems. Since sea transport is the most economical method in international trade, cargo ships are extensively used. These ships with large engines have a very high energy consumption. Apart from running the engines of commercial ships, there is a continuous need for energy to keep other electronic systems operational. Therefore, recovering energy from the exhaust of the ship’s engine and converting it back into electrical energy through recycling will significantly enhance energy efficiency. One system that can be designed for this purpose is the organic Rankine cycle (ORC), which has some differences from the traditional Rankine cycle [
1].
The Rankine cycle converts mechanical energy into electrical energy using a working fluid. This cycle is widely employed in various industrial thermodynamic applications and energy production [
2]. The Rankine cycle consists of four main components: the boiler, turbine, condenser, and pump [
3]. The boiler is where water in the cycle is vaporized at high temperatures and pressure. The flow energy of the steam obtained from the boiler is converted into mechanical energy by turning the turbine blades. The rotating blades move the turbine rotor. After the work is completed in the turbine, the steam goes to the condenser at low pressure and temperature. The condenser condenses the steam back into liquid water. The heat generated during this process is exchanged with the working fluid and released into the surroundings. The pump transports the condensed water back to the steam generator. The condensed water is subjected to vaporization again by bringing it back to high temperature and pressure. The Rankine cycle generally operates with this loop. In the steam generator, water is vaporized, and the resulting steam expands in the turbine, producing mechanical energy [
4]. This cycle is commonly used to generate electricity in thermal power plants, nuclear power plants, and some industrial facilities.
ORC is a cycle used in low-temperature applications and can convert heat from low-temperature sources such as solar energy, geothermal energy, and industrial waste heat recovery into electrical energy [
5]. Like the Rankine cycle, the ORC is a vapor-based thermodynamic cycle. The fundamental difference between these two cycles arises from the characteristics of the liquid refrigerant they use. The Rankine cycle typically utilizes inorganic liquid refrigerants like water or water-based mixtures. In this cycle, water is vaporized and is commonly used to generate electricity in thermal power plants. On the other hand, ORC uses organic liquid refrigerants. These fluids have low boiling points and vaporization characteristics. The Rankine cycle generally harnesses heat from high-temperature sources. In contrast, ORC can more effectively utilize heat from low-temperature sources. The ORC has the advantage of efficiently converting heat from low-temperature sources into electrical energy, making it an ideal choice for applications such as industrial waste heat recovery. Due to these features, ORC plays a significant role in sustainable energy production and efficiency [
6].
The Rankine cycle typically utilizes inorganic liquid refrigerants. Water is the most common and economically used inorganic working fluid. Water-based mixtures can also be used, especially in the evaporator, to reduce the risk of freezing at low temperatures. On the other hand, the ORC uses organic working fluids. These fluids are used in low-temperature applications due to their low boiling point characteristics [
7]. Examples of organic working fluids include components such as R134a, R245fa, R600, and pentane. The ORC is designed to harness heat from low-temperature sources more effectively. Therefore, organic working fluids are more commonly used in low-temperature applications. In contrast, inorganic working fluids like water and water-based mixtures are used in high-temperature applications and industrial thermal power plants. The preferred refrigerants for these cycles are selected based on the application’s requirements, environmental conditions, and efficiency goals [
8].
The working fluids used in RC can generally be classified into three categories: dry, isentropic, and wet. These terms are used based on various physical properties of the fluid. Dry fluid refers to the refrigerant in a completely vaporized state. In other words, if the fluid exists solely in the vapor phase and has no presence in the liquid phase, it is termed a dry fluid. Dry fluids are typically used in evaporator devices, where the refrigerant is entirely in the vapor phase [
9]. For instance, dry fluids are commonly used in household cooling systems such as air conditioners and refrigerators. Dry fluids are also generally used in gas turbines.
The term isentropic describes the condition where a fluid undergoes an isentropic process. An isentropic process is a process where entropy is kept constant. An isentropic fluid is a fluid in thermodynamic equilibrium, where its entropy is constant. Isentropic fluids are often used to model ideal conditions (e.g., the Brayton cycle in jet engines) [
10]. They can also be used in steam engines.
Wet fluids represent fluids that exist in both the liquid and vapor phases. In other words, it signifies a mixture state with some portion in the liquid phase and some in the vapor phase. Wet fluids often represent a situation observed in real engineering applications and systems [
11]. For example, it could denote the state where liquid refrigerant begins to evaporate in an evaporator or where gas starts to condense in a compressor. Wet fluid systems are typically observed in extensive industrial cooling facilities, energy production plants, and large-scale air conditioning systems. Also, they are generally used in the liquefaction of gases (e.g., in cryogenic cooling systems).
