Zeotropic Refrigerant Mixtures Research Articles

Comparative Exergy Analysis of Series and Parallel Dual-Pressure Auto-Cascade Organic Rankine Cycles

The organic Rankine cycle (ORC) is a valuable method for harnessing low-temperature waste heat to generate electricity. In this study, two dual-pressure auto-cascade ORC systems driven by low-grade geothermal water are proposed in series and parallel configurations to ensure high thermal efficiency and power output. The energy and exergy analysis models for two systems are developed for comparative and parametric analysis, which uses a zeotropic refrigerant mixture of R134a and R245fa. The findings indicate that, with a heat source temperature of 393.15 K, the thermal efficiency and exergy efficiency of the series auto-cascade ORC reach 10.12% and 42.07%, respectively, which are 27% and 21.9% higher than those of the parallel auto-cascade ORC. However, the parallel cycle exhibits a higher net power output, indicating a better heat source utilization. The exergy analysis shows that evaporator 1 and the condenser possess the highest exergy destruction in both cycles. Finally, the parameter analysis reveals that the system performance is affected significantly by the heat source and heat sink temperature, the pinch temperature difference, and the refrigerant mixture concentration. These findings could provide valuable insights for improving the overall performance of ORCs driven by low-grade energy when using zeotropic refrigerant mixtures.

Comparative analysis of a modified cascade refrigeration cycle including an auto-cascade refrigeration cycle using different zeotropic refrigerant mixtures for reducing the environmental impact

The two primary challenges in auto-cascade refrigeration (ACR) cycles are high compressor discharge temperatures and low efficiency at ultra-low temperatures. This study proposes a modified cascade refrigeration cycle (MCR) combined with an ACR cycle for cooling to –60 ℃. The innovative aspect of this work is to improve the ACR performance with simpler designs without increasing the system complexity, contrary to the common trend. For the high-temperature cycle (HTC), a dual evaporator refrigeration cycle with R1234yf is used while an ACR cycle is used for the low-temperature cycle (LTC). Environmentally friendly refrigerant mixtures, such as R170/R290, R170/R600a, and R170/R600, were analyzed using energy, exergy, and exergoeconomic methods. The results show that compared to R23/R134a, the R170/R600 significantly improves the performance of the MCR by increasing the COP and exergy efficiency by 24.0 %, and reducing the exergy destruction by 26 %, unit cooling cost by 11.2 %, and total investment cost by 9.7 %. Moreover, when compared with theoretical studies COP improvements range from 38.7 % to 94.1 %, demonstrating the importance and superiority of the system proposed in this study.

Condensation heat transfer of zeotropic refrigerant mixtures R407C and R448A in a horizontal smooth tube

Zeotropic refrigerant blends have attracted wide interest due to their promises to reduce global warming potential as well as their potential ability to improve the performance of heat pumps and air conditioners. The complex phenomenon of non-isothermal phase change complicates condensation of zeotropic refrigerant mixtures and merits extensive investigation to better understand their performance profiles. The present work experimentally studied condensation heat transfer of R407C and R448A – two R134a-based zeotropic refrigerant mixtures – in a horizontal smooth tube, condensed by counter-current water cooling. Measurements for mass flux were in the range of 120 to 320 kg m−2s−1 and saturation temperature in the range of 42 to 64 °C. Variation in refrigerant temperature and composition shift during condensation were quantitatively presented. The experimental data showed the effects of mass flux and saturation temperature on the heat transfer coefficients of R407C and R448A in different vapor quality regions. The analysis of heat and mass transfer resistances related to the flow regimes and refrigerant thermophysical properties provides a deep understanding of the condensation heat transfer of zeotropic refrigerant mixtures. The experimental data presented in this work are of value for the design of condensers.

