Gas Cooler Pressure Research Articles

Performance improvement of CO2 air conditioner by integrating photonic radiative cooler as sub-cooler or/and roof envelope

Passive radiative cooling, which creates a direct in-situ cooling effect, along with a natural refrigerant-based vapor compression system, could be a potential alternative for building air conditioning to reduce the heat island effect and global warming. Hence, the radiative cooler is integrated with transcritical CO2 air conditioner as a sub-cooler or/and roof envelope. Three configurations of radiative cooler integrated CO2 system (Case 1: radiative cooler as roof envelope only, Case 2: radiative cooler as sub-cooler only and Case 3: radiative cooler as sandwiched roof envelope and sub-cooler) are proposed and simulated for tropical Indian summer climate. Energy, exergy and economic analyses along with gas cooler pressure optimization are performed. The Monte Carlo method is used to calculate annual energy savings for all three cases. Results show that case 3 exceeds the performance of other cases. Case 3 also reduces the variations of optimum gas cooler pressure and cooling load, leading to the smooth operation of the CO2 air conditioner. COP increment of 33.8% and exergy efficiency improvement of 29.1% are observed for case 3. A simple payback period of 5.3 years is worked out for case 3. Annual energy saving per unit area for case 3 is found comparable to solar photovoltaic integration.

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.

Experimental studies on transcritical CO2 heat pump system for simultaneous water cooling and heating application

ABSTRACT An in-house transcritical CO2 heat pump experimental facility was developed for simultaneous cooling and heating of water. Hand-operated (Needle) valve is used as an expansion device to control the gas cooler pressure and mass flow rate of refrigerant. The tests were conducted to study the performance of the system and components at various operating conditions. Four independent input operating variables and four output variables are considered for investigation. The factorial experimental design was employed to achieve the optimum set of experiments. Taguchi L9 array is used to design experiments for chosen variables and results were analyzed by using response surface methodology (RSM). It is observed that system charging pressure and degree of expansion valve opening affect the system performance greatly. Maximum cooling and heating output from the system was recorded as 3.8 kW and 6.6 kW, respectively, and the corresponding system COP obtained was 3.1. Empirical correlations are developed employing RSM technique to predict the system performance.

Two-phase ejector enhanced carbon dioxide transcritical heat pump for cold climate

Among the solutions to current energy efficiency and climate change issues, the use of natural refrigerants in mechanical compression heat pumps is an important measure for an effective response. However, the heat pump’s performance decline as the ambient temperature drops is well known. The use of the two-phase ejector under these conditions can be a simple and easy to implement way to address this problem.In this respect, a thermodynamics model to simulate a CO2 transcritical heat pump cycle integrating a two-phase ejector is first presented. The modeling allows implicit adjustments of the ejector geometry with varying operating conditions, to adapt its operation in the cycle for maximised overall efficiency. The interaction between the main cycle components is highlighted and one can clearly see that the optimal operation of the cycle does not necessarily coincide with that of its components.For the simulated ejector conditions, the results show the importance of maintaining a particular entrainment ratio (0.5–0.65) in order to avoid mass balance issues in the separator. With lower Tevap, an enhanced ejector efficiency is achieved by striking a balance between decreased entrainment and improved compression ratios. The results clearly show the need to control the ejector geometry in order to meet capacity variations. Thus, decreasing Tevap (from 5 °C to −20 °C), needs to reduce the throat and the mixing diameters by 36% and 22% respectively.Cycle simulations show a definite advantage with the ejector use in terms of COP and heating capacity for low evaporator temperatures. In this case, the compression ratio of the ejector improves, resulting in a higher compressor pressure suction, compared to the conventional cycle. Thus, at optimal gas cooler pressure and for an evaporator temperature of −20 °C, gas cooler and evaporator capacities as well as the cycle COP improve by 14%, 23% and 9% respectively. These improvements are associated with a slight increase in the compressor work (≈4%).

