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Thermodynamic analysis of auto-cascade refrigeration cycles, with and without ejector, for ultra low temperature freezing using a mixture of refrigerants R600a and R1150
Abstract
The temperatures required by ultra-low temperature applications, ranging from -50°C to -100°C, cannot be reached economically with single stage systems because of the limitation of the compression ratio. Different types of solutions such as cascade or two-stage systems could be implemented to achieve the desired working conditions. However, these systems are usually complex or too expensive. The solution might be found in the use of auto-cascade systems working with zeotropic mixtures. The present article proposes two modifications of the auto-cascade system by including an ejector device to improve the COP. The first modification includes the ejector as an expansion device at the outlet of the phase-separator, while the second includes the ejector as a pre-compression stage. A mixture of the hydrocarbons iso-butane (R600a) and ethylene (R1150) was used as an alternative to conventional refrigerants, which have a very high GWP. The study performed firstly assessed the sensitivity analysis of the free variables on the operating conditions of each cycle. The evaluated variables were the compressor pressure discharge, the mass fraction of the mixture and the phase separator temperature. In the case of the ejector enhanced cycles, the ejector efficiency and the motive pressure were additionally included. Then, the optimal operating conditions were found by means of an optimization process. The results showed a potential improvement in the COP of 12% for the case of the ejector as an expansion device, with an optimal mass fraction of 0.45 of ethylene. On the other hand, the ejector as a pre-compression stage did not show any improvement with regard to the reference case. The present study concludes that the mixture of ethylene and iso-butane is a suitable combination for auto-cascade cycles and that the ejector can be implemented to improve the COP without adding excessive complexity and cost.
... Different refrigeration cycles can be utilised for ULT refrigeration, but limitations exist. For example, a single-stage system is limited for ULT refrigeration due to a high compression ratio and an excessive compressor discharge temperature [7]. One of the solutions is the two-stage system, which utilises a single refrigerant with two compression stages. ...
... Its temperature increases while exchanging heat with the low-NBP refrigerant. The low-NBP refrigerant is then condensed (7)(8) and flows through the evaporator, providing the required refrigeration effect (9-10). However, in real systems, it is not possible to completely separate the pure refrigerants in the condenser because some of the low-NBP refrigerant condenses and some of the high-NBP refrigerant leaves the condenser in vapour form. ...
During the last decade, many industrial and medical applications have shown a requirement for low-temperature-cooling usage (from −40 to −80 °C), which cannot be efficiently obtained via the conventional refrigeration systems usually employed for medium-temperature applications (from 0 to −40 °C). A proper ultra-low-temperature (ULT) refrigeration system design is essential to achieve the desired output. The performance can be maximised via the suitable selection of the configuration and refrigerant for a specific temperature range. This work contributes a detailed overview of the different systems and refrigerants used in ultra-low-temperature applications. Different systems, such as single-stage vapour compression, multi-stage, cascade, auto-cascade, and air refrigeration cycles, are presented and discussed. An energy analysis is then carried out for these systems identifying the optimal system design and refrigerant selection to achieve the highest performance. This paper aims to provide the reader with a comprehensive background through an exhaustive review of refrigeration systems suitable for ultra-low-temperature applications. The effectiveness of these systems is proven numerically, mainly based on the temperature level and purpose of the application.
... In recent years, in response to the current COVID-19 pandemic, the ultralow temperature (UTL) around − 80 °C is increasingly demanded in biomedical areas such as the cryopreservation of new vaccines and cryosurgery. Economical refrigeration in the ULT range cannot be achieved with standard single-stage systems because of the required high compression ratio, which leads to a low efficiency and high discharge temperature [2]. Principally, temperatures below − 40 °C can be obtained by multi-stage compression refrigeration cycles, cascade refrigeration cycles and auto-cascade refrigeration cycles (ARC) [3]. ...
... Similarly, Bai et al. [14,15] also studied a two-stage ARC, and results show that by adding an ejector to the system can boost the exergy efficiency and COP by 25.1% and 9.6%, respectively. Rodríguez-Jara et al. [2] used an ejector as the expansion valve in ARC and found that the COP was increased by 12%. This study also indicates that the mixture of R1150/R600a is a suitable combination for ARC. ...
This study proposed a modified three-stage auto-cascade refrigeration cycle (MTARC) operating with environmentally benign zeotropic mixture of R1234yf/R170/R14 at the refrigeration temperature level of − 80 °C. Compared with the conventional three-stage auto-cascade refrigeration cycle (CTARC), MTARC incorporates an additional pressure regulator between the condenser and separator to realize phase separation at a lower pressure and temperature. A comprehensive evaluation of energy and exergy performance of the two cycles was conducted theoretically. Under a typical working condition, the cooling capacity, COP and exergy efficiency of the MTARC are improved by 15.85%, 11.69% and 7.65% in comparison with the CTARC, respectively. In addition, a lower evaporating temperature was also obtained by the MTARC under the same operating condition. When the intermediate pressure drops from 2 to 1 MPa, the cooling capacity, COP and exergy efficiency are improved by 35.43%, 25.25% and 16.74%, respectively, for the MTARC, meanwhile the compressor outlet temperature increases 19.93 °C from 92.27 to 112.20 °C. Therefore, the selection of the intermediate pressure should be comprehensively considered to ensure a desirable cycle performance and a proper working condition for the compressor. The proposed modified cycle offers new pathways for designing innovative cryogenic refrigeration systems, thereby potentially improving the energy economy in a myriad of modern energy applications for sustainability concerns.
... ACRS often operates with refrigerant mixture, and many researchers have done related research. Rodríguez-Jara et al. [3] used an ejector instead of economizer expansion valve in an ACRS, and Liu et al. [4] added a self-heat exchanger to ACRS. Both were proved to be effective in improving COP of refrigeration system. ...
... The mesh quantity details and independence analysis are shown in Table 2. The left air outlet was set to be the velocity inlet, and the air supply temperature was set according to three different experimental groups: (1) −18 • C (the temperature for general food cold chain transportation); (2) −35 • C (the temperature of the fast-freezing equipment); and (3) −60 • C (the ultra-low temperature freezing temperature required for transporting tuna) [29][30][31]. ...
In order to predict the regular temperature change in tuna during the freezing process for cold chain transportation, improve the quality of frozen tuna, and reduce the energy consumption of freezing equipment, a three-dimensional numerical model for freezing tuna of different sizes was established. An unsteady numerical simulation of the air velocity and flow field was combined with an analysis of the freezing process of tuna. This paper also studied the effect of air velocity, temperature, and tuna size on the freezing process. The numerical results show that there was a positive correlation between the cold source environment and the tuna-freezing process. Lower temperatures and higher air increased the velocity at which the tuna moved through the maximum ice crystal formation zone, maintaining a better aquatic product quality. In some cases, however, the smaller tuna models achieved a longer freezing time. Due to the difficulty of obtaining the whole tuna sample, the temperature curve and freezing rate over time obtained during the freezing process were tested using a tuna block of a specific size. The maximum error did not exceed 6.67%, verifying the authenticity and feasibility of the simulation.
... Yan et al. [6,7] considered mixtures with propane (R-290/R-600, R-290/R-600a) for a dual temperature refrigeration system based on a single-AC system for applications at -30ºC. Rodríguez-Jara et al. [8], Liu et al. [9,10], Li et al. [11] and He et al. [5] used different AC layouts and considered the mixture R-600a/R-1150 for temperatures up to -80ºC, and recently extended the investigation to the use of other zeotropic refrigerant mixtures [12]. Finally, Chen et al. [13] released a 4E analysis about the use of a simple AC system in relation to advanced AC systems including two-phase separators and ejectors, highlighting the opportunities of COP enhancement using new but more complex layouts. ...
... The prototype of above system was built by Bai et al [15,16], which respectively improved the COP and exergy efficiency by up to 9.6% and 25.1% over the baseline system. Similar research results were also concluded by Rodríguez-Jara [17] and Boyaghchi [18]. Liu et al. [19] presented another novel ejector-enhanced autocascade refrigeration system, and declared that the novel system could further increase the COP by 18.1% with respect to the above ejector enhanced system. ...
As known, insufficient component separation of zeotropic refrigerant and massive throttling losses are two major causes for the poor performance of autocascade system. Aiming to combine the benefits of applying two-stage component separation to improve the component separation efficiency and applying ejector to recover partial expansion work, a novel ejector enhanced autocascade refrigeration cycle with two-stage component separation is proposed. The comparative study on the novel cycle and other three autocascade refrigeration cycles are conducted by simulation method, based on energy, exergy, economic and environmental analyses method. The simulation results show that the novel cycle always yields remarkable performance advantage. Under the given condition, the novel cycle respectively improves the coefficient of performance and cooling capacity by 40.8% and 60.8% compared with the convenient autocascade refrigeration cycle. Furthermore, the exergy performance and irreversibility distributions of four cycles are compared. Moreover, the performance variation of the novel cycle with main operation parameters is simulated to investigate the cycle characteristics. The novel cycle achieves the longest payback period of 2.27 years and produces the lowest full life cycle cost of 5268 $, when applied in a low-temperature freezer. Meanwhile, the novel cycle remarkably reduces the carbon dioxide and air pollution emissions.
... HCs have certain energy efficiency and as the new generation of refrigerants that are non-toxic, environmentally friendly, and harmless to the ozone layer [27]. In the context of an ejector-enhanced refrigeration cycle, to which Rodríguez-Jara et al. [28] applied R1150/R600a, it was shown that HCs can increase COP without adding excessive complexity and cost. From the existing literature, we can conclude that a suitable selection of zeotropic mixtures will contribute to the system's performance. ...