Thermodynamic analysis of a dual-pressure evaporation high-temperature heat pump with low GWP zeotropic mixtures for steam generation

High-temperature heat pumps (HTHPs) have gained attention due to their economic and environmental benefits in utilizing industrial waste heat for steam generation. This study focuses on improving the HTHPs’ performance by employing two-stage separation and dual-pressure evaporation technology in the auto-cascade heat pump cycle. Additionally, binary zeotropic refrigerant mixtures with low-GWP are selected using the polar perturbed-chain statistical associated fluid theory (PC-SAFT) equation of state. The findings indicate that the R1234ze(E)&R1233zd(E) refrigerant mixture outperforms other potential alternatives, exhibiting a thermodynamic effectiveness 0.85%–1.86% higher than the benchmark mixture, R134a&R245fa. The improved cycle demonstrates significant enhancements, achieving a 45.17% increase in heat source utilization efficiency and a 24.48% improvement in COP compared to the basic auto-cascade cycle. Effective temperature matching between refrigerant mixtures and heat sources/sinks is guaranteed in the improved cycle. Moreover, a parameter analysis reveals that increasing the subcooling degree of the cascaded heat exchanger and the separation dryness fraction at separator 2 enables improvements in both COP and heat source utilization efficiency. These findings provide valuable insights into the design of steam generation heat pumps and the selection of refrigerant mixtures.

Experimental investigation of a heat pump tumble dryer with a zeotropic refrigerant mixture

Due to their high global warming potential (GWP), fluorinated refrigerants are being replaced in many heat pump and refrigeration systems. Especially in small systems such as heat pump tumble dryers (HPTD), the replacement of the globally used working fluid in this application, R134a, appears to be manageable. One established alternative is the use of hydrocarbon refrigerants. Within this research project, new zeotropic refrigerant blends of hydrocarbons are theoretically evaluated as retrofit substitutes for R134a in heat pump tumble dryers. The aim is to use the temperature glide inherent to zeotropic mixtures during phase change to increase the heat transfer efficiency and thus, the COP of the system. A mixture of R290 (propane) and RE170 (dimethyl ether) was selected and experimentally investigated in a commercially available HPTD. To assess the new mixture, parameters relevant to the cycle such as pressure, temperature, compressor power, and the dryer moisture extraction rate are compared to those values achieved using the refrigerant R134a. Retaining the heat exchanger and compressor from the R134a system, the system charge with the new mixture was 40 % lower, while the thermal capacity was equal to or higher than that of the R134a system. The increased temperature level supplied by the mixture reduced the drying time and the energy consumption of the HPTD. The investigation showed that the heat exchanger design needed to be adapted to the temperature glide properties of the mixture in order to realize meaningful heat transfer efficiency gains.

Circuitry optimization using genetic programming for the advancement of next generation refrigerants

In this study, a new evolutionary method, which can handle the implementation of genetic operators with unrestrained number and locations of splitting and merging nodes for the optimization of heat exchanger circuitries, is developed. Accordingly, this technique expands the search space of previous optimization studies. To this end, a finned-tube heat exchanger simulator is structured around a bijective mathematical representation of a refrigerant circuitry (the tube–tube adjacency matrix), which is used in combination with traversing algorithms from graph theory to recognize infeasible circuitries and constrain the evolutionary search to coherent and feasible offspring. The performance of three refrigerants, namely R32, R410A, and R454C, commonly used in air-conditioning applications was assessed for the optimized circuitries of a 36-tube evaporator while converging to a given cooling capacity, degree of superheating, and heat source boundary conditions. At a given output capacity and air outlet temperature, larger coefficient-of-performance improvements (up to 9.99% with reference to a common serpentine configuration) were realized for zeotropic refrigerant mixtures, such as R454C, where appropriate matching of the temperature glide with the temperature variation of the air yielded the possibility of further reducing the required compression ratio under the corresponding operating conditions. Hence, it was demonstrated that low-GWP zeotropic mixtures with temperature glide can realize a performance comparable to that of R32 and higher than that of R410A by approaching the Lorenz cycle operation.

