Transcritical CO2 Cycle Research Articles

Experimental comparison of cycle modifications and ejector control methods using variable geometry and CO2 pump in a multi-evaporator transcritical CO2 refrigeration system

To reduce the direct global warming impact of refrigerants in HVAC&R applications, low-global warming potential (GWP) refrigerants, including natural refrigerants, have been extensively investigated as alternatives to hydrofluorocarbon (HFC) refrigerants. Among the natural refrigerants, Carbon Dioxide (CO2) offers several advantages, such as excellent transport and thermo-physical properties, being neither toxic nor flammable, and having a low price and high availability around the world. However, the high critical pressure and low critical temperature of CO2 often lead to transcritical operation, resulting in lower efficiency due to the additional compressor power necessary to achieve transcritical operation relative to subcritical HFC cycles. Therefore, a number of cycle modifications are used to enhance the coefficient of performance (COP) of transcritical CO2 cycles to meet or surpass those of HFC cycles. This paper provides a systematic experimental investigation of four such cycle architectures by employing the same multi-stage, two-evaporator CO2 refrigeration cycle test stand, 3 of these configurations in transcritical and 1 in subcritical conditions. The four cycles architectures included intercooling, open economization, an internal heat exchanger and two different ejector control approaches. Specifically, a variable-diameter motive nozzle and a variable-speed liquid CO2 pump located directly upstream of the ejector motive nozzle inlet were analyzed. Based on the experimental data, the maximum COP improvements are 4.64 % and 9.47 % when the ejector and the internal heat exchanger are used, respectively. The CO2 pump, once successfully stabilized, can control the ejector, increase its efficiency by up to 15 % and increase the cooling capacity to a maximum of 6.2 %. Nevertheless, a reduction in COP is measured when the pump is in use; however, unlike the other three different configurations, it was only analyzed under subcritical conditions.

Comparative and optimal study of energy, exergy, and exergoeconomic performance in supercritical CO2 recompression combined cycles with organic rankine, trans-critical CO2, and kalina cycle

The bottom cycle in supercritical carbon dioxide (sCO2) recompression Brayton combined cycle (RCBC) effectively recovers substantial waste heat for electricity generation, enhancing energy utilization. The recovery efficiency is significantly influenced by the bottom cycle configurations and specific working conditions. Besides, implementing these bottom cycles introduces challenges related to increased system dimensions and design complexity. The present study aims to understand and evaluate the characteristics and optimal trade-offs of various bottom cycles, including trans-critical, subcritical, and non-azeotropic mixed working fluids, by establishing 3E (energy, exergy and Exergoeconomic) models for sCO2/tCO2, sCO2/ORC, and sCO2/KC combined cycle systems. To analysis and enhance interpretability, statistical Global Sensitivity Analysis (GSA) alongside unsupervised learning-based Self-Organizing Map (SOM) techniques and parametric analysis are employed. These methods identify key parameters and discern system patterns. Subsequently, the Non-Dominated Sorting Whale Optimization Algorithm (NSWOA) and entropy weight-based Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) are applied to establish a Pareto-optimal decision space and identify compromised ideal design points for each combined cycle. The results quantitatively rank sensitivity for objective variables within the design space in different combined cycles and visually represent system nonlinearity and coupling relationships in 2D weight planes using SOM. The TOPSIS trade-off points based on the Pareto frontier for the three combined cycles are ηth = 45.08 %, cp,tot = 8.5199 $/GJ, ηth = 45.13 %, cp,tot = 8.3739 $/GJ, and ηth = 48.97 %, cp,tot = 8.4783 $/GJ, respectively. Integrating machine learning techniques provides a comprehensive understanding of system patterns, offering valuable insights for decision-makers in the design and optimization of combined cogeneration cycles.

