Waste Heat Utilization in Marine Energy Systems for Enhanced Efficiency

Abstract

The maritime industry, central to global trade, faces critical challenges related to energy efficiency and environmental sustainability due to significant energy loss from waste heat in marine engines. This review investigates the potential of waste heat recovery (WHR) technologies to enhance operational efficiency and reduce emissions in marine systems. By analyzing major WHR methods, such as heat exchangers, Organic Rankine Cycle (ORC) systems, thermoelectric generators, and combined heat and power (CHP) systems, this work highlights the specific advantages, limitations, and practical considerations of each approach. Unique to this review is an examination of WHR performance in confined marine spaces and compatibility with existing ship components, providing essential insights for practical implementation. Findings emphasize WHR as a viable strategy to reduce fuel consumption and meet environmental regulations, contributing to a more sustainable maritime industry.

1. Introduction

1.1. Background on Marine Energy Systems

Marine energy systems are integral to the global maritime industry, serving as the primary means for propulsion and onboard power generation in commercial vessels, naval ships, and offshore platforms. These systems predominantly rely on large-scale internal combustion engines, gas turbines, and auxiliary machinery to convert chemical energy from fossil fuels into the mechanical and electrical energy required for vessel operations. The efficiency and reliability of these energy systems are crucial, as they directly influence operational costs, vessel performance, and environmental impact [1,2,3].

The maritime industry has witnessed significant growth over the past decades, correlating with increased global trade and the expansion of offshore activities. This growth has heightened the demand for energy-efficient solutions to mitigate fuel consumption and reduce greenhouse gas emissions. Energy efficiency in marine operations not only contributes to cost savings but also aligns with the international regulations aimed at minimizing the environmental footprint of maritime activities. Regulatory bodies such as the International Maritime Organization (IMO) have established guidelines and standards, including the Energy Efficiency Design Index (EEDI) and the Ship Energy Efficiency Management Plan (SEEMP), to promote energy conservation and environmental stewardship within the industry [4,5,6].

1.2. The Problem of Waste Heat

Despite advancements in engine technology and energy management practices, a significant portion of the energy generated in marine systems is lost as waste heat. Internal combustion engines, which are the backbone of marine propulsion, typically exhibit thermal efficiencies ranging from 35% to 50%, meaning that up to 65% of the fuel’s energy content is dissipated as heat through exhaust gases, engine cooling systems, and other thermal losses. This waste heat represents a substantial untapped energy resource that, if harnessed effectively, could enhance overall system efficiency and reduce fuel consumption [6,7,8].

The unutilized waste heat in marine systems has both environmental and economic ramifications. Environmentally, the dissipation of waste heat contributes to thermal pollution and exacerbates the emission of greenhouse gases and other pollutants due to increased fuel usage. Economically, the loss of potential energy leads to higher operational costs associated with fuel consumption, which constitutes a significant portion of a vessel’s operating expenses. Moreover, the rising costs of marine fuels and the implementation of stricter emission regulations intensify the need for innovative solutions to recover and utilize waste heat effectively [9,10,11].

1.3. Purpose and Scope of the Paper

The primary objective of this paper is to explore and evaluate methods for enhancing the efficiency of marine energy systems through the utilization of waste heat. By analyzing the current technologies and implementation strategies for waste heat recovery (WHR) in maritime applications, the study aims to identify viable solutions that can be integrated into existing and new vessels to improve energy efficiency and reduce environmental impact [12,13].

The scope of this paper encompasses a comprehensive review of waste heat sources in marine systems, an examination of various WHR technologies such as heat exchangers, Organic Rankine Cycle (ORC) systems, thermoelectric generators, and combined heat and power systems, and an assessment of their applicability and performance in maritime environments. The study also addresses the challenges associated with implementing WHR technologies, including the technical, economic, and regulatory considerations, and discusses potential solutions and future research directions [14,15].

By providing insights into waste heat utilization in marine energy systems, this paper contributes to the broader efforts of enhancing energy efficiency in the maritime industry, supporting environmental sustainability, and promoting compliance with international regulations.

To thoroughly explore waste heat recovery (WHR) in marine systems, this paper is organized into ten sections. Section 2 provides an overview of primary waste heat sources in marine engines and auxiliary systems. Section 3 examines WHR technologies, including heat exchangers, Organic Rankine Cycle (ORC) systems, thermoelectric generators, and combined heat and power (CHP) systems, and assesses their suitability for maritime applications. In Section 4, the performance and efficiency analyses of these technologies are presented, while Section 5 evaluates their economic implications through cost–benefit and payback analyses. Environmental impacts are addressed in Section 6, focusing on emission reduction and compliance with regulations. Design challenges are detailed in Section 7, covering space constraints, weight considerations, and material durability. Section 8 discusses the operational adjustments for optimal WHR system integration on vessels. The local and global environmental benefits of WHR adoption are reviewed in Section 9 and, finally, Section 10 outlines the study limitations and proposes future research directions, aiming to advance WHR applications for sustainable maritime operations.

2. Overview of Waste Heat Sources in Marine Systems

Marine vessels are equipped with various energy systems that generate significant amounts of waste heat during operation. Understanding the sources of this waste heat is essential for developing effective recovery and utilization strategies [16,17]. The primary sources of waste heat in marine systems include main propulsion engines, auxiliary engines, exhaust gases, and cooling systems such as jacket water and charge air cooling (

Table 1. Sources of Waste Heat in Marine Systems.

Source	System Component	Average Temperature Range (°C)	Percentage of Total Heat Loss
Main Engine Exhaust [18]	Exhaust gases	300–500	25–35% (Lion i in. 2020). [19]
Cooling Systems [20]	Jacket water, Charge air	40–90	15–25% (Liu i in. 2020). [21]
Auxiliary Engines [22]	Exhaust gases, Cooling	200–400	10–20% (Konur i in. 2022). [23]

).

2.1. Main Engines and Auxiliary Engines

Marine propulsion is predominantly achieved through internal combustion engines (ICEs), which can be categorized into the main engines for propulsion and the auxiliary engines for onboard power generation. Both types of engines operate on thermodynamic cycles, primarily the Diesel cycle for compression–ignition engines, which inherently produce waste heat as a byproduct of fuel combustion [24,25] (Figure 1).

During combustion, the chemical energy of the fuel is converted into thermal energy, producing high-pressure and high-temperature gases that perform work by expanding against the engine’s pistons or turbine blades. However, due to the second law of thermodynamics and inherent inefficiencies in the energy conversion processes, not all the thermal energy can be transformed into mechanical work [21]. A substantial portion of the energy input is lost as waste heat through the following pathways:

• Exhaust Gases: Combustion gases expelled from the engine cylinders carry away a significant amount of thermal energy [26];
• Engine Cooling Systems: To maintain optimal operating temperatures and prevent overheating, engines utilize cooling systems that absorb excess heat from engine components [27];
• Radiation and Convection: Heat is also lost to the surrounding environment through radiation from engine surfaces and convection with ambient air [28].

The main engines, due to their large size and high power output, contribute the majority of waste heat in marine vessels. The auxiliary engines, although smaller, collectively produce substantial waste heat when operating continuously to supply electrical power and support the systems onboard [22,29].

2.2. Exhaust Gases

Exhaust gases are one of the most prominent carriers of waste heat in marine engines. After combustion, the exhaust gases exit the engine cylinders at high temperatures, typically ranging from 300 °C to 500 °C, depending on the engine load and fuel type. These gases possess considerable thermal energy due to their high enthalpy, representing approximately 25% to 35% of the total energy input from the fuel [21,30].

The high energy content in exhaust gases presents a significant opportunity for heat recovery. By implementing waste heat recovery (WHR) systems, such as exhaust gas boilers (also known as economizers) and heat exchangers, it is possible to capture this thermal energy and utilize it for the following purposes:

• Steam Generation: Exhaust heat can be used to produce steam in boilers, which can then drive steam turbines for additional power generation or supply thermal energy for heating systems [31];
• Electricity Generation: Through technologies like the Organic Rankine Cycle (ORC), low-grade heat from exhaust gases can be converted into electrical energy;
• Heating Applications: Recovered heat can be utilized for preheating fuel, lubricants, or providing heating for crew accommodations and onboard facilities [32].