The cooling capacity needed by ultra-low temperature apparatus cannot be reached economically with a single vapor compression refrigeration cycle due to the constraint of the high compressor pressure ratio. The auto-cascade refrigeration cycle is a good alternative. In this work, a novel concept that applies the principle of the auto-cascade refrigeration cycle to store cold energy is conducted. The environment-friendly refrigerants of R600a/R290/R170 zeotropic mixtures are used to study the performance of the modified auto-cascade refrigeration cycle (MACRC) as an alternative for cold-energy applications. The simulation results show that a cooling capacity of 500 W can be provided below −60 °C. The mixture with a mass fraction of 0.25/0.35/0.40 yields a COP of 0.695 and an exergy efficiency of 0.262 at −66 °C. The performance of the MACRC system was investigated at an ambient temperature of 20 to 40 °C for indoor small-scale applications. It is concluded that the performance would be improved by decreasing the ambient temperature. The results of the work should be helpful for the design and optimization of auto-cascade systems.
... Mota-Babiloni et al. [25] concluded that using ejectors has recently been considered in research papers in auto-cascade architectures. Rodríguez-Jara et al. [26] thermodynamically evaluated natural refrigerants R-600a and R-1150 in auto-cascade ultra-low temperature applications, ranging from −50 °C to −100 °C. The ejector as an expansion device improved the COP up to 12%, with an optimal R-1150 mass fraction of 0.45. ...
Research in ultra-low temperature refrigeration applications has intensified in recent years after the appearance of vaccines in response to the COVID-19 pandemic. There are few current technologies for this low-temperature range, with reduced energy performance and high global warming potential refrigerants. This work analyses the introduction of the ejector in two-stage cascade cycles for ultra-low temperature refrigeration. The proposal includes the assessment of the behaviour of the ejector while implementing it in a single stage or simultaneously in both stages. The study is carried out with refrigerants R-290 in the high-temperature stage and R-170 in the low-temperature stage since these are natural refrigerants with very low global warming potential. The results show that the ejector is a component that causes improvements in the cycle when placed in the high-temperature and low-temperature stages. On the other hand, given the change in evaporation and condensation temperatures, the evaporation temperature is more critical regarding cycle energy performance. With the results obtained, a cascade cycle with an ejector in both stages is proposed, obtaining a 21% higher coefficient of performance than the standard cascade cycle. Also, the cycle with the ejector in both stages causes an improvement of 13.6 % compared to the previous generation's refrigerants (R-23 and R-507A) in the same cycle. The carbon footprint analysis shows that this cycle emits less than half of the equivalent CO2 than actual cycles for ultra-low temperatures, also with a new refrigerant like R-472A.
... These fluids can also be found in mixtures proposed in auto cascade refrigeration (ACR) systems, another type of cycle proposed in ULT applications. An R-600a/1150 mixture was evaluated by Rodríguez-Jara et al. (2022) in an ACR which included an ejector as an expansion device at the outlet of the phase-separator or as a pre-compression stage. The results showed a 12% potential COP improvement for the case of the ejector as an expansion device, with an 0.45 optimal R-1150 mass fraction. ...
The absence of GWP limitation for refrigerants operating in vapour compression systems with a target temperature below –50°C has caused slower market development. Therefore, while typical refrigeration applications have several mixtures offering different characteristics, a few mixtures already exist for –80°C refrigeration (R-469A, R-472A, and R-473A). This paper explores the combination of pure fluids with varied characteristics as three-component mixtures for ultra-low temperature refrigeration. Different parameters have been considered in the theoretical screening, such as volumetric cooling capacity, coefficient of performance, global warming potential, and flammability at constant operating conditions. R-32, R-41, R-125, R-134a, R-152a, R-170, R-227ea, R-290, R-744, R-1132a, R-1150, R-1234ze(E) and RE-170 have been combined at steps of 5%. Mixtures with the lowest global warming potential and highest coefficient of performance result in high flammability, particularly at −80°C. Environmental and energy aspects would require a lower priority in trade-off selections to reduce the flammability classification.
... It was discovered that when using an ejector as an expansion device, the coefficient of performance is likely to improve. However, the ejector's performance in the pre-compression stage was unimpressive [27]. A planetary-driven freezing process vapor ejector using the CFD model to get down the flow structure and performance was investigated. ...
Because of the attributes of no influencing elements, cost efficiency, increased dependability, and good efficiency, the applications of an add-on energy development ejector have great interest. This paper has discussed about design theories and methods of ejectors with various working fluids. The design methodology in the ejector fields observed constant area mixing (CAM) and constant pressure mixing (CPM). One of the alternatives is to utilize an ejector as an expansion mechanism. This review examines ejector development freezing systems using a combination ejector with constant pressure and constant area mixing as an expansion mechanism. That improves the efficiency and coefficient of performance and reduces power consumption.
The present study introduces a novel cascade ejector intercooler refrigeration cycle (CEIRC) and a novel cascade ejector booster refrigeration cycle (CEBRC), which are designed to improve ultralow temperature (ULT) refrigeration system performance based on various refrigerants in terms of energy, exergy, and environment. Through the analysis conducted with eco‐friendly refrigerants for CEIRC, the refrigerant pair exhibits the highest performance that is found to consist of RE170 for high‐temperature cycles and R170 for low‐temperature cycles. These refrigerants have very low global warming potential and are eco‐friendly. The coefficient of performance (COP) reveals improvements of 3.20–9.51%, 10.55–19.12%, and 31.96–65.97% over the CEBRC, cascade ejector refrigeration cycle (CERC), and cascade vapor compression refrigeration cycle (CVCRC), respectively, resulting in corresponding exergy efficiency improvements of 3.18–9.49%, 10.48–19.12%, and 31.87–65.98%. The total compressor power and exergy destruction are also compared, indicating that the CEIRC exhibits the best performance among the four. CEIRC achieves a COP increase of 11.6% compared with the cascade refrigeration cycle, which is considered the cycle with the highest performance operating at ultralow temperatures in the literature, which is CERC. The kg CO2 effect of the CEIRC is 11.35% lower than that of the CERC.
This paper focuses on evaluating and analyzing the performance of an auto-cascade refrigeration system with the zeotropic mixture of R600a/R1150/R14 refrigerant mixture using grey correlation theory and response surface methodology (RSM). The correlation degree among various factors is analyzed using the grey correlation theory. The interactive influences which have significant impacts on the six output parameters are illustrated, and a regression model is further validated by five confirmation parameters. The results show that the vapor quality of the evaporator outlet is the greatest factor that affecting the refrigerating capacity and COP. However, the evaporation temperature has the greatest effect on compressor’s discharge temperature, pressure ratio, exergy destruction rates and exergy efficiency. Lower vapor quality of the condenser outlet and condensation temperature can improve the performances of refrigerating capacity, COP, compressor’s discharge temperature, compression ratio, total exergy destruction rate and exergy efficiency based on the RSM results. The random five groups of validation parameters indicate that the errors between the predicted modeling and calculation results are all less than 5%, presenting that the RSM model has good predictability. The results achieved in this work will assist in the optimizing analysis and the application of cryogenic refrigerators.
The cascade refrigeration system (CRS) uses two types of refrigerants with different boiling points which individually work as the higher-temperature cycle (HTC) and the low-temperature cycle (LTC), with a heat exchanger combining them. In this study, refrigerant R170 is used for LTC, while refrigerants R32, R515B, and R466A are used for HTC in the cascade refrigeration systems to reduce carbon dioxide emission to the environment. Computer software was used to do the numerical calculations and simulation techniques for the cascade refrigeration cycle. The three refrigerant pairings R32/R170, R466A/R170, and R515B/R170 that combine zeotropes, azeotropes, and natural refrigerants were employed in this study to analyze the cascade refrigeration system. In comparison with other refrigerant pairs, the R515B/R170 refrigerant pair offers better coefficient of performance (COP) of 3.781 by raising the evaporator temperature. According to numerical simulations, R515B/R170 is found to be preferable for CRS in terms of maximizing COP and thermodynamic performance. Also, carbon emission to environment has reduced by 15% with the help of alternative refrigerants.
Auto-cascade refrigeration (ACR) systems are widely used at low temperatures. The selection and ratio optimization of zeotropic refrigerants in ACR systems have an important impact on system performance and environmental protection. This study proposes a three-stage regenerator-enhanced auto-cascade refrigeration (RACR) system. Under certain working conditions, the theoretical performances of the three-stage R134a/R23/R14 ACR and RACR systems were compared, and the influence of the refrigerant mixture ratio on the RACR system performance was explored. The system performances of replacing R134a and R23 with different hydrocarbons were compared under different working conditions. The results demonstrated that RACR performed better than ACR alone. A reasonable adjustment to the mixed refrigerant ratio improved the energy efficiency of the RACR system. Water-cooling conditions for the RACR system were suggested, under which the coefficient of performance, thermodynamic perfectness, and exergy efficiency of R600/R170/R14 were 13.8 %, 12.4 %, and 8.4 % higher than those of R600/R23/R14, respectively. The environmental policy, safety requirements, and energy efficiency were considered to obtain −100 °C, R600/R170/R14 with mass fractions of 0.24/0.12/0.64 was the best choice in a water-cooled RACR system.