Analysis of green energy-saving retrofit for air-conditioning system in UHVDC converter stations

Abstract The air-conditioning system in ultra-high-voltage direct current (UHVDC) converter stations uses hydrofluorocarbons (HFCs) as refrigerants currently, which pose a certain environmental hazard. Moreover, the cooling system of the converter valves contains a great deal of low-grade thermal energy, with waste heat being directly discharged into the air, resulting in significant energy wastage and air pollution. In light of the functional characteristics and energy wastage situation of current converter station buildings, a green energy-saving retrofit scheme for the air-conditioning system in UHVDC converter stations is proposed. It adopts a transcritical CO2 cycle system for building cooling and introduces an ejector into the system to improve its performance. Simultaneously, the heat pump technology is used to elevate the grade of waste heat from the converter valves, which is then utilized for heating in the control building. Simulation results demonstrate that this scheme is effective in enhancing ecological and environmental benefits while promoting green development of the converter station.

A Novel Fuel-Based CO2 Transcritical Cycle for Combined Cooling and Power Generation on Hypersonic Aircrafts

A Novel Fuel-Based CO2 Transcritical Cycle for Combined Cooling and Power Generation on Hypersonic Aircrafts

Optimal high-pressure correlation for transcritical CO2 cycle in direct expansion solar assisted heat pumps

Heat demand may be met sustainably by a solar-assisted heat pump that CO2 as a refrigerant. This is made possible by the employment of an ecologically benign refrigerant and a renewable energy source that enhances system performance. At the ideal high pressure, the CO2 heat pump running in a transcritical cycle will function at its highest coefficient of performance (COP). In that way, this study aims to present correlations for calculating the optimum high pressure in a CO2 direct expansion solar assisted heat pump (DX-SAHP). In this study a mathematical model for compressor, solar evaporator and gas cooler was developed and validated experimentally. The mean difference between the mathematical model and experimental results are −3.9%. Two new equations are proposed to calculate the optimum high pressure in a CO2 DX-SAHP. The first one considered environment temperature, water outlet temperature and evaporating temperature, which are easy to measure. These facilities the use of correlation to control the heat pump. The second one considered environment temperature, water outlet temperature and solar radiation, which are more suitable for designing CO2 DX-SAHP. A data base with 100 optimum points was used to curve fitting. Another data base with 50 optimum points was used to compare the results obtained from curve fitting and correlations for the optimum high pressure available in the literature. The proposed correlations shown a maximum error lower than 10.2% despite of the correlations available in the literature from which the errors are about 30%.

Performance analysis and prediction of a modified precompression transcritical CO2 power generation cycle

The efficient utilization of biomass energy and the optimal operation of integrated renewable energy sources in virtual power plants have become critical issues to achieve carbon neutrality targets in rural areas. In the rural areas of Northeast China, a large amount of electricity can be generated from biomass. It is very important to develop biomass power generation equipment for this rural area. Due to the high thermal efficiency and its compact system without freezing process like water, CO2 thermal cycle is an appropriate option for this kind of power generation equipment. According to whether the flow is separated, CO2 cycles can be divided into the single flow layouts and split flow layouts. Compared to the single flow layouts, split flow layouts have higher efficiency, but more complex structures and lower heat transfer power per unit area. Considering the climatic characteristics of Heilongjiang Province and the seasonality of straw biomass utilization, this study proposes a modified scheme for the precompression transcritical CO2 cycle in the single flow layout to meet the simple structure requirement of the small power plants in cold areas like Northeast China. Through energy analysis, the study calculates the thermal efficiencies of the two cycles when the inlet pressure of the main compressor is 5 MPa, the turbine inlet temperature is 550 °C and the pressure ratios of the main compressor are in the range of 4.5–5.5. The optimal efficiency of the traditional precompression cycle is 43.55 % when the main compressor pressure ratio is 5.5, while the modified precompression cycle is 44.86 % when the pressure ratio is 5.0266. The modified precompression cycle has a higher efficiency than the traditional precompression cycle at a lower pressure ratio of the main compressor.

