Power Generation with Renewable Energy and Advanced Supercritical CO₂ Thermodynamic Power Cycles: A Review

Abstract

Supercritical CO₂ (S-CO₂) thermodynamic power cycles have been considerably investigated in the applications of fossil fuel and nuclear power generation systems, considering their superior characteristics such as compactness, sustainability, cost-effectiveness, environmentally friendly working fluid and high thermal efficiency. They can be potentially integrated and applied with various renewable energy systems for low-carbon power generation, so extensive studies in these areas have also been conducted substantially. However, there is a shortage of reviews that specifically concentrate on the integrations of S-CO₂ with renewable energy, encompassing biomass, solar, geothermal and waste heat. It is thus necessary to provide an update and overview of the development of S-CO₂ renewable energy systems and identify technology and integration opportunities for different types of renewable resources. Correspondingly, this paper not only summarizes the advantages of CO₂ working fluid, design layouts of S-CO₂ cycles and classifications of renewable energies to be integrated but also reviews the recent research activities and studies carried out worldwide on advanced S-CO₂ power cycles with renewable energy. Moreover, the performance and development of various systems are well grouped and discussed.

1. Introduction and Motivation

Global energy demand is on the rise in numerous countries as the population increases and the economy grows. One of the primary and representative facts of economic growth in all countries worldwide is the increased electricity demand and generation. From 2019 to 2022, total primary energy consumption experienced a 3% increase, while the proportion of fossil fuel consumption in relation to primary energy consumption remained high at approximately 82% [1]. Growing energy consumption makes it a significant challenge to transition our energy sources and supplies away from fossil fuels and move towards low-carbon ones. Extensive fossil fuel consumption has caused environmental issues such as global warming, ozone depletion and atmospheric pollution. Therefore, it is critically important to achieve energy conversion efficiency improvement and increase renewable energy utilization and thus diminish the reliance on fossil fuels.

A decentralized and localized power generation system is an effective option to reduce energy consumption since energy loss from long-distance transmission or transportation can be avoided. Steam Rankine and gas turbine cycles have been significantly involved in large-scale power plants for electricity generation. The steam Rankine cycle can, however, achieve relatively higher energy conversion efficiency at lower turbine inlet temperature, considering the fact that the working fluid is pumped at a liquid state before being heated by a steam boiler. This is significant because liquid water is essentially incompressible, which can reduce the work required for pumping compared to compressing a gas by a compressor. According to the theory of the Carnot cycle for a heat engine, the higher the temperature at which heat is supplied and the lower the temperature at which heat is rejected, the greater efficient the cycle will be. It reveals the importance of a power cycle operating at a higher heat supply temperature whenever possible to maximize its thermal efficiency. The gas turbine cycle, also known as the Brayton Cycle, utilizes air as a working fluid at a higher turbine inlet temperature. However, in the Brayton Cycle, the compression process consumes more work compared to the steam Rankine cycle since air is compressible. This leads to lower efficiency for the gas turbine cycle. On the other hand, over the last decades, as a mature energy conversion technology, the organic Rankine cycle (ORC) has been generally used in small-scale power generation. Industrial waste heat, biomass combustion, solar energy and geothermal heat can be utilized as heat sources for ORCs [2,3]. The ORC is superior to the traditional steam Rankine cycle in terms of performance when employing a low-temperature heat source. However, the selection of a working fluid for an ORC is a challenge due to the fact that some organic compounds have high global warming potentials (GWPs). Therefore, the investigation of alternative working fluids in applicable power generation cycles attracted more attention [4,5]. CO₂ is a natural, non-toxic, non-flammable, abundant and zero ozone depletion potential (ODP) working fluid, making it a noteworthy competitive candidate to be utilized in a power generation system [6]. According to its low critical point, the CO₂ power cycle can easily extend to the supercritical region and turn it into a transcritical or supercritical CO₂ power cycle. The supercritical CO₂ (S-CO₂) power cycle was originally proposed as a format of partial condensation Brayton cycle by Sulzer [7] in 1940. A simple recuperated supercritical Brayton cycle was propositioned by Feher [8] in 1967, and thereafter, Angelino [9,10,11] conducted a thorough study of CO₂ power cycles. The S-CO₂ Brayton cycle integrates the strengths of the steam Rankine and the gas turbine cycles by compressing the working fluid in the incompressible region and achieving higher thermal efficiency at higher turbine inlet temperatures.

S-CO₂ power cycles equipped with advantages of simplicity, compactness, sustainability and cost-effectiveness have therefore been significantly researched for various applications, including fossil fuel power plants, nuclear power plants, and integrations with renewable energies. The existing power cycles, such as Rankine and gas turbine cycles, face the challenges of working fluid selections and less cycle thermal efficiencies. Driven by the better thermal performance of energy conversion systems, CO₂ has attracted more attention to be utilized as an alternative working fluid. Another reason is the lower critical temperature at 31 °C which enables CO₂ working fluid and its associated system to be applicable in a variety range of heat sources. A power cycle with CO₂ working fluid could be classified as either a direct or indirect Brayton cycle. The semi-closed direct-fire oxyfuel Brayton cycle was suitable for fossil fuel power generation systems, while the indirect Brayton cycle was adapted for applications involving nuclear and renewable energy sources [12]. There have been many studies investigating the applications of direct S-CO₂ cycles. A design was conceptualized by Le Moullec [13] for a coal-fired power plant with 90% post-combustion CO₂ capture. It showed that the cost of electricity and the cost of CO₂ reduction could be reduced by 15% and 45%, respectively. Mecheri and Le Moullec [14] conducted additional simulations on S-CO₂ coal-fired power plants, concluding that by combining a coal-fired power plant with a double reheated single recompression Brayton cycle, a net thermal efficiency of 47.8% could be attained. This efficiency was significantly higher than that of traditional coal-fired power plants. Xu et al. [15] reached a similar conclusion, stating that the net power generation efficiency of a triple-compression S-CO₂ cycle could attain 49.01%, compared to 48.12% achieved by the water–steam Rankine cycle. In addition, although the fabrication cost could be increased, the levelized cost of electricity achieved a reduction of 1.32% compared to the water–steam Rankine cycle. Optimizing the recuperator in the system is crucial to reducing the total cost of the entire S-CO₂ cycle. To further enhance the performance of the S-CO₂ coal-fired power plant, employing a partial flow mode in the S-CO₂ boiler was demonstrated to effectively decrease pressure drop, thereby increasing the system’s thermal efficiency [16]. As for the indirect Brayton cycle, significant attention was paid to the S-CO₂ applications in nuclear reactors, as they could offer higher turbine inlet temperatures, potentially enhancing the system’s thermal efficiency. Various countries have undertaken efforts to develop Generation IV nuclear reactors, given that these reactors operate in the temperature range of 500 °C to 900 °C, compared to water-cooled reactors with operating temperatures at around 300 °C [17]. Dostal [18] conducted an in-depth analysis of the S-CO₂ cycle for nuclear reactors. The study revealed that the S-CO₂ cycle exhibited competitive system efficiency compared to the helium Brayton cycle at the same operating condition. Additionally, it was concluded that the S-CO₂ power cycle was apt for all nuclear reactors with a core CO₂ gas heater outlet temperature exceeding 500 °C. Moisseytsev and Sienicki [19] investigated several alternative cycle layouts for S-CO₂ Brayton cycles coupled to the sodium-cooled fast reactor (SFR). It was found that no advantages could be gained from utilizing a double recompression cycle, incorporating intercooling between the main compressor stages or applying reheating between the high-pressure and low-pressure turbine cycles. However, optimizing the minimum cycle temperature down to 20 °C could lead to improvement in cycle efficiency. Wright et al. [20] demonstrated that integrating light water reactors (LWRs) with an S-CO₂ cycle could potentially improve the LWR power cycle efficiency and save capital costs. In order to further evaluate the benefits of coupling an S-CO₂ Brayton cycle with a small- and medium-sized water-cooled nuclear reactor (SWR), the effects of different operating conditions on system performance were studied by Yoon et al. [21]. The results revealed that even though the S-CO₂ cycle was previously recognized for having superior efficiency in high operating temperature regions, the efficiency of S-CO₂ with SMR cycle still demonstrated competitive efficiency to the existing steam-Rankine cycle with SMRs (around 30%) at the optimum pressure ratio. Additionally, a conclusion aligning with Wright [20] was reached that by combining the S-CO₂ cycle with SWR, capital costs could also be reduced. In 2021, about 440 nuclear power reactors were in operation in 32 countries worldwide with a total generated power of 2653 TWh, which took about 10% of the global electricity supply [22].

