CO2-based Mixtures Research Articles

Performance of a CO2-based mixture cycled transcritical pumped thermal energy storage system

The transcritical CO2 cycle has the characteristics of high power density, high stability and high thermal energy transfer efficiency. It has been employed in pumped thermal energy storage system and ice slurry often serves as the cold storage material. However, ice slurry is not an optimal choice for cold storage due to its huge volume and high cost. In this study, an innovative transcritical pumped thermal energy storage system cycled with CO2-based mixtures is proposed. Temperature of the CO2-based mixture changes during the phase transition process, which means that simple two-tank cold energy storage arrangement with fluid heat materials is feasible to achieve the phase change of working fluid. R32, R161, propane, butane and pentane are screened to mix with CO2, respectively. Multi-parametric analysis of proposed system is conducted to assess its technical and economic performance. The Monte-Carlo method is employed to investigate the system uncertainty. Results display that for a 10 MW/8h scale the system cycled with CO2/R32 has the minimum levelized cost of storage of 0.1655 $/kWh, with a round trip efficiency of 65.51 %. The optimal levelized cost of storage is 0.1678 $/kWh, 0.1801 $/kWh, 0.1781 $/kWh and 0.1801 $/kWh for R161, propane, butane, and pentane, in sequence. The system investment cost decreases as the operating durations increase. The mean levelized cost of electricity and energy capacity cost are separately reduced by 29.35 % and 48.24 % as the installed capacity is extended from 10 MW to 10000 MW.

Thermally integrated innovative Carnot batteries to upgrade and dispatch low temperature sensible waste heat

Abstract This work focuses on innovative thermally integrated Carnot batteries exploiting low temperature sensible waste heat, available at temperatures between 70°C and 100°C, while adopting a sensible solid-based thermal energy storage system. The charging cycle is based on transcritical heat pumps operating with CO2-based mixtures, representing the most innovative aspect of the work, whereas the discharging cycle runs with conventional pure fluid ORC. The heat pumps adopting CO2 doped with a low fraction of hydrocarbons can achieve very high second law efficiencies (around 70%), even in a simplified cycle layout with a sensible heat source, storing heat in the low-cost storage operating between 50°C and 150°C. Similarly, an ORC with cyclopropane as working fluid is identified as the most promising solution for the discharging phase of the Carnot battery. Results show RTE of around 62% and 84% for the thermally integrated systems with waste heat sources at 70°C and 100°C, respectively, and an air-cooled ORC. Finally, calculations show that RTE close to 100% is possible with a water-cooled ORC. Both the computed values of RTEs and the high compactness and effectiveness of the innovative heat pumps highlight the necessity for additional research into thermally integrated Carnot batteries as a way to exploit and dispatch sensible waste heat.

Optimisation of Brayton cycle CO2-based binary mixtures: An application for waste heat recovery of marine low-speed diesel engines exhaust gas

The energy-saving capabilities and efficient operation of marine low-speed diesel engines (MLDE) is a key emphasis for the main power source for ocean transportation. The purpose of the supercritical carbon dioxide recompression Brayton cycle (SCRBC) is to capture and utilise waste heat emitted by the engine's exhaust gas. However, the SCRBC performance will be severely affected by the large temperature fluctuations of ocean-going vessels during operation and high ambient temperatures in the cabin. A SCRBC model was built using the exhaust gas test data as the boundary conditions and validated using the Sandia National Laboratory (SNL) test data. The physical characteristics of the working fluids were evaluated by adding other fluids to CO2 in specific proportions to modify the critical point and increase cycle efficiency. The results demonstrated that employing CO2-based binary working fluids with low alkane and hydrogen sulfide (H2S) enhanced the recovery power, with the most significant increase obtained by the addition of 16.48 % H2S, which increased the power by 9.72 kW and improved the Brayton cycle efficiency by 3.31 %. Compared to the MLDE at 100 % load, the total efficiency increased by 1.77 % and the BSFC decreased by 6.76 (g kW−1 h−1) using CO2-H2S as the working fluid. The analysis of the SCRBC system component exergy losses showed that the cooler had the highest exergy losses. Adding other fluids to CO2 reduced the exergy losses of each component with the SCRBC system exergy losses decreasing from 162.97 to 129.90 kW and the exergy loss efficiency decreasing from 24.24 % to 22.65 %. The use of CO2-based binary working fluids specifically designed for ambient temperature may be expanded to other engines to enhance the efficiency of waste heat recovery.

