ENTROPY CHARACTERISTICS OF MAGNETIC REFRIGERANTS

Mathematical models for predicting magnetic entropy,lattice entropy and electron entropy of magnetic refrigerants are carried out from the molecular field appoximation,the Debye model and the electron energy level theory respectively of quantum mechanics.Using theabove models the temperature and magnetic field dependences of the calculated magnetic entropy,lattice entropy,electron entropy and the total entropy of gadolinium,which is a typical magnetic refrigerant near room temperature,are obtained.The Influences of kinds of entropy on the performance of magnetic refrigeration are discussed,and some general rules for selecting magnetic refrlyerants are drawn to hely the develoyment of highly economic magnetic refrigerators.

Thermodynamic Analysis of Hydrocarbon Refrigerants-Based Ethylene BOG Re-liquefaction System

The present study aims to make a thermodynamic analysis of an ethylene cascade re-liquefaction system that consists of the following two subsystems: a liquefaction cycle using ethylene as the working fluid and a refrigeration cycle operating with a hydrocarbon refrigerant. The hydrocarbon refrigerants considered are propane(R290), butane(R600), isobutane(R600a), and propylene(R1270). A computer program written in FORTRAN is developed to compute parameters for characteristic points of the cycles and the system's performance, which is determined and analyzed using numerical solutions for the refrigerant condensation temperature, temperature in tank, and temperature difference in the cascade condenser. Results show that R600 a gives the best performance, followed by(in order) R600, R290, and R1270. Furthermore, it is found that an increase in tank temperature improves system performance but that an increase in refrigerant condensation temperature causes deterioration. In addition, it is found that running the system at a low temperature difference in the cascade condenser is advantageous.

Numerical Investigations on Supercritical Heat Transfer Characteristics of Environmental Friendly Refrigerants

The heat transfer of supercritical fluids is a vastly growing field, specifically to find suitable alternatives to replace conventional R134a, which can be beneficial for climate change. Most of the experimental and numerical investigations have been conducted to explore supercritical water, carbon dioxide and R134a as heat transfer working fluids. Hydrofluoroolefin (HFO) and refrigerants blends have been considered the most environment-friendly refrigerants to replace Chlorofluorocarbons (CFCs), Hydrochlorofluoro-carbons (HCFCs) and Hydrofluorocarbons (HFCs). Their main advantage of zero Ozone Depletion Potential (ODP) and comparatively lower Global Warming Potential (GWP) have attracted growing amount of attention to mitigate environmental issues. This work adopts the computational method and takes the environmentally friendly refrigerants to investigate the heat transfer characteristics under widely used shear-stress transport (SST) model. A comprehensive comparison was performed at reduced pressure of 1.10 for supercritical fluids R515A, R1234ze(E) and R134a. The peaks of heat transfer coefficient occurred in the vicinity of pseudo critical temperature for all of these considered fluids;however, R134a resulted in higher heat transfer coefficient, Reynolds number and Prandtl number in comparison with R515A and R1234ze(E). The higher heat transfer coefficient of supercritical fluid R134a is owing to its thermophysical properties and the specific heat plays crucial role in the heat transfer of supercritical fluids. Owing to environmental issues, R515A can be a considerable replacement of R134a. R1234ze(E) is also promising alternative to R134a;however, safety issues should thoroughly concern its mild flammable characteristics.

Theoretical and Experimental Study of Explosion Limits and the Inhibition of Flammable Refrigerants

In this study, we analyze factors affecting the explosion limits of flammable refrigerants. We conclude that any method used for measuring flammable refrigerant explosion limits has its conditional restrictions. Flammable refrigerants in the atmosphere can also explode under certain conditions, when the concentration is approaching the explosion limits. An experimental study on the explosion limits of six kinds of flammable refrigerants is carried out with a mixture of refrigerant and combustible refrigerant, which has a similar effect to a flame retardant. An experimental apparatus was designed to test the explosion limits of mixtures made from three different nonflammable refrigerants and six different flammable refrigerants. Two practical models were developed to estimate the critical concentration for inhibiting explosion of refrigerant mixtures: one was made up of two flammable components with one nonflammable component, and the second was made up of one flammable component with two nonflammable components.

