Low-temperature Refrigeration Research Articles

Refrigerants for eutectic refrigerating systems–Experimental results and future consideration

The new European F-Gas and PFAS Directives threaten to dramatically reduce quotas or even outright ban synthetic refrigerants, and transition to natural refrigerants in the near future seems inevitable. This article examines eutectic refrigerating systems used in low-temperature transport refrigerators for retail distribution considering transition to natural refrigerants. The main limitation for the operation of eutectic systems is the low evaporation pressure, and various designs addressing this limitation were already tried in the last few years during transition from R507A to R452A and other alternatives. In this article 5 systems with R452A, two with R290 and two with R1270 are compared. The greatest effect was demonstrated by addition of roll-bond plates – it reduced the difference between the inside air and evaporation temperatures by about 3 K. However, this solution is not promising with hydrocarbons due to the higher refrigerant charge. All other solutions together give no more effect than roll-bond plates in terms of evaporation temperature at the end of pull-down. Based on experimental results, the prospects of eutectic refrigeration system with natural refrigerants are reviewed. R290 is not suitable for such systems, but R1270 could be used in systems where eutectic plates just need to be crystallized. However, in weight-optimized systems used in refrigerated vans, the evaporation pressure of R1270 would be lower than atmospheric, which is considered unacceptable due to safety requirements. If this provision remains, the eutectic refrigerators with a single-stage vapour compression refrigerating system become impossible, and more complicated cascade system has to be developed.

The effect of Co addition on the enhanced amorphous forming ability and magnetocaloric properties of Gd55Ni30Al15 amorphous alloy

In this work, quaternary amorphous alloy Gd55−xNi30Al15Cox (x = 5, 10, 20) alloy ribbons were designed to systematically research the effect of Co addition on both GFA and magnetocaloric performances of the alloy. The Trg and γ parameters indicated that the GFA of the alloys was effectively improved with the incorporation of Co. The magnetic performances and magnetocaloric effects of the alloys were also effectively enhanced by the addition of ferromagnetic element Co. All the alloys show SO-MPT characteristics, with the Tc remaining around 83 K. Under a 3 T external magnetic field, the Gd50Ni30Al15Co5 amorphous alloydemonstrated a −ΔSmpeakof 6.73 J kg−1 K−1, accompanied by a high RCP of 496.2 J kg−1. These results are competitive with excellent magnetic cooling materials in the same temperature region, indicating its potential as an outstanding low-temperature refrigeration material.

Comparative exergy and energy analyses of compression–absorption cascade refrigeration cycles with varied configurations and ejector implementations

This study aims to assess the feasibility and performance, as well as the exergy loss, of utilizing ejectors in absorption–compression cascade configurations for low-temperature refrigeration systems. The coefficient of performance (COP) is used to measure system efficiency, including COP.A (COP of the absorption chiller), COP.C (COP of the compression side) and COP.Ca (COP of the cascade configuration). The results highlight the importance of ejector placement on the absorption side for optimal performance. Configuration D1, with an ejector on the absorption side, demonstrates an 85% improvement in COP.A, approximately 10% in COP.C and nearly 100% in COP.Ca compared to configurations without an ejector. However, in certain cases, using an ejector on the compression side is not feasible from a second-law perspective, particularly for higher compression-side evaporator temperatures. Nonetheless, the study suggests that configuration D0, which includes an ejector on the absorption side coupled with a simple compression cycle, is applicable for these conditions and performs similarly to D1 in other temperature ranges. These findings provide valuable insights for optimizing ejector placement in absorption–compression cascade configurations and advancing the development of more energy-efficient refrigeration systems.