In this paper, a modified ejector-enhanced auto-cascade refrigeration cycle with R290/R170 is proposed on the basis of a basic ejector-enhanced auto-cascade refrigeration cycle proposed previously. An additional expansion valve and internal heat exchanger are utilized in the modified cycle and are located after condenser. Refrigerant leaves condenser with smaller quality and then is throttling in expansion valve. Thus, the quality at inlet of separator can be adjusted to meet varying refrigeration capacity as varying ambient temperature. The thermodynamic analyses based on energy and exergy methods are conducted to compare the performances of two cycles. The influences of several critical operating parameters on cycle performances are investigated in detail. The results indicate that the modified cycle can obtain higher COP under the given operating conditions than the basic cycle. The modified cycle presents average improvement of 7.52%–8.86% in COP and 9.28%–11.33% improvement in volumetric refrigeration capacity. Besides, the exergy destruction of condenser, ejector and expansion valves in modified cycle is much less than that in basic cycle. The total exergy destruction of the modified cycle is decreased by 6.65%–8.35%, and exergy efficiency is increased by 7.54%–8.85%. Therefore, the modified cycle shows its energy-saving advantage and application potential in low-temperature refrigeration.
This work reports new experimental data and detailed equilibrium molecular dynamics (EMD) simulations of viscosity for liquid ethene. The measurements were carried out by using an improved vibrating-wire viscometer for temperatures from (173.150 to 233.150) K and pressures up to 5.5 MPa, with a combined expanded relative uncertainty of 5.3 % (k = 2). The experimental data were compared with the values calculated by the correlation of Holland et al. (with an uncertainty of 10 % for liquid), the deviations vary from −7.2 % to 0.8 % and the AARD is 3.6 %. Furthermore, the Assael and Dymond scheme based on the hard-sphere model was used to correlate the experimental results with an AARD of 0.1 %. For understanding the structure and properties of liquid ethene at the molecular level, a comprehensive evaluation of ten different ethene force fields with four types of molecular configurations (two-site, three-site, four-site, and six-site models) was then performed to predict density and viscosity at a wide temperature range from (133.150 to 233.150) K and pressure of 5 MPa. The selected force fields were generally developed for the vapor-liquid equilibrium properties, without explicit consideration of viscosity and other transport properties. Overall, three rigid two-site force fields (SET, TraPPE-UA, and AUA4) gave the best predictions with the AARD within 0.32 % for density and 8.5 % for viscosity comparing with the correlations of Smukala et al. and Holland et al., separately. Mie force field with 16-6 potential and TraPPE-UA2 force field with two negative partial charges can accurately predict density but severely overestimate viscosity greater than 20 %. The larger intermolecular repulsion and electrostatic interaction may have contributed to the overestimation of viscosity especially at low temperatures.
In this study, a model is established for an ejector with a zeotropic refrigerant as the working medium, and the refrigeration cycle of the ejector is analysed. Depending on the boiling point, different refrigerants are combined to prepare R123/R245fa, R245fa/R141b, R141b/RC318, R245fa/R134a, R245fa/R22, R141b/R134a, R245fa/R143a, and R141b/R22. The temperature and pressure in the generator, evaporator, and condenser are calculated using these refrigerants, and the influence of the temperature glide on these conditions is investigated. The performance of the ejector refrigeration cycle is also studied. The results indicate that the outlet temperatures of both the generator and evaporator can be increased using a zeotropic mixture compared to that using a pure refrigerant. In the condenser, the average temperature difference of the heat transfer is larger than that of the pure refrigerant because of the temperature glide characteristics of the zeotropic refrigerant, which reduces the heat transfer area of the condenser. Among the studied refrigerants, using R245fa/R22 (0.3/0.7) as the working medium yields the refrigeration cycle with the best COP (0.293), which is 4% and 22% higher than those using R22 and R245fa, respectively. The results reveal the advantages of zeotropic refrigerants and provide a theoretical foundation for further research.
To improve the performance of the conventional auto-cascade refrigeration cycle, a modified auto-cascade refrigeration cycle with vapor injection is proposed for the application of a -80 ℃ freezer in this study. The particle swarm optimization algorithm is used to obtain the cycle performance at different operation conditions. The mixture concentration, compression ratio and condenser outlet quality are optimized globally. The binary mixture selection is carried out among the R1150/R290, R150/R600 and R1150/R600a. The results indicate that R1150/R600 is proposed as the alternative refrigerant. The performance comparisons between conventional and vapor injection auto-cascade refrigeration cycles show that the vapor injection technique efficiently improves system performance, especially at high condensation and low evaporation temperatures. Compared with the conventional auto-cascade refrigeration cycle, the optimal COP in the modified auto-cascade refrigeration cycles show a 9.3∼53.8% improvement in COP and the maximum improvement in the volumetric refrigeration capacity reaches 58.2% at low evaporation temperature. The statistical results of the optimal concentration of the mixture R1150/R600 at the selected operating conditions show that 88% of the optimal R600 concentration in the VARC cycle is in the range of 0.657∼0.699. The optimal concentration of R600 in the CARC cycle varies in a wide range of 0.57∼0.75. The optimal intermediate pressure is 38.1∼13.4% lower than the geometric mean of discharge pressure and evaporation pressure. The exergetic analyses illustrate that the largest exergy destruction of the VARC cycle occurs in the recuperator-2 instead of the compressor in CARC, contributing to 22.6∼23.5% of the total exergy destruction.
The excessively high discharge pressure of mixture-based Joule–Thomson (J–T) refrigeration system for low-temperature freezers at the start-up process without active control leads to the reduction of operation reliability of compressors. Start-up characteristics of a J–T refrigeration system with an expansion tank for pressure control were investigated in this study. The binary mixture R1150/R600a was selected as the working fluid, and influences of the control pressure, compressor speed, throttle valve opening, and mixture concentration on the system’s dynamic performances were experimentally explored. Results demonstrated that the discharge pressure could be effectively limited below the compressor’s safety pressure with an expansion tank. Meanwhile, the close relationship between pull-down rate and time of the pressure control phase was associated with the control pressure, compressor speed, throttle valve opening, and mixture concentration. The pull-down rate can be accelerated by increasing the control pressure and throttle valve opening and reducing the compressor speed and the R1150 concentration at the start-up process. The excessively rich R1150 concentration caused the continuous action of the pressure switch and the malfunction of the low-temperature freezing at an acceptable time range. The build-up of the heat recuperation played a significant role in pressure control time and freezing temperature reduction rate.
This paper proposed a modified three-stage auto-cascade refrigeration cycle (MTARC) associated with an intermediate pressure regulator. The energy and exergy analyses indicate that the MTARC can acquire lower evaporating temperature, greater cooling capacity and higher COP. Under a typical operating condition, the MTARC using the alternative ternary zeotropic mixture of R1234yf/R41/R14(0.66/0.17/0.17) has an evaporator inlet temperature of −90.90 °C, COP of 0.4509 and exergy efficiency of 40.67%, which are 4.63 °C lower, 32.93% and 15.38% higher than those of the conventional three-stage ARC, respectively. Further research shows that with the decreasing intermediate pressure, the thermodynamic performance of the MARC can be improved significantly, but the compressor exhaust temperature also increases. When the intermediate pressure decreases from 1.8 MPa to 1 MPa, the cooling capacity, COP and exergy efficiency of the MTARC increase by 30.34%, 22.10% and 13.79% respectively, while the exhaust temperature increases 18.59 °C from 116.66 °C to 135.25 °C, which indicates that the selection of intermediate pressure needs to be comprehensively considered. The proposed new approach is valuable for optimizing the design and application of the refrigerator at an ultra-low temperature level of −80 °C.
Previous studies have pointed out problems with the dual-temperature air source heat pump. Thus, this paper proposes a novel dual-temperature air source heat pump cycle with a self-defrosting method for simultaneous production of heat sources at different temperatures. An ejector was added to the novel system, which reduced the heat transfer temperature difference of the low-temperature condenser and the utilization of multiple heat sources. In addition, a new type of defrosting, which utilizes the heat from the hot liquid refrigerant to defrost the evaporator using two evaporators and a four-way valve, is used to reduce the energy needed for defrosting and decrease temperature fluctuations. Thermodynamic modeling using the energetic and exergetic analysis method was employed to evaluate the modified cycle performance and compare it with that of the basic heat pump cycle. Eco-friendly refrigerants, such as R134a, R600a, R290, and R1234yf, were adopted as the working fluid. The simulation results show that the heating coefficient and exergy efficiency in the proposed cycle were improved by 29.34% and 43.52%, respectively, compared with those of the standard cycle under typical operating conditions. Among the refrigerants, the eco-friendly refrigerant R600a exhibited the best performance under various operating conditions. Moreover, the COP in the new system was 20.72–44.47% higher than that of a traditional system, and the exergy efficiency improvement was 29.70–49.19% compared to the standard system. In summary, this study confirms the performance enhancement potential of an ejector-based dual-temperature air-source heat-pump cycle and provides theoretical support for its practical implementation.
Auto-cascade refrigeration cycle (ARC) is widely used in low-temperature applications. The selection and proportion of refrigerant mixtures in an ARC system have a significant impact on the environment and system performance. In this study, the theoretical performance of a two-stage ARC system using R170/R600a, R170/R600, R1150/R600a, R1150/R600, and R23/R134a was compared. The results showed that, under certain working conditions in the ARC, an optimal refrigerant ratio maximized the COP of the system. At lower evaporator outlet temperatures or higher condenser outlet temperatures, the performance of the system using R170/R600a, R170/R600, R1150/R600a, and R1150/R600 offers advantages over that when using R23/R134a. Among these, R170/R600 afforded the best performance. Exergy analysis was also performed, and the exergy loss ratios of the heat exchanger in the R170/R600 and R23/R134a systems were the highest at 52.3% and 56.7%, respectively.