In recent years, academic studies on ORC systems have increased, turning a topic that was less explored into a significant research interest. Zhao and colleagues, for example, designed a new system using the ORC to recover waste heat generated during silicon production [
12]. CFD analysis showed that this design increased energy efficiency by 64.14% and exergy efficiency by 72.13%. Their economic evaluation indicated that the system would pay for itself in approximately four years. They concluded that the ORC provides a significant advantage in recovering system waste heat. Kong and team developed an energy integration scheme using the ORC to recover energy generated during the purification process of isopropanol–butanol–ethanol fermentation [
13]. Their study showed a 100% reduction in low-pressure steam consumption and a 58.80% reduction in cooling water consumption using the ORC for energy optimization. They utilized R600a as the working fluid in their cycle. Mubashir and colleagues proposed a new system for waste heat recovery in an internal combustion engine [
14]. They conducted energy and exergy analyses for their system, evaluating configurations using both the Brayton cycle and ORC to generate power from the waste heat of a 470 °C engine. Their results indicated that the ORC was the most efficient system for waste heat recovery in internal combustion engines. Zakeralhoseini and Schiffmann conducted a waste heat recovery calculation using an ORC-based system for a high-speed turbopump at a small scale [
15]. Their experimental and numerical studies showed that using the ORC increased the turbo pump’s efficiency by 30% and reduced the reverse flow ratio by 50%. They emphasized the usefulness of the ORC system in recovering waste heat from turbo pumps. Tsimpoukis and team investigated the integration of an ORC unit into an injector-assisted CO
2 cooling system with a standard discharge for compressors [
16]. According to their yearly performance analysis for the city of Athens, Greece, they found that working with isopentane resulted in the highest energy production of 10.36 MWh annually, leading to a 3.69% decrease in energy consumption for the cooling system. Anastasovski and colleagues examined various approaches for integrating the ORC into thermal process systems [
17]. They compared and systematized all the methods they studied in a general scheme for integrating the ORC with low-temperature industrial waste heat sources. Their findings indicated that using ORC improved efficiency in low-temperature waste heat processes. Mondejar and team evaluated using ORC systems for waste heat recovery on ships [
18]. Their results showed that using ORC in engines using low-sulfur fuel reduced fuel consumption by 10% to 15%. Holik and colleagues performed a two-objective optimization using commercial software to increase power output and reduce the total heat exchanger surface area [
19]. They compared configurations using exhaust gas temperatures of 394.2 °C and mass flow rates of 0.306 kg/s, analyzing fluids such as water, ethanol, toluene, and hexamethyl disiloxane. Their study concluded that ethanol was the most efficient fluid for power output and heat exchanger characteristics in ORC systems. In summary, these studies collectively demonstrate the versatility and efficiency of the ORC in various applications, emphasizing its potential in waste heat recovery and energy optimization across different industries.
Toluene is an organic component commonly used in the R-410A mixture, a refrigerant widely employed in air conditioning systems and some commercial and industrial cooling applications [
20]. Toluene is included in this mixture to enhance its performance. R600 (isobutane) is a natural hydrocarbon gas frequently used in household refrigeration devices such as refrigerators and freezers [
21]. It is considered an environmentally friendly option among natural refrigerants. Isopentane is another hydrocarbon gas used as a refrigerant, especially in industrial cooling systems and commercial refrigeration devices [
22]. Natural refrigerants like isopentane are valued for their eco-friendly and energy-efficient characteristics. n-hexane, also known as R-40A, is a type of hydrocarbon refrigerant [
23]. This refrigerant can be used in industrial cooling systems, particularly industrial air conditioning systems. Hydrocarbons like hexane are among the options for natural refrigerants. R-717, commonly known as ammonia, is a natural refrigerant. It is typically used in industrial cooling systems, especially in the food industry [
24]. This refrigerant is preferred due to its low-cost, high-energy efficiency, and environmentally friendly characteristics. R-718 represents water. Water is a coolant in various industrial applications and steam-based energy systems [
25]. For instance, water can be used in energy production with steam turbines and industrial cooling systems. R-123 is a dichlorotrifluoroethane compound commonly used in large-scale commercial and industrial air conditioning systems [
24]. This refrigerant is preferred for its high efficiency and low toxicity. R-142b is a chlorodifluoromethane compound that was once widely used but has now been replaced by environmentally friendly alternatives [
24]. It has been especially utilized in commercial refrigeration and air conditioning systems. R-600a is a natural hydrocarbon gas known as isobutane [
25]. This refrigerant is used in small cooling devices such as household refrigerators and freezers. R-245fa is a pentafluoropropane compound often used in industrial waste heat recovery and the ORC as a refrigerant [
23]. Although R-245fa has been used extensively in ORC systems, its high greenhouse gas content has caused it to be phased out by several environmental restrictions, such as the European Union’s F-Gas Regulation. Manufacturers favor substitutes with less of an impact on the environment over R-245fa. R-141b is a dichlorofluoromethane compound and used to be one of the older types of refrigerants [
26]. However, it has generally been replaced by environmentally friendly alternatives and was previously used in commercial refrigeration systems and some industrial applications.