Performance analysis and multi-objective optimization of the offshore renewable energy powered integrated energy supply system

Offshore renewable energy technologies offer a promising prospect for energy supply on offshore islands. However, traditional wind and solar energy sources exhibit strong variability and cannot fully meet the energy demands of users. At the same time, the energy demand of users also has great volatility, so the supply side and demand side of energy cannot match well. Existing renewable energy systems often utilize thermal power generation as a stabilizer for the system, however, this approach is difficult to apply in offshore conditions. This study introduces an innovative multi-energy complementary system that integrates seawater freezing desalination, refrigeration and power-hydrogen-ammonia co-generation, driven by ocean thermal energy, wind energy, and solar energy. The system incorporates the use of a new developed small temperature difference ejector refrigeration cycle to stabilize the entire system and produce low-temperature refrigeration. By utilizing the seawater freezing desalination process to minimize energy conversion losses and balancing fluctuations in multiple energy inputs and user demands through ammonia-hydrogen co-production, this approach effectively minimizes energy waste, enhances energy utilization efficiency, and reduces excess power handling. The purpose is to optimize the configuration of the energy conversion subsystem within the multi-energy complementary multi-effect co-generation system, thus improving the economic feasibility and reliability of the system in providing energy and freshwater supply, ultimately achieving the goals of reducing energy waste, achieving high conversion efficiency, and minimizing equipment investment. This study conducts an economic analysis and optimization of a multi-energy complementary system, utilizing a 2 MW-level OTEC subsystem as a stabilizer. Both equipment investment and operating costs are considered in the economy analysis. The carbon emission reduction performance of this system is evaluated. The performance optimization results show that the system achieves an energy storage efficiency of 65.96 %, and the curtailment rate of this system is only 3.99 %, significantly lower than the 10 % curtailment rate in traditional system. The proposed system can meet the annual electricity demand of 33.09 million kWh, as well as a cooling demand of 546.81 million kWh. Under typical conditions, the ocean thermal energy conversion cycle exhibits a power efficiency of 0.85 % and a refrigeration efficiency of 29.10 %. Meanwhile, it annually reduces CO2 emissions by 2.54 million tons, demonstrating significant environmental benefits. This proposed system is mainly applicable to tropical waters with high surface water temperatures and depths of no less than 600 m worldwide. This study offers a practical and economically beneficial solution for energy and freshwater supply on offshore islands.

Low-temperature ammonia absorption refrigeration system based on the temperature difference uniformity principle: Optimization analysis

AARS (ammonia absorption refrigeration system) is one of the effective ways to utilize low-temperature waste heat below 100 °C for industrial applications. This novel system of LT-AARS is designed based on the principle of temperature difference uniformity for improving the overall COP. The partial condenser at the distillation section is adopted to achieve the purity of ammonia to more than 99.9 % with a small reflux ratio. A thermodynamic model is established to analyze the influence of the optimized distillation process on system performance and exergy loss. These modifications reduce the temperature difference uniformity factor from 1.12 to 1.025 and improve the COP from 0.414 to 065. Meanwhile, it is proved that the LT-AARS protocol is reasonable and effective by comparing the experimental values with the theoretical model. The exergy loss of the absorber and distiller accounts for about 55 % of the total exergy losses of LT-AARS. With the increases in refrigeration temperature from −15 to 5 °C and heat source temperature from 80 to 100 ℃, the COP can reach a maximum of 0.77. ECOP first increases sharply and then decreases slowly with the increase of refrigeration and heat source temperature, and ECOP has a maximum value. The optimal range of △x for LT-AARS varies from 5 % to 10 %. The COP and ECOP of the LT-AARS are more than 0.6 and 0.3, respectively.

Intestinal pathogens detected in cockroach species within different food-related environment in Pudong, China

Cockroaches are considered mechanical transmitters of infectious diseases, posing a threat to human health. This study assessed the potential of cockroaches in food-related environments to mechanically transmit intestinal pathogens. Cockroaches captured with traps were placed together into a low temperature refrigerator at − 80° for 2 h. Standard taxonomic keys and Fluorescent quantitative PCR techniques were applied for species identification and digestive tract etiological examination. A total of 360 cockroach traps were placed, with a positive rate of 20.8%, and 266 cockroaches were captured. In general, compared with other places and areas, the degree of infestation of cockroaches was more serious in catering places and kitchens. Blattella germanica were most found in catering places (40.2%), followed by Periplaneta fuliginosa in schools (22.2%). According to the life stage, among the 128 cockroach samples, 23 were positive for nymphs and 13 were positive for adults. There were statistically significant differences in the intestinal pathogen detection rates between nymphs and adults (P < 0.05). A total of eight intestinal pathogens were detected, and enterovirus infections were the main ones, with sapovirus being the most detected in Blattella germanica or nymph. Shiga toxin-producing Escherichia coli (STEC) was the most frequently isolated bacterium. Blastocystis hominis had the highest isolation rate. In contrast, 12 diarrhoeal disease pathogens were isolated, and the viruses and bacteria with the highest frequencies were norovirus and E. coli, respectively; no parasites were found. Blattella germanica and Periplaneta fuliginosa in food-related environments can act as potential vectors for the spread of intestinal pathogens and may pose a significant threat to public health.

