Organic Rankine Cycle System Review: Thermodynamic Configurations, Working Fluids, and Future Challenges in Low-Temperature Power Generation

Abstract

In the context of the zero-carbon transition, this article provides a comprehensive review of Organic Rankine Cycle (ORC) technologies for low-grade heat recovery and conversion to power. It surveys a wide range of renewable and waste heat sources—including geothermal, solar thermal, biomass, internal combustion engine exhaust, and industrial process heat—and discusses the integration of ORC systems to enhance energy recovery and thermal efficiency. The analysis examines various configurations, from basic and regenerative cycles to advanced transcritical and supercritical designs, cascaded systems, and multi-source integration, evaluating their thermodynamic performance for different heat source profiles. A critical focus is placed on working fluid selection, where the landscape is being reshaped by stringent regulatory frameworks such as the EU F-Gas regulation, driving a shift towards low-GWP hydrofluoroolefins, natural refrigerants, and tailored zeotropic mixtures. The review benchmarks ORC against competing technologies such as the Kalina cycle, Stirling engines, and thermoelectric generators, highlighting relative performance characteristics. Furthermore, it identifies key trends, including the move beyond single-source applications toward integrated hybrid systems and the use of multi-objective optimization to balance thermodynamic, economic, and environmental criteria, despite persistent challenges related to computational cost and real-time control. Key findings confirm that ORC systems significantly improve low-grade heat utilization and overall thermal efficiency, positioning them as vital components for integrated zero-carbon power plants. The study concludes that synergistically optimizing ORC design, refrigerant choice in line with regulations, and system integration strategies is crucial for maximizing energy recovery and supporting the broader zero-carbon energy transition.

1. Introduction

1.1. General Context and Motivation

The growing global energy demand, combined with increasing environmental concerns, has intensified the need to reduce greenhouse gas emissions and improve energy efficiency [1]. A large proportion of primary energy is lost in the form of low-grade heat, originating mainly from industrial processes and renewable thermal sources such as geothermal and solar energy [2]. This low-enthalpy heat remains underutilized because of technological and economic barriers that limit its effective recovery [2].

Technological progress is making the conversion of this low-temperature energy increasingly feasible [1]. Recovering waste heat can reduce fossil fuel dependence, lower operational costs, and mitigate environmental impacts, providing a cost-effective pathway toward decarbonization [1]. Exploiting low-grade thermal energy and recovering industrial waste heat in Europe could provide an additional 8 EJ of energy annually [3].

The Organic Rankine Cycle (ORC) offers a reliable and efficient solution for converting low-enthalpy heat into mechanical power and electricity [4]. It is particularly well suited to low- and medium-temperature applications, including industrial waste heat recovery [5], solar thermal utilization [6], geothermal resources [7], and ocean thermal energy [8]. Advanced ORC configurations have been developed to improve thermal efficiency and better match heat source characteristics [9].

The technology has also been successfully applied in biomass energy systems [10], combined heat and power generation [11], and other industrial processes [12]. By exploiting this untapped energy potential, ORC contributes to improving overall energy efficiency and supports the broader energy transition toward low-carbon power generation. The ORC has emerged as a viable solution for converting this low-grade heat into electricity, offering a practical way to reuse waste heat that would otherwise be lost [13].

This review aims to address the limitations identified in previous surveys of ORC technology by providing an integrated and comparative analysis across configurations, working-fluid selection, applications, and techno-economic performance. Unlike earlier reviews that focused mainly on fluid thermodynamic behavior or system topology in isolation, this work consolidates results from more than 170 studies to compare efficiency ranges, LCOE trends, environmental implications, and configuration-specific advantages. Moreover, the review incorporates emerging directions such as ORC 4.0, hybrid integration, advanced control, and future refrigerant regulation—topics that are only briefly mentioned or absent in earlier publications. Therefore, the main contribution of this work lies not only in summarizing recent developments but in offering structured comparison criteria and decision-oriented guidance that may assist designers and researchers in selecting ORC layouts, working fluids, and integration strategies for different operating conditions.

1.2. Review Methodology

To ensure a structured and reproducible review, a systematic screening and selection process was adopted. The Scopus, Web of Science, and Google Scholar databases were searched using the keywords “Organic Rankine Cycle”, “ORC configurations”, “working fluids”, “low-temperature power generation”, and “waste heat recovery”. Additional terms such as “supercritical ORC”, “cascaded ORC”, “regenerative ORC”, “LCOE”, “techno-economic analysis”, and “hybrid ORC systems” were included to capture emerging research topics.

Studies published between 2000 and 2025 were considered. The initial search yielded ~3200 records, which were screened based on title and abstract. Duplicates and papers without thermodynamic, configurational, or applied relevance were removed, resulting in 412 documents for detailed evaluation. From these, a core set of 178 peer-reviewed articles, reviews, and reports was included in the final synthesis, prioritizing works that presented comparative performance data, novel ORC layouts, experimental results, or techno-economic metrics (efficiency, LCOE, environmental impact, CO₂ savings).

The review was structured in four analytical axes:

(i)

thermodynamic configurations (basic, regenerative, supercritical, cascaded, multi-source),

(ii)

working-fluid selection and regulatory constraints,

(iii)

application domains and scale-dependent performance,

(iv)

future challenges, research gaps, and ORC 4.0 digital transition.

This methodology ensures transparency in literature selection and reinforces that the conclusions derive from a representative and diversified body of published research.

1.3. Low-Enthalpy Thermal Energy Sources

Low-enthalpy thermal sources, typically below 150 °C, represent a significant but still underexploited energy potential in both renewable and industrial contexts [14,15]. They include geothermal reservoirs, solar thermal energy, biomass combustion, and a wide variety of industrial waste heat streams. Although these sources are widely available, their lower temperature levels have traditionally limited their direct use for electricity generation. In recent years, however, advances in energy conversion technologies—particularly Organic Rankine Cycle (ORC) systems—have made their exploitation increasingly feasible [16,17].

Geothermal energy provides one of the most stable and predictable low-temperature heat supplies [17,18]. Its temperature levels, generally between 80 and 200 °C, match well with subcritical or regenerative ORC configurations, which can achieve good conversion efficiencies under these conditions. Unlike most renewables, geothermal energy is dispatchable, making it highly suitable for continuous power generation at small and medium scales [19].

Solar thermal energy also offers a valuable low-enthalpy resource through flat-plate and evacuated-tube collectors, though its intermittency requires either storage or hybridization strategies to ensure stable operation [20,21]. ORC systems are particularly suitable for this temperature range, allowing efficient conversion of solar heat in distributed generation or off-grid applications where steam cycles are not technically or economically viable [22].

Biomass combustion represents another widely available low-enthalpy source. Its thermal output is stable and compatible with small-scale ORC plants [23,24], enabling local power generation with low operational complexity and the possibility of cogeneration [25]. This flexibility has driven growing interest in biomass–ORC systems for rural electrification, district heating, and combined heat and power applications.

Industrial processes constitute one of the largest reservoirs of low-enthalpy heat, with significant potential for energy recovery [26,27]. Waste heat from sectors such as cement, steel, chemical, or oil refining is often available continuously and on-site, which enhances the technical and economic viability of ORC installations [13,28]. The recovery of this waste heat can reduce primary energy consumption and emissions while improving overall process efficiency [29].

Although these sources differ in temperature profiles, availability, and intermittency, they share a common characteristic: they can be effectively harnessed through ORC technology. Its operational flexibility and broad compatibility with different thermal inputs have made ORC one of the most promising options for converting low-grade heat into useful power, supporting decarbonization targets and improving overall energy system efficiency [30,31,32].

Table 1. Common Energy Sources for Organic Rankine Cycle Systems.

Energy Source	Description	Cited Works (2021 Onwards)
Renewable Energy Sources
Geothermal Energy	Harnesses heat from the Earth’s interior for power generation through ORC systems. Studies focus on performance optimization with different working fluids and exergetic analysis of polygeneration arrangements.	[3,14,17,32]
Solar Energy	Utilizes solar thermal collectors to generate heat for ORC electricity production. Research includes optimization with evacuated tube collectors, robust system design, and integration with thermal storage.	[10,20,22,33,34]
Biomass Energy	Converts organic materials through combustion for ORC power generation. Applications range from small-scale plants using agricultural waste to thermoeconomic analysis of hybrid systems.	[17,23,24,25,35]
Ocean Thermal Energy Conversion	Exploits temperature differences between surface and deep seawater to drive ORCs. Research covers site selection, working fluid optimization, and system performance modeling.	[3,36,37]
Waste Heat Recovery
Industrial Processes	Recovers excess heat from industrial operations (e.g., iron/steel, manufacturing) using ORC technology. Studies review heat integration approaches and provide techno-economic optimization case studies.	[5,13,15,25,26,29]
Gas Turbines and Engines	Captures exhaust heat from internal combustion engines and gas turbines for additional electricity generation. Research includes turbo-expander design, marine applications, and comprehensive system reviews.	[13,38,39,40,41,42,43]

shows the most common renewable sources and waste heat recovery that have been used in ORCs.

1.4. Overview of Technologies for Low-Enthalpy Energy Conversion

Different authors have proposed diverse classification schemes for ORC systems depending on the application context and technological focus.

Table 2. Summary of Key Heat-to-Power Technologies: Organic Rankine Cycle and Alternatives.

