Recent Developments in Supercritical CO₂-Based Sustainable Power Generation Technologies

Abstract

Global warming and environmental pollution from greenhouse gas emissions are hitting an all-time high consistently year after year. In 2022, energy-related emissions accounted for 87% of the overall global emissions. The fossil fuel-based conventional power systems also need timely upgrades to improve their cycle efficiency and reduce their impact on the environment. Supercritical CO₂ systems and cycles are gaining attention because of their higher efficiencies and their compatibility with varied energy sources. The present work is a detailed overview of the recent developments in supercritical CO₂-based power generation technologies. The supercritical CO₂-based Brayton and Rankine power cycles and their improvisations in industrial applications are also discussed in detail. The advances in heat exchanger technology for supercritical CO₂ systems are another focus of the study. The energy, exergy, and economical (3E) analysis is carried out on various supercritical CO₂ power cycles reported in the literature and the data are concisely and intuitively presented. The review concludes by listing the identified directions for future technology development and areas of immediate research interest. A roadmap is proposed for easing the commercialization of supercritical CO₂ technologies to immediately address the growing challenges and concerns arising from energy-related emissions.

1. Introduction

One of the major global challenges confronting the power sector is the ever-growing energy demand, coupled with the environmental impact of emissions from energy generation systems. The Statistical Review of World Energy 2023 reports a growth of 1.1% (6.6 EJ) in the primary energy demand for the year 2022 [1]. That year also shows a record increase in power generation from renewable sources, which contributed to 14% of the global electricity production. However, 82% of the global energy demand was met through fossil fuels. Greenhouse gas emissions reached an all-time high in 2022, with energy emissions hitting a new peak of 39.3 billion tons of carbon dioxide (CO₂) equivalent, contributing to 87% of the total global emissions. The Global Head for Energy Natural Resources and Chemicals suggests the need for urgent actions to bend the global temperature curve, especially by scaling up the low-carbon resources [1].

The reliance on fossil fuels for power generation since the Industrial Revolution has had a serious environmental impact, leading to greenhouse gas emissions, global warming, and air pollution. With this growing energy demand, environmental concern, and global oath towards achieving the United Nations sustainable development goals for ‘Affordable and clean energy’, ‘Industry, innovation and infrastructure’, and ‘Climate action’ (UN SDGs 7, 9, and 13, respectively) [2], it is imperative to develop clean and cost-efficient power generation methods [3]. The development of sustainable power generation systems and their implementation on a wider scale is one of the most effective ways to tackle this challenge. Sustainable power generation systems use energy sources that have negligible contribution towards environmental degradation and have a natural replenishment of energy sources such as solar, nuclear, and geothermal power plants [4,5,6].

The implementation of advanced energy conversion technologies and sustainable system modifications is essential for boosting the overall efficiency of such power generation systems to reduce the environmental impact. Thermodynamic power cycles continue to be a critical element in ensuring a sustainable future. In closed-loop power cycles, it is important to choose a working fluid that places a lower load on the compressor, thereby improving the overall efficiency. This is where supercritical carbon dioxide (sCO₂) emerges as the most appropriate candidate because of its suitable thermophysical properties in power cycles that enable improved heat transfer with both low-grade and high-grade heat sources [7]. The sCO₂-based Brayton and Rankine cycles provide high thermal efficiencies in power generation systems leading to compactness and modularity in the systems [8,9]. These two cycles offer heat source flexibility and are suitable for recovering waste heat from various systems. The cycle’s higher thermal efficiency reduces the fuel consumption and lowers the power generation system’s impact on the environment [8].

The present article is a highlighted overview of the recent advances in the application of sCO₂-based Brayton and Rankine cycles and their different versions in sustainable power generation systems, such as solar, geothermal, nuclear, etc. The recent improvisations to these two sCO₂-based power cycles are also discussed in detail. The work also provides a brief review of the progress in sCO₂-based heat exchangers in power systems. The review concludes by proposing the future directions of research related to sCO₂ power generation technologies and provides a road map for the commercialization of these systems. In a global scenario that necessitates an immediate shift to sustainable systems, this review offers an overview of the state-of-the-art sCO₂ technology for sustainable power systems and projects the research opportunities and the right steps to faster commercialization of the technology.

2. Relevance of Supercritical CO₂ Cycles in Power Generation Systems

The brief review on thermodynamic cycles for current power generation systems conducted by Yu et al. [10] reported that most of the thermodynamic cycles employ steam [11] or air [12] as the working fluid. However, when a working fluid approaches its critical point, the compression work is substantially reduced. Therefore, to achieve a greater efficiency with less environmental impact, consideration was given to supercritical CO₂ (sCO₂) as the working fluid for power cycles. This is because CO₂ as a working fluid has a more appropriate critical temperature of 304.7 K, which is very close to the ambient temperature, and a critical pressure of 7.38 MPa. Furthermore, CO₂ is inexpensive, abundant, non-toxic, and thermally stable at high temperatures [13]. In addition, sCO₂ has significant advantages in terms of thermophysical properties and a wider operating temperature range, allowing it to operate in both low-grade heat conversion and higher-temperature nuclear applications.

Figure 1 depicts the variations in the thermophysical properties of sCO₂ at 8 MPa pressure. The thermophysical properties were obtained from the National Institute of Standards and Technology (NIST) [14]. The property curves show that in the supercritical state, specific heat and density are significantly higher than in the gaseous state, while the viscosity is lower than in the liquid state. This results in a higher compressibility factor and significantly reduced compressor work [15].

Figure 2a shows the statistics of the number of research and review articles published on sCO₂-based power generation systems in the past decade [Source: Scopus database 2010–2023]. From 2016 onwards, a substantial increase is observed in the number of related works, which demonstrates the growing significance and research interest globally for sCO₂ technologies. As shown in Figure 2b, several countries contribute substantially to this field. While China holds the major share of publications, the United States and the United Kingdom also contribute more than a 5% share individually.

A typical power cycle consists of four essential processes: compression, heat absorption, expansion, and heat rejection. Further, it is classified according to the state of the working fluid, such as subcritical, supercritical, and transcritical. The Brayton and Rankine cycles are prominent thermodynamic cycles that are widely employed in power generation industries. The working fluid in the Brayton cycle is in the vapor phase, whereas in the Rankine cycle, two-phase changes occur to the fluid, one during the addition of heat and the other during its rejection. The Brayton cycle utilizes CO₂ for power generation in its supercritical state, whereas the Rankine cycle’s heat rejection occurs below the critical point. Therefore, the CO₂ Rankine cycle is a transcritical state, while the CO₂ Brayton cycle is a fully supercritical state [13].

The sCO₂ power cycle is considered a potentially viable substitute for the conventional ideal gas Brayton cycle or water/steam Rankine cycle due to its enhanced efficiency and compact design [5,6]. Nuclear power generation currently plays an important role in providing stable and clean energy. Nevertheless, since the 2011 accident in Fukushima Daiichi’s boiling water reactors, the development of reliable decay heat removal has become an unavoidable requirement. In general, the steam Rankine cycle or the gas turbine cycle is utilized to power large plants. The ultrasteam Rankine (USR) cycle is utilized when the temperature increases above 550 °C, causing the system material to degrade due to extremely high temperatures and pressures. When the USR is coupled with a nuclear reactor, plant reliability becomes a major concern. As an effective alternative, the sCO₂ power cycle may be considered in light of this safety concern. To address such cases, the concept of a self-propelled sCO₂-based Brayton cycle heat removal system is being developed [15].

