CO2 Transcritical Power Cycle Research Articles

A Novel Thermally Integrated CO2-Carnot Battery (TI-PTES) utilizing Cold Thermal Storage

Abstract The growing integration of renewable energy sources in the energy grid presents intermittency and negative pricing challenges, necessitating large-scale energy storage solutions. Pumped Thermal Energy Storage (PTES) can address these issues by storing and delivering substantial energy whenever required. High-temperature heat pump development is crucial to deploying PTES for storing heat at sink temperatures that are well above the ambient temperature(>450°C) to ensure a reasonable round-trip efficiency(RTE). Currently, however, it is not a technological possibility for heat pumps to achieve these temperatures even with the support of freely available heat (200°C to 400°C) as source temperatures. This study explores a potential layout of the TI-PTES system that exploits commercially available equipment by storing heat below the ambient temperature while still being able to utilize the freely available heat source(Solar, Waste heat, biomass, etc.) to support the overall RTE. The charging phase employs a well-established CO2-refrigeration cycle to accumulate energy below the ambient temperature in cold thermal storage. While the discharging phase runs a trans-critical CO2 power cycle between the freely available heat source and the cold thermal storage. Overall, offering a practically implementable model for the PTES system with market-available components. The study investigates the design of this innovative system presenting the relevance of different operating and machine parameters as well as the contribution of freely available heat sources to the overall performance. Finally benchmarking the technology with other long-duration energy storages.

Thermodynamic and parametric analyses of a zero-carbon emission SOFC-based CCHP system using LNG cold energy

A novel zero-carbon emission combined cooling, heating and power (CCHP) system is currently proposed. The thermal energy of the solid oxide fuel cell (SOFC) exhaust is recovered in a transcritical CO2 (T- CO2) power cycle. Relying on the efficient utilization of liquefied natural gas (LNG) cold energy, CO2 capture has been achieved at low energy consumption. The thermodynamic performance is evaluated by using energy and exergy analysis methods for this hybrid process. The electrical, exergy, and CCHP efficiencies are obtained as 60.37 %, 62.83 %, and 79.09 %, respectively. The CO2 condenser, with the largest exergy destruction ratio (18.60 %) and a low exergy efficiency (37.16 %), is regarded as the weakest point in terms of thermodynamic perfectibility. In addition, the influences of the current density, stack temperature and pressure, and fuel utilization rate have also been studied from different aspects of system performance. The current density, stack temperature, stack pressure, and fuel utilization rate are recommended as 0.3 A/cm2∼0.4 A/cm2, 900 °C, 5 bar, and 85 %, respectively. This research provides practical reference and pragmatic guidance for the integration, analysis, and optimization of SOFC-based CCHP systems.

Simulation and comprehensive study of a new trigeneration process combined with a gas turbine cycle, involving transcritical and supercritical CO2 power cycles and Goswami cycle

Simulation and comprehensive study of a new trigeneration process combined with a gas turbine cycle, involving transcritical and supercritical CO2 power cycles and Goswami cycle

Heat Recuperation for the Self-Condensing CO2 Transcritical Power Cycle

Heat Recuperation for the Self-Condensing CO2 Transcritical Power Cycle

Numerical investigation on thermal-hydraulic performance of variable cross section printed circuit heat exchanger

CO2 transcritical power cycle (CTPC) systems have attracted considerable research focus in the fields of thermoelectric conversion and waste heat recovery. The regenerator is a key component affecting the CTPC system's efficiency. To improve the comprehensive performance of the regenerator, extensive research has been conducted to optimize the regenerator flow channel design. However, the optimization of the traditional Z-channel printed circuit heat exchanger structure (ZPCHE) is limited to constant cross-sectional configurations along the flow direction, which can lead to low channel space utilization. To solve this problem, an efficient variable cross section Z-channel structure (UAPCHE) is proposed in this study. The structure is designed with different cross-sectional shapes along the flow direction to fit the flow path of the main fluid. UAPCHE achieves a coordinated optimization of the heat transfer (Nu), flow (dP), and compactness performance (Q/V) by increasing the effective utilization of the channel space and weakening the damage to the fluid boundary layer. The design principle of the UAPCHE is introduced, and based on this, the structural parameters of the UAPCHE are optimized to achieve the best comprehensive performance. The results show that, compared with the ZPCHE, Nu of UAPCHE can be increased by 16.79%, dP can be reduced by 19.48%, and Q/V can be increased by 22.65%.

