Applications and New Technologies Pertaining to Waste Heat Recovery: A Vision Article

1. Introduction

Industrial processes account for a substantial fraction of primary energy consumption worldwide, and they are often characterized by the large amounts of heat they discharge into the environment. Recent data from the European Commission indicate that industrial processes are responsible for about 26% of the European Union’s final energy consumption [1]. Similarly, the Intergovernmental Panel on Climate Change (IPCC) [2] has highlighted the urgent need for large-scale reductions in greenhouse gas emissions by 2050, a goal that can be significantly aided by improving energy efficiency and harnessing waste heat. This has led researchers and industry practitioners alike to emphasize effective waste heat recovery (WHR) strategies to reduce fossil-fuel dependence, enhance sustainability, and bolster the economic performance of energy-intensive sectors.

The incentive for WHR is especially strong in industries such as the chemical, iron and steel [3], paper, cement, oil and gas, and food-processing sectors, which collectively consume vast amounts of thermal energy [4,5]. Several studies have quantified the significant availability of industrial excess heat at different scales, in different regions [5,6,7,8,9], and at different temperatures, ranging from low-grade (below 100 °C) to high-grade (above 400 °C) streams [10]. The values for EU industrial waste heat were estimated to be 304.13 TWh/year [7] for the year 2018; this has since been reduced, with values of 221.32 TWh/year [6] estimated for the year 2021. The iron and steel industries contribute the most waste heat, with approximately 50 TWh/year [6], with waste heat stream temperatures above 500 °C. In the case of marine waste heat recovery, approximately half of a two-stroke engine’s fuel energy, or around 63% of a four-stroke engine’s fuel energy, is lost to the environment [11,12].

While direct process integration and district heating solutions are widely employed [13,14], there is growing interest in harnessing even-lower-temperature streams for power generation, absorption refrigeration, and other non-traditional applications.

These new directions in WHR are motivated by notable advances in techniques technologies [15] such as zeotropic mixtures for organic Rankine cycles [16], partial evaporating cycles [17], heat pipes [18,19], and advanced supercritical carbon dioxide (sCO₂) power systems [20]. They are also driven by the desire to develop flexible, cost-effective, and high-performance solutions that can withstand the often-harsh operating conditions of industrial exhaust and effluents. For instance, the incorporation of cold-energy exploitation in liquefied natural gas (LNG) regasification processes has introduced new horizons for achieving zero-emission targets, as exemplified in the development of organic Rankine cycle (ORC)–open cycle (OC) systems on floating storage regasification units [21]. Moreover, the use of data-driven controls and machine learning for predicting optimal setpoints in WHR-based systems, such as marine diesel engine absorption refrigeration plants, has expanded the possibilities of stable operation under fluctuating loads [22].

Nevertheless, despite growing evidence of strong technical and environmental benefits, the adoption of WHR solutions can be constrained by non-technical barriers and site-specific complexities [23]. The capital-intensiveness of advanced heat exchangers, as well as concerns about integrating new cycles into tightly configured processes, highlights the continuing need for advanced technoeconomic feasibility studies. Faced with increasingly strict regulations and corporate sustainability mandates, industries are actively seeking solutions that facilitate fast return on investment while also future-proofing their operations against uncertain energy markets. This overview highlights new advances and applications, illustrating how the field of WHR is evolving toward new technological frontiers.

2. Recent Advances, Applications, and Integrations

Recent years have witnessed remarkable progress in the technological, methodological, and scientific underpinnings of WHR. The recent advances, as well as applications and integrations, are categorized here into industrial and marine waste heat recovery. The technology, however, is interchangeable between the industries and sectors, as the main factors governing the use of WH are the quality and the quantity of the waste streams and accessibility.

2.1. Advances in the Marine Waste Heat Recovery Sector

A very high percentage of global trade is conducted via seaborne transport, which relies heavily on diesel engines for propulsion, leading to substantial fuel consumption and emissions [24]. WHR thermodynamic cycles, such as dual-pressure cycles and organic Rankine cycles, could offer a promising future for the marine WHR sector, for both diesel and dual-fuel engines [25], by reducing its environmental footprint through capturing and converting exhaust heat into usable energy [26].

Multi-criteria evaluation and optimization methods have emerged as essential tools for enhancing the performance of marine WHR systems. Recent advancements have focused on dual-pressure organic Rankine cycles (DPORCs), revealing that carefully selecting working fluids, combined with optimizing key operating parameters, significantly improves overall efficiency, reduces electricity generation costs, and minimizes environmental impacts. In particular, studies have confirmed that cyclopentane exhibits superior thermodynamic performance and a lower environmental footprint compared to other fluids commonly used in marine applications [27].