Numerical modelling of a single-compression multi-temperature ejector-supported R744 refrigeration unit for last mile delivery

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

Low temperature phase change material for cold storage and its application to refrigerated transportation

This paper applies the phase-change cold storage technology to refrigerated transportation to reduce the energy consumption. Experiment data showed that the electronic expansion valve can be randomly adjusted to simulate the temperature within negative 25?C to negative 5?C, and a system for defrosting at low temperature and auxiliary refrigeration based on phase-change thermal energy storage of diethylene glycol were developed to guarantee the reliability of the cold chain system.

Large magnetocaloric effect in gadolinium-rich silicate NaGd9(SiO4)6O2

Magnetic cooling has aroused great interest in cryogenic research due to its high energy efficiency. Due to the large magnetic moment and completely quenched orbital momentum, Gd-based oxides have been promising candidates for cryogenic magnetic refrigerant. In this article, the magnetic properties and magnetocaloric effect of NaGd9(SiO4)6O2 (NGSO) single crystal are investigated. The maximum magnetic entropy change of 60 J·kg−1K−1 is observed with the magnetic field changing from 0–7 T at 2.6 K. The large magnetic entropy change can be attributed to high metal/ligand ratio and weak exchange interaction. Particularly, due to the large density of NGSO, the volumetric entropy change is identified to be 382.8 mJ·cm−3K−1, which surpasses most reported magnetic refrigerants in volume units. Besides, the excellent refrigerant capacity and adiabatic temperature change have been determined to be as high as 370.4 J·kg−1 and 7.4 K under the applied field change of 0–7 T and 0–2 T, respectively. These performances suggest that NGSO is a promising candidate for low-temperature magnetic refrigeration applications.

Structure and cryogenic magnetocaloric effects of NaREO2 (RE = Er, Ho) compounds

Rare-earth (RE) based oxides have presented great application prospects in low-temperature magnetic refrigeration technology. Thus far, studies on magnetic and especially magnetocaloric properties of alkali-metal RE oxides are limited, largely due to the difficulty in synthesizing these oxides. In this work, NaREO2 (RE = Er, Ho) compounds with both the rhombohedral (R3‾m space group) and cubic (Fm3‾m space group) structures were prepared by a facile ball-milling-aided sintering approach. The cubic structural compounds exhibit a spherical microstructure composed of vertically-grown irregular hexahedra, which sharply contrast with a loose lamellar microstructure of the rhombohedral structural compound. The as-prepared NaHoO2 or NaErO2 compound exhibits an antiferromagnetic ordering of RE3+ moments at a temperature of 2.4 K or lower than 2 K respectively. An excellent comprehensive magnetic refrigeration performance at cryogenic temperatures in a low magnetic field change of 0–20 kOe was obtained in these compounds, including a complete reversibility of the magnetocaloric effect, a maximal magnetic-entropy change ΔSM at 2 K of 12 J/kg·K for the rhombohedral NaErO2 and a relative cooling power (RCP) of 79.7 J/kg for the cubic NaHoO2.