Today, vapor compression refrigeration (VCR) systems are commonly used for cooling and heating in buildings, industrial and many other applications. Given the high-energy costs, it is essential to use highly efficient VCR systems. This study presents an experimental performance analysis of a refrigeration cycle with a dual evaporator ejector system (DEES). For this aim, an experimental system was designed and established on a laboratory scale. DEES system was organized for regarding different and possible applications via operating the individual evaporators with different heat sources as air and water. Within the scope of the work performed in this study, the DEES was tested by varying the condensing temperature and ejector Entrainment Ratio (ER) values. The investigated DEES was also contrasted with a conventional vapor compression refrigeration cycle as a baseline system considering different operating conditions. The findings obtained in this study showed that a 13.24% enhancement in coefficient of performance (COP) was achieved when comparing with the baseline system. The highest exergy destruction rate for the compressor in ER#3 was 0.79 kW, but the total cooling capacity significantly improved compared to the baseline system. Additionally, the total exergy destruction rate of the baseline system was the lowest compared to the other modes, while the cooling capacity of this mode was always the lowest. It was also found that when the ER value varied from ER#3 to ER#1, the total cooling capacity increased by up to 11.79%. The results show that the ejector works more efficiently at low ER values and condenser temperatures.
The absence of GWP limitation for refrigerants operating in vapour compression systems with a target temperature below-50 °C has caused slower market development. Therefore, while typical refrigeration applications count with several mixtures offering different characteristics, a few mixtures already exist for-80 °C refrigeration (R-469A, R-472A, and R-473A). This paper explores the combination of pure fluids with varied characteristics as mixtures for ultra-low temperature refrigeration. Different parameters have been considered in the theoretical screening, such as volumetric cooling capacity, coefficient of performance, global warming potential, and flammability while keeping operating conditions constant. The refrigerants R-744, R-32, R-125, R-744, R-134a, R-1132a, R-41, and R-23, and R-1150, have been combined at steps of 5%. Mixtures with the lowest global warming potential and highest coefficient of performance result in high flammability. Environmental and energetic aspects should have a lower priority to reduce the flammability classification.
In this paper, parametric investigation of energy, exergy, economic and environmental performances has been presented for natural refrigerants employed in cascade refrigeration systems (CRS). A Mathematical simulation model of CRS has been developed and variations of exergy efficiency, total annual cost, COP, exergy destruction rate, the mass of GHG emission, and environmental cost with operating temperatures have been shown for fifteen natural refrigerant couples. A single objective thermodynamic optimization has also been carried out on the top three couples which include NH3-C3H6, C4H10-C3H6, and C3H8-C3H6. Comparative analyses based on parametric study and optimization reveal that NH3-C3H6 is the most promising refrigerant couple showing the highest COP and exergy efficiency of 1.326 and 33.42%, respectively, and the lowest total annual cost and environmental cost of 1.048 and 23,003 $/yr. C4H10-C3H6 is slightly better than C3H8-C3H6 refrigerant couple showing 0.5% higher COP and exergy efficiency, and 0.5% lower environmental cost but the overall cost corresponding to C4H10-C3H6 is 13.97% higher than that of C3H8-C3H6. It is found that COP and exergy efficiency improve by 5.57 and 5.20%, respectively, whereas annual environmental and total costs decrease by 4.93 and 3.06%, respectively, when the NH3-CO2 refrigerant couple is replaced by NH3-C3H6 couple under similar operating conditions.
The main goal of this study was to determine the effects of the entrainment ratio (ER) and condensing temperature (CT) on the performance of a dual evaporator ejector system (DEES) operating on environmentally friendly, low-GWP (Global Warming Potential) refrigerants of R1234ze(E), ND, R515a, R456a, and R516a as a replacement for R134a. The effects of various CTs and ERs on the DEES system performance were evaluated. It was revealed that when the ER increased from 0.1 to 0.9, the Coefficient of Performance (COP) decreased by 26%. Besides, in ejector refrigeration systems, ER values can be changed by using smart technologies, so the desired cooling capacities can be adjusted from DEES. The experiments based on condensing temperature were carried at the ER where the cooling capacities of both evaporators were very close to each other. The results demonstrated that the COP of R1234ze(E) and R515a were higher than that of R134a, and their performance was insufficient in terms of cooling capacity. The total cooling capacities of the DEES with R515a and R1234ze(E) were 11% and 5% lower, respectively, than that of the DEES with R134a, and the COPs of the DEES with R515a and R1234ze(E) were 1% and 9% higher, respectively, than that of the DEES with R134a. Furthermore, the performance parameters obtained from R516a were found to be very close to those of R134a. The COP and cooling capacity of the DEES with R516a were 7% higher and 2% lower, respectively, than that of the DEES with R134a. Finally, considering the GWP values, R516a was the best alternative to R134a among the considered refrigerants.
Research on the reduction of refrigeration systems energy consumption is a topic in which popularity benefits from the contextual growth of the refrigeration market. Solar or waste heat-driven ejector refrigeration systems are very promising in this context. However, the lack of operational flexibility of ejectors penalizes the performance of such systems. Since the mixing chamber has a fixed cross-section, for condensation temperatures above critical, the cooling capacity vanishes and, for temperatures lower than critical, the cooling capacity stagnates. The paper proposes a new variable mixing chamber solution that overcomes these limitations. It is a range extender that allows ejectors to operate at high cooling capacity all over the operation range of the system. This range extender is based on the concept of lateral cylindrical moving slots. The paper compares the new ejector to a reference ejector from the literature. First, the Computational Fluid Mechanics (CFD) model viability is evaluated thanks to the experimental measurements on the reference ejector. Then, the new system’s performance is evaluated. The results show that the adaptation of the mixing chamber section to the condensation temperature allows increasing the cooling capacity without supplementary motive energy. That leads to an increase of up to 120% of the cooling capacity at low condensation temperatures and a shifting of 8°C of the critical temperature. Such extension of the operational range was never obtained before. That means that an ejector stalling at 33°C could operate at 41°C using the proposed range extender.
This paper proposes a modified auto-cascade refrigeration cycle (MARC) with a self-recuperator. The introduced self-recuperator and associated expansion valve effectively increases the refrigerant enriched with more low-boiling component in the evaporator. This case could improve the cycle performance by further choosing appropriate design-dependent cycle parameters. The energy and exergy analysis methods are used to compare and evaluate the performance of MARC with conventional auto-cascade refrigeration cycle (CARC). The simulation results show that under all given working conditions, the COP and exergy efficiency of MARC are superior to those of CARC. Among R290/R170 and R600a/R1150, R600a/R1150 is much better refrigerant mixture. Under a typical working condition, the COP of MARC using R600a/R1150 is 68.17% higher than that using R290/R170. The performance improvement of MARC is more obvious when R600a/R1150 is used. Under the typical working condition, the COP of MARC is increased by 6.24% and 24.17% using R290/R170 and R600a/R1150, respectively. When initial mass fraction of R1150 is about 0.6, the two cycles have maximum COP and exergy efficiency, and the maximum COP of MARC is 24.26% higher than that of CARC. In addition, the increase in the two-phase flow vapor quality at the condenser outlet can improve the COP, but reduce the refrigeration capacity. The COP of MARC increases by 37.56% when the quality increases from 0.4 to 0.6. It is also found for MARC that COP and refrigeration capacity are positively correlated with intermediate pressure.
While the global refrigeration capacity installed in ultra-low temperature applications is considerably lower than in standard refrigeration, the interest in such systems has risen during the last months due to their emerging demand for the preservation of mRNA vaccines. Several theoretical studies can be found in the literature for ultra-low temperature applications. However, there is still a lack of reliable experimental data that may be used for the models validation and, thereby, to perform more rigorous investigations. In light of this, the present manuscript proposes an ultra-low temperature refrigeration unit based on an indirect cascade system, developed by retrofitting a standard low-temperature R290 packaged unit. In this investigation, firstly, the suitability of the original components is assessed and, then, the unit is experimentally tested within a wide range of operating conditions. Thereby, the system's reliability is evaluated by assessing different parameters, such as discharge, condensing and evaporating temperatures and performance ratios. The main results showed a successful behaviour at the operating conditions tested, exhibiting a COP that ranged from 0.6 to 1.6 for cold room temperatures between -80 °C and -65 °C, respectively.
In few years, stand-alone systems will rely on low-GWP refrigerants in order to reduce their direct impact, which agrees with the recent regulations and standards. However, the energy performance of the system is dependent on the refrigerant charge and thus on their annual leakage ratio. The leakage effect is not considered in the suggested methods to evaluate the environmental impact and requires an in deep analysis. We aim to extend the TEWI analysis of stand-alone refrigeration systems by considering a realistic evolution of their charge during its lifetime and to quantify the discrepancies of the classical methodology. For that, the performance of a stand-alone cabinet (four low-GWP refrigerants) has been considered as reference. It has been concluded that leakage consideration makes classical TEWI to underestimate emissions for any country, refrigerant or annual leakage ratio, up to 20% in this study. Deviation increases with higher leakage ratio, but especially for those values that do not imply a refilling during the lifetime of the system. Deviations are higher as mass and energy spans of the systems are.
