Transcritical CO2 Research Articles

Performance analysis of a modified dual-ejector and dual-evaporator transcritical CO2 refrigeration cycle for supermarket application

This paper proposes a modified dual-ejector and dual-evaporator transcritical CO2 refrigeration cycle for supermarket application. Based on the conventional dual-evaporator transcritical CO2 refrigeration cycle, the modified cycle introduces two ejectors and a flash tank. The first liquid-vapor ejector sucks partial throttling flash vapor flowing to the low-temperature evaporator into the high-temperature evaporator; the second vapor-vapor ejector takes the vapor from the high-temperature evaporator as the primary flow to entrain the vapor from the low-temperature evaporator, and then the compressor suction pressure is lifted. Furthermore, the introduction of the first ejector increases the primary flow of the second ejector, and then improves the ability of the second ejector to lift the suction pressure of the compressor. The performance comparison between the two cycles is conducted by adopting energy and exergy analysis methods, and the efficiencies of the two ejectors are also analyzed. The comparison results demonstrate that the use of dual-ejector reduces the compressor pressure ratio by up to 19.1% under a typical working condition, and the improvements of the COP and exergy efficiency could reach up to 15.9–27.1% and 15.5–27.5% under all given working conditions. In addition, the modified cycle has an optimal gas cooler pressure of around 8.15 MPa, which is lower than the 8.3 MPa of the conventional cycle. The performance comparison results state clearly the energy-saving potential of the modified cycle and its application prospect in supermarket refrigeration.

Two-objective optimization of a hybrid solar-geothermal system with thermal energy storage for power, hydrogen and freshwater production based on transcritical CO2 cycle

In the light of increasing negative impacts on environment of fossil fuels, development of renewable energy utilization is a high priority. In this regard, hybridization of geothermal and solar energies (as two types of abundant renewable resources) has been proved to be a promising combination for renewable-based power generation systems. For power generation from low-grade renewable resources, the transcritical CO2 (TRCC) power cycle has been considered as an ambitious competitor against ORC and Kalina cycle. This paper aims at development and proposal of a novel efficient trigeneration system based on TRCC cycle for production of power, hydrogen, and freshwater driven by hybrid solar-geothermal energies. In proposed system, thermoelectric generators and HDH desalination unit are employed for waste heat utilization of TRCC cycle. Thermodynamic and thermoeconomic analyses and two-objective optimization are conducted and monthly performance of the system is estimated for real environmental data. Results showed that, the system yields highest values of power, hydrogen and freshwater in May, respectively as 1286 kW, 1.989 kg/h, and 13.38 m3/day. Under the optimized conditions, the system yields energy efficiency of 23.35% with a unit product cost of 17.07 $/GJ. Also, the results revealed efficiency improvement via increasing the solar energy share. In addition it is shown that, developed system in this work can yield higher efficiency compared to a similar trigeneration system based on the Kalina cycle.

Ejector optimization and performance analysis of electric vehicle CO2 heat pump with dual ejectors

The performance of fixed ejector is limited by working conditions, so it is difficult to meet the operating requirements of electric vehicle heat pump system under varying conditions and wide temperature range. In this paper, a transcritical CO2 refrigeration cycle with dual ejectors in parallel (DEP) for electric vehicle heat pump system is established and its operation condition weight statistics method in all climate is proposed. On this basis, a fixed ejector optimization method based on the genetic algorithm is proposed, which takes the increment of integrated part load value (ΔIPLV) compared to the conventional transcritical CO2 refrigeration cycle (CON) system as the evaluation index. The performance analysis results show that under the cooling conditions, the coefficient of performance (COP) of DEP system with the optimized ejector for cooling is increased by 17.32%–23.42% compared to the CON system, and the COP of the transcritical CO2 refrigeration cycle with a single ejector (SEJ) system is increased by 7.31%–9.47%. In the heating mode, the COP of DEP system with the optimized ejector for heating is increased by 18%–19.79%, while the COP of SEJ system with the unoptimized ejector is decreased by 0.07%–2.43%.