Measurement of vapour pressure, miscibility and thermal conductivity for binary and ternary refrigerant lubricant mixtures in the context of heat pump tumble dryers

Household appliances need to be both efficient and cost-effective. As a result, electrically driven tumble dryers were converted to heat pump driven ones several years ago in order to comply with energy directives, in this case in Germany. In order to increase the efficiency of heat pumps of different performance classes, the use of zeotropic refrigerant mixtures can be useful, since the temperature glide during the phase change of the refrigerant mixture can be adapted to the temperature gradients of the secondary fluids. In order to take advantage of this unique property, a refrigerant mixture selection was carried out by a screening procedure using REFPROP. Within the investigation presented here, two different zeotropic refrigerant blends, propane (R290)/dimethyl ether (DME or RE170) (30 wt.%/70 wt.%) and R152a/carbon dioxide (R744) (90 wt.%/10 wt.%) are focused on as substitutes with low global warming potential (GWP) for R134a in heat pump tumble dryers. The focus is on the interaction between the refrigerant mixtures and the respective lubricant to ensure usability in the heat pump tumble dryer. A specialised polyolefin-based (POE) lubricant was chosen for each refrigerant blend. Because the numerous combinations of possible refrigerant mixtures can lead to unanticipated challenges with oil mixtures, additional experimental investigations are necessary to ensure the compatibility of the lubricant and refrigerant mixture combination in the temperature ranges associated with the investigated application. Therefore, measurements of the vapor pressure, miscibility gaps and thermal conductivity of the refrigerant-lubricant mixture have been carried out and are the focus of this paper.

A comprehensive review and analysis on CO2 heat pump water heaters

• Existing CO 2 HPWHs in industrial and residential sectors are reviewed. • Performance restricted by the pinch point effect is analyzed. • Limitations in CO 2 compressors and lubrication are revealed for the first time. • Current enhancements to overcome the limitations are summarized. In the context of the Montreal Protocol, CO 2 is considered as an appropriate natural refrigerant with an Ozone Depletion Potential (ODP) of zero and a Global Warming Potential (GWP) of one. A CO 2 heat pump water heater (HPWH) can provide hot water with a relatively high COP and consume significantly less energy than all electric or gas water heaters. This study reviews and analyses the existing CO 2 HPWHs both in industrial and residential sectors, where the low-temperature and high-temperature systems are classified by the heat delivery temperature of below and above 80 °C, respectively. The significance and features of existing CO 2 HPWHs are also summarised in this study. Moreover, the performance limitations of current CO 2 HPWHs are discussed considering the temperature gliding matching, and the technical constraints in both CO 2 compressors and heat exchangers. Due to the drastic variation of CO 2 heat capacity with temperature and pressure, the heat exchange process between sCO 2 and water is affected by the pinch point. The temperature gliding matching can be enhanced by optimising the parameters in both the CO 2 and water sides, using zeotropic refrigerant mixtures, or adding new devices or cycles to the existing CO 2 system. Another way to improve the system performance is increasing the discharge pressure or discharge temperature, while current CO 2 reciprocating compressors have an upper-temperature limit of 140 °C and upper-pressure limit of 140 bar, as well as some constraints in the heat exchangers. Due to the decomposition risk of lubricant in the compressor, two types of oil-free compressors (turbo and scroll) have been described. But it is found that the turbo-compressor is hard to provide a competitive efficiency when the pressure is high, and there is a significant leakage flow in a scroll compressor.

Influence of mixing section inlet and diffuser outlet velocities on the performance of ejector-expansion refrigeration system using zeotropic mixture