Energy and Exergy Analysis of Transcritical CO2 Cycles for Heat Pump Applications

Energy and Exergy Analysis of Transcritical CO2 Cycles for Heat Pump Applications

Assessment of carbon dioxide transcritical cycles for electrothermal energy storage with geological storage in salt cavities

Carbon capture, utilisation, and storage (CCUS) technologies are envisaged as critical actors in the energy transition and climate change mitigation framework. Despite the recent advances in CCUS implementation, there is still significant scope for their growth and improvement. On the other hand, developing new large-scale energy storage systems is a key factor for the massive deployment of renewable energy systems. Technologies such as the electrothermal energy storage system based on carbon dioxide transcritical cycles incorporating geological storage (CEEGS) can contribute to both fields. Preliminary studies have shown that the integrated storage system can operate with roundtrip efficiencies exceeding 50%, injecting over 1 million tons of captured carbon dioxide (CO2) annually. It is an early-stage technology with open challenges for successful development, such as an adequate balance between the high- and low-temperature thermal storage reservoirs or the operation definition between the electrothermal system and the carbon dioxide injection and recovery. Underground pressures will oscillate with the charge/discharge process, with operational implications and constraints. Advancing in the concept requires the definition of optimised operation. This work presents a novel analysis of the adaptation of CO2 transcritical cycles to include injection and production processes with the specific scenario of salt cavities. It considers new modifications to adapt the transcritical cycles in closed-mode operation to the conditions required by the salt cavities injection and production processes. Different case studies are evaluated, evaluating the impact of injection and geological storage, production and surface storage, and reinjection into the geological formation. The analyses show roundtrip efficiencies within the range of 49.1–73.0% when the injection, production, and reinjection processes are executed.

Thermo-economic examination of ocean heat-assisted pumped thermal energy storage system using transcritical CO2 cycles

In response to the challenges posed by the growth of renewable energy electricity to the security of the power grid, energy storage technology has rapidly developed in recent years. Pumped thermal energy has been recognized as an attractive massive energy storage technology with the superiority being independent of site. An ocean heat-assisted pumped thermal energy storage system using transcritical CO2 cycles is proposed in this study. State-of-the-art thermo-economic models are developed and described to simulate the system processes and conduct cost estimates. The originality of this system lies in the ability to effectively improve its efficiency by configuring seawater at different depths at the low temperature end of the system. Research has found that, if the seawater used in the charge process always comes from the surface of the ocean, the system efficiencies are 48.1 %, 61.3 % and 66.4 % respectively when the depth of seawater during the discharge process are 50 m, 100 m and 1000 m. When the seawater employed in the entire system comes from 1000 m underwater, the efficiency of the system is only 53.5 %. In addition, for the 50 MW/10 h system, the minimum value of levelized cost of storage is 0.143 $/kWh. The uncertainty analysis of cost for the proposed system is carried out using the Monte Carlo method. Results show that with the charge/discharge duration ratio rising from 4 h/10 h to 10 h/4h, the total capital cost falls from 245 ± 76 M$ to 102 ± 34 M$ and the levelized cost of storage reaches the minimum value when this ratio is 1. The larger the energy storage capacity, the lower the levelized cost of storage. In summary, the system has shown satisfactory results in both thermodynamics and economic performance, and further research is worthwhile in the future.

SENSITIVITY ANALYSIS ON HEAT EXCHANGERS SELECTION AND ITS INFLUENCE ON THE EFFICIENCY OF A TRANSCRITICAL CO2 CYCLE WITH INDIRECT DEDICATED MECHANICAL SUBCOOLING

This work evaluates through numerical modeling, the efficiency of an air-to-water heat pump that operates using a transcritical CO2 cycle assisted by an indirect dedicated mechanical subcooling (IDMS) cycle, with R1234ze(E) as the refrigerant. The research aims to analyze how the size of the heat exchangers used as the evaporator and the condenser for the IDMS cycle affects the overall efficiency of the system. A numerical model previously developed in Matlab® is used to modify the refrigerant and compressor of the IDMS cycle and to determine the optimal number of evaporator and condenser plates that maximize the efficiency of the whole system. The compressor model is derived from manufacturer correlations, whereas the models for the heat exchangers are obtained through a parametric study developed using IMST-ART® to characterize their operating parameters based on their number of plates and the inlet conditions of water and refrigerant. Once the models are developed, they are integrated into the model of the overall system to find the heat exchanger sizes that maximize the efficiency of the system.