The turbine inlet temperature is greater in gas turbines compared with Rankine, requiring more compression work and leading to a low overall efficiency. The minimum pressure of sCO₂ beyond the critical point is 7.3 MPa, which exceeds the pressures of the steam cycle and the gas turbine cycle. This eliminates the compressor work in the system, resulting in improved overall efficiency in the Rankine cycle. However, the sCO₂ Brayton cycle combines the benefits of both the Rankine and the gas turbine cycles. In this, the fluid density remains in the liquid-like region throughout the cycle. A lower volume flow rate is adequate to generate an equivalent amount of energy as the Rankine cycle for a high-density fluid. Consequently, the sCO₂ Brayton cycle requires only one-tenth of the turbomachinery size than that of the Rankine cycle, resulting in more compact turbo equipment [16]. Therefore, the higher efficiency, higher operating temperature, compact structure, and dependability make the sCO₂ Brayton cycle an appropriate choice for Generation IV nuclear reactors. Furthermore, the researchers believe that the sCO₂-based Brayton cycle is a cutting-edge technique that could lead to a revolutionary breakthrough in energy conversion technology. As a result, the current review concentrates on these two primary sCO₂ cycles. The potential applications of sCO₂ cycles in diverse power generation systems are illustrated in Figure 3.

3. sCO₂ Brayton Cycle

The sCO₂-based Brayton cycle is a closed power cycle that includes turbomachinery (turbine and compressor) and heat exchangers (intercooler, reheater, and recuperator). The Brayton power cycle typically includes four processes: a constant pressure heat addition and a constant pressure heat rejection, and two isentropic processes. In the sCO₂-based power cycle, sCO₂ enters the compressor nearer to the critical point. After compression, heat is added by a regenerator (recuperator) and an external heat source. After the sCO₂ is expanded in the turbine, it has some amount of heat that can be utilized by the regenerator. Moreover, the thermal efficiency of the power cycle is determined by the system temperatures. The fluid is maintained at a temperature and pressure above its critical point throughout the cycle. The heat sources are generally provided by coal fire, a nuclear reactor, combustible gas, or extracted heat from a different cycle. Figure 4 illustrates the simple sCO₂ Brayton cycle with a recuperator and its thermodynamic cycle.

The turbine and compressor are the two most vital components of the sCO₂ Brayton cycle. To optimize the cycle, it is necessary to evaluate the performance of the turbine and the compressor components. To improve the thermal efficiency, multiple compression stages, reheating, and additional thermodynamic processes are incorporated. Combining these processes, typical sCO₂ layouts are depicted in Figure 5. It will provide a quick comprehension of the development of the sCO₂ power cycle.

3.1. Classification of the sCO₂ Brayton Cycles

The simple power cycle is inefficient due to the energy lost to the environment as waste heat. To utilize this waste heat, a regenerative component (recuperator) is added to the simple power cycle. In general, the basic recuperated cycle is the foundation of all the advanced power cycles, and it is used as a reference layout for all the complex layouts. Figure 6 illustrates the improvement paths of the sCO₂ Brayton cycle. Although a simple recuperated cycle is capable of recovering a substantial quantity of waste heat, its operational capabilities are constrained by the pinch point issue (i.e., the pinch point refers to the point where the temperature difference between fluids is the smallest). By utilizing the recompression cycle, the pinch point effect is circumvented. In this scenario, the recuperator is divided into two parts (low-temperature recuperators and high-temperature recuperators), with a compressor sandwiched between the two. By decreasing the high-pressure stream’s mass flow rate, the recompression cycle reduces the variation in the heat capacity. Another method for minimizing the pinch point effect and increasing the recovery heat is to use the precompression cycle. It is accomplished by increasing the pressure of a low-pressure flow stream, which reduces the difference between the hot and cold heat capacity. The turbine work increases during the recompression cycle with precompression, because sCO₂ expands to a subcritical state from the turbine outlet.

Furthermore, the efficiency can be enhanced through the reduction in compressor work. To accomplish this, the classic approach is used, and an intercooler is placed between the multistage compressors. The intercooling cycle is added in two different configurations to enhance the recompression cycle’s efficiency. They are the recompression cycle with main compression intercooling (also known as the intercooling cycle) and the recompression cycle with pre-cooling. The position where the flow splits differentiates these two layouts. It was discovered that adding reheat and intercooling to the simple sCO₂ cycle improves the thermal efficiency [20].

In a split expansion cycle, an additional turbine (also known as a split turbine) is installed between the high-temperature recuperator and the main heater. This layout not only increases the efficiency but also lowers the stress in the main heater. In turbine split flows, a portion of the compressor flow is tapped and routed through a low-temperature recuperator or a high-temperature recuperator before being fed directly to the heater. In all turbine split-flow cases, the energy utilization increased, but the system layout is complex and expensive. During the preheating cycle process, a portion of the flow is preheated by a heat source, while the remainder is recuperated. This method is ideal for applications that require multiple heat sources. Therefore, the sCO₂ Brayton cycle’s performance can be improved during the heating process, the recuperation process, the compression process, and the expansion process.

There have been numerous complex sCO₂ cycles proposed in different studies. Particularly, the regeneration sCO₂ cycle and the intercooler regeneration cycle provide a greater cycle efficiency. Figures S1 and S2 depict this general classification of the advanced sCO₂ cycle. Based on the design, they are divided into single-flow and split-flow configurations. Single-flow configurations include those with intercooling, inter-recuperation, reheating, precompression, etc. The split-flow design incorporates recompression, modified compression, partial cooling, and preheating [21].

Moreover, in addition to the standard layout, several researchers have also proposed several new advanced layouts. Fan et al. [22] suggested a 1000 MW sCO₂ Brayton cycle integration with a coal-fired boiler. The proposed new layout achieved the highest thermal efficiency of 47.57% after a comprehensive thermal–hydraulic analysis of various layouts. Zhou et al. [23] proposed two aspects of a new layout for optimizing the thermodynamic and economic performance of offshore waste heat recovery systems, which is residual heat recovered by the bottoming transcritical CO₂ (T-CO₂) cycle and adding the bottoming cycle with a split-flow branch. The results of the study revealed that the proposed system resulted in a 10.53% increase in the net power output and a 7.87% increase in the net present value (NPV). Furthermore, multi-objective optimization results show at least an 8.30% improvement in the optimized net power output and a 3.79% rise in the optimized NPV at the same time. A similar study of the sCO₂ bottoming cycle in cascade arrangement reported that sCO₂ as the primary cycle provides better energy and exergy efficiency [24].

3.2. Recent Advances in the sCO₂ Brayton Cycle

A strategy for the system component coupled optimization of the sCO₂-based power cycle was put forth by Wang et al. [25]. The sCO₂ radial inflow turbine and sCO₂ centrifugal compressor are represented by a one-dimensional model and it is constructed using a genetic algorithm. Additionally, it is reported that this method can analyze the cycle performance with greater precision. In addition, the net power output and optimized thermal efficiency of the cycle are 19.47% and 987.3 kW, respectively. The optimized turbine efficiency stands at 82.57%, whereas the compressor efficiency is 81.25%. Ma et al. [26] proposed a parametric optimization for the operation of an off-design sCO₂ cycle with an intercooled main compressor. The outcome indicates that the variable pressure control option increases the cycle efficiency by 0.02% to 23% when compared with the fixed pressure control mode. Xu et al. [8] performed an economical comparison of the sCO₂ and water–steam power cycles. According to their findings, sCO₂ power generation exhibits an efficiency that is 0.89% higher than that of the water steam cycle. In terms of the cost, the sCO₂ turbine cost decreased by 30%, while the boiler cost increased by 36.3%. The sCO₂ recuperator is more expensive than a Rankine heater. Moreover, the final result demonstrated that the specific cost of sCO₂ is 29% higher than that of the water steam cycle. By optimizing the recuperator, the sCO₂ cost can be reduced. It is noteworthy to mention that the sCO₂ cycle exhibits a 1.32% reduction in the levelized cost of electricity (LCOE) when compared with the water steam cycle. Hu et al. [27] investigated the sCO₂ Brayton cycle efficiency using various cooling systems, including wet cooling systems (WCSs), dry cooling systems (DCSs), and hybrid cooling systems (HCSs). The results indicate that the DCS produces electricity with the highest efficiency, followed by HCS and WCS. The HCS has the largest water-saving capacity (95.56%), and even under challenging circumstances, the capacity is 59.08%. An exergy analysis reveals that the furnace experiences a major exergy loss, followed by the recuperator, cooling water, compressor, and so forth. Mecheri et al. [28] examined the sCO₂ Brayton cycle’s efficiency in power plants that utilize coal. Importantly, even for low-temperature combustible flue gas, the recompression cycle is crucial. Furthermore, they discovered that adding a reheat cycle only enhances the plant efficiency by 1.5%. On the other hand, incorporating a recompression cycle enhances the plant efficiency by 4.5%. Siddiqui et al. [29,30] investigated the energy and exergy analysis of four different sCO₂ Brayton layouts with and without reheating. They reported that in both configurations, the recompression Brayton cycle with partial cooling and improved heat recovery achieved the best thermal performance.

