Zeotropic Research Articles

Thermodynamic analysis of auto-cascade refrigeration cycles, with and without ejector, for ultra low temperature freezing using a mixture of refrigerants R600a and R1150

• A mixture of R600a and R1150 was thermodynamically evaluated in auto-cascade cooling cycles. • The influence of the mass fraction, the discharge pressure and the phase-separator temperature were evaluated. • The results are especially sensitive to the temperature at the phase-separator and the mass fraction of the refrigerants. • The ejector as an expansion device can improve the COP by up to 12% with regard to the reference case. 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.

Numerical Analysis of Heat Transfer Characteristics of a Novel Heat Exchanger for Lorenz-Meutzner Cycle with Zeotropic Mixtures

Numerical Analysis of Heat Transfer Characteristics of a Novel Heat Exchanger for Lorenz-Meutzner Cycle with Zeotropic Mixtures

Numerical Analysis of Heat Transfer Characteristics of a Novel Heat Exchanger for Lorenz-Meutzner Cycle with Zeotropic Mixtures

Numerical Analysis of Heat Transfer Characteristics of a Novel Heat Exchanger for Lorenz-Meutzner Cycle with Zeotropic Mixtures

A review of fluorocarbon sorption on porous materials

Fluorocarbons have commonly been used for refrigerants, coolants, fire-suppressants, propellants, semiconductors, fluoropolymers, and other applications since the early 20th century. However, starting in the late 1980s these chemicals have been highly regulated due to environmental concerns over ozone depletion potential (ODP) and global warming potential (GWP). Porous materials such as zeolites, activated carbons, and metal-organic frameworks (MOFs) are among the current sorbents being studied to reduce emissions of greenhouse gases (GHGs) including fluorocarbons and carbon dioxide. Recent legislation now requires the reduction of hydrofluorocarbon refrigerants over the next two decades to reduce global warming which makes environmental remediation a relevant issue. An extensive literature search has been conducted for fluorocarbon sorption in porous materials to better understand current and prior separation technologies that can reduce climate change. This review article discusses applications for which fluorocarbon sorption is reported and encompasses both academic and patent literature from the late 1940s to present. Pure gas sorption of straight-chain paraffinic and olefinic fluorocarbons as well as zeotropic, azeotropic, and isomeric mixture separations is reviewed. Additional topics including molecular interactions, reactivity, and molecular modeling are discussed in detail. • Comprehensive review of fluorocarbon sorption with 367 sources from 1947 to present. • Discusses capture, purification, separation, adsorption-refrigeration, nanofluids, catalysis, and fundamental studies. • More than 10 sorbents mentioned including zeolites, activated carbons, and metal-organic frameworks (MOFs). • Includes over 100 fluorocarbons consisting of CFCs, CFOs, HCFCs, HCFOs, HFCs, HFOs, PFCs, and PFOs. • Highlights the use of over 15 adsorption isotherms for modeling fluorocarbon sorption with different sorbents.

Advanced exergy analysis of renewable heat source driven ORC-VCR system

The advanced exergy analysis (AEA) of the organic Ranking cycle-driven vapor compression refrigeration (ORC-VCR) system is carried out in this study. AEA is a method that can be used to identify the endogenous and exogenous causes of the exergy destruction taking place in the system components and thus helps in improving the exergetic efficiency of the system. The zeotropic mixture of hexane and R245fa is used in the system. R245fa is used to suppress the flammable characteristics of the hexane without compromising its thermal performance. The results indicate that 53.75% of exergy input to the system is destroyed. Further, 32.19% of the endogenousiexergy destruction rate is avoidable, to which the condenser contributes 33.42%, followed by the ORC evaporator at 22.82%.