In light of growing environmental concerns, the utilization of the S-CO₂ power cycle in the renewable energy sector has recently emerged as an appealing option. While several researchers have provided extensive reviews of S-CO₂ cycles, as outlined in

Table 1. Recent reviews on S-CO₂ power generation technologies.

Ref.	Year	Main Energy	Thermodynamic Equations	Summary Points	Limitations
Ahn et al. [17]	2015	Coal-fired power plant Nuclear Waste heat recovery	n/a	Performance of various layouts of S-CO2 Braton cycles were compared. Progress in the development of S-CO2 was introduced.	Application reviews of various heat sources are not sufficient.
Kumar and Srinivasan [23]	2016	Solar power	n/a	The potential and limitation of thermodynamics were reviewed when CO2 was used either alone or as a component in a mixture of the working fluid. Heat transfer issue in recuperator was reviewed.	The study mainly focuses on the application of solar power generation without specifically considering other renewable energies.
Crespi et al. [24]	2017	Nuclear Solar power Waste heat	n/a	42 standalone layouts of the S-CO2 cycle and 38 combined cycle configurations were reviewed. Operating conditions of different layouts were reviewed.	The limited thermodynamic equations or T-S diagrams aim to assist readers in comprehending distinctions among different S-CO2 configurations.
Marchionni et al. [25]	2020	Waste heat	n/a	Technological challenges of high-grade waste heat recovery were reviewed. Main components of S-CO2 cycle were reviewed.	The illustrations of various cycles are not sufficient.
White et al. [26]	2021	Fossil fuel Nuclear Solar power	n/a	State-of-the-art S-CO2 cycles, along with their technical and operational issues, were reviewed. Development status of turbomachinery, heat exchanger, material selection and control system designs were reviewed.	The advantages of integrating the S-CO2 cycle with renewable energies are not thoroughly outlined.
Guo et al. [27]	2022	Coal-fired power plant Nuclear Solar power	n/a	The challenges of state-of-the-art S-CO2 technologies were reviewed. Research progress of S-CO2 power cycles and components was reviewed. The review explores the thermodynamic, economic, environmental and flexible feasibility of the technology.	The thermodynamic performance of system application needs to be further analyzed.

, the majority have focused on the development status of S-CO₂ cycles and the progress in optimizing components for applications in fossil fuel, nuclear, and solar power regions. Renewable energy resources are also known as alternative, sustainable or nonconventional energy supplies, including solar, geothermal, biomass and waste heat. However, reviews exclusively addressing S-CO₂ cycles in the context of these renewable energies are limited. Therefore, the objective of this paper is to review the most recent development of supercritical CO₂ cycles in power generation systems with renewable energy by offering a comprehensive view of the advantages of supercritical CO₂ working fluid, the landscapes of renewable energy, the options of S-CO₂ cycles, and application status of S-CO₂ renewable energies. Below are the key points highlighted in each section:

(i)

In Section 2, the superior thermal–physical properties of CO₂ are outlined, along with the benefits of incorporating CO₂ in supercritical power generation cycles.

(ii)

Section 3 demonstrates the advantage characteristics and categorizations of renewable energy as a promising source of heat, encompassing biomass, solar, geothermal and waste heat.

(iii)

In Section 4, representative S-CO₂ cycles are summarized with emphasis on features of each layout, T-S diagrams and thermodynamic equations.

(iv)

In Section 5, a review of recent applications of S-CO₂ renewable power systems is presented, including S-CO₂ for biomass power systems, S-CO₂ cycle for concentrating solar power systems, S-CO₂ cycle for geothermal power systems and S-CO₂ cycle for waste heat recovery. This focuses on various technologies, operating conditions and efficiencies. In addition, the barriers to S-CO₂ technology are also concluded.

2. Superior Thermal–Physical Properties of CO₂

CO₂ is a natural, non-toxic and non-flammable working fluid that possesses excellent thermophysical properties, including higher density, latent heat, specific heat, thermal conductivity and volumetric cooling capacity, along with lower viscosity [28]. These attributes make CO₂ a significant player in various energy conversion systems. It has a low critical temperature of 31 °C but quite a high critical pressure of 7.4 MPa. As shown in Figure 1, near the pseudocritical region, the thermophysical properties of CO₂ undergo rapid changes due to its high density and low compressibility factor close to its critical point. This leads to significant fluctuations in density and specific heat capacity with only slight variations in pressure or temperature. The compressibility factor is characterized as the ratio of the actual volume of a substance to its ideal volume. As observed in Figure 2, around the critical point, the compressibility factor of CO₂ fluctuates between 0.2 and 0.5, which leads to a reduction in the power consumption of the compressor. Consequently, the S-CO₂ cycle is distinguished by its high thermal efficiency, simple cycle configuration and compactness of system components. At the same power generation, the overall size of a steam Rankine cycle is estimated to be approximately four times larger than that of an S-CO₂ Brayton cycle [17]. The benefits of utilizing CO₂ working fluid in thermodynamic cycles include (1) environmentally friendly nature, with no ozone-depleting potential (ODP) and neglectable global warming potential (GWP = 1), (2) abundance, non-toxicity and non-combustibility, (3) non-reactivity with component materials, and (4) superb thermodynamic and transport properties. According to the low critical temperature of CO₂ working fluid, its associated thermodynamic cycle can easily traverse both subcritical and supercritical regions with high-temperature heat sources, naming the cycles as either transcritical or supercritical cycles. Three options exist for utilizing CO₂ in the Brayton cycle as shown in Figure 3: (1) the classic cycle (‘SN’), which operates entirely under critical pressure, known as the subcritical cycle; (2) the transcritical cycle (‘TN’), the highest pressure operates above critical pressure, allowing CO₂ to pass through both subcritical and supercritical regions; and (3) the supercritical cycle (‘S’), which operates entirely above the critical pressure. However, the difference between the transcritical and supercritical Rankine cycles is not strict. The transcritical Rankine cycle, under some conditions, can also be called the supercritical Rankine cycle. The concept of the supercritical CO₂ Rankine cycle pertains to the situation where heat addition happens at CO₂ pressure above the critical point, while heat rejection occurs at CO₂ pressure below the critical point, as illustrated in Figure 4. It has been demonstrated that the efficiency of the S-CO₂ Brayton cycle surpasses that of the superheated steam Rankine cycle when the turbine inlet temperature exceeds 470 °C [29]. The potential for maximizing power output in an ORC is hindered by the evaporation process with constant temperature, making it a less favourable option for sensible heat sources. By avoiding the isothermal boiling process, CO₂ in the transcritical/supercritical cycle achieves a more effective thermal match with the heat source, thereby attaining higher thermal efficiency compared to ORC, as depicted in Figure 5. The issue of pinching points between the heat source and working fluid temperature along the heat exchanger is effectively addressed by utilizing a transcritical or supercritical CO₂ cycle.