Theoretical basis and practical implementation of “bi-refrigerant” and “two-compressor” refrigeration machine schemes

The operating conditions of a refrigeration machine determine its energy efficiency. Some industries, particularly the food industry, are characterized by seasonal (short-term) consumption of cold in the low-temperature range (−40℃ and below) when rapid freezing of the product is required to achieve high quality because of finely dispersed crystallization of moisture and a “shock” temperature for psychrophilic microorganisms. To solve this problem, the periodic use of a low-temperature refrigerant, which, from a rational technical point of view, is not always combined with the constant required temperature of refrigerated storage because of possible operation under vacuum conditions and other difficulties, is advised. The difference between the temperature regime of the required short-term (seasonal or periodic) processing and the constant (main) temperature regime (e.g., refrigerated storage of products) often reaches 20℃ or more, which imposes certain restrictions on the design or operation of the refrigeration machine. In this regard, small and medium-sized refrigeration machines with the capability to operate on two refrigerants (e.g., low and medium temperatures, with periodic automatic mutual replacement) are promising. Such a refrigeration machine is referred to as “bi-refrigerant” in this study. The aforementioned difficulties are also solved by the proposed scheme of a “two-compressor” refrigeration machine, in which the possibility of its operation is realized during periodic change of modes from one stage to two stages, and vice versa, according to the need for the corresponding differences in boiling and condensation pressures.&#x0D; BACKGROUND: Environmental conditions and boiling point impose certain requirements on the selection of refrigerant and the design of the refrigeration machine, which often necessitates the division of the design period of operation of the refrigeration machine into two seasons, i.e., summer and winter, or the main operating mode (e.g., refrigerated storage) and nonessential (e.g., freezing seasonal or periodically supplied products to the refrigerator). This need may be especially acute in seasonal refrigerators and refrigerators operating under the conditions of large differences in ambient temperatures that periodically occur in some geographic areas. The need for a two-season (“bi-refrigerant”) refrigeration machine arises not due to fluctuations in the conversion coefficient under different environmental conditions (although this does occur) or a change in operating mode from the refrigeration cycle to the heat cycle (heat pump) but due to the seasonality of the supply of products subject to primary processing (quick freezing).&#x0D; AIMS: This work aims to provide theoretical justification for the practical implementation of the “bi-refrigerant” and “two-compressor” refrigeration machine schemes.&#x0D; MATERIALS AND METHODS: A theoretical investigation method was used to propose schemes for “bi-refrigerant” and “two-compressor” refrigeration machines, was used. The objects of the study were the diagrams of vapor compression refrigeration machines.&#x0D; RESULTS: A schematic diagram of a “bi-refrigerant” refrigeration machine operating by periodically replacing “seasonal” refrigerants has been developed. A schematic diagram of a “two-compressor” refrigeration machine, which provides the possibility of periodically changing a one-stage to a two-stage cycle, has been presented. A diagram of the automatic control of the proposed solutions is presented using the example of a “bi-refrigerant” refrigeration machine. Mathematical support and justification of the developed schemes are also presented.&#x0D; CONCLUSIONS: Our investigation revealed the theoretically justified possibility of using “bi-refrigerant” and “two-compressor” refrigeration machine schemes, e.g., in industries with seasonal short-term supply of short-term products subject to rapid (low-temperature) freezing or in other industries that require periodic changes in the boiling point of the refrigerant. Delaying factors in the application of the proposed “bi-refrigerant” refrigeration machine design may be the selection of compressor oil capable of operating on two refrigerants and the need to carefully solve the problem of circulation and its return to the compressor. However, this problem can be solved.

Energy, modified exergy, exergo-economic and exergo-environmental analyses of a separation-enhanced auto-cascade refrigeration cycle

Auto-cascade refrigeration technology has broad application prospects in the field of low-temperature refrigeration. The separation efficiency of refrigerants is one of the main factors affecting system performance. This paper proposes a separation-enhanced auto-cascade refrigeration cycle. An additional cascade heat exchanger and an auxiliary phase separator are introduced before the original cascade heat exchanger. The configuration enhances cooling capacity and cycle performance. The energy, modified exergy, exergo-economic and exergo-environmental performance of the cycle are evaluated through simulation and compared with the basic cycle and previously modified cycle. The results demonstrate that the proposed cycle outperforms both the basic cycle and the modified cycle in both energy and exergy efficiency. The maximum coefficient of performance and exergy efficiency of the separation-enhanced cycle is 44.4% and 44.3% higher than those of the basic cycle, 17.1% and 24.8% higher than those of the modified cycle, respectively. Furthermore, the modified exergy analysis method identifies the exergy destruction caused by the components and zeotropic refrigerant. Actually, the exergy destruction of refrigerant contributes to 39.7% of the overall exergy destruction under typical conditions. Moreover, the exergo-economic analysis indicates a 7.6% reduction in the total cost rate for the proposed cycle compared to the basic cycle and a 2.35% reduction compared to the modified cycle. The exergo-environmental analysis highlights a decrease of 25.4% in overall environmental impact for proposed cycle compared to basic cycle and 12.1% reduction compared to modified cycle. In general, the comparative analysis reveals the improvement potential of the proposed cycle in terms of energy-saving, economic and environmental performance.