Analysis of the CO2 + C2Cl4 mixture in high temperature heat pumps: Experimental thermal stability, liquid densities and cycle simulations

Transcritical heat pumps working with CO2-based mixtures with a low-volatility dopant are found to achieve good performances in thermally integrated heat pumps, especially when sensible heat sources and heat sinks are considered. This paper introduces in literature tetrachloroethylene, C2Cl4, as CO2-dopant for the mixture to be adopted as working fluid in high temperature heat pumps. To calibrate the thermodynamic model used in the cycle simulations, an experimental characterization on the mixture is proposed: liquid densities of the mixtures are measured, in a wide range of concentration, optimizing the binary interaction parameter of the Peng Robinson equation of state. Moreover, the thermal stability of pure C2Cl4 is experimentally evaluated, identifying the maximum allowable compressor outlet temperature between 200 °C and 250 °C, with a decomposition rate below 1 %/year if the fluid is kept at temperatures around 200 °C. Then, the potentialities of this very high temperature heat pump are assessed in spray dryer applications: a coefficient of performance around 3.38 is obtained for a conventional spray dryer plant, corresponding to 73 % of second law efficiency, considering an air flow heated from ambient temperature to 200 °C as the sink, while cooling the sensible heat source, available at 76 °C, below 30 °C. As term of comparison, the same system adopting propane, instead of the CO2 + C2Cl4 mixture, would achieve a coefficient of performance and second law efficiency of 2.94 and 64 %, respectively.

Thermodynamic analysis of supercritical CO2 power generation system for waste heat recovery with impurities in CO2

Waste heat recovery from gas turbines is an effective method for meeting the demand for a more efficient and cleaner energy system with decreased CO2 emissions and lower environmental pollution. The supercritical CO2 Brayton cycle is a promising power conversion system for waste heat recovery. The advantage of the supercritical CO2 Brayton cycle can be attained based on its physical characteristics. However, when another fluid is mixed as an impurity in a system using pure CO2, physical characteristics of working fluid was changed and operation at the design point can be challenging. Therefore, it is important to verify the purity of CO2 and understand the effect of impurities in CO2 in advance. This study examined the effects of impurities in the working fluid of a supercritical CO2 power generation system for actual demonstration facilities. The effect of impurity, a CO2-based mixture, in a supercritical CO2 power generation system on waste heat recovery from an LM500 gas turbine was investigated. A purity test of industrial grade CO2, which was noted as 99.9 % pure by the supplier, was conducted to verify the composition of the working fluid. The results of the purity test showed that the supplied CO2 consisted of 99.076 % CO2 along with nitrogen, argon, oxygen, water, and methane. Based on the results of the purity test, sensitivity analyses of the CO2-based binary mixtures and industrial mixtures were performed. A design point analysis was conducted for the CO2-based industrial mixture in comparison with pure CO2, which has cycle efficiency of 16.15 %, gross power of 510.03 kW, mass flow rate of 12.55 kg/s, and waste heat recovery of 2119.06 kW. In the case of fixed waste heat with mass flow rate of 14.417 kg/s, the cycle efficiency decreased by 0.39 % and gross power increased by 66.14 kW. In the case of fixed mass flow rate with 1844.598 kW waste heat, the cycle efficiency decreased by 0.32 % and gross power decreased by 9.84 kW. In addition, the influence of the performance of the major components on the system performance was investigated.

Pyrolysis mechanism of 1,1,1,2-tetrafluoroethane and 1,1,1,2-tetrafluoroethane/carbon dioxide as working fluids for transcritical power cycles: Insights from reactive force field molecular dynamics and density functional theory studies

