Transcritical CO2 Refrigeration System Research Articles

Comparative thermodynamic study of CO2 transcritical supermarket booster refrigeration system combined with vapor injection and parallel compressionÉtude thermodynamique Comparative du système de réfrigération transcritique de supermarché au CO2 combiné à l'injection de vapeur et à la compression parallèle

The use of CO2 refrigerants in supermarket refrigeration has recently demonstrated great potential owing to their excellent thermophysical properties and environmental protection. To further improve the efficiency of transcritical CO2 refrigeration systems, a parallel compression cycle with vapor injection system (SRC) is proposed. Thermodynamic model is established to investigate the influence of working condition parameters and compared with basic booster refrigeration system (BRC) and parallel compression refrigeration system (PRC). In addition, the effect of vapor injection parameters on the characteristics of the proposed system is investigated. The results show that the medium pressure has little effect on BRC, while the optimum value exists for both PRC and SRC. The performance of the three systems can be significantly enhanced by optimizing the discharge pressure and medium pressure. The optimal value of the relative vapor injection pressure is constant at about 1.15. The maximum coefficient of performance (COPmax) of the SRC is significantly improved by 23.99%∼32.14% and 14.44%∼20.07% at disccussed working conditions compared with BRC and PRC, respectively. In summary, this paper confirms the performance enhancement potential of the proposed cycle and provides theoretical support for its practical implementation.

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.

Performance analysis of sub-cooled transcritical CO2 refrigeration system using vapour absorption refrigeration system and dew point evaporative cooling

Shifting towards natural refrigerants is expected to reduce the environmental damage in terms of refrigerants leaks, as they have zero ozone depletion potential as well as minimal global warming potential. A CO2 transcritical cycle is analysed thermodynamically in this study. Further, to improve the performance of the CO2 system, a vapour absorption refrigeration system (driven by the heat available at the exit of the compressor) has been integrated with it. Also, to counter the effects of hot climatic conditions, in Indian context, a dew point evaporative cooling system is further utilized to take up the heat from the gas-cooler. A thermodynamic and economic study of the proposed integrated system is performed. The effect of various performance parameters; such as ambient dry bulb temperature, relative humidity, evaporator temperature has been studied on the overall performance of the system. Compared to the base case, the coefficient of performance is improved in the range of 40%–270 %, for the hybrid systems, under the different climatic conditions. Moreover, the proposed system provided a low-cost performance of about 0.024 $/kWh, as compared to the base case consumption of about 0.05 $/kWh.

Experimental investigation of a transcritical CO2 refrigeration system incorporating rotary gas pressure exchanger and low lift ejectors

Natural refrigerants like CO2 are playing a significant role in making refrigeration and heat pump systems climate-friendly by slowly phasing out the high global warming refrigerants like hydrofluorocarbons (HFCs). However, the efficiency of a transcritical CO2 refrigeration system declines significantly when the ambient temperature increases, primarily attributed to the high-pressure lift and the losses incurred during expansion. To remedy this issue, this paper presents a novel rotary gas pressure exchanger (PXG) device, which simultaneously achieves high differential pressure expansion work recovery and the “free compression” of the portion of the flash gas in a compact, rotary machine. For this, a PXG device is designed, fabricated, and tested to achieve free compression of CO2 over the entire differential pressure of approximately 70 bar between a receiver and a gas cooler. This is one of the highest free-pressure lift provided by any device to date in CO2 refrigeration. However, there is a small pressure loss of approximately 1–2 bar in the system due to viscous and inertia losses in the piping and in the PXG itself, which needs to be overcome by an external booster device. Results on a baseline PXG integrated system with two low lift booster compressors are presented, which show up to 60 bar free pressure lift and up to 18.2 % COP improvement provided by PXG. Additionally, key performance characteristics of the PXG, like the expansion work recovery, the mass boost ratio, direct fluid-to-fluid contact, and no pass-through operation are experimentally quantified. This work also presents a novel method to integrate two low lift ejectors with PXG to eliminate the need for separate low lift compressors. The low lift ejectors are designed, fabricated, and tested in-house, followed by their integration with the PXG device. A new type of transcritical CO2 refrigeration system is designed to integrate these low lift ejectors with PXG, and experiments are conducted at various evaporator thermal duties and gas cooler exit temperatures, simulating varying ambient temperature conditions. A novel control system to control the gas cooler pressure to optimal thermodynamic levels using PXG rotational speed is demonstrated experimentally. Further, automated control of high-pressure low lift ejector mass flow using an in-built needle design has been successfully demonstrated to optimise PXG mass boost performance. The LP low lift ejector achieved a successful pressure lift of 3.8 bar, and the HP low lift ejector showed a lift of 5.7 bar on the top of 42 bar free pressure lift provided by PXG for up to 5.8 kg/min mass flow delivered by free PXG compression. The results from this study demonstrate that the PXG device provides a significant energy efficiency improvement to the transcritical CO2 refrigeration system, and the novel low lift ejectors, when integrated with PXG, provide a successful method to maximise PXG’s thermodynamic potential.