Numerical simulation on constituent separation and mass transfer of binary zeotropic mixtures in a branching T-junction

The study of constituent separation in T-junction is important to realize the operation of the constituent adjustable system. This paper presents a numerical simulation on the constituent separation and mass transfer of R134a and R600a in T-junction. The multiphase flow model of volume of fluid (VOF) is adopted, and the interphase mass transfer of R134a and R600a is achieved by adding source terms in governing equations. The quality at inlet varies from 0.51 to 0.92. The mass flow rate at inlet and flow rate weighting of branch outlet are 10 g⋅s−1 and 0.3, respectively. Results show that the error of numerical simulation with mass transfer is lower than that of numerical simulation without mass transfer. When the quality at inlet is 0.81, the error of numerical simulation with mass transfer can be reduced by 50.07% compared with the numerical simulation without mass transfer. The higher the quality at inlet, the higher the separation efficiency of R134a when the quality at inlet is less than 0.81. When the quality at inlet is 0.81, the separation efficiency of R134a in the numerical simulation with mass transfer is the largest, whose value is 1.47%. Most of the flow patterns are annular flow. The pressure and flow pattern have an important influence on the constituent separation and the interphase mass transfer rate. A correlation with 10% error band of interphase mass transfer for annular flow in T-junction is proposed.

Enabling optimization of complex distillation configurations in GAMS with CAPE-OPEN thermodynamic models

Optimization of complex process models, e.g., those, resulting from conceptual design of distillation processes, with optimization environments such as GAMS, AMPL, or AIMMS is still thwarted by the manual implementation necessary for the interface to external thermo engines. In this contribution, we address this issue by automatically generating all code for interfacing GAMS with an external CAPE-OPEN thermodynamic property package. The distillation models in our approach are assembled in a modular way by combining multiple general vapor-liquid equilibrium stage models. A method, where all existing vapor-liquid-equilibria are relaxed is proposed and examined for enhanced model stability. The ability of our optimization approach is shown by the results of two examples that determine an optimum energy-targeted design of a fully thermally coupled dividing wall column for the separation of a zeotropic mixture of n-pentane, n-hexane, and n-heptane and an azeotropic mixture of acetone, methanol, and water.

A numerical model for the flow condensation of the binary zeotropic mixture

Inspired by the quasi-static equilibrium state theory and the method of computational fluid dynamics, a mass and heat transfer model is proposed to simulate the condensation of the binary zeotropic mixtures. Combining the VOF method and species transport model, the mass transfer and heat transfer of the components in the liquid phase and vapor phase are analyzed, separately. The experiments of R32/R1234ze(E) and R134a/R1234ze(E) mixtures with different mass fluxes and mass compositions were chosen to validate this model. The results show the numerical data of heat transfer coefficients agree well with the experimental results. The condensation of R32/R1234ze(E) mixtures in mini-channels were simulated. The effects of mass flux, tube diameter and mass composition on the heat transfer coefficient, film thickness, velocity distribution, temperature distribution, and concentration distribution were investigated. The results show the mixture with more R32 has a larger HTC. The concentration of R32 vapor near the vapor-liquid interface increases with the decrease of vapor quality. And the mass flux has little effect on the concentration of R32 vapor.

An experimental study of flow boiling heat transfer of zeotropic mixture R32/R134a in a microchannel heat exchanger

This paper presents experimental data on heat transfer of a binary zeotropic mixture of refrigerants R32/R134a in a microchannel heat exchanger with a high specific surface within a range of parameters that is practically important for the development of cooling systems for microelectronics and space technology. The experiments were carried out in a horizontal heat exchanger with one-sided heating of a copper microchannel plate 20x40 mm, containing 21 rectangular microchannels with a cross-section of 335x930 μm, within the range of mass fluxes from 80 to 250 kg/m2s, and at an absolute pressure in the system ranged from 12 to 14 bar. A zeotropic mixture of refrigerants R32/R134a with a molar concentration of the initial mixture of 65%/35% was used as a working fluid. Experimental data were compared with model-based calculations that take into account the influence of changes in the concentrations of components in the liquid and gas phases.