The effective recovery of exhaust gas heat can significantly enhance the overall thermal efficiency of marine engines, reduce fuel consumption, and lower emissions by decreasing the workload on primary energy sources [33,34].

2.3. Cooling Systems

Marine engines require efficient cooling systems to dissipate the excess heat generated during operation and maintain the engine components within safe temperature limits. The primary cooling systems include jacket water cooling and charge air cooling, both of which extract substantial amounts of thermal energy from the engine [35,36].

• Jacket Water Cooling: This system circulates a coolant (usually treated fresh water) around the engine block and cylinder liners to absorb heat from combustion. The jacket water exits the engine at temperatures typically between 70 °C and 90 °C, carrying away approximately 15% to 25% of the total waste heat produced by the engine [37,38].
• Charge Air Cooling: After compression in turbochargers, the charge air (combustion air) becomes heated, which can reduce engine efficiency and increase the risk of knocking. Charge air coolers (intercoolers or aftercoolers) lower the temperature of the compressed air before it enters the combustion chamber, enhancing engine performance and efficiency. The cooling process absorbs heat from the charge air, with outlet temperatures usually around 40 °C to 50 °C [34,39,40].

The waste heat extracted by cooling systems represents a significant thermal energy resource that is often dissipated into the sea or atmosphere without utilization. By integrating heat exchangers or heat pumps, this low-grade heat can be recovered for applications such as the following:

• Freshwater Production: Utilizing thermal desalination processes like multi-stage flash (MSF) distillation or multi-effect distillation (MED) to produce freshwater from seawater [41];
• Space Heating: Providing heating for crew living quarters and operational spaces onboard the vessel [42];
• Domestic Hot Water: Supplying hot water for sanitary and galley uses [43].

Recovering waste heat from cooling systems poses challenges due to the relatively low temperatures involved, which may limit the efficiency of heat recovery technologies. However, advancements in thermoelectric materials and heat pump technologies are expanding the potential for the effective utilization of low-grade waste heat [44,45].

The main engines and auxiliary engines of marine vessels generate substantial amounts of waste heat through exhaust gases and cooling systems. The high energy content of exhaust gases offers significant opportunities for heat recovery and power generation, while waste heat from cooling systems can be harnessed for thermal applications onboard. Understanding these waste heat sources is crucial for designing and implementing effective waste heat recovery systems that enhance the energy efficiency of marine operations, reduce fuel consumption, and mitigate environmental impacts [23,46].

3. Technologies for Waste Heat Recovery (WHR)

Effective waste heat recovery (WHR) in marine energy systems is essential for enhancing overall energy efficiency, reducing fuel consumption, and minimizing environmental impacts. Various technologies have been developed to harness waste heat from marine engines and auxiliary systems. This section provides an in-depth analysis of the principal WHR technologies applicable to marine environments, including their operational principles, applicability, advantages, and limitations (

Table 2. Waste Heat Recovery Technologies Comparison.

Technology	Suitable Heat Source	Energy Recovery Efficiency	Advantages	Limitations
Heat Exchangers (Men i in. 2021). [47]	Exhaust gases, Cooling	High	Reliable, compact	Susceptible to fouling
Organic Rankine Cycle (ORC) (Loni I in. 2021). [48]	Low-grade heat (80–350 °C)	Moderate	Works with low-temperature heat	Higher cost, complex system
Thermoelectric Generators (Ochieng i in. 2022). [49]	Hot surfaces (exhaust)	Low	No moving parts, scalable	Low efficiency
Steam Turbine Systems (Qu I in. 2021). [50]	High-temperature exhaust	High	Generates additional power	High space and weight requirements

) (Figure 2).

3.1. Heat Exchangers

Heat exchangers are fundamental devices used in waste heat recovery, facilitating the transfer of thermal energy between fluids without mixing them. In marine applications, they play a critical role in capturing waste heat from engine exhaust gases and cooling systems, which can then be repurposed for heating or power generation [51].

There are several types of heat exchangers utilized in marine systems, including the following:

• Shell and Tube Heat Exchangers: This type consists of a series of tubes enclosed within a cylindrical shell. One fluid flows through the tubes, while another flows over the tubes within the shell. The large surface area facilitates efficient heat transfer between the hot and cold fluids. Shell and tube exchangers are robust and can handle high pressures and temperatures, making them suitable for recovering heat from high-temperature exhaust gases [52];
• Plate Heat Exchangers: Comprising multiple thin, corrugated metal plates stacked together, plate heat exchangers allow fluids to flow in alternate channels between the plates. The corrugations induce turbulence, enhancing heat transfer efficiency. They are compact and offer a high heat transfer coefficient, making them ideal for applications where space is limited, such as recovering heat from engine cooling systems [53];
• Finned Tube Heat Exchangers: These exchangers have fins attached to the tubes to increase the surface area for heat exchange. The fins enhance heat transfer between the tube surface and the surrounding fluid, improving the efficiency of heat recovery from gases with lower heat transfer coefficients [54].

In recovering heat from exhaust gases, heat exchangers are installed in the exhaust system downstream of the engine. The high-temperature exhaust gases pass over the heat exchanger surfaces, transferring heat to the secondary fluid, often water or thermal oil. This recovered heat can generate steam for auxiliary power or heating purposes. In cooling systems, heat exchangers capture heat from engine jacket water or lubricating oil, which can be used for domestic heating, freshwater production through desalination, or preheating fuel [55,56].

Advantages of heat exchangers include their simple design, reliability, and high efficiency in transferring heat. They can be customized to meet specific operational requirements and can handle a wide range of temperatures and pressures. Limitations involve the potential for fouling and corrosion due to exposure to marine environments and exhaust gases, necessitating regular maintenance. Additionally, their physical size and weight may pose installation challenges on vessels with limited space.

3.2. Organic Rankine Cycle (ORC) Systems

The Organic Rankine Cycle (ORC) system is a thermodynamic process that converts waste heat into mechanical power using an organic working fluid with a low boiling point, such as refrigerants or hydrocarbons. The ORC is particularly suited for recovering low-grade waste heat where traditional steam Rankine cycles are inefficient [57,58].

3.2.1. Principle of Operation and Suitability for Low-Grade Heat Sources

In an ORC system, waste heat is transferred to the organic working fluid in an evaporator, causing it to vaporize. The high-pressure vapor then expands through a turbine, producing mechanical work that drives an electrical generator. The vapor exiting the turbine is condensed back into a liquid in a condenser and pumped back to the evaporator, completing the cycle [48,59].

ORC systems are highly effective for low-temperature heat sources ranging from 80 °C to 350 °C, making them suitable for marine engine exhaust gases and cooling systems. Their ability to operate efficiently at lower temperatures allows for the recovery of waste heat that would otherwise be impractical to utilize [50].

3.2.2. Integration with Marine Engines

Integrating ORC systems with marine engines involves installing heat exchangers in the exhaust gas path or cooling circuits to capture waste heat. The compact nature of ORC equipment facilitates its installation within the spatial constraints of a vessel. The recovered energy contributes to the vessel’s electrical power supply, reducing the load on the main engines and lowering fuel consumption [8,57].

Advantages of ORC systems include improved energy efficiency, reduced emissions, and operational flexibility. They can enhance the overall efficiency of marine propulsion systems by 5–15%. Limitations involve higher capital costs compared to simpler WHR technologies, the complexity of system integration, and the need for careful selection and management of the working fluid due to potential environmental and safety concerns [8,60].

3.3. Thermoelectric Generators

Thermoelectric generators (TEGs) convert thermal energy directly into electrical energy through the Seebeck effect. When a temperature difference exists across thermoelectric materials, charge carriers (electrons or holes) diffuse from the hot side to the cold side, generating an electric current [36,61].