OPTIMIZATION OF THE COMPOSITIONS FOR CFC ALTERNATIVE MIXTURE REFRIGERANTS

In this work,eight commonly used and recently developed cubic EOSs have beenextensively tested for the calculation of thermodynamic properties,including vapor pressure,vaporand liquid densities and heat of vaporization for 26 pure CFCs and their alternatives.The modifiedDu-Guo EOS is recommended for the vapor-liquid equilibria calculations of mixtures for its goodaccuracy.A method for the development of the optimized mixture compositions of CFC alternativeshas been proposed by using minimization of deviations between the vapor pressures of CFC and thealternative mixtures of interest.As examples,two binary mixtures R22-R142b and R22-R152a,and aternary R22-R142b-R152a have been tested.The results show that the vapor pressure of R12 can bewell duplicated by a mixture refrigerant with the optimized composition.On the other hand,thedeviations between the heat of vaporization are somewhat apparent within 10 percent.Moreover,allthe mixtures discussed here are of the characteristics of being near azeotropic。

AIR CONDITIONING REFRIGERANTS AND OZONE PROTECTION

Until recently, CFC-11 and CFC-12 were the most widely used refrigerant gases in air conditioning systems. Environmental agencies around the world, however, have specifical concern for these gases as a direct cause of the depletion of the Earth’s ozone layer. Consequently, the world’s industrial nations are taking concerted action to decrease the use of CFCs, calling for attention of the engineers and designers involved in the manufacture and application of refrigeration systems for air conditioning. This paper analyses the available alternatives of CFCs in the industry and examines the problem found in process application.

Development of Environmentally Friendly and Energy Efficient Refrigerants for Refrigeration Systems

This paper presents the improvement of eco-friendly and power consumption saving refrigerants for refrigeration systems.The novel azeotropic refrigerant mixtures of HFCs and HCs can replace refrigeration systems,and using the R134,R32,R125,and R1270 refrigerants in several compositions found using the decision tree function of the RapidMiner software(which camefirst in the KDnuggets annual software poll).All refrigerant results are mixed of POE,which is A1 classification refrigerant,non-flammable,and innocuous refrigerant,and using REFPROP software and CYCLE_D-HX software are under the CAN/ANSI/AHRI540 standards.The boiling point of the new refrigerant mix R-No.595 is 4.58%,lower than that of R404A,with a higher refrigerant effect and 50.34%lower GWP value than R404A.The proposed mix R-No.595 can be operated in hot environmental country and has high critical temperature and heat-rejection effects,due to the presence of R32 and R1270.The COPc of R463A is 13.49%,higher than R404A in freeze condition.The novel refrigerant mixes provide alternate refrig-erant options mixed of 1%R1270,and which are related with the development of current refrigerants,containing a compose of HFOs and eco refrigerants for producing low-GWP,zero ODP,high-refrigerant effect,low-operating pressure,and innocuous refrigerants.

A New Excess Free Energy Mixing Rule for Representing Vapor-Liquid Equilibria of Mixed Refrigerants

A suitable mixing rule is important for vapor liquid equilibrium(VLE)investigations for mixed refrigerants.In this work,a new excess free energy mixing rule(MRv)was proposed at zero pressure based on the linear relationship between dimensionless parameter 1/(u-1)and a.MRv mixing rule was explicit adopted variable liquid molar volume.The applicable temperature range of MRv could be extended by means of an empirical method to estimate the liquid molar volume for components at high temperatures.Three mixing rules modified Huron-Vidal mixing rule(MHV1),Wong-Sandler mixing rule(WS),and MRv at two reference pressures were used to compare the VLE data in the calculation of 37 mixed refrigerants.Results demonstrated that MRv had a relatively similar accuracy to MHV1 and WS for component and pressure calculation.Moreover,the average excess Gibbs free energy using the MRv mixing rule for the 37 selected mixed refrigerants(0.0013)was much lower than those using the MHV1(0.0078)and WS(0.0809)mixing rules,which was very valuable for the design and optimization of thermodynamic systems using mixed refrigerants.

Evaluation of zeotropic refrigerants based on nonlinear relationship between temperature and enthalpy

In order to evaluate cycling characters of zeotropic refrigerants in air-con- ditioning operation, and to reveal distribution rules of temperature difference between refrigerants and heat transfer fluids in condenser and evaporator, theoretical researches were carried out based on nonlinear relationship between temperature and enthalpy in period of refrigerants’ phase change. Firstly, a phase changing model of refrigerants was built, and refrigerants state parameters were decided in the air-conditioning operation. Secondly, the state equation of refrigerants was applied for computing relationship be- tween temperature and enthalpy, else based on some suppositions, temperature differ- ences between 15 sorts of refrigerants and heat-transfer fluids were gotten too. Through concluding those temperature differences changing in condenser and evaporator, some rules were found. Lastly, after calculating and comparing the additive exergy loss among 15 sorts of refrigerants, which resulted from the changing of temperature difference, their cycling characters evaluation were presented.

