Power Generation with Renewable Energy and Advanced Supercritical CO 2 Thermodynamic Power Cycles: A Review

: Supercritical CO 2 (S-CO 2 ) 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-CO 2 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-CO 2 renewable energy systems and identify technology and integration opportunities for different types of renewable resources. Correspondingly, this paper not only summarizes the advantages of CO 2 working ﬂuid, design layouts of S-CO 2 cycles and classiﬁcations of renewable energies to be integrated but also reviews the recent research activities and studies carried out worldwide on advanced S-CO 2 power cycles with renewable energy. Moreover, the performance and development of various systems are well grouped and discussed.


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 Energies 2023, 16, 7781 2 of 32 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].CO 2 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 CO 2 power cycle can easily extend to the supercritical region and turn it into a transcritical or supercritical CO 2 power cycle.The supercritical CO 2 (S-CO 2 ) 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 CO 2 power cycles.The S-CO 2 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-CO 2 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, CO 2 has attracted more attention to be utilized as an alternative working fluid.Another reason is the lower critical temperature at 31 • C which enables CO 2 working fluid and its associated system to be applicable in a variety range of heat sources.A power cycle with CO 2 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-CO 2 cycles.A design was conceptualized by Le Moullec [13] for a coal-fired power plant with 90% post-combustion CO 2 capture.It showed that the cost of electricity and the cost of CO 2 reduction could be reduced by 15% and 45%, respectively.Mecheri and Le Moullec [14] conducted additional simulations on S-CO 2 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-CO 2 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-CO 2 cycle.To further enhance the performance of the S-CO 2 coal-fired power plant, employing a partial flow mode in the S-CO 2 boiler was demonstrated to effectively decrease pressure drop, thereby increasing the system's thermal efficiency [16].As for the indirect Brayton Energies 2023, 16, 7781 3 of 32 cycle, significant attention was paid to the S-CO 2 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-CO 2 cycle for nuclear reactors.The study revealed that the S-CO 2 cycle exhibited competitive system efficiency compared to the helium Brayton cycle at the same operating condition.Additionally, it was concluded that the S-CO 2 power cycle was apt for all nuclear reactors with a core CO 2 gas heater outlet temperature exceeding 500 • C. Moisseytsev and Sienicki [19] investigated several alternative cycle layouts for S-CO 2 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-CO 2 cycle could potentially improve the LWR power cycle efficiency and save capital costs.In order to further evaluate the benefits of coupling an S-CO 2 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-CO 2 cycle was previously recognized for having superior efficiency in high operating temperature regions, the efficiency of S-CO 2 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-CO 2 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-CO 2 power cycle in the renewable energy sector has recently emerged as an appealing option.While several researchers have provided extensive reviews of S-CO 2 cycles, as outlined in Table 1, the majority have focused on the development status of S-CO 2 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-CO 2 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 CO 2 cycles in power generation systems with renewable energy by offering a comprehensive view of the advantages of supercritical CO 2 working fluid, the landscapes of renewable energy, the options of S-CO 2 cycles, and application status of S-CO 2 renewable energies.Below are the key points highlighted in each section: (i) In Section 2, the superior thermal-physical properties of CO 2 are outlined, along with the benefits of incorporating CO 2 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-CO 2 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-CO 2 renewable power systems is presented, including S-CO 2 for biomass power systems, S-CO 2 cycle for concentrating solar power systems, S-CO 2 cycle for geothermal power systems and S-CO 2 cycle for waste heat recovery.This focuses on various technologies, operating conditions and efficiencies.In addition, the barriers to S-CO 2 technology are also concluded.

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Research progress of S-CO 2 power cycles and components was reviewed.

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The review explores the thermodynamic, economic, environmental and flexible feasibility of the technology.
The thermodynamic performance of system application needs to be further analyzed.