3.3.1. Use of Thermoelectric Materials to Convert Heat Directly into Electricity

TEGs are composed of thermoelectric modules made of n-type and p-type semiconductor elements arranged electrically in series and thermally in parallel. In marine applications, TEGs can be affixed to surfaces with significant temperature gradients, such as exhaust manifolds or engine components. The waste heat from these surfaces establishes the required temperature differential across the thermoelectric materials [49,61].

3.3.2. Advantages and Challenges in Marine Environments

The primary advantages of TEGs include their solid-state nature with no moving parts, leading to high reliability and minimal maintenance. They are silent, compact, and scalable, making them suitable for integration into various parts of a vessel. TEGs can provide power for low-energy devices, such as sensors and control systems [2,62].

However, TEGs face challenges in marine environments. The conversion efficiency of current thermoelectric materials is relatively low, typically less than 5%, limiting their applicability to small-scale power generation. Marine conditions, such as high humidity, saltwater exposure, and mechanical vibrations, can degrade thermoelectric materials and connections. Advances in materials science are necessary to develop thermoelectric materials with higher efficiency and durability suitable for marine applications [2,62,63].

3.4. Combined Heat and Power (CHP) Systems

Combined Heat and Power (CHP) systems or cogeneration systems, simultaneously generate electricity and useful heat from the same energy source. In marine settings, CHP systems optimize fuel utilization by recovering waste heat from engines to meet the onboard thermal energy demands [64].

3.4.1. Simultaneous Generation of Electricity and Useful Heat

In a marine CHP system, an internal combustion engine or turbine drives an electrical generator. The waste heat from the engine’s exhaust and cooling systems is captured using heat exchangers and utilized for heating purposes, such as domestic hot water, space heating, or providing thermal energy for auxiliary processes like desalination [65].

3.4.2. Benefits for Onboard Energy Demands

CHP systems significantly enhance overall energy efficiency, often achieving total efficiencies of up to 80–90%. By utilizing waste heat, they reduce the need for additional fuel consumption to generate thermal energy, leading to lower operating costs and reduced emissions. CHP systems are particularly beneficial on vessels with substantial simultaneous electrical and thermal energy requirements, such as cruise ships, ferries, and offshore platforms [33,42,66].

Limitations include the complexity of the system design and integration, as the thermal and electrical outputs must be carefully balanced to match the vessel’s demand profiles. Space and weight constraints can also be a concern, as CHP systems require additional equipment. Furthermore, specialized maintenance and operational expertise are necessary to ensure optimal performance [18,51].

3.5. Steam Turbine Systems

Steam turbine systems exploit high-temperature waste heat to generate steam, which then drives a turbine connected to an electrical generator. This process converts thermal energy from waste heat into mechanical and electrical energy [7,67].

3.5.1. Utilization of High-Temperature Waste Heat to Produce Additional Power

In marine applications, exhaust gas boilers (also known as economizers) are installed in the engine’s exhaust system to recover heat from the high-temperature exhaust gases, typically ranging from 300 °C to 500 °C. The heat generates steam, which is directed to a steam turbine. The turbine converts the thermal energy of the steam into mechanical work, driving an electrical generator. After passing through the turbine, the steam is condensed and returned to the boiler feedwater system [18,68].

Advantages of steam turbine systems include their ability to generate significant amounts of additional power, making them suitable for large vessels with high energy demands, such as container ships, tankers, and cruise liners. The technology is mature and well understood, offering reliable operation and integration with the existing marine power systems [69,70].

Limitations involve the substantial space and weight requirements of steam turbine equipment, which can impact vessel design and cargo capacity. The systems are complex and require skilled personnel for operation and maintenance. Additionally, the high initial capital investment and installation costs may be prohibitive for some vessels. The efficiency of steam turbine systems is highly dependent on the temperature and quantity of the available waste heat; thus, they are most effective on ships with large engines producing significant high-temperature waste heat [32,41].

Selection of the appropriate waste heat recovery technology for a marine vessel depends on several factors, including the temperature and availability of waste heat sources, the vessel’s energy demands, spatial constraints, and economic considerations. Heat exchangers offer a straightforward and reliable method for heat transfer and are suitable for a variety of applications. ORC systems are advantageous for converting low-grade waste heat into electrical power, enhancing overall efficiency without the need for high-temperature sources. Thermoelectric generators provide a compact solution for direct heat-to-electricity conversion but are limited by their low efficiency. CHP systems maximize fuel utilization by meeting both electrical and thermal energy needs, making them ideal for vessels with significant heating requirements. Steam turbine systems are effective for large-scale power generation from high-temperature waste heat but require considerable space and investment [71,72].

Integrating these WHR technologies contributes to improved energy efficiency, reduced fuel consumption, and compliance with environmental regulations, ultimately supporting the maritime industry’s sustainability goals. Ongoing research and development are essential to enhance the performance, reliability, and cost-effectiveness of these technologies, ensuring their broader adoption across the various classes of vessels.

3.5.2. Comparative Analysis with Recent Studies on Waste Heat Recovery

Recent studies offer valuable perspectives on waste heat recovery (WHR) technologies, although they often emphasize different aspects of energy optimization and system integration. For instance, ref. [73] examined a multi-objective energy dispatch approach for a multi-energy ship microgrid, focusing on coordinated energy management across various energy sources to reduce the operating costs and emissions of this multi-energy vessel. The study highlighted the importance of optimizing energy utilization within multi-energy configurations, contrasting with our broader assessment of standalone WHR systems, such as Organic Rankine Cycle (ORC) and Combined Heat and Power (CHP). In the context of confined marine spaces, our work provides distinct insights into the specific limitations and integration challenges faced by conventional WHR systems, which are particularly pertinent for vessels with limited capacity for extensive energy system modifications.

Similarly, ref. [74] addressed energy optimization within a multi-energy ship microgrid, employing a coordinated model to integrate power, thermal, hydrogen, and freshwater flows, along with underwater radiated noise (URN) considerations. Their copula-based two-stage operation structure was used to manage the unique constraints and uncertainties in multi-energy ship power distribution networks (SPDNs). Unlike our review, which primarily evaluates the applicability and limitations of traditional WHR technologies within confined spaces, Fei et al. focused on the need for adaptive scheduling within multi-energy systems. By examining WHR system compatibility with the existing ship components and the challenges posed by confined spaces, our review provides practical insights that complement the multi-energy perspectives discussed by Fei et al., especially for applications where spatial constraints and existing ship infrastructure are central concerns.

By addressing WHR implementation from the perspective of confined spaces and compatibility with legacy ship components, our review advances the understanding of how WHR can be practically applied to various vessel types. This approach provides valuable insights for both traditional WHR systems and emerging multi-energy configurations, potentially guiding future advancements in sustainable marine energy solutions.

By addressing WHR implementation from the perspective of confined spaces and existing component compatibility, our review advances the understanding of how WHR can be practically applied to various ship types. This approach provides valuable insights for both traditional WHR systems and emerging multi-energy configurations, potentially guiding future advancements in sustainable marine energy solutions.

4. Strategies for Waste Heat Recovery in Marine Systems

The implementation of waste heat recovery (WHR) technologies in marine systems demands a strategic approach that encompasses the integration with the existing ship systems, meticulous design considerations, and adjustments to the operational procedures. Effective retrofitting of the existing vessels with WHR technologies can significantly enhance energy efficiency and reduce environmental impact, but it requires overcoming the various technical and logistical challenges [14,75].

4.1. Integration with Existing Systems

Retrofitting existing ships with waste heat recovery technologies involves integrating new equipment into established propulsion and auxiliary systems without compromising their functionality or safety. The process begins with a comprehensive assessment of the vessel’s current energy systems to identify viable waste heat sources, such as exhaust gases from main and auxiliary engines, or heat from cooling systems like jacket water and charge air coolers [76,77].

To retrofit WHR technologies, engineers must design systems that are compatible with the ship’s existing infrastructure. This may involve modifying piping networks, electrical systems, and control mechanisms to accommodate additional components like heat exchangers, Organic Rankine Cycle (ORC) units, or thermoelectric generators. The integration process must ensure that the new systems operate harmoniously with the existing engines and do not introduce undue stress or interference [78,79].