Prospect Evaluation of Low-GWP Refrigerants R1233zd(E)and R1336mzz(Z)Used in Solar-Driven Ejector-Vapor Compression Hybrid Refrigeration System

In this paper,the entrainment ratio,pump work,heat loads of heat exchangers and COPthermal were theoretically evaluated for a solar-driven ejector-vapor compression hybrid refrigeration system with R1233zd(E)and R1336mzz(Z)as the working fluids.The evaluation of the utilization potentials of R1233zd(E)and R1336mzz(Z)was presented by comparing the system performance with that of R245fa,a commonly used refrigerant in the ejector system.The results indicated that the systems with R1233zd(E)and R1336mzz(Z)had a higher entrainment ratio and lower pump work.The pump works when using R1233zd(E)and R1336mzz(Z)can be up to 14.59%and 38.05%lower than those of R245fa,respectively.Meanwhile,the system showed the highest COPthermal utilizing R1233zd(E)followed by that of R245fa,with the R1336mzz(Z)system having the lowest value.The differences between R1233zd(E)and R1336mzz(Z)systems,R1233zd(E)and R245fa systems were 4.33%and 2.0%,respectively.This paper was expected to provide a good reference for the utilizing prospect of R1233zd(E)and R1336mzz(Z)in ejector refrigeration systems.

Magnetic and magnetocaloric effect of Er_(20)Ho_(20)Dy_(20)Cu_(20)Ni_(20)high-entropy metallic glass

Er_(20)Ho_(20)Dy_(20)Cu_(20)Ni_(20)high-entropy metallic glass exhibited excellent magnetic refrigeration material with a wide temperature range and high refrigeration capacity(RC)was reported.Er_(20)Ho_(20)Dy_(20)Cu_(20)Ni_(20)high-entropy metallic glass was observed with typical spin glass behavior around 15.5 K.In addition,we find that the magnetic entropy change(-△S_(M))originates from the sample undergoing a ferromagnetic(FM)to paramagnetic(PM)transition around 20 K.Under a field change from 0 T to 7 T,the value of maximum magnetic entropy change(-△S_(M)^(max))reaches 12.5 J/kg·K,and the corresponding value of RC reaches 487.7 J/kg in the temperature range from 6 K to 60 K.The large RC and wide temperature range make the Er_(20)Ho_(20)Dy_(20)Cu_(20)Ni_(20)high-entropy metallic glass be a promising material for application in magnetic refrigerators.

Simulation study on performance of a dual-source hybrid heat pump unit with alternative refrigerants

To solve the problems of single heat source heat pump systems in severe cold regions,a dual-source hybrid heat pump unit(DSHHPU)is proposed.The mathematical models of the DSHHPU when charging R134a or its alternative refrigerants R32,R290 and R600a were established respectively,and the performance was simulated and analysed.The results showed that the four refrigerants have different performance characteristics in different aspects.In heat pipe mode,the heating capacity and evaporating pressure of R32 are 36.94%and 59.94%higher than those of R134a.The heating capacity and evaporating pressure of R290 are 5.73%and 22.99%lower than those of R134a.The heating capacity and evaporating pressure of R600a are 43.29%and 68.08%lower than those of R134a.In vapour compression heating mode,the discharge temperature of R32,R290 and R600a are 184.88,72.98 and 66.44%of that of R134a.The coefficient of performance(COP)of R32,R290 and R600a are 72.65,111.59 and 117.94%of that of R134a.Finally,the effects of radiation intensity and ambient temperature on key performance parameters of the different refrigerants were analysed.The research results provide a reference for research on refrigerant replacements for multi-heat source composite heat pump systems.

Intermodal Competition: Cargo Airships versus Long-Haul Trucking for Perishable Commodities

Intermodal competition changes with changes in technology, economics, and environmental concerns. Trucks and airships are generally considered not to be competitors, but this depends on the distance of haul. The tonne-kilometer cost of trucking rises much more quickly with distance than it does the cost of a cargo airship. At some distance, the two modes are direct substitutes. The costs of the Mexico-Canada refrigerated truck supply chain are compared with the costs of a 100t-lift, electrically-powered airship. The flight characteristics of the Hindenburg Zeppelin are used as a model for a modern cargo airship. The supply chain cost of trucking tomatoes is used to test the theorical proposition. The cost difference works out to about US10￠/kg (5￠/lb) advantage for trucking Mexican tomatoes to Canada. However, this cost disadvantage of the airship could be made up by their vibrationless ride, better air circulation and one-day service versus four days by truck. This alternative form of transportation could have a positive impact on worldwide north-south distribution of food. Airships can overcome trade barriers and distance to open new markets for perishable food exports. In addition, they would reduce the carbon emissions of transport. Canada imports 160,000 refrigerated truckloads of fruits and vegetables by from the southern US and Mexico. With an average driving distance of 3,000 km, these trucks emit 606,000 MT of CO2 annually. Airships powered by hydrogen fuel cells would have zero-carbon emissions. Markets are not yet incorporating the environmental advantage of airships in any freight comparison, but inevitably this will be important.