Carbon dioxide-based mixtures in transcritical power cycle systems can enhance thermodynamic performance but may pose risks of thermal decomposition, potentially compromising system performance and safety. This study investigates the pyrolysis mechanism of 1,1,1,2-tetrafluoroethane (R134a)/carbon dioxide (CO₂), a typical CO₂-based mixture, using reactive force field molecular dynamics (ReaxFF-MD) simulations and density functional theory (DFT). ReaxFF-MD simulations are conducted at pressures ranging from 4 to 12 MPa and temperatures between 1800 K and 3200 K for various fluid compositions, including pure R134a and R134a/CO₂ mixtures at mole ratios of 0.7/0.3, 0.5/0.5, and 0.3/0.7. The effects of temperature, pressure, and composition on the thermal decomposition of both pure R134a and R134a/CO₂ mixtures are examined, with particular focus on behavior at 8 MPa. In the thermal decomposition of R134a/CO₂ mixtures, CO₂ inhibits the formation of F radicals and reduces their concentration through chemical reactions, thereby suppressing R134a decomposition. Pure R134a decomposes into primary products such as hydrogen fluoride (HF), fluorine (F), tetrafluoroethylene (C2HF4), trifluoromethyl radicals (CF3), and diatomic carbon (C2). The addition of CO2 results in the formation of additional products, including carbonyl fluoride (COF), oxygen (O), hydroxyl (HO), and formyl radicals (CHO). The decomposition pathways involve two reaction types: self-decomposition reactions dominate initially, while extraction reactions become more prominent later. Using the DFT approach, reaction energy barriers are analyzed to corroborate the ReaxFF-MD simulation findings. Moreover, the apparent activation energies for these reactions are quantified using first-order kinetics based on the Arrhenius equation, indicating that the thermal decomposition of R134a/CO2 mixtures is more challenging than that of pure R134a.

Comparison analysis of three CO2-based binary mixtures performing in the supercritical Brayton cycle for molten salt energy storage

It is generally acknowledged that using CO2-based binary mixtures as working fluids in supercritical recompression Brayton cycle for waste heat recovery is a promising technique. This study examines the performance of CO2-Kr (0.76/0.24) and CO2-Xe (0.56/0.44) mixtures, comparing them with pure CO2. Both thermodynamic and thermo-economic considerations are taken into account to establish optimal parameter settings. The findings suggest that the minimum system temperature should be around but not below the critical temperature of the working fluid. The system performs badly at lower pressures and shows nearly the same at higher pressures, with 25 MPa being the recommended value for the main compressor. When the system split ratio and the low temperature recuperator temperature difference are well matched, the heat exchangers exhibit an optimal temperature distribution. The exergy loss distributions of the exchangers in the CO2-Kr and CO2-Xe systems are likewise rather uniform when the input pressure of the main compressor is above the critical pressure of the fluid, typically between 0.2 MPa and 0.4 MPa, enhancing system efficiency. The corresponding temperature difference of the LTR should be within 5 °C above or below the critical temperature of the fluid. This advantageous feature is not immediately apparent in the pure SCO2 cycle.

Performance assessment of residential heat pumps operating in extreme cold climates using zeotropic mixtures

Extremely cold climates subject residential heat pumps to significant temperature differences between the heat source and the ambient being heated, which may lead to system failure and reduced compressor performance. The present study considers the possibility of improving the performance of heat pump systems that are simultaneously used for residential space and domestic water heating while subjected to climates varying from -25 °C to 5 °C by exploring the use of zeotropic mixtures. CO2-based binary mixtures composed of low-GWP (global warming potential) refrigerants are considered – R32, R1234yf, and R290 – aiming to benefit from environmentally friendly and flame suppressants characteristics of CO2, as well as the improved thermal efficiency granted by the addition of a secondary refrigerant. A thermodynamic model was developed for a standard vapor-compression heat pump cycle and used to maximize the coefficient of performance limited by the minimum pinch point in the heat exchangers. Our analysis explores the effect of several parameters, such as, the mixture components, mass fractions, space and water heating demands, and heat source temperature on the heat pump's performance. For cold climates, the mixture of R32 (90%)/CO2 (10%) yields the highest COP and CO2-rich mixtures exhibit the lowest. However, by increasing the mass fraction of CO2 within the zeotropic mixture, the pressure ratio of the heat pump was improved. When considering combined performance criteria, such as the volumetric heating effect and the total heat delivered related to heat pump size, CO2-rich mixtures tend to allow more compact systems, especially in colder climates.

Exploring eco-friendly gas mixtures for resistive plate chambers: A comprehensive study on performance and aging

Resistive Plate Chambers (RPCs) are gaseous detectors widely used in high energy physics experiments, operating with a gas mixture primarily containing Tetrafluoroethane (C2H2F4), commonly known as R-134a, which has a global warming potential (GWP) of 1430. To comply with European regulations, the RPC EcoGas@GIF++ collaboration, involving ALICE, ATLAS, CMS, LHCb/SHiP, and EP-DT communities, has undertaken intensive R&D efforts to explore new environmentally friendly alternative gas mixtures for RPC technology.A leading alternative under investigation is HFO1234ze, boasting a low GWP of 6 and demonstrating reasonable performance compared to R-134a. Over the past few years, RPC detectors with slightly different characteristics and electronics have been studied using HFO and CO2-based gas mixtures at the CERN Gamma Irradiation Facility. An aging test campaign was launched in August 2022, and during the latest test beam in July 2023, all detector systems underwent evaluation. This contribution will report the results of the aging studies and the performance evaluations of the detectors with and without irradiation.