To explore the technical alternatives of refrigerant substitution and analyze the application of low GWP refrigerant in three-stage cascade refrigeration system (TCRS), the change of operation parameters with the evaporation temperature has been studied by thermodynamic analysis. The results show that R1150 can replace R14 in low-temperature cycle. R41 and R170 can replace R23 in medium-temperature cycle and R170 will be recommended as a priority. In high-temperature cycle, the refrigerants of R717, R152a and R161 are recommended. Taking into account environmental protection, the natural refrigerant of R717 is a good choice in the large refrigeration system. In particular, the refrigerant groups of R1150/R41/R717, R1150/R41/R152a, R1150/R41/R161, R1150/R170/R717, R1150/R170/R152a and R1150/R170/R161 are recognized. The results provide a basic theoretical analysis of the selection and replacement of refrigerant in TCRS.
The applications of ejectors are many and encompass the refrigeration, the power generation and the chemical sectors. On one hand, ejector technology needs limited maintenance, has low operational costs and has no restrictions concerning the working fluids; on the other hand, the complex single- and multi-phase fluid dynamics make ejector design and performance prediction a real challenge. This perspective explores the main advancements in ejector technology and proposes a critical discussion with an outlook for the future research; the proposed discussion is grounded on the multi-scale relationship between the “local-scale” phenomena and the “component-scale” performances. After a look at the past, this perspective examines the ongoing research activities and achievements regarding four state-of-the-art research areas, namely refrigeration systems, power conversion plants, chemical process and technology and computational methods. For the different research areas discussed, directions and opportunities for the future research activities are appraised. Finally, this perspective defines a fundamental challenge that needs be addressed in the forthcoming long-term activities: “the multi-scale ejector challenge”.
The present work focuses on analytical computation of thermodynamic performance of actual vapour compression refrigeration system by using six pure refrigerants. The refrigerants are namely R22, R32, R134a, R152a, R290 and R1270 respectively. A MATLAB code is developed to compute the thermodynamic performance parameters of actual vapour compression system such as refrigeration effect, compressor work, COP, power per ton of refrigeration, compressor discharge temperature and volumetric refrigeration capacity at condensing and evaporating temperatures of 54.4oC and 7.2oC respectively. Analytical results exhibited that COP of both R32 and R134a are 15.95% and 11.71% higher among the six investigated refrigerants. However R32 and R134a cannot be replaced directly into R22 system. This is due to their higher compressor discharge temperature and poor volumetric capacity respectively. The discharge temperature of both R1270 and R290 are lower than R22 by 20-26oC. Volumetric refrigeration capacity of R1270 (3197 kJ/m3) is very close to that of volumetric capacity of R22 (3251 kJ/m3). Both R1270 and R290 shows good miscibility with R22 mineral oil. Overall R1270 would be a suitable ecofriendly refrigerant to replace R22 from the stand point of ODP, GWP, volumetric capacity, discharge temperature and miscibility with mineral oil although its COP is lower.
The present work focuses on analytical computation of thermodynamic performance of actual vapour compression refrigeration system by using six pure refrigerants. The refrigerants are namely R22, R32, R134a, R152a, R290 and R1270 respectively. A MATLAB code is developed to compute the thermodynamic performance parameters of actual vapour compression system such as refrigeration effect, compressor work, COP, power per ton of refrigeration, compressor discharge temperature and volumetric refrigeration capacity at condensing and evaporating temperatures of 54.4oC and 7.2oC respectively. Analytical results exhibited that COP of both R32 and R134a are 15.95% and 11.71% higher among the six investigated refrigerants. However R32 and R134a cannot be replaced directly into R22 system. This is due to their higher compressor discharge temperature and poor volumetric capacity respectively. The discharge temperature of both R1270 and R290 are lower than R22 by 20-26oC. Volumetric refrigeration capacity of R1270 (3197 kJ/m3) is very close to that of volumetric capacity of R22 (3251 kJ/m3). Both R1270 and R290 shows good miscibility with R22 mineral oil. Overall R1270 would be a suitable ecofriendly refrigerant to replace R22 from the stand point of ODP, GWP, volumetric capacity, discharge temperature and miscibility with mineral oil although its COP is lower.
Thermodynamic analysis of a transcritical N2O ejector expansion refrigeration cycle is analysed. A two phase ejector, as an expansion device, is used in place of a conventional expansion valve. Effect of three performance parameters for cycle, namely COP, refrigerating effect, compressor work and two performance parameters of ejectors namely entrainment ratio and pressure recovery ratio are evaluated for various motive nozzle inlet conditions and evaporator temperatures. Further, both energetic and exergetic comparison of the proposed cycle is presented with respect to conventional CO2 ejector expansion refrigeration cycle. The N2O ejector expansion system is found to have higher COP, lower compressor discharge pressure and higher entrainment ratio but have disadvantage of lower volumetric cooling capacity. Maximum COP of N2O ejector cycle is found to be about 10% higher than maximum COP of CO2 ejector cycle. Exergetic output of N2O ejector cycle is higher whereas losses occurred due to irreversibility during expansion is lower.
The main results of a theoretical and experimental investigation of the performance
characteristics of an ejector and an ejector refrigeration machine (ERM) operating with
refrigerant R245fa at design and off-design working conditions are presented. The ejector
and ERM were explored theoretically using improved 1D model and the calculated results
were validated experimentally on ejector test rig that has been assembled and operated at
National Taiwan University. For typical cases, the performance characteristics variation
with condensing, generating and evaporating temperatures along with performance maps
are presented. The theoretical results are compared with the results of a set of experiments
and good qualitative and quantitative agreement is observed.
Over the last few decades, researchers have developed a number of empirical and theoretical models for the correlation and prediction of the thermophysical properties of pure fluids and mixtures treated as pseudo-pure fluids. In this paper, a survey of all the state-of-the-art formulations of thermophysical properties is presented. The most-accurate thermodynamic properties are obtained from multiparameter Helmholtz-energy-explicit-type formulations. For the transport properties, a wider range of methods has been employed, including the extended corresponding states method. All of the thermophysical property correlations described here have been implemented into CoolProp, an open-source thermophysical property library. This library is written in C++, with wrappers available for the majority of programming languages and platforms of technical interest. As of publication, 110 pure and pseudo-pure fluids are included in the library, as well as properties of 40 incompressible fluids and humid air. The source code for the CoolProp library is included as an electronic annex.
Theoretical analyses and optimisation are carried out with ethane, ethylene and nitrous oxide as the lowtemperature
(LT) fluids in a cascade system for ultra-low-temperature refrigeration applications to examine
the effects of design and operating parameters. Finally, performance improvement has been investigated
employing an internal heat exchanger. Optimal intermediate temperature (IT) correlations have been developed.
Ethane is superior in terms of coefficient of performance (COP), whereas nitrous oxide is superior in
terms of volumetric cooling capacity as an LT fluid.With increase in compressor efficiency, COP increases;
however, with little influence on the optimum IT. Using an internal heat exchanger in the LT circuit, the
cooling COP can be increased for ethane and ethylene; however, there is marginal decrease in COP for
N2O. Ammonia is not suitable as a HT fluid for some operating conditions where the optimum IT is lower
than the normal boiling point and propylene may be a suitable substitute with a penalty on COP.
In this paper the experimental results of an autocascade refrigeration system for achieving ultra low temperature are presented. The plant is used to preserve tissue and cells. When the air temperature is equal to −150°C in 0.25 m3 space, the required refrigeration power is about 250 W. The influence of the most meaningful variables is discussed with regard to the design of the plant. The experimental results show an acceptable time to reach the steady state in dependence of the finality of the plant. The working substance is a non-azeotropic mixture consisting of hydrofluorocarbon (HFC) refrigerants in addition to argon and methane. Copyright © 2009 John Wiley & Sons, Ltd.
Employing an ejector to recover the expansion work of the auto-cascade refrigeration cycle is a feasible method to improve the system performance. The system operation characteristics are closely relevant to the ejector geometry parameters. In order to obtain the critical structure parameters influencing the freezer's pull-down performance, experimental research was conducted on an ejector-enhanced auto-cascade refrigeration cycle applied in a low-temperature freezer. The impacts of the ejector nozzle throat, mixing chamber diameter and length, and the nozzle exit position on the system's pull-down and steady operation characteristics were explored. The experimental results illustrated that the cooling rate and the attainable freezing temperature were mainly influenced by the nozzle throat and mixing chamber length instead of the nozzle exit position and mixing chamber diameter. The nozzle throat diameter of 0.52 mm and the mixing chamber length of 25 mm was optimal concerning the fastest cool-down rate and lowest freezing temperature of -61.2 °C. At the early phase of the pull-down process, a small mixing chamber diameter would cause the ejector malfunction of the pressure lift. There was a worst nozzle exit position of slowing down the pull-down speed, rising the freezing temperature, and reducing the system COP and exergy efficiency at the given operations. The ejector yielded the maximum pressure lift ratio of 1.196 and the entrainment ratio of 0.523 at the optimal ejector geometries. This work would be helpful to guide the ejector structure optimization for the ejector-enhanced auto-cascade low-temperature freezers.