One of the key considerations is the selection of the appropriate WHR technologies that align with the vessel’s operational profile and energy demands. For instance, installing an ORC system may be suitable for ships with consistent engine loads and sufficient high-temperature waste heat, while thermoelectric generators might be more appropriate for vessels seeking to harness low-grade heat for auxiliary power [39,80].

Effective integration also requires adherence to the regulatory standards set by classification societies and international maritime organizations. Compliance with safety regulations, emission standards, and environmental guidelines is mandatory, necessitating thorough documentation and approval processes before installation.

Performance and Integration of Waste Heat Recovery Equipment in Confined Marine Spaces

The confined environments of marine vessels present significant challenges for the implementation of waste heat recovery (WHR) systems, influencing both the feasibility and the efficiency of various recovery technologies. The spatial limitations inherent to ship design necessitate meticulous planning and strategic adaptation to ensure that the WHR equipment can be integrated without disrupting the existing infrastructure or compromising essential vessel functions. In this context, confined space restrictions impact WHR performance, ease of maintenance, and the compatibility of these systems with the ship’s established layout [23,33].

Firstly, the design and operation of WHR equipment, including heat exchangers, Organic Rankine Cycle (ORC) systems, and thermoelectric generators, are inherently impacted by space limitations. Larger systems, such as those utilizing steam turbines or combined heat and power (CHP) units, are particularly challenging to implement onboard due to their substantial footprint and the requirement for auxiliary components such as piping and thermal insulation. The lack of adequate space may limit the size of the heat exchange surfaces or restrict the arrangement of necessary components, potentially leading to reduced efficiency. In compact spaces, it is essential to optimize the layout to maximize heat transfer while minimizing the potential for thermal losses or mechanical wear, which may be exacerbated by restricted airflow and proximity to other machinery [81,82].

Moreover, integrating WHR equipment into existing ship systems often requires overcoming significant compatibility challenges. Maritime vessels are typically equipped with well-established propulsion and auxiliary power systems that function as cohesive units; retrofitting WHR systems into these frameworks demands careful coordination with the ship’s energy and mechanical configurations. For example, piping modifications are frequently required to direct waste heat from the main engines and auxiliary engines to the WHR units while ensuring no disruption to the cooling circuits or other critical pathways. To avoid operational conflicts, the WHR systems must be designed to harmonize with the existing power distribution, monitoring, and safety protocols [83,84].

Confined spaces also restrict maintenance access, which can affect the long-term reliability and operational consistency of WHR equipment. Heat exchangers, for instance, are prone to fouling in marine environments, requiring regular cleaning to maintain optimal performance. However, limited accessibility may hinder routine maintenance tasks, leading to an accumulation of deposits that reduce thermal efficiency. Additionally, confined spaces can amplify the effects of vibration and heat on WHR components, potentially accelerating wear on materials and increasing the likelihood of mechanical failures. Innovations in materials science, such as corrosion-resistant alloys or thermally durable composites, offer potential solutions to these issues, although their adoption may necessitate higher initial investments [84,85].

Lastly, confined spaces present an increased need for compact and modular WHR systems that can be tailored to fit specific vessel layouts without necessitating extensive structural modifications. Emerging technologies, such as advanced thermoelectric materials or high-efficiency micro heat exchangers, may provide viable solutions for compact WHR applications. However, more research is needed to assess the long-term viability and performance of these technologies under real-world maritime conditions. Integrating WHR systems into confined ship spaces thus requires a balance between optimizing spatial efficiency and ensuring compatibility with the ship’s overall operational ecosystem, underscoring the need for adaptive designs that address the unique challenges of marine applications [83,85].

4.2. Design Considerations

Designing WHR systems for marine applications involves addressing space constraints, weight limitations, and the potential impacts on the vessel’s stability. Ships have finite space available, and the addition of new equipment must be planned carefully to avoid encroaching on cargo areas, accommodation spaces, or essential operational zones.

Space constraints necessitate the use of compact and efficient equipment. Engineers may opt for plate heat exchangers instead of shell-and-tube designs due to their smaller footprint and higher heat transfer efficiency per unit volume. Similarly, modular ORC systems can be tailored to fit within the available spaces without significant alterations to the ship’s structure [86,87].

Weight is another critical factor, as additional mass can affect the ship’s draft, trim, and overall stability. The weight distribution of WHR equipment must be calculated meticulously to prevent negative impacts on the vessel’s center of gravity. In some cases, structural reinforcements or ballast adjustments may be required to maintain stability and comply with the stability criteria defined by maritime regulations [45,88].

Material selection plays a vital role in ensuring the durability and reliability of WHR systems in the harsh marine environment. Components must withstand exposure to saltwater, humidity, temperature variations, and vibrations. Corrosion-resistant materials such as stainless steel, titanium, or specialized alloys are often employed to extend the lifespan of the equipment and reduce maintenance requirements [40,89].

Thermal expansion and mechanical stresses induced by temperature fluctuations must also be considered in the design phase. Expansion joints, flexible couplings, and appropriate mounting strategies can mitigate these issues, ensuring the structural integrity of the WHR systems over time.

4.3. Operational Adjustments

The successful operation of waste heat recovery systems may necessitate adjustments to the vessel’s standard operating procedures. Crew members require training to understand the principles of the new technologies, their operational parameters, and maintenance needs. This training ensures that the systems are operated safely and efficiently, maximizing the benefits of the investment [90,91].

Operational adjustments may include optimizing engine loads to enhance waste heat availability. Running engines at optimal efficiency points not only reduces fuel consumption but also increases the temperature and flow rate of exhaust gases, improving the performance of WHR systems like ORC units or exhaust gas boilers. However, these adjustments must be balanced against other operational requirements, such as speed schedules, fuel economy, and emission control areas (ECAs) [92,93].

Monitoring and control systems play a crucial role in the efficient operation of WHR technologies. Integrating advanced sensors and automation allows for real-time data collection on temperatures, pressures, flow rates, and energy outputs. These data enable the crew to make informed decisions regarding system adjustments, preventive maintenance, and troubleshooting [21,26].

Maintenance procedures must be updated to include the new WHR equipment. Regular inspections, cleaning, and component replacements are essential to prevent fouling, corrosion, and mechanical failures. Establishing a maintenance schedule aligned with the vessel’s operational timetable minimizes downtime and ensures continuous system performance [11,94].

Moreover, contingency plans should be developed to address potential malfunctions or emergencies involving the WHR systems. Safety protocols must be in place to isolate and shut down equipment if necessary, preventing accidents and ensuring the safety of the vessel and crew [28,30].

Implementing waste heat recovery technologies in marine systems requires a holistic approach that addresses technical integration, design challenges, and operational practices. By carefully planning retrofits, considering the unique constraints of each vessel, and adjusting operational procedures, shipowners can successfully enhance energy efficiency and reduce emissions. The collaboration between naval architects, marine engineers, ship operators, and classification societies is essential to navigate the complexities of WHR implementation, ultimately contributing to more sustainable maritime operations (

Table 3. Summary of WHR Technologies.

Technology	Applicability	Advantages	Limitations
Heat Exchangers [20]	Exhaust and cooling systems	High efficiency, reliability	Fouling, size constraints
Organic Rankine Cycle [89]	Low-grade heat sources	Efficient with low temperatures	Cost, working fluid handling
Thermoelectric Generators [49]	Hot surfaces and exhaust	No moving parts, scalable	Low efficiency, material costs
Combined Heat and Power [38]	Vessels with heat and power needs	High fuel efficiency, emissions reduction	Complex integration, space
Steam Turbine Systems [95]	High-temperature exhaust	High power output, mature technology	Size, complexity, cost

).