Technology	Working Principle	Typical Temperature (°C/ΔT)	Typical Efficiency	Technical Maturity	Main Advantages	Main Drawbacks	References
ORC	Rankine cycle with organic fluid	60–150	8–18%	High (commercial)	Mature, flexible, scalable	Moderate efficiency vs. high-T cycles	[1,5,11,15,44,45,46,47]
Kalina cycle	Rankine with ammonia–water mixture	90–200	10–20%	Medium (pilot/demo)	Better temperature matching, higher potential efficiency	Complexity, corrosion, O&M issues	[5,45,48,49]
Stirling engine	External combustion engine	80–200	10–15%	Medium–low	Compact, fuel-flexible	High cost, lower efficiency	[50,51,52,53]
TEG	Thermoelectric conversion (solid state)	50–150	2–7%	Low (niche)	No moving parts, reliable	Very low efficiency, very high cost	[13,54,55,56,57]
OTEC	Rankine using the ocean temperature gradient	ΔT 20–25	2–5%	Low (pilot)	Stable renewable source, continuous operation	Low efficiency, high infrastructure cost, location dependent	[36,58,59]
Goswami cycle	Combined Rankine–absorption cycle using ammonia–water mixture	60–350	up to 30%	Laboratory-scale	Simultaneous power and cooling; good for low–medium heat sources	Toxic working fluid; limited operational flexibility	[60,61,62,63]

compares representative examples from the literature.

This classification landscape provides the foundation for the more detailed categorizations discussed below:

• Organic Rankine Cycle

In contrast to conventional steam Rankine cycles, ORC systems employ organic fluids characterized by lower boiling points [12], rendering them effective for recuperating waste heat from industrial processes [5,44,64], geothermal resources [32], ocean thermal energy, and solar thermal power [13,33]. In the cycle, waste heat vaporizes the organic fluid, and the high-pressure vapor expands through a turbine to generate electricity [65]. The vapor condenses and returns, completing the cycle. The working fluid’s selection is critical [38,66], considering its thermodynamic properties, environmental impact, safety, and cost [66]. Key improvements include internal heat exchangers [13] and system optimization to enhance efficiency [26].

• Kalina Cycle

The Kalina Cycle is another promising technology for low-enthalpy conversion, particularly for geothermal and industrial waste heat [5]. Its key distinction from the ORC is the use of a zeotropic ammonia-water mixture as the working fluid. This allows for a variable composition during the cycle, enabling a better thermal match with the heat source’s temperature profile (temperature glide) during evaporation and condensation, which reduces exergy destruction and improves thermodynamic efficiency [48]. The cycle involves separating the mixture into ammonia-rich and water-rich streams to optimize heat transfer and recovery. While the Kalina Cycle has demonstrated 20–40% higher efficiency than ORC in some applications, especially those with varying temperature profiles [5,45], its design and operation are more complex, requiring sophisticated control systems and equipment [48,49].

• Reversed Stirling Engine

The reversed Stirling engine utilizes mechanical work or high-temperature heat to pump thermal energy from a low-temperature source to a higher-temperature sink, making it suitable for cooling or heating applications driven by waste heat. Its advantages include operation with relatively low-temperature heat sources, high potential efficiency, and low emissions. Free Piston Stirling Engines are newer variants with marked advantages. Research focuses on improving efficiency and reducing costs [50]. Stirling engines are fuel-agnostic [50], perform well under partial loads [51], and can achieve Carnot efficiency under ideal conditions [52]. Their use in combined cycles is postulated to yield higher plant efficiency [53], and they are favored for moderate temperatures [53].

• Thermoelectric Generators

Thermoelectric Generators (TEGs) provide a solid-state approach to direct heat-to-electricity conversion based on the Seebeck effect [12]. They consist of p-type and n-type semiconductor modules; a temperature gradient causes charge carrier migration, generating a voltage. TEGs are highly reliable, environmentally friendly, silent, have a long operating life [56], and require low maintenance due to the absence of moving parts, making them suitable for automotive exhaust, industrial processes, and electronics [41,54]. Their efficiency is relatively low compared to other technologies, such as Organic Rankine Cycles (ORCs), Kalina cycles, and Stirling engines (around 2.9%, lower than conventional generators [12]), limited by the properties of thermoelectric materials [55]. Ongoing research aims to develop better materials and reduce costs [36], with nanotechnology enabling efficiencies of 15% or greater [64]. Despite lower efficiency, TEGs are valuable in niche applications where reliability and low maintenance are paramount, such as in space probes and medical equipment [13,54]. TEGs are environmentally friendly, silent, and have a long operating life [56], creating electricity from a temperature difference without moving parts [64].

• Ocean Thermal Energy Conversion

Ocean Thermal Energy Conversion (OTEC) harnesses the temperature difference between warm surface seawater and cold deep seawater (typically 20–25 °C in the tropics) to generate electricity, tapping into the vast solar energy stored in the oceans [36]. The OTEC resource spans over 100 million km² in tropical oceans [58]. OTEC Systems can be two primary types [37], with closed-Cycle OTEC (similar to ORC) [67], or open-Cycle OTEC, where the steam is condensed by direct contact with cold deep seawater or via a surface condenser. Challenges include high capital costs, environmental concerns related to seawater intake/discharge, and the need for large-scale, corrosion-resistant infrastructure [36]. The thermal efficiency of OTEC systems is low (3–5%) due to the small temperature differences [68]. Further research and development efforts are needed to enhance efficiency and reduce costs [36], with working fluid optimization being one avenue [3,5].

• Goswami cycle

The Goswami cycle is a combined power and cooling thermodynamic cycle that integrates a Rankine power cycle with an ammonia–water absorption refrigeration cycle, using a single ammonia–water working fluid loop to simultaneously produce electricity and refrigeration. It typically operates with low-to-medium temperature heat sources and achieves modest thermal efficiencies on the order of ~25–30%, while exergy efficiencies can reach around 47–48% under optimized conditions. The cycle enables cogeneration or even trigeneration of energy outputs, generating power and cooling in one process, which makes it attractive for applications such as industrial waste heat recovery, geothermal energy utilization, and integration into industrial processes or chemical plants. Key advantages of the Goswami cycle include its ability to harness low- and medium-grade heat sources that are unusable by conventional steam cycles, the simultaneous production of cooling and power from the same heat input, and the potential for implementation in relatively compact systems. However, important drawbacks are also noted: the working fluid ammonia is toxic, the cycle has limited operational flexibility in adjusting the balance between cooling and power output, and its cooling performance tends to drop off at heat-source temperatures below ~200 °C [62,69,70,71]. Overall, this cycle is highly relevant today because it offers an efficient means to convert abundant low-grade thermal energy into multiple useful outputs (power and cooling), aligning with current goals in sustainable energy utilization and waste-heat recovery.

Overall, recent trends in low-temperature power conversion show a growing interest in hybrid and multifunctional cycles, particularly those capable of simultaneously producing power and cooling, such as the Goswami cycle [3,69,70,72]. Emerging technologies focus on improving adaptability to variable and ultra-low-grade heat sources, integrating renewable inputs, and enhancing system compactness [73,74]. Although several concepts remain at the research stage [72], these advancements demonstrate a clear movement toward more flexible, efficient, and application-specific architectures for low-enthalpy heat recovery [2,3].

1.5. Commercial Case Studies and Operational Plants

The practical feasibility of Organic Rankine Cycle systems for low-enthalpy energy conversion is demonstrated by commercial installations worldwide, highlighting the technology’s operational flexibility and its capacity to integrate with various heat sources [17,22,27]. As an example, some existing installations with ORC systems are listed below.

The integration of ORC systems in steel plants to mitigate CO₂ emissions and improve energy efficiency remains a focus of current research [75,76]. One of the most emblematic examples is the 14 MW ORC plant by Turboden in Brescia (Italy), which recovers high-temperature exhaust gases from a steel plant. The facility achieves electrical efficiencies above 20% and offsets approximately 10,000 tons of CO₂ per year.

Feasibility studies and plans for geothermal electricity co-production are underway in locations like the Blackburn Oil Field (Pine Valley, NV, USA), where modular ORC units are being considered to utilize co-produced hot waters [77], targeting resources with temperatures around 175 °C [78,79].

Biomass-fired ORC cogeneration facilities delivering both electrical power and thermal energy for district heating represent a viable application of ORC technology, promoting decentralized renewable heat-to-power solutions [80]. A notable example is the installation in Lienz (Austria), which provides 1 MW of electricity and 5 MW of heat for the local district heating network.

This review, therefore, provides not only a structured overview of ORC fundamentals but also a design-oriented discussion of configurations, working fluids, and performance trade-offs. The following sections present the system classifications, component-level considerations, and emerging research directions that currently shape ORC development.

2. Organic Rankine Cycle Technologies

2.1. Variations and Enhancements

The basic Organic Rankine Cycle (ORC) layout consists of four primary components—a pump, an evaporator, an expander, and a condenser—arranged in a closed loop (as illustrated in Figure 1 [44]). This configuration follows the thermodynamic principles of a conventional steam Rankine cycle, but it is adapted for low-enthalpy heat sources by using organic working fluids and moderate operating pressures [15,49].

Several modifications and enhancements have been introduced to improve performance and tailor the ORC to specific applications [81]. These variations—which include recuperative (regenerative) cycles [82], reheated cycles, use of multiple heat sources, and cascaded multi-cycle systems—are all aimed at boosting thermal efficiency, increasing power output, or optimizing the cycle for particular heat-source characteristics [13]. They provide significant design flexibility, enabling ORC systems to be customized for various requirements and heat-source profiles and thereby maximizing energy recovery while minimizing waste [83]. Figure 2 illustrates a representative example of a regenerative ORC configuration, integrated with a linear Fresnel solar collector. The following subsections describe the major ORC variations and their thermodynamic enhancements, with attention to typical operating conditions and reported efficiency gains.