Li et al. [31] found that the integration of the LiBr-H₂O and ammonia absorption power production systems improves the performance of the sCO₂ cycle. This study employs an exergy analysis, a parameter analysis, and genetic algorithm optimization. The results reveal that, when compared with sCO₂ alone, sCO₂ paired with the absorption system, the absorption system improves the thermodynamic performance and economic efficiency. The comparison of the absorption systems revealed the higher potential of the LiBr-H₂O subsystem to boost the overall performance of the sCO₂ cycle over the ammonia–water subsystem. The exergy analysis found that the suggested absorption power generation using sCO₂ might minimize the exergy loss inside the cooler of the standalone sCO₂ system by half. A thermodynamic analysis and economic evaluation of an sCO₂ coal-fired power production system were conducted by Guo et al. [32]. In this study, three distinct water-cooling wall layouts were investigated, and the optimum layout was paired with the sCO₂ cycles. The results revealed that the sCO₂ boiler’s diameter and mass flow rate are eight times more than those of a traditional boiler. Due to its wider diameter, water cooling is inefficient in the sCO₂ boiler. The sCO₂ coal-fired power system with inter-cooling has the highest thermal and exergy efficiencies despite all obstacles, at 47.69% to 50.56% and 47.69% to 50.5%, respectively. Sun et al. [33] used the cascade approach to design a novel way of absorbing the heat in the sCO₂ Brayton cycle over a wide temperature range (120 °C to 1500 °C). The cycles are classified into three types: the recompression cycle (RC) + double reheating (DRH) + flue gas cooler (FGC), the separate top–bottom cycle (STB), and the connected top–bottom cycle (CTB). The findings indicate that the CTB (RC) exhibits a power generation efficiency of 51.82%, which is significantly greater than the Rankine cycle powerplant’s available supercritical water steam. These observations demonstrated that incorporating the sCO₂ Brayton cycle into an existing system produces considerable improvements in both thermal efficiency and net power output.

These studies have shown that the compressor and turbine are critical components that can greatly affect performance. To maximize the efficiency and performance, it is essential to adopt different optimization techniques, such as coupled optimization, which considers both the compressor and turbine simultaneously. These methods improve the accuracy of the cycle performance analysis. The turbine cost decreases significantly in sCO₂ power cycles, while the cost of the recuperator increases, making the system more expensive compared with existing systems. The cooling system also helps to enhance the system performance and reduce costs. Therefore, improvements are needed in the recuperator and cooling system. Furthermore, integrating a sCO₂ power system with an absorption system significantly enhances the thermodynamic efficiency and reduces exergy loss. In addition, identifying the optimal pressure and temperature ranges is vital for maximizing efficiency. Hence, current development efforts focus on system efficiency and cost-effectiveness simultaneously.

3.3. Methods to Enhance the Efficiency and Performance of the sCO₂ Brayton Cycle

There are three primary methods to improve the sCO₂ power cycle performance. The first method is to organize various combinations of the thermodynamic processes in the advanced sCO₂ power cycle. The second method involves combining the various cycles with the sCO₂ power cycle. The third method involves recovering waste heat from the power cycle, including efficient heat exchangers that do not interfere with the main cycle as shown in Figure 7.

The sCO₂ Brayton cycle exhibits a higher efficiency in power generation when compared with the supercritical steam Rankine cycle. Comparing the component costs, the turbine cost is minimal due to the lower turbine inlet temperature than the steam Rankine cycle. However, the boiler and recuperator are expensive. Consequently, the sCO₂ power cycle’s overall specific cost is higher than the steam Rankine cycle. The LCOE is negligible due to the increased efficiency of the sCO₂ Brayton cycle.

4. sCO₂ Rankine Cycle

In the supercritical Brayton cycle, both heat addition and heat rejection occur at a supercritical pressure, whereas only heat addition occurs at a supercritical pressure in the sCO₂ Rankine cycle. In the sCO₂ Rankine cycle, as shown in Figure 8a, liquid CO₂ is pumped to a vapor generator and heat is added to this fluid under constant pressure. This process continues to a point where the CO₂ crosses beyond its critical point. The CO₂ entering the turbine is fully in the superheated vapor state. In the turbine, the superheated vapor undergoes an isentropic expansion as shown in Figure 8b. After the expansion, the CO₂ is in a saturated vapor state and it is then fed to a condenser, where the heat is removed from the fluid. Saturated liquid CO₂ exits the condenser under a constant pressure condition. The cycle diagram of a typical Rankine cycle is shown in Figure 8 as illustrated by the work [34], which proposes the flexibility of low-grade heat conversion.

In terms of the thermal performance, compactness, and environmental advantages, the sCO₂ Rankine cycle surpasses both the Rankine steam cycle and the organic Rankine cycle (ORC). The sCO₂ Rankine cycle is preferable over the Brayton cycle for power output and exergy with low-temperature sources. To further increase the efficiency, power, and thermal performance, the simple Rankine cycle is upgraded by introducing a reheat system, a multi-compression system, advanced heat exchangers, and even combined with other non-Rankine cycles. Kim et al. [34] developed a novel tCO₂ Rankine cycle by utilizing low-temperature and high-temperature heat sources in order to maximize the power output of the CO₂ power cycle. The tCO₂ Rankine cycle produces heat at a lower temperature more efficiently than the tCO₂ Brayton cycle due to a reduction in the compressor work. Moreover, in terms of load levelling, the outcomes demonstrate that the proposed tCO₂ cycle, in conjunction with low-temperature thermal energy storage, surpasses alternative CO₂ cycles utilizing the designated high-temperature heat sources.

Notwithstanding the tCO₂ Rankine cycle’s reduced compressor work and exergy loss, the observed data suggest that its thermal efficiency is inferior to that of the tCO₂ Brayton cycle. Furthermore, when utilizing a lower-temperature heat source, the tCO₂ Rankine cycle produces 25% more power and 10% greater exergy efficiency than when employing a high-temperature heat source, as compared with the tCO₂ Brayton cycle. The conversion and transfer of energy and exergy in a power plant were examined by Chen et al. [35] using single-reheat and double-reheat processes and it was discovered that the energy efficiency did not vary considerably between the single-reheat and double-reheat processes. Additionally, a substantial quantity of exergy is dissipated in the course of the combustion procedure, 20.45% and 21.12% of the total exergy inputs of the power plants are lost during single reheating and double reheating, respectively.