• Theoretical analysis of an ejector expansion refrigeration cycle . • Effects of mixing section inlet and diffuser outlet velocities on the system. • Effects of zeotropic mixture mass ratio on system performance. It is known that the performance of vapor compression cooling systems can be increased thanks to the ejector. In the literature, there are many studies on the subject, in which mathematical models of ejector-expansion cooling systems are developed. However, the refrigerant velocities at the mixing section inlet for the secondary flow (suction stream) and diffuser outlet were neglected in these studies. In the mathematical model used in this study, the velocities in both sections are considered and the system performance is calculated. The performance assessment of an ejector-expansion refrigeration system was realized in the case of using the R600a/R290 refrigerant mixture as a working fluid. Thus, the effect of zeotropic refrigerant mixtures on the system performance is also investigated, considering the refrigerant velocities at the mixing section inlet and diffuser outlet. According to the obtained results, it has been found that the cooling performance of the system that uses R600a/R290 mixture as a refrigerant is 5.8%-7% higher when the velocities at the mixing section inlet and diffuser outlet are considered compared to the neglected one. The exergy efficiency is also calculated to be around 4.6% lower when the ejector mixing section inlet and diffuser outlet velocities are neglected.

Current Research Trends in the Process of Condensation of Cooling Zeotropic Mixtures in Compact Condensers

This paper is an introduction to the cycle proposed by the authors related to research directions concerning the problems of condensation of zeotropic refrigerant mixtures. For over a hundred years, research has been conducted on the search for new working fluids in the cycles for cooling devices and heat pumps. Initially, the natural refrigerants used were replaced with homogeneous synthetic refrigerants, followed by mixtures of two or more refrigerants. Among the mixtures, there are azeotropic and zeotropic mixtures. In the case of an azeotrope mixture, a liquid solution of two or more chemical compounds is in thermodynamic equilibrium with the saturated vapor resulting from this mixture. The chemical composition of the liquid and vapor is identical. A zeotropic mixture is a liquid-vapor system in which the composition of a liquid mixture (solution) of two or more chemical compounds is always different from that of the saturated vapor generated from this liquid. This is due to the different boiling and condensation temperatures of the individual components of the mixture at the same pressure. There is a so-called temperature glide. The phase transformations of individual components do not run simultaneously, which means that the boiling or condensation phase transition temperature changes during the process being carried out. This raises a number of computational, design, and operational problems for power equipment. Today, however, zeotropic mixtures find an alternative to refrigerants with a high GWP potential. Despite the disadvantage of temperature glide, they also have advantages. These include ecological, energy, and economic indicators. As a result, they are increasingly used in the energy economy. This prompts researchers to conduct further research in the field of a detailed description of the phenomenon of boiling and condensation phase transformations of zeotropic mixtures under temperature glide, searching for new computational relationships, new design solutions, and applications. It is still an insufficiently recognized research problem. Bearing the above in mind, the authors made an attempt to review the state of knowledge in this area. Particular attention was paid to the progress in modeling the condensation phenomenon of zeotropic mixtures for application in compact heat exchangers. Miniaturization of cooling devices creates great application possibilities in this area.

Condensation heat and mass transfer characteristics of low GWP zeotropic refrigerant mixture R1234yf/R32 inside a horizontal smooth tube: An experimental study and non-equilibrium film model development

Condensation heat and mass transfer characteristics of low GWP zeotropic refrigerant mixtures R1234yf/R32 (mass fractions of 0.52:0.48 and 0.77:0.23, respectively) inside a horizontal smooth tube (inner diameter 4 mm) were studied experimentally. Moreover, a non-equilibrium film heat and mass transfer model was also developed by considering the mass transfer resistance on the vapor and the liquid side. Effects of mass flux, vapor quality, thermophysical properties and mass fraction on the heat transfer coefficients (HTCs) were analyzed. The heat transfer characteristics, especially the heat transfer deterioration caused by the mass transfer difference of the zeotropic refrigerant mixture, were evaluated. The experiment data show the HTCs of R1234yf/R32 (0.48:0.52) are lower than that of R1234yf at the inlet nearby of the condenser, then that becomes higher gradually than R1234yf due to mass transfer resistance decreasing. The non-equilibrium film model shows good agreement with the experimental results with the mean deviation in HTC of 22.9% at a mass fraction of 0.77:0.23 and 17.8% at a mass fraction of 0.48:0.52. Furthermore, the heat transfer degradation coefficient, interface temperature, diffusion flux of R32 and mass transfer Nusselt number were also reasonably determined with the non-equilibrium film model.