CO2 Storage in Deep Oceanic Sediments in the form of Hydrates: Energy Evaluation and Advantages Related to the Use of N2-Containing Mixtures

Permanent storage in suitable geological sites and/or deep aquifers is emerging as the most concrete and effective solution to mitigate its increasing concentration in the atmosphere. This article experimentally investigated its storage in deep marine environments in the form of hydrates. Gas hydrates were formed into a small-scale reactor, designed to reproduce marine seafloors. Hydrates were formed with pure carbon dioxide and with CO2-based gaseous mixture containing nitrogen at different concentrations, equal, respectively, to 30, 50 and 60 vol%. The results obtained for each mixture were then compared to each other. In particular, the quantity of hydrates formed was evaluated as a function of the thermodynamic conditions selected for the experiments. The energy spent for the process, calculated by considering the cooling and compression phases, was calculated for the unit quantity of hydrates formed and for the unit quantity of carbon dioxide stored. Finally, the energy requirements for gas cooling and for gas compression were calculated separately in order to comprehend the contribution of the single process for each mixture.

Transcritical Cycles With CO2-based Mixtures for CSP Applications: An Overview of the SCARABEUS Findings

This work summarizes the methodology developed within the H2020 EU project SCARABEUS for the analysis of innovative CO2-based mixtures used as working fluid in transcritical cycles for CSP applications. By adding a specific quantity of carefully selected dopant to CO2 it is possible to reach a high mixture critical temperature, suitable for air cooled cycles at very high minimum temperature in hot environments with high efficiencies. Along the methodology, some results related to the design of the turbine and the heat exchangers for the innovative mixtures are presented, including a focus on the system integration between the power block and the solar plant: as such, these results can be considered as interesting research outcomes to widen the knowledge on innovative solutions for efficient power cycles. The new power cycles are foreseen to drastically increase the profitability and cut the specific cost of CSP systems with respect to the state-of-the-art

Investigations on cooling heat transfer of CO2-based mixtures in a novel airfoil fin mini-channel at supercritical pressure

Thermal-hydraulic characteristics of three supercritical CO2-based mixtures (CO2/H2S, CO2/Xe and CO2/propane) in an airfoil fin (AFF) mini-channel adopted in the printed circuit heat exchanger (PCHE) under cooling conditions are comprehensively investigated through numerical simulations. The results show that the CO2/H2S mixture, which has higher critical temperature and larger critical pressure than the pure CO2, exhibits the greatest heat transfer performance among three mixtures. This advantage is particularly pronounced at relatively large Reynolds number, with small mass fraction of the H2S and under near-critical conditions. The overall friction loss of mixtures with the additive mass fraction smaller than 0.3 is comparable to that of the pure CO2. The heat transfer coefficient of the mixtures presents fluctuations during the cooling processes, and this goes against the stable and safe operation of the PCHEs. These fluctuations could be mitigated at relatively small mass flow rate and with small mass fraction of the additive. Three widely-recognized buoyancy criteria are employed to predict the onset of the buoyancy effects in the AFF channel, and the superior heat transfer of the CO2/H2S mixture could be attributed to the fiercer buoyancy effects. Considering the buoyancy effects, modified correlations for the flow and heat transfer of pure CO2 and the CO2-based mixtures in the novel mini-channel are proposed. The maximum deviations of the modified Nusselt number and fanning friction factor correlations are ± 25 % and ± 20 %, respectively.