Superior Thermal-Physical Properties of CO 2
CO 2 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 CO 2 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 CO 2 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 Energies 2023, 16, 7781 5 of 32 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 CO 2 fluctuates between 0.2 and 0.5, which leads to a reduction in the power consumption of the compressor.Consequently, the S-CO 2 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-CO 2 Brayton cycle [17].The benefits of utilizing CO 2 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 noncombustibility, (3) non-reactivity with component materials, and (4) superb thermodynamic and transport properties.According to the low critical temperature of CO 2 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 CO 2 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 CO 2 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 CO 2 Rankine cycle pertains to the situation where heat addition happens at CO 2 pressure above the critical point, while heat rejection occurs at CO 2 pressure below the critical point, as illustrated in Figure 4.It has been demonstrated that the efficiency of the S-CO 2 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, CO 2 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 CO 2 cycle.(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.

Biomass
Biomass, a type of non-fossilized and biodegradable organic material originatin from plants, animals and microorganisms, has emerged as a global frontrunner for th development of low-carbon energy.It encompasses products, by-products, residues an waste from agriculture, industry and forestry.The energy stored in biomass is initial derived from the sun, with photosynthesis being the primary process through whic plants convert the sun's radiant energy into chemical energy, stored as glucose or suga Biomass is categorized into two main groups: virgin biomass, which includes fore biomass, energy crops and grasses; and waste biomass, which comprises municipal soli waste, agricultural crop residues and leaves [33].Serving as a renewable and sustainab energy source, biomass can be utilized to generate electricity or other forms of energ Unlike fossil fuels, the CO2 produced from the complete combustion of biomass equivalent to the amount it absorbs from the atmosphere, resulting in no net contributio to atmospheric carbon dioxide levels and emitting low levels of SOx and NOx.There a two main thermochemical conversion routes to use biomass for supplying electricity an heating, i.e., gasification and combustion.Combustion is the most mature technology convert biomass to useful electricity and heating.Using biomass as a heat source can n only effectively reduce biomass waste but also provide high temperatures definite higher than 900 °C [34] during the combustion process.Figure 6 shows the biomass energ conversion processes and temperature ranges of the thermochemical route.
Biomass has already become a crucial element in the UK's energy suppl contributing to 11% of the total electricity generated in 2022.According to the most recen energy statistics from the same year, it was estimated that bioenergy made u approximately 8.6% of the UK's overall energy supply [35].The key advantages an disadvantages of biomass applications are outlined in Table 2 below.

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 CO 2 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 below.

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

Solar Power
Solar energy is the most abundant and widely distributed form of renewable energy available for utilization.Out of the 1.75 × 10 5 terawatts (TW) representing the total energy from the sun reaching Earth's atmosphere, approximately 1 × 10 5 TW consistently reaches the Earth's surface [36].In 2022, solar energy production reached a total of 1289.27terawatt-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 largerscale 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 lowtemperature heat sources [39].In addition, oil, salt and steam are typical heat transfer

Solar Power
Solar energy is the most abundant and widely distributed form of renewable energy available for utilization.Out of the 1.75 × 10 5 terawatts (TW) representing the total energy from the sun reaching Earth's atmosphere, approximately 1 × 10 5 TW consistently reaches the Earth's surface [36].In 2022, solar energy production reached a total of 1289.27terawatthours (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 largerscale 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 respectively, and direct steam generation has limitations in terms of storage capacity [40].From the study of Dostal et al. [29], the S-CO 2 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-CO 2 cycle has become more favourable for converting solar power to electricity since it has higher thermal efficiency.
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.

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

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

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 CO 2 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 CO 2 Rankine and R123 ORC cycles, utilizing a low-grade heat source with a temperature of 150 • C. Their investigation established that the transcritical CO 2 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 CO 2 cycle.Moreover, if the heat temperature is as low as approximately 112 • C, the transcritical CO 2 Rankine cycle exhibits greater efficiency than that of the transcritical CO 2 Brayton cycle due to reduced compression work while achieving the same temperature increase.The supercritical CO 2 Brayton cycle is expected to replace the organic Rankine cycle to improve thermal efficiency for waste heat recovery.In addition, the S-CO 2 cycle can recover waste heat from small turbines.