• Regenerative (Recuperative) ORC

A regenerative ORC employs an internal heat exchanger—often called a regenerator or recuperator—to recover residual heat from the turbine exhaust and use it to preheat the working fluid before it enters the evaporator [45,82]. In a basic ORC, the hot vapor exiting the expander is normally rejected to the condenser, wasting a considerable amount of heat. By contrast, in a regenerative configuration (as shown in Figure 2), this waste heat is intercepted and transferred to the high-pressure liquid coming from the pump, raising its temperature prior to evaporation. This internal heat exchange reduces the amount of external heat input required from the primary source to vaporize the fluid, thereby improving the cycle’s thermal efficiency [82]. The effectiveness of the regenerator is a key design parameter: higher effectiveness yields greater heat recovery and efficiency gains, although it also introduces additional cost, complexity, and potential pressure drops that must be carefully managed [26,45].

Several studies have quantified the benefits of regeneration under various conditions. For example, Bouhamady et al. [82] integrated a regenerator into a solar-ORC system with a Fresnel linear concentrator using refrigerants R245fa/R123 (heat source 150–300 °C). Their results showed that regeneration improved the thermal efficiency by approximately 12–15% compared to a basic ORC, especially for the high-latent-heat fluid R123. Tarrad (2021) [84] investigated a dual-temperature waste heat recovery ORC using R134a, where low-grade heat at 80–200 °C and medium-grade heat at 200–350 °C were both utilized. The regenerative cycle in this case achieved about 8–10% higher efficiency by flexibly capturing heat from the two temperature levels. Likewise, Kosmadakis and Manolakos (2016) [8] demonstrated a regenerative ORC on a small-scale diesel engine (90–180 °C heat source) with R245fa, and observed a 7–9% increase in net power output despite the system’s spatial constraints. These examples confirm that internal heat recuperation can substantially boost performance across various scales and source types. In practice, fully capitalizing on regeneration requires careful recuperator design and an appropriate choice of working fluid [45] (see Section 3.3 for further discussion of fluid selection criteria).

• Reheated ORC

In high-temperature applications, a reheated ORC (RH-ORC) can be employed to further enhance power output and efficiency [81]. This configuration is analogous to reheat in conventional steam cycles: the working fluid undergoes an initial expansion in a high-pressure turbine, is then routed back to the heater or a secondary evaporator for reheating, and subsequently expanded again in a low-pressure turbine [31]. By splitting the expansion into two stages with intermediate heat addition, the cycle can extract more energy at a higher average temperature, thus improving thermal efficiency. Reheating also reduces the moisture content at the final turbine exhaust (a concern for certain fluids), protecting expander blades and improving performance [85]. Hemadri and Subbarao (2021) [85] investigated an RH-ORC integrated with a gas turbine exhaust, reporting notable gains in power recovery relative to a single-expansion ORC. Similarly, a recent thermodynamic analysis by Sakina (2023) [86] compared basic, recuperative, and reheated ORC modes for high-grade waste heat recovery, finding that the reheated cycle achieved the highest efficiency among the three configurations. These studies indicate that adding a reheat stage is beneficial for ORCs utilizing heat sources at the upper end of the temperature spectrum, albeit at the expense of additional hardware and complexity for the extra heating and expansion stages.

• Trilateral Flash Cycle (TFC)

The Trilateral Flash Cycle (TFC) is a specialized ORC variation designed to better handle heat sources that undergo a large temperature drop (a significant temperature glide) during heat transfer [3]. In a standard ORC, the working fluid is fully evaporated in the heater, which may not be ideal for sources that cool gradually (for instance, thermal oil or exhaust gas with a wide temperature glide). The TFC addresses this by heating the pressurized working fluid only up to its saturation point at the maximum source temperature (without complete evaporation) [3,38]. The resulting high-pressure liquid, just at the brink of vaporization, is then expanded directly in a specially designed two-phase expander (e.g., a screw expander or two-phase turbine). During this expansion, a portion of the liquid flashes into vapor as the pressure drops, producing useful work [3]. This approach maintains a closer thermal match between the working fluid and the sensible heat source throughout the heating process, which can reduce exergy destruction compared to a conventional cycle that might incur large temperature differences in the evaporator [87]. Studies have suggested that TFCs can improve performance for certain low- to medium-temperature applications with significant source glides [3]. However, the two-phase expansion process is challenging: the expander must handle a liquid–vapor mixture, which can impact efficiency and mechanical reliability. These practical challenges mean TFC implementations are less common, and their benefits are typically realized only in niche scenarios where the heat source profile is a good match [26].

2.2. Configurations of Organic Rankine Cycles

Beyond the above cycle variations, more fundamental alterations in the thermodynamic configuration of ORCs can yield further performance improvements. A key distinction is the cycle’s operation relative to the working fluid’s critical point. In subcritical ORCs, the entire evaporation process occurs below the fluid’s critical pressure and temperature, as is the case for most conventional designs. By contrast, transcritical ORC systems operate at a maximum pressure above the fluid’s critical pressure [3]. A special subset of transcritical operation is the supercritical ORC, wherein the fluid is not only pressurized above the critical point but also heated beyond the critical temperature in the evaporator. In a supercritical evaporator, there is no sharp phase change—the fluid transitions continuously from liquid-like to gas-like states [88]. This regime allows a closer thermal match with the heat-source profile, which can enhance cycle efficiency (see Section 3.3 for a detailed discussion on matching fluid properties to heat sources). Transcritical cycles (including supercritical as an extreme case) thus provide flexibility for high-pressure operation even when the source temperature is not high enough to achieve a fully supercritical condition throughout the heating process. The following subsections discuss several important ORC configurations in this category—namely supercritical, transcritical, dual-pressure, and cascaded ORCs—highlighting their operating conditions and reported performance gains.

• Supercritical ORC

A supercritical ORC is characterized by an evaporator pressure above the working fluid’s critical pressure and a heating process that exceeds the critical temperature, so that the fluid enters a supercritical state [88]. Under these conditions, the fluid does not undergo a distinct liquid-to-vapor phase change; instead, it smoothly transitions into a supercritical fluid, which exhibits properties of both liquid and gas [89]. One advantage of supercritical operation is improved thermal matching between the heat source and the working fluid, thereby reducing irreversibilities [88]. This can lead to higher cycle efficiencies compared to subcritical ORCs, particularly for high-temperature heat sources that can drive the fluid well beyond its critical point [87]. On the other hand, supercritical ORCs require significantly higher operating pressures and often more robust equipment (pumps, heat exchangers, piping), which increases capital cost and complexity [26]. Careful design is needed to ensure that the efficiency gains outweigh these practical considerations.

Recent studies underscore both the potential and challenges of supercritical ORC systems. Marzouk [90] numerically investigated a supercritical Rankine cycle in a power generation context using water as the working fluid (critical point of water ~374 °C, 22.1 MPa) with heat source temperatures of 500–600 °C. The analysis focused on how varying the condenser pressure influences performance. The results showed that operating in the supercritical regime boosted the thermal efficiency by about 8–12% relative to an equivalent subcritical cycle. However, this came with the need for specialized high-pressure equipment and careful control of the cycle conditions. In an applied industrial setting, Delgado [26] evaluated a supercritical ORC implemented as part of a cogeneration system at a sugar processing plant. Using refrigerant R1233zd, the ORC recovered medium-grade waste heat (280–350 °C) from the process. The supercritical cycle was found to augment net power output by 18–22% compared to a subcritical ORC operating under the same conditions. This substantial improvement helped address the facility’s cogeneration shortfall, although the authors note that the higher pressures and temperatures necessitated robust components and a thorough economic justification for the added investment. These findings illustrate that supercritical ORCs can deliver markedly better performance in suitable high-temperature applications, provided the engineering challenges are properly managed.

• Transcritical ORC

Transcritical ORC systems also operate with high-side pressures above the fluid’s critical pressure, but unlike fully supercritical cycles, the fluid may not be heated completely past the critical temperature. In other words, the evaporator in a transcritical ORC can straddle the critical point: part of the heat addition occurs in the liquid phase up to the critical temperature, after which the fluid becomes supercritical for the remainder of the heating process. This configuration is advantageous when the heat-source temperature is moderately high but not enough to push the working fluid far into the supercritical region. By operating at supercritical pressure without requiring extreme source temperatures, transcritical cycles can improve thermal efficiency over subcritical cycles while avoiding some of the material and design challenges of a fully supercritical system [91]. However, transcritical ORCs demand precise control near the critical point to maintain stability and avoid large performance swings, since fluid properties change rapidly in this region.

Empirical comparisons between subcritical and transcritical ORCs demonstrate the efficiency gains of the latter. Oyewunmi et al. [91] designed a transcritical ORC for industrial waste-heat recovery using R245fa as the working fluid, with a heat source temperature of 120–180 °C. Their experiments and simulations showed that transcritical operation improved the thermal efficiency by roughly 12–18% relative to a conventional subcritical cycle under the same conditions. This benefit, however, was contingent on fine-tuning the operating pressure near the critical point, underscoring the need for tight pressure control. In another study, Lecompte et al. [92] tested a transcritical ORC in a biomass-fueled cogeneration plant using refrigerant R1233zd(E). With heat-source temperatures of 150–200 °C, they achieved partial supercritical heating of the working fluid. The transcritical ORC yielded a 15–20% higher power output compared to a subcritical cycle, although the authors noted challenges in expander design due to the fluid’s changing properties as it crossed the critical threshold. These examples confirm that transcritical cycles can offer significant performance advantages for medium- to high-temperature heat sources, bridging the gap between conventional and supercritical ORC designs.