One of the most transformative areas is the study of combined cycles, including the incorporation of ORCs in unconventional applications. In the offshore regasification sector, for example, the cold energy of LNG has been successfully harnessed to power a series of ORCs working in tandem with an OC, yielding zero greenhouse gas emissions during the regasification process. As described in [21], employing up to three stages of ORCs with zeotropic mixtures and combining them with LNG direct expansion can simultaneously address environmental performance and on-board power production requirements. While multiple configurations (two or three stages) can meet zero-emissions targets, the authors identified that the two-stage ORC-OC arrangement provides a more favorable economic outlook. In parallel, partial evaporating ORC cycles have emerged as an innovative approach for low-temperature heat sources. Traditional ORCs often rely on full evaporation and superheating, but partial-evaporation ORCs allow a controlled two-phase flow at the turbine inlet to maximize heat absorption from a limited heat source. These systems can reduce equipment size, particularly in the evaporator section, thereby lowering capital costs while maintaining a relatively high conversion efficiency. Laboratory-scale experiments have demonstrated that allowing partial evaporation in an axial turbine reduces electrical efficiency only slightly and yet still increases the heat-source utilization rate [28]. This technology can be readily adapted to small- and medium-scale facilities seeking to reduce their payback periods for WHR investments.

Another technique applicable to marine WHR is the integration of thermoelectric generator (TEG) systems, which convert exhaust heat from marine diesel engines directly into electricity using solid-state materials. Despite the aforementioned TEG-WHR systems’ potential for enhancing energy efficiency and reducing emissions, their application in marine vessels remains limited. The marine environment offers a distinct advantage for integrating TEG technology with the availability of seawater as a cooling medium. Nevertheless, the literature on marine-specific TEG-WHR systems is limited [29]. Recent research efforts, such as the development and testing of nanoscale bismuth telluride thermoelectric materials [30], have made significant progress, including increased power outputs and improved system efficiency, yet they also highlight critical challenges, such as low material durability and contact resistances, which must be addressed to fully realize this technology’s potential.

2.2. Advances in the Industry Waste Heat Recovery Sector

In the industrial sector, as noted in Section 1, WHR has a great deal of potential, and there is a large amount of waste energy available. A specific area of active interest is the advanced control of waste-heat-based systems. The fluctuating nature of heat sources in certain industries, especially those wherein processes or engine loads vary with demand, calls for robust control solutions that can stabilize the system and maintain desired outputs. In marine diesel engines, for instance, the dynamic exhaust gas conditions hamper continuous absorption cooling. Machine learning algorithms, including backpropagation neural networks (BPNN), extreme learning machine (ELM), and Elman neural networks, make it possible to predict system variables and adjust critical flows accordingly [22]. The ELM algorithm was found to have excellent generalization capabilities and short training times, keeping refrigeration output nearly constant for a prescribed cooling demand. This capacity lays the foundation for applying advanced, data-driven strategies to other sectors where reliable WHR operation depends on a steady output despite variable heat inputs.

Further innovations center on heat exchange devices and power cycles better suited to high-particulate, corrosive, or otherwise demanding environments. Heat pipes, especially flat-heat-pipe-based systems, have attracted considerable attention for their compactness and ability to handle large heat fluxes over a wide temperature range. The coverage of such technologies in recent reviews [1,18] underscores the flexibility of heat pipes for different industrial flue gases, including applications where standard heat exchangers would require extensive maintenance. In some cases, the addition of condensing economizers with specialized coatings can recover both sensible and latent heat, a capacity of paramount significance in industries featuring damp or acidic exhaust streams.

Meanwhile, systematic approaches to mapping low- and medium-temperature industrial excess heat, particularly in sectors such as kraft pulping, have become more refined. Methodologies combining process integration and regression analysis show that substantial potential remains in harnessing streams at 60–140 °C for either direct heating or electricity generation [10]. Related studies highlight how carefully derived “excess heat temperature signatures” for typical mill configurations can be extrapolated to broader national sectors, providing more precise estimates of technical and economic feasibility than conventional top-down surveys. Outside the pulp and paper industry, these techniques can be transferred to other process-intensive sectors, allowing managers of industrial sites to make informed decisions about investing in ORCs or alternative low-temperature power systems.

Another domain exhibiting increasing promise and garnering ever-increasing interest regarding WHR is data centers [31,32,33], where the ever-growing demand for cloud computing has led to large-scale facilities generating substantial heat. Cooling systems for servers often generate a concentrated but relatively low-grade waste heat stream—typically near 30–40 °C or even lower [34]—making direct reuse challenging. Nonetheless, recent technological advances and innovative system designs now explore heat upgrading through heat pumps or low-temperature ORCs, thereby converting data centers’ thermal byproducts into usable energy for nearby industrial processes, fourth- or fifth-generation district heating networks, or even agriculture (e.g., greenhouse heating). The success of such initiatives depends on the use of careful thermal integration, compact heat exchangers, and robust designs to handle continuous operation while ensuring reliability for mission-critical digital infrastructure. As data centers continue to thrive, harnessing these low-temperature heat flows could deliver notable energy and environmental benefits.