S-CO 2 Layouts
The progressions of the five representative S-CO 2 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-CO 2 cycles, building upon the fundamental S-CO 2 power system depicted in Figure 8a.Thermodynamic equations of each component and thermal efficiencies of different layouts are summarized in Tables 4 and 5, respectively.The introduction of a recuperator allows for the recovery of more waste heat, resulting in what is termed the recuperation S-CO 2 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 CO 2 , leading to a pinch-point problem [49].The smaller the pinchpoint, the better the heat transfer efficiency; however, a larger heat transfer area could be required.To address the pinch-point problem, a recompression S-CO 2 cycle was developed, as shown in Figure 8c.In this configuration, an additional recuperator and compressor are introduced.A portion of CO 2 is cooled by a gas cooler and then compressed to the highest pressure by the main compressor.The remaining amount of CO 2 is compressed by the secondary compressor.These two streams are combined at the inlet of the hightemperature recuperator (HTR or Recuperator 1) on the high-pressure side.This minimizes the temperature difference between the hot and cold fluid sides of CO 2 , thereby improving heat transfer performance and resolving the pinch-point issue.Recompression S-CO 2 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 precompression 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-CO 2 cycle.Increasing the number of stages results in the compression process approaching nearisothermal conditions at the compressor's inlet temperature [54].However, intercooling (Figure 8e) does not hold much appeal in S-CO 2 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-CO 2 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-CO 2 cycle.Nonetheless, it is exclusively applicable to indirect cycles, and utilizing more than one reheat stage is not economically practicable.

Gas cooler
Energies 2023, 16, x FOR PEER REVIEW Gas cooler Thermal efficiencies of each S-CO2 layout.

Layout First Law Efficiency Equation
Basic S-CO2 Reheating S-CO2

S-CO2 for Biomass Power Systems
Typical biomass power generation technologies have been listed in Table 6.can be combusted directly within waste-to-energy facilities for electricity pro Biomass co-firing is the substitution of a portion of the fuel with biomass within co thermal power plants.It is an economical approach to convert biomass effectiv environmentally into electricity and involves integrating biomass as a partial repl for fuel in high-efficiency coal boilers [55].Sweden has successfully operated th integrated gasification combined cycle (IGCC) plant that utilizes 100% b specifically straw [56].However, biomass poses a constraint on the adoption of lar steam cycles or IGCCs, which are designed to achieve higher efficiencies.The ma biomass plants are typically small-scale and rely on internal combustion engi ORCs.The electrical efficiency of a biomass-fired ORC system was between 7 13.5% [57].Subsequently, in an experimental study conducted by Qiu et al. [58 observed that the electricity generation efficiency of this biomass-fired ORC sys

Layout
First Law Efficiency Equation

S-CO 2 for Biomass Power Systems
Typical biomass power generation technologies have been listed in Table 6.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 coalfired 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 CO 2 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 CO 2 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 H 2 , CO and CH 4 .Subsequently, this high-temperature hydrogen-rich syngas flows through a CO 2 gas heater, elevating the temperature of CO 2 to drive the S-CO 2 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-CO 2 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-CO 2 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-CO 2 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-CO 2 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-CO 2 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 CO 2 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-CO 2 cycle may utilize high-temperature flue gas insufficiently, a concept of cascaded supercritical CO 2 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-CO 2 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-CO 2 cycle and a simple recuperation S-CO 2 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-CO 2 cycles have been summarized in Table 7.Although higher efficiency in biomass conversion to electricity can be obtained by using the supercritical CO 2 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.A S-CO2 biomass system for bio-synthetic natural gas, power and freshwater productions [59].Figure 9.A S-CO 2 biomass system for bio-synthetic natural gas, power and freshwater productions [59].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

S-CO 2 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.

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.    [76].