Evaluation on Excess Entropy Scaling Method Predicting Thermal Transport Properties of Liquid HFC/HFO Refrigerants

The application of the excess entropy scaling(EES)method to predict the viscosity,thermal conductivity and thermal diffusivity of HFC/HFO refrigerants is evaluated in this paper.The universal coefficients of the EES model were firstly obtained from the properties of HFC refrigerants,and the accuracy of the model was further investigated with HFO properties.It was suggested that the EES model correlated the viscosity very well with the average absolute deviations(AADs)of most HFC refrigerants lower than 6.55%except R32.The correlations also provided very good prediction on the viscosity for R1234yf and R1234ze(E),but not for R1336mzz(Z).The prediction of thermal conductivity for both HFC and HFO refrigerants was generally well with the maximum AAD of 11.44%.However,the paper also indicated that there were no universal thermal diffusivity coefficients for even HFC refrigerants,and the linear function could not fit the thermal diffusivity curve very well.Therefore,the exclusively two-order polynomial correlations based on the EES model were presented for each HFC/HFO refrigerant.

Table-like magnetocaloric effect and large refrigerant capacity of composite magnetic refrigerants based on LaFe_(11.6)Si_(1.4)H_y alloys

Composite magnetic refrigerants were prepared by physical mixing LaFeSiHalloys with different Curie temperatures(Tc). The phase structures of these LaFeSiHalloys were analyzed by X-ray diffraction(XRD) and the magnetocaloric effect(MCE) and refrigerant capacity(RC) of these composite magnetic refrigerants were investigated by experiment and calculation in this paper. The magnetocaloric effect(MCE) and refrigerant capacity(RC) of these composite magnetic refrigerants were investigated by experiment and calculation in this paper. The results indicate the experimental magnetic entropy change(-△S)-Tcurve corresponds reasonably with the(-△S)-Tcurve calculated by the linear combination of(-△S)-T curves of the single material. An optimal mixing ratio can make the composite magnetic refrigerant possess a table-like(-△S)-Tcurve which is beneficial to magnetic Ericsson cycle. When three LaFeSiHalloys with different Tare mixed, the full width at half maximum(△T) of(-△S)-T curves is about 48.7 K and the RC is about 177.76 J/kg under a magnetic field change of 2 T. The composite magnetic refrigerants based on LaFeSiHalloys can be promising candidates for near room temperature magnetic refrigeration and the work will be helpful to develop novel composite magnetic refrigerants with table-like MCE and large RC.

Evaluation of non-azeotropic mixtures containing HFOs as potential refrigerants in refrigeration and high-temperature heat pump systems

With the increasing environmental concern on global warming, hydrofluoro-olefin (HFOs), possessing low GWP, has attracted great attention of many researchers recently. In this study, non-azeotropic mixtures composed of HFOs (HFO-1234yf, HFO-1234ze(z), HFO-1234ze(e) and HFO-1234zf) are developed to substitute for HFC-134a and CFC-114 in air-conditioning and high-temperature heat pump systems, respectively. The cycle performances were evaluated by an improved theoretical cy-cle evaluation methodology. The results showed that all the mixtures proposed herein were favorable refrigerants with excel-lent thermodynamic cycle performances. M1A presented lower discharge temperature and pressure ratio and higher COPc than that of HFC-134a. The volumetric cooling capacity was similar to HFC-134a. It can be served as a good environmentally friendly alternative to replace HFC-134a. M3H delivered similar discharge temperature as CFC-114 did. And the COPh was 3% higher. It exhibits excellent cycle performance in high-temperature heat pump and is a promising refrigerant to substitute for CFC-114. And the gliding temperature differences enable them to exhibit better coefficient of performance by matching the sink/source temperature in practice. Because the toxicity, flammability and other properties are not investigated in detail, ex-tensive toxicity and flammability testing needs to be conducted before they are used in a particular application.