Rational assessment and selection of air source heat pump system operating with CO2 and R407C for electric bus

Heat pump technology has gained surging interests due to its abilities in reducing the battery burden of the electric bus. Here, three heat pump systems for an 8 m bus are compared, including a R407C system, a basic transcritical CO2 system (CO2-BASE), and a transcritical CO2 system with an expansion-compression integrated machine (CO2-Advanced). The results demonstrate that the optimal ratio of the theoretical volume of the expander to the compressor in expander-compressor machine can improve the coefficient of performance (COP) of the CO2-BASE system in all cooling conditions, with a range of 0.165–0.190. The COP of the CO2-Advanced system has 13.59% improvement than that of the R407C system at most in cooling mode. In addition, the performances of the three systems run in five typical climates in China are compared. The comparison result shows that the CO2-BASE system is more suitable for Severe Cold Zone and Mild Zone, and the R407C system performs better in Hot summer and Warm winter Zone. However, the CO2-Advanced system, which has the best energy-saving effect in all climate zones in China, has a great potential to replace the existing R407C system for electric buses.

Energy, exergy and exergoeconomic evaluation of the air source transcritical CO2 heat pump with internal heat exchanger for space heating

• The system performance of CO 2 heat pump with internal heat exchanger is compared. • The system performance is evaluated with energy, exergy and exergoeconomic analysis. • The COP and exergy efficiency is significantly enhanced with the proposed system. • The cost per unit of exergy production is obviously reduced with the proposed system. • The internal heat exchanger effectiveness and sensitivity analysis are conducted. Currently, the air source heat pump is widely used for space heating. Although the CO 2 has great potential in the refrigeration system, the basic transcritical CO 2 heat pump performance is restricted by the huge throttling irreversibility caused by the higher gas cooler outlet temperature. The internal heat exchanger has been investigated extensively from the energetic perspective. However, little information is available about the exergy analysis, let alone the exergoeconomic analysis. In this paper, the transcritical CO 2 heat pump with/without the internal heat exchanger is studied with the energy, exergy, and exergoeconomic analysis. Results show that the optimal coefficient of performance can be increased to 2.96 and 2.17 in the different heating terminals. The enlarged internal heat exchanger effectiveness is also beneficial to reduce the optimal discharge pressure and increased the system coefficient of performance. The throttling valve irreversibility is decreased from 0.067 to 0.145 kW·kW −1 to 0.040–0.071 kW·kW −1 . With total irreversibility decrement, the exergy efficiency can be effectively improved, resulting in the 55.65%-73.42% reduction of the throttle valve cost rate. Considering the CO 2 compressor investment, its total exergy cost rate increment is reached up to 34.06%-50.21%, leading to the higher exergoeconomic factor. However, the sensitivity analysis results show that the cost per unit of exergy production is more sensitive to the electricity price. Moreover, the optimal cost per unit of exergy production reduction is declined by 58.71% with the return water temperature of 30 °C.

Design and commissioning of a modular multi-stage two-evaporator transcritical CO2 test stand

As the adverse effects of global warming become increasingly apparent, the need for low-GWP solutions in the HVAC&R industry is its highest in decades. Alternative working fluids, including natural refrigerants such as Carbon Dioxide (CO2), and high efficiency systems are ways to mitigate the negative environmental impact of refrigeration systems. Researchers have focused on investigating several transcritical CO2 cycles for supermarket and transport applications. Yet, a systematic experimental comparison between multiple advanced cycle architectures is still lacking. To this end, a novel test stand capable of comparing two methods of expansion as well as open-economization at one or both evaporator pressure levels, allowing six cycle configurations that represent much of the state-of-the-art has been developed. Cycle designs, thermodynamic modeling for component sizing, and experimental validation of test stand operation and energy balances are discussed in-depth. Limitations and modifications to the test stand are discussed, as are next steps.

Refrigerant distributions and dynamic migration characteristics of the transcritical CO2 air conditioning system

Research on refrigerant mass migration and distribution could not only provide deep understanding of the system performance enhancement, but also contribute to searching the high risk components caused by the Boiling Liquid Expanding Vapor Explosion (BLEVE) due to the high operation pressure of transcritical CO2 system. An experimental bench and a mathematical model of the transcritical CO2 automotive air-conditioning system were set up to explore the refrigerant migration characteristics at different operation conditions. Simulated results of the system were validated by experiments. The distribution characteristics of the refrigerant under different charge were studied and the refrigerant charge suggestions were given. Then, under the appropriate refrigerant charge, the migration characteristics of the refrigerant in the automobile air conditioning system were simulated in the process of startup and shutdown of the compressor and conversion of cooling mode. Furthermore, influence of the parameters, such as ambient temperature, air flow rate, compressor speed on the characteristics of the refrigerant distribution in the air-conditioning system were studied. The results could provide reference for the system design and system safety.

