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Optimal operation and design of ejectors are the subject of recent concerns, especially for the enhancement of refrigeration and heat pump cycles based on natural refrigerants like carbon dioxide. In this study, a thermodynamic analysis of an ejector-based refrigeration cycle is performed to determine both what operating pressures lead to the highest physically possible performance depending on the ambient conditions, and what are the main dimensions of the ejector leading to the best performance at a given ambient temperature. A state-of-the-art thermodynamic model for the prediction of the ejector performance is for the first time utilized to generate reliable operation and performance maps of the ejector cycle. Most notably, it is found that the optimal coefficient of performance is not necessarily found when the ejector operates at critical conditions but mostly when the device is under off-design regime, depending on the ejector internal efficiency and the hot side temperature. In addition, the analysis reveals that the performance of the cycle is not highly sensitive to the throat area ratio of the ejector given that the latter lies within an acceptable range. Those findings contribute to getting a better understanding of how the cycle benefits from the ejector and define design and control strategies for the cycle. [ABSTRACT FROM AUTHOR]

• Boiling coefficient increased with nozzle inlet pressure and specific enthalpy. • Boiling coefficient ranged from 300 to 4000 in the investigated trans-critical region. • Modified HNB showed an accuracy of 3 % in predicting the nozzle flow rates. • Proposed method for determining saturation pressure was superior to constant value. A modified homogeneous non-equilibrium boiling approach (HNB) was proposed for high-fidelity modelling of the trans-critical flashing process in two-phase R744 ejector nozzles. In the modified approach, the method of determining the metastable liquid saturation pressure was improved by integrating the metastable liquid enthalpy. Experimental tests were conducted on two different nozzles to calibrate the lumped accommodation coefficient in the modified HNB. The calibration dataset consisted of 30 trans-critical test points, with the motive inlet pressure ranging from 7.5 MPa to 11.5 MPa and temperature varying from 21 °C to 45 °C. It was demonstrated that the boiling coefficient was strongly related to nozzle inlet pressure and specific enthalpy. The coefficient was therefore fitted as a function of the operating condition. The modified HNB with the fitted function was then validated against another set of test data composed of 29 test points. The results demonstrated that the relative errors of the nozzle mass-flow rates were within 3 %. Finally, the modified HNB with the improved metastable liquid saturation pressure was compared and discussed with the existing HNB approach in the literature. [ABSTRACT FROM AUTHOR]

A R744 (CO 2) refrigeration system has been designed to cool down the Large Hadron Collider (LHC) silicon detectors ATLAS and CMS, located at CERN, Switzerland. The silicon detectors are subjected to high radiation levels. The system is composed of a pri- mary CO 2 trans-critical booster vapor compression loop operated with piston compressors, and an oil-free liquid pumped loop on the evaporation side, to preserve the detectors. To ensure the system's reliability, the cooling facility is designed to operate under a parallel operation mode of several modular 70 kW plant units providing evaporation temperature as low as -53 °C. This layout, is also useful in case of components failure and maintenance. A numerical model is developed using a dynamic simulation software Dymola that is based on the open source Modelica modelling language. The simulation results are proven on a first demonstration plant (System A) experimentally to explore the systems control logic and to validate the reliability of the system before it is built on the detectors side. In this paper the models development is explained and the results of the experimental validation of the numerical model are shown. [ABSTRACT FROM AUTHOR]

• Ejector cycles achieved higher performance than booster cycle within the gas cooler outlet temperature range of 31 and 50°C. • Multi-ejectors can be applied to ejector cycles with different configurations. • Up to 17% annual energy consumption reduction was found using ejector cycles. • Unit product exergy costs of ejector cycles are up to 28% lower than booster cycle. Restrictions on high-GWP refrigerants have made the use of transcritical CO 2 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 CO 2 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. [Display omitted] [ABSTRACT FROM AUTHOR]

• A novel general method for the construction and interpolation of the equation of state is described. • A density-based formulation of the Homogeneous Relaxation Model is implemented for the first time in a CFD frame. • The impact of the metastability on local flow quantities and flow rates is studied for CO 2 flashing nozzles. • It is found that the experimental pressure profile is best fitted by the Homogeneous Relaxation Model in the near-throat region. Accurate prediction of the flow physics within nozzles and ejectors working with two-phase carbon dioxide remains particularly challenging. Indeed, the flashing phenomenon in the device often comes along with the formation of the metastable phase, which can severely affect both the flow physics, and the flow rate. Due to the particular challenges associated with building and operating two-phase carbon dioxide experimental setups, computational fluid dynamics constitutes a compelling alternative to study and predict nozzle and ejector flows. In this work, the metastability effects are modeled through a Homogeneous Relaxation Model for the first time expressed in density-based formulation, within the SU2 solver. The thermodynamic relations for the equation of state are tabulated using a novel quad-tree algorithm and directly coupled to the solver, ensuring fast and accurate predictions. The simulation tool is then validated against experimental nozzle pressure profiles and used to compare the commonly used Homogeneous Equilibrium Model to the Homogeneous Relaxation Model in terms of local flow topology and mass flow rates. Most notably, it was found that the Homogeneous Relaxation Model was able to better fit the experimental pressure profiles within the first part of the expansion. [ABSTRACT FROM AUTHOR]

• A novel R744 multi-temperature unit for last mile delivery is presented. • MT and LT ejectors are specifically designed for this application. • LT ejector allows using a single stage compressor at high ambient temperature. • Ejector cycle leads to significant increase of system efficiency. • Back-pressure cycle substantially increases flexibility in cooling power range. A novel R744 vapor-compression refrigeration system has been developed to meet the cooling needs of a medium-size refrigerated truck at two temperature levels: 4-5 kW for medium-temperature (MT) refrigeration at 0°C and 1-2 kW for low-temperature (LT) refrigeration at -20°C. This system is designed for transporting chilled and frozen goods efficiently during last-mile deliveries in urban areas. The key innovation of this system lies in its single compression stage with two different evaporation levels. Firstly, the unit incorporates an MT ejector to enhance energy efficiency by reducing the compressor pressure ratio. Furthermore, an LT ejector is employed to allow providing LT cooling with a single-stage compressor by pre-compressing vapor before it enters the compressor, avoiding excessive compression ratios. This extends the LT operational range with a single stage of compression from 34°C to 40°C ambient temperature. When the ambient temperature is not sufficient to sustain the ejector cycle, the system can switch to a back-pressure cycle. A numerical model of this refrigeration system has been developed to evaluate its steady-state and dynamic performance in both back-pressure and ejector configurations. For a specific cooling effect production (3.8 kW in MT, 1.1 kW in LT operation), the ejector cycle increases the COP by 25.8% for MT and 42.0% for LT operation compared to the back-pressure cycle. However, the back-pressure cycle offers greater flexibility in cooling power production, ranging from -32.8% to +8.2% for MT and -8.9% to +82.5% for LT, making it useful for pulldown and part-load operations. [ABSTRACT FROM AUTHOR]