Thermodynamic modeling and performance optimization of transcritical CO2 dual-evaporator refrigeration system enhanced with ejectors

The utilization of ejectors in transcritical CO2 refrigeration systems is known to enhance system performance significantly. However, the operational conditions play a crucial role in determining the system’s overall efficiency. This study introduces a thermodynamic model for transcritical CO2 ejector refrigeration systems and investigates the influence of operating conditions on system performance for four different refrigeration systems, and the optimization of the Nakagawa sound velocity model coefficient in the ejector modeling is carried out to enhance the accuracy of the model. Meanwhile, these research contents are also the innovation of the research. Key conclusions include: (1) Increasing the Nakagawa coefficient to 1.1 improves model accuracy. (2) Ejector inclusion enhances system COP, with one configuration showing a 43.36 % increase and another a 53.23 % increase, recommending different system setups for large spaces like supermarkets and smaller areas like refrigerated vehicles. (3) Higher evaporating pressures result in better COP for ejector systems, with a suggested maximum air cooler pressure of 9 MPa. COP also benefits from increased ejector outlet pressures. (4) AR (area ratio) of the ejector affects mass flow rates and cooling capacity significantly, while its impact on COP is minimal. Adjusting the primary throat area can control cooling capacity for varied needs. The study provides actionable insights for improving the design and efficiency of transcritical CO2 ejector refrigeration systems.

Thermodynamic, environmental, and exergoeconomic analysis of multi-ejector expansion transcritical CO2 supermarket refrigeration cycles in different climate regions of Türkiye

Restrictions on high-GWP refrigerants have made the use of transcritical CO2 refrigeration systems widespread. Using transcritical booster refrigeration cycle in warm climates is unsatisfactory due to its high energy consumption. This paper presents theoretical analysis and performance comparison of three different transcritical CO2 supermarket refrigeration cycle configurations with ejector expansion in Türkiye, which has different climatic regions. Bin-hour data were derived using hourly dry-bulb temperature values for provinces from 7 different regions in Türkiye. The applicability of multi-ejectors to each modeled cycle was also investigated. Annual energy consumption and environmental impact reductions of up to 17% were obtained using ejector expansion cycle compared to booster cycle. Ejector expansion cycles achieved higher performance than booster cycle up to 46% in terms of exergy efficiency at investigated ambient temperatures. Unit product exergy costs of the ejector cycles were found up to 28% lower than booster cycle.

A new CO2 refrigeration system with two-phase ejector and parallel compression for supermarkets