Generation First Generation Second Generation Third Generation
Receiver outlet temperature ( 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-CO 2 technology in CSP applications is crucial for improving the efficiency of solar plants.Replacing the steam Rankine power block with an S-CO 2 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-CO 2 power cycles in CSP systems [78].
Comprehensive modelling of various SPT systems that are coupled with S-CO 2 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-CO 2 cycle.For further improving the efficiency of S-CO 2 SCP, modified cycles have been investigated at different operating conditions.The thermal efficiency of the supercritical CO 2 Brayton cycle consistently rises with the temperature of the cycle, as seen in Figure 13, by comparing with different S-CO 2 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-CO 2 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-CO 2 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-CO 2 CSP power systems has been summarized and presented in Table 9.Despite the extensive analysis conducted on the feasibility and efficacy of the S-CO 2 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.
Energies 2023, 16, x FOR PEER REVIEW 26 of 42 considerably higher magnitude in comparison to alternative renewable energy sources and technologies such as PV panels or conventional fossil fuel power plants.

S-CO 2 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 permeability [93].The performance of EGS based on working fluid of CO 2 and water was investigated, and it was found that a CO 2 -EGS was more efficient than a water-EGS [94][95][96][97].A CO 2 -Plume Geothermal (CPG) system was proposed by Randolph et al. [98], as shown in Figure 14, CO 2 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 CO 2 stored in a CO 2 -CPG system was significantly greater than what was typically seen in CO 2 -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, CO 2 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-CO 2 cycles, including pre-compression, inter-cooling and reheating for utilizing geothermal energy via model simulations.Subsequently, the S-CO 2 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-CO 2 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-CO 2 thermodynamic cycles, as depicted in Figure 15.In this process, external CO 2 was introduced into the deep natural gas reservoir.As it travels from the injection well to the production well, the CO 2 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-CO 2 cycle and an ORC.The simulation results demonstrated that under conditions of a low CO 2 circulation rate in the CPG stage, a net electricity generation of 0.656 MWe could be sustained over 42 years.Conversely, when the CO 2 circulation rate is high, the system could generate 1.187 MWe over a period of 32 years.The higher flow rate of CO 2 contributed to higher power output with less timeframe and thus fewer capital costs could be achieved.In addition to the investigation of different S-CO 2 configurations, researchers also carried out the comparison between pure CO 2 and CO 2 mixtures.Wright et al. [102] developed an S-CO 2 cycle for low-temperature geothermal heat sources and conducted a system performance comparison when either CO 2 -mixture (CO 2 /butane) or pure CO 2 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 CO 2 -mixture cycle was 18.1%, while the efficiency of the pure CO 2 cycle was 15%.Another investigation on CO 2 mixture containing SF6 was conducted by Yin et al. [103].It was observed that CO 2 concentrations of 15 mol% led to the highest Brayton cycle efficiency, while 20 mol% resulted in the highest Rankine cycle efficiency.Some recent S-CO 2 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-CO 2 -CPG demonstrates superior thermal performance compared to conventional hydrothermal geothermal power systems, it is important to note that the TRL of CO 2 -CPG is relatively low and less mature in comparison to traditional approaches.
Energies 2023, 16, x FOR PEER REVIEW 28 of 42 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.

S-CO 2 Cycle for Waste Heat Recovery
Plenty of waste heat exists in industries, which can be potentially recovered, as shown in Table 11.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 Sources
Energy End Use 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-CO 2 Brayton, as shown in Figure 17.As the S-CO 2 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 CO 2 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-CO 2 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-CO 2 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-CO 2 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-CO 2 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-CO 2 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-CO 2 layouts for recovering waste heat of temperature at 600 • C, finding that the S-CO 2 duo-expansion cycle had the highest energy conversion efficiency.More research regarding waste heat recovery technologies has been summarized in Table 12.
Energies 2023, 16, x FOR PEER REVIEW 31 of 42 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.

Barriers to Take Up of the S-CO 2 Technology
The application of supercritical CO 2 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 CO 2 , it allows for more compact turbine, compressor and heat exchanger components.The design, production and selection of turbomachinery is still a challenge for operating CO 2 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-CO 2 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.