Experimental investigation of critical parameters for the binary mixture of CO2 + R1336mzz(Z) (cis-1,1,1,4,4,4-hexafluoro-2-butene) and analysis of critical locus for CO2 + hydrofluoroolefin systems

The transcritical CO2 power cycle has been recognized as a highly promising technique for reducing carbon emissions in heat utilization. However, this technology is limited because of the critical parameters of pure CO2 (including low critical temperature and high critical pressure). To overcome these limitations, one potential solution is to introduce a new working fluid consisting of a blend of hydrofluoroolefins (HFOs) and CO2. Precise knowledge of the critical parameters is essential for accurately evaluating the effectiveness and capacity of CO2+HFO mixtures. In this study, the critical properties of the CO2 + R1336mzz(Z) (cis-1,1,1,4,4,4-hexafluoro-2-butene) blend were determined using a metal-bellows variable volumeter. The critical point was identified by visually observing the critical opalescence and the reappearance of the vapor-liquid meniscus. The Modified Wilson method and the Redlich-Kister method were used to fit critical data. Our results reveal that the CO2 + R1336mzz(Z) mixture can elevate the critical temperature in comparison to pure CO2. The highest critical pressure is achieved when the mole fraction of CO2 is approximately 0.82. For the critical temperature, critical pressure, critical density, and mole fraction, the expanded combined uncertainties were below 50 mK, 21 kPa, 0.6 %, and 0.004 (k = 2, 95 %), respectively. Meanwhile, the Modified Extended Chueh-Prausnitz (MECP) method and a simplified MECP method were used to predict critical properties of the CO2 + R1336mzz(Z) binary mixture. Finally, the critical locus of the CO2 + R1336mzz(Z) mixture was compared with other CO2 + HFO mixtures.

Demonstration of a small‐scale power generator using supercritical CO2

AbstractThe supercritical CO2 (sCO2) power cycle could improve efficiencies for a wide range of thermal power plants. The sCO2 turbine generator plays an important role in the sCO2 power cycle by directly converting thermal energy into mechanical work and electric power. The operation of the generator encounters challenges, including high temperature, high pressure, high rotational speed, and other engineering problems, such as leakage. Experimental studies of sCO2 turbines are insufficient because of the significant difficulties in turbine manufacturing and system construction. Unlike most experimental investigations that primarily focus on 100 kW‐ or MW‐scale power generation systems, we consider, for the first time, a small‐scale power generator using sCO2. A partial admission axial turbine was designed and manufactured with a rated rotational speed of 40,000 rpm, and a CO2 transcritical power cycle test loop was constructed to validate the performance of our manufactured generator. A resistant gas was proposed in the constructed turbine expander to solve the leakage issue. Both dynamic and steady performances were investigated. The results indicated that a peak electric power of 11.55 kW was achieved at 29,369 rpm. The maximum total efficiency of the turbo‐generator was 58.98%, which was affected by both the turbine rotational speed and pressure ratio, according to the proposed performance map.

Dynamic performance analysis of transcritical power cycle with preheater and regenerator using CO2-based binary zeotropic mixtures

The CO2 transcritical Rankine cycle is considered as an ideal solution in the application of waste heat recovery due to its safety, environmental friendliness and compactness characteristics. In this work, for the heat recovery from internal combustion engine, a dynamic model of an improved CO2 transcritical Rankine cycle system containing both a preheater and regenerator with CO2-based binary zeotropic mixtures as working fluid is developed. The performance of CO2 transcritical power cycle with preheater and regenerator was compared and analyzed for mixture working fluids with different proportions under the disturbance of heat source conditions and system input parameters, which can provide references for system control strategy to ensure efficient and stable operating conditions when selecting control variables. The results show that there is a certain pump speed to obtain the optimal value of the net output work and thermal efficiency of the system. And the net output power and thermal efficiency of system with CO2-based binary zeotropic mixtures at optimum point are higher 6. 34 kW and 2.19% respectively than pure CO2 as working fluid by adjusting pump speed.