This study explores the integration of parallel compression and a two-phase ejector in transcritical CO2 refrigeration systems, aiming to improve efficiency and performance. This innovative approach bridges the gap between conventional approaches and explores new energy-saving potential. The study uses thermodynamic modeling, mathematical simulation, and in-depth analysis to look at energy and exergy performance in a new configuration for applications in the retail sector at medium evaporation temperatures. The work investigates thermodynamic phenomena in a novel cycle with steady-state conditions, low pressure differentials, and adiabatic efficiency. The model is validated against experimental and theoretical published data, revealing component-specific exergy destruction and key parameters. The novel cycle efficiently extracts heat at higher temperatures, outperforming conventional and parallel cycles. Exergetic efficiency surpasses the standard cycle, with gas cooler pressure and temperature dependence enhancing efficiency by 40%–45%. The distribution of exergy destruction percentages reveals efficiency determinants, emphasizing heat exchange optimization and ejector responsiveness in energy dissipation dynamics. The study investigates the coefficient of performance (COP) dependence on gas cooler pressure and temperature, revealing superior performance compared to conventional cycles. COP increases by 50% at 80 bars, indicating enhanced efficiency. The new cycle offers exceptional efficiency gains, with a COP enhancement of over 75% for evaporator temperature transitions. Comparative analysis shows a COP superiority of up to 53% for lower evaporator temperatures and 20% for higher evaporator temperatures, demonstrating substantial energy savings and improved performance across various operating conditions.

Energy and Conventional Exergy Analysis of an Integrated Transcritical CO2 (R-744) Refrigeration System

This study analyses the performance of an integrated transcritical CO2 (R-744) refrigeration system operating in winter conditions within a supermarket in Trento, north Italy. This system fulfils multiple functions, providing the heating (for domestic hot water and space heating), cooling, and freezing capabilities for the supermarket. Energy analysis reveals that the average value of the total coefficient of performance, total COP, over the entire study period is calculated at 2.47. Notably, the medium-temperature (MT) compressor rack exhibits the highest power consumption, especially in sub −5 °C conditions. The auxiliary (AUX) compressor rack and the gas cooler (GC) fan contribute significantly to the electrical power usage. The air conditioning (AC) heating load is consistently high, averaging 41.6 kW, while the domestic hot water (DHW) heating load remains stable at approximately 5 kW. The refrigeration demands include an average MT cooling load of 25.86 kW and a low-temperature (LT) freezing load of 10–15 kW, with an average of 13.76 kW. The current study also delves into exergy analysis, disclosing an overall system exergy efficiency of 22.4%. The AUX compressor rack is identified as the primary exergy destructor, followed by the GC, AC coils, MT compressor rack, and the ejector. The LT compressor rack has the highest exergy efficiency, followed by the MT and LT expansion valves.

Simulation study of key cross-sectional area dimensions of CO2 ejector on performance

In this paper, based on the homogeneous equilibrium model, the effects of nozzle inlet cross-sectional area, nozzle throat cross-sectional area, pre-mixing chamber cross-sectional area, mixing chamber cross-sectional area, and diffusion chamber outlet cross-sectional area on the performance of the ejector of a CO2 transcritical refrigeration system are analyzed by using Ansys Fluent. The simulation results show that the primary flow rate of the ejector is sensitive only to the nozzle throat cross-sectional area, while the entrainment ratio is strongly influenced by the nozzle throat and the mixing chamber cross-sectional area, and there is an optimum value. The nozzle inlet cross-sectional area has no significant effect on the performance of the ejector. The pre-mixing and diffusion chamber outlet cross-sectional areas do not have a significant effect on the performance within a limited range, but deviating too much from the optimum value can also lead to degradation of ejector performance.

Effect of spindle position on the performance and two-phase characteristics of controllable ejector for transcritical CO[formula omitted] refrigeration system

Spindle-controlled variable-geometry ejectors can adjust the system capacity to meet varying load requirements in transcritical-CO2 refrigeration systems. The present study numerically explores the effect of spindle position on the performance and flow characteristics of a controllable two-phase CO2 ejector. Spindle-controlled ejectors achieved entrainment ratios higher than that of fixed geometry ejector. Motive and suction flows exhibited high sensitivity to the spindle position near the critical operating range because of the transition between operating modes. The two-phase and shock characteristics associated with ejector flow and the variations observed for different spindle positions are discussed. Critical discharge pressures and entrainment ratios are estimated for various spindle positions. The critical compression and entrainment ratios correlate with the respective normalized spindle position in the ejector. The impact of the effective nozzle throat area on the flow entrainment range and achievable pressure lift is investigated. The results showed that the critical entrainment ratio bears a common linear relationship with the critical compression ratio for all positions of the spindle. The study relates the most significant performance and geometric parameters of the spindle-controlled two-phase CO2 ejector.