The usage of specific chemicals in various industries, including those utilized as working fluids in ORC systems, has been increasingly impacted by PFAS (Per- and Polyfluoroalkyl Substances) regulations. PFAS production, usage, and disposal are subject to stricter regulations from regulatory agencies such as the European Chemicals Agency (ECHA) and the U.S. Environmental Protection Agency (EPA). This includes restricting the amount of PFAS in products and outright outlawing specific PFAS compounds.
The detailed descriptions of each fluid’s characteristics and applications are provided in
Table 1.
The International Maritime Organization (IMO) amended MARPOL Annex VI in 2011 to require operational and technological energy efficiency measures to lower the quantity of CO2 emissions from international vessels. On 1 January 2013, the Ship Energy Efficiency Management Plan (SEEMP) and the Energy Efficiency Design Index (EEDI) were enacted. In 2021, IMO member states enacted additional energy efficiency measures to lower international shipping’s carbon intensity by a minimum of 40% by 2030 compared to 2008.
Additionally, within the scope of this study, the effect of adding a recuperator to the designed RC system on efficiency has been investigated. These calculations were performed separately for each working fluid used in the RC design, and the results were compared. Recuperators can be employed to enhance efficiency in specific thermodynamic cycles. In the RC, a recuperator can heat the feedwater by using the heat of the water leaving the condenser. This ensures the vapor has higher temperatures and more energy [
1].
Recuperators are particularly useful in systems with reduced heat emissions to the environment, facilities where energy recovery occurs, and continuously operating industrial applications [
27]. This enhances system efficiency, reduces energy consumption, and minimizes environmental impacts. Recuperators recover waste heat from a system and convert this heat into usable energy for beneficial purposes. This results in energy savings and increases the overall efficiency of the system. Recuperators enable the system to operate more efficiently by recovering the energy of hot gases or liquids within the system instead of losing it as waste heat. This translates to reduced energy consumption. The recovery of waste heat can help reduce energy costs, significantly saving long-term operating expenses. By promoting energy savings, recuperators contribute to a decrease in fossil fuel consumption, subsequently reducing environmental impacts. Lower energy consumption can contribute to reducing greenhouse gas emissions and environmental pollution. In large-scale HVAC systems, recuperators can recover waste heat from indoor air units or outdoor air intakes for use in beneficial tasks such as heating or hot water production [
1].
Furthermore, exergy analysis and irreversibility calculations have been conducted in another section of this study. These calculations were performed separately for each fluid used in the ORC design, and the results were compared. Exergy is a concept in thermodynamics that measures the quality or availability of energy [
28]. It indicates how efficiently energy is utilized compared to the conditions of the system’s surroundings. Higher exergy indicates that energy is more practical and of higher quality, whereas lower exergy suggests less valuable energy. Exergy analysis of a cooling system is used to assess the system’s energy efficiency, identify energy losses, and optimize system performance. Exergy analysis is a crucial tool for improving the energy efficiency of cooling systems and using resources more effectively. Irreversibility, in thermodynamics, indicates how effective a thermodynamic process is or how efficiently a process occurs. It shows how much loss or energy waste is inherent in a process. In thermodynamics, the more irreversible a process is, the less efficient it is. The irreversibility value of a cooling system is used to determine how effective the system is or how efficient the process is. This value is obtained by comparing the actual and ideal processes [
29].