• Two-Pressure Evaporation ORC

For heat sources that release energy over a broad temperature range (for instance, a source with a high initial temperature but a substantial glide as it cools), a single-pressure ORC may not capture the available exergy efficiently. In such cases, an ORC with two-pressure evaporation can be employed to improve thermal matching. This configuration is analogous to a dual-pressure steam Rankine cycle: it features two separate evaporators (or evaporation levels) operating at different pressure levels—one high-pressure and one low-pressure—using the same working fluid [3]. The heat source fluid first passes through the high-pressure evaporator, vaporizing a portion of the working fluid at the higher pressure. Downstream of that, the now-cooler heat source fluid is still hot enough to boil additional working fluid in a second evaporator at a lower pressure. The generated high-pressure and low-pressure vapors can then be expanded either in two separate turbines staged appropriately or in a single turbine with dual inlets designed to accommodate both pressure levels. By sequentially evaporating the working fluid at two pressure levels, this approach maintains a closer temperature profile match between the heat source and the working fluid, thereby reducing exergy losses across the heat exchangers. The net result is an increase in total energy extracted from the source and a higher overall cycle efficiency [93], at the cost of additional system complexity and equipment (an extra evaporator and potentially a second expander) [45,87].

Dual-pressure ORC setups have demonstrated notable performance improvements in simulations and pilot projects. For example, Markides et al. [3] describe a dual-evaporation ORC designed for a heat source with a large glide, which achieved better thermal utilization than a comparable single-pressure cycle. Similarly, Surendran and Seshadri (2020) [94] investigated two-stage ORC architectures with induction turbine layouts for dual-source waste heat recovery, reporting enhanced thermal efficiency and flexibility in managing variable heat inputs. By incorporating two evaporation pressures, the system was able to boost the heat recovery efficiency significantly, confirming the thermodynamic advantage of this configuration (specific quantitative gains depend on the source profile and working fluid and were summarized by the authors for various case studies [3]. Despite these benefits, it is important to acknowledge the engineering trade-offs: the added heat exchanger, piping, and possible multi-stage expansion increase the system’s capital cost and control complexity. Thus, two-pressure ORCs tend to be justified only when the heat source characteristics (e.g., very wide temperature range) allow a substantial efficiency payoff to outweigh the extra complexity.

• Cascaded ORC

A cascaded ORC system involves coupling the basic ORC with one or more additional thermodynamic cycles in series, so that the waste heat from one cycle is used to drive another. Often this takes the form of an ORC combined with a refrigeration or cooling cycle (absorption or vapor-compression), effectively creating a power-and-cooling cogeneration system. In a typical cascaded setup, the ORC produces electricity from a high-temperature heat source, and its low-grade thermal reject heat (e.g., the condenser heat) is then supplied to a secondary cycle, such as an absorption chiller, to provide cooling (or to another lower-temperature power cycle). This cascading approach boosts the overall utilization of the heat source, often achieving higher combined efficiencies than either process alone, enabling the simultaneous production of power and refrigeration. The concept is especially attractive in industrial contexts with both electrical and cooling demands, or in trigeneration systems. However, cascaded systems are inherently more complex and require careful design optimization to ensure that the two (or more) coupled cycles are well matched. The operating conditions must be balanced such that the temperature of the ORC’s waste heat matches the input requirements of the subsequent cycle, and control strategies must ensure the integrated system is managed effectively [3,93].

Several case studies have highlighted the benefits of cascaded ORC configurations. Patel et al. [95] implemented an ORC–refrigeration cascade for industrial furnace exhaust heat recovery. In their system, a toluene-based ORC using 350 °C flue gas produced power, and the ORC’s condenser heat (at about 80–90 °C) drove a hybrid cooling cycle consisting of a LiBr–H₂O absorption chiller combined with a vapor-compression stage (using R134a). By optimizing the distribution of heat between power generation and cooling, the cascaded system achieved about 25% higher exergy efficiency than two standalone systems working separately. The best performance was obtained when the ORC’s condensation temperature was closely aligned with the chiller’s driving temperature, illustrating the importance of thermal matching between the cascaded stages. In a more recent study, Markides et al. [3] analyzed a cascaded ORC–absorption chiller for a geothermal district energy plant. The ORC (using isopentane as the working fluid) generated power from a 140 °C geothermal brine, while its reject heat powered an ammonia–water absorption cooling cycle. Through computational optimization, including advanced working-fluid selection techniques, the integrated system was predicted to achieve 30–40% higher overall energy efficiency compared to a single-generation system. This considerable gain demonstrates the appeal of cascaded ORCs, although the study also emphasized that precise coordination of the two cycles (in terms of heat exchanger sizing and load following) is crucial to minimize exergy losses at the interface between them.

Table 3. Table of advanced ORC configurations with their improvement percentages.

Configuration ORC	Description	Working Fluid	Improvement (%)
Regenerative	Uses an internal heat exchanger to preheat fluid with turbine exhaust heat	R245fa	12–15% [82]
Regenerative	Dual-temperature waste heat recovery with R134a	R134a	8–10% [84]
Regenerative	Small-scale diesel engine waste heat recovery with R245fa	R245fa	7–9% [8]
Supercritical	Operates above the critical point; simulated with water at 500–600 °C	Water	8–12% [90]
Supercritical	Industrial sugar plant cogeneration with R1233zd	R1233zd	18–22% [26]
Transcritical	Industrial waste heat recovery with R245fa (120–180 °C)	R245fa	12–18% [91]
Transcritical	Biomass-powered cogeneration with R1233zd(E)	R1233zd	15–20% [92]
Multiple Heat Sources	Sugar plant cogeneration with dual waste heat streams	Benzene	32% [87]
Multiple Heat Sources	Fischer-Tropsch plant with cyclopentane	Cyclopentane	21–24% [89]
Cascaded	ORC-refrigeration system with toluene and a hybrid chiller	Toluene	20–23% [94]
Cascaded	Geothermal ORC with absorption chiller	Methyl-formate for ORC, Ammonia-water for absorption chiller	30–40% [3]

provides a comparative summary of the overall efficiency improvements reported for the various ORC configurations discussed above (e.g., regenerative, reheated, supercritical, transcritical, dual-pressure, cascaded, etc.) under typical operating conditions. To enhance clarity and complement the discussion of advanced ORC architectures, Figure 3 illustrates the main configurations—including transcritical, regenerative, supercritical, multiple heat-source, and cascaded ORCs—together with their operating principles, representative working fluids, and reported performance improvements.

These results highlight the range of efficiency gains achievable, ranging from approximately 5–10% for internal modifications up to 20–30% or more for advanced multi-cycle arrangements. The choice of an optimal configuration is highly dependent on the specific application and heat source characteristics. In the next section, we examine how these ORC system configurations are applied across different sectors and use cases.

To enhance the comparative understanding of thermodynamic performance across technologies, Figure 4 summarizes the expected efficiency range of ORC technologies relative to alternative low-temperature conversion systems. This synthesized visualization supports the quantified gains reported in Section 2 and Table 3, making explicit how the efficiency window evolves as the heat-source temperature increases.

In recent years, research on ORC configurations has shifted toward systems with higher flexibility and improved thermodynamic matching to low- and medium-grade heat sources. Recuperative and regenerative layouts continue to demonstrate solid performance gains, while multi-stage, cascaded, and hybrid solar–ORC configurations are emerging as promising solutions for maximizing exergy recovery. Current trends also emphasize dynamic operation, part-load optimization, and modular architectures, reinforcing the transition toward ORC systems that can efficiently operate under highly variable heat-source conditions.

2.3. Applications

The flexibility of ORC technology to utilize low- and medium-grade heat sources (roughly 80 °C to 300 °C) [81,96]—in contrast to traditional steam cycles that require much higher temperatures—has led to a broad range of applications in both waste heat recovery and renewable energy conversion. ORC systems can be reconfigured or customized (for example, by selecting regenerative cycle layouts or tailoring the working fluid) to suit specific heat-source profiles, thereby maximizing energy extraction from streams that are variable, intermittent, or have suboptimal thermodynamic qualities. This adaptability makes ORC a uniquely versatile platform for improving the utilization of energy that would otherwise be lost [97]. For instance, ORC units have been successfully integrated with sources as diverse as industrial exhaust gases, geothermal reservoirs, biomass boilers, and solar thermal collectors, each time adjusting the cycle parameters or design to match the source characteristics [98].

Another important emerging trend is the integration of ORC systems with energy storage solutions to mitigate the intermittency of renewable heat sources and to buffer fluctuations in waste heat availability. By coupling an ORC with thermal energy storage (e.g., molten salt tanks or phase-change materials) or other storage technologies, the system can operate more continuously and at optimal load even when the heat supply is uneven [97]. Indeed, the combination of ORC units with storage and the use of low-boiling-point working fluids extends efficient power generation into temperature ranges and operating profiles that steam-based systems cannot accommodate [12,13]. This dual capability not only expands the applicability of ORCs but also improves overall energy efficiency by allowing more of the recovered heat to be converted into useful work. Studies have reported that such approaches can improve energy efficiency by approximately 20–40% in various industrial scenarios while also reducing thermal pollution (excess heat rejection to the environment) [26,99]. In the following subsections, we highlight several key application areas for ORC technology—including industrial waste heat recovery, multi-source heat integration, and renewable energy utilization—and discuss how the aforementioned cycle configurations are employed to maximize performance in each case.