An efficient biogas-base tri-generation of power, heating and cooling integrating inverted Brayton and ejector transcritical CO2 cycles: exergoeconomic evaluation

An efficient biogas-base tri-generation of power, heating and cooling integrating inverted Brayton and ejector transcritical CO2 cycles: exergoeconomic evaluation

An investigation of the thermo-hydraulic performance of a novel double tube heat exchanger with variable cross-section inner tube for trans-critical CO2 cycle

Double tube heat exchangers (DTHE) is one of the most significant components in the trans-critical CO2 cycle system, whose performance significantly affects the efficiency and compactness of the system. This work aims to improve the heat transfer performance of CO2 at supercritical pressure in DTHE. In this paper, the cylindrical inner tube of traditional DTHE is changed into the diverging/converging tube as an innovative design without increment of its heat transfer areas. The thermal–hydraulic performance of traditional DTHE with uniform cross-section inner tube (UIT), DTHE with diverging (DIT)/converging (CIT) inner tube were numerically investigated by using SST k-ω turbulence model. The results show that under the same operating conditions, the DIT and CIT can effectively enhance the heat transfer compared with the UIT. Utilization of DIT and CIT instead of UIT, the maximum increase of the total heat transfer coefficient are 27.3 % and 24.2% can be obtained, respectively. However, the pressure drop of DIT and CIT also increases accordingly. The performance evaluation criterion (PEC) of DIT is always greater than 1, and the maximum is 1.16. In most cases, the PEC of CIT is greater than 1, but with the increase of SCO2-side mass flow rate, it gradually decreases to less than 1. The comprehensive performance of DIT is the best. The heat transfer mechanism is analyzed from the aspects of flow field distribution, thermos-physical property and field synergy theory. The large turbulent kinetic energy near the wall, high thermal conductivity and large specific heat of SCO2 in the tube are the main reasons for the enhanced heat transfer of DIT and CIT. Finally, the influence of SCO2-side pressure and mass flow rate on local heat transfer coefficient hCO2 in DIT are also further studied. This study can provide some guidance for the design and optimization of DTHE.

Performance advantages of transcritical CO2 cycle in the marine environment

Low-temperature seawater is far away from CO2 critical temperature, which has significant impacts on the performance of CO2 closed cycle in the marine environment. The temperature adaptability of CO2 closed cycle to the marine environment and its performance remain open issues. In this paper, we compare performance advantages of transcritical/supercritical CO2 cycle based on thermodynamic and dynamic models. Compared with supercritical CO2 Brayton cycle, transcritical gas-phase CO2 Brayton cycle exhibits higher thermal efficiency and specific power at low cycle maximum pressure, and its reduced heat load and thermal inertia of regenerator facilitate cycle rapid response. However, transcritical liquid-phase CO2 Brayton cycle and transcritical CO2 Rankine cycle demonstrate higher thermal efficiency and specific power at high cycle maximum pressure. Lower compressor inlet temperature causes CO2 pseudo-critical point to migrate into regenerator, and the intersection point of cp curves of CO2 on both sides is located within regenerator. This can lead to pinch point and non-physical design within regenerator that inhibits cycle response. It can be avoided by adjusting cycle matching parameters so that the temperature corresponding to intersection point is lower than regenerator hot side outlet temperature. This study provides insights into performance advantages of transcritical CO2 cycles in marine environments.

Thermodynamic analysis of an enhanced ejector vapor injection refrigeration cycle for CO2 transcritical operation at low evaporating temperatures

The main drawback associated with CO2 refrigeration systems is related to their performance reduction during transcritical operation at warm climate conditions, which may be compensated by better cycle architectures such as the split-cycle with subcooling or the flash-tank configuration, among others. Specifically, the use of standard gas-ejectors together with parallel compressors provides even better efficiency improvements, not being able to use them with low-temperature evaporators to prevent the triple point inside the ejector. This paper proposes an enhanced cycle with a gas ejector for two-stage compressor architectures with vapor injection from the flash-tank, which is able to operate at low evaporating temperatures and that provides a greater performance improvement the more severe the climate conditions are. The methodology conducted is based on a thermodynamic analysis that includes parametric evaluation and cycle optimization, comparing the results to a conventional CO2 transcritical cycle with flash-tank and dynamic vapor injection architecture. The main results show that a maximum Coefficient of Performance improvement of 17.5% is achievable for transcritical operation at -40 °C evaporating temperature. The compressor displacement capacity required with the enhanced cycle is up to 9% lower for the same refrigeration demand, reducing the electrical consumption as well as the compressor expenditure. Moreover, greater vapor injection mass flow rates are obtained by the gas-ejector injection with discharge temperature reductions up to 18%, enhancing the system reliability.