Experimental study of a two-phase ejector for CO2 transcritical refrigeration system

Experimental study of a two-phase ejector for CO2 transcritical refrigeration system

Performance evaluation of transcritical CO2 refrigeration system for simultaneous heating and cooling applications

Performance evaluation of transcritical CO2 refrigeration system for simultaneous heating and cooling applications

Performance evaluation of absorption cooling assisted transcritical CO2 refrigeration systems

Studies have shown appreciable improvements in system performance by subcooling the working fluid in transcritical CO2 refrigeration systems. The use of a dedicated vapour absorption refrigeration system (VARS) for subcooling the high side working fluid downstream of the heat rejection device is proved to be a viable option. The heat available in the discharged gas downstream of the compressor in the transcritical CO2 system is used to drive a LiBr-H2O VARS. This study examines the performance of an absorption cooling-assisted transcritical CO2 system. Three different configurations of the absorption system are considered, namely, single effect (SE), series double effect (DE), and series double effect with dual heat input (DEDH). The performance analysis is carried out for gas cooler exit temperatures (Tgc) ranging from 30 to 50 °C and evaporation temperatures ranging from -10 to 5 °C. The average COP improvement of the CO2 system is found to be 29 and 27% using SE and DEDH systems respectively.

Exergetic and energetic evaluation of a modified tc-CO2 refrigeration system assisted with an absorption chiller using natural refrigerants

Transcritical CO2 (tc-CO2) refrigeration system is a pertinent option for refrigeration at low temperature, but it has high energy consumption with low coefficient of performance (COP) and exergetic efficiency. Thermodynamic properties of CO2 are favorable to be used as refrigerant with zero ozone depletion potential (ODP) and very low global warming potential (GWP). The tc-CO2 system rejects a huge amount of energy at higher temperature in the gas cooler which can be utilized to run the absorption chiller and produce desired cooling effect, in various combinations. The current research focuses on the energy and exergy analysis of a tc-CO2 refrigeration system that aspires to accomplish low-temperature refrigeration. A single-effect absorption chiller is used to give subcooling to the tc-CO2 refrigeration system, which is powered by the heat available at high temperatures and pressures at the exit of the compression process of the former system, rather than any other external heat source. A work recovery turbine is also used as the expansion device to reduce throttling losses and electricity consumption to power the integrated system. The parametric study reveals that there is a significant improvement in the refrigerating capacity, COP and exergetic efficiency of the system.

Enhancing energy efficiency and sustainability in ejector expansion transcritical CO2 and lithium bromide water vapour absorption refrigeration systems

Enhancing energy efficiency and sustainability in ejector expansion transcritical CO2 and lithium bromide water vapour absorption refrigeration systems

Optimization analysis of Organic Rankine Cycle powered by waste heat of a supermarket transcritical CO2 multi-ejector refrigeration cycle

The exploitation of low-grade heat sources that are available from existing thermal systems processes consists a promising concept for the production of energy. In this research, the integration of an Organic Rankine Cycle (ORC) unit on a supermarket transcritical CO2 refrigeration system with ejectors is investigated, which is supplied with low-grade heat by the common discharge of the compressors. The optimization analysis proves that for different combinations of CO2 temperature and pressure in the ORC evaporator, as well as for different ORC condensation temperatures, there is a different optimum ORC evaporation temperature that leads to the highest energy production. Concerning the annual performance analysis conducted for the city of Athens, Greece, operation with isopentane is found to provide the greatest energy production with 10.36 MWh annually, which corresponds to a 3.69% reduction in the refrigeration system's energy consumption. For operation under the ambient temperature of 40 °C, the corresponding reduction of the refrigeration system consumption due to the presence of ORC is 7.54%. Finally, the financial analysis reveals that toluene is the most advantageous solution, leading to a payback period of 3.9 years, with a net present value by the end of the project's lifetime equal to 34.32 k€.