3. Superior Characteristics of Renewable Energy

3.1. Biomass

Biomass, a type of non-fossilized and biodegradable organic material originating from plants, animals and microorganisms, has emerged as a global frontrunner for the development of low-carbon energy. It encompasses products, by-products, residues and waste from agriculture, industry and forestry. The energy stored in biomass is initially derived from the sun, with photosynthesis being the primary process through which plants convert the sun’s radiant energy into chemical energy, stored as glucose or sugar. Biomass is categorized into two main groups: virgin biomass, which includes forest biomass, energy crops and grasses; and waste biomass, which comprises municipal solid waste, agricultural crop residues and leaves [33]. Serving as a renewable and sustainable energy source, biomass can be utilized to generate electricity or other forms of energy. Unlike fossil fuels, the CO₂ produced from the complete combustion of biomass is equivalent to the amount it absorbs from the atmosphere, resulting in no net contribution to atmospheric carbon dioxide levels and emitting low levels of SOx and NOx. There are two main thermochemical conversion routes to use biomass for supplying electricity and heating, i.e., gasification and combustion. Combustion is the most mature technology to convert biomass to useful electricity and heating. Using biomass as a heat source can not only effectively reduce biomass waste but also provide high temperatures definitely higher than 900 °C [34] during the combustion process. Figure 6 shows the biomass energy conversion processes and temperature ranges of the thermochemical route.

Biomass has already become a crucial element in the UK’s energy supply, contributing to 11% of the total electricity generated in 2022. According to the most recent energy statistics from the same year, it was estimated that bioenergy made up approximately 8.6% of the UK’s overall energy supply [35]. The key advantages and disadvantages of biomass applications are outlined in

Table 2. Major advantages and disadvantages of biomass.

Advantages	Disadvantages
Renewable and inexhaustible source	Low energy density
Low content of ash, C, S, N and trace elements	Potential competition with food and feed production
During combustion, ash can capture some hazardous components	Great harvesting, collection, transportation and storage cost
Cheap resource	Could lead to global warming if burned directly

below.

3.2. Solar Power

Solar energy is the most abundant and widely distributed form of renewable energy available for utilization. Out of the 1.75 × 10⁵ terawatts (TW) representing the total energy from the sun reaching Earth’s atmosphere, approximately 1 × 10⁵ TW consistently reaches the Earth’s surface [36]. In 2022, solar energy production reached a total of 1289.27 terawatt-hours (TWh), contributing to approximately 4.6% of the global electricity generation [37].

There are two approaches to converting solar energy for electricity generation: the photovoltaic (PV) cell system and the solar thermal system. Generally, multiple PV cells are linked in a series to capture sunlight and transform it into direct current (DC) electrical power. Nevertheless, PV systems are primarily employed on a smaller scale and can be affected by weather conditions. In contrast, solar thermal systems are suitable for larger-scale applications. Conventional concentrating solar power (CSP) is a well-known sunlight conversion technology. Typically, a CSP consists of a central receiver system, parabolic trough, dish Stirling unit and integrated gas cycles. The CSP plant generates electricity by using a linear or punctual collector to focus on radiation energy and convert it into high-temperature heat. This high-temperature heat is then transferred into a working fluid, and the absorbed heat is thus converted into electricity by a generator via a power cycle. The Rankine cycle is the most common technology for converting solar energy to electricity [38]. The main components of a solar ORC system are the solar collector and evaporator, energy storage, turbine, generator and condenser, as shown in Figure 7. However, as explained previously, the ORC is more appropriate for low-temperature heat sources [39]. In addition, oil, salt and steam are typical heat transfer fluids used in CSP systems for converting solar energy into electricity. Nevertheless, the properties of these heat transfer mediums can impose constraints on the performance of CSP plants. For instance, synthetic oil and salt can only withstand temperatures up to 400 °C and 560 °C, respectively, and direct steam generation has limitations in terms of storage capacity [40]. From the study of Dostal et al. [29], the S-CO₂ cycle outperformed the steam Rankine cycle when the inlet temperature of the turbine exceeded 550 °C. This temperature range falls within the attainable parameters of solar power systems, with the hot-end temperatures having reached between 800 °C and 1000 °C [41] or even above 1000 °C [42]. The challenges of the development of solar thermal power systems are the lower efficiency and high capital cost. As an alternative promising technology, the S-CO₂ cycle has become more favourable for converting solar power to electricity since it has higher thermal efficiency.

3.3. Geothermal Resource

Geothermal energy is globally available, constituting a clean, plentiful and sustainable energy source originating primarily from the natural decay of radioactive isotopes during the Earth’s formation, deeply embedded within its layers. There is approximately 43,000,000 EJ of geothermal energy stored at depths reaching as far as 3000 m below the surface [44].

Hydrothermal resources represent the dominant form of geothermal energy harnessed for large-scale electricity generation. In addition to hydrothermal sources, there are five other categories of geothermal energy: hot dry rock, geopressured, magma energy, deep hydrothermal and low-temperature systems [45]. The temperature range of the geothermal heat resources is between 50 °C and 350 °C [46]. Geothermal resources can also be classified into various temperature grades, which are high-temperature (>180 °C), intermediate temperature (100–180 °C) and low-temperature (<100 °C) [39]. In order to extract heat at a suitable temperature, it is typically necessary to drill holes into the ground, creating both production and injection wells.

Table 3. Potential of different heat source temperature ranges of geothermal energy in terms of heating (MW_th) and electricity (MW_e) in Europe [47].

Temperature °C	MWth	MWe
65–90	147,736	10,462
90–120	75,421	7503
120–150	22,819	1268
150–225	42,703	4745
225–350	66,897	11,150

shows the potential of different heat source temperature ranges of geothermal energy in Europe.

3.4. Waste Heat Resource

Low-temperature exhaust stream emission is still a notable issue. In many manufacturing industries, 20~50% of the energy consumed by manufacturing is lost as waste heat [48]. However, this heat cannot be recovered completely on-site and used for district heating. It is then discharged into the ambient, which has a significant negative impact on human health, biodiversity, and the environment. Generally, waste heat can be categorized as low-temperature (<230 °C), medium-temperature (230–650 °C) and high-temperature (>650 °C) [3]. Recovery of low-grade waste heat for electricity production is a promising technology to protect the environment and meet electricity demand, although it is a big challenge for power plants. There are several technologies for waste heat recovery based on the heat transfer between working fluid and waste heat. The organic Rankine cycle is a common method utilized for low-grade waste heat recovery, in which heat is immediately recovered by a heat transfer loop to evaporate the working fluid. Persichilli et al. [11,25] discovered that the transcritical CO₂ power cycle could attain superior efficiency across a broad spectrum of heat source temperatures, ranging from 204 °C to 650 °C, and be more cost effective in comparison to ORC and steam Rankine cycles. Similarly, Chen et al. [32] conducted a comparative analysis between transcritical CO₂ Rankine and R123 ORC cycles, utilizing a low-grade heat source with a temperature of 150 °C. Their investigation established that the transcritical CO₂ cycle outperformed the subcritical R123 ORC one in terms of efficiency. However, for the low-grade heat source temperature of around 100 °C, the power output of the transcritical R125 cycle was approximately 14% higher than that of the transcritical CO₂ cycle. Moreover, if the heat temperature is as low as approximately 112 °C, the transcritical CO₂ Rankine cycle exhibits greater efficiency than that of the transcritical CO₂ Brayton cycle due to reduced compression work while achieving the same temperature increase. The supercritical CO₂ Brayton cycle is expected to replace the organic Rankine cycle to improve thermal efficiency for waste heat recovery. In addition, the S-CO₂ cycle can recover waste heat from small turbines.