Thermodynamic, economic, and environmental analyses of various novel ejector refrigeration subcooled transcritical CO 2 systems

The ejector refrigeration cycle has gained special interest due to its simplicity and hence can be a good choice as a subcooling device for the transcritical CO2 refrigeration system for performance improvement, which is an emerging research field. Hence, the aim of this study is the proposal and comparison of various ejector refrigeration-based dedicated subcooling methods in the CO2 refrigeration system. Six different cycle configurations (one is existing and five are novel) using the ejector refrigeration cycle for subcooling the CO2 after the gas cooler are examined. All cycles are analyzed energetically, exegetically, economically, and environmentally by developed model and compared with the reference CO2 refrigeration system (without subcooling). Effects of gas cooler pressure, generator temperature, evaporator temperature, ambient temperature, and degree of subcooling are discussed. Gas cooler pressure is optimized for maximum COP. Cycle 4 (with internal heat exchanger and gas cooler waste heat-driven generator only) yields maximum COP and exergy efficiency among all subcooling cycles at optimum operating conditions. However, the environmental pollution for cycles without an internal heat exchanger is less. In terms of cost per cooling capacity, Cycle 4 is best but having operational restriction (better for sufficiently higher temperature lift). Cycle 1 and Cycle 2 may be the better choice if heat is available.

Exergy analysis of a vortex tube expansion two-stage transcritical N2O refrigeration cycle

In the present article, two-stage transcritical cycle with expansion valve (TSTCEV) and two-stage transcritical cycle with vortex tube (TSTCVT) are compared mainly on the basis of the second law of thermodynamics. Natural refrigerant nitrous oxide (N2O) is used in both cycles for thermodynamic analysis. The gas cooler and evaporator temperatures are varied from 35°C to 55°C and −55°C to 5°C, respectively. The effects of various operating parameters on the optimum intercooler and gas cooler pressures, the coefficient of performance (COP), exergy destruction and the exergetic efficiency are presented. A component-wise exergy analysis of TSTCEV and TSTCVT shows that the use of the vortex tube instead of the expansion valve reduces the total exergy destruction and increases the exergetic efficiency as well as COP. The exergetic efficiency of TSTCVT is on average 9.17% to 11.68% higher than that of TSTCEV for considered operating conditions. Furthermore, the optimum intercooler and gas cooler pressures in TSTCVT are lower than those of TSTCEV. These results offer significant help for optimum design and the selection of appropriate operating conditions of TSTCVT using refrigerant N2O.

An experimental study of an ejector-boosted transcritical R744 refrigeration system including an exergy analysis

The field of refrigeration witness a massive transition in the supermarket with a strong focus reflected on energy consumption. The use of ejector allows for overcoming the significant exergy destruction lays on the expansion processes of the cooling systems and led to spark improvement in the system performance by recovering some of the expansion work. In this study, a detailed experimental work and exergy analysis on the R744 transcritical ejector cooling system was investigated. The experiment was implemented on the commercial ejector cartridge type (032F7045 CTM ELP60 by Danfoss). The impact of different operating conditions determined by exit gas cooler pressure and temperature, evaporation temperature and receiver pressure was examined. The ejector performance of the pressure lift, mass entrainment ratio, work rate recovery and efficiency were evaluated. In addition, exergy efficiency and the variation of exergy produced, consumed, and destruction were assessed based on the transiting exergy. The result revealed better overall performance when the ejector operated at transcritical conditions. The ejector was able to recover up to 36.9% of the available work rate and provide a maximum pressure lift of 9.51 bar. Moreover, it was found out that the overall available work recovery potential increased by rising the gas cooler pressure. Out of the findings, the ejector could deliver maximum exergy efficiency of 23% when working at higher motive nozzle flow temperatures along with providing lower exergy destruction. The experiment results show that the amount of the exergy consumed and destruction were gradually increased with higher gas cooler pressure and, in contrast, decreasing with higher motive nozzle flow temperature.