A critical review on waste heat recovery utilization with special focus on Organic Rankine Cycle applications

In the recent era of development, energy consumption increased many folds. While tapping this energy from various resources, almost half of this energy is dumped back into the environment causing adverse environmental imbalance. This can be mitigated by the recovery and utilization of waste heat which will also lead to the conservation of fossil fuel. In the present study, various waste heat recovery (WHR) technologies are explored and a state-of-the-art review is conducted with a particular focus on Organic Rankine Cycle (ORC). It is concluded that ORC technology is a reliable and promising solution. In the present study research challenges associated with the selection of ORC working fluid are evaluated and described. It is found that zeotropic mixtures could be good alternatives because of their temperature glide reducing the irreversibility of the system. The research is conducted on different configurations of ORC such as Regenerative cycle, CO 2 transcritical power cycle (CTPC), and cascade ORC system. It is explored from the literature that the high thermal decomposition temperature of carbon dioxide reduces the irreversibility in CTPC systems. Multi-grade waste heat is efficiently recovered by the Cascade-Organic Rankine Cycle (C-ORC). Technical challenges in the application of Phase Change Material (PCM) into WHR and utilization of WHR in adsorption cooling and Hybrid vehicle is critically reviewed to identify the potential knowledge gap between the present status and futuristic research. The study also discussed the present Indian scenario of WHR utilization for ORC applications. • Different architectures of ORC based on WHR are investigated. • Selection of working fluid, system optimization in the purview of ORC are discussed. • Technical challenges associated with the application of PCM in WHR are identified. • Utilization of WHR in adsorption cooling and hybrid vehicles are studied.

Energy, exergy, and exergoeconomic analyses and optimization of a novel thermal and compressed air energy storage integrated with a dual-pressure organic Rankine cycle and ejector refrigeration cycle

The current work analyzes an innovative thermal and compressed air energy storage cycle integrated with a dual-pressure organic Rankine cycle combined with an ejector refrigeration cycle for power and cooling cogeneration. The bottoming cycle employs zeotropic mixtures, including Pentane (0.5)/Butane (0.5) and Pentane (0.5)/Trans-2-butene (0.5), as working fluids for performance improvement. The system assessment includes three crucial viewpoints: energy, exergy, and exergoeconomic. A parametric study is performed to examine the effects of some key parameters on the performance of the devised system. Also, a multi-objective optimization program based on the MOPSO algorithm is conducted to find an optimal trade-off between thermodynamic performance and economic attractiveness of the system considering two decision-making techniques of TOPSIS and LINMAP. The results showed that the sum unit cost of the product has less value for Pentane (0.5)/Trans-2-butene (0.5). Also, it is found that the performance of Pentane (0.5)/Trans-2-butene (0.5) as the zeotropic working fluid is better. Therefore, this fluid is considered as the working fluid for the optimization procedure. Besides, based on the achieved results, the optimum round trip efficiency and sum unit cost of the product are 54.25% and 13.51 $.GJ−1, respectively.

Thermodynamic analysis of double flash organic flash cycle using R600a/R601a mixtures

Double flash organic flash cycle (OFC) exhibits perfect temperature matching during heat absorption and low throttling loss. Zeotropic mixtures could decrease the irreversible loss during the phase change process due to their non-isothermal characteristic. This study combines double flash OFC with R600a/R601a mixtures to efficiently utilize low-grade thermal energy. The high- and low-stage flash pressures are optimized for maximal net power output. The influences of the R600a mole fraction and the heat source temperature on net power output, exergy efficiency, and cycle parameters are analyzed. Optimal mixture compositions and flash pressures are obtained, and the exergy loss distribution is investigated. The proposed system is compared with the basic single flash OFC using R600a/R601a mixtures. Research shows that the optimized flash pressures have upper limits as heat source temperature increases. The net power output decreases with increasing R600a mole fraction when the heat source temperature is high. The proposed system has more net power output than that using pure working fluid for the 100 °C–180 °C heat source, and the relative net power output growth rates are 3.22%–5.24%. Moreover, the net power outputs of double flash OFC are 15.13%–55.70% higher than basic single flash OFC.