4. S-CO₂ Layouts

The progressions of the five representative S-CO₂ cycles are delineated in Figure 8, including the recuperation cycle [8], recompression cycle [18,29,49,50], pre-compression cycle [10,51], intercooling cycle [52] and reheating cycle [53]. Various configurations have been explored to enhance the performance of S-CO₂ cycles, building upon the fundamental S-CO₂ power system depicted in Figure 8a. Thermodynamic equations of each component and thermal efficiencies of different layouts are summarized in

Table 4. Thermodynamic equations of each component.

Component		Thermodynamic Equation
Heater		$Q_H=\dot{m(}{h}_4-h_3)$
Turbine		$W_T=\dot{m(}{h}_1-h_2)$, ${ⴄ}_T={(h}_1-h_2)/{(h}_1-h_{2,s})$
Recuperator		$Q_{rec,max}=min\left\{\genfrac{}{}{0pt}{}{\dot{m}\left(h_1-h_2\right),assumming{T}_2=T_3}{\dot{m}\left(h_4-h_3\right),assumming{T}_4=T_1}\right\}$ ${Ꜫ}_{rec}=\frac{h_1-h_2}{Q_{rec,max}}=\frac{h_4-h_3}{Q_{rec,max}}$
Gas cooler		$Q_c=\dot{m(}{h}_1-h_2)$
Compressor		$W_P=\dot{m(}{h}_2-h_1)$ ${ⴄ}_P={(h}_{2,s}-h_1)/{(h}_2-h_1)$

and

Table 5. Thermal efficiencies of each S-CO₂ layout.

Layout	First Law Efficiency Equation
Basic S-CO2	${ⴄ}_{th}=\frac{W_T-W_P}{Q_H}$
Recuperation S-CO2	${ⴄ}_{th}=\frac{W_T-W_P}{Q_H}$
Recompression S-CO2	${ⴄ}_{th}=\frac{W_T-\sum W_P}{Q_H}$=$\frac{W_T-(W_{compressor}+W_{re-compressor})}{Q_H}$
Pre-compression S-CO2	${ⴄ}_{th}=\frac{W_T-\sum W_P}{Q_H}$=$\frac{W_T-(W_{pre-compressor}+W_{compressor})}{Q_H}$
Intercooling S-CO2	${ⴄ}_{th}=\frac{W_T-\sum W_P}{Q_H}$=$\frac{W_T-(W_{P,1}+W_{P,2})}{Q_H}$
Reheating S-CO2	${ⴄ}_{th}=\frac{\sum W_T-W_P}{Q_{in}}$=$\frac{\left(W_{T,1}+W_{T,2}\right)-W_P}{Q_{H,1}+Q_{H,2}}$

, respectively. The introduction of a recuperator allows for the recovery of more waste heat, resulting in what is termed the recuperation S-CO₂ cycle, as illustrated in Figure 8b. However, the adoption of an internal heat exchanger or recuperator, while boosting electrical efficiency, has revealed a significant internal irreversibility issue within the recuperator. This is primarily due to the substantial difference in specific heat capacity between the cold and hot fluid sides of CO₂, leading to a pinch-point problem [49]. The smaller the pinch-point, the better the heat transfer efficiency; however, a larger heat transfer area could be required. To address the pinch-point problem, a recompression S-CO₂ cycle was developed, as shown in Figure 8c. In this configuration, an additional recuperator and compressor are introduced. A portion of CO₂ is cooled by a gas cooler and then compressed to the highest pressure by the main compressor. The remaining amount of CO₂ is compressed by the secondary compressor. These two streams are combined at the inlet of the high-temperature recuperator (HTR or Recuperator 1) on the high-pressure side. This minimizes the temperature difference between the hot and cold fluid sides of CO₂, thereby improving heat transfer performance and resolving the pinch-point issue. Recompression S-CO₂ is the most efficient cycle compared to internal cooling, reheating and pre-compression cycles [18]. The pre-compression configuration offers an alternative method to mitigate the impact of the pinch-point problem and enhance regeneration, as seen in Figure 8d. The pre-compression cycle was initially introduced by Angelino [10]. It involves the placement of a pre-compressor between the high-temperature recuperator (HTR) and the low-temperature recuperator (LTR or Recuperator 2). This pre-compression cycle works by narrowing the gap in specific heat capacity between the low-pressure and high-pressure streams via the elevation of pressure in the low-pressure stream. Intercooling is the traditional method to minimize the load of the compressor and increase the thermal efficiency of the S-CO₂ cycle. Increasing the number of stages results in the compression process approaching near-isothermal conditions at the compressor’s inlet temperature [54]. However, intercooling (Figure 8e) does not hold much appeal in S-CO₂ cycles due to the minimal efficiency enhancement it provides [18]. Reheating (Figure 8f) is the technology to improve the turbine work and thus enhance the thermal efficiency of the S-CO₂ cycle since work output from the turbine could be increased without increasing the maximum temperature in the cycle and keeping the compressor work constant. Reheating holds substantial promise for the development of the S-CO₂ cycle. Nonetheless, it is exclusively applicable to indirect cycles, and utilizing more than one reheat stage is not economically practicable.

5. Application Status of S-CO₂ Renewable Power Systems

5.1. S-CO₂ for Biomass Power Systems

Typical biomass power generation technologies have been listed in

Table 6. Typical data for power generation from biomass [56].

Technologies	Efficiency % (LHV)	Typical Size (MWe)	Typical Costs
			Capital Costs ($/kW)	Electricity ($/kWh)
Co-firing	35–40	10–50	1100–1300	0.05
Dedicated steam cycles	30–35	5–25	3000–5000	0.11
IGCC	30–40	10–30	2500–5500	0.11–0.13
Gasification + engine CHP	25–30	0.2–1	3000–4000	0.11
Stirling engine CHP	11–20	<0.1	5000–7000	0.13

. Biomass can be combusted directly within waste-to-energy facilities for electricity production. Biomass co-firing is the substitution of a portion of the fuel with biomass within coal-fired thermal power plants. It is an economical approach to convert biomass effectively and environmentally into electricity and involves integrating biomass as a partial replacement for fuel in high-efficiency coal boilers [55]. Sweden has successfully operated the initial integrated gasification combined cycle (IGCC) plant that utilizes 100% biomass, specifically straw [56]. However, biomass poses a constraint on the adoption of large-scale steam cycles or IGCCs, which are designed to achieve higher efficiencies. The majority of biomass plants are typically small-scale and rely on internal combustion engines and ORCs. The electrical efficiency of a biomass-fired ORC system was between 7.5% and 13.5% [57]. Subsequently, in an experimental study conducted by Qiu et al. [58], it was observed that the electricity generation efficiency of this biomass-fired ORC system was 1.41%, primarily attributed to the lower efficiency of the expander and alternator during the experiments.

For the further development of biomass conversion technologies, a supercritical CO₂ power system is a promising option for the efficient utilization of abundant renewable resources. Biomass has the potential to be used for power generation and bio-synthetic production with zero CO₂ emission. Chitsaz et al. [59] proposed a novel tri-generation system of bio-synthetic nature gas, fresh water and power, as illustrated in Figure 9. The operational concept of this system involves introducing biomass into the fluidized bed gasifier, where it undergoes a reaction with steam to produce a mixture of highly combustible species such as H₂, CO and CH₄. Subsequently, this high-temperature hydrogen-rich syngas flows through a CO₂ gas heater, elevating the temperature of CO₂ to drive the S-CO₂ Brayton cycle, and subsequently enters the methanation reactor. By utilizing heat from heat exchangers 3 and 4 within the humidification–dehumidification loop, seawater can be transformed into freshwater. By carrying out a comprehensive simulation of this system, it was found that the power production of 172.6 kW could be achieved under multi-objective optimum conditions. An alternative approach to utilize biomass gasification combined with the S-CO₂ Brayton power cycle is the combustion of biofuel. Cao et al. [60] introduced a heat and power system that integrates biomass gasification with an advanced solid oxide fuel cell-CO₂ supercritical Brayton cycle, as shown in Figure 10. After the biofuel (solid oxide fuel cell) is produced via the peach stone gasification process, it is burned in post-combustion to generate high-temperature gas up to 600 °C~700 °C. The results indicated that the optimal conditions for achieving maximum power (138 kW) and heat (195 kW) were as follows: an equivalence ratio of 4, a fuel cell temperature of 680 °C, a fuel utilization factor of 0.82 and a pressure ratio of 5.11. To enhance the electrical power output even further, biomass gasification with CHP plants can be equipped with multiple power cycles. Ji-chao and Sobhani [61] conducted a mathematical modelling study on an integrated power and heat system that merged biomass gasification with both the S-CO₂ Brayton cycle and the Kalina cycle. They achieved a peak net power output of 7.375 MW when the pressure ratio was set at 3.5. Moradi et al. [62] conducted a comparison study between a gas turbine and an S-CO₂ cycle, both of which were coupled with bottom ORCs and heated via biomass gasification. The results indicated that the average net electric power output of the entire integrated S-CO₂ system was approximately 126 kW, which was about 25% higher than the power output of the gas turbine system.