Exploring the Potential of Silicon Tetrachloride as an Additive in CO2-Based Binary Mixtures in Transcritical Organic Rankine Cycle—A Comparative Study with Traditional Hydrocarbons

Exploring the Potential of Silicon Tetrachloride as an Additive in CO2-Based Binary Mixtures in Transcritical Organic Rankine Cycle—A Comparative Study with Traditional Hydrocarbons

Empirical modelling of a semihermetic reciprocating compressor for different CO2-based mixtures

Mixing CO2 with other refrigerants gives carbon dioxide new thermophysical properties, such as higher critical temperature or lower working pressures.However, most of these new mixtures havenotyetbeenevaluatedexperimentally, introducing a lack of knowledge about how the main components of a refrigerating plant will operate with these blends.One clear example is the compressor, whose operationdirectly affects the performance and energy consumption of theplant. Accordingly, this work evaluates experimentally the behaviour of a CO2 semi-hermetic reciprocating compressor working with four different CO2-based mixtures of CO2/R32, CO2/R290, CO2/R1270 and CO2/R1234yf. The analysis covers the parameters of volumetric and global efficiency, discharge temperature, mass flow rate, and electrical power consumption, where noticeable differences are shown in terms of efficiencies, affecting the mass flow rate and the electrical power consumption significantly, and, to a lesser extent, the discharge temperature. The experimental results are compared with the standardized models from UNE-12900 and AHRI-540, proposing a simplified empirical model with a prediction error below 4% for all analysed mixtures and an extensive model for other CO2-based mixtures.

Turbulent Rayleigh–Bénard convection in a supercritical [formula omitted]-based binary mixture with cross-diffusion effects

CO2-based binary mixtures are promising in energy engineering to improve efficiencies of thermodynamic cycles where supercritical CO2 prevails. Indeed, under supercritical pressure and transcritical temperature conditions, special thermophysical properties of binary mixtures make the heat transfer process accompanied by enhanced cross-diffusion effects, i.e., the Soret effect and the Dufour effect. Their influences on heat transfer have not been well understood yet. This paper numerically investigates turbulent Rayleigh–Bénard convection in CO2-C2H6 binary mixture. Calculations are performed based on a spectral element method solver considering various temperature differences and pressures. Results indicate that the flow field presents a double-vortex structure. The thickness of the boundary layer is negatively correlated with the flow intensity. Under a positive separation ratio, the concentration gradient induced by cross-diffusion effects enhances buoyancy, which in turn reinforces the natural convection and heat transfer. The enhancement becomes stronger as the critical pressure is approached. Turbulent fluctuations are also strengthened by cross-diffusion effects, except for temperature fluctuations. The unusual reduction in temperature fluctuations is attributed to the increased size of the thermal plume caused by the Dufour effect.

Experimental investigation on CO2-based zeotropic mixture composition-adjustable system

CO2 is a promising natural working fluid for its environment-friendly, property-stable and low-cost, and CO2-based zeotropic mixture could enhance the comprehensive performance of combined cooling and power cycle (CCP). Generally, CCP operates in a wide temperature range, and the fixed operating composition could not realize the optimal match for two sub-cycles. Zeotropic mixture composition adjustment could realize targeted thermal match of sub-cycles and further enhance the performance of CCP. However, systematic research on composition-adjustable CCP is limited in theoretical simulation, and there is still a lack of experimental evidence for the performance enhancement of composition adjustment on CCP. Besides, the feasibility of composition adjustment on CCP has not been fully verified through experimental method yet. In this study, an experimental prototype of CO2-based composition-adjustable CCP is established, the approach to realize composition adjustment is investigated, and the systematic test is carried out. Experimental results show that the introduction of composition adjustment is beneficial to the performance of CCP. Under the same boundary conditions and similar refrigeration conditions, composition-adjustable CCP could export 13.72 % more net power than composition-fixed CCP when the initial charged CO2 mass fraction equals 0.246, and the improvement is more apparent when the initial charged composition gets larger.

Role of CO2-based mixtures in the organic Rankine cycle using LNG cold energy

Organic Rankine cycle (ORC) is an attractive method for recovery of liquefied natural gas (LNG). Zeotropic mixtures are optimal working fluids for improving the ORC power generation performance. In this study, eight CO2-based binary mixtures were selected as alternative working fluids for the utilization of medium-temperature LNG cold energy for ORC. The effect of CO2-based binary mixture on thermodynamic performance of ORC was presented and investigated. The genetic algorithm was applied to optimize the thermodynamic performance of ORC so as to determine the optimal working fluid among CO2-based binary mixtures. The results indicate that CO2-based binary mixtures significantly improve the thermodynamic performance of ORC in the recovery of LNG cold energy when compared with pure-CO2 ORC. There is a specific CO2 mass fraction that leads to the highest net power output of the ORC, and the mass fraction of CO2 increases with higher condensation temperatures (For CO2/Propane, the condensation temperatures are −55 °C, −40 °C, and −30 °C, and this CO2 mass fraction is 0.35, 0.65, and 0.8, respectively). Furthermore, the ORC using CO2/R134a exhibits the highest net power output at 13.50 kW among the eight alternative CO2-based binary mixtures.