• A novel linear carbon dioxide compressor is introduced. • Resonance springs can be replaced with gas springs and magnetic springs. • The dynamic characteristics of the proposed compressor are studied. • The designed compressor achieves over 20 % reduction in leakage. In this study, an innovative two-stage linear CO 2 compressor with dual piston is proposed and its simulation model is developed. The dynamic characteristics of the two-stage dual-piston compressor are comprehensively analyzed. The simulation results indicate that the dual-piston configuration leads to gas forces oscillating around the zero axis, resulting in a reduction in the mean gas force, allowing for the replacement of the resonance spring with gas springs and magnetic springs. The dynamic performance of the two-stage linear compressor is compared with that of a single-stage linear compressor. The moving center offset of the dual-piston two-stage compressor is reduced by approximately 90 %. The gas equivalent stiffness and the equivalent damping at the TDC (Top Dead Center) get lower than the single-stage one by 25.7 % and 11.6 % as the pressure ratio is set at 2.5. The leakage during the compression process gets lower by more than 20%. The dual-piston compressor demonstrates excellent capacity and modulation capabilities, with a theoretical increase in cooling capacity of over 300 W when the stroke is increased from 6 mm to 8 mm. [ABSTRACT FROM AUTHOR]

• A traditional R134a and a novel R744 cooling units are compared over their lifetime. • Dynamic numerical simulations are used to assess the performance during operation. • The total carbon footprint of the units is evaluated from a life cycle perspective. • On annual basis, the R744 unit COP is 27.5 % higher than the R134a unit. • The lifetime carbon footprint of the R744 unit is 34.8 % lower than the R134a unit. In recent years, the temperature-controlled transport sector has experienced a substantial growth, and this trend is expected to accelerate in the coming years. However, as the sector expands, it becomes imperative to address and mitigate the carbon footprint of transport refrigeration systems. The carbon footprint of a refrigeration system comprises both direct emissions from refrigerant leakage and indirect emissions derived from energy consumption. In this study, dynamic numerical simulations are used to evaluate the annual performance of a conventional HFC (R134a) transport refrigeration unit and of a newly developed natural refrigerant (R744) one employed in a multi-drop delivery mission in urban environment. Additionally, a simplified life cycle approach is used to compare the carbon footprint of these cooling units across various stages of the system supply chain, including refrigerant production and recycling, unit manufacturing, operation and disposal. Results show that the R744 unit presents significantly higher COP for high ambient temperatures, while its performance is degraded for low ambient temperatures due to low duty cycle. On annual basis, the R744 unit COP is 27.5 % higher than the R134a one. However, the increased weight of the R744 unit results in significantly higher emissions associated with fuel consumption required to transport the unit weight, leading to slightly higher fuel related emissions (+9.3 %) for the R744 unit. Nevertheless, the high GWP of R134a contributes to substantial direct emissions due to refrigerant leakages, resulting in an overall 31.9 % reduction in the lifetime CO 2 equivalent emissions for the R744 unit when considering the total emissions. [ABSTRACT FROM AUTHOR]

• A novel CO2/NH3 cascade refrigeration system with subcooling in LTC and HTC is proposed. • The proposed system is compared with conventional HFC-404A and CO2/NH3 cascade refrigeration systems. • Mathematical models are based on the actual compressors equations. CO 2 /NH 3 cascade refrigeration system is an all-natural refrigeration option which can be specifically designed and gainfully operated for low temperature applications in warm ambient conditions. In the present study, a novel CO 2 /NH 3 cascade refrigeration system with subcooling is proposed and evaluated as an alternative to the HFC-404A refrigeration system, which is typically used for blast freezer applications in the seafood processing industry. The performance of the proposed system is also compared with a conventional CO 2 /NH 3 cascade refrigeration system without any subcooling arrangement. Real compressor specific equations were used to simulate the performance of the systems under various conditions. The proposed system yields 26% higher performance ratio (COP) than the conventional HFC-404A system at the design condition. In addition, the proposed subcooling arrangement enhances COP of the conventional CO 2 /NH 3 cascade unit by around 5%. The system performance is investigated theoretically under various condensing temperatures and subcooling degrees. The annual energy demand and total equivalent warming impact (TEWI) of the proposed system are 15% and 48% lower than for the conventional HFC-404A system which results in significant economic and environmental benefits. Overall, the proposed system exhibits superior energy efficiency and offers a climate and environmental friendly alternative to the conventional environmentally harmful systems within the seafood industry. Adoption of such energy efficient all natural and clean refrigerant technology is crucial for the future phase out of hydrofluorocarbon (HFC) fluids, as ratified in accordance with the Kigali Amendment to the Montreal Protocol in India and other tropical countries. [ABSTRACT FROM AUTHOR]

• A novel R744 multi-temperature unit for refrigerated road transport is presented. • An ejector is used to replace the low-temperature compressor. • Experimental tests on the ejector operating in LT suction conditions are performed. • The pressure lift requirement significantly influences the ejector performance. • The unit operating conditions choice should consider both cooling effect and COP. A novel vapor-compression system concept employing carbon dioxide as the refrigerant is proposed to serve the needs of a typical medium-size refrigerated truck used for multi-temperature (MT and LT) goods delivery. The system design is based on the implementation of an ejector as the only component increasing the refrigerant pressure from the LT evaporation pressure to the MT evaporation pressure, thus providing cooling effect at two different temperature levels with only one stage of compression. The ejector was experimentally tested and its ability to effectively entrain mass flow rate from low pressure suction conditions (corresponding to a LT evaporation temperature of −25 °C) was assessed. Lower external ambient temperatures and consequent lower expansion energy available at the ejector motive nozzle leads to a reduction of the maximum achievable pressure lift. Moreover, a significant degradation of the ejector performance towards the highest pressure lifts is experienced. Based on the ejector experimental data, a numerical evaluation of the proposed cooling unit performance has been performed, highlighting that in design conditions (LT evaporation at −25 °C) the cooling unit provides a LT freezing power ranging between 1.1 kW and 2.3 kW and a corresponding minimum MT cooling power ranging between 5.1 kW and 3.8 kW, depending on the chosen ejector lift. The MT cooling power can be further increased by increasing the compressor mass flow rate. The system COP is maximized at the maximum available lift provided by the ejector. [ABSTRACT FROM AUTHOR]

• Viscosity, thermal conductivity, and interfacial tension of (CO 2 + R32) have been determined • Performance of existing models has been evaluated at industrial-relevant conditions • Accuracy of models implemented in REFPROP 10.0 has been improved Reliable understanding of viscosity (η), thermal conductivity (λ), and interfacial tension (γ) are demanded in the refrigeration process, especially in the heat, mass and momentum transfer calculations. In this work, measurements of these thermophysical properties for (CO 2 + R32) have been conducted by vibrating wire viscometry, transient hot-wire technique, and differential capillary rise approach. The experimental condition ranges from (208.4 to 344.4) K and pressures up to 7.58 MPa at x (CO 2) = 0.7, 0.8 and 0.9, including those in the single-phase and near the melting curves. The standard uncertainties (k = 1) are between (0.21 and 6.80) μPa·s, (0.00012 and 0.00290) W·m–1·K–1, and (0.13 and 0.67) mN·m–1 for viscosity, thermal conductivity, and interfacial tension, respectively. The achieved results and the literature data (if applicable) were utilised to regress the extended corresponding states correlation and Parachor approach implemented in REFPROP 10.0. With the regressed models, most viscosity and thermal conductivity results can be described within 4%. The determined data and improved model provided here should contribute significantly to the design margin minimisation in the refrigeration cycle. [ABSTRACT FROM AUTHOR]