Optimization of Operating Conditions of Vapor-compression Type Air-conditioning Systems Using Zeotropic Refrigerant Mixture

Optimization of Operating Conditions of Vapor-compression Type Air-conditioning Systems Using Zeotropic Refrigerant Mixture

Studies on alternative hybrid materials for replacement of R134a in space heating process

Abstract This paper exhibits the performance analysis of a heat pump framework utilizing pure refrigerants and zeotropic refrigerant mixtures. In the current work R407c has been blended with R134a and the performance and energy consumption patterns are studied. The work is carried out without doing any modifications to the compressor. The main idea behind the work is to replace the R134a with R407c to some extent in the existing systems without doing any modifications. It can be concluded that refrigerant mixtures have the potential of competing with pure R134a. In case of mixture the COP is less due to the lesser critical temperature of the mixture which reduces the heat absorbing capacity and hence COP. Thinking about the ongoing patterns of substitution of R134a, in the present examination, RR407c can be a potential HFC refrigerant trade for new and existing frameworks by and by utilizing R134a with least investment and efforts.

In-Tube condensation of zeotropic refrigerant R454C from superheated vapor to subcooled liquid

This article investigates the in-tube, superheated, saturated, and subcooled condensation of zeotropic refrigerant mixture R454C. R454C is proposed to replace R404A for commercial refrigeration applications. Quasi-local heat transfer coefficients were measured in a 4.7 mm horizontal tube at mass fluxes ranging from 100 to 500 kg m−2 s−1 at three different saturation conditions (40, 50, and 50 °C). The resulting data was compared with the non-equilibrium condensation models of Agarwal and Hrnjak, Kondou and Hrnjak, and Xiao and Hrnjak, as well as the equilibrium model of Cavallini et al. with the Gnielinski correlation for predictions in subcooled and superheated regions. The additional mass transfer effects were accounted for by applying the Silver, Bell and Ghaly mixture correction. The non-equilibrium Kondou and Hrnjak model, with the predictions in the subcooled region from Gnielinski correlation, agrees best with the data (mean average percent error = 9%). An air-cooled condenser for a 1055 kW refrigeration system is designed by following both the non-equilibrium and equilibrium approaches. This non-equilibrium approach leads to a 4.8 and 9.1% reduction in heat transfer area for R454C and R404A, respectively. R454C condenser area is 17–21% larger than that of a R404A condenser.

Comparison of R404A condensation heat transfer and pressure drop with low global warming potential replacement candidates R448A and R452A

The objective of this paper was to measure and compare the heat transfer coefficient and pressure drop of R404A and two potential replacement candidate refrigerants with a lower global warming potential, R448A and R452A. Experiments were conducted inside a 4.7 mm horizontal tube at mass fluxes ranging from 100 to 800 kg m−2 s−1, and at three different saturation conditions (40, 50, and 60 ∘C). R448A and R452A are zeotropic refrigerant mixtures, with temperature glides of approximately 4 ∘C and 3 ∘C, respectively. The Cavallini et al. (2006) correlation predicted the heat transfer data best for all three refrigerants, once mixture effects were accounted for using the equilibrium Silver (1947), Bell and Ghaly (1973) method. The Haraguchi et al. (1994) correlation predicted the R484A frictional pressure drop data best (mean average percent error, MAPE = 15%), while the Friedel (1979) correlation predicted the R404A (MAPE = 15%) and R452A data (MAPE = 14%) pressure drop the best. The heat transfer coefficients of R404A were slightly higher than those of R452A and slightly lower than those of R448A at equivalent conditions. At the same mass flux, (G = 600 kg m−2 s−1), R448A and R452A pressure drops are on average 81% and 3.3% higher than that of R404A. For the same cooling capacity and condenser design, the pressure drop of R404A and R452A are approximately the same and the pressure drop of R448A is approximately 16% lower than that of R404A.