Sizing and control rules of dedicated mechanical subcooler in transcritical CO2 booster systems for commercial refrigeration

Use of Dedicated Mechanical Subcoolers gives rise to the need of an optimisation process involving gas cooler pressure, activation and setpoint temperature values, subcooling degree and subcooler size, with the aim of further improving the energy efficiency while considering costs. In this paper a thermoeconomic analysis is performed on a commercial refrigeration plant, at four climate conditions from warm to hot, with time-dependent refrigerating load depending on the location, resulting in some design and control rules for the DMS. In terms of energy efficiency, the size of the DMS appears to be more crucial at hot climate conditions (difference up to 3.5% in energy saving), with sizes ranging from 35% to 45% of the total cooling capacity. Optimal control rules should be preferentially adopted at mild-warm conditions, suggesting to exploit the highest subcooling rate available. The economic analysis shows that energy use is the most important cost item.

Experimental assessment of a CO2 direct-expansion solar-assisted heat pump operating with capillary tubes and air-solar heat source

This paper presents an experimental study on a direct-expansion solar-assisted heat pump (DX-SAHP) operating with capillary tubes in a transcritical carbon dioxide cycle. An air-solar dual-source evaporator is also proposed as an alternative to mitigate the performance drop in low solar radiation conditions. The experiments followed a factorial design, considering capillary tubes (number 1 and number 2) and operating schemes (Low Solar Radiation, Low Solar Radiation with fan, and High Solar Radiation) as factors. The results showed that altering from Low Solar Radiation (7 W/m2) to High Solar Radiation (969 W/m2) caused an augment on the heating capacity (58%), on coefficient of performance (from 2.19 to 3.12; 42%), on the refrigerant mass flow rate (35%), and on the degree of superheat (from 4.4 K to 30.6 K). Changing from Low Solar Radiation to Low Solar Radiation with fan, a 17% improvement on the heating capacity was found, with no variation on coefficient of performance. Besides the evaporating pressure, the gas cooler pressure was also sensitive to solar radiation variations. It is estimated that for lower levels of solar radiation, up to 570 W/m2, a single capillary tube could be a reasonable choice for a DX-SAHP. This work aims to be the first to experimentally investigate the use of capillary tubes in a DX-SAHP under solar radiation variations greater than 960 W/m2.

INVESTIGATING THE PERFORMANCE OF A TRANSCRITICAL BOOSTER REFRIGERATION SYSTEM WITHCARBON DIOXIDE IN TROPICAL CLIMATES: THE CASE OF BENIN

This article presents a prospective study of low carbon footprint refrigeration systems in tropical climates. A transcritical carbon dioxide booster refrigeration system is simulated within the Engineering Equation Solver (EES) environment. The results are discussed under the weather conditions Cotonou, Benin. The performance of the transcritical booster cycle increases with low ambient temperatures the Coefficient of Performance (COP) increased by 94.2% when the ambient temperature went from 40°C to 25°C). The gas cooler pressure and the medium pressure were ctoptimised and two correlations were developed to predict the optimal gas cooler pressure and intermediate pressure for which the COP value is maximum. It should be noted that these pressures are very sensitive to changes in ambient temperature and they vary proportionally with the temperature at the end of cooling and in a linear fashion.

Design and optimization strategy for ejectors applied in refrigeration cycles

Implementation of an ejector for expansion work recovery in transcritical Carbon Dioxide (CO2) cycles provides an opportunity to improve the efficiency of these environmentally-friendly refrigeration systems. However, literature outlining an approach to ejector design for a given application is lacking. This paper presents a tool to design a complete ejector applied in a vapor compression cycle. In this work, the developed design tool was validated using experimentally-derived polynomials at air conditioning conditions, then efficiencies were input to broaden the analysis to study a transcritical CO2 system with an ejector operating in the evaporating temperature range of −15 °C to 20 °C and gas cooler pressure in the range of 80–110 bar. The design tool allows for the calculation of the motive and suction nozzle throat diameters, the mixing section diameter, and the diffuser outlet diameter, as well as the lengths of each section, to output a full internal geometry of the ejector based on performance requirements. Individual component sub-models are presented within the proposed model structure. The model which forms the basis of the design tool was experimentally validated with a mean absolute error (MAE) between 3% and 4%. Additionally, the sensitivity of the ejector geometry and performance to component efficiencies, operating conditions, and component versus system optimization was investigated. The optimization and parametric studies conducted provided novel insights into the impact of desired efficiency and operating conditions on ejector geometry, thus allowing a designer to make decisions based on the tradeoff between ejector size and performance. For example, as the diffuser length increased by 5.1 mm to obtain an efficiency increase, to obtain a further efficiency increase of the same amount would require a 17.1 mm length increase in diffuser length. Potential model improvements and other future work are also discussed.