Multidimensional numerical simulation of thermodynamic and oscillating gas flow processes of a Gifford-McMahon cryocooler

Abstract The Gifford-McMahon cryocoolers are considered to be prominent candidates for the cooling of high-temperature superconducting magnets, liquefaction of permanent gases, helium recondensation in magnetic resonance imaging machines, cooling of superconducting quantum interference device, etc. In this paper, multi-dimensional numerical simulation is performed to visualize the oscillating heat and fluid flow processes that happen in a mechanically driven GM cryocooler. Influence of the ideal gas equation and real gas equation of states on the cooling behaviour is explained. The minimum achievable refrigeration temperature of a uniform mesh regenerator is compared with a multi-mesh regenerator. It is noticed that a multi-mesh regenerator produces a lower refrigeration temperature as compared to a uniform mesh regenerator. In addition to this, a one-dimensional simulation is conducted and results are compared with multi-dimensional numerical simulation. The no-load temperature value calculated by the one-dimensional model and multi-dimensional model with ideal gas is lower than that of real gas equations. Additionally, the refrigerating capacity calculated by the one-dimensional model and multi-dimensional model with the ideal gas equation is higher than those of the real gas equation of state.

Numerical study on infrared detectors cooling by multi-stage thermoelectric cooler combined with microchannel heat sink

Lightweight, noiseless, and constant low-temperature refrigeration is crucial for cooled infrared (IR) detectors, especially short-wave-infrared (SWIR) detectors. However, some SWIR detectors, such as HgCdTe photovoltaic detectors are working below 210 K. For this application, multi-stage thermoelectric coolers (TEC) are preferable to single-stage thermoelectric coolers because they can generate larger temperature differences. This paper presents a refrigeration design that integrates SWIR and TEC into a vacuum Dewar package and combines them with a microchannel heat sink (MHS) to improve the detection performance and better responsiveness of SWIR. Two different refrigeration models, SWIR + TEC + AHS (air-cooled heat sink) and SWIR + TEC + MHS, were established to evaluate the refrigeration performance of the proposed SWIR + TEC + MHS scheme using the finite element method. The influences of multi-stage TEC leg length and current, inlet water temperature, and water velocity on the refrigeration performance of the TEC are analyzed. The results show that the SWIR + TEC + MHS design scheme has the best comprehensive performance and minimum volume. The influences of all the factors are obvious while the current has the most important effect on the performance. The temperature of SWIR reached 201.69 K at standard working conditions. It shows that the cooling scheme can not only meet the needs of flexible deployment of SWIR but also provide better thermal security. The proposed cooling design has an excellent prospect for short-wave-infrared detectors.

Proposal and analysis of a concentrating photovoltaic-driven system for combined production of electricity, hydrogen, and low-temperature refrigeration

A trigeneration system that uses concentrating photovoltaic (CPV) waste heat for low-temperature refrigeration and surplus electricity to power an electrolyzer for hydrogen synthesis is presented in this research to address the energy demands of a multi-unit building. Energy and exergy assessments evaluate system performance under various situations. The ANSYS-FLUENT and Engineering Equation Solver are used. To evaluate its energetic and exergetic performance, the system is studied at different direct normal irradiation (DNI), photovoltaic areas, and absorption chiller driving temperatures. At a constant DNI, increasing the coil diameter from 400 to 600 mm may raise channel water temperature by 3–5%. Ejector-absorption cycle driving temperature enhancement from 65 to 95 °C increased refrigeration capacity from 22 to 34 kW, hydrogen production from 0.76 to 2.8 kg/h, and power generation from 5.9 kW to 4.1 kW. By increasing the photovoltaics area from 78 to 126 m2 and using an absorption chiller and electrolyzer, system energy and exergy efficiencies increase by 23.47% and 8.02%, respectively, while CPV exergy efficiency decreases by 18.06%. The CPV destroys the most exergy in the system, whereas the electrolyzer destroys the least. Combining photovoltaics and the refrigeration machine with an electrolyzer is a novel approach to harnessing sunshine energy.