5. Efficiency and Performance Analysis

5.1. Thermodynamic Analysis

Evaluating the efficiency improvements achieved through waste heat recovery (WHR) in marine energy systems necessitates a thorough energy analysis. This analysis is grounded in the First Law of Thermodynamics, which focuses on the conservation of energy within a system. It assesses how effectively the input energy, such as heat from engine exhaust gases and cooling systems, is converted into useful work. The energy balance for such a process can be mathematically expressed as follows:

$$\dot{Q}-\dot{W}=\sum {\dot{m}}_i{h}_i- \sum {\dot{m}}_e{h}_e$$

where $\dot{Q}$ and $W$ represent the heat added to and the work carried out by the system, respectively, while ${\dot{m}}_i$ and ${\dot{m}}_e$ represent the mass flow rates into and out of the system, and $h_i$ and $h_e$ denote the specific enthalpies of these inflows and outflows. Implementing WHR technologies allows the system to capture and convert waste heat into additional useful energy, thereby enhancing the overall energy efficiency of the marine propulsion system without additional fuel consumption [46,87].

5.1.1. Exergy Analysis

In complement to the energy analysis, exergy analysis provides a more comprehensive evaluation of system performance by considering the quality of energy and system irreversibilities. Grounded in the Second Law of Thermodynamics, exergy represents the maximum potential work as a system moves towards equilibrium with its surroundings. The exergy flow within a system is defined by the following equation:

$$\dot{B}=\dot{m}(h-h_0-T_0\left(s-s_0\right))$$

In this equation, $\dot{B}$ indicates the exergy flow, $T_0$ and $s_0$ represent the ambient temperature and entropy, respectively, and $h$ and $s$ are the specific enthalpy and entropy of the stream. Exergy analysis helps identify where significant losses due to entropy generation occur and allows for the targeting of these areas to minimize exergy destruction in critical components such as heat exchangers, turbines, and condensers. This leads to an optimized system performance, enhancing the practical implementation of WHR technologies [44,88] (Figure 3).

5.1.2. Computational Tools and Simulation

Both energy and exergy analyses employ various computational tools and simulation software, such as Aspen HYSYS V14.0, MATLAB R2024b, and Python 3.10, to model the thermodynamic processes and calculate efficiency improvements. These tools are instrumental in handling parameters like temperature, pressure, mass flow rates, and specific heats, which are integral to optimizing the WHR system design and operational strategies.

5.2. Fuel Consumption Reduction

The integration of WHR technologies in marine systems leads to a direct reduction in fuel consumption by harnessing energy that would otherwise be wasted. By converting waste heat into additional mechanical or electrical power, the vessel’s reliance on fuel for meeting its energy demands decreases [96].

The quantification of fuel savings can be calculated using the relationship between the energy recovered and the fuel’s lower heating value (LHV) [97]. The fuel consumption reduction (ΔF) can be expressed as follows:

$$\mathsf{\Delta }F={\displaystyle \frac{E_{WHR}}{{\eta }_{engine}\times Q_{LHV}}}$$

where E_WHR is the energy recovered by the waste heat recovery system (in watts), η_engine is the thermal efficiency of the engine, and Q_LHV is the lower heating value of the fuel (in joules per kilogram).

For example, if a WHR system recovers 1 MW (megawatt) of power and the engine has a thermal efficiency of 45%, with a fuel LHV of 42 MJ/kg (megajoules per kilogram), the fuel savings are calculated as follows:

$$\mathsf{\Delta }F={\displaystyle \frac{1\times {10}^6\mathrm{W}}{0.45\times 42\times {10}^6 \raisebox{1ex}{$\mathrm{J}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{k}\mathrm{g}$}\right.}} \approx 0.0527 \raisebox{1ex}{$\mathrm{k}\mathrm{g}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{s}$}\right.$$

Over extended operational periods, such as a year of continuous sailing, the cumulative fuel savings become substantial, leading to significant cost reductions for ship operators. This also extends the vessel’s range and operational flexibility.

5.3. Emission Reductions

Reducing fuel consumption inherently decreases the emission of greenhouse gases (GHGs) and other pollutants. Carbon dioxide (CO₂) emissions are directly proportional to the amount of fuel burned, as CO₂ is a primary product of hydrocarbon combustion. Other harmful emissions, including nitrogen oxides (NOx), sulfur oxides (SOx), and particulate matter (PM), are also reduced when less fuel is consumed [25].

The reduction in CO₂ emissions (ΔCO₂) can be estimated using the following formula:

$$\mathsf{\Delta }C{O}_2=\mathsf{\Delta }F\times C_c\times {\displaystyle \frac{44}{12}}$$

where ΔF is the fuel savings (kg/s), C_c is the carbon content fraction of the fuel (typically around 0.86 for marine diesel oil), and ${\displaystyle \frac{44}{12}}$ is the molecular weight ratio of CO₂ to carbon.

Using the previous example with ΔF = 0.0527 kg/s and C_c = 0.86, we obtain the following equation:

$$\mathsf{\Delta }C{O}_2=0.0527 \mathrm{k}\mathrm{g}/\mathrm{s}\times 0.86\times {\displaystyle \frac{44}{12}}\approx 0.166 \mathrm{k}\mathrm{g}/\mathrm{s}$$

Over a year, this results in a reduction of approximately 5240.00 metric tons of CO₂. Such significant emission reductions contribute to global efforts to mitigate climate change and help ship operators comply with stringent environmental regulations set by the International Maritime Organization (IMO) under MARPOL Annex VI [96,97].

The thermodynamic analysis of waste heat recovery in marine systems reveals substantial opportunities for enhancing energy efficiency. By applying energy and exergy methodologies, engineers can optimize system designs to minimize losses and maximize performance. The quantifiable reductions in fuel consumption and emissions underscore the economic and environmental benefits of WHR technologies. Real-world case studies validate these advantages, demonstrating the practical feasibility and effectiveness of WHR implementations in improving the sustainability of marine operations.

6. Economic Evaluation

The implementation of waste heat recovery [98] (WHR) technologies in marine energy systems necessitates a thorough economic evaluation to determine their financial viability. This evaluation encompasses a cost–benefit analysis, an assessment of the return on investment (ROI), and a consideration of operational and maintenance costs associated with the WHR systems (

Table 4. Economic Evaluation of WHR Technologies.

Technology	Initial Investment	Payback Period (Years)	Annual Fuel Savings (%)	Maintenance Complexity
Heat Exchangers [99]	Low	2–4	5–10%	Low
Organic Rankine Cycle (ORC) [50]	High	5–10	4–8%	High
Thermoelectric Generators [36]	Moderate	4–6	1–2%	Moderate
Steam Turbine Systems [68]	High	6–8	7–12%	High

).

6.1. Cost–Benefit Analysis

A cost–benefit analysis involves comparing the initial capital investment required for the installation of WHR technologies against the projected long-term savings from reduced fuel consumption and lower emissions. The initial investment includes costs for equipment procurement, engineering design, system integration, installation labor, and potential modifications to the vessel’s infrastructure. Advanced WHR systems, such as Organic Rankine Cycle (ORC) units or steam turbine systems, generally entail higher upfront costs due to their complexity and the need for specialized components [32,100].

Long-term savings are primarily derived from decreased fuel consumption, as WHR systems convert waste heat into useful energy, thereby reducing the load on primary engines. This reduction in fuel usage not only leads to direct cost savings but also minimizes exposure to fuel price volatility. Additionally, savings may accrue from lower maintenance costs on primary engines due to reduced operational stress and from compliance with environmental regulations that might otherwise incur penalties or necessitate the purchase of emissions credits [100,101].

To quantify these benefits, the net present value (NPV) method is often employed. This method discounts future cash flows to present values using an appropriate discount rate, allowing for a direct comparison between the initial investment and the sum of future savings over the WHR system’s operational lifespan. A positive NPV indicates that the investment is economically justified.

6.2. Return on Investment (ROI)

The return on investment (ROI) and the payback period are critical metrics in assessing the financial attractiveness of WHR technologies. ROI is calculated by dividing the net profit (total savings minus total costs) by the total investment cost, often expressed as a percentage. A higher ROI signifies a more profitable investment.

The payback period is the time required for the cumulative savings from reduced fuel consumption and operational efficiencies to equal the initial capital expenditure. Shorter payback periods are generally preferred, as they imply a quicker recovery of the investment and lower financial risk [31,102].