Conclusions
The S-CO 2 cycle development has attracted considerable attention for its applications in renewable energy sectors.This paper reviews advanced S-CO 2 technologies and cycles integrated with renewables such as biomass, solar and geothermal, as well as waste heat for power generation.The S-CO 2 system can achieve higher efficiency compared to other conventional power generation systems, as CO 2 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-CO 2 layouts.By reviewing recent years' studies, thermal efficiencies of S-CO 2 biomass systems are in the range of 21~78%, and the S-CO 2 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-CO 2 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 CO 2 -Plume Geothermal demonstrates superior thermal performance compared to conventional hydrothermal geothermal power systems.The thermal efficiency of different S-CO 2 Brayton cycles for geothermal utilization ranges from 8.92% to 18%.
Advanced S-CO 2 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-CO 2 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-CO 2 renewable energy power generation systems is expected to focus on achieving both economic viability and high efficiency.

Figure 1 .
Figure 1.CO2 thermal-physical properties vary with temperature at different pressures.

Figure 1 .
Figure 1.CO 2 thermal-physical properties vary with temperature at different pressures.

Figure 4 .
Figure 4. Configuration and processes of a CO2 supercritical Rankine cycle.(a) The configuration.(b) The process in a T-S diagram [31].

Figure 4 .Figure 5 .
Figure 4. Configuration and processes of a CO 2 supercritical Rankine cycle.(a) The configuration.(b) The process in a T-S diagram [31].

Figure 5 .
Figure 5. Diagram illustrating the heat transfer between the heat source and working fluid in the high-temperature main heat exchanger: (a) ORC cycle.(b) CO 2 transcritical/supercritical power cycle [32].

Figure 8 .
Figure 8. Layouts of S-CO 2 Brayton cycle and T-S diagrams.

Figure 10 .
Figure 10.The integration of proton-conducting solid oxide fuel cell and supercritical CO 2 Brayton cycle [60].

Figure 11 .
Figure 11.A small-scale power generation and heat recovery test rig based on biomass with CO2 transcritical Brayton cycle [64].

Figure 11 .
Figure 11.A small-scale power generation and heat recovery test rig based on biomass with CO 2 transcritical Brayton cycle [64].

Figure 13 .
Figure 13.Variations in thermal and exergy efficiency with different turbine inlet temperatures for five S-CO2 CSP cycles [81].

Figure 13 .
Figure 13.Variations in thermal and exergy efficiency with different turbine inlet temperatures for five S-CO 2 CSP cycles [81].

Figure 16 .
Figure 16.Basic process of a WHP system.

Figure 16 .
Figure 16.Basic process of a WHP system.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.

Figure 17 .
Figure 17.Thermodynamic cycles for waste heat recovery at different temperatures and scales [118].

Figure 19 .
Figure19.Different layouts of S-CO2 waste heat recovery power systems for gas turbine waste heat recovery[116].

Figure 19 .
Figure19.Different layouts of S-CO 2 waste heat recovery power systems for gas turbine waste heat recovery[116].

Table 1 .
Recent reviews on S-CO 2 power generation technologies.

Table 2 .
Major advantages and disadvantages of biomass.

Table 2 .
Major advantages and disadvantages of biomass.
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

Table 3 .
[47]ntial of different heat source temperature ranges of geothermal energy in terms of heating (MW th ) and electricity (MW e ) in Europe[47].

Table 4 .
Thermodynamic equations of each component.
Energies 2023, 16, x FOR PEER REVIEW

Table 4 .
Thermodynamic equations of each component.

Table 5 .
Thermal efficiencies of each S-CO 2 layout.

Table 7 .
Reviews in biomass conversion technologies.

Table 7 .
Reviews in biomass conversion technologies.

Table 8 .
Categories of CSP technologies

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

Table 9 .
Summary of typical applications of S-CO 2 in CSP and modelling/experimental analysis.

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

Table 10 .
S-CO 2 geothermal systems studies in recent years.

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