Shi et al. [36] evaluated four CO₂-based transcritical Rankine cycles (CTRCs), that included a conventional CTRC (B-CTRC), one with a regenerator (R-CTRC), one that pre-heats the coolant (P-CTRC), and one that employs a preheater and a regenerator (PR-CTRC). These were utilized for coolant and heat recovery from the engine exhaust. On the basis of the expansion valve, comparisons between the four cycles are performed. In addition, the exergy, energy, and cooling load were evaluated for several pressure ratios. The findings show that the PR-CTRC has a higher net production of 3.47 kW of power and an exergy efficiency potential of 17.1%. By increasing the bottoming cycle’s output power, the PR-thermal CTRC’s efficiency increased from 39.4% to 41.4%. Niu et al. [37] experimented to investigate the heat transmission properties of sCO₂ in a solar Rankine cycle (SRC) collector system. The local heat transfer behavior was significantly altered in the vicinity of the sCO₂ critical point. An increase in mass flow rate can enhance the heat transfer of sCO₂, while simultaneously reducing the heat flux at the intake and the pressure at the inlet [38]. In order to improve the heat transfer of sCO₂ in the collector, the analysis indicated that the operating conditions of the SRCs should be set near to the critical pressure and at a higher flow rate.

Mohammadia et al. [39] proposed an innovative triple-power cycle in which a recuperative ORC and a recompression sCO₂ cycle are powered by the waste heat from a gas turbine. sCO₂ cycle characteristics and the inlet temperature of the turbine have a major impact on the thermo-economic performance of the triple cycle. The LCOE and the overall thermal efficiency of the triple cycle for the 100 MW cycle are, respectively, $52.819 and 0.521 per megawatt-hour. Akbari et al. [40] investigated the exergo-economic analysis of a new combination of the sCO₂ recompression Brayton/organic Rankine cycle (SCRBC/ORC). The ORC, which generates electricity, is powered by the waste heat from the SCRBC and eight distinct working fluids are evaluated for this purpose. When the SCRBC/ORC and the SCRBC are compared, the SCRBC/ORC is up to 11.7% more efficient than the SCRBC. Ochoa et al. [41] studied the supercritical Brayton cycle in conjunction with the ORC. An investigation was conducted into the turbine inlet temperature, high pressure, turbine efficiency, and evaporation pressure. Their findings show that the turbine inlet temperature has the highest impact on the economic and energy metrics, whereas the pressure has the least. The ORC was recently proposed for several applications, including the use of solar energy during the day and cotton waste biomass at night, to provide a flexible energy solution [42]. The system was analyzed for different weather conditions in India, and the required collector area for various cities varied from 750 to 2750 m². The system achieves a peak electricity rate of 220 kW. During non-solar periods, the cotton waste biomass was utilized, with the optimal flow rate being about 0.013 kg/s. It was able to operate the 20-ton refrigeration dairy plant throughout the period, reducing CO₂ emissions from 364 kg/h to 43 kg/h compared with a traditional electricity-powered system. Therefore, utilizing sCO₂ will significantly improve performance by allowing for multiple applications simultaneously. Hence, integrating sCO₂ with the Rankine cycle for different applications presents a significant method to improve overall efficiency. This system offers a solution that is both efficient and eco-friendly. Further, in a cascade refrigeration system for ultra-low temperature medical applications that can simultaneously cool, heat, and generate electricity [43]. The combination of sCO₂ will aid in the development of such systems, which are particularly useful in rural areas and power outages. The Brayton/simple ORC configuration outperforms the Brayton/regenerator ORC combination in terms of economic performance.

5. Heat Exchanger Technology for sCO₂ Systems

Heat exchangers constitute 30% of the cost of the power cycle. Heat exchangers are often employed as heaters, recuperators, and precoolers in the power cycle. Therefore, heat exchangers are among the most crucial components that can affect the performance of any power generation system [44]. sCO₂-based systems are ideal candidates for meeting compact space constraints and can withstand high temperatures and high pressures. However, compared with water, CO₂ has a relatively lower critical temperature and excellent thermodynamic properties at the supercritical state. This is why sCO₂ is often used in energy conversion systems as a heat exchange fluid for a wider temperature range, including heat pumps, the Brayton cycle, the Rankine cycle, and numerous other power cycles [45]. The use of sCO₂ heat exchangers effectively balances the efficiency and cost of the power generation system because of the significantly lower compressor work, along with numerous other benefits [46]. Moreover, there are numerous applications found for supercritical convective heat transfer, such as regenerative cooling and supercritical water-cooled reactors.

In the case of sCO₂ systems, heat exchangers must be capable of withstanding pressures of at least 30 MPa and temperatures ranging from 550 °C to 750 °C. It is subject to severe thermal shock and stress. As a result, the sCO₂ system employs a limited variety of heat exchangers, as illustrated in Figure 9. It includes shell-and-tube heat exchangers, printed circuit heat exchangers (PCHEs), plate fine heat exchangers (PFHEs), hybrid printed circuit heat exchangers (H-PCHEs), and supercritical natural circulation loops (SCNCLs). The recuperator is a critical element within the Brayton cycle, exerting a substantial influence on the overall efficiency of the apparatus. When a fluid reaches its pseudo-critical point, the recuperator heat transfer mechanism becomes more complex. Numerous researchers are working to optimize the designs of recuperators. Additionally, in the Brayton cycle, the sCO₂ heat exchanger is commonly heated by high-temperature water [47] or other liquids [48] or gases [49]. The PCHE, shell-and-tube heat exchangers have potential applications in this field, which can handle high pressures and high temperatures [50]. Seo et al. [49] conducted a numerical analysis of tubular-type sCO₂ heat exchangers for sCO₂ systems. The desired outlet temperature and pressure were established at 600 °C and 200 bar, respectively, using a staggered tube array featuring counter-crossflow configurations. The thermal stress was determined to be less than the allowable limit proposed by the American Society of Mechanical Engineers (ASME), suggesting the suitability of tubular-type heat exchangers in high-temperature and high-pressure scenarios.

Traditional heat exchangers, such as shell-and-tube and spiral-wound heat exchangers require high manufacturing costs, and have a low operational reliability, among other disadvantages. Heat exchangers of the next generation, or Generation IV reactors, must be more resistant to extreme conditions than those found in PWRs. Heat exchangers that can operate at temperatures as high as 900 °C and pressures as high as 7 MPa [51]. The traditional shell-and-tube construction is three to five times more costly at a high pressure.

The literature indicates that the Brayton cycle gas turbine is more effective than the Rankine cycle steam turbine. Further, the Brayton cycle is more efficient and straightforward than the Rankine cycle. However, using a recuperator as a heat exchanger in the system necessitates more volume. Therefore, numerous studies have been conducted to find a solution to this issue. The recuperator must be compact and thermally efficient. Studies have observed that compact heat exchangers are one of the viable solutions to these issues [52]. Therefore, PCHE was introduced as a recuperator in the sCO₂ Brayton cycle by Cheng et al. [53]. Using a zigzag channel in PCHE increased the heat transfer rate in this experiment. Additionally, the findings demonstrated that the exergy efficiency increased when the Reynolds number was low and the inlet temperature was high on the cold side of PCHE. The dynamic response characteristics of thermodynamic parameters for PCHE, such as the total surface heat flux and the surface heat transfer coefficient, were investigated numerically by Teng Ma et al. [54] on a 1000 MW sCO₂ coal-fired power plant. A neural network is utilized to forecast PCHE’s performance. The result indicates that a disparity of less than 1.5% exists between the projected and actual temperatures of the hot and cold fluids.