One particularly compelling recent development involves the design of adaptive supersonic micro turbines capable of maintaining high cycle pressure even under fluctuating mass flow rates. The authors of [35] created a variable-geometry micro turbine for ORC systems in industrial scenarios with time-varying heat sources. By adjusting the turbine’s nozzles to match the available flow, the system can sustain a higher inlet pressure and thus closer-to-design efficiency. Experimental tests using a digital twin of the ORC confirmed that an adaptive nozzle arrangement can outperform conventional fixed-geometry turbines during part-load operation, improving the overall energy output by up to 18% under certain off-design conditions. This innovation is particularly relevant for waste heat recovery from processes like electric arc furnaces or other intermittently available streams, where the temperature or flow rate can fluctuate widely.

Solid-state thermoelectric generation technology has emerged as a promising solution for low-temperature waste heat recovery. Despite having lower efficiency than organic Rankine cycles, thermoelectric generators (TEGs) require smaller construction areas, increasing their market acceptance and applicability [24]. TEGs have been successfully applied in various areas, including automotive exhaust recovery systems, the cement industry, and concentrated solar heat exchangers [25]. Interestingly, some contradictions and challenges exist in the field of WHR. While thermoelectric technology shows promise, its current commercial conversion efficiency is only about 5% [24]. Additionally, the ceramic sector, which is energy-intensive, has implemented various WHR strategies, including high-efficiency burners, hot-air recycling, and combined heat and power systems, achieving energy savings of up to 50–60% in some cases [26].

Overall, the recent advances depict a trend towards complete assessments and integrated solutions. Hybrid and multi-stage processes, flexible partial-evaporation cycles, advanced heat pipe designs, specialized coatings for corrosive environments, and digital controls are converging to provide a suite of WHR technologies that can be deployed in increasingly diverse contexts. This progression suggests a maturation of the WHR field from stand-alone hardware retrofits to systemic interventions that couple process optimization with robust economic and environmental gains. Moreover, many studies indicate comparatively short payback times once integration challenges are overcome, reinforcing the notion that the adoption of these new solutions is becoming increasingly viable.

3. Conclusions

The growing global emphasis on sustainability, combined with technological breakthroughs, has driven waste heat recovery (WHR) to the forefront of industrial energy-efficiency research. This overview article highlights how targeted design, advanced control strategies, and innovative materials can effectively address critical barriers, including economic constraints, fluctuating heat sources, and environmental concerns.

In maritime WHR applications, dual-pressure organic Rankine cycles (DPORCs) optimized with suitable working fluids such as cyclopentane offer significant improvements in efficiency and environmental footprint reduction. The integration of organic Rankine cycles (ORCs) with LNG cold energy has enabled near-zero-emission regasification processes, while partial-evaporation ORCs provide cost-effective solutions for power generation based on lower-grade heat streams. Additionally, thermoelectric generator systems (TEGs), despite their current limitations, show promising advances in power output and integration capabilities for marine environments.

At an industry-wide scale, refined process integration methods and temperature profiling reveal that a significant portion of thermal energy—once considered low-grade—can in fact be used for power production or directly used internally as thermal energy, substantially enhancing both energy and cost performance. Furthermore, emerging data-driven machine learning algorithms enhance the operational stability and predictability of WHR systems, particularly under fluctuating loads. Moreover, recent developments in flat heat pipe exchangers, condensing economizers, trilateral flash cycles, and supercritical CO₂ systems have collectively broadened the range of temperature levels that can be feasibly targeted for heat recovery, bringing theoretical potential closer to practical implementation. These innovative technologies continue to push efficiency higher, reduce greenhouse gas emissions, and exhibit lower return on investment, giving industries a credible and increasingly attractive outline for harnessing heat that would otherwise be wasted.

However, it is equally important to acknowledge the need for supportive frameworks that account for economic, regulatory, and operational realities. Investments in advanced WHR technologies must be complemented by flexible market conditions, stable fuel-price scenarios, and proactive organizational cultures. Researchers and industrial experts should continue to collaborate across disciplines to ensure that WHR solutions evolve in parallel with the shifting demands and constraints of industrial operations. As demonstrated in this collection of articles in the Special Issue “Applications and New Technologies of Waste Heat Recover”, the path forward requires the combination of cutting-edge research, thoughtful deployment, and mutual understanding among technology providers, policymakers, and end users. With continued innovation, the role of industrial WHR will only expand, contributing considerably to a more resource-efficient and environmentally responsible industrial future.

Finally, the development of new WHR technologies and the optimization of existing ones are crucial for achieving sustainable energy development. The integration of these technologies in various industrial processes can lead to significant energy savings, reduced carbon emissions, and improved overall energy efficiency [1,27]. As research continues to advance, it is expected that more efficient and cost-effective WHR solutions will emerge, further enhancing the potential for widespread adoption across different industries.