Two-stage Transcritical CO2 Research Articles

Effect of Inter-Stage Pressure on Performances of Two-Stage Transcritical CO2 Refrigeration Cycle with Dedicated Absorption Dual-Subcooling and Mechanical Recooling

Effect of Inter-Stage Pressure on Performances of Two-Stage Transcritical CO2 Refrigeration Cycle with Dedicated Absorption Dual-Subcooling and Mechanical Recooling

Thermodynamic analysis of a modified two-stage transcritical CO2 refrigeration cycle with an ejector and a subcooler

In the application scope of large-scale supermarkets, the practicality of transcritical CO2 refrigeration device has been confirmed to be quite satisfying. A modified two-stage transcritical CO2 refrigeration cycle with an ejector and a subcooler (MTC) is proposed in this paper. In the modified cycle, the ejector recovers expansion works and reduces irreversible losses in the throttling process. The subcooler provides subcooling degree for the CO2 entering the low-temperature (LT) evaporator, thus increasing the refrigeration capacity and improving the COP of the modified cycle. Thermodynamic analysis has shown that the exergy efficiency (ηex) and COP of MTC under given operating condition has been enhanced by 13.7 % and 14.2 % compared to BTC. The discharge temperature at the outlet of high-pressure compressor in MTC is decreased by 8.3℃. Under typical operating condition, the optimal discharge pressure of MTC is 9.07 MPa, which is lower than that of BTC. The correlation to calculate optimal discharge pressure for single-stage CO2 refrigeration cycle is also suitable for MTC under given operating conditions. The MTC has also shown better performance under variable operating conditions. For MTC, when the temperature at the outlet of gas cooler increases from 35 – 45℃, the COP and ηex are enhanced by 14.1 % - 16.1 % and 12.8 % - 14.2 % compared to BTC, respectively. When gas cooler outlet pressure decreases from 11.0 to 7.5 MPa, the COP and ηex are enhanced by 14.0 % - 22.8 % and 13.4 % - 18.7 %. As the evaporating temperature at the cold side of subcooler increases from -25 to -15℃, the COP and ηex increase from 1.82 to 1.98 and 27.4 % to 29.7 %. With the ratio of refrigeration capacity (Rec) between medium-temperature evaporator and low-temperature evaporator varies from 0.7 to 1.2, the COP and ηex are improved by 10.7 % - 16.3 % and 10.7 % - 15.4 % compared to BTC. There is the maximum exergy loss at gas cooler in the MTC, whereas that of BTC is located in the expansion valve before LT evaporator. The economic analysis shows the cost per unit of exergy of MTC is decreased by 11.5 % under typical operation condition. According to simulation results, the modified cycle has better performance in severe working conditions such as high gas cooler outlet temperature and low gas cooler outlet pressure in the given range of working conditions compared to BTC.

Transcritical [formula omitted] precooled single mixed refrigerant natural gas liquefaction process: Exergy and Exergoeconomic optimization

This study proposes a two-stage transcritical CO2 pre-cooled single mixed refrigerant process to investigate the potential improvement in exergy efficiency and exergoeconomic performance. Two different objective functions, namely maximizing exergy efficiency and minimizing the unit cost exergy of LNG, were optimized using the particle swarm optimization algorithm to compare the performance of the CO2 pre-cooled single mixed refrigerant process with single mixed refrigerant process. The results of the first objective function indicate a higher exergy efficiency of the CO2 pre-cooled single mixed refrigerant process, with an increment of 7.56%. This improvement can be attributed to not only the lower total power consumption but also the better matching of the temperature-difference composite curve, particularly the location of the pinch temperature at the warm end of the heat exchanger. Additionally, the exergy destruction of the CO2 pre-cooled single mixed refrigerant process was 30.12% lower than that of the single mixed refrigerant process. The second objective function demonstrated that the CO2 pre-cooled single mixed refrigerant process had a unit exergy cost value of 7.46 $/GJ, which was 11.98% lower than the single mixed refrigerant process. In conclusion, the proposed process contributes to energy saving throughout the LNG chain. Previous studies on the transcritical CO2 precooling cycle were limited to only one-stage configuration paving the way for thermodynamic efficiency enhancement by modifying the design of the process. This study also considers the effects of two objective functions maximizing exergy efficiency and minimizing the unit exergy cost of LNG on the optimization of decision variables which has not been discussed yet in the literature. Moreover, innovative exergoeconomic analysis presents a comprehensive criterion for evaluating economic performance, providing guidance for the initial construction and sustainable development of LNG plants.