The field of refrigeration witness a massive transition in the supermarket with a strong focus reflected on energy consumption. The use of ejector allows for overcoming the significant exergy destruction lays on the expansion processes of the cooling systems and led to spark improvement in the system performance by recovering some of the expansion work. In this study, a detailed experimental work and exergy analysis on the R744 transcritical ejector cooling system was investigated. The experiment was implemented on the commercial ejector cartridge type (032F7045 CTM ELP60 by Danfoss). The impact of different operating conditions determined by exit gas cooler pressure and temperature, evaporation temperature and receiver pressure was examined. The ejector performance of the pressure lift, mass entrainment ratio, work rate recovery and efficiency were evaluated. In addition, exergy efficiency and the variation of exergy produced, consumed, and destruction were assessed based on the transiting exergy. The result revealed better overall performance when the ejector operated at transcritical conditions. The ejector was able to recover up to 36.9% of the available work rate and provide a maximum pressure lift of 9.51 bar. Moreover, it was found out that the overall available work recovery potential increased by rising the gas cooler pressure. Out of the findings, the ejector could deliver maximum exergy efficiency of 23% when working at higher motive nozzle flow temperatures along with providing lower exergy destruction. The experiment results show that the amount of the exergy consumed and destruction were gradually increased with higher gas cooler pressure and, in contrast, decreasing with higher motive nozzle flow temperature.
Refrigeration industry is adopting a proactive strategy to phase out fluorinated greenhouse gases by more sustainable working fluids. R744 is a natural refrigerant widely proposed for commercial refrigeration. Its use in cascade and booster cycles allows a combined cooling and freezing production. However, when single-stage evaporation at low temperature is required, the adoption of R744 in transcritical cycles is scarce. The main reasons are due to the low Coefficient of Performance (COP) achieved, as well as the technical limitations to reach extreme pressure ratios using commercial compressors. In light of this, this paper proposes to use the CO2 Ejector-Expansion Refrigeration Cycle (EERC) to overcome these drawbacks. To assess the feasibility of the proposal, a thermoeconomic optimization is conducted for low-temperature refrigeration in warm climates. The analysis has been conducted considering a two-phase flow ejector, a commercial double-stage compressor, and evaporating conditions ranging from -10 °C to -38.11 °C, which was revealed the minimum temperature to avoid the triple point inside the ejector. The results showed that the EERC allows using smaller commercial compressors within a broader operating envelope, improving the annual average COP about 5.5% compared to the reference cycle, besides reducing investment and yearly energy consumption costs.
In order to improve the cycle performance of a conventional single-stage autocascade refrigeration cycle (ARC), an auxiliary separator is considered to be introduced. In the modified autocascade refrigeration cycle (MARC), the auxiliary separator located after an expansion device is used to further collect the vapor enriched with low-boiling components. In this case, the MARC enables to improve the cycle performances by utilizing more zeotropic mixture enriched with the low-boiling components to realize a higher evaporation pressure at the given evaporation temperature in the evaporator of the cycle. The performances of the MARC and ARC are compared utilizing energy, exergy and exergo-economic analysis methods, and several important parameters are also discussed in detail. The results indicate that the MARC using zeotropic mixture R290/R170 is feasible and there are obvious improvements in terms of the COP, volumetric refrigeration capacity and exergy efficiency. It is found that compared to the ARC, the COP improvement of MARC can reach up to 16.1%. The exergy efficiency of MARC is increased by 10.23% and the overall cost rate of MARC is decreased by 2.51% under a typical operating condition. In addition, the COP of MARC has a maximum value at given conditions when the mass fraction of R290 at the compressor inlet is around 0.3. In general, the performance characteristics of the proposed cycle demonstrate its potential applications in low-temperature freezers.
The natural refrigerant R744 seems to be the long term solution to be imposed in the food retail industry, where low and medium refrigeration temperatures are usually required, despite the technical difficulties related to high the pressure, especially in warm climates, leading to trans-critical operation with the consequent COP reduction. To overcome such difficulties, different approaches have been proposed in literature, being very popular the use of parallel compressor or its combination with ejector expansion devices. The efficient operation of trans-critical R744 systems requires the control of the gas cooler pressure, as well as the pressure of the flash-tank. In the case of ejector expansion devices, the ratio of entrained mass flow rate must be also included in the control variables and, at the same time, its use implies additional restrictions. The current paper presents the optimization methodology for the operating conditions of a two-stage CO2 refrigeration unit for both cases, with and without ejector. The curves of optimal combinations of gas cooler and flash-tank pressures, understood as operational control laws, are provided together with the COP achieved. The operating conditions and performance of the ejector case are compared showing COP improvements of up to 13%. The results also show that the operating region of the control variables is limited due to the use of the ejector.
The ejector-expansion freezer refrigeration system (ERS) has been proposed to recover part of the expansion work in the throttling process and improve cycle efficiency. In this paper, the performance characteristics of a -40 °C low-temperature freezer adopted with the standard ERS are experimentally investigated under various operating conditions, and several important parameters are also examined in detail. Additionally, the performance of the freezer prototype is compared between using the ERS and a conventional vapor compression freezer refrigeration system (CRS). The experimental results show that the lower evaporation temperature can be obtained by ERS, the pressure lift ratio of ejector can reach more than 1.25, the compressor compression ratio and power consumption are reduced due to the effective pressure lifting effect of the ejector. Furthermore, when the ambient temperature is 20/25/32 °C, the refrigeration capacity and COP of the ERS based prototype can be improved by 112.4% and 20.2%, respectively. Meanwhile, the energy consumption of ERS can be reduced by 6.2%, 21.7% and 38.9% compared to that of the CRS based prototype, respectively. This validate that the ERS offers a significant energy-saving potential to low-temperature freezers.
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 CO2'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 N2O 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 N2O and CO2 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 N2O could successfully be supressed by various measures.
CAUTION: Pure N2O as well as mixtures of N2O and CO2 with lubricants based on hydrocarbons can cause violent explosive decomposition reactions abruptly increasing the pressure inside a refrigeration system approximately tenfold.
Heat pumps have been widely applied owing to their high energy efficiency. Two-stage and quasi-two-stage systems are effective in enhancing the performance of heat pumps under large pressure ratio. Meanwhile, refrigerant mixtures are commonly regarded as potential substitutes for heat pumps because they can provide more flexibility in searching for new alternatives. In contrast to previous conclusions regarding gas-injected heat pumps with pure or azeotropic refrigerants, the type of economizer has an important effect on the system performance. In this study, a novel double internal auto-cascade two-stage compression system suitable for refrigerant mixtures is proposed, which can effectively modulate the concentration of the refrigerant going to the evaporator and the injection port, drive the refrigerant rich in high-pressure composition to enter the evaporator, thereby improving the system performance. The results indicate that under the operational working conditions, the flashing pressure is the only independent parameter and the optimal value exists in this novel system. Compared with the flash tank system, the novel system exhibits a 9.6% higher heating capacity and a 6.1% COP higher. The largest increases in the heating capacity and COP relative to the intermediate heat exchanger system are 2.1% and 2.5%, respectively.
Several environmental protection policies have been enforced restricting working fluids with high global warming potential (GWP) values used in many types of refrigeration and heat pump systems. However, ultralow-temperature (ULT) refrigeration has not been included, which commonly uses refrigerants with very high GWP values (such as R23 and R508B). Therefore, publicly available research programs seeking low GWP alternative refrigerants do not cover this application and the transition to more environmentally friendly fluids is slowed down. This work presents a comprehensive review that summarizes and discusses the available studies about ULT refrigeration systems. The current status of the technology, system architectures and refrigerants are analyzed. Moreover, the transition towards low GWP refrigerants is proposed, presenting the most promising low GWP alternatives. The most commonly used architectures for ULT refrigeration are the two-stage cascade and auto-cascade, in which the use of ejector has recently been considered in research papers. R170 and R1150 are the available natural refrigerants suitable for ULT, but they have not yet been included in many flammability and risk assessment studies. The A2 hydrofluoroolefin R1132a has been recently proposed as a blend component to avoid problems of stability. However, more information is still necessary to start with simulation and experimental studies. R41 could be an alternative due to its low GWP and suitable normal boiling point, but it has not been thoroughly investigated yet. Overall, there is a gap in the literature in terms of developing alternative refrigerants for ULT refrigeration. This study aims at shedding light on this gap to direct future research in this field towards reliable, environmentally friendly and marketable alternative refrigerants.
In this study, an ejector subcooling refrigeration cycle (ESRC) with zeotropic mixture R290/R170 for low-temperature freezer applications is proposed. In the ESRC, an ejector is introduced with a new cycle configuration to reduce the throttling loss and lift the suction pressure of compressor. A major advantage of the proposed cycle is that it does not need any a vapour-liquid separator. The performances of the ESRC, the baseline vapor-compression refrigeration cycles (BVRC) and the standard ejector-expansion refrigeration cycle (EERC) are compared utilizing energy and exergy analysis methods, and several important parameters are also discussed in detail. The results indicate that the ESRC is feasible and there are obvious performance improvements in terms of the COP, volumetric refrigeration capacity and exergic efficiency. Especially, the ESRC can achieve COP improvements of 17.4% and 26.6% compared with the EERC and BVRC in typical operating conditions, respectively. In general, the performance characteristics of the proposed cycle demonstrate its potential applications in low-temperature freezers.
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%.
To increase the coefficient of performance of a simple vapor-vapor ejector refrigeration cycle, this cycle is combined with a simple liquid-vapor ejector refrigeration cycle. The new combined ejector refrigeration cycle is modeled in energy, exergy, environmental and economic aspects. Then two-objective optimization of this cycle (with exergy efficiency and total annual cost as two objective functions) is performed and optimum values of five design parameters for combined vapor-vapor and liquid-vapor ejector refrigeration cycle are obtained. Moreover, from three different refrigerants for vapor-vapor ejector cycle and four different refrigerants for liquid-vapor ejector cycle (sum of twelve different cases), two refrigerants are also selected for combined vapor-vapor and liquid-vapor ejector cycle at the optimum point. Analysis of the new proposed cycle shows 18% higher coefficient of performance, 25% higher exergy efficiency, 31% lower electricity consumption and 8% lower annual cost for a specific cooling capacity in comparison with another combined (hybrid) ejector-compressor refrigeration cycle.