Numerical investigation on the performance of transcritical CO2 two-phase ejector with a novel non-equilibrium CFD model

Flashing flow in the transcritical CO2 ejector was simulated using a novel non-equilibrium evaporation-cavitation CFD model. The novel model was verified using the experimental results of three ejectors with nozzle diverging angles (NDAs) of 0°, 2.00° and 6.70°. Furthermore, the effects of dimensions of mixing chamber and diffuser on the ejector performance were disclosed. The results suggested that prediction errors of secondary flow mass flow rate using the proposed model were less than 9%, and the maximum error of primary flow mass flow rate was within the range of 15%, which was much smaller than that of −34.55% predicted by homogenous equilibrium model. The results also showed that the mass transfer rates increased with increasing NDAs, and the maximum mass transfer rate of evaporation was approximately 3–13 times that of cavitation as the NDAs increased from 0° to 6.70°, proving evaporation dominates the flashing process in the CO2 ejector. Besides, the results showed that the entrainment ratio increased first and then decreased with increasing the mixing chamber width, mixing chamber length and diffuser angle, and the entrainment ratio was improved by 36.55%. This study is helpful to understand flashing mechanisms inside the CO2 ejector and improve the ejector performance.

Performance Optimizations of the Transcritical CO2 Two-Stage Compression Refrigeration System and Influences of the Auxiliary Gas Cooler

To optimize the performance of the transcritical CO2 two-stage compression refrigeration system, the energy analysis and the exergy analysis are conducted. It is found that higher COP, lower compression power, and less exergy destruction can be achieved when the auxiliary gas cooler is applied. Moreover, the discharge temperature of the compound compressor (HPS) can be reduced by decreasing the temperature at the outlet of the auxiliary gas cooler (Tagc,out). When the Tagc,out is reduced from 30 to 12 °C, the discharge temperature of the compound compressor (HPS) can be decreased by 13.83 °C. Furthermore, the COP and the exergy efficiency can be raised by enhancing the intermediate pressure. Based on these results, the optimizations of system design and system operation are put forward. The application of the auxiliary gas cooler can improve the performance of the transcritical CO2 two-stage compression refrigeration system. Operators can decrease the discharge temperature of the compound compressor (HPS) by reducing the Tagc,out, and increase the COP and the exergy efficiency by enhancing the intermediate pressure.

Numerical study on sulfur-bearing natural gas oxy-fuel combustion power plant

• A sulfur-bearing natural gas power plant adopting transcritical CO 2 cycle is built. • Oxy-fuel combustion technology is employed to reach near zero CO 2 emission. • The effect of sulfur content of fuel and combustion diluent are studied. • The efficiency of the proposed systems with recycled flue gas can reach 67.59%. Waste heat recovery technology can significantly improve the efficiency of oxy-fuel combustion system. The restriction of the acid dew point of the flue gas is essential to be considered due to the low-temperature corrosion during the waste heat recovery process. In this paper, a sulfur-bearing natural gas oxy-fuel combustion power plant is established to explore the effect of sulfur content on the system performance. Sulfur content (taking H 2 S as an example) in natural gas is set to 1 ppmv-1 vol%. Three different diluents of recycled flue gas, H 2 O and CO 2 are adopted to moderate the combustion temperature. Transcritical CO 2 cycle is introduced to efficiently utilize flue gas waste heat. As H 2 S content increases from 1 ppmv to 1 vol%, the power generation efficiency of recycled flue gas system, H 2 O system and CO 2 system decreases by 0.61%, 1.15% and 1.65%, respectively. When H 2 S is 1 ppmv, the energy efficiency can reach 67.59% in recycled flue gas system, 18.74% and 12.09% respectively higher than H 2 O and CO 2 system. The recycled flue gas system is preferred to be applied in the oxy-fuel power generation system driven by sulfur-bearing natural gas, which can improve energy efficiency and reduce environmental impact.