• Waste Heat Recovery

One of the most significant applications of ORC technology is the conversion of industrial waste heat into electricity [15,45]. By tapping into waste heat streams from processes such as metal smelting [27], cement manufacturing, glass production [27,45], and internal combustion engines [13], ORC systems can yield substantial energy savings (often in the range of 15–30% of the waste heat stream’s energy) and concomitantly reduce greenhouse gas emissions [15,45]. In internal combustion engine plants, for example, harnessing the exhaust and coolant waste heat via an ORC bottoming cycle can noticeably improve the overall thermal efficiency of the system. As demonstrated by Li et al. [13], a hybrid dual-cycle configuration can be particularly effective: in their work, a high-temperature CO₂ cycle was used to capture the prime mover’s hot exhaust (350–500 °C) while a secondary ORC was tasked with the lower-grade heat from the engine intercooler (80–120 °C). This combined CO₂–ORC arrangement, optimized via coordinated expander and condenser pressure settings, achieved an overall 12–15% increase in engine thermal efficiency relative to the baseline engine, demonstrating the value of ORC augmentation in engine waste heat recovery.

Similarly, Brancaleoni et al. [42] implemented ORC technology on a hydrogen-fueled internal combustion engine to convert its exhaust heat (around 200–450 °C) into additional power. The ORC in this case was integrated into a turbocompound engine system. It delivered an 8–10% increase in net power output from the engine–ORC combined system. The authors observed some control challenges during rapid load changes (transients), as the ORC must be regulated in tandem with the engine, but overall, the system remained stable. These examples underscore the attractiveness of ORCs for waste heat to power conversion in transport and stationary engine applications. Nevertheless, despite clear technical potential, widespread commercial adoption of ORC-based waste heat recovery is still hindered by economic factors and by operational complexities under off-design conditions [42]. Continued advancements in expander technology, control systems, and working-fluid optimization are expected to further improve the viability of ORCs in the waste-heat recovery market.

• ORC with Multiple Heat Sources

ORC systems can also be configured to utilize multiple heat sources simultaneously [5,98], opening opportunities for integrated energy recovery in facilities where several waste streams or renewable sources are available [15]. In a multi-heat-source ORC, different heat inputs (at different temperatures or from different processes) feed into the cycle at appropriate points [98] (for example, one source might serve the main evaporator while another lower-grade source preheats the working fluid). An illustrative scenario is a cogeneration plant where high-temperature exhaust gas is used in the primary evaporator and a lower-temperature source (such as engine jacket water or process cooling water) is used to preheat or regenerate within the cycle [26,98]. By harvesting heat from multiple streams, the ORC maximizes overall energy utilization and can significantly improve the system’s combined efficiency, especially in cogeneration or polygeneration contexts [5,100,101]. Crucially, the cycle must be customized to match the composite temperature–enthalpy profile of the combined sources; doing so can substantially boost the conversion efficiency relative to what could be achieved using each source independently [102].

A recent example of a dual-source ORC is provided by Delgado et al. [26], who implemented an ORC in a sugar mill cogeneration facility that drew heat from two waste streams: (1) high-temperature flue gases from bagasse (biomass) combustion at 250–350 °C, and (2) lower-temperature process steam condensate at 80–120 °C. The ORC used R1233zd(E) as the working fluid and was designed with a matching two-level heat input structure. Through careful heat cascade utilization and system optimization, the integrated ORC increased the net power output of the plant by 32% compared to the original cogeneration setup, while also shortening the economic payback period to under 5 years. This substantial improvement helped address a power generation deficit at the sugar plant by efficiently using energy that was previously wasted or used only for low-grade heating. In another study, Di Genova et al. [102] developed a custom ORC to recover heat from a pair of sources in a Fischer–Tropsch chemical plant: hot syngas cooling effluent (~300–400 °C) and a lower-temperature reactor exhaust stream (~150–200 °C). By selecting cyclopentane as the working fluid and fine-tuning the cycle to accommodate the two input streams, the system achieved about 28% higher exergy efficiency than a comparable single-source ORC. The authors noted that advanced control logic was required to handle dynamic variations when the two heat sources did not fluctuate in unison, but the overall concept proved highly effective in maximizing energy recovery. These multi-source applications highlight the ORC’s strength in integrative energy systems, where its inherent flexibility allows it to serve as a nexus for converging heat streams.

• Renewable Energy Utilization

ORC technology is increasingly employed to harness renewable thermal energy sources—such as solar, geothermal, and biomass—for power generation, particularly in situations where conventional steam-based systems are not feasible due to the moderate temperatures involved [5,15,33]. ORCs can efficiently convert solar thermal energy collected by non-concentrating or low-concentration collectors, as well as by higher-temperature concentrated solar power (CSP) systems, into electricity [33,37]. In solar-ORC installations, a working fluid with an appropriate boiling point is chosen to match the available temperature (which might range from ~120 °C in flat-plate collector systems up to 300 °C or more in CSP systems) [33]. Thermal energy storage (e.g., molten salt storage tanks) is often integrated to buffer the solar input’s intermittency, allowing the ORC to continue operating during cloud cover or at night [37]. For example, Salem et al. [34] analyzed a CSP-driven ORC plant in a Mediterranean climate that included molten salt storage. Operating with collector outlet temperatures of 250–390 °C and using the storage to provide heat when sunshine was not available, the ORC plant achieved a 35% increase in annual capacity factor compared to an equivalent system without storage. This improvement translates to a much more stable and continuous power output, demonstrating how pairing ORCs with thermal storage can overcome renewable intermittency and enhance capacity utilization.

Biomass-fired ORC systems are another important application, particularly for distributed generation and combined heat and power (CHP). These systems burn solid biomass (wood chips, agricultural residues, etc.) to provide a steady thermal input (often 150–300 °C hot oil or steam) to an ORC, which then generates electricity and possibly useful heat for local use. ORCs are well-suited to biomass plants because of their efficiency at the relevant temperature levels and their ability to operate in small-scale facilities with simpler maintenance than steam turbines. One challenge in biomass ORC (and generally in any renewable ORC) is handling fluctuating heat input or load variations. The dynamic behavior of ORC systems under unsteady conditions has been studied to ensure reliability and efficiency. Wang et al. [103] investigated a biomass-fueled ORC system with source temperatures oscillating between 180 °C and 280 °C. They found that when the heat input fluctuated with an amplitude exceeding ~20% of the nominal value, the ORC’s thermal efficiency could decline by 12–18%, primarily due to off-design operation of the expander and heat exchangers. Interestingly, maintaining a higher average source temperature (above ~250 °C) helped to widen the operational stability margin, partially mitigating the efficiency drop during fluctuations.

To further enhance the resilience of ORC systems in the face of renewable source variability, hybrid configurations with storage or supplemental control strategies are being explored. For instance, Bokelman et al. [97] implemented a hybrid thermal–battery storage system in a geothermal ORC plant (source temperature 140–220 °C). The ORC drew from a thermal reservoir when the geothermal supply fluctuated [97], and this was coupled with an active control scheme adjusting working fluid pressures in real time [97]. The result was a 22% improvement in grid responsiveness during load transients [97], meaning the ORC could ramp output more quickly and maintain stable operation without trips. At the same time, safety constraints (e.g., avoiding excessive pressures) were respected through the real-time pressure modulation strategy [97]. This example shows how integrating energy storage and advanced control can allow ORC systems based on intermittent renewable heat to perform with reliability akin to conventional power plants. Overall, whether in solar, geothermal, or biomass domains, ORCs have proven to be effective in expanding the reach of renewable energy by efficiently converting moderate-temperature heat into electricity and by offering operational flexibility through hybridization and intelligent control. The continued refinement of ORC technology in these applications is expected to contribute significantly to decentralized renewable power generation and the global transition toward low-carbon energy systems.

To provide a clear overview of how the different elements discussed above interact within ORC systems, Figure 5 presents a conceptual diagram summarizing the main components of the ORC ecosystem [22]. The figure illustrates the logical connection between heat sources, cycle configurations, and working fluid families, highlighting how these technical choices influence the final applications and their techno-economic and environmental performance [22]. This integrated view helps to contextualize the discussion in the next section, where the selection and properties of working fluids are analyzed in detail.

2.4. Comparative Assessment of ORC Configurations

One critical consideration in ORC design is the performance and limitations of key subcomponents—namely the expander, pump, and heat exchangers (evaporator, condenser, and any recuperative economizer)—as these characteristics strongly influence the optimal cycle configuration and working fluid choice. For example, the selection of the expander (turbine or positive-displacement machine) depends on the system scale and desired expansion ratio. Medium- to large-scale ORC units typically employ high-efficiency radial-inflow or axial turbines, often achieving isentropic efficiencies above 80% under design conditions, whereas small-scale ORCs (<50 kW) often use volumetric expanders (e.g., scroll or screw expanders) with lower efficiency (~60–70%) but simpler design and lower cost [8,47]. The expander’s design is closely tied to working fluid properties: high molecular-weight fluids yield lower volumetric expansion ratios and acoustic speeds, allowing lower rotational speeds or single-stage turbines [46], while fluids that tend to condense (“wet” fluids) require either superheating or selection of “dry” fluids to avoid liquid droplets eroding the turbine blades [104]. Pump characteristics are also important, even though the pump consumes only a small fraction of the turbine output in subcritical cycles. In high-pressure supercritical ORC designs, the feed pump can absorb up to 20–30% of the gross power output—especially with high-critical-pressure fluids—making pump efficiency and proper sizing crucial for overall performance [7]. This is even more pronounced at micro-scales, where both pump and expander efficiencies are penalized by miniaturization [7]. Designers must therefore balance expander and pump selections based on efficiency versus cost, ensuring that gains in thermodynamic performance outweigh the complexity or losses introduced by these components [11,47].