A Theoretical Study on the Thermal Performance of an Increasing Pressure Endothermic Cycle for Geothermal Power Generation

In this study, a power cycle (IPEC), with an increasing pressure endothermic process in a downhole heat exchanger (DHE) and a CO2-based working fluid mixture, was developed for geothermal power generation. The increasing pressure endothermic process, which cannot be achieved in a conventional evaporator on the ground, was realized using the gravitational potential energy in the DHE. The parameters of the power cycle and the structural size of the DHE were optimized simultaneously. Using CO2-R32 as the working fluid of the IPEC provides the highest net power output. The net power generated with the IPEC was compared with a single-flash (SF) system, a trans-critical CO2 (t-CO2) system, and an organic Rankine cycle (ORC) under the same heat source and sink conditions. Six selection maps were generated for choosing the optimum power cycle for electricity production, in which four power generation systems (ORC, t-CO2, IPEC, and SF) were included, and two DHE diameters (0.155 m and 0.22 m) were investigated. It was found that the IPEC system had more net power output than the other three systems (ORC, t-CO2, and SF) under the conditions that the geofluid’s mass flow rate was less than 10 kg/s and its temperature was lower than 180 °C.

Experimental evaluation of CO2/R-152a mixtures in a refrigeration plant with and without IHX

In recent years, CO2-based mixtures have been considered as a way to improve the performance of refrigerating plants using pure CO2 as refrigerant. Combining CO2 with a fluid with higher critical temperature generates a blend which allows to run a refrigeration plant in subcritical conditions at higher heat rejection temperature when compared to pure carbon dioxide, and consequently reach higher COP. This work evaluates from an experimental point of view two CO2/R-152a mixtures, ([90/10 %] and [95/5 %]), used as refrigerants in a single-stage refrigeration plant with and without internal heat exchanger, and compares the results to those obtained using pure CO2. The work analyses the main energy parameters of the plant for secondary fluid inlet temperature of 2.5 °C at the evaporator, and in the range from 20 °C to 40 °C with 5 °C step at the condenser/gas-cooler. The use of such mixtures, compared to the use of pure CO2, allowed to obtain a higher COP in the base cycle for heat rejection temperature above 25 °C, reaching the largest increment at the highest temperature while, when working with IHX cycle, no improvements in the COP were measured.

Feasibility assessment of trough concentrated solar power plants with transcritical power cycles based on carbon dioxide mixtures: A 4E analysis and systematic comparison

Medium and low-temperature solar thermal power generation, integrating parabolic trough collectors with transcritical carbon dioxide (CO2)-based mixture power cycles, is explored for enhanced solar energy utilization and improved efficiency. This study assesses the feasibility of parabolic trough concentrated solar power plants utilizing transcritical CO2-based mixtures, considering energy output, efficiency, economics, and environmental (4E) aspects. Seven candidate organic working fluids (R32, R134a, R152a, R1234yf, R161, R290, and R1270) are evaluated within a two-layer decision-making framework based on multi-objective optimization. Results indicate a significant performance improvement with an increased mass fraction of the organic working fluid. Particularly, systems employing CO2/R32 or CO2/R161 demonstrate superior overall performance compared to those with other CO2-based mixtures. Under the base case (at a hot tank temperature of 350 °C), the CO2/R32 (0.3/0.7 wt%) system achieves the maximum energy output (355.22 kW), while the CO2/R161 (0.3/0.7 wt%) system attains the highest efficiency with the lowest levelized cost of electricity and CO2 emissions (9.78%, 0.576 $/kW·h, and 0.0328 kg/kW·h, respectively). Exergy analysis emphasizes the predominant role of the solar collector in exergy destruction (approximately 70%), with the CO2/R161 (0.3/0.7 wt%) system yielding the highest exergy efficiency (14.96%). Across a hot tank temperature range of 250–350 °C and an organic fluid mass fraction of 0.3–0.7, the working fluids are generally ranked as CO2/R161, CO2/R32, CO2/R152a, CO2/R1270, CO2/R134a, CO2/R290, and CO2/R1234yf in terms of the 4E aspects via a multi-objective optimization decision-making framework.