Water is the preferred working fluid for large-scale Rankine cycles that use high-temperature energy sources in a wide range of efficient cycle configurations, from saturated nuclear power plants to coal-fired ultra-supercritical cycles. However, steam’s thermodynamic features require multistage, capital-intensive turbines, complex plant systems, and liquid formation during expansion; these limitations render steam an unsuitable working fluid for low temperature and power output applications.
The ORC aims to generate power from moderate-temperature, distributed energy sources cost effectively. This is accomplished by selecting a suitable working fluid capable of facilitating a basic plant arrangement and a low-cost turbine without liquid formation.
Although the traditional Rankine cycle (RC) and the organic Rankine cycle (ORC) share the same principle, they have many differences [
30]. A comparison between the RC and ORC is provided in
Table 2.
In this study, a traditional RC and ORC system designed to recover the energy released during the full-load operation of a ship engine is presented. The designed system incorporates eleven working fluids, comprising four dry, two wet, and five isentropic conditions. In this work, the Engineering Equation Solver (EES) [M1] [HK2] (V11.898) and NIST Reference Fluid Thermodynamic and Transport Properties Database (REFPROP) (V10) commercial software are employed to obtain the thermophysical properties of each refrigerant used in the RC, and calculations are performed accordingly.
3. Results
In this study, the condenser temperature of 35 °C for effective heat transfer was chosen because the maximum seawater temperature is 32 °C, according to the Loydu Rules for the Classification of Naval Ships [
36]. The mass flow rate of seawater through the condenser changes depending on the seawater inlet temperature.
The efficiency of a simple cycle and a cycle with a recuperator for dry fluid is illustrated in
Figure 3. The following can be seen in the figure:
If the turbine inlet temperature is above 200 °C, the highest efficiency among dry fluids is achieved in the RC system with a recuperator using toluene;
For the turbine inlet temperature of 300 °C, the efficiency of the toluene in the recuperator system is calculated to be 38%;
If the turbine inlet temperature is below 200 °C, the recuperator system with n-hexane yields the highest efficiency;
For a turbine inlet temperature of 200 °C, the efficiency of the n-hexane in the recuperator system is 37%;
It has been observed that using a recuperator in dry fluids increases the cycle efficiency by approximately 2%.
Figure 4 illustrates the efficiency of a simple RC for wet fluid. The highest efficiency among the wet fluids compared in this study is obtained from R718.
For a turbine inlet temperature of 300 °C, the efficiency of the R718 fluid is calculated to be 33%;
Additionally, due to the significantly broader operating range of R718 compared to R717, using R718 as the wet fluid in RC systems is seen as more advantageous.
The efficiency of a simple RC for isentropic fluid is illustrated in
Figure 5. The following can be seen in the figure:
If the turbine inlet temperature is above 160 °C, the highest efficiency among isentropic fluids is obtained from R141b;
For a turbine inlet temperature of 200 °C, the efficiency of the R141b fluid is calculated to be 23%;
If the turbine inlet temperature is below 160 °C, the highest efficiency among isentropic fluids is obtained from R123;
For a turbine inlet temperature of 160 °C, the efficiency of R123 is calculated to be 23%;
In situations where the turbine inlet temperature is below 160 °C, the efficiency ranking among isentropic fluids is as follows: R123, R245fa, R600a, R141b, and R142b.
The exergy efficiency of a simple cycle and a cycle with a recuperator for dry fluids is illustrated in
Figure 6. The following can be seen in the figure:
If the turbine inlet temperature is above 200 °C, the highest exergy efficiency among dry fluids is obtained from the recuperator-based RC system using toluene;
For a turbine inlet temperature of 300 °C, the exergy efficiency of toluene fluid in the recuperator-based ORC system is calculated as 90%;
If the turbine inlet temperature is below 200 °C, the highest exergy efficiency among dry fluids is obtained from the recuperator-based RC system using n-hexane;
For a turbine inlet temperature of 200 °C, the exergy efficiency of n-hexane fluid in the recuperator-based RC system is calculated to be 86%;
Additionally, it has been observed that using a recuperator in RC systems increases the exergy efficiency by approximately 5%.
The exergy efficiency for wet fluid is illustrated in
Figure 7. The following can be seen in the figure:
The highest exergy efficiency among the wet fluids compared in this study belongs to R718;
For a turbine inlet temperature of 300 °C, the exergy efficiency of the R718 is calculated to be 71%.