There are currently many drivers pushing the transport refrigeration sector towards more sustainable refrigeration systems, being at the same time more efficient and adopting Natural Working Fluids (NWF) in place of synthetics. Due to the variety of involved means of transport, the different and challenging boundary conditions and safety requirements and not last the fact of not being the major user of refrigerants, the refrigeration systems for mobile refrigeration have not always been dedicated the same R&D effort as stationary refrigeration and air conditioning. However, it is now becoming more and more urgent to identify sustainable future proof solutions for the sector. To this extent, transport refrigeration can benefit from the experience gained in other sectors and achieve sustainable solutions in a short time. There are many examples of efficient NWF based systems already out in the market; some others are on the way, also supported by international projects, and others still need some regulatory provisions to become widespread, mainly those involving flammable refrigerants. Innovations in the refrigeration systems need finally to keep the pace and harmonize with the revolution that the transport sector is undergoing towards hybrid and electric vehicles. [ABSTRACT FROM AUTHOR]

• The critical properties data for R1234ze(E) + CO 2 binary system were measured. • Measurements were carried out based on the metal-bellows variable-volume method. • Experimental data were correlated by the Modified Chueh-Prausnitz (MCP) method, the Modified Wilson (MW) method, and the Redlich-Kister (RK) method. In the context of global warming and demanding alternative refrigerants, the transcritical CO 2 refrigeration cycle was considered a potentially energy-saving and carbon-reducing technology. The use of CO 2 mixtures with higher critical temperatures and lower critical pressures has been demonstrated as an effective way to improve the performance of transcritical CO 2 refrigeration cycles. To achieve this approach, accurate critical locus parameters are needed for establishing cross-over state equations and calculating thermodynamic properties in the near-critical region. Currently, experimental methods are the most effective way to obtain high-precision critical locus parameters of mixtures. In this work, the critical properties including critical temperature, critical pressure, critical density, and mole fraction of the R744 (carbon dioxide) + R1234ze(E) (trans-1,3,3,3-tetrafluoropropene) binary mixture were measured according to a variable-volume method with a metal-bellows volumeter. The critical point was determined by visual observation of the critical opalescence and reappearance of the vapor-liquid meniscus. The combined expanded measurement (relative) uncertainties of the critical temperature, critical pressure, critical density, and mole fraction were within 50 mK, 0.021 MPa, 0.6%, and 0.012 (k = 2, 0.95 level of confidence), respectively. After comparing the fitting results of the Modified Chueh-Prausnitz (MCP) method, the Modified Wilson (MW) method, and the Redlich-Kister (RK) method. We recommended the Modified Wilson method to correlate the critical properties of the binary system of R1234ze(E)+R744. Our research showed that R1234ze(E)+R744 mixture can achieve a higher critical temperature and critical density simultaneously and a lower critical pressure compared with pure CO 2 when the composition of R744 is less than 0.23. [ABSTRACT FROM AUTHOR]

• Four kinds of ice-making refrigeration cycles were studied with 1-D models. • 2-stage compression and ejector systems were analyzed with exergy methods. • 2-stage compression and medium-pressure ejector system is optimal in heat recovery. • 2-stage compression and low-pressure ejector system is better for ice making only. • The optimized order of components is obtained by using the exergy analysis. The transcritical CO 2 direct cooling technology was used for the first time in the indoor venues of the 2022 Beijing Winter Olympic Games. The technology has a series of advantages, such as compact structure, uniform ice temperature, energy saving and environmental protection. In order to further reduce the power consumption and improve the performance, two-stage compression and low-pressure ejector direct cooling ice making system (TCLES) and two-stage compression and intermediate-pressure ejector direct cooling ice making system (TCIES) are studied. The performance of the systems is simulated by constructing a one-dimensional thermodynamic model. The impacts of the gas cooler pressure, gas cooler outlet temperature, evaporation temperature and intermediate pressure on these systems are considered. It is showed that TCLES and TCIES have their own advantages when the system is without and with heat recovery, and found that two-stage compression improves the system performance more than the ejector when only the cooling performance is considered. On this basis, conventional exergy and advanced exergy analysis of both systems are carried out. Advanced exergy analysis shows that exergy destruction can be avoided mainly in low-pressure and high-pressure compressors of both systems. Besides, the influence of gas cooler pressure on exergy destruction characteristics is discussed. When controlling the gas cooler pressure at 9.0 MPa, the avoidable endogenous exergy destruction of compressors and ejector is reduced by a maximum of 4.14% and 13.92% for TCLES and reduced by a maximum of 16.15% and 28.04% for TCIES. [ABSTRACT FROM AUTHOR]

Une personne qui parle, qui tousse ou qui éternue émet dans l'air des gouttelettes respiratoires de différentes tailles. Les plus grosses sédimentent après l'émission, les plus petites se déshydratent pour former des particules fines qui vont rester en suspension dans l'air, sous forme d'un aérosol. En cas d'infection respiratoire, les gouttelettes peuvent contenir des agents infectieux, surtout viraux ou bactériens. Selon la survie de l'agent infectieux dans l'air, la transmission par inhalation est possible entre deux personnes, soit en proximité, soit à distance. L'aération des locaux permet de diluer et d'éliminer les particules infectantes en suspension dans l'air. La mesure du dioxyde de carbone (CO 2), traceur du renouvellement de l'air intérieur, permet de définir la durée et la fréquence d'ouverture des fenêtres. A person who speaks, coughs or sneezes emits respiratory droplets of various sizes into the air. The larger sediments after the emission, the smaller ones dehydrate to form fine particles that will remain airborne like an aerosol. In case of respiratory infection, droplets may contain infectious agents, especially viral or bacterial. Depending on the survival of the infectious agent in the air, inhalation transmission is possible between two people, either in proximity or at a distance. The aeration of the premises makes it possible to dilute and eliminate infectious particles suspended in the air. Measurement of carbon dioxide (CO2), an indoor air renewal tracer, allows you to define the window opening time and frequency. [ABSTRACT FROM AUTHOR]

• Thermodynamic properties of (CO 2 + R32) have been determined. • The determined properties of (CO 2 + R32) reveal the deficiency of existing models. • The Helmholtz energy model was significantly improved based on the available results. Accurate thermodynamic property data and models are demanded to reduce the design margins of industrial processes based on these fluids. In this work, measurements of density (ρ), heat capacity (c p), and vapour-liquid equilibrium (VLE) for CO 2 + difluoromethane (R32) have been performed by the vibrating tube densimeter and the magnetic suspension balance densimeter, the visual cell and gas chromatograph, and the differential scanning calorimeter, respectively. Experiments were conducted over the temperatures ranging from (208.3 to 334.3) K and pressures reaching 10.04 MPa at CO 2 concentrations of 0.7, 0.8 and 0.9. The measured data, together with the results reported in literature where applicable, were subsequently applied to regress the binary interaction parameters utilised in the mixture functions of Helmholtz energy model from REFPROP 10 software package. Noticeable improvements have been achieved for the model's ability to represent thermodynamic property data. The most significant achievement exists in density description: compared with the default parameters, the root-mean-square (RMS) deviation has been decreased by half. [ABSTRACT FROM AUTHOR]