Experimental determination of the optimum intermediate and gas-cooler pressures of a commercial transcritical CO2 refrigeration plant with parallel compression

CO2 systems used in refrigeration are becoming more complex with the aim of improving their energy performance. Parallel compression is one of the implemented solutions to enhance the performance of the plants. However, an optimization process is required to operate this system at high performance and its operation is subjected to physical limitations in real plants.This work presents the experimental optimization of a transcritical CO2 plant working with parallel compression. The plant is tested at different discharge pressures and different secondary compressor speeds in order to optimize the COP of the plant and determine the optimal conditions for three gas-cooler exit temperatures 27.5 °C, 32.5 °C and 37.5 °C and three evaporation levels: −15.0 °C, −10.0 °C and −5.0 °C.The optimal working conditions that can be achieved in a real plant have been determined, obtaining COP from 1.71 to 2.63 for −5.0 °C, from 1.50 to 2.22 for −10.0 °C and from 1.25 to 1.84 for −15.0 °C. Cooling capacity ranges from 8.94 kW to 11.34 for −5.0 °C, from 7.71 kW to 9.47 kW for −10.0 °C and from 6.22 kW to 7.76 kW for −15.0 °C. The trends observed in theoretical results have been corroborated and the optimum gas-cooler and intermediate pressures have been determined and discussed.

Thermodynamic analysis of the optimal operating conditions for a two-stage CO2 refrigeration unit in warm climates with and without ejector

The natural refrigerant R744 seems to be the long term solution to be imposed in the food retail industry, where low and medium refrigeration temperatures are usually required, despite the technical difficulties related to high the pressure, especially in warm climates, leading to trans-critical operation with the consequent COP reduction. To overcome such difficulties, different approaches have been proposed in literature, being very popular the use of parallel compressor or its combination with ejector expansion devices. The efficient operation of trans-critical R744 systems requires the control of the gas cooler pressure, as well as the pressure of the flash-tank. In the case of ejector expansion devices, the ratio of entrained mass flow rate must be also included in the control variables and, at the same time, its use implies additional restrictions. The current paper presents the optimization methodology for the operating conditions of a two-stage CO2 refrigeration unit for both cases, with and without ejector. The curves of optimal combinations of gas cooler and flash-tank pressures, understood as operational control laws, are provided together with the COP achieved. The operating conditions and performance of the ejector case are compared showing COP improvements of up to 13%. The results also show that the operating region of the control variables is limited due to the use of the ejector.

Experimental determination of the optimum working conditions of a commercial transcritical CO2 refrigeration plant with a R-152a dedicated mechanical subcooling.

Abstract Transcritical CO2 plants combined with subcooling systems are the focus of several researches in the last years with the objective of improving their performance. Among the subcooling systems, the Dedicated Mechanical Subcooling system (DMS) is one of the most interesting because it greatly improves the overall COP and the cooling capacity of the system. This work presents the experimental study of a transcritical CO2 plant working with an R-152a DMS. The plant was tested at different pressure and subcooling conditions in order to determine the working conditions where the COP of the plant is maximum. The optimal operation conditions are determined for three ambient temperatures 25.0 °C, 30.4 °C and 35.1 °C and three cold sink temperatures (-1.3 °C, 3.8 °C and 10.0 °C). The measured values go from 6.5 kW to 7.3 kW for glycol inlet temperature -1.3 °C, from 7.6 kW to 8.4 kW for 3.8 °C and from 8.8 kW to 9.8 kW for 10.0 °C. Optimum COP goes from 1.51 to 1.95 for -1.3 °C, from 1.69 to 2.21 for 3.8 °C and from 1.86 to 2.52 for 10.0 °C. Optimum gas-cooler pressure has a higher dependence on the heat rejection level, being higher when higher the heat rejection level is, but it slightly depends on the evaporation level. Optimum subcooling degree is both dependent on the water inlet temperature and on the glycol inlet temperature. Two correlations are proposed to determine the optimal pressure and subcooling degree for the CO2 plants working with DMS as a function of the gas-cooler outlet temperature and the evaporation level.