Performance analysis of indirect-expansion solar assisted heat pump using CO2 as refrigerant for space heating in cold climate

The indirect-expansion solar assisted heat pump (SAHP) is one of the main configurations for SAHP systems. Previous work on this configuration has focused only on one-stage heat pump cycles using traditional refrigerants, which will be phased out in the future. In addition, one-stage heat pumps may not work properly in cold climate regions. To evaluate the performance and understand the potential of indirect-expansion SAHPs using CO2 as a refrigerant for space heating in cold climates, models of an indirect-expansion SAHP with a one-stage transcritical CO2 cycle, with a two-stage transcritical CO2 cycle, and with the presently popular one-stage R410a cycle for space heating in Toronto, Canada were developed using the combined platforms of EES and TRNSYS. Based on these models, the performance variations of the three systems with solar thermal collector area, storage tank volume, and heat pump compressor capacity were simulated, compared, and presented. The comparison of the systems showed that the performance of the two-stage cycle peaked at a much smaller compressor capacity than that of the other cycles – 0.4 m3/h for the two-stage cycle, 0.7 m3/h for the one-stage cycle, and 1.7 m3/h for the R410a cycle. When the three systems operated at the optimal compressor capacity, the free energy ratio (FER) of the two-stage cycle could reach 0.772, which was higher than that of the one-stage cycle by 19.0% and higher than that of the R410a cycle by 11.6% at the given conditions. The potential cost reduction benefits from the smaller compressor and higher FER could make the two-stage cycle more competitive than the other cycles in cold climates.

Effects of Intercooling and Inter-Stage Heat Recovery on the Performance of Two-Stage Transcritical CO2 Cycles for Residential Heating Applications

Due to the harmful effects of synthetic refrigerants, such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs)n the environment, natural refrigerants like carbon dioxide (CO2) have been attracting great interest. The higher inter-stage superheating of CO2 makes it difficult to predict the effects of the intercooling on heating performance of a two-stage transcritical CO2 cycle. In addition, very little is known about the potential of inter-stage heat rejection recovery in the heating performance enhancement of this cycle. In order to explore the effects of intercooling and inter-stage heat rejection recovery potential, three “sub-cycles”—(1) a sub-cycle with heat recovery, (2) a sub-cycle without heat recovery, and (3) a sub-cycle without intercooling—were modeled in Engineering Equation Solver (EES) software for three commonly-used two-stage transcritical cycles: (1) an intercooler cycle, (2) a flash cycle, and (3) a split cycle. Then, the discharge pressure and intermediate pressure were simultaneously optimized. Based on the optimization results, the heating performance of the sub-cycles for each cycle were compared. The results demonstrate that the incorporation of intercooling without heat recovery was detrimental to the heating performance in comparison to the absence of intercooling. It is also clear that there is a great potential for heating performance improvement through inter-stage heat recovery.

Performance of a new two-stage transcritical CO2 refrigeration cycle with two ejectors

The present study describes a new two-stage transcritical CO2 refrigeration cycle with two ejectors (NERC). At this studied cycle (NERC), vapor compression is performed in two stages and each vapor compression line includes separate ejector. Thermodynamic and exergetic analysis of the proposed cycle are performed using Engineering Equation Solver (EES) software. The new cycle (NERC) is compared with a conventional cycle. It is found that depended on conditions the new cycle (NERC) improves the COP from 20% to 80% compared to the conventional cycle. The effects of main cycle parameters i.e. the high-side pressure of a compressor (discharge pressure), intermediate pressure, the outlet temperature of gas cooler and evaporator temperature on COP are studied.

Performance analysis of a modified trans-critical CO2 refrigeration cycle

Performance of a two-stage trans-critical CO2 refrigeration cycle, which contains internal heat exchanger, intercooler, ejector, and separator, has been analyzed after modification. Original cycle has been modified with extracting a line of saturated vapor flow from separator and feeding to the intercooler. The impacts of operational parameters such as gas cooler temperature, evaporator temperature, and gas cooler pressure, on the cycle performance have been investigated. Computed results showed that modified cycle improved the maximum coefficient of performance, COPmax, by 26.89% compared to the original cycle at a particular operation conditions. Meanwhile, this modification averagely decreased the exergy destruction rate by 18.6% compared to the original cycle.