Different WHR technologies exhibit varying ROIs and payback periods due to differences in capital costs, efficiency gains, and operational complexities. For instance, heat exchangers and economizers typically have lower capital costs and can achieve payback periods of two to four years, owing to their relative simplicity and immediate impact on fuel efficiency. In contrast, ORC systems and steam turbine setups involve higher initial costs and may have payback periods extending from five to ten years, depending on factors such as the vessel’s operational profile, fuel prices, and the magnitude of recoverable waste heat [94,103].

An analysis by MAN Energy Solutions (2016) demonstrated that the payback period for a WHR system on a large container ship could be as short as three years when fuel prices are high, but it could extend to over six years if fuel prices decrease significantly. Therefore, sensitivity analyses accounting for fuel price fluctuations are essential in ROI assessments [71,72].

6.3. Operational and Maintenance Costs

Operational and maintenance (O&M) costs are ongoing expenses associated with the operation and upkeep of WHR systems. These costs include the following:

• Operational Costs: Energy consumption by auxiliary equipment (pumps, fans, control systems), crew training, and any additional labor required for system monitoring and management [91];
• Maintenance Costs: Regular inspections, cleaning to prevent fouling and corrosion, repairs, and the replacement of worn or damaged components. Maintenance activities are crucial for ensuring the reliability and longevity of WHR systems, especially given the harsh marine environment characterized by saltwater exposure, vibrations, and thermal stresses [104].

Advanced WHR technologies may incur higher O&M costs due to their complexity. For example, ORC systems require specialized working fluids that may be costly or necessitate careful handling due to toxicity or flammability. Steam turbine systems demand rigorous maintenance schedules to address issues related to high-pressure steam, potential scaling, and thermal fatigue [105,106].

Moreover, downtime due to maintenance or unexpected failures can lead to indirect costs, such as the loss of revenue from delays or increased fuel consumption if the WHR system is offline. Implementing predictive maintenance strategies and investing in high-quality materials can mitigate some of these costs but may increase the initial expenses [17,107,108].

The economic evaluation of WHR technologies in marine systems must balance the initial investment against long-term operational savings. A comprehensive cost–benefit analysis should account for the capital costs of installation and integration, projected fuel savings, and potential revenue from emissions reductions. The ROI and payback period provide insight into the financial feasibility and attractiveness of different WHR options [24,109].

Operational and maintenance costs are significant factors that influence the net economic benefit of WHR systems. While these systems can lead to substantial fuel savings and emissions reductions, they also introduce additional complexities and expenses that must be managed effectively [110,111].

In conclusion, the decision to implement WHR technologies should be based on a detailed economic analysis tailored to the specific vessel and operational context. Factors such as fuel prices, voyage patterns, engine types, and regulatory environments play pivotal roles in determining the financial viability of WHR investments. By carefully considering these aspects, ship owners and operators can make informed decisions that enhance energy efficiency, reduce environmental impact, and improve profitability.

7. Challenges and Solutions

The implementation of waste heat recovery (WHR) systems in marine energy applications presents several challenges that must be addressed to ensure their effectiveness, reliability, and compliance with international regulations. Technical issues such as material degradation due to corrosion and high temperatures, as well as reliability and maintenance concerns, can impede the performance and longevity of WHR systems. Additionally, adherence to the regulatory standards set by international maritime organizations is crucial for the lawful operation of vessels equipped with these technologies. To overcome these challenges, advances in materials science and the development of smart monitoring and control systems offer promising solutions [13,63,112].

7.1. Technical Challenges

One of the primary technical challenges in marine WHR systems is material degradation caused by corrosion and exposure to high temperatures. Marine environments are inherently harsh due to the presence of saltwater, humidity, and temperature fluctuations, which accelerate the corrosion of metals and other materials used in WHR equipment. Components such as heat exchangers, boilers, and piping are particularly susceptible to corrosion, leading to reduced efficiency, increased maintenance requirements, and potential system failures [108,113].

High-temperature waste heat sources, such as engine exhaust gases, subject materials to thermal stresses that can cause deformation, cracking, and other forms of degradation. Prolonged exposure to elevated temperatures can alter the mechanical properties of materials, reducing their strength and resistance to wear. Thermal fatigue resulting from cyclic temperature variations during operation can further exacerbate material degradation [41,114].

Reliability and maintenance issues are also significant concerns in the operation of WHR systems. The complexity of these systems, which often involve intricate components operating under high pressure and high temperature conditions, increases the likelihood of mechanical failures and operational disruptions. Regular maintenance is essential to prevent fouling, scaling, and deposition of contaminants on heat transfer surfaces, which can impair heat exchanger performance and reduce overall system efficiency. However, maintenance activities can be challenging due to the limited accessibility of equipment on board ships and the need for specialized knowledge and tools [41,63].

7.2. Regulatory and Compliance

Compliance with international maritime regulations is imperative for the operation of vessels equipped with WHR systems. The International Maritime Organization (IMO) has set standards to ensure the safety, security, and environmental performance of international shipping. Regulations such as the International Convention for the Safety of Life at Sea (SOLAS) and the International Convention for the Prevention of Pollution from Ships (MARPOL) have imposed stringent requirements on the design, construction, and operation of shipboard equipment [25].

WHR systems must be designed and installed in accordance with these regulations to ensure they do not compromise the vessel’s safety or environmental compliance. For instance, the addition of WHR equipment must not interfere with critical ship systems or obstruct essential pathways. Moreover, any modifications to the vessel’s structure or machinery must be approved by classification societies and flag states to verify adherence to the regulatory standards [80,115].

Meeting the emission reduction targets established by the IMO, such as those outlined in the Energy Efficiency Design Index (EEDI) and the Ship Energy Efficiency Management Plan (SEEMP), is also a key driver for the adoption of WHR technologies. However, achieving compliance can be challenging due to the technical complexities and financial investments required to integrate these systems effectively [75,76].

7.3. Solutions and Innovations

Advancements in materials science offer significant potential to address the technical challenges associated with material degradation in WHR systems. The development of corrosion-resistant alloys and composite materials can enhance the durability and longevity of components exposed to harsh marine environments and high temperatures. For example, the use of stainless-steel alloys with high chromium and molybdenum content can improve resistance to pitting and crevice corrosion. Additionally, coatings and surface treatments, such as thermal spray coatings and ceramic linings, provide protective barriers that mitigate corrosion and wear [6,31].

Innovations in material technology also include the exploration of high-temperature materials capable of maintaining structural integrity under thermal stress. Nickel-based superalloys and advanced ceramics exhibit excellent high-temperature performance and are being considered for critical components within WHR systems [110].

The implementation of smart monitoring and control systems is another solution to enhance the reliability and efficiency of WHR technologies. These systems utilize sensors, data acquisition, and advanced algorithms to provide real-time monitoring of operational parameters such as temperature, pressure, flow rates, and system performance indicators. By continuously tracking these variables, operators can detect anomalies, predict potential failures, and implement preventive maintenance strategies before critical issues arise [116,117,118].

The integration of automation and control technologies enables the adaptive operation of WHR systems, optimizing performance under varying operational conditions. For instance, control systems can adjust the flow of working fluids or modulate heat exchanger surfaces to maintain optimal heat transfer efficiency. Machine learning algorithms and predictive analytics further enhance the system’s ability to respond to complex patterns and trends in operational data, leading to improved efficiency and reduced downtime [15,116,118].

Overcoming the challenges associated with the implementation of waste heat recovery systems in marine energy applications requires a multi-faceted approach that addresses technical, regulatory, and operational aspects. Advances in materials science provide solutions to material degradation issues, enhancing the durability and performance of WHR components in corrosive and high-temperature environments. The adoption of smart monitoring and control systems enhances reliability, facilitates maintenance, and optimizes system performance [15,28,119].

Ensuring compliance with international maritime regulations necessitates careful planning, design, and collaboration with regulatory bodies and classification societies. By integrating innovative technologies and adhering to regulatory standards, the maritime industry can effectively harness waste heat recovery systems to improve energy efficiency, reduce emissions, and contribute to sustainable shipping practices.