Cooling performance enhancement for the automobile transcritical CO2 air conditioning system with various internal heat exchanger effectiveness

• CO 2 system with internal heat exchanger (IHX) were investigated. • An experiment to change IHX under different conditions was designed. • Effects of IHX on system performance and parameters were studied. • Effect of various IHX under a wide working condition was analyzed. • Energy and exergy analyses with different IHX were conducted. Carbon dioxide (CO 2 ) is one of the promising refrigerant substitutes in mobile air conditioning system. To enhance the cooling performance, this paper presented an experimental investigation on the influence of the various internal heat exchanger (IHX) effectiveness on a transcritical CO 2 mobile air conditioning system. An experimental apparatus with regulatable IHX effectiveness was set up and tested. The effects of IHX effectiveness on the cooling capacities and coefficient of performance (COP) were studied under various outdoor air temperatures (30, 35 and 40 ℃) and compressor speeds (2000 to 7000 rev·min −1 ). Optimal discharge pressure and cycle state parameters with various IHX effectiveness were analyzed. Based on exergy perspective, the irreversibility of the main components and the cycle were discussed to give further reference for system optimization. Results showed that using IHX enhanced the COP of CO 2 system by the rates from 14.5% to 18.5% at various outdoor air temperatures. With increasing IHX effectiveness, the COP enhancement decreased, and the discharge temperature easily increased to its restriction value. The compressor and gas cooler occupied the main exergy losses of the system with IHX. The results could provide reference for the design and optimization of the CO 2 mobile air conditioning system.

Comparison between reverse cycle and hot gas bypass defrosting methods in a transcritical CO2 heat pump water heater

• Reverse cycle defrosting method showed lower power consumption and shorter time. • System returned to steady heating faster with hot gas bypass defrosting method. • Averaged heating capacity with reverse cycle defrosting method was 8.24% lower. • Averaged COP with the reverse cycle defrosting method was 4.61% higher. Defrosting is an important issue that should be considered when an air source heat pump is operating in a cold climate. For the transcritical CO 2 heat pump (TCHP) water heater, the high operating pressure affects the application of defrosting methods. In this paper, the reverse cycle defrosting (RCD) and hot gas bypass defrosting (HGBD) methods were experimentally investigated in an air source TCHP water heater. The dynamic defrosting characteristics, influences on system operation and overall energy performances of the two methods were analyzed. The results suggested that the RCD method presented more complex parameter variations, lower power consumption, shorter defrosting time and larger influence on the system operation; while the HGBD method displayed less impact on the system heating capacity. The electric energy used for the RCD method was only 17.5% of that for the HGBD method. Moreover, the averaged COP with the RCD method was higher than that with the HGBD method by 4.61%. In summary, the RCD method showed evidently higher energy performance and could be a promising solution for the TCHP water heater, and the HGBD method demonstrated its robustness but had a limited energy performance.

Advanced exergy and exergoeconomic analyses and a case study of a novel trans-critical CO2 cycle with pressurization process for hot dry rock

Using hot dry rocks instead of fossil fuels can greatly alleviate the problem of environmental pollution. However, the low thermoelectric conversion efficiency of dry hot rock resources may hinder its commercial development. Therefore, an in-depth analysis of the operating efficiency and the relationship between the components in the hot dry rock power generation system can provide a basis for improving the system performance. In this study, the influence of the initial operating parameters on the thermodynamic performance of a trans-critical CO2 cycle with pressurization process applied to the utilization of hot dry rock resources is analyzed, so as to make the system more universal. On the other hand, the thermo-economic performance of the system is evaluated by traditional and advanced exergy/ exergoeconomic analysis respectively. The results show that the increase in production well-head temperature and CO2 capture pressure, the decrease in the CO2 capture temperature will improve the thermodynamic performance of the system to a certain extent. In general, the ratio of endogenous avoidable exergy destruction in trans-critical CO2 generation cycle and CO2 pressurization process is 46.48% and 34.41%, respectively. The exergy efficiency of condensers and intercoolers in the system is low, which have high performance improvement potential. The exergoeconomic analysis shows that the investment cost of each component is the main factor affecting the economic performance. Meanwhile, advanced exergy/ exergoeconomic analysis provides the valuable information for a clearer understanding of the interactions between different components of the system and the potential for improvement.