Thermodynamic analysis on a two-stage transcritical CO2 heat pump cycle with double ejectors

In this study, two ejectors are proposed as expansion devices for a two-stage transcritical CO2 heat pump cycle to enhance the cycle performance. The two ejectors are arranged at the low- and high-pressure stages, respectively, to recover more available expansion work, and significantly reduce the throttling loss at each stage. The performance of the improved two-stage cycle is evaluated by using the developed mathematical model, and then compared with those of the basic two-stage cycle with a flash tank. The simulation results show that the improved two-stage cycle exhibits higher heating COP and volumetric heating capacity compared to the basic two-stage cycle. By further incorporating an internal heat exchanger, the heating COP can be increased by 10.5–30.6% above that of the baseline cycle when the subcooling degree varied from 0 to 15°C under given operation conditions of −15°C evaporating temperature, 10MPa gas cooler pressure and 35°C outlet temperature. Additionally, the effects of the gas cooler pressure and intermediate pressure on the maximal heating COP are also discussed.

Vortex Tube Expansion Two-Stage Transcritical CO<sub>2</sub> Refrigeration Cycle

Use of vortex tube as an expansion device in transcritical CO2 cycle could reduce the throttle loss and improve the coefficient of performance. In this paper, a vortex tube expansion two-stage transcritical CO2 refrigeration cycle(VTTC) is established and compared to that of the two-stage transcritical CO2 refrigeration cycle with throttle valve(TVTC). Thermodynamic analysis results indicate that there is also an optimum heat rejection pressure for the vortex tube cycle, and the COP improvement is 2.4%~16.3% at given conditions. Decrease in evaporation temperature or increase in gas-cooler outlet temperature decrease the COP, but the COP improvement will increase. The effect of cold mass fraction on the COP is negligible, but the COP improvement will increase fast with the increase of cold mass fraction.

Second law optimization of two-stage transcritical CO2 refrigeration cycles in the cooling mode operation

Second law optimization studies of two-stage transcritical CO2 (TRCC) refrigeration cycles, incorporating options such as a new ejector-expansion with internal heat exchanger (IHE) and intercooler (IC), flash gas bypass, flash gas intercooling, compressor intercooling with IHE, are presented based on cycle simulation. To validate the simulations, the available numerical data in open literature are used. It is found that the coefficient of performance (COP) and second law efficiency of the new two-stage TRCC cycle are on average 16.5, 18.4, and 28.4 per cent higher than that of the two-stage TRCC with IHE and IC, the two-stage TRCC with flash gas bypass, and the two-stage TRCC with flash gas intercooling cycles, respectively. Hence, the new two-stage refrigeration cycle is a promising refrigeration cycle from the thermodynamic point of view. It is also concluded that for cases of the flash gas bypass and flash gas intercooling the optimum inter-stage pressure deviates significantly from the geometric mean pressure of the gas cooler and evaporator pressure. While for the new two-stage TRCC and the two-stage TRCC with IHE and IC, the optimum inter-stage pressure is approximately equal to geometric mean pressure. Finally, a regression analysis was employed in terms of evaporator and gas cooler exit temperatures to develop mathematical expressions for maximum COP, optimum discharge, and inter-stage pressures and entrainment ratio.

The performance analysis of a two-stage transcritical CO2cooling cycle with various gas cooler pressures

A theoretical analysis of a two-stage transcritical CO2 cooling cycle is presented. The effect of a two-stage cycle with intercooling process on the system coefficient of cooling performance is presented for various gas cooler pressures. However, the performance comparison between one-stage and two-stage cycles is presented for same operating conditions. Gas cooler pressure, compressor isentropic efficiency, gas cooler efficiency, intercooling quantity and refrigerant outlet temperature from the gas cooler are used as variable parameters in the analysis. It is concluded that the performance of the two-stage transcritical CO2 cycle is approximately 30% higher than that of the one-stage transcritical CO2 cycle. Hence, the two-stage compression and intercooling processes can be assumed as valuable applications to improve the transcritical CO2 cycle performance. Copyright © 2008 John Wiley & Sons, Ltd.