Review of Recent Advances in Transcritical CO2 Heat Pump and Refrigeration Cycles and Their Development in the Vehicle Field

Refrigerant substitution is an urgent need in the context of reducing carbon emissions and slowing global warming. CO2 is now being proposed as a promising solution based on its excellent properties and system performance, especially in low-temperature environments. This paper presents an overview of recent advances in system configuration and operation characteristics to improve the performance of transcritical CO2 heat pump and refrigeration systems. The paper first introduces the basic research background, system cycle, and thermodynamic characteristics. Secondly, CO2 cycle improvements with single modifications and modification combinations are reviewed. Then, some important operation characteristics and control methods are discussed. Additionally, the paper provides a detailed description of the development of transcritical CO2 heat pump and refrigeration systems in the vehicle field. At the end of this review, conclusions and opportunities for future work in this field are presented.

Transkritik Soğutma Çevrimlerinde Gaz Soğutucu Geometrisinin Sayısal Optimizasyonu

Traditional halocarbon – based refrigerants tend to considerably increase global warming and ozone depletion factors. Therefore, CO2 is fast becoming a key instrument as a natural refrigerant which was widely applied and attracted the consideration of the research community. The gas cooler is an important component in the CO2 transcritical refrigeration system and plays a key role in the performance due to the determination of operating pressure consequently power consumption. In this research, the performance characteristics of a CO2 gas cooler having wavy fin geometry, which is currently used in industries such as air conditioning, automotive and aviation, was determined experimentally in a calorimetric test room. The experimental results was used as benchmark data to validate the three – dimensional numerical model. Laminar model and realizable k - ɛ turbulent model were employed for analyses. Moreover, the second order upwind scheme was considered to discretize momentum and energy equations. Accordingly, a multi-objective optimization process has been performed employing Response Surface Method (RSM) to determine the optimum wavy fin geometry in CO2 transcritical refrigeration system. Four geometrical parameters namely longitudinal pitch, half transverse pitch, tube outer diameter, and fin pitch of the gas cooler were optimized. According to results, the new optimized CO2 gas cooler exhibited lesser pressure drop and higher heat transfer capacity in comparison with the tested gas cooler geometry used in the industry. It was found that the overall heat transfer coefficient enhancement is between 5.4 – 12.2 % while pressure drop decreases about 175.08 – 188.58 % for three different inlet velocities.

Dynamic simulation study on regulation characteristics of transcritical CO2 multi-ejector refrigeration system

This paper presents a dynamic model of transcritical CO2 refrigeration system with multi-ejector which is developed based on the real CO2 properties and considering the internal transonic flow, phase change and choking, and its predictions are in good agreement with experimental data from the literature. The transient response of the system is studied by adjusting the speed of compressor, water flow of heat exchanger, opening of expansion valve and combination of ejector as disturbance conditions. The presented results show that the developed dynamic model has similar response rules with the same type of system. Changes of cooling water flow of gas cooler and chilled water flow of flooded evaporator can cause significant response of other pressures in the system except low-temperature evaporator, and can regulate the cooling and heating load to some extent. Opening of medium and low temperature expansion valve plays a decisive role in the system's medium and low temperature refrigeration capacity respectively, and the low-temperature expansion valve is the only regulation means of the low-temperature cooling capacity. Optimal operating combination of multi-ejector maximizes the system performance and ejector pressure lift ratio. Furthermore, the optimal ejector combination of the system under the refrigeration capacity of 75∼115 kW is studied and compared, a control strategy of adjusting ejector combination by "nozzle mass flow rate" is put forward to adapt to the high-performance operation of the system under different loads. Under the research conditions, the optimal ejector combination satisfies that the nozzle mass flow rate is stable in 0.066∼0.070 kg·s-1.

Design and performance simulation of a novel waste heat-driven liquid desiccant combined with transcritical CO2 refrigeration system for ocean-going vessels in typical shipping routes

Design and performance simulation of a novel waste heat-driven liquid desiccant combined with transcritical CO2 refrigeration system for ocean-going vessels in typical shipping routes