Experimental and numerical investigation on the dynamic characteristics of a lab-scale transcritical CO2 loop

Supercritical carbon dioxide (sCO2) Brayton cycle attracts much attention due to its high efficiency, compactness and low water consumption. In a sCO2 cycle, a transcritical CO2 (tCO2) dynamic looping is unavoidable during processes of startup, shutdown and unexpected conditions, which brings many challenges in the system design and operation due to the drastic physical property change of CO2 near the critical point, and its effects on the system dynamic characteristics are still not clear. In this study, we carry out experimental and numerical studies to explore the dynamic characteristics of a simplified tCO2 loop. The numerical model is developed by combining heat exchangers, a pump and a back-pressure valve with a detailed consideration of the coupled CFD and heat transfer processes. We validate the model by comparing the simulation results with the experimental data and obtain a small root-mean-square error (<3%). We investigate the system response and dynamic characteristics in the case studies, where the developed model has a higher accuracy in capturing the dynamic characteristics compared with the experimental sensors, and the change of ambient temperature resulted in a significant fluctuation of gas source pressure between 3.8 and 8.0 MPa. We also observe a pressure overdose of > 1.0 MPa in a short time with the frequency rising from 20 Hz to 50 Hz, which is about 40% of the target pressure change. We find a large acquisition time gap could provide stable results but may lost transient characteristics during the operation.

Integration of energy storage systems based on transcritical CO2: Concept of CO2 based electrothermal energy and geological storage

Energy storage systems are crucial for the massive deployment of renewable energy at a large scale. This paper presents a conceptual large-scale thermoelectrical energy storage system based on a transcritical CO2 cycle. The concept is developed through the analysis of three high-efficiency systems: renewable energy storage using a thermoelectric energy storage system based on a reversible heat pump; a CO2 storage system; and novel integration of energy storage using a reversible heat pump and geological injection of CO2. The latter system efficiently integrates energy and CO2 storage, taking advantage of the synergies between the operational requirements of both systems. The system uses CO2 captured in stationary sources as a working fluid to store energy from renewables. The energy is stored and recovered in geological formation and heat/cold tanks, with energy storage based on sensible or latent heat of ice and water. A fraction of the CO2 is expected to be permanently sequestered in the geological formation. The analysis of the time evolution of the system, under different operation profiles, shows the interest of the concept as a feasible integration for energy storage and CO2 capture based on renewable energy, with an electric-to-electric efficiency varying between 40 and 50 %.

Energy and Exergy Analysis of the Air Source Transcritical CO2 Heat Pump Water Heater Using CO2-Based Mixture as Working Fluid

Given the large demand nowadays for domestic hot water, and its impact on modern building energy consumption, air source transcritical CO2 heat pumps have been extensively adopted for hot water production. Since their system efficiency is limited by significant irreversibility, a CO2-based mixture could offer a promising drop-in technology to overcome this deficiency without increasing system complexity. Although many CO2 blends have been studied in previously published literature, little has been presented about the CO2/R32 mixture. Therefore, a proposed mixture for use in transcritical CO2 heat pumps was analyzed using energy and exergy analysis. Results showed that the coefficient of performance and exergy efficiency variation displayed an “M” shape trend, and the optimal CO2/R32 mixture concentration was determined as 0.9/0.1 with regard to flammability and efficiency. The irreversibility of the throttling valve was reduced from 0.031 to 0.009 kW⋅kW−1 and the total irreversibility reduction was more notable with ambient temperature variation. A case study was also conducted to examine domestic hot water demand during the year. Pure CO2 and the proposed CO2 blend were compared with regard to annual performance factor and annual exergy efficiency, and the findings could provide guidance for practical applications in the future.