Performances of Transcritical Power Cycles with CO2-Based Mixtures for the Waste Heat Recovery of ICE.

In the waste heat recovery of the internal combustion engine (ICE), the transcritical CO2 power cycle still faces the high operation pressure and difficulty in condensation. To overcome these challenges, CO2 is mixed with organic fluids to form zeotropic mixtures. Thus, in this work, five organic fluids, namely R290, R600a, R600, R601a, and R601, are mixed with CO2. Mixture performance in the waste heat recovery of ICE is evaluated, based on two transcritical power cycles, namely the recuperative cycle and split cycle. The results show that the split cycle always has better performance than the recuperative cycle. Under design conditions, CO2/R290(0.3/0.7) has the best performance in the split cycle. The corresponding net work and cycle efficiency are respectively 21.05 kW and 20.44%. Furthermore, effects of key parameters such as turbine inlet temperature, turbine inlet pressure, and split ratio on the cycle performance are studied. With the increase of turbine inlet temperature, the net works of the recuperative cycle and split cycle firstly increase and then decrease. There exist peak values of net work in both cycles. Meanwhile, the net work of the split cycle firstly increases and then decreases with the increase of the split ratio. Thereafter, with the target of maximizing net work, these key parameters are optimized at different mass fractions of CO2. The optimization results show that CO2/R600 obtains the highest net work of 27.43 kW at the CO2 mass fraction 0.9 in the split cycle.

Adjusting the pinch point of heat transfer to improve the thermal performance of the organic Rankine cycle

For the thermal performance of the organic Rankine cycle (ORC), there exists a monotonic increase or a maximum value with an optimum point because of the relationship between the heat source temperature and the critical temperature of working fluid. The performance with a monotonic increase is better than that with an optimum value. However, the ORC with a low-grade heat source generally falls into the condition with an optimum value when working fluids have a high critical temperature, for example, R245fa. Early research revealed that this phenomenon is affected by the location of the pinch point of the heat transfer (PPHT) in the heat addition process. Based on this consideration, an approach of adjusting the PPHT and a theoretical cycle: the ORC with adjustable pinch point of heat transfer (APPORC) are proposed, and two new configurations of the ORC with the adjustment of the PPHT are proposed: the organic Rankine cycle with an ejector (EORC) and organic Rankine cycle with a flasher (FORC). The theoretical performance of the APPORC is deduced, and its two innovative reconfiguration cycles are simulated and compared to the basic ORC (BORC). From the theoretical derivation as well as simulation results, the APPORC and its innovative cycles: The EORC and FORC can achieve higher net power output and lower thermal efficiency. The results also show that when the heat source temperature is in a range of 100°C to 160°C, the maximum net power outputs of the EORC and FORC with R245fa as working fluid increase 34.99%-22.57% and 16.96%-11.08%, respectively, compared to the BORC. The exergy destructions of the heat addition process of the EORC and FORC are lower compared to the BORC due to the better temperature matching. For the EORC with zeotropic mixtures R134a/R245fa (0.7/0.3) as working fluid, the increases in maximum net power output of 7.12% and 9.45% are obtained compared to pure fluids R245fa and R134a at a heat source temperature of 120°C. Furthermore, an additional benefit for the EORC with zeotropic mixtures was obtained: The condensation temperature glide can be adjusted to approach the temperature rise of the cooling water.