Effect of dedicated mechanical subcooler size and gas cooler pressure control on transcritical CO2 booster systems

Dedicated Mechanical Subcooling (DMS) is one of the most investigated and effective strategies applied to increase the performance of CO2 commercial refrigeration systems in transcritical operation. Further performance benefits can be obtained by a reduction of the gas cooler pressure of the main cycle at transcritical conditions. In this work the most important parameters for the design and operation of such a system, i.e. the DMS cooling capacity, the subcooling degree and the gas cooler pressure, are considered and their effect on the annual energy use of the plant is estimated in warm and hot climate conditions by means of a validated model. DMS is also compared to the parallel compression scheme and subcooling performed through a water chiller dedicated to HVAC. DMS results to be the most effective solution among those investigated, and the choice of the best design and operating parameters allows further energy saving and cost reduction.

Analysis of the optimal gas cooler pressure of a CO2 heat pump with gas bypass for hot water generation

This paper presents an experimental and numerical model-based approach for the estimation and control of the optimal heat rejection pressure in a one-stage transcritical CO2 heat pump equipped with an internal heat exchanger (IHX) and a liquid receiver. Once mass flow rate and compressor power consumption has been experimentally characterized, the correlations obtained have been introduced in a numerical model used to study the influence that evaporation temperature, superheating degree, gas cooler outlet temperature, IHX efficiency, liquid receiver pressure and compressor characteristics have on the optimal pressure. The results obtained show that only the gas cooler outlet temperature has a decisive influence on the optimal pressure, so it is possible to obtain a correlation for the optimal pressure based only on that temperature. However, that correlation does not limit the compressor discharge temperature, which can reach values near to 170 °C, leading to lubricating oil degradation. In order to limit that temperature to 140 °C, an alternative correlation for the optimal gas cooler pressure is proposed. This correlation takes into account not only the gas cooler outlet temperature but also the evaporation temperature and the compressor suction temperature.

Thermoeconomic and feasibility analysis of novel transcritical vapor compression-absorption integrated refrigeration system

In this paper, a novel structure of transcritical vapor compression-absorption integrated refrigeration system (TVCAIRS) is proposed based on its thermoeconomic viability. The integration of single effect vapor absorption refrigeration system (VARS) (with H2O-LiBr fluid couple) in transcritical vapor compression refrigeration system (TVCRS) (with R744 refrigerant) provides a subcooling of 5 °C in the proposed configuration. Hence, the overall COP and exergetic efficiency of TVCAIRS is 28.6% and 26.9%, respectively higher than the equivalent standalone TVCRS with optimum gas cooler pressure and generator temperature. Besides, the requirement of external cooling water for the gas cooler, absorber and condenser cooling of TVCAIRS is 12.3% less than the equivalent TVCRS. The direction for the further process improvement of TVCAIRS is also presented. Further, the thermodynamic performance of proposed system is also investigated for low temperature refrigeration application and hot climate conditions. The comparative results showed that the present TVCAIRS is thermodynamically more efficient as compared to the equivalent TVCRS and conserves more water resources with effective recovery of waste heat for the sustainable development.The techno-economic analysis of proposed TVCAIRS shows a shorter breakeven point and payback period of 1.3 yr. and 1.8 yr., respectively with total annualized cost of INR 5,641,614 (17.8% less than the equivalent TVCRS). Further, selection diagram for TVCAIRS and equivalent TVCRS shows that TVCAIRS is more feasible as compared to the TVCRS under low temperature and hot climate conditions and its viability strongly increases, if financial incentives of reduced interest rate are offered with longer life designs and operation duty.