Thermodynamic analysis of rotary pressure exchanger and ejectors for CO2 refrigeration system

Natural refrigerant CO2 has become a viable choice for refrigeration units for land-based and offshore applications due to its environment-friendly nature and compactness. The CO2 transcritical cycle allows operating in colder climates and in elevated ambient temperature conditions and with significant heat recovery. However, the energy efficiency of the system suffers at higher heat rejection conditions mainly due to expansion losses. This work theoretically investigates and proposes the implementation of a new expansion work recovery device, a pressure exchanger (CO2-PX), for the transcritical CO2 cycle. The numerical models are developed in the Engineering Equation solver (EES) to compare the performance of CO2-PX configuration with standard booster-, parallel-, and ejector- configurations for various conditions. The analysis is carried out for the evaporation temperature of 0 ℃ and the gas cooler outlet temperature of 33 ℃ to 37 ℃. The results indicate that the coefficient of performance (COP) is improved by 17.7–23.5%, 16.3–20.3%, and 2.4–5.5% to the standard booster, parallel and ejector configurations, respectively, at the investigated conditions.

Thermodynamic and optimization analysis of a fuel cell-based combined cooling, heating, and power system integrated with LNG-fueled chemical looping hydrogen generation

Chemical looping combustion technology is essential to achieve efficient decarbonized electricity generation for fossil-fueled power plants. However, the existing chemical looping hydrogen generation (CLHG)-driven cogeneration systems have complex refrigeration units and energy-consuming carbon capture. This study proposes a novel solid oxide fuel cell (SOFC)-based combined cooling, heating, and power (CCHP) system, which is integrated with the CLHG and fueled by a liquid natural gas (LNG) regasification process. The system is designed to enhance the overall performance by directly recovering LNG cold energy for cooling and liquefying the resulting CO2. The energy and exergy performance of the system under the baseline design case are evaluated. Subsequently, we discuss the impact of key design parameters on system performance and weigh between cooling/heating power and net power generation for the system dominated by the combined heat and power (CHP)/combined cooling and power (CCP) mode. Our findings reveal that the proposed system exhibits an electrical efficiency of 66.92 % and an exergy efficiency of 53.94 % under the baseline design case. The exergy loss ratio for condenser1 is identified as the highest among all components, accounting for 29.50 %. The constituent unit with the largest exergy loss contribution is the LNG regasification unit, followed by the SOFC unit, CLHG unit, transcritical CO2 cycle unit, and heating unit. More hydrogen needs to be replenished when fuel flow in the CLHG and fuel utilization in the SOFC are used to improve system performance. The optimal electrical and exergy efficiencies of the system predominantly designed in CCP mode surpass those in CHP mode by 4.27 % and 0.57 %, respectively. The results can guide potential applications of CLHG-based cogeneration systems.

A comprehensive 6E analysis of a novel multigeneration system powered by solar-biomass energies

This study presents a novel multigeneration system comprising five different cycles driven by a combination of solar and biomass energy resources. This system includes a modified gas turbine cycle, a trans-critical CO2 cycle (TCO2), a steam Rankine cycle, a biomass-fueled boiler, and a solar collector. The gas turbines exhibit limited thermal efficiency, primarily due to substantial heat losses. Consequently, the TCO2 cycle and the steam Rankine cycle are integrated into the system to improve performance. An innovative aspect of this study is the utilization of a unique configuration of transcritical CO2 and steam Rankine cycles, combined with biomass and solar energies, aimed at maximizing energy recovery and minimizing waste. Furthermore, a proton membrane electrolyzer (PEME) is integrated into the system for hydrogen production, along with a proton membrane fuel cell (PEMFC) to increase electricity generation. In this comprehensive analysis, in addition to energy and exergy analyses, exergoeconomic, exergoenvironmental, emergoeconomic, and emergoenvironmental analyses are conducted. The results indicate that the proposed system achieves notable energy and exergy efficiencies of 20.2 % and 15.2 %, respectively. The total investment cost rate for the system equipment is 0.007554 $/s and the system's environmental impact is 0.0008611 pt/s. The results of the emergy analysis reveal that the emergoeconomic values of the streams are lower than the corresponding environmental values, primarily due to the higher environmental impact throughout the components' life cycle compared to their economic value.