For the combustion biomass conversion route, a small-scale power generation test system with biomass and CO₂ transcritical Brayton cycles has been designed and constructed with purposely selected and manufactured system components [63,64,65], as depicted in Figure 11. Based on their modelling findings, it was discovered that there was an ideal pressure ratio that maximizes the thermal efficiency of the system. Furthermore, a higher temperature of the biomass flue gas was associated with higher thermal efficiency. As a single S-CO₂ cycle may utilize high-temperature flue gas insufficiently, a concept of cascaded supercritical CO₂ Brayton cycles was proposed to optimize the conversion efficiency of biomass energy into electricity, with a potential maximum efficiency of up to 36% [66]. Wang et al. [67] conducted a thermodynamic analysis of a biomass-solar combined with S-CO₂ Brayton power generation system. The combined use of solar energy and biomass enabled the continuous operation of the system. This is because biomass steps in to supply heat, either partially or entirely, when solar irradiation falls short. This system consists of a solar island, a biomass burner, a recompression S-CO₂ cycle and a simple recuperation S-CO₂ cycle. Results showed that the solar-to-electric efficiency could achieve up to 27.85%. More studies of biomass energy conversion technologies based on S-CO₂ cycles have been summarized in

Table 7. Reviews in biomass conversion technologies.

Refs	Year	Biomass Conversion	Thermodynamic Cycle	Optimum Power Production (kWe)	Energy Conversion Efficiency
Manente et al. [66]	2014	Combustion	Cascaded supercritical CO2 Brayton cycles	5359	36%
Wang et al. [67]	2018	Combustion	Recompression S-CO2 Brayton cycle combined with Recuperation S-CO2 Brayton cycle	11,250	21%
Ge et al. [63,64,65]	2020	Combustion	Recuperation T-CO2 Brayton cycle	11.9	22%
Nkhonjera et al. [68]	2020	Gasification	Recuperation S-CO2 Brayton cycle combined with Steam Rankine cycle	-	60%
Ji-chao et al. [61]	2021	Gasification	Recuperation S-CO2 Brayton cycle combined with Kalina cycles	7400	78.15%
Chein et al. [69]	2021	Gasification	Recuperation S-CO2 Brayton cycle	-	21%
Chitsaz et al. [59]	2022	Gasification	Recuperation S-CO2 Brayton cycle	172.6	-
Cao et al. [60]	2022	Gasification	Recuperation S-CO2 Brayton cycle	138	-
Wang et al. [70]	2022	Gasification	semi-closed S-CO2 cycle with a bottom ORC	70,210	38.76%
Moradi et al. [62]	2023	Gasification	Gas turbine, S-CO2 Brayton cycle, ORC	126	48%
Zhang et al. [71]	2023	Gasification	Gas turbine cycle, S-CO2 cycle, Organic Flash Cycle integrated	8210	75.80%
Zhang et al. [72]	2023	Gasification	Recuperation S-CO2 Brayton cycle	68,200	67.98%

. Although higher efficiency in biomass conversion to electricity can be obtained by using the supercritical CO₂ Brayton cycle, some barriers still exist for bioenergy development. The limited large-scale application of biomass can be attributed to several factors, including the need for logistics for feedstock collection and transportation, elevated feedstock costs compared to fossil fuels and greater upfront capital investment requirements.

5.2. S-CO₂ Cycle for Concentrating Solar Power Systems

Globally, there are 144 concentrated solar power (CSP) projects running in 22 countries, where Spain, the United States and China are at the forefront in terms of construction and operation [73]. However, among those projects, steam Rankine cycles or organic Rankine cycles are still the primary technologies for converting solar energy to electricity. In comparison to photovoltaic (PV) panel technologies, CSP exhibits an inherent capacity to retain thermal energy for a short time, allowing for its subsequent conversion into electricity. CSP plants, when equipped with thermal storage capacity, have the capability to generate electrical power even in situations where sunlight is blocked by cloud cover or during post-sunset hours. CSP can be classified into three categories, including the first generation with a receiver temperature range of 250 °C~450 °C, the second generation with a receiver temperature range of 500 °C~720 °C, and the third generation with a receiver temperature above 700 °C. Moreover, based on the collector’s type, CSP technologies can be divided into four types, including linear Fresnel reflector (LFR), solar power tower (SPT), parabolic power dish collector (PDC) and parabolic trough collector (PTC) [74], as illustrated in Figure 12. Detailed information regarding the CSP category is listed in

Table 8. Categories of CSP technologies [76].

Generation	First Generation	Second Generation	Third Generation
Receiver outlet temperature (°C)	250–450	500–720	>700
Typical technology	Parabolic trough collector Solar power tower Linear Fresnel reflector	Parabolic trough collector Solar power tower Linear Fresnel reflector Power dish collector	Particles Gas
Heat transfer medium	Oil Steam	Salt Steam Gas	Salt Air Helium CO2
Thermodynamic cycle	Steam Rankine cycle	Steam Rankine cycle/Stirling	Brayton cycle
Cycle efficiency (%)	28–38	38–44	>50

.

The levelized cost of electricity (LCOE) is widely recognized as a crucial metric for assessing different power generation systems, as it encompasses all relevant aspects, such as initial capital investments, installation expenses, continuous operational costs and maintenance expenditures across the full lifespan of a power station [77]. The development of S-CO₂ technology in CSP applications is crucial for improving the efficiency of solar plants. Replacing the steam Rankine power block with an S-CO₂ cycle can enhance the LCOE of conventional molten salt tower technology. Operating at salt temperatures close to 600 °C is projected to yield an 8% enhancement in LCOE, while further advancements in temperature and efficiency can be achieved by employing S-CO₂ power cycles in CSP systems [78].

Comprehensive modelling of various SPT systems that are coupled with S-CO₂ Brayton cycles was conducted by Wang et al. [79]. Results showed that the intercooling cycle delivered the highest level of efficiency, with the partial-cooling cycle ranking the next, followed by the recompression cycle. Similarly, Neises and Turchi [80] concluded that the implementation of a partial cooling cycle had the potential to generate a larger temperature difference across the primary heat exchanger, thus leading to cost savings on the heat exchanger and enhancing the efficiency of the CSP receiver. Zhu et al. [81] further investigated the effects of different turbine inlet temperatures on the thermal efficiency of this system which was the same as Wang et al.’s [79]. The findings indicated that the turbine inlet temperature had a parabolic impact on the overall efficiencies of each S-CO₂ cycle. For further improving the efficiency of S-CO₂ SCP, modified cycles have been investigated at different operating conditions. The thermal efficiency of the supercritical CO₂ Brayton cycle consistently rises with the temperature of the cycle, as seen in Figure 13, by comparing with different S-CO₂ cycles, including recuperation, recompression, partial cooling with recompression and recompression with main compression. The recompression cycle with main compression intercooling achieved the best thermal efficiency of 55.2% at 850 °C [82]. Binotti et al. [83] also arrived at similar findings, indicating further that the recompression with main compression intercooling S-CO₂ cycle could attain a solar-to-electric efficiency of 24.5%. The studies mentioned previously relied on steady-state design conditions. However, other researchers have conducted dynamic analyses of S-CO₂ CSP performance with varied solar irradiance levels during different seasons, including spring, summer, fall and winter [84,85]. More research conducted by researchers to assess the performance of S-CO₂ CSP power systems has been summarized and presented in

Table 9. Summary of typical applications of S-CO₂ in CSP and modelling/experimental analysis.