• A novel CO 2 booster system for commercial refrigeration is presented. • The objective is to simplify ejector-supported booster systems keeping efficiency. • The system layout was experimentally validated and benchmarked. • Efficiency increased up to 40 % compared to conventional booster systems. • The novel configuration performed positively under all ambient conditions. The pernicious effects of synthetic refrigerants on different environmental aspects leave natural refrigerants as the only alternative for vapour compressions systems. Among natural refrigerants, CO 2 (R744) has become the preferred choice for commercial refrigeration at almost any location and climate. However, efficient R744 refrigeration systems for warm climates have a great level of complexity, implementing technologies such as mechanical subcooling or ejector that increase the investment costs. The goal of the novel hybrid configuration presented in this work is to simplify ejector-supported R744 commercial refrigeration systems while maintaining all the benefits of the ejector implementation in transcritical operation mode. Moreover, utilization of a low-pressure accumulator layout in the subcritical mode removes all the practical challenges related to the booster layout operating at low ambient temperatures. This solution is based on: (i) MT and LT compressor suction groups, (ii) non-superheated MT evaporation with increased evaporation temperature, and (iii) ejector utilization throughout the year. The ejector is actively operated as a high-pressure-control device at elevated ambient temperatures ('summer mode'), while it is passive and acts as a check-valve at lower ambient temperatures ('winter mode'). The experimental campaign performed proved that this novel system configuration is more energy efficient than booster systems with parallel compressors at any condition, improving the COP by around 40 % at the most extreme gas cooler outlet temperatures tested, i.e., 40 °C (summer mode) and 10 °C (winter mode). [ABSTRACT FROM AUTHOR]

Since the last decade of the 20th century, there has been increasing attention to the use of CO 2 as a working fluid in transcritical refrigeration and cooling cycles. Transcritical cycles suffer from a heavy expansion loss, requiring specific means for its mitigation. Moreover, most applications for cooling or refrigeration are cooling a stream that is not changing phase, therefore has a finite heat capacity and does not well match with the evaporation curve of any pure fluid. In this work, to overcome these limitations (low COP and poor matching of heat exchanges curves) the potential of using a second component, mixed with CO 2 , has been assessed. For this purpose, several fluids have been investigated, from synthetic refrigerants, such as R1233zd(E) or R1234yf, to hydrocarbons, such as n-butane, or n-pentane and alcohols, like dimethyl ether. To investigate the effects of the addition of the second element in the CO 2 transcritical cycles, several configurations have been investigated, from the base refrigeration cycle to the double evaporator ejector cycle. All examined mixtures show a COP improvement with respect to pure CO 2. However, the mixture with R1234yf has the flattest evaporation curve and hence, in the case of a constant specific heat of the fluid to be cooled, gives the best result. The quantity of R1234yf that must be added to the mixture never exceeds 15%. [ABSTRACT FROM AUTHOR]

• CO 2 refrigeration systems operate in both subcritical and transcritical modes. • Consistent control strategy can be independent of operation modes. • Average COP losses of typical systems with new control strategy are within −0.5%. CO 2 refrigeration systems operate in both subcritical and transcritical modes throughout a year. Different control strategies have been developed under different operation modes. However, this leads to complexity in implementation. In this work, we proposed a consistent control strategy (CCS) independent of operation modes for CO 2 refrigeration systems to provide stable operation in a simple way. This strategy is grounded on the fact that discharge pressure control in transcritical mode is essentially regulating refrigerant charge in a running system. The active pressure control can be avoided with fine refrigerant charge management, thereby employing the same control strategy as in subcritical mode. Through numerical simulations, the characteristics of optimum charge in three typical CO 2 refrigeration configurations were identified, which are single-stage system, two-stage system, and booster system. The optimum charges of three systems under different operating conditions all vary less than 22%, particularly the booster system fluctuates by 6% only. By adopting CCS, the average COP losses for three systems are all within -0.5% relative to their ideal optimum performance. Compared with re-optimized conventional multi-zone control, the maximum deviation is -0.3%. Therefore, the proposed new strategy can achieve comparable quasi-optimum performance, while significantly simplifying control scheme. It can be an attractive solution for small units which require simple and robust control. [ABSTRACT FROM AUTHOR]

Growing season surface–atmosphere exchange of carbon dioxide and methane were quantified at Fish Island, a wetland site in the lower northeast Mackenzie River Delta, Northwest Territories, Canada. The terrain consists of low-center polygonal tundra and is subject to infrequent flooding in high water years. Carbon dioxide and methane fluxes were continuously measured using eddy covariance and the relevance of different environmental controls were identified using neural networks. Net daily carbon dioxide uptake peaked in mid-July before gradually decreasing and transitioning to net daily emissions by September. Variations in light level and temperature were the main controls over diurnal net carbon dioxide uptake, whereas thaw depth and phenology were the main seasonal controls. Methane emissions measured at Fish Island were higher than comparable studies on river delta sites in the Arctic and were influenced by the interaction of a large number of factors including thaw and water table depth, soil temperatures, and net radiation. Spikes in methane emissions were associated with strong winds and turbulence. The Fish Island tundra was a net sink for carbon during the growing season and methane emissions only slightly reduced the overall sink strength.

• Low-GWP refrigerants were tested in a cascade refrigeration system as replacements in the high-temperature cycle. • The cascade-condenser temperature difference varied from 2.5 to 5.5 K. • Cooling capacity varied from 2.3 to 3.8 kW and cold room temperature from −23.1 to −9.4 °C. • All tested refrigerants proved to be adequate replacements for R134a. • R436A and R1234yf reduce equivalent CO 2 emissions by 46 and 42%, respectively. This work presents an experimental analysis of the energy and environmental performance of low-GWP refrigerants R513A, R1234yf and R436A as alternatives for R134a in a cascade refrigeration system that operates with R744 in low-temperature cycle. An apparatus was built using a well-insulated room, where tests were carried out in a steady state condition. In addition to the typical components of a cascade cycle, an intermediate heat exchanger was used to thermally communicate the low and high temperature cycles, thus providing additional superheating and subcooling, respectively. The cooling capacity of the system ranged from 2.2 to 3.8 kW, the evaporation temperature varied from -34 to -24.5 °C and the air temperature inside the cold room ranged from -23.1 to -9.4 °C. The results revealed that the alternative refrigerants are potential substitutes for R134a in cascade refrigeration systems, showing an average increase in the CRC coefficient of performance of 3.1% for R436A, while R1234yf and R513A exhibited a reduction of 3.7% and 4.4%, respectively. The environmental impact analysis considered the scenarios of Chicago (USA) and Curitiba (Brazil), the latter being a southern Brazilian city with a wet climate throughout the year, a warm and mostly cloudy summer, and a short and cold winter. Total TEWI values, compared to those obtained with the originally designed R134a system, showed reductions in CO 2 equivalent emission of 46%, 42.1% and 22.5% for refrigerants R436A, R1234yf and R513A, respectively, for the Curitiba scenario, while the values for Chicago were increments of 0.3%, 2.4% and 5.8%, respectively. [ABSTRACT FROM AUTHOR]