PCHE is very often preferred in various power generation plants as it operates over a broad pressure and temperature range compared with others. Due to its compact dimensions and notable efficiency, PCHE is being considered as a prospective precooler in the sCO₂ Brayton cycle. In the sCO₂ Brayton cycle, a significant amount of heat is transferred in the regenerator after the turbine exhausts, accounting for 60–70% of the total heat transfer in the cycle. The performance of the system is significantly influenced by the regenerator’s efficiency. The PCHE is the best choice for regenerators as a result of its high surface-to-volume ratio [55]. Baik et al. [56] developed a lab-scale PCHE to use as a precooler in a sCO₂ power cycle. In the trial, both the on-design and off-design situations revealed an outstanding heat transfer performance. In addition, the CFD analysis was conducted on two cases: PCHEs with sharp corners and rounded corners. The observations indicate that a channel with rounded corners reduces the pressure drop by 40 to 65% compared with one with sharp corners. Gkountas et al. [57] conducted a thermodynamic analysis of a 600 MW recompression sCO₂ power cycle. The results indicate that the recuperator performance is crucial for achieving a system performance of 46% or higher.

Similarly, for the sCO₂ system, a novel PFHE proposal with Inconel 625 was developed specifically for the sCO₂ system’s concentrated solar power system, as shown in Figure 9 [58]. Apart from this, Thar Energy, LLC created the micro-shell and tube heat exchangers, which can withstand the high temperature and pressure required for sCO₂ recuperator applications [59]. Over the course of the past twenty years, sCO₂ has become increasingly significant in the SCNCL. Moreover, the SCNCL is capable of providing any configuration to meet the needs of the system, provided that it generates an adequate buoyancy field along the loop [60,61,62]. Because of this, the SCNCL loop has various applications in energy conversion systems in both conventional and modern engineering applications, such as solar water heaters [63], geothermal power extraction [64], nuclear reactor cooling [65,66], and refrigeration [67].

6. sCO₂ Cycle Application in Power Generation Industries

In this section, a detailed overview of the various power generation applications of the sCO₂ cycle is discussed.

Table 1. Energy, exergy, and economical (3E) analysis of various sCO₂ power cycles.

Author(s)	sCO2 System Layout	First Law Analysis	Second Law Analysis	Economic Analysis
Pérez-Pichel et al. [74]	sCO2 + sodium fast reactor	43.31%	-	-
Turchi et al. [68]	sCO2 + CSP	>50%	-	-
Floyd et al. [75]	sCO2 recompression + sodium fast reactor	42.2%	-	-
Le Moullec [76]	sCO2 + coal-fired plant	41.3% net plant efficiency (LHV)	-	Cost of electricity of 67 euro/MWh
Akbari et al. [40]	sCO2 Brayton cycle (SCRBC) recompression	38.3%	53.03%	7819 $/h capital cost rate
	sCO2/ORC different fluids	41.85–42.16%	57.95–58.38%	7957–8568 $/h Capital cost rate ($/h)
Manente et al. [17]	Cascaded sCO2 Brayton cycle (biomass—part flow topping cycle)	36%	-	-
Li et al. [77]	tCO2	5.5%	54.40%	Cost per net power output (CPP) −1750 $/W
	tCO2 (IHX)	6.1%	51.09%	1870 $/W
Serrano et al. [78]	sCO2 Brayton + fusion reactor	46.5%	-	-
Li et al. [69]	1. Supercritical Rankine cycle 2. Transcritical Rankine cycle without/with ejector	7.88% 8.18%/6.23%	-	-
Baronci et al. [79]	sCO2 + molten carbonate fuel cell	56.25% (electrical efficiency)	-	-
Linares et al. [80]	sCO2 Brayton + fusion reactor	40.59% (cycle efficiency)	-	-
Hu et al. [81]	sCO2 Brayton + recompression CO2 + He mixture CO2 + butane	47.82% >41.5% >35%	-	-
Wang et al. [82]	sCO2 + ORC Scheme 1: Pcomp inlet = 6.61 MPa, Pcomp outlet = 20 MPa, Ppump outlet = 0.97 MPa, ΔTevapor.hot = 56 K Scheme 2: Pcomp inlet = 4.69 MPa, Pcomp outlet = 7.65 MPa, Ppump outlet = 0.43 MPa, ΔTevapor.hot = 29 K Scheme 3: Pcomp inlet = 5.305 MPa, Pcomp outlet = 7.65 MPa, Ppump outlet = 0.66 MPa, ΔTevapor.hot = 33 K Scheme 4: Pcomp inlet = 3.783 MPa, Ppump outlet = 3.491 MPa, ΔTevapor.hot = 37.1 K	41.40% 40.48% 40.85% 42.85%	56.96% 55.69% 56.21% 58.95%	-
Mecheri et al. [28]	sCO2 Brayton cycle for coal-fired powerplant (net cycle efficiency) Case 1: Simple regenerative cycle without recompression Case 2: Simple regenerative cycle without recompression with a bypass pipe Case 3: Single recompression + 2 recuperator + 2 compressor + 1 bypass pipe Case 4: Single recompression + 3 compressor + 3 recuperator + 1 bypass pipe	41.9% 42% 46.8% 47.1%	-	-
Zhang et al. [83]	sCO2 Brayton cycle for coal-fired plant	50.71%	-	-
Shi et al. [36]	B-CTRC R-CTRC P-CTRC PR-CTRC	4.24% 5.52% 5.02% 7.19%	8.84% 10.63% 12.38% 15.87%	-
Xia et al. [84]	Solid oxide fuel cell/gas turbine/sCO2 hybrid power system	60.42% energy efficiency	63.03%	0.079$/kWh Electricity production cost
Sun et al. [33]	Coal-fired plant − top bottom cycle cascade (CTB recompression cycle)	51.82%	-	-
Hou et al. [85]	Cogeneration system (gas turbine + sCO2 recompression cycle + steam power cycle + organic Rankine cycle) − Optimum	-	69.33%	Total product unit cost = 10.77 $/GJ
Singh et al. [70]	Solar-powered combined cycle (Recompression sCO2 cycle + organic Rankine cycle) R 123 based R-sCO2 + ORC at various conditions	63.86–85.83%	35.57–47.82%	-
Singh et al. [86]	sCO2 & organic Rankine cycle (solar trough collector) organic working fluid R407c	43.49%	78.07%
Song et al. [87]	sCO2 with bottoming organic Rankine cycle Organic working fluid R245fa	Standalone regenerative sCO2 = 16.4% sCO2 + ORC = 17.7% sCO2 + ORC with vaporize at high temperature = 18.1% Recompression sCO2 + ORC + precooler = 19.1%	-	-
Xu et al. [20]	sCO2 coal-fired plant (Anthracite coal)	Thermal efficiency = 51.22% Electricity efficiency = 48.37%	-	-
Jose J et al. [73]	sCO2 combined cooling, heating, and power system with regeneration	37% improvement in energy efficiency	0.6% reduction in exergy destruction rate	-
Park J et al. [88]	sCO2 Brayton recompression cycle for various small modular reactors	Pressurized water reactor = 30.6% Sodium cooled fast reactor = 46.38% High-temperature gas cooled reactor = 50.04%	-	-
Li et al. [89]	sCO2 cycle with LFR	Reheating recompression sCO2 Brayton cycle = 43.72% LFR with reheating recompression sCO2 Brayton cycle = 41.53%	-	Electricity production cost = 0.0536 $/kWh
Weiland et al. [90]	Coal-fueled, oxy-fired direct supercritical CO2 (sCO2) power cycle	Plant thermal efficiency = 40.6%	-	Cost of electricity (COE) = 122.7 $/MWh
Mishra et al. [91]	Combined sCO2 & VAR cycle (at max cycle temp. = 650 K)	41.89%	75.2%	-
Ma et al. [92]	sCO2 cycle in coal-fired plant	1. Temperature major control method—37.42% 2. Mass flow rate major control method—34.57% 3. Novel synergetic control method—36.88%	-	-
Xiao et al. [93]	Nuclear-driven sCO2 block with membrane distillation block	Power block—48.18% Desalination block—37.10%	67.82%	1. LCOE = 0.0527 $/kWh 2. LCOW = 0.445 $/m3
Xu et al. [8]	sCO2 power cycle for coal-fired power plant	49.01%	-	LCOE = 60.56 $/MWh
Wang et al. [94]	Concentrated solar-driven power and desalination (CSPD) system using sCO2 Brayton cycle and multi-stage flash (MSF)	36.6%	-	1. LCOE of 0.059 $∙kW/h 2. LCOW = 1.15 $∙t−1
Guo et al. [32]	sCO2 Brayton cycle integrated coal power plants	47.69–49.09%	47.69–50.55%	LCOE = 0.0397 $·(kWh)−1
Wang et al. [25]	sCO2 power cycle	19.47%
Sun et al. [95]	Multi-compressions sCO2 power cycle	47.43% for recompression cycle 49.47% for tri compression cycle
Chen et al. [35]	sCO2 coal-fired power plant with reheating	49.06% for double-reheat 48.72% for single reheat	48.02% for double reheat 47.69% for single reheat	-
Mohammadi et al. [39]	Triple power cycle is suggested for waste heat recovery from turbine for driving the sCO2 recompression cycle and recuperative ORC	52.1%	-	LCOE = $52.819/MWh
Mohammadi et al. [71]	Recompression sCO2 cycle	-	1. 16.63% for real conditions 2. 17.13% for unavoidable conditions	-
Ehsan et al. [96]	Dry-cooled sCO2 recompression cycle	50.9%	-	-
Li et al. [97]	Fossil-fired sCO2 power cycle pilot loop [recompression and reheat cycle with two split ratio]	33.49%	-	-
Xu et al. [72]	Solar hybrid coal-based direct-fired supercritical carbon dioxide power cycle	43.4%	44.6%	-