A transcritical carbon dioxide power cycle enhanced by ejector refrigeration for engine waste heat recovery: Comprehensive analysis and optimization

A novel approach to realizing and enhancing a transcritical carbon dioxide (CO2) power cycle can be achieved by simultaneously utilizing waste heat from an internal combustion engine at two different temperature levels. This paper introduces a combined power system integrating a transcritical CO2 power cycle and an ejector refrigeration cycle (tCO2-ERC) via internal condensation for engine waster heat recovery, where the ERC driven by jacket water provides cooling energy for the condensation of CO2 in the tCO2 subsystem driven by exhaust gas. Comprehensive analyses, including energy, exergy, and economic aspects, are conducted through parameter analysis and system optimization to explore the mechanisms underlying the performance improvement of the combined power system. The results of refrigerant screening demonstrate that implementing R717 in the ERC of the tCO2-ERC system yields better overall thermoeconomic performance. Exergy analysis demonstrates that the exergy losses from the exhaust and jacket water to the environment in the tCO2-ERC system are reduced to 17.62% and 30.27% of the total exergy input, respectively, which represents a reduction of 7.32%pt (percentage point) and 12.51%pt compared to the standalone reference CO2 power system. Multi-objective optimization results indicate that the optimized net power output and levelized cost of electricity of the tCO2-ERC system are 388.79 kW and 0.09023 $/(kW·h), respectively, which are 13.01% higher and 6.91% lower than those of the reference CO2 power cycle system. Furthermore, the net power output of the tCO2-ERC system exceeds that of the reference CO2 power system by 5.44% to 14.34% when the ambient temperature ranges from 10 ℃ to 30 °C. Additionally, the tCO2 subsystem consistently operates in a transcritical mode within an ambient temperature range of 10–30 °C, thereby expanding its application range.

Exergoeconomic analysis and multi-objective optimization of a CCHP system based on SOFC/GT and transcritical CO2 power/refrigeration cycles

A CCHP (Combined Cooling, Heating and Power) system based on SOFC/GT (Solid Oxide Fuel Cell/Gas Turbine) and transcritical CO2 power/refrigeration cycles is proposed to efficiently provide cooling, heating and power for users under the principle of energy cascade utilization. The energy, exergy and exergoeconomic analysis is conducted to assess the effects of key parameters on the system’s thermal-economic performance. The simulation results demonstrate that the cooling, heating and net electricity outputs could reach 48.37 kW, 240.65 kW and 250.95 kW with a trigeneration cost per unit exergy of 37.12$/GJ, while the power generation and exergetic efficiencies could reach 62.65% and 62.27%. The annualized capital cost, O&M cost and fuel cost are 131 549$, 90 532$ and 162 375$, respectively. The NPV and payback period of the CCHP system are 502 824$ and 12.5 years. The effects of SOFC pressure, air flowrate, refrigerant flowrate and CO2 flowrate of the transcritical CO2 power cycle on the CCHP system characteristics are discussed. Furthermore, exergoeconomic and multi-objective optimization methods are used to find the final optimum operation condition with best results. A solution set in Pareto frontier with each fitness value superior to that of the design condition is obtained after optimization. Through the decision-making process, the total energy output and the exergy efficiency increase by 35.58 kW and 0.20%, respectively, while the unit exergy cost of trigeneration rises by 0.21$/GJ.

A novel self-condensing transcritical CO2 power cycle with a vortex tube: Thermoeconomic assessment study and comparison

The low critical temperature of carbon dioxide (CO2) requires low-temperature heat sinks for condensation, which limits the application of the transcritical carbon dioxide (tCO2) power cycle system. This paper proposes a novel self-condensing tCO2 power cycle system with a vortex tube, which uses the vortex tube to achieve CO2 condensation without an external low-temperature heat sink. Firstly, the detailed thermodynamic and economic models of the proposed tCO2 power cycle system are established and verified. Then, the effects of decision variables on the thermoeconomic performances of the system are studied through parametric analysis. Finally, the performances of the self-condensing tCO2 power cycle system and the simple regenerative supercritical carbon dioxide (sCO2) power cycle are optimized and compared by single-objective and multi-objective optimizations. The single-objective optimization results reveal that the optimal thermal efficiency, exergy efficiency, and the levelized cost of electricity of the self-condensing tCO2 power cycle system are 41.42%, 69.01%, and 0.0739 $/(kW·h), which are 3.06%pt (percentage point) higher, 3.89%pt higher and 3.90% lower than those of the simple regenerative sCO2 power cycle system. The multi-objective optimization results show that compared with the simple regenerative sCO2 power cycle system, the self-condensing tCO2 power cycle system improves the thermal and exergy efficiencies by 1.81%pt and 2.41%pt at the expense of 3.37% higher in the levelized cost of electricity. Exergy destruction analysis reveals that the high temperature recuperator has the largest exergy destruction, which should be improved first.