Ref.	Year	Approach	CSP type	Thermodynamic Cycles	Pmax (MPa)	Pmin (MPa)	TH (°C)	TL (°C)	ⴄth	Wnet (MW)
Dyreby et al. [85]	2013	Modelling	-	Recuperation; Recompression	25	8.14~9.17	700	45	47.6~49.4%	10
Iverson et al. [86]	2013	Experiment	Six immersion heaters	Split-flow recompression	14.091	7.688	538	32.4	15.2%	0.176
Neises and Turchi [80]	2014	Modelling	Solar power tower	Recuperation; Recompression; Partial Cooling	25		650	50	44.6~49.5%	35
Padilla et al. [82]	2015	Modelling	Solar power tower	Recuperation; Recompression; Partial cooling with recompression; Recompression with main compression intercooling	25	6.25~16.1	500~800	55.5	35.1~55.2%	-
Osorio et al. [84]	2016	Modelling	Solar power tower	Recompression with multi-stage expansion and intercooling	20	8	497.1~515.2	38.2~44.9	44.3~48.1%	1.516~1.855
Binotti et al. [83]	2017	Modelling	molten salts solar tower plants	Recompression; Partial-cooling; Recompression with main compression intercooling	25	5.23~9.37	740~780	51	-	23.81~24.78
Wang et al. [53,79,81]	2017	Modelling	Molten salt solar power towers	Recompression; Intercooling; Partial-cooling; Split-expansion	25	7.6	450~800	35	38%~58%	1
Khan et al. [87]	2019	Modelling	Parabolic dish solar	Recompression with reheat	20	7.6	549.9	31.85	33.7%	-
Sun et al. [88]	2019	Experiment	Heater	Recuperation with spray-assisted dry cooling	20	8	610	42~57	39.4~40.9%	0.79~0.9
Liu et al. [89]	2021	Modelling	Molten salt solar power tower	Split-recompression with bottom Rankine Cycle	31.81	8.14	893.2	35	44.5~49.5%	50
Chen et al. [90]	2023	Modelling	-	Recompression	25	7.615~7.646	500~700	32	44.5~53.7%	-

. Despite the extensive analysis conducted on the feasibility and efficacy of the S-CO₂ CSP solar power system, its implementation on a commercial scale has not yet been achieved due to the challenges associated with operating under high-temperature and high-pressure conditions, which are necessary for optimal performance and analysis. Attaining and sustaining these conditions can pose difficulties to material selection and component production. In addition, the initial capital cost may exhibit a considerably higher magnitude in comparison to alternative renewable energy sources and technologies such as PV panels or conventional fossil fuel power plants.

5.3. S-CO₂ Cycle for Geothermal Power Systems

Geothermal power generation is the utilization of underground thermal energy to generate electricity. In general, brine is the most common working fluid to extract heat from underground earth and convert it into electricity via thermodynamic power cycles. However, conventional geothermal energy conversion systems are constrained by size and location, while brines have the potential for scaling and erosion of injection systems and heat exchangers. Therefore, improved monitoring and system management are essential [91]. Furthermore, the majority of geothermal energy is trapped within rocks characterized by low fracture permeability and a lack of fluid circulation. Consequently, the development of new technologies is imperative.

To extract energy from hard dry rock (HDR) to generate electricity, enhanced geothermal systems (EGS) involve the extraction of thermal energy via the creation of artificial geothermal reservoirs [92]. Typically, an EGS requires hydrofracturing of rock with low natural permeability [93]. The performance of EGS based on working fluid of CO₂ and water was investigated, and it was found that a CO₂-EGS was more efficient than a water-EGS [94,95,96,97]. A CO₂-Plume Geothermal (CPG) system was proposed by Randolph et al. [98], as shown in Figure 14, CO₂ was sent to the subsurface to recover the energy, and then a small portion of it was piped back to the surface to undergo turbine, generator and heat exchangers for electricity generation, or it was used to provide heat for a power cycle. A CPG was applied as an alternative technology to EGS without hydrofracturing, and the amount of CO₂ stored in a CO₂-CPG system was significantly greater than what was typically seen in CO₂-EGS [93]. Adams et al. [99] conducted a comprehensive comparison between CPG and brine geothermal systems under the conditions of different reservoirs. The results showed that in comparison to brine systems, CO₂ direct systems generated a higher net power output when dealing with reservoir depths and permeabilities that fall within the low to moderate range. Wang [92] compared the performance of different S-CO₂ cycles, including pre-compression, inter-cooling and reheating for utilizing geothermal energy via model simulations. Subsequently, the S-CO₂ Cycle with reheating has the highest net power output of 6.9 MW. Similarly, with low-grade geothermal heat sources, Ruiz-Casanova et al. [100] numerically analyzed the performance of four S-CO₂ Brayton cycles, including simple, recuperation, intercooling and intercooled recuperation. The results revealed that the intercooled recuperated Brayton cycle achieved the best performance with the highest power output and highest thermal efficiency. Furthermore, an enhanced natural gas recovery (EGR) reservoir has a larger size than an EGS reservoir, and EGR-CPG can be a promising technology to efficiently extract heat from geothermal. In order to enhance the total producible energy from the gas field while mitigating electricity costs, Ezekiel et al. [101] explored the potential of CPG and the enhanced natural gas recovery (EGR) in a high-temperature reservoir using S-CO₂ thermodynamic cycles, as depicted in Figure 15. In this process, external CO₂ was introduced into the deep natural gas reservoir. As it travels from the injection well to the production well, the CO₂ fluid undergoes heating due to the presence of hot natural gas. The resulting mixture of high-temperature gases was then transported to the land surface, where they were separated for use in a combined system consisting of an S-CO₂ cycle and an ORC. The simulation results demonstrated that under conditions of a low CO₂ circulation rate in the CPG stage, a net electricity generation of 0.656 MWe could be sustained over 42 years. Conversely, when the CO₂ circulation rate is high, the system could generate 1.187 MWe over a period of 32 years. The higher flow rate of CO₂ contributed to higher power output with less timeframe and thus fewer capital costs could be achieved. In addition to the investigation of different S-CO₂ configurations, researchers also carried out the comparison between pure CO₂ and CO₂ mixtures. Wright et al. [102] developed an S-CO₂ cycle for low-temperature geothermal heat sources and conducted a system performance comparison when either CO₂-mixture (CO₂/butane) or pure CO₂ as a working fluid. The findings indicated that, at a turbine inlet temperature of 160 °C and a dry heat rejection temperature of 46.7 °C, the efficiency of the CO₂-mixture cycle was 18.1%, while the efficiency of the pure CO₂ cycle was 15%. Another investigation on CO₂ mixture containing SF6 was conducted by Yin et al. [103]. It was observed that CO₂ concentrations of 15 mol% led to the highest Brayton cycle efficiency, while 20 mol% resulted in the highest Rankine cycle efficiency. Some recent S-CO₂ geothermal systems studies are listed in Table 10.