In this paper, an advanced ejector-expansion autocascade refrigeration cycle (AEARC) using hydrocarbon mixture R290/R170 for applications in low-temperature freezers is proposed. In the AEARC, a two-phase flow driven ejector is introduced with new cycle configuration to reduce the thermodynamic loss in throttling process and lift the suction pressure of compressor significantly. The performances of the AEARC and traditional autocascade refrigeration cycle (ARC) are compared using energy and exergy analysis methods, and several important parameters for AEARC are also discussed in detail. The results indicate that AEARC is feasible and there are obvious performances improvements in theCOP, volumetric refrigeration capacity and exergic efficiency. Especially, in AEARC, the COP and volumetric refrigeration capacity increase by 80.0% and 78.5% at most compared to that of ARC, respectively. In general, the AEARC can provide significant performance improvement and produce better actual operation benefit. The potential practical advantages may be worth further attention in future research.
This paper proposed a double ejector-expansion autocascade refrigeration cycle (DEARC) using hydrocarbon mixture R290/R170 for applications in low-temperature freezers. In DEARC, the two ejectors are introduced to reduce the thermodynamic loss in throttling process. The performances of the traditional autocascade refrigeration cycle (ARC), single ejector-expansion autocascade refrigeration cycle (EARC) and DEARC are compared, and several important parameters for DEARC are also discussed in detail. The results indicate that DEARC is feasible and there are obvious performances improvements on theCOP, volumetric refrigeration capacity and exergic efficiency. Especially, in DEARC, the COP increases by 97.9% and 29.6% at most compared to that of ARC and EARC, respectively, the volumetric refrigeration capacity is 334.8% and 188.6% higher than that of the two cycles. In general, the DEARC provides obvious performance improvement and produces better actual operation benefit. The potential practical advantages may be worth further attention in future studies.
The hybrid auto-cascade refrigeration system with an integrated ejector cooling cycle (HACRS) driven by high-grade power and low-grade heat simultaneously is developed in this paper. The working fluid applied in the system is a zeotropic refrigerant mixture of R170/R600a. The heat-driven ejector cooling cycle is employed to the auto-cascade refrigeration cycle to form a novel hybrid auto-cascade refrigeration system coupled with an ejector cycle (HACRS). The ejector is applied to increase the suction pressure of the compressor, and cooling capacity from the ejector cycle is also utilized by the evaporative-condenser and dephlegmator in the HACRS. The system performance is evaluated, based on the mathematical model of the system from the principle of mass and energy conservation. The results indicate that energy consumption of the compressor in HACRS could be reduced by 50% as compared to that in the conventional auto-cascade refrigeration cycle (ACRC). The HACRS can use the heat-driven ejector cycle and the recovery of exhaust waste heat of the compressor to improve its mechanical coefficient of performance (COPme) effectively.
The performance of an ejector refrigeration system (ERS) with R134a ejectors was investigated experimentally. More stable operating states under overall modes were achieved by adopting shell-and-tube heat exchangers and setting the branch of pipe to the expansion valve between the condenser and the liquid receiver. It is found that the change of the ejector operational mode caused by varying the operating and geometric parameters of ejector has significant impact on ejector performance. With a rising generating temperature, the entrainment ratio can first increase and then decrease because the operational mode changes from subcritical mode to critical mode. A rise of the ejector area ratio can cause an increase in the critical entrainment ratio at the expense of a decrease in the critical condensing temperature. Moreover, the COP and the cooling capacity of the ERS behave similarly to the entrainment ratio. Empirical correlations obtained from the experimental data can accurately predict the critical entrainment ratio, the critical condensing temperature and the breakdown condensing temperature.
This study presents the experimental investigation of an ejector-enhanced auto-cascade refrigeration cycle (EARC) with zeotropic refrigerant R134a/R23. Performance comparisons among the EARC and two conventional cycles were conducted at selected operating conditions. The effects of ambient temperature, charged mass fraction ratio of the mixture, throttle valve opening, and heat load on the performance characteristics of the EARC were investigated. The results indicated that the EARC had more advantages in terms of lower refrigeration temperature and higher energy utilization efficiency over the conventional cycles, and the coefficient of performance (COP) and exergy efficiency improvements of the EARC reached up to 9.6% and 25.1%, respectively. The throttle valve opening was optimal with respect to the maximum system exergy efficiency determination. The refrigerant R134a/R23 with the optimal mass fraction ratio of 0.70/0.30 was proposed.
Natural substances are becoming very promising for long term alternative for refrigeration purposes. In this paper, two natural refrigerants have been proposed and analyzed for a novel ejector expansion transcritical cascade refrigeration (NEETCR) system. Nitrous oxide (N2O) is used in the low temperature circuit (LTC) whereas carbon dioxide (CO2) is used in the high temperature circuit (HTC) of the NEETCR system. The reject of refrigerant vapor heat in the HTC is carried out through the use of transcritical carbon dioxide Rankine cycle. This produces work, which will be used to reduce the consumption work of compressors and feed pump thereby resulting in the improvement of the energy efficiency of the whole system. The simulation results were obtained by a computer FORTRAN program, where REFPROP 9 database was used to get the refrigerant thermodynamic properties. The simulation results showed that the (NEETCR) system had higher coefficient of performance and higher system second law efficiency compared to the EETCR system.
A new ejector enhanced auto-cascade refrigeration cycle using R134a/R23 refrigerant mixture is proposed in this paper. In the new cycle, an ejector is used to recover part of the work that would otherwise be lost in the throttling processes. The performance comparison between the new cycle and a basic auto-cascade refrigeration cycle is carried out based on the first and second laws of thermodynamics. The simulation results show that both the coefficient of performance and exergy efficiency of the new cycle can be improved by 8.42-18.02% compared with those of the basic cycle at the same operation conditions as the ejector has achieved pressure lift ratios of 1.12-1.23. It is found that in the new cycle, the highest exergy destruction occurs in the compressor followed by the condenser, cascade condenser, expansion valve, ejector and evaporator. The effect of some main parameters on the cycle performance is further investigated. The results show that for the new cycle, the achieved performance improvement over the basic cycle is also dependent on the mixture composition and the vapor quality at the condenser outlet. The coefficient of performance improvement of the new cycle over the basic cycle degrades with increasing vapor quality. In addition, there exists an optimum mixture composition to obtain the maximum coefficient of performance for the new cycle when other operation conditions are given. The optimum mixture composition of both cycles may be fixed at about 0.5 under the given evaporating temperature.
This paper proposes an internal auto-cascade refrigeration cycle (IARC) operating with the zeotropic mixture of R290/R600a or R290/R600 for domestic refrigerator-freezers. In the IARC system, a cascade heat exchanger associated with a phase separator is introduced to achieve auto refrigerant cascade refrigeration and enhance the overall system performance. Performances of the IARC are evaluated by using a developed mathematical model, and then compared with that of the conventional refrigeration cycle (CRC). According to the simulation results, the IARC with R290/R600a has 7.8-13.3% improvement in coefficient of performance (COP), 10.2-17.1% improvement in volumetric refrigeration capacity and 7.4-12.3% reduction in pressure ratio of compressor compared with those of the CRC under the same given operating conditions. The further simulation results of the IARC using refrigerant R290/R600 also indicate that the performances of COP, volumetric refrigeration capacity and pressure ratio of compressor could be further improved. The performance characteristics of the IARC may show its promise in domestic refrigerator-freezers applications.
A mathematical model of the compressible transonic single- and two-phase flow of a real fluid is discussed in this paper. The model was originally developed to simulate a refrigerant flow through a heat pump ejector. In the proposed approach, a temperature-based energy equation is replaced with an enthalpy-based formulation, in which the specific enthalpy, instead of the temperature, is an independent variable. A thermodynamic and mechanical equilibrium between gaseous and liquid phases is assumed for the two-phase flow. Consequently, real fluid properties, such as the density, the dynamic viscosity and the diffusion coefficient, are defined as functions of the pressure and the specific enthalpy. The energy equation formulation is implemented in commercial CFD software using subroutines that were developed in-house. The formulations was tested extensively for a single-phase flow of the R141b refrigerant, and for a two-phase flow of the R744 fluid (carbon dioxide) that occurred in a 3-D model of the ejector motive nozzle. In the model validation procedure, a satisfactory comparison between the experimental and computational results of the primary and secondary mass flow rates was obtained for both flow regimes. In addition, in the case of the R744 flow, the pressure distribution along the centre line of the ejector was accurately predicted as well. Furthermore, the results also shows that geometry modelling and measurement accuracy play an important in the final numerical results. As a result of the reasonable computational times, this method can be effectively used for the design of ejectors and also in geometric optimisation computations.
In the present study an experimental comparison between the standard CO2 expansion valve refrigeration cycle and the ejector refrigeration cycle is presented. High pressure variations for different evaporation pressures and gas cooler outlet temperatures were performed. Additionally, the ejector efficiency, the entrainment ratio, and the pressure recovery by the ejector were investigated at these conditions. Both investigated cycles where realized using the same test rig, thus minimizing comparison errors. Compared to the maximal COP of the expansion valve cycle, COP improvements of the ejector cycle of 17% were reached with ejector efficiencies of up to 22%.