Computer-aided molecular design of CO2-based mixture working fluid harvesting engine waste heat

The CO2 transcritical power cycle is an important method for recovering waste heat. The cycle performance can be further improved by using CO2-based mixtures instead of pure CO2. To avoid the empirical screening of CO2 mixtures, a computer-aided molecular design method, which is based on the group contribution method and perturbed chain-statistical associating fluid theory equations of state, is used for the first time in this study to design CO2-based mixtures for power cycles. The calculation method for group to system thermodynamic performance is established and verified. A molecular identification model is used to obtain all compounds based on the group library, calculate their thermodynamic properties for ranking, and find the best CO2-based mixture in the global range. The performance and cost of the expander are also briefly discussed. Taking the simple transcritical power cycle as an example, the results show that when the mole fraction of CO2 is greater than 0.70, the optimal CO2-based mixture, CH3-C≡CH, can increase the system efficiency and net output work by 16.58% (9.07%) and 17.88% (17.87 kW), respectively, compared to those of pure CO2. The peak system efficiency is 9.83% at an evaporation pressure of 24 MPa. Shorter-chain additives can improve thermodynamic performance, which is demonstrated by extracting the contribution of each group to the performance of the system using statistical methods. This method enables the ranking of CO2 mixtures on a global scale and provides some rules. It is also suggested that further economic analysis is necessary.

Thermo-economic analysis of combined transcritical CO2 power cycle based on a novel liquefied-biomethane energy storage system

The Chinese government has been encouraging to accelerate the development of energy storage systems to support the rapid growth of renewable energy power generation. For the purpose of achieving flexible and economic energy storage, in this study, a combined transcritical CO2 power cycle is added to biomethane plants to form a liquefied-biomethane energy storage system. A mathematical model that includes terms of energy, exergy, and economy is established to assess the performance of the combined transcritical CO2 power cycle. The effects of critical parameters on the thermo-economic performance are studied. A sensitivity analysis is used to evaluate the extent of the influence of critical parameters. According to the simulation results, the characteristics of the liquefied-biomethane energy storage system are calculated and evaluated. The results show that at a liquefied-biomethane mass flow rate of 1 kg/s, the maximum net power output of the combined transcritical CO2 power cycle reaches 909.81 kW by adjusting the pressure and temperature. High turbine inlet temperature, pump pressure ratio, and methane purity have positive effects on system performance. Among these, the inlet temperature of turbine 2 is the most sensitive factor. Simultaneously, the maximum energy storage density and round-trip efficiency of the liquefied-biomethane energy storage system are 106.8 Wh/L and 52.7 %, respectively. Benefiting from less upfront equipment investment, the lowest power capital cost of the liquefied-biomethane energy storage system is 885.3 $/kW. In conclusion, the liquefied-biomethane energy storage system integrated with a combined transcritical CO2 power cycle exhibits good performance in terms of power output and economy.

A goal-oriented Design Method of CO2 Power Cycle (CPC) System

The CO2 power cycle (CPC) system is an efficient and environmentally friendly method for waste heat recovery (WHR). However, the traditional design and optimization process of a CPC system is very complex and time-consuming. This paper proposes a novel goal-oriented design method based on machine-learning methods for quickly designing an optimized CPC system with given performance indicators. And taking the design of the CO2 transcritical power cycle (CTPC) system for internal combustion engines (ICEs) as an example. Firstly, the net output power and the total cost of the system prediction models are trained by simulated data. Then the multi-objective optimization of the system is carried out by using the genetic algorithm coupled with the prediction models, and the optimization results are used to train a classification model. Finally, the given target indicators are input into the classification model for goal-oriented designing and getting the optimal configuration. The results of the goal-oriented design validation show that the goal-oriented design method can design the CTPC system well. And, once the classification model is trained, the CTPC system's future goal-oriented design process only needs to be calculated once, significantly reducing design time. In conclusion, the goal-oriented design method based on machine-learning proposed is a novel and promising method. This is a technology that combines computer science and energy science and can provide users with a quick and reliable CPC system design method.