The exergy efficiency for isentropic fluid is illustrated in
Figure 8. The following can be seen in the figure:
If the turbine inlet temperature is above 160 °C, the highest exergy efficiency among isentropic fluids is obtained from R141b;
For a turbine inlet temperature of 200 °C, the exergy efficiency of the R141b fluid is calculated to be 50%;
If the turbine inlet temperature is below 160 °C, the highest exergy efficiency among isentropic fluids is obtained from R123;
For a turbine inlet temperature of 160 °C, the exergy efficiency of the R123 fluid is calculated to be 50%.
The net power and irreversibility for a simple RC with dry fluid are illustrated in
Figure 9. The following can be seen in the figure:
The highest net power among dry fluids above a turbine inlet temperature of 200 °C is obtained from toluene;
For a turbine inlet temperature of 300 °C, the net power obtained from the toluene is calculated to be 201.094 kW;
If the turbine inlet temperature is below 200 °C, the highest net power among dry fluids is obtained from n-hexane;
For a turbine inlet temperature of 200 °C, the net power obtained from the n-hexane is calculated to be 193.93 kW;
Additionally,
Figure 9 shows that irreversibility and net power are inversely proportional. Irreversibility decreases as net power increases.
The net power and irreversibility for a dry fluid in RC with a recuperator are illustrated in
Figure 10. The following can be seen in the figure:
The highest net power among dry fluids with a turbine inlet temperature above 200 °C is obtained from toluene;
For a turbine inlet temperature of 300 °C, the net power obtained from the toluene fluid is calculated to be 215.512 kW;
If the turbine inlet temperature is below 200 °C, the highest net power among dry fluids is obtained from n-hexane;
For a turbine inlet temperature of 200 °C, the net power obtained from the n-hexane is calculated to be 207.894 kW;
Additionally, as shown in
Figure 10, irreversibility and net power are inversely proportional. Irreversibility decreases as net power increases.
The net power and irreversibility for a wet fluid in RC are illustrated in
Figure 11. The following can be seen in the figure:
The net power and irreversibility of an isentropic fluid in a simple RC are illustrated in
Figure 12. The following can be seen in the figure:
For turbine inlet temperatures above 160 °C, the highest net power among isentropic fluids is obtained from R141b;
In cases where the turbine inlet temperature is below 130 °C, the highest net power is obtained from R123.
The proposed system design, with waste heat of diesel engine onboard ships enables not only improved energy efficiency, fuel savings, reduction in greenhouse gas emissions, and compliance with environmental regulations but also economic competitiveness, promoting technological innovation, increased operational flexibility, broader impact, contribution to the circular economy, potential for retrofitting existing fleets, and stimulating job creation and industry growth.
4. Conclusions
In this study, thermodynamic analysis for a seawater-cooled and marine diesel engine exhaust-driven organic and inorganic RC has been carried out. The cycle efficiencies for a simple RC and an RC with a recuperator for the marine engine have been investigated. The systems’ cycle efficiencies, exergy efficiencies, net power, and irreversibility have been computed versus turbine inlet temperature by keeping the condensing temperature constant. The net power and irreversibility of the system have been calculated at full load for marine diesel engines.
The cycle and exergy efficiencies of RC are highly dependent on the working fluid.
For dry fluids, the most efficient fluid at low turbine inlet temperatures is n-hexane, while at high turbine inlet temperatures it is toluene.
For isentropic fluids, the most efficient fluid at low turbine inlet temperatures is R123, while at high turbine inlet temperatures it is R141b. The results demonstrate that when the turbine inlet temperature in the RC system with a recuperator is above 200 °C, the dry fluid with the highest efficiency is toluene. When it is below 200 °C, dry fluid with the highest efficiency is n-hexane. Although toluene and n-hexane have an ODP of 0 and a low GWP of 3, they have higher flammability. R123, which features higher toxicity, and R141b have a high GWP.
For wet fluids, R718 is one of the most suitable working fluids at low and high turbine inlet temperatures for marine diesel engine exhaust heat onboard ships with a safety group classification of A1, ODP of 0, and GWP100 of 0 at high temperatures among compared fluids. For a turbine inlet temperature of 300 °C, R718 has a cycle efficiency of 33%, exergy efficiency of 71%, net power of 171.544 kW, and irreversibility of 67.773 kW at a whole load of marine diesel engine. R718 is environmentally friendly, abundant in nature, and can be used safely onboard ships.
This study contributes to significant improvements in fuel efficiency and reductions in greenhouse gas emissions, leading to more sustainable and cost-effective maritime operations.