While geothermal energy is characterized by its cleanliness, sustainability and low operating costs, its utilization is constrained by geographic factors. Only a few countries with ample geothermal resources can effectively harness this energy source. The construction of a geothermal power plant demands substantial initial investments since the drilling and exploration stages are the main project-related risks. Additionally, when assessing a technology’s potential, the technology readiness level (TRL) is often utilized [104]. Although S-CO₂-CPG demonstrates superior thermal performance compared to conventional hydrothermal geothermal power systems, it is important to note that the TRL of CO₂-CPG is relatively low and less mature in comparison to traditional approaches.

Figure 14. Simplified schematic of a CO₂-plume geothermal (CPG) system [98].

Figure 14. Simplified schematic of a CO₂-plume geothermal (CPG) system [98].

Figure 15. The integrated CO₂ EGR-CPG system designed for electricity generation [101].

Figure 15. The integrated CO₂ EGR-CPG system designed for electricity generation [101].

Table 10. S-CO₂ geothermal systems studies in recent years.

Ref.	Year	Technology		Tsource (°C)	Pmax (MPa)	Pmin (Mpa)	Wnet (kW)	ⴄth
Ezekiel et al. [101]	2019	CO2 EGR-CPG		150	30		1187
Ruiz-Casanova et al. [100]	2020	Simple Brayton cycle		150	23.942	7.904	725.34	10.71%
		Recuperated Brayton cycle			17.919	8	748.95	11.1%
		Intercooled Brayton cycle			24.74	7.939	719.31	10.62%
		Intercooled recuperated Brayton cycle			18.332	8.13	779.99	11.51%
Wang [92]	2018	Simply sCO₂ cycle		195	22.5	7.8	2758	13.92%
		Recuperative sCO₂ cycle					2584	8.92%
		sCO2 cycle with pre-compression and inter-cooling					3194	11.02%
		sCO2 cycle with reheating					5970	10.3%
		sCO2 Cycle with pre-compression, inter-cooling and reheating					6904	11.91%
Wright [102]	2017	sCO2 cycle with recompression, reheating and intercooling		>160	22.2	8.62	2369	15%
		sCO2/10% Butane cycle with recompression, reheating and intercooling					2675	18%
Glos et al. [105]	2019	s-CO2 Rankine cycle		>102	24.5	6	3240	5%
Levy et al. [106]	2018	Direct turbine expansion system		225	14.5	8.34	30,000	-
Tagliaferri et al. [107]	2022	Direct sCO2 cycle		District heating system located between turbine: T hot water = 35 °C T cold water = 60 °C Recovery heat exchanger located after the production well: T hot water = 50 °C T cold water = 80 °C 35 °C ≤ Tinjection ≤ 55 °C			1630	-
		Indirect sCO2 cycle with ORC (binary cycle)					2612	-
		Direct S-CO2 with cogeneration	District heating system located between turbine stages				1556	-
			District heating system located after the production well				1055	-
		Combined direct sCO2 with ORC	Recovery heat exchanger located before the injection well				2918	-
			Recovery heat exchanger located after the production well				2663	-
Sun et al. [108]	2023	T-CO2 Rankine Cycle + power and heat generation unit		1966	20	5.73	67.5	42.5%

Table 10. S-CO₂ geothermal systems studies in recent years.

Ref.	Year	Technology		Tsource (°C)	Pmax (MPa)	Pmin (Mpa)	Wnet (kW)	ⴄth
Ezekiel et al. [101]	2019	CO2 EGR-CPG		150	30		1187
Ruiz-Casanova et al. [100]	2020	Simple Brayton cycle		150	23.942	7.904	725.34	10.71%
		Recuperated Brayton cycle			17.919	8	748.95	11.1%
		Intercooled Brayton cycle			24.74	7.939	719.31	10.62%
		Intercooled recuperated Brayton cycle			18.332	8.13	779.99	11.51%
Wang [92]	2018	Simply sCO₂ cycle		195	22.5	7.8	2758	13.92%
		Recuperative sCO₂ cycle					2584	8.92%
		sCO2 cycle with pre-compression and inter-cooling					3194	11.02%
		sCO2 cycle with reheating					5970	10.3%
		sCO2 Cycle with pre-compression, inter-cooling and reheating					6904	11.91%
Wright [102]	2017	sCO2 cycle with recompression, reheating and intercooling		>160	22.2	8.62	2369	15%
		sCO2/10% Butane cycle with recompression, reheating and intercooling					2675	18%
Glos et al. [105]	2019	s-CO2 Rankine cycle		>102	24.5	6	3240	5%
Levy et al. [106]	2018	Direct turbine expansion system		225	14.5	8.34	30,000	-
Tagliaferri et al. [107]	2022	Direct sCO2 cycle		District heating system located between turbine: T hot water = 35 °C T cold water = 60 °C Recovery heat exchanger located after the production well: T hot water = 50 °C T cold water = 80 °C 35 °C ≤ Tinjection ≤ 55 °C			1630	-
		Indirect sCO2 cycle with ORC (binary cycle)					2612	-
		Direct S-CO2 with cogeneration	District heating system located between turbine stages				1556	-
			District heating system located after the production well				1055	-
		Combined direct sCO2 with ORC	Recovery heat exchanger located before the injection well				2918	-
			Recovery heat exchanger located after the production well				2663	-
Sun et al. [108]	2023	T-CO2 Rankine Cycle + power and heat generation unit		1966	20	5.73	67.5	42.5%

5.4. S-CO₂ Cycle for Waste Heat Recovery

Plenty of waste heat exists in industries, which can be potentially recovered, as shown in

Table 11. Waste heat sources and end uses [110].

Waste Heat Sources	Energy End Use
Combustion exhausts: Glass melting furnace; Cement kiln; Fume incinerator; Aluminum reverberatory furnace; Boiler. Process off-gas: Steel electric arc furnace. Cooling water from Furnaces; Air compressors; Internal combustion engines. Conductive, convective and radiative losses from equipment and heat products	Combustion air preheating; Boiler feedwater preheating; Load preheating; Power generation; Steam generation for use in power generation; mechanical power; process steam; Space heating; Water preheating; Transfer to liquid or gaseous process streams.

. Waste heat recovery represents a promising approach for minimizing energy consumption and channelling additional energy towards end-use applications. Given the substantial abundance of waste heat in process industries, advanced thermodynamic cycles present greater potential to increase power generation and improve energy efficiency [109].

Waste heat to power (WHP) involves harnessing the heat that is typically wasted by an ongoing process and converting it into electrical energy. The primary conversion route is shown in Figure 16, with applicable technologies and thermodynamic cycles such as steam Rankine, ORC, Stirling, Kalina, and S-CO₂ Brayton, as shown in Figure 17. As the S-CO₂ Brayton cycle outperforms the existing Rankine cycles in terms of adapting for higher grade heat sources, higher efficiency, system compactness and environmentally friendly working fluid, it has been considered in numerous applications, including waste heat recovery. The initial commercial implementation of a supercritical CO₂ power cycle emerged in Alberta. This system captured residual heat from a gas-fired turbine and subsequently transformed this heat into electricity utilizing an innovative S-CO₂ power cycle based on the patented technology developed by Echogen and General Electric [111,112], as shown in Figure 18. The power generation system consists of two cycles: a gas turbine cycle and a steam Rankine cycle. Typically, the exhaust gas temperature emanating from a gas turbine exceeds 450 °C, and the conventional steam Rankine cycle makes use of this exhaust gas to enhance thermal efficiency. Ahnv et al. [113] conducted research on the application of a basic S-CO₂ cycle for recovering waste heat from a gas turbine shipboard. Their study revealed that it was possible to recover 16.7% of the wasted energy. Zhang et al. [114] carried out a novel recompression S-CO₂ system to recover the waste heat from an internal combustion engine in which one more heater and one more turbine were added in the cycle to continuously recover the waste heat. Results revealed that the recovery efficiency of this novel system increased by about 18%. Song et al. [115] pointed out that preheating the S-CO₂ cycle can improve net power output in the waste heat recovery system. Wright et al. [116] compared the performance of four different layouts of S-CO₂ for waste heat recovery from a gas turbine, including simple recuperation, cascaded, dual recuperated and recuperated with preheating Brayton cycles, as shown in Figure 19. The results pointed out that the simple recuperation cycle had the lowest waste recovery efficiency of 61.2%, while the cascaded cycle had the highest efficiency of 85.64%. Nonetheless, the cascaded cycle demonstrates the lowest thermal efficiency and net cycle efficiency, at 26.5% and 24.7%, respectively. In contrast, the simple recuperated cycle attains the highest values, with thermal efficiency and net cycle efficiency reaching 30.42% and 28.3%, respectively. Manente et al. [117] further investigated three other S-CO₂ layouts for recovering waste heat of temperature at 600 °C, finding that the S-CO₂ duo-expansion cycle had the highest energy conversion efficiency. More research regarding waste heat recovery technologies has been summarized in