Sensitivity Analysis of Transcritical CO2 Cycle Performance Regarding Isentropic Efficiencies of Turbomachinery for Low Temperature Heat Sources

The transcritical CO2 (T-CO2) power cycle using low temperature waste heat is a promising technique for energy saving and environmental protection. However, according to the literature, there is no commercialized unit in service yet. This study provides developers a reference to shorten the design phase of the T-CO2 cycle commercialization process. A sensitivity analysis of the system performance, i.e., thermal efficiency and net power output, regarding the isentropic efficiencies of pump (ηp) and expander (ηe) and the heat source temperature (Th,in) has been carried out using MATLAB and NIST REFPROP database. Simple and recuperative configurations are investigated based on their own optimal working pressures. The results show that the enhancement of ηe has a greater influence on improving the system performance, but the improvement will diminish as ηp, ηe, and Th,in increase. Although better system performance can be achieved with higher ηp, ηe, and Th,in, the cost of the system equipment will also increase due to the higher optimal working pressure. In addition, increasing ηp and ηe will negatively affect the effectiveness of the recuperator. Therefore, the turbomachinery efficiencies and the heat source temperature should be considered simultaneously for the most cost-effective system design.

Thermodynamic analysis and optimisation of a novel transcritical CO2 cycle

Transcritical and supercritical CO2 cycles are simple, low-cost, environmentally friendly, and compact solutions for harnessing a wide range of thermal energy sources such as waste heat, solar-thermal, geothermal, and nuclear reactors. In a supercritical CO2 cycle, compression begins near the critical-state (30.98 °C and 7.3773 MPa), whereas CO2 condensation occurs below the critical-state in a transcritical CO2 cycle, and the required sink temperatures are difficult to attain in the hot arid climates. The current study presents and optimises a novel transcritical CO2 power cycle that can be used in hot climates and offers very compact system which is vital for engine waste heat recovery. The proposed system achieves CO2 condensation by expanding pre-cooled CO2 from some intermediate pressure. It uses phase-wise CO2 separation in the separator and two recuperators to recycle waste heat from the hot CO2 stream leaving the turbine, reducing heat input into the heater and heat rejection by the pre-cooler. To achieve the best thermal efficiency, a differential evolution optimisation is used, and the phase-wise split of CO2 is tuned to provide excellent matching of temperature profiles while achieving the required pinch-point conditions in both the recuperators, resulting in a minimal low exergy destruction. The maximum thermal efficiency of the proposed cycle is 39.82%, while the optimal supercritical recuperative-Brayton cycle offers efficiency of 37.77%, under the same initial conditions. Under optimal current cycle conditions, thermodynamic processes avoid operating close to the CO2 critical point, necessitating less demanding and simpler turbomachinery designs.

A partial heating supercritical CO2 nested transcritical CO2 cascade power cycle for marine engine waste heat recovery: Thermodynamic, economic, and footprint analysis

A novel cascade system, in which the exhaust CO2 residual heat from a partial heating supercritical CO2 (SCO2) power cycle is reclaimed by a transcritical CO2 (TCO2) power cycle, is developed for onboard engine exhaust gas waste heat recovery. Firstly, thermodynamic analysis of the cascade system is carried out with the investigation of pinch points in heat exchangers. Then the influence of the topping cycle on the heat source condition of the bottom cycle is found, and the parametric study of the cascade system is performed from the viewpoint of thermodynamics, economy, and footprint. Further, the system comparison analysis with single-optimization is carried out to prove the superiority of the proposed system. Finally, the three-objective optimization is performed to maximize power output, minimize heat exchanger area per unit power and levelized cost of electricity. The results indicate that by integrating a TCO2 power cycle with the partial heating SCO2 cycle, system thermodynamic performance can increase by 15.35%. The recuperator effectiveness has a great impact on the bottoming cycle heat source temperature. The total heat recovery efficiency of the proposed system is improved by 22.64% and 20.24% respectively, compared with the simple SCO2-TCO2 combined cycle and the recuperative SCO2-TCO2 combined cycle. The multi-objective optimization results of the system performance are 841.84 kW, 0.2028 m2/kW, and 7.434 cent/kWh, respectively, with the 136 °C discharge temperature of the exhaust gas and 29.26% thermal efficiency. These results confirm the cascade system is attractive to waste heat recovery engineering, especially in space-constrained applications.