• Existing two-phase ejector analysis does not take oil into account. • 1D simulation to consider the impact of oil in two-phase ejector. • Two-phase ejector internal oil obstructs flow and affects mass flow rate. • Ejector flow analysis according to OCR with varying external conditions. • Validation of simulation study through experiment with uncertainty analysis. The use of traditional refrigerants such as HCFCs and HFCs has a significant impact on global warming. On the other hand, natural refrigerants have little environmental impact with essential characteristics such as low global warming potential (GWP) and zero ozone depletion potential (ODP). Among various natural refrigerants, carbon dioxide is used as an alternative natural refrigerant due to its excellent heat transfer efficiency in a supercritical state. However, its COP is low due to its inherent high compressor work. To reduce this high compressor work, additional components such as ejectors are used for carbon dioxide cycles. In this study, a two-phase ejector was investigated. In particular, ejector analysis was performed in consideration of the influence of lubricant oil. In vapor compression cycle, lubrication is essential. This lubricant oil is discharged to the system through a compressor. When oil flows to system and enters the ejector, it forms a layer that can obstruct the flow inside the ejector. Oil circulation ratio (OCR) change can affect the mass flow rate of the ejector as if the inner diameter of the ejector is reduced. Therefore, it is essential to consider effects of OCR in a two-phase ejector analysis. As a result of the simulation, considering the effect of oil, a lower mass flow rate was predicted compared to existing analysis. Compared to previous studies without considering oil, there was an average mass flow rate difference of 13.6%. This simulation result was validated through experiments. [ABSTRACT FROM AUTHOR]

• Multiple factors affecting the sCO 2 -oil mixture cooling heat transfer are commented. • The formation of oil films is the primary reason that weakens the heat transfer. • The heat transfer correlations of sCO 2 -oil mixtures are summarized and evaluated. • Development directions for the sCO 2 -oil mixture cooling heat transfer are proposed. The presence of lubricating oil has been shown to have a significant effect on the heat transfer process of the CO 2 gas cooler. Previous review work has not comprehensively summarized and discussed the effect of lubricating oil on supercritical carbon dioxide (sCO 2) cooling heat transfer (CHT) in gas coolers. This study provides a comprehensive review of the effect of lubricants on the CHT of sCO 2. This study aims to provide a good insight for evaluating the influence of lubricating oil on the heat transfer performance of gas coolers and provide a reference to guide the actual design and optimization of gas coolers for oil-entrained cases. In the present work, the effects of temperature, the type and concentration of oil, mass flux, operating pressure, tube diameter, and tube configuration on the CHT of sCO 2 -oil mixtures are explored in detail. The results show that the presence of lubricant contributes to the formation of oil films and droplets, thereby impairing heat transfer. Compared to oil droplets, the oil film has a more profound effect on weakening the heat transfer. Moreover, this paper also summarizes and evaluates the sCO 2 -oil mixture heat transfer correlations and presents the recommended correlations. Finally, given the limitations in existing studies, two future research directions are put forward: research on CHT of sCO 2 -oil mixtures under different heat fluxes and development of heat transfer correlations. [ABSTRACT FROM AUTHOR]

According to the phase change property of carbon dioxide and the rapid cooling ability of liquid nitrogen, an experimental system of quick-frozen strawberry by combined jet of carbon dioxide and liquid nitrogen was designed. Through a series of experiments, the pressure changes and jet outlet temperature at different nitrogen jet speeds at different temperatures were studied. Then, the effects of nitrogen injection speed at different low temperatures and different inlet positions in the quick-frozen room on the quick-frozen strawberry freezing rate was investigated. The final results show that when the jet speed of low-temperature nitrogen increases, the pressure loss in the flow process also increases gradually. At the same time, the speed of mixing will increase, and the disturbance will increase, and the outlet jet temperature will also decrease. When the nitrogen inlet speed was 0.2 m/s, the temperature between the strawberry was uniform. Different inlet and outlet positions had no influence on the freezing time of strawberry. Quick-freezing room (c) improves the heat transfer efficiency and makes the temperature of strawberries more uniform. The results obtained lay a foundation for further research and development of an efficient quick-frozen product device. [ABSTRACT FROM AUTHOR]

• CO 2 GSHP vapor-compression cycle w/ liquid-line/suction-line heat exchanger (LLSL-HX). • Experimentally tested in cooling mode in an environmental chamber. • The COP ranged (7.3 to 2.4) for entering liquid temperature (10 to 39) °C. • An exergy analysis showed the compressor had the largest defect. • LLSL-HX gave small benefit to transcritical cycle and penalized subcritical cycle. A prototype CO 2 ground-source heat pump (GSHP), i.e. a water-to-air heat pump designed to couple with a ground heat exchanger, was tested in cooling mode per ISO standard 13256-1 to provide experimental data for assessing and improving CO 2 -based GSHPs. The GSHP consisted of a vapor-compression cycle with a liquid-line/suction-line heat exchanger (LLSL-HX). The system operated in a subcritical cycle for antifreeze entering liquid temperature (ELT) ≤ 25 °C and a transcritical cycle for ELT > 25 °C. The 'Standard' condition metrics were: coefficient of performance (COP) 4.14, total capacity 6690 W, sensible capacity 5400 W, and sensible-heat ratio (SHR) 0.81. The 'Part-load' performance was: COP 4.92, total capacity 7240 W, sensible capacity 5640 W, and SHR 0.78. The GSHP was also tested at additional ELTs ranging (10 to 39) °C, where COP ranged (7.3 to 2.4). Compressor efficiency correlations were shown for these and 118 additional tests. The LLSL-HX was estimated to reduce COP by (0 to 2)% for ELTs ranging (10 to 25) °C, and increase COP by (0 to 5)% for ELTs ranging (30 to 39) °C. At the 'Standard' condition the major exergy defects were compressor 0.32, ground heat exchanger 0.19, condenser 0.11, evaporator 0.11, EEV 0.07, pump 0.07, and fan 0.04. The exergy defects were sensitive to ELT, though the compressor was always the largest. Estimated performance of the cycle without the LLSL-HX showed for ELT (10 to 39) °C the HX reduced the EEV throttling defect by (0 to 0.08) and compressor defect by (0 to 0.015), but increased the condenser/gas-cooler defect by (0 to 0.06). [ABSTRACT FROM AUTHOR]