Performance evaluation of ejector based CO2 system for simultaneous heating and cooling application in an Indian dairy industry

India is ranked first in milk production, and its goal is to increase annual production to 300 million tons by 2024. The dairy industry requires multi-temperature cooling and simultaneous heating. In the present study, the ejector based transcritical CO2 system with two evaporators (LT at −10°C, MT at 0°C) is analyzed and proposed for fulfilling both cooling and heating demands in the dairy industry. Ejector based systems with and without internal heat exchanger are analyzed and compared. The effects of gas cooler outlet temperature, receiver pressure and gas cooler pressure on the performance of the system are investigated by energy and exergy analysis. The gas cooler pressure is analyzed for achieving the desired pasteurization temperature. The internal heat exchanger not only reduces the minimum gas cooler pressure required for the pasteurization temperature but also increases the system COP and exergy efficiency by 6.4% and 4.5% respectively. Second law analysis shows that the maximum exergy efficiency of the system is 38.4%.

Theoretical analysis on a novel two-stage compression transcritical CO2 dual-evaporator refrigeration cycle with an ejector

This paper proposes a novel two-stage compression transcritical CO2 dual-evaporator refrigeration cycle with an ejector. The ejector and flash tank introduced in the novel system is used to reduce the flashed vapor flowing through the low-temperature evaporator and thus reduce the input power of the low-temperature compressor, rather than traditionally reducing the compressor input power by lifting the compressor suction pressure. Thermodynamic analysis results indicate that under all given operating conditions, the performances of novel system are better than those of conventional system. Compared with the conventional system, the COP and exergy efficiency of the novel system under a typical operating condition is increased by 19.6% and 15.9% respectively, and the discharge temperature of high-temperature compressor in the novel system is reduced by 10.5 °C. In addition, the optimum gas cooler pressure of the novel system is 10.3 MPa, which is lower than that of the conventional system. Moreover, exergy analysis results display that the overall exergy destruction in all expansion devices for both systems accounts for the largest part of the total exergy destruction, but its proportion of the novel system is smaller than that of the conventional system.

Influence of geometric and operating parameters on the performance of the helical capillary tube for the R744 refrigerant

This paper reveals the numerical investigation of the helical capillary tube. The study is carried out for an adiabatic, homogenous capillary tube with the R744 refrigerant. The basic principles of conservation of mass, momentum, and energy are used to develop the mathematical model. The results of the present coiled capillary tube model are verified with previously published test results. The refrigerant properties of R744 are employed from CO2PROP. The influence of various geometric parameters of the capillary tube-like tube diameter, roughness, and coil diameter on the performance of the tube been computed. The consequence of the tube diameter on the tube performance is larger than other geometric parameters. As the tube diameter increase by 18%, the mass flow rate increase by nearly 55%. Similarly, as an increment in the coil diameter took place by 10%, the mass flow rate increase by 5%. The negligible change is observed owing to the change in surface roughness. While the surface roughness increases by 18%, the mass increases by 1%. Moreover, the influence of various operating factors is evaluated. A significant variation in the tube and system performance is seen owing to the change in gas cooler temperature. As the gas cooler temperature rises by 5%, the drop in mass flow rate is about 18%. Comparatively, less effect is recorded due to an evaporator temperature. As the evaporator temperature rises by 15%, the mass flow decreases by nearly 11%. The considerable impact is perceived owing to the change in pressure of the gas cooler. As the gas cooler pressure increases by 4%, the mass flow rate increases by nearly 7%. Similarly, the influence of operating and geometric parameters on the coefficient of performance and cooling capacity of the transcritical R744 system is evaluated. For optimum performance of the R744 cycle, the selection of proper gas cooler temperature and gas cooler pressure is a key factor. This work is useful to design the helical capillary tube with R744 refrigerant.