summarizes the energy, exergy, and economic (3E) analysis of various sCO₂ power cycles in different power generation industries. The first step of any research work that involves thermal or power systems is an energy and exergy analysis [32,35,54,68,69,70,71,72]. While the energy analysis quantifies the system performance and heat absorption/rejection rate, the exergy analysis provides a clear understanding of the second law efficiency of the system and the contribution of individual components towards the overall system exergy destruction rate. To bring in more clarity, a recent study by Jose et al. [73] could be taken as an example. The authors reported that adding a regeneration cycle to the conventional combined cooling, heating, and power system improved the system performance by around 37% under the given operating conditions. However, it is evident from their exergy analysis that the regeneration cycle is dominant over the conventional cycle only for a lower compressor discharge pressure (170 bar). This observation provides more understanding of the allowable operating regimes of the system. The data presented in the table are discussed in the following subsections in association with the respective industries.

6.1. sCO₂ Cycle in the Solar Power Industry

In recent decades, the globe has faced two big challenges: environmental difficulties and energy crises. Scientists and researchers are working to find suitable renewable energy sources to substitute dwindling energy sources, as well as ecologically undesirable ones such as fossil fuels. In terms of affordability, accessibility, and clean energy, solar energy is the best alternative for addressing all these challenges [52,53]. When solar-powered cycles are integrated with other cycles like the sCO₂ Brayton or Rankine cycles, the entire system becomes more efficient, uses less primary cycle fuels, and emits fewer or no pollutants. The solar-powered cycle’s efficiency was enhanced even further through the implementation of multi-heating in the solar tower.

A novel concept, the solar hybrid coal direct-fired sCO₂ power cycle was developed by Xu et al. [72] to achieve zero emissions and efficient solar energy use. Following the development of an exhaustive analytical model, a process simulation was utilized to ascertain the thermodynamic performance. By gasifying coal, concentrated solar energy is utilized in this process to generate syngas. For energy production in the semi-closed sCO₂ power cycle, the syngas is subsequently burned with pure oxygen. The results demonstrate that this hybrid system may reduce coal use by 29.9% while enhancing energy and exergy efficiencies by 43.4% and 44.6%, respectively. In addition, this novel technology enhances the solar–coal hybridization of energy production while releasing nearly no emissions.

Wang et al. [94] created a new concentrated solar-powered energy and desalination system based on the sCO₂ Brayton cycle. In this system, the sCO₂ Brayton cycle produced 4050.8 tons of fresh water per day and 50.1 MW of power with a 36.1% operational efficiency. Exergy estimates also demonstrate that it is economically viable. Also, Wang et al. [98] examined a molten salt solar power tower (SPT) system integrated with a sCO₂ Brayton cycle to determine the influence of thermodynamic parameters (hot salt temperature, cycle high pressure, cycle low pressure, and cycle intermediate pressure) on exergy efficiency. The studies revealed that solar salt (a 60% NaNO₃ + 40% KNO₃ mixture) is the most suitable fluid for heat transfer and for use as a medium for the storage of thermal energy up to 565 °C. It was also shown that 680 °C is the highest allowed molten salt temperature for the SPT system when combined with the sCO₂ recompression Brayton cycle. Gonzalez-Portillo et al. [99] devised a novel solar field configuration that bears a resemblance to a solar tower, with the exception of a bifurcation in the receiver and heliostats. Furthermore, each part meets the specific requirements for the concentration ratio, fluid temperature, and absorbed heat flux. The results indicated that multi-heating sCO₂ solar tower efficiency is 3.1% larger than conventional sCO₂ solar tower efficiency.

A solar parabolic trough collector-driven sCO₂ cycle and ORC power plant was presented by Singh et al. [86]. Five organic fluids (R134a, R1234yf, R407c, R1234ze, and R245fa) are utilized to undertake energy and exergy evaluations in the ORC. According to the findings, the sCO₂ turbine and evaporator exergy destruction are 9.72% and 8.54% of the input exergy, respectively. Furthermore, for a solar collector, sCO₂ coupled with R407c generates the highest power output of 3740 kW and the lowest fuel depletion ratio of 0.2583. Singh et al. [70] proposed a solar-powered combined cycle of the recompression sCO₂ cycle with ORC for power generation and waste heat recovery and they conducted exergy and energy analyses in ORC with eight different organic fluids (R123, R290, C₄H₁₀, R1234yf, R1234ze, C₆H₅CH₃, C₅H₁₂, and C₆H₁₂). The analysis reveals that sCO₂ mixed with R123 has the maximum thermal and exergy efficiency, whereas sCO₂ combined with R290 has the lowest thermal and exergy efficiency. Furthermore, the data unveil that the solar collector experienced the highest degree of exergy loss.

sCO₂ technology represents a promising advancement in the solar power industry, offering enhanced efficiency, reduced costs, and greater environmental benefits compared with traditional methods. However, the commercialization of sCO₂ technology can be expensive due to the costs associated with research, development, and initial deployment. Additionally, challenges related to cost, complexity, and market readiness need to be overcome for widespread adoption in the solar power industry. In recent decades, there has been a noticeable increase in research interest in this technology, which has accelerated its commercialization.

6.2. sCO₂ Cycle in the Nuclear Industry

Generation IV reactors are chosen based on factors such as nuclear safety, sustainability, and economic competitiveness. The selected Generation IV reactors are gas-cooled fast reactors (GFRs), lead-cooled fast reactors (LFRs), sodium-cooled fast reactors (SFRs), molten salt reactors (MSRs), supercritical water-cooled reactor (SCWRs), and very-high-temperature reactors (VHTRs). The thermal efficiency increases with the operating temperature in the Generation IV reactor. However, the efficiency of nuclear power plants is lower than that of conventional water-cooled reactors. The sCO₂ cycle is an excellent match for the nuclear power cycle for further advancement in Generation IV reactors due to qualities like compactness, simplicity, less exergy loss, and high power conversion efficiency.