The heat exchangers in an ORC (evaporator, condenser, and internal heat recuperator) likewise play a pivotal role in configuration selection and performance outcomes. The heat transfer characteristics and allowable pressure drops of these exchangers often dictate whether certain cycle enhancements are practical. For instance, incorporating an internal heat exchanger (recuperator/economizer) to recover turbine exhaust heat can boost the thermal efficiency by roughly 10–15% for cycles using dry fluids that exit the expander as superheated vapor [104,105]. This improvement is particularly notable for high molecular-weight “dry” fluids, which leave substantial sensible heat at the turbine outlet, and thus benefit from regeneration. However, the added heat exchanger incurs extra cost, volume, and complexity, and its advantages diminish for “wet” fluids that have little superheat [13]. Evaporator and condenser design must be tailored to the heat source and sink characteristics: for example, shell-and-tube or finned tube exchangers are favored in larger systems or when utilizing exhaust gases, due to their robustness at high temperatures, whereas compact plate heat exchangers are common in small ORCs for their high effectiveness and compactness [7]. The choice of exchanger technology influences achievable pinch points and thus cycle efficiency, but also the system’s cost and allowable operating pressures [11]. Moreover, different ORC architectures are better suited to specific temperature ranges and heat source profiles in light of these subcomponent considerations. High-temperature heat sources (e.g., >300 °C) often justify specialized configurations like reheated cycles (with a two-stage turbine) or supercritical cycles, which achieve higher efficiency but demand expander materials and heat exchangers capable of withstanding increased thermal and mechanical stress [80,85]. By contrast, low-temperature heat sources (<150 °C) yield small ideal efficiencies, so simple regenerative ORCs are commonly employed with fluids that have low boiling points—this maximizes heat recovery while keeping the system compact and economical [47,106]. Likewise, the scale of the installation guides design choices: micro-ORC systems tend to use fewer stages and may omit a recuperator to reduce capital cost and complexity, whereas multi-megawatt ORC plants can economically utilize multi-stage expanders and multiple pressure levels or recuperative loops to improve efficiency [8,11]. In summary, the technical characteristics of each subcomponent impose certain trade-offs that directly impact ORC configuration decisions. Optimizing an ORC for a given application, therefore, requires an integrated approach—matching the working fluid and cycle layout to the expander type, pump capabilities, and heat exchanger design—in order to balance thermal performance, economic viability, and operational reliability [7,11,46].

To consolidate the configuration-performance trade-offs discussed in Section 2.1, Section 2.2 and Section 2.3, Figure 6 provides a radial comparison of ORC architectures. Rather than ranking technologies, the visualization highlights how design improvements (efficiency, glide utilization) often increase complexity and cost. This directly addresses the need for decision-oriented selection criteria in ORC design.

3. Working Fluids in Organic Rankine Cycles

3.1. Introduction to Types of Working Fluids Used in ORC Systems

The working fluids in ORC systems are typically refrigerants. The selection of an appropriate working fluid is paramount for system efficiency and environmental sustainability in Organic Rankine Cycle systems. Given the myriad of working fluids available, this section aims to categorize and analyze those commonly employed in ORC (Organic Rankine Cycle). This analysis encompasses traditional, natural, and synthetic working fluids, as well as working fluid mixtures, evaluating their properties and performance within the context of ORC applications [107]. The choice of this fluid is not arbitrary; it is dictated by the temperature range of the available heat source, which can vary from the relatively tepid waste heat of industrial processes to the more elevated temperatures of geothermal reservoirs or solar concentrators [3].

This rigorous selection process must carefully weigh factors such as critical temperature, pressure, molecular weight, latent heat, thermal stability, and compatibility with system components, all of which significantly affect the performance and efficiency of the ORC system [108]. In recent years, the Global Warming Potential (GWP) and Ozone Depletion Potential (ODP) of the refrigerant are critical factors in the choice of a refrigerant, which has led to a significant trend towards the use of natural refrigerants with minimal environmental impact [31].

The European Union has established regulations introducing progressive bans to reduce the use of refrigerants with high Global Warming Potential (GWP). Starting in 2032, gases with GWP ≥ 750 will be banned, and by 2050, all HFCs will be prohibited [109,110]. These regulations drive the adoption of climate-friendly alternatives like carbon dioxide, propane, and isobutane in commercial refrigeration [111].

This section categorizes refrigerants used in ORC systems, providing a foundational understanding for subsequent analysis, and also discusses the performance analysis of different refrigerants [112]. Based on this review, we can begin to determine which fluids are most commonly used and for what applications.

Table 4. Summary of Key Refrigerant Categories for Organic Rankine Cycle Applications.

Category	Common Examples	Optimal Temperature Range	Key Advantages	Key Disadvantages	Typical GWP
Natural Refrigerants	R717 (Ammonia), R744 (CO2), R718 (Water)	Medium to High (NH3, H2O); Transcritical (CO2)	Zero or negligible GWP, zero ODP. Ammonia: high efficiency. CO2: low operating pressure, non-flammable	Ammonia: toxic, mildly flammable. CO2: requires high pressures for high temperatures. Water: unsuitable for low temperatures	0–1
Hydrocarbons (HCs)	R290 (Propane), R600a (Isobutane), R601 (Pentane)	Low to High	Very low GWP, zero ODP, excellent thermodynamic properties, low cost	Highly flammable (A3), requires stringent safety protocols	~3–20
Hydrofluorocarbons (HFCs)	R245fa, R134a	Low to Medium	Well-established, good thermal stability, non-flammable (A1), extensive performance data	High to very high GWP, being phased down by regulations (e.g., EU F-gas)	700–10,000+
Hydrofluoroolefins (HFOs)	R1234ze(E), R1234yf, R1233zd(E)	Low to Medium	Very low GWP, zero ODP, non-flammable (A1) or mildly flammable (A2L), good thermodynamic performance	Higher cost, relatively new, some with mild flammability require safety measures	<10
Zeotropic Mixtures	R445A, R447A	Tailored to the application	Temperature glide improves thermal match, potentially higher efficiency, and can tailor properties	Higher complexity, risk of composition shift if leaked, may require specialized heat exchangers	Varies (typically low)

provides an overview of the most relevant working fluid families in the context of current and future ORC development, with a focus on environmental sustainability. Handbooks such as the one by Ebnesajjad (2021) [113] provide concise yet thorough coverage of fluorocarbon gases and their applications in refrigeration and ORC systems, serving as a practical reference for engineers and researchers. For a comprehensive, detailed list of the numerous refrigerants investigated in the scientific literature and their occurrence across major review studies, the reader is referred to Supplementary Materials. It serves as a valuable reference for researchers and engineers seeking to identify the full spectrum of working fluids considered for ORC applications.

As can be seen in the table, there are several fundamental aspects to consider when selecting a working fluid for Organic Rankine Cycle systems. Next, we will examine each of these aspects in detail.

• Natural refrigerants typically have very low GWPs and zero ODP, aligning with increasingly stringent environmental regulations [113]. For this reason, natural refrigerants are gaining traction as sustainable alternatives to synthetic and traditional refrigerants [107]. Fluids like carbon dioxide and water exhibit high thermal stability [114]. Carbon dioxide is often used in transcritical cycles, while hydrocarbons are effective in lower-temperature waste heat recovery [31]. Another natural alternative refrigerant is the use of ammonia (R717), which offers excellent thermodynamic properties but requires careful handling due to its toxicity and flammability [45].
• Hydrocarbon refrigerants, including propane, butane, isobutane, and propylene, demonstrate efficient performance within low to medium-temperature ranges, typically from 80 °C to 250 °C, making them well-suited for waste heat recovery and solar thermal applications [115]. For example, in a specific integrated Organic Rankine Cycle and Vapor Compression Refrigeration (ORC-VCR) system designed for low-grade thermal energy, butane was identified as an optimal refrigerant when the boiler exit temperature, representing the heat source, was between 60 and 90 °C [115]. The use of these flammable refrigerants necessitates specific safety measures and system designs, such as leak detection and explosion-proof equipment, to mitigate potential hazards [116].
• Synthetic refrigerants include all those fluids specifically created for use as refrigerants, so their properties adapt more easily to thermodynamic cycles. They are hydrofluorinated fluids, and currently only hydrofluorocarbons (HFCs) are permitted, such as R-134a, R-245fa and R-236fa, which have a high GWP and will be eliminated by 2050 [96] and hydrofluoroolefins (HFOs), such as R-1234yf and R-1234ze, which represent the latest advance, combining very low GWP with favorable thermodynamic properties, offering a reduced environmental footprint coupled with satisfactory system performance characteristics [23,117].
• Refrigerant mixtures, particularly zeotropic blends, offer a way to tailor the working fluid’s properties for optimal ORC performance [3,98,118]. Their key feature is a temperature glide—a temperature shift that occurs during evaporation and condensation, unlike the constant-temperature phase change of pure fluids [46]. This non-isothermal behavior enhances heat transfer efficiency by allowing the refrigerant’s temperature profile to better align with that of the heat source (in the evaporator) and the heat sink (in the condenser). This improved thermal matching reduces exergy destruction (irreversibility), thereby enhancing the system’s overall exergy efficiency [119]. For instance, carefully tuning the blend ratio in a mixture of R245fa and R152a has been shown to increase net power output [120].

Beyond thermodynamic advantages, mixtures provide design flexibility. They allow engineers to adjust physical, environmental, and safety properties. For example, adding non-flammable components can mitigate the flammability concerns associated with some hydrocarbons [46,119]. This has driven recent research toward developing sustainable, low-GWP refrigerant blends that comply with stringent environmental regulations [113,121].

However, the long-term use of mixtures requires careful consideration of their stability and durability. Factors such as thermal stability, chemical compatibility with system components, and the potential for phase separation are crucial to ensure consistent performance over the system’s lifetime.