Comparative analysis on design and off-design performance of novel cascade CO2 combined cycles for gas turbine waste heat utilization

To provide a higher energy conversion efficiency for gas turbines (GT), this paper proposes a novel two-stage cascaded supercritical CO2 and transcritical CO2 (sCO2-tCO2) power cycle for waste heat recovery (WHR) of GT exhaust. A comparative study on system design and off-design performance is conducted between the typical GT-sCO2 cycles and the novel GT-cascaded CO2 (GT-CCO2) cycles based on the detailed steady-state mathematical models and self-built simulation platform. The simple layout (SSBC) and recompression layout (RSBC) are chosen to represent the typical sCO2 Brayton cycle configurations. The simulation results show that the SSBC-tCO2 cycle is more suitable than the RSBC-tCO2 cycle to work as the bottoming cycle for GT since the SSBC-tCO2 cycle can produce higher power with a simpler configuration. Compared with the traditional GT-RSBC and GT-SSBC, the optimal GT-CCO2 cycle (GT-SSBC-tCO2) gains an improvement of 5.32% and 4.32% for the thermal efficiency, and a decrement by 4.08% and 2.42% for the Levelized cost of electricity respectively. The combined GT-CCO2 cycles show superior performance with the variable inlet guide vane modulation during the off-design conditions because this control strategy increases the GT exhaust gas temperature and thus brings about a large improvement of the bottoming cycle output power. These findings could provide references for high efficiently utilizing gas turbine exhaust and verify the commercial viability of the CO2 system.

Reducing the operational fluctuation via splitting CO2 transcritical power cycle in engine waste heat recovery

CO2 power cycle has been considered a promising technology to recover the engine waste heat at a fixed condition. Nevertheless, the large variation of engine conditions may bring about the operational fluctuation, which is reflected on the system performance deterioration. In this study, a splitting CO2 power cycle layout is proposed and optimized from the point of both design and off-design phases to match the large-fluctuated engine conditions. The mass flow through different heat exchangers can be adjusted when the engine heat sources are altered, which means that a higher system degree of freedom can be achieved. Results showed that the system operational fluctuation could be reduced effectively when the operation parameters were deviated from the design phase. Specifically, the total mass flow rate decreased apparently when the engine speed and torque decreased, whereas a moderate decrease was found in the splitting layout thanks to the flexible mass flow through each splitting branch. In addition, the deviation fluctuation of pressure ratio was also reduced especially at higher engine conditions. Average improvements of 12.2%, 17.5% and 8.5% in net generated power, exhaust utilization and exergy efficiency could be achieved, respectively. And the minimum brake specific fuel consumption could be improved by 5.5%.

Theoretical Analysis of a self-condensing CO2 Transcritical Power Cycle with regeneration involving DPS in a Solar Thermal Power Plant

Abstract The choice of heat transfer fluids(HTF) significantly dictates the thermal efficiency of the solar power plant. Presently, molten salt is widely used choice owing to its phase, low cost and non-toxic nature. Along with other alternatives like liquid metals, multiphase fluids, these HTFs are limited to peak operating temperatures ranging from 300-550°C. With the introduction of Dense Particle Suspensions (DPS) as an HTF, the highest operating temperatures in a Solar thermal power plant can reach up to 700°C, offering considerable scope for improving thermal efficiency. Due to the higher average specific heat as compared to the alternatives, CO2 is a promising working fluid in the considered range of moderately high operating temperatures. The cost of the components and size of the power block makes the transcritical CO2 cycle an attractive alternative. The present work analyses the theoretical efficiency of the proposed cycle, with the peak operating temperature ranging from 550°C to 700°C. The effects of the variation in the lower operating pressure and the condensation temperature have also been analysed. It is observed that thermodynamic efficiencies as high as 40% can be reached at the maximum operating temperature. The optimum combination of the lower operating pressure and the condensation temperature is also noted.