Fan et al. [100] examined a combination cycle that included a supercritical sCO₂ topping cycle and a transcritical sCO₂ bottoming cycle. A genetic algorithm was employed to accomplish multi-objective optimization in order to attain optimal thermodynamic and economic performance. The optimal thermal efficiency and exergy efficiency were discovered using multi-objective optimization to be 43.94% and 65.03%, respectively, from a thermodynamic and economic perspective. Li et al. [101] addressed a fictitious design for a relatively small sCO₂ power cycle equipped with LFR for the thermo-economic analysis. In this study, five different sCO₂ Brayton cycles are integrated with the reduced LFR model and their relative performance is analyzed. The results indicate that the integration of the recompression cycle with the LFR power generation system should be a top priority due to the outstanding exergo-economic performance of the recompression cycle. The study also showed that the integration of LFR into the sCO₂ cycle provides a high compactness to the system.

Luo et al. [6] investigated six different layouts of the Brayton cycle utilized in nuclear systems to analyze the thermodynamic and exergo-economic behavior. The results demonstrated that the intercooling cycle exhibited the highest efficiency among the cycles, even exceeding that of the recompression cycle. The findings indicate that the recompression and partial cooling cycles significantly affect the efficiency of the cycle and the overall cost per unit of the product for the high-temperature reactor (HTR). The system’s regenerators influence the intercooling cycle, subsequent in importance to the recompression cycle. Perez-Pichel G et al. [74] examined the sCO₂ recompression cycle in the sodium-cooled fast reactor (SFR). The study found that thermal efficiency reached as high as 43.31% and it was also highlighted that the SFR supports the balance of the plant. In summary, the above studies have demonstrated that the thermodynamic and economic performance of nuclear reactors integrated with the SCO₂ power cycle has improved. In particular, the thermal efficiency exceeds 50% with turbine inlet temperatures above 650 °C [18]. However, the sCO₂ system performance varies significantly with the system layout. Therefore, many system layouts need to be investigated and optimized for better efficiency.

6.3. sCO₂ Cycle in Miscellaneous Power Generation Industries

The overall cycle efficiency can be increased by integrating molten carbonate fuel cells (MCFCs) with other cycles like the sCO₂ Brayton or the sCO₂ ORC, which can be carried out using less expensive machinery. Song J. et al. [102] proposed four different configurations of the sCO₂–ORC for hybrid and geothermal power generation. This combined system’s total electricity generation ranges from 22% to 45% over the standalone sCO₂ plant, which is illustrated in Figure 10.

A novel pumped thermal energy storage method was introduced by Tafur-Escanta et al. [103]. This approach utilized molten salts as the thermal storage fluid and sCO₂ as the working fluid.

It achieved an efficiency rate of 80.26%. A low wear per duty cycle, scalability of power, and portability are the principal benefits of this method. In addition, unlike chemical batteries, its performance is not affected by the sequence of the cycle. The geothermal system’s performance is significantly enhanced through the incorporation of the sCO₂ power cycle in the system.

Three alternative sCO₂ configurations as a bottoming cycle were examined by Ryu et al. [104]. According to the findings, the MCFC standalone system net power efficiency decreased by 3.1% to 4.6%. When heating costs are less than $28/Gcal, this system is more cost-effective. Mohammadi et al. [71] examined a conventional and advanced exergy analysis on the recompression sCO₂ cycle. Both approaches proposed distinct component priority orders in an effort to enhance the overall performance of the system. The conventional exergy analysis suggested the reactor as a high priority while the advanced exergy analysis recommended a high-temperature recuperator as a high priority. In addition, the advanced exergy analysis revealed that the rate of exergy destruction is less in the endogenous portion than in the exogenous portion. Furthermore, Li et al. [77] conducted an analysis on the thermo-economic performance of the ORC and the tCO₂ power cycle in comparison with various alternative working fluids. These systems were run by geothermal sources in the temperature range 90–120 °C. The results demonstrated that regeneration could enhance the thermodynamic performance of the cycle. The ORC employing R600a generates the highest net power output, whereas the regenerative ORC utilizing R601 achieves the highest levels of thermal and exergy efficiency. In addition, the outcome demonstrated that CDTPC exhibits a superior economic performance relative to the ORC in terms of the cost per net power output.

The sCO₂ power production system is a prospective replacement for the traditional systems. This is owing to the sCO₂ power cycle’s improved efficiency and compactness. It is more feasible to combine the sCO₂ cycle with existing systems as it improves the thermal efficiency and net power generation significantly. The Rankine cycle exhibits a superior performance in systems when it comes to the conversion of low-grade heat compared with the Brayton cycle. However, the Brayton cycle is more appropriate in other industries, such as nuclear and coal power plants. The sCO₂ power cycle is applicable to numerous industries, such as fuel cells, solar power generation, nuclear power generation, and power plants. Nuclear reactors integrated with sCO₂ power cycles offer compactness, low exergy loss, and an excellent exergo-economic performance. Furthermore, exergo-economics and energy production are optimized by integrating the sCO₂ power cycle with other power cycles, such as the organic Rankine cycle, which demonstrates an excellent thermal performance. Such a system can utilize a variety of established energy generation methods.

6.4. sCO₂ Cycles Economic Analysis

The improvement in power cycle efficiency sometimes leads to poor economic performance. Reports indicate that the increased costs of the recuperator and boiler in sCO₂ power cycles can elevate the total system cost, sometimes surpassing that of the existing systems. Therefore, it is crucial to also consider the economic perspective. This section discusses the economic perspective of sCO₂ power cycles based on the available data. Le Moullec [76] investigated the sCO2 Brayton cycle in coal-fired power plants, comparing scenarios with and without carbon capture. The study shows that the net efficiency improves by 41.3%, and the cost of electricity is 67 euros per MWh, approximately 45% of the cost is reduced. Akbari et al. [40] reported that SCRBC/ORC has a lower product unit cost than the SCRBC by up to 5.7%. Further, from the table, the LCOEs and COEs for various sCO₂ system layouts are plotted in a graph, as shown in Figure 11.

The LCOE analysis indicates that the sCO₂ Brayton cycle, when integrated with a coal-fired power plant, has a lower LCOE compared with the other systems. In contrast, the sCO₂ Brayton cycle on its own exhibits a higher LCOE. Similarly, the COE plot reveals that the sCO₂ cycle with a lead-cooled fast reactor has a lower COE compared with the other configurations, while the coal-fueled oxy-fired direct sCO₂ power cycle has a higher COE. The graphs demonstrate that the sCO₂ cycle is more efficient when integrated with other power systems, and nuclear applications also show a significant economic improvement.

7. Future Directions and Roadmap for the Commercialization of sCO₂ Technologies

Sustainability goals are no longer an option. It is essential to take action to boost efficiency and reduce or eliminate the environmental impact of energy production. Renewable energy technologies must be significantly enhanced to achieve greater efficiency, for which sCO₂ technologies offer promising advantages. Figure 12 depicts the future areas of development for the sCO₂ systems.

Since the behaviors of supercritical fluids are more complex, heat exchanger and turbomachinery designs for sCO₂ systems must be carefully considered. The selection of materials that can withstand the pressure and temperature of supercritical conditions is also crucial. The carbon nanotube coating improves the component strength, offering a potential solution to this issue. Goyel et al. [105] studied its application in thermal power plants at high temperatures, noting an improved corrosion resistance without added weight. Ramachandran et al. [106,107] further discussed the significance of this method across multiple applications.

The sCO₂ technologies can be used to achieve a variety of long-term objectives proposed by various agencies and governments. The United Nations SDGs 7 and 13, the Montreal Protocol, India’s PM-KUSUM (Pradhan Mantri Kisan Urja Suraksha evam Utthaan Mahabhiyan) scheme, various countries’ Net Zero Emissions policies, and the International Atomic Energy Agency’s Generation IV nuclear reactor are just a few examples.