8. Future Trends and Research Directions

8.1. Emerging Technologies

The advancement of materials science plays a pivotal role in enhancing waste heat recovery (WHR) systems within marine energy applications. The development of advanced materials with superior thermal, mechanical, and chemical properties is crucial for improving the efficiency and durability of WHR technologies. Researchers are focusing on novel alloys and composite materials that can withstand the harsh marine environment, characterized by high salinity, humidity, and temperature fluctuations (

Table 5. Future Trends in Waste Heat Recovery.

Technology/Innovation	Description	Expected Efficiency Improvements	Potential Challenges
Advanced Thermoelectrics [36]	Higher efficiency thermoelectric materials	10–20% improvement	Material costs, integration issues
Nanotechnology in Heat Exchangers [120]	Enhanced surface area for heat exchange	15–25% improvement	Manufacturing complexities
Digital Twin Models [28]	Real-time system optimization using simulations	10–15% operational efficiency	Data processing, system complexity

).

Nanostructured materials and high-performance thermoelectric materials are at the forefront of this development. The incorporation of nanotechnology in thermoelectric generators can significantly increase their efficiency by enhancing the Seebeck coefficient and electrical conductivity while reducing thermal conductivity. Materials such as skutterudites, half-Heusler compounds, and complex chalcogenides are being investigated for their potential to achieve higher thermoelectric performance. Additionally, advances in high-temperature ceramics and corrosion-resistant coatings are expected to extend the lifespan of heat exchangers and other WHR components, thereby reducing maintenance costs and improving reliability [36].

The exploration of phase change materials (PCMs) for thermal energy storage is another promising area. PCMs can absorb and release large amounts of latent heat during phase transitions, providing a means to store waste heat for later use. The integration of PCMs into WHR systems could enhance energy management on board ships, allowing for greater flexibility and efficiency in energy utilization [121].

8.2. Digitalization and Automation

The integration of digital technologies, including artificial intelligence (AI) and the Internet of Things (IoT), is transforming the optimization and management of waste heat recovery systems in marine settings. AI algorithms, particularly machine learning models, can analyze vast amounts of operational data from WHR systems to identify patterns, predict equipment failures, and optimize performance parameters in real time. This predictive maintenance approach minimizes unplanned downtime and extends the operational lifespan of WHR equipment [39,70].

IoT devices enable the interconnection of sensors and actuators within WHR systems, facilitating real-time monitoring and control over the internet or dedicated networks. This connectivity allows for the implementation of advanced control strategies that can adjust system operations dynamically in response to changing conditions, such as variations in engine load or environmental factors. For example, adaptive control systems can modulate the flow rates in heat exchangers or adjust the operating points of Organic Rankine Cycle (ORC) systems to maintain optimal efficiency [39,122].

Digital twins, which are virtual replicas of physical systems, are emerging as valuable tools for the simulation and optimization of WHR systems. By creating a digital twin of a ship’s energy system, engineers can model the interactions between different components, predict the impacts of modifications, and optimize system design and operation without the risks associated with physical experimentation. This approach accelerates innovation and improves the integration of WHR technologies into marine vessels [41,68].

8.3. Policy and Incentives

Governmental policies and incentives are critical in promoting the adoption of energy-efficient technologies, including waste heat recovery systems, in maritime transport. International regulations, such as those established by the International Maritime Organization (IMO), set targets for reducing greenhouse gas emissions and improving energy efficiency. The IMO’s Energy Efficiency Design Index (EEDI) and Ship Energy Efficiency Management Plan (SEEMP) are examples of regulatory frameworks that encourage shipowners to adopt technologies that reduce fuel consumption and emissions [23,59].

National governments and regional authorities can bolster these efforts by implementing policies that provide financial incentives for the adoption of WHR technologies. These incentives may include tax credits, grants, low-interest loans, or subsidies that offset the initial capital costs associated with installing WHR systems. Additionally, policies that impose higher costs on emissions, such as carbon taxes or emissions trading schemes, create economic drivers for shipowners to invest in energy-efficient technologies to reduce operational expenses [23,47].

Collaborative research and development initiatives supported by government funding can accelerate the advancement of WHR technologies. By fostering partnerships between academia, industry, and government agencies, these programs can facilitate knowledge transfer, innovation, and the commercialization of emerging technologies. Standards and certification programs can also play a role by establishing guidelines for the performance and safety of WHR systems, thereby increasing confidence among shipowners and operators in adopting these technologies [95,122].

Moreover, international cooperation is essential to harmonize regulations and incentives across different jurisdictions, reducing barriers to the global implementation of WHR technologies. Unified standards and shared best practices enable shipbuilders and operators to streamline the integration of energy-efficient systems, contributing to the overall sustainability of the maritime industry [38,48].

The future of waste heat recovery in marine energy systems is characterized by significant opportunities and advancements driven by technological innovation, digital transformation, and supportive policy frameworks. The development of advanced materials enhances the efficiency and durability of WHR components, enabling better heat recovery under challenging marine conditions. The adoption of digitalization and automation, through AI and IoT technologies, optimizes system performance and maintenance, leading to more reliable and efficient operations [33,99].

Governmental policies and incentives play a pivotal role in encouraging the maritime industry to invest in WHR technologies. By aligning economic interests with environmental objectives, these policies promote the widespread adoption of energy-efficient solutions that reduce fuel consumption and emissions [61,123].

Continued research and collaboration among stakeholders are essential to address the remaining challenges and to fully realize the potential of waste heat recovery in maritime transport. By embracing these future trends and research directions, the maritime industry can significantly contribute to global efforts in energy conservation and environmental protection.

9. Environmental Impact Assessment

9.1. Global Environmental Benefits

The implementation of waste heat utilization in marine energy systems presents significant global environmental benefits by contributing to a reduction in greenhouse gas (GHG) emissions, which are a primary driver of climate change. The maritime industry accounts for approximately 2.5% of the global CO₂ emissions, a figure projected to increase if mitigation strategies are not employed. By enhancing the energy efficiency of vessels through waste heat recovery (WHR) technologies, fuel consumption is reduced, directly leading to lower CO₂ emissions (

Table 6. Environmental Impact of WHR Systems.

Technology	CO2 Emission Reduction (%)	NOx Reduction (%)	SOx Reduction (%)	Other Environmental Benefits
Heat Exchangers [124]	8–12%	5–8%	4–6%	Reduced thermal pollution
Organic Rankine Cycle (ORC) [89]	6–10%	3–5%	3–5%	Minimizes emissions, supports sustainability
Thermoelectric Generators [61]	2–4%	1–3%	1–3%	Reduced reliance on fuel, low noise
Steam Turbine Systems [18]	10–15%	7–10%	6–9%	Enhanced energy efficiency

).

Waste heat recovery systems improve the overall thermal efficiency of marine engines by converting energy that would otherwise be lost into useful work, thereby decreasing the specific fuel oil consumption (SFOC). Studies have demonstrated that the integration of WHR technologies can lead to fuel savings of up to 10%, depending on the vessel type and operational profile. This reduction in fuel consumption corresponds to a proportional decrease in CO₂ emissions, supporting international efforts to mitigate climate change [7,51,53].

The International Maritime Organization (IMO) has set ambitious targets to reduce GHG emissions from international shipping by at least 50% by 2050 compared to 2008 levels, with the vision of phasing them out entirely within this century. The adoption of WHR technologies is a critical component in achieving these goals. By enhancing energy efficiency, ships contribute to the IMO’s Energy Efficiency Design Index (EEDI) and comply with the Ship Energy Efficiency Management Plan (SEEMP), both of which are instrumental in driving the maritime industry towards sustainability [19,23,99].

Furthermore, reducing fuel consumption also leads to lower emissions of other harmful pollutants such as nitrogen oxides (NO_x), sulfur oxides (SO_x), and particulate matter (PM), which have detrimental effects on air quality and human health. The global environmental benefits of waste heat utilization thus extend beyond GHG emission reductions, contributing to the overall improvement in environmental quality and public health.

9.2. Local Environmental Effects

At the local level, the impact of waste heat utilization on marine ecosystems is multifaceted [125]. The operation of ships involves the discharge of thermal energy into the marine environment, primarily through cooling water systems. Elevated temperatures in the discharged water can lead to thermal pollution, adversely affecting marine life by altering metabolic rates, reproductive cycles, and disrupting local habitats [59,92,126].