• Micro-grooves on scroll wrap tips limit the radial leakage. • Increasing groove number leads to the decrease of the radial leakage. • Appropriate groove depth brings optimal flow control effect of the radial leakage. • The number of groove partitions determines the availability of micro-grooves. Radial leakage was commonly regarded as the primary factor restricting the aero-thermodynamic performance of scroll compressors. In this paper, a passive flow control approach of micro-grooves in scroll wrap tip for the radial leakage in the scroll compressor was proposed, and its flow characteristics were numerically investigated. The impact of micro-grooves geometric parameters, such as groove number, depth, and partitions, on the flow control effect of the radial leakage was discussed, which was obtained by analyzing the evolution of the flow field and its influence on the performance of the scroll compressor. Results showed that the adoption of micro-grooves in scroll wrap tip yielded a significant limitation on the radial leakage, and geometrical parameters of the micro-grooves directly affected the flow control performance. Increasing groove numbers and partitions decreased the radial leakage effectively, while the leakage decreased first and then increased with the increase of the groove depth. The energy dissipation induced by the continuous throttling effect, jet expansion, and vortex in the groove cavity was the main flow control mechanism. The quadruple-grooves with the groove width of 0.5 mm and the groove depth of 100 μm were adopted in this study eventually. The volumetric and isentropic efficiency of the scroll compressor raised up 2.1 and 1.0 percentage point compared to the original case, respectively. Through reasonable working conditions and micro-grooves geometry, the aero-thermodynamic performance of the oil-free scroll compressor could be improved effectively by the scroll wrap tip with micro-grooves. [ABSTRACT FROM AUTHOR]

• A cascade system with a CO 2 sublimation cycle in the lower stage is experimentally investigated. • Continuous operation below the triple pressure of CO 2 has been realized. • The transient behaviors and blocking issues as an example case are discussed. • In addition, HFE 7500 is used as carrier-fluid to reduce the blockages. • The performance of the system both with and without the carrier-fluid is investigated at different sublimation temperatures. • Optimization potentials of the system are analyzed. Some studies in recent years have shown the possibility of extending the application temperature of the natural refrigerant carbon dioxide (CO 2) below its triple point. Thus, CO 2 could be a potential alternative refrigerant for R-23 in ultralow-temperature (ULT) refrigeration systems. This paper introduces a special cascade system, whose lower stage can be switched between a CO 2 sublimation cycle and a conventional R-23 cycle, so that both cycles can be compared directly under the same ambient conditions. The sublimation system features a unique sublimation heat exchanger, which allows the operation with an extra carrier fluid in order to improve the heat transfer and reduce the blockage by solid CO 2. In this study, experiments were carried out using HFE-7500 as the carrier fluid ("wet sublimation") as well as without the carrier fluid ("dry" sublimation). The system was able to be operated continuously in both operating mode. A refrigerating capacity of 600 W was achieved at a sublimation temperature of -79°C. The transient behavior and blocking issues of the system are discussed. The efficiency of the sublimation system during stable operations are compared with the result of the reference R-23 system. It shows that there is still large potential for improvement on the system performance of the CO 2 sublimation system. [ABSTRACT FROM AUTHOR]

l A CO 2 cryogenic separation process at high pressure is proposed and designed for the new LNG purification cold box. l The main parameters are obtained through the mixture phase diagram and solubility curve, and then verified and analyzed by HYSYS simulation. l The temperature curves of the heat exchangers in the process are analyzed. l The exergy losses of the major equipment in the process are calculated and compared with the original process. A CO 2 cryogenic separation process is proposed and designed for the new liquefied natural gas (LNG) purification cold box. This process is based on the liquefaction process using brazed plate heat exchanger (BPHE) and two separators are embedded between the liquefaction and subcooling heat exchangers to remove frozen CO 2. The separator adopts one-use one-standby mode to ensure uninterrupted operation. The main parameters are first obtained through the phase diagram and solubility curve, and then verified by HYSYS simulation. The simulation results show that the preset values are in good agreement with the simulation. LNG enters the separator at 163 K and then is cooled down to 132.5 K to freeze and separate the CO 2 , so that the CO 2 solids will not block the heat exchanger when they are further subcooled. When the mole fraction of CO 2 inside the separator accumulates to 10%, the process will switch to another separator to work. The mixed refrigerants (MRs) heat the frozen separator to above 189 K, and the mixtures (solid CO 2 and LNG) become liquid and flow out. The change of the process does not cause the temperature crossover inside the heat exchanger. Furthermore, the exergy losses of the proposed process are slightly increased by 1.3% compared to the original process. [ABSTRACT FROM AUTHOR]

• Experimental performance analysis of a CO 2 DX-SAHP. • COP decreases when the water inlet temperature increases. • The augment of solar radiation increases COP. • COP slightly increases due to the humidity increases. In the past decades, combined solar energy with heat pump systems has been one of the very widespread ways to improve the efficiency of conventional heat pumps. In that way, this paper presents an experimental analysis of the influence of environmental conditions in a small size DX-SAHP operating with CO 2 as refrigeration fluid to heat water. The variation of the water inlet temperature, the solar radiation flux and the relative humidity are analyzed and 88 different experimental results are presented. To support the analysis, a thermographic camera was also used. The results show that the gas cooler outlet pressure and temperature are strongly influenced by the water inlet temperature. As the water inlet temperature increases, the gas cooler outlet pressure and temperature increase, and consequently the COP decreases. The COP decreases in 45.8% for the variation of the water inlet temperature to 15–35 oC. The variation of the solar radiation of 876.9–30.17 W.m-2 presents a reduction of performance in about 30%. For the same conditions for the water heat exchange, the pressure ratio of the cycle decreases with an augment on the solar radiation flux. In addition, the COP is higher in 6% for a change in the mean relative humidity of 31.6–55.8 %. [ABSTRACT FROM AUTHOR]

The introduction of extruders, in particular how it works and its principles, must be started from the definition of the extrusion process, the description of its development and types, as well as its functions and advantages. In order to understand the process of extrusion technology, it is necessary to go into detailed descriptions and discussions together with theoretical insights of the subject. In the present article, we discuss a theoretical perspective of production of porous granules from oil crops by extrusion technology. Let’s divide this theoretical perspective into three steps. At first, we study the modeling of statics of oil crop mix compressibility together with carbon dioxide. At second, we study the heat exchange and phase transformation during the stroke of oil crop extruder filled with CO2. At third, we observe the deformation of the oil crops during the extrusion process. There is a probability that extrusion process of oil crop mix with the hard phase of carbon dioxide provides technological way that does not overheat the cellular structure. Potentially, the equation of Van der Waals is able to describe the change in volume of oil crops, which may be related with range of factors associated with volume expansion, calculated by equations as the expansion of granule material takes place at the outlet of extruder matrix. The extrusion processing of a mix of oil crops and carbon dioxide in hard phase at initial stage may likely occur as temperature declines. The resulting values show good compliance of theoretical results to experimental data on the example of coriander seeds. [ABSTRACT FROM AUTHOR]