Comprehensive thermodynamic and exergoeconomic analyses and multi-objective optimization of a compressed air energy storage hybridized with a parabolic trough solar collectors

This paper designs a novel power plant consisting of a medium-temperature solar field based on parabolic trough solar collectors, an organic Rankine cycle, and a compressed air energy storage unit. The solar field supplies the energy required by the organic Rankine cycle at the charging period, providing the power for the compressors and high-temperature energy storage. Also, for the performance improvement of the system, zeotropic mixtures are employed as the working fluids in the Rankine cycle. The system was assessed from energy, exergy, and exergoeconomic viewpoints. A comprehensive parametric study was performed to examine the effects of some key parameters on the performance of the devised system. And, the MOPSO optimization algorithm is selected to optimize the calculations, wherein the TOPSIS decision-making method identifies the optimal solutions. According to the results, Isopentane/R142b mixture was selected as the efficient zeotropic working fluid of the Rankine cycle. Hence, the optimum energy efficiency and exergy efficiency were computed to be 5.08% and 5.04%, respectively. Besides, the optimum round trip efficiency and exergetic round trip efficiency were obtained by 57.06% and 66.20%, respectively. Moreover, considering electricity cost of 0.36 $.GJ−1, the lowest payback period of 4.39 years, and the highest profitability were obtained.

Thermodynamic optimization and comparative economic analysis of four organic Rankine cycle configurations with a zeotropic mixture

The current paper aims to compare the four fundamental Organic Rankine Cycle (ORC) structures with a zeotropic mixture of Pentane and Hexane, utilized as the working fluid. The studied variants of ORCS include simple single and dual-pressure ORCs alongside their recuperative configurations. Each configuration is optimized in different mole fractions to obtain the parameters that generate the highest power. The energy and exergy efficiencies are then calculated and compared besides a detailed view of the exergy destructions of each component. Next, the Levelized Cost of electricity is calculated for each mole fraction to carry out the thermoeconomic analysis. The optimum mole fractions of each structure are then obtained to conduct a techno-economic analysis to elaborate more on the superiority of cycles than each other, considering both economic and thermodynamic results. According to the results, the recuperative dual-pressure ORC is the most efficient and has the most generated power (2038 kW), while the other’s performances decline regarding their complexity level. Its performance is found to be more than 16% better than the simple ORC structure which proves a significant improvement. In addition, using a zeotropic mixture can boost the cycle performance and reduce the exergy destructions. Moreover, it is revealed that the dual-pressure ORC is not an economically acceptable option due to its relatively lower net present value, 4.29 M$, while the recuperative structures result in a NPV of 4.53 M$ after the 20-year lifetime. Finally, by conducting a sensitivity analysis, it is concluded that either of the recuperative configurations can be of more desire depending on the power demand and electricity price.

Flow boiling of hydrocarbons and their zeotropic binary mixtures under pre- and post-dryout conditions

This paper concerns an experimental evaluation of the flow boiling heat transfer coefficient of R290, R600a and R1270 focusing on the replacement of R134a in commercial, and residential air-conditioning and refrigeration applications. Moreover, new zeotropic binary mixtures (R600a/R290, 70/30% molar fraction, and R600a/R1270, 75/25% molar fraction) with similar range of temperature applications than R134a are proposed and evaluated. Experiments are performed inside a conduit with a diameter of 9.43 mm for mass velocity ranging from 50 to 300 kg/(m2 s), local heat flux from 10 to 30 kW/m2, vapor quality from 0 to 1, and saturation temperature of 5 °C. In general, higher heat transfer coefficients are obtained for pure hydrocarbons, followed by the mixtures, therefore, showing a better heat transfer performance than R134a. Similar heat transfer coefficients are noticed among pure hydrocarbons, with R600a presenting slightly higher values under certain conditions. A behavior typical of heat transfer dominated by convective effects is noticed for the hydrocarbons with increasing quality at mass velocities higher than 100 kg/(m2 s), while for R134a such a behavior is verified only for mass velocities higher than 200 kg/(m2 s). Nucleate boiling dominated heat transfer is noticed only for mass velocities lower than 100 kg/(m2 s) and 200 kg/(m2 s) for hydrocarbons and R134a, respectively. In general, the heat transfer coefficient increases with mass velocity and heat flux. The heat transfer coefficient and dryout incipience vapor quality data are compared against prediction methods from literature, however, none predicting the current data. Generally, lower dryout incipience vapor qualities are noticed for the hydrocarbons compared to R134a, with the R600a presenting the lowest values. The dryout is delayed in terms of vapor quality for the mixtures compared to the pure hydrocarbons.