Thermodynamic and environmental analyses of a hydrogen-enriched natural gas oxy-fuel combustion power plant

Hydrogen-enriched natural gas is regarded as a promising low-carbon fuel in clean energy field. A hydrogen-enriched natural gas oxy-fuel combustion power plant is established to study the effect of hydrogen content on system thermodynamic performance. Three different diluents including recycled flue gas (RFG), H2O, and CO2 are used to moderate the high combustion temperature. The transcritical CO2 cycle is adopted to efficiently utilize flue gas waste heat. As the hydrogen blending ratio increases from 0 to 0.5, the efficiency of the O2/RFG system, O2/H2O system and O2/CO2 system increases by 1.45%, 1.13% and 1.16%, respectively. The RFG system is preferable to realize high efficiency that can reach 50.23%. In the proposed systems, the carbon capture rate (CCR) decreases while specific CO2 emission (SECO2) increases with the hydrogen addition. In O2/RFG system, the CCR reaches a minimum of 99.09% when SECO2 is 3.81. The study results can have a positive influence on the utilization of hydrogen energy.

Thermodynamic assessment of an innovative biomass-driven system for generating power, heat, and hydrogen

Due to the widespread use of multi-generation systems utilizing renewable energy sources and the growing global demand for such systems from both economic and environmental considerations, numerous researchers have focused on the design and evaluation of their performance. To this end, this research presents a biomass-based multi-generation system with an innovative and practical design that can generate electricity, heat, and hydrogen. This system includes a modified gas turbine cycle, a supercritical CO2 (SCO2) cycle, a transcritical CO2 (TCO2) cycle, a proton exchange membrane (PEM) electrolyzer, and a PEM fuel cell unit. This study aims to evaluate the impact of various biomass sources (paper, wood, paddy husk, and municipal solid waste) on the system performance. The proposed system has been analyzed using the first and second laws of thermodynamics. This system uses the maximum capacity to produce power, heat and hydrogen. A fuel cell unit has been used to consume hydrogen and generate more electricity. In the basic mode, the system has energy and exergy efficiencies of 47.89% and 32.26%, respectively, and can produce 2.74 kg/h of hydrogen. The biomass fuel consumption rate within the system is 0.055 kg/s. The overall exergy destruction of the system amounts to 1240 kW, with the biomass boiler and the condenser being the components that experience the greatest exergy destruction, registering values of 535.5 kW and 432.3 kW, respectively. Notably, employing municipal waste as biomass increases the system's exergy efficiency to 33.16%.

Investigation on the integration of supercritical CO2 cycle with natural gas oxy-fuel combustion power plant

The utilization of LNG cold energy is promising to offset the energy penalty of oxy-fuel combustion technology and achieve efficient carbon emissions reduction. In this paper, natural gas oxy-fuel power systems utilizing LNG cold energy are investigated. Supercritical and transcritical CO2 cycles are adopted to deeply recover flue gas waste heat. Results show that the thermal efficiency of the supercritical CO2 recompression cycle is 6% higher than that of the regenerative cycle, whereas the net power output of the recompression cycle is lower. With transcritical CO2 cycle as the secondary subsystem, the efficiency of both integrated systems is enhanced by approximately 9%. Meanwhile, 95% CO2 is captured with the power consumption of 0.07 kWh/kg-CO2. The total investment costs of the integrated regenerative and recompression cycle are 53.96 M$ and 54.62 M$. The levelized cost of electricity of the two integrated systems is 3.21 $/MWh and 3.24 $/MWh, indicating great financial availability. Therefore, applying supercritical and transcritical CO2 cycles contributes to efficient energy conversion and carbon capture. The research could provide meaningful information on the oxy-fuel combustion power system integrated with supercritical and transcritical CO2 cycles from thermodynamic and economic insights.