Experimental investigation and comparison on heating performance characteristics of two transcritical CO2 heat pump systems

An experimental investigation and comparison of the heating performance of the transcritical CO2 single-stage and vapor injection cycles were conducted. The effects of operating parameters on the heating performance of the two cycles were investigated. The experimental results showed that the heating capacity of the vapor injection cycle was 7.0% higher and the discharge temperature of the refrigerant was 7.7–11.8 ℃ lower than that of the single-stage cycle under the same condition. Besides, both the compressor input power and heating capacity were increased with the increasing temperature of the glycol aqueous solution. However, the growth rate of heating capacity was greater than that of the compressor input power. Therefore, the coefficient of performance of the vapor injection cycle was 4.4% greater than that of the single-stage cycle. So the vapor injection cycle has a wider range of applications in cold conditions. When the vapor injection pressure increased, the coefficient of performance of the vapor injection cycle increased then decreased, so there was a peak of the coefficient of performance. Therefore, a new correlation of the optimal vapor injection pressure was proposed. The new correlation has a significant meaning for designing a transcritical CO2 vapor injection cycle.

Model predictive control for the operation of a transcritical CO2 air source heat pump water heater

This paper presents a model predictive control (MPC) approach to optimize the operation of a transcritical CO2 air source heat pump (ASHP) water heater. Past findings have revealed that the heat rejection pressure is the key factor affecting the system performance of the ASHP. However, the currently used PI controls based on correlations and other control strategies reported in the literature have some limitations. In this paper, we applied the MPC strategy in an ASHP water heater to optimize its operational performance in real-time. Different from the current control strategy aiming to search for the optimal heat rejection pressure, MPC predicts the future operation condition and calculates the optimal inputs based on the control-oriented model and objective function. A high-fidelity physical model is first built in Dymola to produce the operational data, then a data-driven control-oriented model for MPC is derived from the data source via a dynamical system identification. MPC is designed to maximize the coefficient of performance of the ASHP water heater under the state-space equations and constraints. The evaluation is carried out under several cases including fixed ambient temperature, realistic ambient temperature, and variable water outlet temperature. The simulation results prove the control effectiveness of MPC.

Thermodynamic evaluation and modelling of an auto-thermal hybrid system of chemical looping combustion and air separation for power generation coupling with CO2 cycles

Chemical looping technologies have been demonstrated to have favorable economy for CO2 separation and high exergy efficiency. In order to realize the energy cascade utilization, a hybrid system combing autothermal coal-fueled chemical looping combustion (CLC), chemical looping air separation (CLAS), supercritical CO2 Brayton cycle and transcritical CO2 cycle is constructed for high-efficiency power production and CO2 separation. The proposed hybrid system is first modeled and simulated under fundamental condition to investigate the basic performance and exergy efficiency of each component. Then the thermodynamic evaluations of the system performance are conducted under different conditions with five variables. Finally, using these simulation results as training samples, a neural network model (NNM) of the system is constructed to predict the power output under variable working parameters. As the energy is used in different subsystems according to its grade, the net power efficiency of this hybrid system can reach to a high value of 64.59%. The established NNM is accurate for power prediction under random conditions with correlation coefficient of 0.9931.

Multi- objective optimization and life cycle assessment of a cascade system integrating LiBr/H2O absorption refrigeration with transcritical CO2 power cycle

Energy, economic and environmental (3E) analysis of a cascade system combining absorption refrigeration (AR) and transcritical CO2 power cycle (TCPC) for low-grade waste heat recovery (WHR) is conducted. Life cycle assessment (LCA) is applied to evaluate environmental performance. A novel comprehensive index Eco-indicator 16 (EI16) integrating seventeen environmental impacts was proposed which involves human health, ecosystem and resource. A multi-objection optimization model with thermal efficiency, annual total cost and EI16 as three objection functions is established and solved by Non-dominated sorting genetic algorithm II (NSGA-II). The final optimal solution is obtained after decision making with Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) and Shannon entropy method. The results show that payback time can be less than 7 years and there is a synergistic relationship between energy efficiency and environmental performance, while energy efficiency and economic performance are mutually contradictory. The final optimal solution achieves an increase by 13.17% in thermal efficiency and a decrease by 13.49% in Eco-indicator 16 at cost of an 8.94% increase in annual total cost. In all environmental impact categories, the contribution to the climate change is the largest, which is beneficial to the human health and ecosystem protection.