• New thermodynamic data reported for mixtures of CO2 with HFCs and HFO-1234yf. • Data include vapour-liquid equilibrium, density and heat capacity measurements. • Accuracy of reference Helmholtz equations assessed for multi-component mixtures • Performance of Helmholtz equations in NIST's REFPROP 10 software package improved Measurements of the thermodynamic properties for a series of more environmentally-friendly refrigerant mixtures containing hydrofluorocarbons (HFCs), hydrofluoroolefins (HFOs), and carbon dioxide (CO 2) were conducted. These new property data help increase confidence in the design and simulation of refrigeration processes that use CO 2 + HFO + HFC refrigerant mixtures. The HFCs of interest were R32, R125, and R134a and the HFO tested was R1234yf. The measurements collected were prioritised to fill gaps in the available literature data. Vapour-liquid equilibrium plus liquid-phase density and heat capacity data were collected for different binary mixtures containing HFCs, HFOs and CO 2 , with the liquid phase measurements spanning (223 to 323) K and (1 to 5) MPa. The measured data, as well as data from the literature, were then used to tune the mixture parameters in the models used by NIST's REFPROP 10 software package to improve the prediction of thermodynamic properties for these fluids. To test the predictive capabilities of the models tuned to the binary mixtures, thermodynamic property data were also measured for four ternary mixtures and a five-component mixture of HFCs, HFOs and CO 2. The new models developed in this work significantly improved the root mean square deviations of the predicted properties for these multi-component mixtures: the most significant reductions were about a factor of two in density. [ABSTRACT FROM AUTHOR]

The EU F-Gas Regulation grants exceptions from the GWP-related placing on the market prohibition for stationary refrigeration equipment for applications below -50 °C. Nonetheless, non-flammable refrigerants, which can be used for that temperature range, become increasingly expensive and rare inside the EU due to the phase down of HFCs under the regulation. Flammable alternatives based on methane, ethane and ethylene are available, but are not viable for all applications due to their flammability. Carbon dioxide cannot be used for applications below -50 °C due to CO 2 's triple point at -56 °C. Nitrous oxide with a triple point at -92 °C seems to be an alternative. However, possible exothermal decomposition of N 2 O calls for additional measures in order to be able to operate such systems safely. Two low-temperature systems have been developed, built and successfully operated at evaporation temperatures down to -80 °C with mixtures of N 2 O and CO 2 and different lubricants at ILK and Karlsruhe University of Applied Sciences. The units achieved similar energy efficiency as the standard HFC-equipment used for freeze drying. Possible decomposition of N 2 O could successfully be supressed by various measures. CAUTION: Pure N 2 O as well as mixtures of N 2 O and CO 2 with lubricants based on hydrocarbons can cause violent explosive decomposition reactions abruptly increasing the pressure inside a refrigeration system approximately tenfold. [ABSTRACT FROM AUTHOR]

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• Development of a dynamic numerical model of a CO 2 cooling unit. • 3 layouts, including a back-pressure valve, ejector and auxiliary evaporator. • The ejector can give a COP increase of +15.9% at T amb = 42°C, T i = −5°C. • The auxiliary evaporator can extend the operating range of the ejector to lower T amb. • The auxiliary evaporator gives a max COP increase of 21.0% for T amb = 25°C, T i = 5°C. This paper provides a theoretical assessment of the thermal performance of a new CO 2 vapour-compression system for refrigerated transport applications. Three different configurations are investigated: the standard back-pressure with low pressure receiver lay-out and two arrangements integrating a two-phase ejector. In particular, the use of an auxiliary evaporator in the outlet line of the ejector is considered, to extend the ejector operating range. A numerical model of the system is developed and its theoretical performance is discussed for different values of internal space temperature and external ambient temperature. Simulations' results show that the ejector cycle configuration is convenient when the system is operating in a hot climate with a maximum COP increase (compared to the traditional configuration) equal to 15.9%, at 42°C ambient temperature and -5°C internal space temperature. The use of an auxiliary evaporator can extend the operating range of the ejector to lower values of ambient temperature, with a maximum COP improvement (over the traditional configuration) equal to 21.0% at 25°C ambient temperature and 5°C internal cargo space. [ABSTRACT FROM AUTHOR]

• R1270 and carbon dioxide was used in the cascade refrigerant system. • A prototype was manufactured and experimental investigation was carried out. • Two semi-hermetic piston compressors were used for high- and low-temperature circuit. • R1270 had good performance on high-temperature circuit and could be used to replace R22 and R134a. Herein, we proposed a c ascade refrigerant system with R1270 and carbon dioxide refrigerants. The natural refrigerant R1270 was adopted in the high-temperature circuit to replace NH 3 and R290. A prototype cascade refrigerant system with two semi-hermetic piston compressors was manufactured, and its performance was investigated experimentally. The results establish R1270 as a good alternative in the high-temperature circuit of the cascade refrigerant system. Moreover, with a constant condensation and evaporation temperature of cascade system, as the R1270 evaporation temperature was decreased (from −7 °C to −19 °C), the coefficient of performance (COP) of the cascade system increased. The isentropic efficiency of R1270 compressor decreased from 78.5% to 74.7% when the R1270 evaporation temperature increased from −19 °C to −7 °C. [ABSTRACT FROM AUTHOR]

• Non-equilibrium approach for supersonic expansion of carbon-dioxide was presented. • Phase-change intensity was calibrated on the basis of 150 experimental points. • High quality of the motive nozzle mass flow rate prediction was obtained. • Field results were analysed having regard vapour quality and velocity distribution. A non-equilibrium approach was proposed for highly accurate modelling of the expansion process during two-phase flow in the convergent-divergent motive nozzle of an R744 ejector. Comprehensive mapping of the coefficients used in the source terms of the additional transport equation of the vapour quality was provided on the basis of four ejector geometries. The calibration range contained motive pressures from 50 bar to 70 bar, where the prediction quality of the homogeneous equilibrium (HEM) and relaxation (HRM) models, was unsatisfactory. The calibrated model was validated on the basis of experimental mass flow rate data collected from 150 operating points. The mapping results were utilised for final model derivation in the form of an approximation function for R744 expansion. The validation process resulted in satisfactory relative error below 10% for the vast majority of the cases. Moreover, 70% of the simulated cases were considered with a mass flow rate discrepancy below 7.5% in the inaccuracy. Finally, the selected cases were compared and discussed with the HEM approach on the basis of field results. [ABSTRACT FROM AUTHOR]

• Four different configurations of adsorption cooling cycles are studied for improved performance for subcritical and transcritical cycles. • Detailed thermodynamic analysis is carried out for 5 different adsorbent materials for all the configurations. • Effect of various parameters are studied on system performance. • Out of all the adsorbents used, Maxsorb III showed better system performance. • Various co-relations have been developed for various intermediate temperatures and COP. In the present investigation, thermodynamic analysis of four different adsorption cooling cycles; basic, internal heat recovery, mass recovery, and heat & mass recovery cycles are carried out employing CO 2 as the refrigerant. The said investigation is done for five different adsorbent materials i.e. a highly microporous activated carbon (Maxsorb III), Maxsorb III based composite, Activated carbon fibre (ACF) A-20, BPL activated carbon (AC), and Norit AC. The study stretches the analysis from subcritical to supercritical zone for evaporator temperature ranging from −5 °C to 15 °C. For subcritical cycle, generator and condenser temperatures are varied between 65 °C to 91 °C and 20 °C to 30 °C, and for transcritical cycle, generator and gas cooler exit temperatures are varied from 90 °C to 120 °C and 35 °C to 40 °C, respectively. The maximum COP and specific cooling effect (SCE) achieved by incorporating both heat and mass recovery to the basic cycle are 0.305 and 105 kJ/kg, respectively. The improvement in COP and 2nd law efficiency of the same cycle w.r.t. other cycles are in the range of 7% to 84% and 38% to 77%, respectively for subcritical cycles. Similarly, for transcritical cycles, improvements in COPs of HMRC are in range of 17% to 75%. Analysis showed better system performance for Maxsorb III and it's composite as compared to other adsorbents. Various regression equations are developed to predict the different intermediate temperatures and COP for the mentioned modified subcritical and transcritical cycles. [ABSTRACT FROM AUTHOR]