An Improved Prediction Equation of Refrigerants Surface Tension Based on the Principle of Corresponding States

The surface tensions of 21 pure refrigerants have been predicted by a new improved equation based on the principle of corresponding states with double referenced fluids.The average absolute deviation between the calculated surface tensions from the equation and the experimental result is-0.015 mN/m.The surface tensions of 9 binary mixtures were calculated in consideration of certain mixing rules.And the average absolute deviation between the calculated surface tensions from the equation and the experimental result is-0.251 mN/m.The new improved prediction equation can be used for calculating the surface tension of environmental friendly refrigerants.

Chemically inducing room temperature spin-crossover in double layered magnetic refrigerants Pr1.4+xSr1.6-xMn2O7(0.0≤x≤0.5)

The height of total entropy(S)for a magnetic refrigerant material is essentially concerned with the magnetic and structural transitions.However,the participation of such transitions in layered materials is not well understood.Therefore,the purpose of this work is to investigate the interplay between double layer lattice with their single perovskite counterpart,to achieve optimal magnetocaloric performance.A series of self-doped Pr_(1.4+x)Sr_(1.6-x)Mn_(2)O_(7)(0.0≤x≤0.5)Ruddlesden-Popper(R-P)perovskite have been prepared through the solid-state sintering method.With increasing the Pr-stoichiometry,the lattice faults have increased and the double layer lattice dramatically disintegrates into single perovskite structure.Due to the reduction of bilayer R-P phase into single perovskite the spin crossover occurs from weak bilayer(T=304 K)interactions towards the strong three-dimensional(T=308 K)interactions respectively.This series consistently develops thermomagnetic irreversibility in zero-field cooled(ZFC)-field cooled(FC)magnetization,which is indicative of a spin-glass state.The glassy nature has been ascribed collectively to the lattice strain produced because of dislocations and to an antiferromagnetic phase segregated at the surface.The maximum value of temperature average entropy change(TEC)and adiabatic temperature(ΔT)has enhanced nearly by 4 folds from 0.53 J kg^(-1)K^(-1),0.59 K(for x=0.0)up to 1.85 J kg^(-1)K^(-1),10 K(for x=0.5)at 2.5 T,respectively.Additionally,the room temperature relative cooling power has improved from 26.94 J/kg up to 77.84 J/kg with an applied field of 2.5 T.Our findings in this work suggest that the controlled reduction of double layer lattice into single perovskite and/or existence of both phases simultaneously in bilayer R-P manganites may be very effective in obtaining the desirable characteristics of magnetocaloric effects.

Effect of an Internal Heat Exchanger on the Performances of a Double Evaporator Ejector Refrigeration Cycle

A theoretical investigation is presented about a double evaporator ejector refrigeration cycle(DEERC).Special attention is paid to take into account the influence of the sub-cooling and superheating effects induced by an internal heat exchanger(IHX).The ejector is introduced into the baseline cycle in order to mitigate the throttling process losses and increase the compressor suction pressure.Moreover,the IHX has the structure of a concentric counter-flow type heat exchanger and is intentionally used to ensure that the fluid at the compressor inlet is vapor.To assess accurately the influence of the IHX on the DEERC performance,a mathematical model is derived in the frame of the dominant one-dimensional theory for ejectors.The model also accounts for the friction effect in the ejector mixing section.The equations of this model are solved using an Engineering Equation Solver(EES)for different fluids.These are:R134a as baseline fluid and other environment friendly refrigerants used for comparison,namely,R1234yf,R1234ze,R600,R600a,R290,R717 and R1270.The simulation results show that the DEERC with an IHX can achieve COP(the coefficient of performance)improvements from 5.2 until 10%.

Magnetocaloric properties of phenolic resin bonded La(Fe,Si)13-based plates and its use in a hybrid magnetic refrigerator

We present a simple hot press-based method for processing La(Fe,Si)13-based compounds consisting of La–Fe–Co–Si–C particles and phenolic resin. The magnetic entropy change △S per unit mass for the La Fe_(10.87)Co_(0.63)Si_(1.5)C_(0.2)/phenolic resin compounds have nearly the same magnitude with the base materials. With the content of phenolic resin of 5.0 wt%, the compound conductivity is 3.13 W·m^(-1)·K^(-1). In order to measure the cooling performance of La(Fe,Si)13-based compounds,the La(Fe_(11.6-x)Co_(x))Si_(1.4)C_(0.15)(x =0.60, 0.65, 0.75, 0.80, 0.85)/phenolic resin compounds were pressed into thin plates and tested in a hybrid refrigerator that combines the active magnetic refrigeration effect with the Stirling cycle refrigeration effect. The test results showed that a maximum cooling power of 41 W was achieved over a temperature span of 30 K.