Table 12. Research on waste heat recovery in recent years.

Ref.	Year	Source	Cycle	Power of Source (kW)	Tsource (°C)	Pmax (Mpa)	Pmin (Mpa)	Wnet (kW)	ⴄth	ⴄrecovery
Ahnb et al. [113]	-	Gas turbine	Recuperation S-CO2	25,000	566	-	-	4175	-	16.70%
Ahmadi et al. [119]	2016	Proton exchange membrane fuel cell	S-CO2 Rankine cycle combined with liquefied natural gas cycle	-	>70	10	0.6	1413	-	66.39%
Wright et al. [116]	2016	Gas turbine	Recuperation S-CO2	40,731	549	24	7.7	7017	30.42%	61.20%
			Cascaded S-CO2 cycle					8214	26.50%	85.64%
			Dual recuperated sCO2					8322	28.17%	78.36%
			Recuperated Brayton cycle with preheating					8601	27.80%	82.10%
Manjunath et al. [120]	2018	Gas turbine	Recuperation S-CO2 with T-CO2 vapour compression cycle	20,600	572	20	>7.37	3138	38.70%	44.50%
Song et al. [115]	2018	Engine waste heat	Preheating S-CO2	996	300	15	7.8	64	-	-
			Preheating S-CO2 with regeneration					68
Zhang et al. [114]	2020	Internal combustion engines	Recompression S-CO2	235.8	519	25	8	33.06	35.86%	58.70%
Manente et al. [117]	2020	Steel industry	S-CO2 dual expansion	2010	600	20	7.63	1000	26.62%	22.30%
		Gas turbine	S-CO2 dual recuperation	2312					28.40%	19.39%
		Fuel cell	S-CO2 partial heating	2073					25.82%	21.63%
Bonalumi et al. [121]	2021	Gas turbine	Partial heating S-CO2	4710	511	26	9.56	1550	25%	70%
Sanchez et al. [122]	2011	Molten carbonate fuel cell	Simple recuperation S-CO2	-	>650	22.5	7.5	583.6	39.90%	59.40%
Marchionni et al. [123]	2021	Simulated waste heat source–Air heater	Simple recuperation S-CO2	830	650	20	7.4	84	23%	-
			Reheating S-CO2					87	25%	-
			Recompression S-CO2					85	24%	-
			Recompression reheating S-CO2					88	27%	-
			Preheating S-CO2					155	26%	-
			Preheating Split-Expansion S-CO2					140	23%	-
			Split-heating Split-Expansion S-CO2					110	17.50%	-
			Preheating pre-compression S-CO2					150	25	-

.

Although waste heat recovery systems are attracting more and more attention, there are limitations due to factors such as temperature constraints and the expenses associated with recovery equipment. The majority of waste heat recovery research focuses on medium temperatures ranging from 230 °C to 650 °C because low temperatures offer limited thermal energy, lower electricity generation and thus lower economic viability. However, recovering high temperatures of waste heat can lead to increased thermal stress on materials used in heat exchangers.

5.5. Barriers to Take Up of the S-CO₂ Technology

The application of supercritical CO₂ in both power cycles or heating and cooling systems [124,125,126,127] have potential advantages in efficiency, size and environmental impact. However, the barriers to taking up this technology can be concluded as follows:

• Although a small power generation system can be constructed owing to the high density of CO₂, it allows for more compact turbine, compressor and heat exchanger components. The design, production and selection of turbomachinery is still a challenge for operating CO₂ in a wide range of temperatures and pressures.
• Enhancing the efficiency of the system via the improvement in heat exchangers remains a compelling aspect for the successful operation of a S-CO₂ Brayton cycle, given the presence of at least two heat exchangers in the basic cycle.
• Insufficient practical experience and performance data from both experimental and commercial applications to provide solid support for applying this technology in the area of renewable energy.

6. Conclusions

The S-CO₂ cycle development has attracted considerable attention for its applications in renewable energy sectors. This paper reviews advanced S-CO₂ technologies and cycles integrated with renewables such as biomass, solar and geothermal, as well as waste heat for power generation. The S-CO₂ system can achieve higher efficiency compared to other conventional power generation systems, as CO₂ working fluid can achieve a more effective thermal match with the applicable heat source. Different renewable energies have been categorized and reviewed according to their characteristics, including temperature ranges, energy conversion routes, and suitable technologies.

Biomass has the capacity to reach temperatures as high as 1400 °C via combustion, while solar energy can achieve temperatures up to 1000 °C, allowing its associated power generation system to achieve higher net power output and enhanced energy conversion efficiency in the range of 0.1~68 MWe by adopting different S-CO₂ layouts. By reviewing recent years’ studies, thermal efficiencies of S-CO₂ biomass systems are in the range of 21~78%, and the S-CO₂ solar system is in the range of 15.2~58%. Combined multiple cycles for biomass energy conversion can achieve higher power output. Partial cooling, intercooling and recompression cycles are considered the most favourable cycle configurations for concentrated solar power. The majority of waste heat recovery research focuses on medium temperatures ranging from 230 °C to 650 °C. From the reviewed latest publications, the thermal efficiency and energy conversion efficiency can be achieved in the range of 17.5~39.9% and 18~85%, respectively. The simple recuperation S-CO₂ Brayton cycle for waste heat recovery is less attractive due to its lower net power output. In terms of a relatively lower temperature heat source geothermal, supercritical CO₂-Plume Geothermal demonstrates superior thermal performance compared to conventional hydrothermal geothermal power systems. The thermal efficiency of different S-CO₂ Brayton cycles for geothermal utilization ranges from 8.92% to 18%.

Advanced S-CO₂ cycles for renewable energy are the promising approach to improve power output and increase thermal efficiency than the conventional Rankine cycle. There are still some barriers that need to be considered. The limited large-scale application of biomass can be attributed to the need for logistics for feedstock collection and transportation, elevated feedstock costs compared to fossil fuels and greater upfront capital investment requirements. The majority of current CSP solar thermal projects worldwide are primarily based on the steam Rankine cycle. This is due to the Rankine cycle being a mature technology with lower initial capital cost compared to the S-CO₂ Brayton cycle. Geothermal power plants are more constrained by location, which must be situated in close proximity to or directly above geothermal resources. The effectiveness of waste heat recovery is heavily dependent on the temperature of the waste heat source. Waste heat recovery faces limitations due to factors such as temperature constraints and the expenses associated with recovery equipment. Moreover, there is a pronounced need for the advancement of turbomachinery and heat exchanger technology. Future research on S-CO₂ renewable energy power generation systems is expected to focus on achieving both economic viability and high efficiency.