3.2. Properties of Refrigerants, Performance, and Efficiency in ORC Systems

Refrigerant properties in ORC systems can be grouped into thermodynamic (e.g., critical temperature T_crit, critical pressure P_crit, boiling point, molecular weight), environmental (ODP, GWP), safety (flammability, toxicity), and heat-transfer (latent heat, thermal conductivity, viscosity) characteristics. These parameters together determine the operating conditions, performance, and efficiency of the cycle.

In general, fluids with a higher T_crit can achieve greater thermal efficiency when properly matched to the heat-source temperature. Liu et al. showed that ORC efficiency increases with rising T_crit and decreases with fluid dryness [101]. They found that R152a (the fluid with the highest T_crit among those studied) yielded the peak efficiency (≈11.2%) for a subcritical ORC [101]. Kim et al. [122] conducted a parametric study and performance evaluation of an ORC system using low-grade heat below 80 °C, providing valuable data for selecting working fluids in ultra-low temperature applications. Fluorocarbon or hydrocarbon fluids with lower T_crit are typically best suited for low-grade sources, whereas fluids with higher T_crit are preferable at higher temperatures [65]. Another study reported that when T_crit closely matches the heat-source inlet temperature, cycle output can increase by about 29% [50].

The boiling point and P_crit establish the required evaporation pressure. Subcritical ORCs often limit the evaporator pressure to around 90% of P_crit for stability, while transcritical cycles deliberately operate near the critical point to maximize efficiency. Fluids with a large latent heat and favorable heat-transfer properties allow more energy to be absorbed per unit mass and reduce heat-exchanger size, improving both efficiency and cost. Higher thermal conductivity reduces pinch losses and pressure drops, which leads to better heat recovery and smaller components.

Environmental properties are also essential. Modern system design requires fluids with negligible ODP and low GWP. Low-GWP refrigerants (e.g., HFOs, CO₂, hydrocarbons) significantly reduce the life-cycle carbon footprint of ORC systems [64]. As a result of evolving environmental regulations, commonly used HFCs such as R134a and R245fa are being phased out due to their high GWP values [64].

Table 5. Key thermophysical and environmental properties of typical ORC working fluids.

Refrigerant	Tcrit (°C)	Pcrit (bar)	GWP (100 yr)	ODP
R245fa (HFC-245fa)	154	36	950	0
R134a (HFC-134a)	101	40.6	1430	0
R1234yf (HFO-1234yf)	94	36	4	0
CO2 (R744)	31	73.8	1	0
Isobutane (R600a)	133	36.5	3	0

summarizes typical thermophysical and environmental properties of representative fluids. CO₂ exhibits a low T_crit (≈31 °C) and high P_crit (≈73.8 bar), whereas most organic fluids have T_crit values between 90 and 155 °C and moderate critical pressures. High T_crit is generally associated with higher cycle efficiency for high-temperature sources [50,65].

Safety aspects are equally important. Ideally, the refrigerant should be non-flammable and non-toxic, but in practice, some flammable fluids (e.g., hydrocarbons, ammonia) are used under controlled safety measures. Flammable fluids like propane and isobutane offer excellent thermodynamic performance but require strict safety systems. Surveys of candidate fluids have identified R134a (non-flammable, low-toxicity) and hydrocarbons such as R600 and R601 as practical choices, striking a balance between performance and safety [50].

In summary, refrigerant selection has a direct impact on ORC efficiency and operability. Designers must balance high T_crit (for efficiency) with source temperature, select low-GWP/ODP fluids for environmental compliance, and consider thermal properties (latent heat and thermal conductivity) to optimize heat-exchanger design. Matching the fluid’s critical properties to the heat-source temperature is a key design strategy [50]. Traditional fluids such as R134a and R245fa offer good efficiency but have high GWP [64], whereas newer HFOs and natural refrigerants are intended to maintain or even improve performance while significantly reducing climate impact [50,64,65].

3.3. Key Considerations for Refrigerant Selection

Choosing the optimal working fluid for ORC systems involves balancing multiple, often competing, criteria: temperature suitability, environmental impact and regulatory compliance, system compatibility and cost, performance, and safety.

From a thermodynamic perspective, the fluid must be well matched to the heat-source temperature. High-T_crit fluids, such as siloxanes or hydrocarbons, are advantageous at higher source temperatures, whereas low-T_crit fluorocarbons or light hydrocarbons are better for low-grade heat recovery [65]. Matching T_crit to the heat-source temperature can yield up to 29% higher power output compared to non-optimized selections [50].

Environmental and regulatory considerations are increasingly decisive. As noted earlier in Section 3.1, international regulations (e.g., the EU F-gas rule) are phasing out high-GWP refrigerants like R134a and R245fa. Albà et al. [123] assessed low-GWP refrigerants for drop-in replacement by linking molecular features to system performance, providing a framework for selecting sustainable alternatives without compromising efficiency. Consequently, low-GWP alternatives such as R1234yf, R1234ze(E), CO₂, and certain hydrocarbons are now preferred in new ORC systems [64]. Life-cycle assessments confirm that adopting these fluids significantly reduces an ORC system’s carbon footprint [64].

System compatibility and cost must also be considered. The chosen refrigerant must be chemically compatible with system materials (e.g., metals, seals, and lubricants) and economically viable. Some advanced HFOs are more expensive or less readily available than conventional HFCs. Using exotic blends may enhance efficiency but can also increase capital expenditures (CAPEX) and maintenance complexity.

Performance trade-offs are inherent to the selection process. The ideal fluid should maximize net power output and thermal efficiency. Zeotropic mixtures can improve thermal matching between the heat source and the working fluid, reducing irreversibilities in evaporation and condensation [65]. For example, blends of R134a and R32 have shown power output improvements of 10–30% over pure R134a [65]. However, mixture stability, fractionation, and additional control requirements must be addressed.

Safety and reliability are fundamental. Refrigerant flammability, toxicity, and chemical stability directly influence engineering controls and installation costs. Fluids such as R134a or hydrocarbons (R600a, R601a) provide a good compromise: no ODP, low toxicity, and manageable flammability [50].

Finally, economic and regulatory trends heavily influence fluid selection. As high-GWP HFCs are phased out, low-GWP HFOs and natural refrigerants will increasingly dominate. Designers must balance thermodynamic performance with regulatory compliance and life-cycle costs. Selecting the ORC working fluid is ultimately a multi-criteria decision aimed at maximizing efficiency, minimizing environmental impact, and ensuring safety and cost-effectiveness [50]. Beyond their thermodynamic implications, these fluid selection decisions have a direct impact on capital and operating costs, environmental performance, and regulatory compliance—aspects that are explored in the next section.

Recent literature also reports notable performance benchmarks that help identify the most competitive ORC solutions in terms of efficiency, cost, and environmental benefit. Regenerative and two-stage ORCs consistently yield the highest thermodynamic efficiencies, reaching ~20% under optimized conditions, approximately 15% higher than those of basic single-stage layouts [96,122]. Cost indicators similarly reveal strong potential: recent techno-economic analyses report LCOE values as low as 0.10 USD/kWh for optimized subcritical ORCs using R1234yf [124] and values near 0.025 USD/kWh for combined ORC–refrigeration configurations [125], positioning ORC as a competitive renewable-heat-to-power technology. Life-cycle studies further support this trend; replacing high-GWP fluids such as R245fa with R1233zd(E) can reduce CO₂-equivalent emissions from 78 to 13 g/kWh [126], while ORC-based waste-heat recovery in biogas engines can avoid up to ~280 t CO₂ per year [127]. These findings indicate that the State-of-the-Art in ORC development converges toward regenerative/hybrid layouts combined with low-GWP refrigerants, delivering higher efficiency, lower LCOE, and substantially improved environmental performance.

Recent developments in working-fluid research indicate a clear transition from traditional HFCs toward low-GWP alternatives such as HFOs, natural refrigerants, and tailored fluid mixtures. Studies increasingly focus on fluid–configuration matching, environmental compliance, and safety considerations. Mixtures and zeotropic fluids are gaining relevance due to their ability to improve thermal matching in heat exchangers, while regulatory pressures and sustainability goals continue to shape the future direction of refrigerant selection. Overall, current trends highlight the need for fluids that balance performance, environmental impact, and long-term regulatory viability [128,129].

4. Economic and Environmental Considerations

The economic performance of ORC systems is largely determined by the balance between capital expenditures (CAPEX), operating expenditures (OPEX), and the revenues generated through electricity production or energy savings. In most commercial ORC installations, the largest share of the total cost corresponds to the heat exchangers, which typically represent 40–60% of the CAPEX depending on the heat-source temperature and system scale. Turbomachinery (expander and pump) accounts for an additional 15–25%, while balance-of-plant components and control systems make up the remainder [3,13,81]. The cost per installed kilowatt strongly depends on both power scale and application context. Small-scale ORCs (<500 kWe) often fall in the range of 2500–4000 EUR/kW, whereas larger utility-scale plants can reduce this to below 2000 EUR/kW [81].

From a profitability standpoint, the most commonly used metric is the Levelized Cost of Energy (LCOE), which captures both investment and operating costs over the system’s lifetime. Reported LCOE values for ORC installations span 0.04–0.14 EUR/kWh, reflecting differences in plant size, heat-source quality, and financing structure [81,102]. Projects using geothermal or biomass heat at medium temperatures often achieve the lowest LCOE due to the stability of their thermal input and high-capacity factors, whereas projects relying on variable solar or industrial waste heat typically face higher LCOE values. Nevertheless, technological advances—particularly in compact heat exchangers and cost-effective expanders—have steadily lowered capital costs over the past decade.