• A hybrid reduced-order model of a R744 liquid ejector is developed. • A satisfactory accuracy of a low-order model is obtained for wide range. • Motive nozzle and mass entrainment ratio maps are generated. • A liquid ejector performance maximisation at different climate zones is done. Performance maps were generated for designed liquid ejectors installed in a multi-ejector module in an R744 integrated supermarket system using a hybrid of the reduced-order model (ROM). Hybrid ROMs of the investigated liquid ejectors were developed based on experimental data and CFD results. The CFD model achieved satisfactory accuracy at motive nozzle pressures above 59 bar, and the hybrid ROM improved the mass flow rate prediction. Maps were generated for the motive nozzle mass flow rate and the mass entrainment ratio at different ambient conditions related to cold, moderate, and hot climate zones. The set of the pressure lift for ejector efficiency maximisation at different ambient conditions is presented for refrigeration and air-conditioning applications. The developed liquid ejectors hybrid ROMs can be used to optimise work of the R744 integrated supermarket system equipped with the vapour and liquid ejectors. [ABSTRACT FROM AUTHOR]

• Thermodynamic performance of a cascade refrigeration system was investigated. • Three mixed refrigerants (R744/R1270, R744/RE170, R744/R717) were tested in both subcycles. • Mixed refrigerants allow a better thermodynamic performance than pure fluids. • R744/RE170 presented the greatest global COP, 2.34, for a fixed cooling rate of 100 kW. • Exergetic efficiency increased up to 30% and refrigerant mass flow rate was reduced up to 34%. This work evaluates the thermodynamic performance of three different mixed refrigerants: R744/R1270, R744/R717 and R744/RE170 in a cascade refrigeration system composed of two vapor compression cycles. Mixture composition and condensation temperature of the cascade heat exchanger were used as inputs in a parametric analysis, while outputs considered were: compressor power, refrigerant mass flow rate in both cycles, exergy destruction rate, exergetic efficiency and coefficient of performance. Systems were optimized for COP maximization, considering a fixed cooling rate of 100 kW in the evaporator. After optimization, COP increased from 18% to 32% when compared to the values obtained for pure refrigerants. R744/RE170 mixture showed the best results with a COP of 2.34, increasing exergetic efficiency up to 30% and reducing refrigerant mass flow rate in the range from 6% to 34%, compressor power between 20% to 23% and exergy destruction rate was decreased around 31% to 36%. [ABSTRACT FROM AUTHOR]

• Fixed expansion device can provide good performance with a 2-stage rotary compressor. • High internal heat exchanger effectiveness increases minimum stable superheating. • Concentric tube and micro channel internal hex provide similar COP with stable superheating control. • Flash-gas bypass provides minor improvements in COP. • Flash-gas bypass increases cooling capacity due to lower evaporator inlet quality. This paper focuses on experimentally investigating different cycle architectures with two-stage variable capacity compressors for light commercial refrigeration CO 2 systems. A system with two-stage rotary compressor and intercooler was used as a baseline. Tests with an iHX showed the existence of a minimum stable/safe superheating. Proper superheating control (>3 K) with an iHX increases the COP by 12.9%, 16.0%, and 17.2% in the compressor frequencies of 45, 60 and 75 Hz, respectively. To prevent instability a capillary tube selected for the lowest frequency can be used, but it reduces performance by up to 15.5% at higher frequencies. A cycle with iHX, two-stage expansion and intermediate flash-gas bypass was also studied. This cycle provided an average increase in cooling capacity of 7.5%, and gains in COP of 15.5%, 18.4% and 18.1%, at 45, 60 and 75 Hz, respectively. Flash-gas bypass control showed little benefit to performance, but improved cooling capacity by up to 12.1% when compared to the cycle with an iHX. [ABSTRACT FROM AUTHOR]

• The sublimation heat transfer of R744/R600a mixtures was investigated. • The effects of carbon dioxide crystallisation, the heat flux, and the vapour quality were examined. • The crystallisation and formation of a solid fraction has a huge influence on the heat transfer characteristics. • The heat transfer coefficient values are approximately ten times higher for wet carbon dioxide sublimation than for a single-phase flow. Carbon dioxide, a natural refrigerant, is attracting attention as a direct solution to the legal restrictions on hydrofluorocarbon (HFC) refrigerants. The use of carbon dioxide as a refrigerant is limited by the triple point temperature of −56.5 °C. However, used along with a carrier fluid, carbon dioxide can provide refrigeration to temperatures lower than the triple point. A blend composed of carbon dioxide (R744) and isobutane (R600a) is experimentally studied. An experimental setup was designed, built, and verified in order to obtain the sublimation heat transfer coefficients of R744/R600a mixtures at temperatures lower than −56.5 °C. The test section comprises a horizontal copper tube with an inner diameter of 10 mm and length of 1 m. The tube is electrically heated by a copper wire heater wrapped uniformly around the tube. The tests were conducted with a novel refrigerant blend at heat fluxes from 3630 W m−2 to 8480 W m−2. The heat transfer coefficient decreases with increasing heat flux. Furthermore, a heat transfer coefficient calculation correlation has been developed. [ABSTRACT FROM AUTHOR]

Highlights • A portable thermoelectric refrigerator was manufactured and tested. • Effect of fan power at not only hot side but also cold side of TE module is studied. • Method of determination of optimum operational conditions is described for max. COP. • Better thermal performance at optimum conditions as promising portable refrigerator. Abstract In this study, a portable mini thermoelectric refrigerator driven by a Peltier element is designed, constructed and tested. The Peltier element combined with fans on both sides is mounted onto a refrigerator box. An optimization for the operating conditions of the thermoelectric refrigerator is performed. The operating conditions of the system are the ambient temperature and voltages of the fans and Peltier element. Performance analysis for the optimum conditions obtained is presented. Results show that the optimum voltages of the Peltier element, inner and outer fans maximizing the cooling coefficient of performance of the system (COP) are 12 V, 3 V and 9 V, respectively. The optimum ambient temperature is 293 K. Results also show that the cooling COP of the system decreases from 0.351 to 0.011 while the temperature of the cooled space changes from 293 K to 254.8 K. The proposed thermoelectric refrigerator is used to keep drinks, foods or medicines fresh and cool. [ABSTRACT FROM AUTHOR]