By implementing WHR systems, the amount of waste heat released into the surrounding water bodies is reduced. Heat that would have been dissipated is instead captured and converted into useful energy. This reduction in thermal discharge mitigates the effects of thermal pollution, helping to preserve local marine ecosystems and biodiversity. The decrease in thermal gradients minimizes the disruption of aquatic organisms and supports the stability of the marine food chain [106,127].

Additionally, the reduction in exhaust gas emissions resulting from lower fuel consumption decreases the deposition of acidifying compounds and toxic substances into the ocean. These substances can lead to ocean acidification and eutrophication, which harm coral reefs, shell-forming organisms, and overall marine biodiversity. By lessening the emissions of NO_x and SO_x, waste heat utilization indirectly protects marine environments from these adverse effects [39,79,91].

However, the installation and operation of WHR systems must be carefully managed to avoid unintended negative impacts. For instance, the use of certain working fluids in Organic Rankine Cycle (ORC) systems may pose environmental risks if leaks occur. The proper selection of environmentally benign working fluids and adherence to strict maintenance protocols are essential to prevent the contamination of marine ecosystems.

9.3. Sustainability Considerations

The alignment of waste heat utilization with the United Nations Sustainable Development Goals (SDGs) underscores its significance in promoting sustainable maritime operations [32,128]. Specifically, the adoption of WHR technologies contributes to several key SDGs, as follows:

• SDG 7: Affordable and Clean Energy—By enhancing energy efficiency and promoting the use of recovered waste heat, WHR systems support the transition to cleaner energy sources and improve energy security in maritime transport;
• SDG 9: Industry, Innovation, and Infrastructure—The implementation of advanced WHR technologies fosters innovation in the maritime industry, leading to more resilient and sustainable infrastructure;
• SDG 13: Climate Action—Reducing GHG emissions from ships directly addresses the challenges of climate change. Waste heat utilization is a tangible measure that contributes to global efforts to limit temperature rise in accordance with the Paris Agreement;
• SDG 14: Life Below Water—By minimizing thermal pollution and reducing the release of harmful pollutants, WHR systems help protect marine ecosystems and promote the sustainable use of oceans and marine resources.

The economic benefits associated with fuel savings enhance the sustainability of maritime operations by reducing operational costs and improving the competitiveness of shipping companies. These savings can be reinvested in further technological advancements and environmental protection measures [15,97].

Moreover, waste heat utilization exemplifies the principles of the circular economy by transforming waste streams into valuable resources. This approach reduces the environmental footprint of shipping activities and promotes resource efficiency, aligning with broader sustainability frameworks and international environmental commitments [12,129].

In summary, the environmental impact assessment of waste heat utilization in marine energy systems reveals substantial benefits at both the global and local levels. Technology contributes significantly to global emission reduction targets by lowering fuel consumption and the associated GHG emissions. Locally, it mitigates the negative effects of thermal pollution and protects marine ecosystems from harmful pollutants. The alignment with the Sustainable Development Goals highlights the strategic importance of waste heat recovery in fostering sustainable maritime transport. Continued investment and innovation in this area are essential for advancing environmental stewardship and achieving long-term sustainability in the maritime industry.

10. Study Limitations and Future Directions

This review provides a detailed examination of waste heat recovery (WHR) technologies for large commercial vessels, focusing primarily on traditional propulsion systems. However, it gives limited attention to smaller vessels, as well as emerging propulsion technologies, such as electric or hybrid systems, which are increasingly relevant for sustainable maritime operations. Future studies could broaden the scope to explore WHR potential in these alternative propulsion methods, assessing how their unique thermal profiles might affect waste heat recovery opportunities [16].

Furthermore, while this paper includes a high-level economic evaluation of WHR technologies, it does not deliver comprehensive cost analyses for specific installations. The economic feasibility of WHR systems can vary significantly based on vessel type [121], operational profile, regional fuel prices, and market conditions [48]. Future research could include detailed case studies or cost–benefit analyses tailored to different vessel classes and operational contexts, providing shipowners with actionable financial insights [64].

Another limitation is the current lack of experimental validation in confined marine spaces. While this review discusses spatial constraints theoretically, empirical research would be valuable to better understand how WHR systems perform under the physical restrictions of marine environments [70]. Field studies focusing on retrofitting WHR systems into existing ship designs could yield practical insights, particularly in terms of performance, maintenance, and durability over time.

The review also highlights significant technological challenges, such as material durability under high temperatures and exposure to corrosive marine conditions, which are not fully addressed here [48]. Future studies could investigate advanced materials, such as corrosion-resistant alloys or high-efficiency thermoelectric materials, and assess their potential to improve the resilience and effectiveness of WHR systems in marine settings [17].

Finally, digitalization and automation offer promising avenues for optimizing WHR system operation through real-time monitoring and adaptive control. Research into the integration of digital twins, predictive maintenance algorithms, and IoT-based monitoring for WHR systems could enhance their performance and reliability [114]. Future work in these areas may further support WHR adoption across diverse vessel types and operating conditions, advancing the maritime industry’s progress toward sustainable energy solutions.

11. Conclusions

The utilization of waste heat in marine energy systems represents a significant opportunity for enhancing the energy efficiency of maritime operations. Through the integration of waste heat recovery (WHR) technologies such as heat exchangers, Organic Rankine Cycle (ORC) systems, thermoelectric generators, combined heat and power (CHP) systems, and steam turbine systems, it is possible to convert waste thermal energy into useful power, thereby reducing fuel consumption and associated emissions.

The thermodynamic analyses conducted demonstrate that employing WHR systems can lead to substantial improvements in overall system efficiency. Energy and exergy assessments reveal that optimizing waste heat utilization minimizes irreversibilities within the energy conversion processes, leading to the enhanced performance of marine propulsion systems. The reduction in fuel consumption not only provides economic benefits by lowering operational costs but also contributes to environmental sustainability by decreasing greenhouse gas emissions and pollutants such as NO_x, SO_x, and particulate matter.

Case studies underscore the practical viability of WHR technologies in real-world applications. For instance, the implementation of ORC systems on commercial vessels has resulted in efficiency gains of up to 15%, with corresponding reductions in fuel usage and emissions. These examples highlight the potential for widespread adoption of waste heat utilization strategies within the maritime industry.

However, the deployment of WHR systems is not without challenges. Technical issues such as material degradation due to corrosion and high temperatures necessitate advances in materials science to develop more resilient components. Reliability and maintenance concerns require the integration of smart monitoring and control systems to ensure optimal operation and longevity of the equipment. Regulatory compliance with international maritime standards, particularly those established by the International Maritime Organization (IMO), is essential to align waste heat utilization practices with global efforts to enhance energy efficiency and environmental protection.

Economic evaluations indicate that while the initial investment in WHR technologies can be significant, the long-term savings from reduced fuel consumption and maintenance costs often justify the expenditure. Return on investment (ROI) analyses and payback periods vary depending on the specific technology and vessel characteristics, but many implementations achieve financial viability within acceptable timeframes.

Looking toward the future, emerging technologies and research directions offer promising avenues for further enhancing waste heat utilization. Developments in advanced materials, such as high-efficiency thermoelectric materials and corrosion-resistant alloys, are expected to improve system performance and durability. The digitalization and automation of WHR systems through artificial intelligence (AI) and the Internet of Things (IoT) will enable more precise control and optimization, leading to greater efficiency gains. Policy measures and governmental incentives will play a crucial role in promoting the adoption of these technologies, supporting the maritime industry’s transition toward sustainability.

In conclusion, waste heat utilization in marine energy systems is a critical strategy for enhancing the efficiency and sustainability of maritime operations. By overcoming technical and regulatory challenges and leveraging technological innovations, the maritime industry can significantly reduce its environmental footprint while realizing economic benefits. Continued research and collaboration among stakeholders are essential to advance these technologies and achieve the broader goals of energy conservation and environmental stewardship.