This study presents experimental results on the cycle characteristics of an auto-cascade refrigeration system. The auto-cascade refrigeration (ACR) system is driven by a single compressor, using zeotropic mixture as refrigerant, which achieves cascade between high and low boiling point components by an evaporative condenser for the purpose of obtaining a lower evaporation temperature comparing with single refrigerant system. The coefficient of performance, cooling capacity, evaporation temperature, and pressures and temperatures of refrigerant at the inlet and outlet were measured and parameter analyses were conducted with different charging concentrations, different temperatures of cooling water and different matches of cycle flux between high and low boiling point components under the same evaporating pressure. The systematic theoretical analyses and experimental results depict the relations between the above parameters in an auto-cascade refrigeration system, which constitute a useful source for the auto-cascade refrigeration system’s design and analysis.
A theoretical investigation was performed concerning the coefficient of performance (COP) of cascade refrigerating systems using N2O as refrigerant for the low temperature cascade stage and various natural refrigerants like ammonia, propane, propene, carbon dioxide and nitrous oxide itself for the high temperature stage. The basis of the comparison was a conventional R23/R134a-cascade refrigerating system for heat rejection temperatures of +55, +35 and +25°C for air cooling, cooling tower water cooling and city water cooling, respectively. It can be stated that such an application of N2O at the primary stage and ammonia or hydrocarbons as refrigerants at the secondary stage in refrigerating systems achieves similar COP-values compared to the R23/R134a-cascade refrigerating system, whereas CO2 and N2O in a transcritical cycle in general perform worse.An application of N2O in a two-stage compression cycle with interstage injection and city water cooling at low and high interstage temperatures has a nearly equal COP as a conventional R23/R134a-cascade refrigerating system and is an interesting alternative for small laboratory refrigerating systems.
This study presents experimental results obtained from a transcritical R744 system using a refrigerant ejector. The results were compared to that of a conventional system with an expansion valve. For the test conditions considered, the cooling capacity and COP simultaneously improved by up to 8% and 7%, respectively. Experiments were analyzed to quantitatively assess the effects on system performance as a result of changes in basic ejector dimensions such as motive nozzle and diffuser sizing. Small angles of 5° yielded best results for the static pressure recovery of the high-speed two-phase flow entering the diffuser. Experiments confirmed that like in a conventional transcritical R744 system with expansion valve, the high-side pressure control integrated into the ejector could be used to maximize the system performance. Numerical simulation results helped identifying this basic trend. Due to difficulties in the ejector throat pressure measurements, a more practical performance metric was introduced in order to quantify overall ejector efficiencies. According to this definition, the prototype ejector was able to recover up to 14.5% of the throttling losses.
A one-dimensional mathematical model was developed using the equations governing the flow and thermodynamics within a jet pump with a mixing region of constant cross-sectional area. The analysis is capable of handling two-phase flows and the resulting flow phenomena such as condensation shocks and the Fabri limit on the secondary mass flowrate. This work presents a technique for quickly achieving first-approximation solutions for two-phase ejectors. The thermodynamic state of the working fluid, R-134a for this analysis, is determined at key locations within the ejector. From these results, performance parameters are calculated and presented for varying inlet conditions. The Fabri limit was found to limit the operational regime of the two-phase ejector because, in the two-phase region, the speed of sound may be orders of magnitude smaller than in a single-phase fluid.
In ejector refrigeration systems, the performance of the ejector is critical to the performance, capability, size and cost of the whole system. Construction of mathematical models has been used as an effective method for analyzing the performance of the ejector as well as the whole refrigeration system. These models can also be used to guide system operation, interpret experimental results and assist in system design and optimization. The overall objective of this paper is to provide a review of various researches of the mathematical model on the hydrodynamic and thermodynamic character within the ejector. The paper first briefly describes ejector including fundamental principle, flowing and mixing mechanism and the method of model establishment. Then various models consisting of ideal assumptions, governing equations, auxiliary conditions, solution methods and main results are presented. The models can be classified into two main categories: (i) steady thermodynamic models which can be further subdivided into single-phase flow model and two-phase flow model and (ii) dynamic models which are also subdivided according to the flowing phases considered. It has been shown that the dynamic models have higher prediction precision and give more information compared with the steady thermodynamic models. In addition, the simplified empirical and semi-empirical models based on measured data are briefly discussed. This review is useful for understanding the evolution process and the current status of the mathematical models on ejector and highlighting the key aspects of model improvement such as the mixing mechanism, the capture of the shock wave, etc.
This paper presented a novel autocascade refrigeration cycle (NARC) with an ejector. In the NARC, the ejector is used to recover some available work to increase the compressor suction pressure. The NARC enables the compressor to operate at lower pressure ratio, which in turn improves the cycle performance. Theoretical computation model based on the constant pressure-mixing model for the ejector is used to perform a thermodynamic cycle analysis for the NARC with the refrigerant mixture of R23/R134a. The effects of some main parameters on cycle performance were investigated. The results show the NARC has an outstanding merit in decreasing the pressure ratio of compressor as well as increasing the COP. For NARC operated at the condenser outlet temperature of 40 °C, the evaporator inlet temperature of −40.3 °C, and the mass fraction of R23 is 0.15, the pressure ratio of the ejector reaches to 1.35, the pressure ratio of compressor is reduced by 25.8% and the COP is improved by 19.1% over the conventional autocascade refrigeration cycle.
The main purpose of this study is to investigate the performance of an autocascade refrigeration system using zeo-tropic refrigerant mixtures of R744/134a and R744/290. One of the advantages of this system is the possibility of keeping the highest pressure of the system within a limit by selecting the composition of a refrigerant mixture as compared to that in the vapor compression system using pure carbon dioxide. Performance test and simulation have been carried out for an autocascade refrigeration system by varying secondary fluid temperatures at evaporator and condenser inlets. Variations of mass flow rate of refrigerant, compressor power, refrigeration capacity, and coefficient of performance (COP) with respect to the mass fraction of R744 in R744/134a and R744/290 mixtures are presented at different operating conditions. Experimental results show similar trends with those from the simulation. As the composition of R744 in the refrigerant mixture increases, cooling capacity is enhanced, but COP tends to decrease while the system pressure rises.
A 1-D analysis for the prediction of ejector performance at critical-mode operation is carried out in the present study. Constant-pressure mixing is assumed to occur inside the constant-area section of the ejector and the entrained flow at choking condition is analyzed. We also carried out an experiment using 11 ejectors and R141b as the working fluid to verify the analytical results. The test results are used to determine the coefficients, ηp, ηs, φp and φm defined in the 1-D model by matching the test data with the analytical results. It is shown that the1-D analysis using the empirical coefficients can accurately predict the performance of the ejectors.RésuméDans cette étude, on a effectué une analyse unidimensionnelle pour prédire la performance d'un éjecteur fonctionnant en mode critique. Les auteurs sont partis du principe que le mélange s'effectue à pression constante dans la partie de l'éjecteur dont la section est constante et ont analysé le flux entraı̂né au niveau de l'onde de choc. Onze éjecteurs utilisant le R141b comme fluide actif ont été utilisés par les auteurs afin de vérifier les résultats analytiques. Les résultats expérimentaux sont utilisés pour déterminer les coefficients ηp, ηs, φp et φm définis dans le modèle unidimensionnel. L'étude a montré que l'analyse unidimensionnelle utilisant les coefficients empiriques peut prédire la performance des éjecteurs de façon précise.
The refrigeration performance parameters of two binary azeotropic mixtures of R170 + R23 and R170 + R116 and a ternary azeotropic mixture of R170 + R23 + R116 were measured systematically in the low-stage loop of a two-stage cascade system. In addition, the performance of the currently used R508B refrigerant was also measured at similar conditions for comparison. A two-stage cascade refrigeration testing apparatus was designed and assembled, in which an R404A system was used as the high-temperature refrigeration stage. Thermodynamic performance parameters of the low-stage loop such as the coefficient of performance (COP), cooling capacity, and the discharge temperature of these four mixtures were measured at various condensation and evaporating temperatures. The results show that the COP of the R170 + R116 binary mixture is up to 10% higher than that of R508B. These mixtures show good potential as low-temperature stage refrigerants for applications in the −80 °C temperature range.
Thermodynamic analyses as well as optimization studies based on maximum cooling COP of a transcritical N2O cycle and both energetic and exergetic comparisons with CO2 cycle are presented in this article. Effects of superheating, internal heat exchanger and expansion turbine are studied as well. An expression for optimum discharge pressure has been developed. Variation trends of optimal parameters for the N2O system are similar to that of a CO2 system. The N2O cycle exhibits higher cooling COP, lower compressor pressure ratio and lower discharge pressure and temperature, and higher second law efficiency when compared to CO2 based systems; however, it is inferior in term of volumetric cooling capacity at the optimum condition. Effect of superheating in evaporator is negligible and effect of introducing an internal heat exchanger is moderate whereas effect of employing a work recovery turbine is significant on COP improvement and discharge pressure reduction at the optimal condition for both working fluids.
Regulation (EU) No 517/2014 of the European Parliament and of the Council of 16
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Ejector characterisation by means of entrainment ratio and compression efficiency | Comparaison thermodynamique de cycles de refroidissement à éjecteur. Caractérisation d’éjecteur aux moyens du ratio d’e
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