VIABILITY AND ANTAGONISM OF CRYOPRESERVED LACTIC ACID BACTERIA

Preserving and advancing bioresources involving industrial microorganisms is of paramount importance for every nation. However, long-term storage of these strains often leads to diminished viability and biological activity. Thus, it is crucial to investigate the properties of cryopreserved strains stored at -80°C in a 10% glycerol solution within low-temperature refrigerators. This study aimed to comparatively analyze the viability of 129 lactic acid bacteria strains, including Lactobacillus sp., Lactococcus sp., and Pediococcus sp., cryopreserved from 2006 to 2020. Among them, 93 Lactobacillus sp. strains were categorized into three groups based on storage dates (2006-2007, 2013-2014, and 2017-2020). Viability titers were determined using the standard serial dilution method, counting microorganisms in CFU/ml. Regardless of the storage duration or species affiliation, the study identified lactic acid bacteria strains exhibiting both high (107 - 109 CFU/ml) and low (104-106 CFU/ml) viability titers. Additionally, the antagonistic activity of 33 Lactobacillus sp. strains was investigated using the delayed antagonism method, subdividing them into 17 strains with sufficiently high viability titers and 16 strains with low viability titers. The results revealed that 20% of strains with high viability titers and 27% with low viability titers exhibited relatively high antagonistic activity (with a zone of inhibition ranging from 10 to 18 mm). In both groups, strains with low antagonistic activity (with a zone of inhibition measuring 5-9 mm), particularly against Gram-positive and Gram-negative bacterial test-cultures, predominated. Significantly, 51% of lactobacilli strains demonstrated pronounced antagonism against Candida albicans ATCC- 885-653 test-culture. These findings underscore the practical importance of the study, emphasizing the necessity to analyze and select optimal concentrations of intracellular or extracellular cryopreservatives and to determine the initial viabilitytiter when storing strains for cryopreservation. Tailoring cryopreservation solutions to each strain can enhance the preservation of their original properties, ultimately improving overall preservation quality.

Zeotropic mixture ejector: Modeling approach, validation, and assessment based on composition ratio

The ejector refrigeration system (ERS) using zeotropic mixtures obtains better performance and lower refrigeration temperature than that of the ERS using pure refrigerants, and the characteristics of the zeotropic mixture ejector are a theoretical basis for its optimal design. However, there are few literatures on the performance mechanism of the zeotropic mixture ejector. From the perspective of the binary zeotropic mixture, the R32/R245fa ejector model is developed in this paper based on computational fluid dynamics (CFD) theory, and an experimental setup is also built to validate the CFD model of the ejector. Firstly, the mixing process characteristics of two streams with different composition ratios entering the ejector are investigated, and the key parameters affecting the mixing process are evaluated. Secondly, the impact of zeotropic mixture composition ratio on the ejector's performance is analyzed in terms of entrainment ratio, velocity distribution, and pressure distribution. Finally, aiming at the entrainment ratio, a performance comparison between the R32/R245fa ejector and the pure refrigerant R245fa ejector is conducted under various primary stream conditions, yielding the optimal composition ratio range for the mixture when the R32/R245fa ejector outperforms the R245fa ejector. The findings demonstrate that the R32 mass fraction in both the primary and secondary streams significantly influences the performance of the R32/R245fa ejector. The composition distribution inside the ejector is primarily influenced by the shock chain. The optimal entrainment ratio is achieved by selecting an appropriate R32 mass fraction in the primary stream, and increasing the R32 mass fraction in the secondary stream leads to a linear enhancement in the entrainment ratio. Moreover, when the R32 mass fraction in the primary stream falls within the range of 0–0.08, the entrainment ratio of the R32/R245fa ejector surpasses that of the R245fa ejector operating in the critical mode. Furthermore, this advantage becomes more pronounced as the R245fa ejector gradually deviates from the critical mode.