Economic viability also depends on the project’s payback time and operational strategy. Typical simple payback periods for commercial ORC installations range from 3 to 7 years, with the lower end achievable in cases where the ORC recovers waste heat that would otherwise be lost, or where the generated electricity displaces expensive on-site consumption [13,87]. Integrating ORCs into hybrid systems or cogeneration setups further improves their economic performance by increasing utilization hours and reducing downtime. For instance, hybrid ORC plants that couple waste heat with renewable sources have demonstrated shorter payback times and higher return on investment compared to single-source installations [26].

On the environmental side, ORCs offer important advantages by enabling the recovery and conversion of low-grade heat that would otherwise be rejected to the environment. This contributes directly to reductions in primary energy consumption and associated greenhouse gas (GHG) emissions. Life Cycle Assessment (LCA) studies consistently show that ORC installations result in GHG emission reductions of 100–350 g CO₂-eq per kWh of electricity generated, depending on the displaced energy mix [99,102]. When ORCs are coupled to renewable sources such as geothermal, solar, or biomass, net emissions can approach zero over the plant’s operational lifetime. Even in industrial waste heat applications, ORCs reduce indirect emissions by displacing grid electricity produced from fossil fuels.

In addition to carbon savings, ORCs mitigate thermal pollution, since the heat recovered from the process is converted into useful energy instead of being dissipated. This is particularly relevant in energy-intensive industries such as steel, cement, or chemical manufacturing, where waste heat represents a significant environmental burden. Moreover, the choice of working fluid has a direct impact on environmental performance. Earlier generations of ORC systems often relied on hydrofluorocarbons (HFCs) with high global warming potential (GWP), but current designs increasingly adopt hydrofluoroolefins (HFOs), hydrocarbons, or natural fluids (e.g., CO₂), significantly lowering the environmental footprint [3,97]. The European regulatory framework is accelerating this transition, aligning economic and environmental considerations with broader decarbonization goals.

In summary, ORC systems are becoming increasingly competitive thanks to falling component costs, higher energy conversion efficiencies, and clear environmental benefits. Their ability to monetize low-grade heat and reduce emissions positions them as a cost-effective and sustainable technology for a wide range of industrial and renewable applications.

Economic and regulatory factors not only shape the current deployment of ORC technologies but also influence future innovation pathways. The next section examines emerging trends, such as digitalization, hybrid integration, and the development of advanced working fluids, which are expected to drive the next generation of ORC systems.

From an economic and environmental standpoint, recent trends emphasize reducing system capital cost through component standardization, improving durability, and integrating ORC units with existing industrial processes to enhance overall profitability. Life-cycle analyses consistently show that low-GWP working fluids and efficient heat-exchanger designs are key contributors to lowering environmental impact. Meanwhile, policy incentives and carbon-reduction targets increasingly drive ORC deployment in waste-heat recovery, geothermal energy, and renewable hybrid systems. These trends underline the growing alignment between ORC technology development and global decarbonization strategies.

5. Challenges and Future Trends: Towards ORC 4.0

The evolution of ORC technology is entering a new stage characterized by digitalization, integration with renewable energy systems, advanced working-fluid engineering, and stricter environmental regulations. This transition—often referred to as ORC 4.0—is driven by the need for more flexible, efficient, and intelligent energy systems capable of operating in dynamic and decentralized environments [3,9,15,97,130,131,132].

One of the most significant shifts is the integration of ORC systems with digital monitoring, advanced control strategies, and data-driven optimization. Traditional ORCs have been designed for relatively stable operating conditions, but as they are increasingly deployed in variable environments (e.g., hybrid renewable systems, fluctuating industrial processes), maintaining high efficiency under off-design conditions becomes critical. Advanced control approaches such as model predictive control (MPC) and AI-based optimization allow ORCs to dynamically adjust turbine inlet conditions, working-fluid mass flow, and heat-exchanger operation in response to changes in the heat-source profile [97,103,130,133,134,135,136,137,138,139,140]. This real-time adaptability improves energy yield, reduces wear on components, and lowers operating costs. Combined with cloud-based monitoring and digital twins, these capabilities are enabling more reliable and economically optimized ORC plants [134,135,136,137,138,141].

In parallel, ORCs are being increasingly embedded in hybrid energy systems, where they operate alongside other technologies such as heat pumps, absorption chillers, energy storage, or complementary power cycles [5,23,26,37,85,107,142,143,144]. Hybridization enhances resilience and utilization rates, addressing one of the historical challenges of ORC technology: dependence on the continuity of the heat source. By coupling ORCs with thermal storage systems (e.g., molten salts, phase-change materials), operators can decouple heat availability from power generation, flattening production curves and stabilizing grid interaction [37,145]. This feature is particularly valuable in solar and biomass applications, where heat availability can vary significantly over time [146]. Moreover, hybrid architectures facilitate sector coupling, allowing ORC units to contribute to both power and thermal grids—a key enabler in future integrated energy systems [5,53,65,85,144].

The development of new working fluids represents another key frontier. As regulatory pressures intensify—especially in the EU and other regions—high-GWP refrigerants are being phased out in favor of low-GWP HFOs, hydrocarbons, and natural fluids [3,121,147,148]. Parallel research is exploring custom zeotropic and azeotropic mixtures tailored to specific source temperature profiles, which can improve thermal matching and cycle performance while complying with environmental regulations [3,81,121]. This trend is also linked to advanced material compatibility studies and safety considerations, as some of these low-GWP alternatives introduce new operational constraints [3,147].

Despite these advances, several technical and economic challenges remain. Turbomachinery optimization for small-scale, low-temperature ORC systems continues to be a bottleneck in terms of efficiency and cost [47,140,147,149]. The economic competitiveness of ORCs in some applications is still sensitive to electricity market conditions, incentives, and carbon pricing [15,47,149,150]. Furthermore, ensuring long-term operational stability under transient loads—particularly in hybrid and renewable applications—requires more robust components and control architectures [130,131,140]. Addressing these issues will be central to unlocking the full potential of the technology.

Finally, the broader energy transition is creating new opportunities for ORC deployment. As industries decarbonize and energy systems become more distributed, ORC units are emerging as flexible building blocks for local energy recovery, microgrids, and integrated industrial clusters [5,15,26,107,143,144]. Their ability to operate autonomously, scale modularly, and interact with other technologies positions them as a strategic element in the future low-carbon energy landscape. The concept of ORC 4.0—an intelligent, hybrid, low-emission power cycle platform—encapsulates this shift from isolated heat-recovery units to smart, integrated energy systems that align technological innovation with environmental and economic imperatives [3,9,15,130,131,132].

Despite the significant technological progress achieved in ORC systems over the last decades, several research gaps and challenges remain. One of the main unresolved issues is the integration of ORC technology into flexible, hybrid energy systems, which requires more advanced control strategies, dynamic modeling tools, and adaptive fluid selection methods. Another critical challenge is the development of new working fluids and mixtures with improved thermodynamic performance, low environmental impact, and full regulatory compliance. Current studies are often limited to steady-state analyses, whereas transient operation, degradation over time, and long-term system reliability remain insufficiently explored. Furthermore, the techno-economic assessment of large-scale ORC deployment is still fragmented, particularly regarding real CAPEX and OPEX data under different policy frameworks. Finally, digitalization and ORC 4.0 approaches (including AI-based optimization and predictive maintenance) are in their infancy and represent a promising but underdeveloped research direction. Addressing these challenges will be essential for ORC technologies to play a decisive role in future low-carbon energy systems. Beyond these challenges, the insights gained throughout this review provide the foundation for identifying key research gaps and strategic directions for future ORC development. The conclusions summarize these findings and highlight the most promising areas for technological and economic advancement.

In addition to the technical and thermodynamic challenges discussed, future development of ORC systems must address several emerging issues. The environmental impact of working fluids has become a primary concern due to increasing global regulations targeting high-GWP refrigerants. Substances such as R1233zd(E) and R1234ze(E) have been proposed as low-impact alternatives that offer a balance between environmental safety and thermodynamic performance [150]. Cost remains another barrier, especially in small- and medium-scale applications where capital and maintenance expenses still outweigh efficiency gains [151]. To address performance variability and improve reliability, the adoption of smart control systems under the ORC 4.0 paradigm is gaining traction. These include digital twins and data-driven diagnostics to support real-time optimization [152]. Furthermore, hybrid systems that integrate ORCs with absorption chillers, solar thermal input, or energy storage solutions are showing strong potential for increasing overall system efficiency and flexibility [153,154]. These innovations represent key directions for future research and development.

6. Conclusions

This review has provided a comprehensive overview of Organic Rankine Cycle (ORC) systems, focusing on heat-source typologies, system configurations, working fluid selection, and emerging trends in techno-economic performance and environmental regulation. The analysis highlights that matching the thermal characteristics of the working fluid with the heat source remains the most critical factor determining system efficiency and cost-effectiveness.

Low- and medium-grade heat sources offer vast potential for ORC deployment, and the range of cycle configurations—from simple subcritical layouts to supercritical and cascaded designs—enables flexible integration into different industrial and renewable energy contexts. Working fluid selection is undergoing a rapid transition: traditional high-GWP HFCs are being replaced by HFOs, natural refrigerants, and tailored mixtures that balance thermodynamic performance with environmental sustainability and regulatory compliance.

From an application perspective, ORC systems are well-positioned to play a key role in waste heat recovery, industrial decarbonization, and hybrid renewable energy systems. However, their large-scale deployment requires addressing technical and economic challenges, including improved component efficiency, cost reduction, and advanced control strategies for variable operating conditions.

Looking ahead, digitalization and ORC 4.0 concepts—involving AI-based optimization, predictive maintenance, and real-time performance monitoring—represent a promising direction for improving reliability and profitability. At the same time, further research into novel working fluids and mixtures, dynamic operation, and long-term reliability will be essential to consolidate ORC technology as a cornerstone of low-carbon energy systems.