Hybrid auto-cascade refrigeration system coupled with a heat-driven ejector cooling cycle

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.

Condensation of superheated R134a and R437A inside a vertical tube

An experimental study is performed to investigate condensation of superheated R134a and R437A inside a vertical tube. Experimental tests have been performed at the normal operating conditions found in a small power vapor compression refrigeration system. The experimental setup is described and the experimental procedure and data reduction method are explained. Experimental results of the condensation heat transfer coefficient are reported and discussed for different pressures, vapor mass flow rates, and vapor degrees of superheating. Different approximate methods to evaluate the condensate heat transfer coefficient of the zeotropic refrigerant mixture R437A are discussed. The experimental heat transfer coefficients are also compared with those obtained using different correlations; it is found that the values are well predicted with the Chen correlation.

Thermodynamic performance of an auto-cascade ejector refrigeration cycle with mixed refrigerant R32 + R236fa

In this paper, an auto-cascade ejector refrigeration cycle (ACERC) is proposed to obtain lower refrigeration temperature based on conventional ejector refrigeration and auto-cascade refrigeration principle. The thermodynamic performance of ACERC is investigated theoretically. The zeotropic refrigerant mixture R32 + R236fa is used as its working fluid. A parametric analysis is conducted to evaluate the effects of some thermodynamic parameters on the cycle performance. The study shows that refrigerant mixture composition, condenser outlet temperature and evaporation pressure have effects on performance of ACERC. The theoretical results also indicate that the ACERC can achieve the lowest refrigeration temperature at the temperature level of −30 °C. The application of zeotropic refrigerant mixture auto-cascade refrigeration in the ejector refrigeration cycle can provide a new way to obtain lower refrigeration temperature utilizing low-grade thermal energy.

Improving cold climate air-source heat pump performance with refrigerant mixtures

Heat pumps (HP) are one of the most adapted heating solutions for meeting low energy consumption requirements of buildings. However, improving the performance of the heat pumps at low ambient temperatures is still an open challenge. This paper assesses the potential benefits of implementing zeotropic refrigerant mixtures in residential air-source heat pumps for cold climates, in order to increase their seasonal performance. The seasonal performance of a heating system with an air-source heat pump, supplemented with an auxiliary electric heater is studied in two cold climate cities of Montreal and Edmonton. To this aim, a detailed screening heat pump model previously developed is modified and used. Furthermore, the performance of the system with a variable composition refrigerant mixture is assessed, the main goal being to illustrate the possibility of applying environmentally friendly zeotropic refrigerant mixtures in conventional heat pumps, with minimal changes in the components, in order to improve their performance in cold climate conditions.

A thermodynamic analysis of an auto-cascade heat pump cycle for heating application in cold regions

The coefficient of performance (COP) of air-source heat pumps (ASHP) decreases toward low outdoor temperatures. This paper presents an auto-cascade heat pump system (ACHPS) with the objective to improve the heating performance of ASHP in cold climate. The auto-cascade process is achieved based on the separation of binary zeotropic refrigerant mixture consisting of pure refrigerants with different boiling points. Fluid selection for the ACHPS is carried out to validate the feasibility for potential application. Based on the performance analysis on COP, 15 qualified binary mixtures are obtained. R143a/R600 (0.8/0.2) tops the potential working fluid with a COP of 2.15. The effects of system parameters, such as circulation composition, heat transfer fluid (HTF) temperature glide and temperatures of heat source and sink on COP and exergy efficiency of ACHPS are thereafter investigated. The simulation results show that there exist a maximum COP of 2.15 and a maximum of exergy efficiency of 37.2% at the R143a mass fraction of 0.8 when the ambient temperature and heating temperature are −10°C and 50°C, respectively. Finally, by comparison with the single-stage heat pump operating with commercially available refrigerants, the proposed ACHPS shows an advantage of compressor isentropic efficiency. The compression isentropic efficiency of ACHPS could be increased by 25.5% compared to the single stage cycle working with R407c when the ambient temperature is −10°C.