Experimental research on the concentration distribution characteristics of dual-temperature refrigeration system using R290/R600a based on separation condensation

• Experiments were studied to obtain R290/R600a concentration distribution behavior. • Linear relationships between circulation and charged concentration are discovered. • Experimental prediction correlations on the circulating concentrations are obtained. • Variations of mixture concentration as operation parameters are analyzed. In this paper, R290/R600a dual-temperature refrigeration system based on separation condensation approach with two evaporators is experimentally investigated. Refrigerant concentration distribution characteristics of zeotropic mixture R290/R600a are obtained through gas chromatography method. Moreover, the difference between charged concentration and circulating concentration are explored. The results show that circulating refrigerant concentration at different sampling positions deviate from charge refrigerant concentration. The circulating refrigerant concentration of low boiling component R290 at compressor discharge port is 11.9% higher than its charged concentration. Based on the analysis of refrigerant concentration deviation relationship, the experimental prediction correlations on circulating refrigerant concentrations are obtained. In addition, the effects of operation parameters including compressor frequency and ambient temperature on concentration distribution are also investigated. The present work provides a guide for refrigerant circulating concentration prediction and selection of charged refrigerant composition. Furthermore, this could reveal an in-depth understanding on refrigerant circulating concentration distribution characteristics.

Experimental characterization of an innovative refrigeration system coupled with Linde-Hampson cycle and auto-cascade cycle for multi-stage refrigeration temperature applications

In this study, a dual mixed-refrigerants refrigeration system coupled with a Linde-Hampson refrigeration (LHR) cycle and a three-stage auto-cascade refrigeration (ACR) cycle was designed and built. The thermodynamic performance of the coupled system was experimentally investigated using low GWP zeotropic mixtures of R1234yf/R32 for the LHR and R170/R14/R50 for the ACR. The experimental results show that when the average ambient temperature was 24.18 °C, it took about 0.5 h for the prototype to reach a no-load temperature level of −120 °C, and 4 h to −146.97 °C, which was quite close to steady state. The corresponding cooling capacity, coefficient of performance (COP) and relative Carnot efficiency were 182.94 W, 0.121 and 16.0%, respectively. The hot fluid outlet temperatures in the condensation evaporator-I (CE-I), CE-II and CE-III were −49.04 °C, −87.12 °C and −113.20 °C, respectively. In addition, the discharge pressure, evaporation temperature and cooling capacity increased slightly with increasing ambient temperature, while the suction pressure and COP remained nearly constant. The LHR-ACR system manifests a promising perspective as a new cooling approach to produce multi-temperatures ranging from −40 °C to −150 °C in the energy cascade utilization fields, such as the multi-temperature cryopreservation and gas liquefaction.

Comparison of shortcut methods for designing of divided wall columns for separating tertiary zeotropic mixtures

Recently, divided wall column has been considered as one of the types of thermally coupled towers for separating multicomponent mixtures. In the structure of this tower, there is a wall that divides the tower into two parts, the Prefractionator and the Main Column. The divided wall column is thermodynamically equivalent of the Petlyuk tower. Therefore, in order to obtain the design and simulation parameters of the Petlyuk tower utilizing the available software, this tower must be divided into simpler towers. The shortcut method is employed to acquire the initial parameters. In the present research, initially, the design of divided wall column has been conducted using shortcut methods to separate three-component zeotropic mixtures. Next, the design and simulation of the divided wall column is performed employing ASPEN PLUS software and the results of the two methods are compared from various angles.