The commercialization roadmap for sCO₂ systems is presented in Figure 13. The incorporation of government policies into system development and collaboration with industry is the initial step in commercialization, allowing research coordination to reach its objective more quickly. The second phase consists of constructing a system facility, conducting a risk assessment, and the experimental validation of the system.

The third phase, which is more crucial for commercialization, entails adapting the system to be compatible with existing plants and evaluating its dependability and resilience. The final step should involve applying laboratory technology to a commercial plant. Thus, it is high time that sCO₂ technology must replace the existing technologies to ensure a sustainable and livable future. The sCO₂ power technologies are in the initial phases of development in most parts of the world, where the technology is either being researched extensively or is being demonstrated and tested at laboratory scales. Most of the developed European nations, the US, China, and Japan, are the frontrunners in sCO₂ research. The extensive implementation of this technology at a commercial level in these countries may take a few more years because of the challenges involved in technology implementation.

However, the Technology Readiness Level (TRL) for any particular sCO₂-based power technology may vary based on the specific design, development, application, and operating conditions. This suggests a methodology adopted to evaluate the level of technical readiness or maturity of a technology during its evaluation phase. The TRLs of various sCO₂-based power technologies and applications are presented in

Table 2. The Technology Readiness Level of various sCO₂-based power technologies.

sCO2 Power Technology	Technology Readiness Level	Comments
Brayton Power Cycle Systems	7–8	This technology has undergone sufficient development, and many such systems are already in operation [108,109].
Waste Heat Recovery	5–6	Waste heat recovery systems using sCO2 have been exhaustively researched and pilot projects are being evaluated for further development [110].
Solar Power Plants	4–5	Research is ongoing to improve the efficiency and for the smooth integration of sCO2 technology into concentrated solar power systems [111].
Nuclear Power Plants	3–4	The nuclear power plants show huge potential in the application of sCO2 technologies. The research and development are still in earlier stages, where the feasibility and safety studies are underway [21,112].
Fossil Fuel Power Plants	4–5	There are crucial challenges related to the integration of sCO2 technologies with the prevailing infrastructure, especially for coal and natural gas applications, and achieving reasonable efficiency [113,114].
Combined Heat and Power (CHP) Systems	3–4	Research on CHP systems is in the early stages with the main focus on small-scale applications and improving the overall system efficiency [115,116].

. The minimum level starts at 1, and the maximum level ends at 9.

8. Conclusions

A detailed overview of the recent advancements in sCO₂ technologies for power generation systems is presented in this review. The research interest in sCO₂ technologies has grown exponentially over the past decade. The heat transfer characteristics of CO₂ in its supercritical state are highly suitable for developing compact systems with high thermal efficiency, which makes sCO₂ an effective and widely accepted working fluid in power generation systems. The sCO₂ cycles also exhibit high flexibility with energy sources for power generation and are very effective in waste heat recovery. sCO₂ power cycles exhibit considerable potential as viable substitutes for traditional gas and steam power cycles owing to their superior efficiency and negligible environmental footprint. While the CO₂ Rankine cycle is a transcritical cycle, the sCO₂ Brayton cycle works entirely in the supercritical regime. The sCO_2-based Brayton cycles provide improved thermal safety for nuclear power systems by effectively removing heat and avoiding excess temperature build-up in the system.

Generally, layouts are preferred based on priorities such as cost, maintenance, simplicity, and efficiency. In the case of split flow, the cycle efficiency can be improved by increasing the energy utilization and expanding the turbine work. Furthermore, more power can be retrieved even with a low thermodynamic efficiency system. For a single-flow layout, due to its simplicity, the cost and maintenance are less than those of split-flow layouts. In other words, split flow is preferred where the efficiency is a higher priority, while single flow is preferred where the cost and simplicity are important.

Reheating and intercooling the sCO₂ Brayton cycle increases its thermal efficiency. While turbine split flow does enhance the energy efficiency, it also increases the complexity of the system and the cost of its layout. The sCO₂ Brayton cycle performance can be improved through heating, recuperation, compression, and expansion processes. A dry cooling system provides the highest generation efficiency for the sCO₂ Brayton cycle, and the highest exergy loss is contributed by the furnace part. Efficient methods to enhance the performance of sCO₂ power cycles include the recovery of waste heat, the synergistic integration of various thermodynamic cycle processes, and the appropriate combination of multiple cycles.

The maximum temperature is a major concern in selecting the layout since the efficiency and power output depend on the temperature. Based on the sCO₂ turbomachinery test loop, when the maximum temperature is around 200 °C, a simple cycle layout is sufficient. If the source temperature is as high as 1150 °C, the other cycle is preferred. However, the cycle involves a recompression process, which ranges from 537 °C to 750 °C, while the recuperator operates between 200 °C to 485 °C.

Although the cost of the turbine for sCO₂ is lower than that of the Rankine cycle for supercritical steam, the sCO₂ requires a more expensive boiler and recuperator. Despite having a higher overall specific cost compared with the supercritical steam Rankine cycle, the sCO₂ Brayton cycle exhibits a lower levelized cost of electricity due to its enhanced efficiency. Reheating, multi-compression, the use of advanced heat exchangers, and combining with suitable thermodynamic cycles are some of the techniques used for enhancing the performance of the sCO₂ Rankine cycle. Despite the lower compressor work and exergy losses, the tCO₂ Rankine cycle is thermally less efficient than the tCO₂ Brayton cycle. The tCO₂ Rankine cycle equipped with a preheater and regenerator provides a high power output and energy efficiency.

Heat exchangers utilized in sCO₂ systems must be resistant to high temperatures and pressures. Around 60–70% of the heat transfer in the sCO₂ Brayton cycle is transferred in the regenerator. The PCHE is among the most suitable for regeneration applications as its effectiveness significantly improves the system performance. The sCO₂ NCL is a passive device and offers high flexibility in the shapes into which it can be fabricated and therefore can be applied in various energy conversion systems.

The incorporation of sCO₂ into solar power systems increases the efficiency of power generation and decreases the amount of fuel consumed during the primary cycle, thereby reducing the plant’s emissions. Higher mass flow rates and operation close to the critical inlet pressure can improve the heat transfer in sCO₂-based solar collectors. The solar hybrid coal-direct-fired sCO₂ power cycle is very effective in reducing coal use by 30% and provides an improvement of more than 40% in energy and exergy efficiencies, causing only near zero emissions. The sCO₂ Brayton cycle is effectively applied in concentrated solar-powered energy generation and desalination systems. Multi-heating solar towers perform more efficiently than the conventional sCO₂ solar tower.

The sCO₂ Brayton cycle with intercooling provides the highest cycle efficiency for nuclear applications, especially in Generation IV nuclear reactors. The generating range was increased by 22 to 45% when hybrid and geothermal power generation was conducted using a sCO₂–ORC combined configuration as opposed to the standalone sCO₂ system. The thermo-economic analysis of the tCO₂ power cycle and the ORC driven by geothermal sources reinforced the contribution of regeneration in improving the cycle performance.

Some of the major areas that need to be explored through research and analysis related to sCO₂ systems are the supercritical flow behavior and the development of suitable and compatible materials. The advancements in heat exchanger and turbomachinery technologies and system integration and modelling can also boost the wide-scale implementation of sCO₂ systems. The government–industry collaboration leading to policies fostering sustainable developmental technologies is the first step in commercializing the sCO₂ systems. Further, focused research and development activities on related technologies and their scaling and testing for real-world applications satisfying safety standards can transform the laboratory-level systems to the industry level, leading to faster commercialization of the technology and a more sustainable future.