High-field magnetic and magnetocaloric properties of pseudo-binary Er1−xHoxNi2 (x = 0.25–0.75) solid solutions

Hydrogen is quickly becoming a desired type of fuel, however, the energy and cost required for liquefaction using today’s cooling technology is excessively high. Magnetic cooling based on the magnetocaloric effect is an energy-efficient and environmentally friendly alternative to commonly used vapor compression, but improvements in refrigerants are crucial for this technology to succeed. Polycrystalline Er1−xHoxNi2 (x = 0.25, 0.5, 0.75) Laves-phase solid solutions obtained by the arc-melting method have been investigated due to their potential for low-temperature refrigerants. Er0.75Ho0.25Ni2 and Er0.5Ho0.5Ni2 crystallize in cubic Laves phase superstructure (space group F-43 m), while Er0.25Ho0.75Ni2, similarly to the initial ErNi2 and HoNi2 binary compounds, crystallizes with the formation of the regular cubic C15 structure (space group Fd-3 m). To evaluate how the structure affects magnetic and magnetocaloric properties, studies in a wide magnetic field range, up to 14 T, were conducted. Measurements show all samples obey the second-order magnetic phase transition from ferromagnetic to paramagnetic state, and their Curie temperatures increase with increasing Ho content from 8 K for Er0.75Ho0.25Ni2 to 12.3 K for Er0.25Ho0.75Ni2. At higher temperatures, all solid solutions are Curie-Weiss paramagnets. However, it has been observed that the formation of the superstructure with lower translational symmetry seems to affect magnetic moments and magnetic entropy values. Er0.25Ho0.75Ni2, with the regular cubic C15 structure, showed the highest entropy changes of 43.5 J/kgK around 12 K, while Er0.75Ho0.25Ni2 and Er0.5Ho0.5Ni2, with cubic superstructure, provided ∼30 % lower results of 30.3 and 35.1 J/kgK, around 8 and 10 K, respectively, for magnetic field change of 14 T. Nevertheless, relatively large and reversible values of magnetic entropy prove that these compounds can be promising candidates for magnetic cooling operating within the low-temperature range needed for hydrogen liquefaction.

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

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

Tunning the magnetism and magnetocaloric effects of EuAl4 single crystals by Si and Cu doping

The effects of Si and Cu doping on crystal structures, magnetism, and the magnetocaloric effect of EuAl4 have been investigated. EuAl4, EuAl3.35Si0.65, and EuAl3.82Cu0.18 single crystals grown by the Al flux method are isotypic and crystallize in BaAl4-type structure with space group I4/mmm. Compared to the pristine compound EuAl4, the lattice parameter a keeps almost unchanged and c decreases obviously by Si doping, while a decreases and c increases upon Cu-substitution. These irregular changes in the lattice parameters may contribute to the electronegativity difference between Si and Cu. Cu and Si doping enhances the ferromagnetic interaction, and the direction of easy magnetization changes from the ab plane to the c axis upon the Cu-substitution. Multiple-step metamagnetic transitions are still observed in EuAl3.35Si0.65 as applying the magnetic field along the c axis, while EuAl3.82Cu0.18 shows ferromagnetic behavior along the ab plane and c axis. As a consequence, the low-field magnetocaloric effect is significantly enhanced. The maximum magnetic entropy change under a field change of 20 kOe increases from − 8.4 J/kg K for EuAl4 to − 12.9 J/kg K for EuAl3.35Si0.65 and − 15.9 J/kg K for EuAl3.82Cu0.18. The corresponding values of refrigerating capacity are 67, 140, and 147 J/kg, respectively. The remarkable magnetocaloric performances suggest that the Cu-doped EuAl4 compound is an excellent candidate material for low-temperature magnetic refrigeration.