A new thermoeconomic approach and parametric study of an irreversible regenerative Brayton refrigeration cycle

The detailed parametric study of an irreversible regenerative Brayton refrigerator cycle using the new thermoeconomic approach is presented in this paper. The external irreversibility is due to finite temperature difference between the cycle and the external reservoirs while the internal irreversibilities are due to the nonisentropic compression and expansion processes and the regenerative loss. The thermoeconomic objective function defined as the cooling load per unit cost is optimized with respect to the state point temperatures for a typical set of operating conditions. The power input and cooling load are found to be decreasing functions of the expansion outlet temperature (T₁), while it is the reverse in the case of COP. On the other hand, there are optimal values of the temperature T₁, cooling load, power input and COP at which the cycle attains the maximum objective function for a typical set of operating parameters. Again, the objective function, COP and cooling load further enhance, while the power input goes down, as the various values of the effectiveness or efficiency components are increased.     

Thermodynamics of magnetic refrigeration

A comprehensive treatment of the thermodynamics of cyclic magnetic refrigeration processes is presented. It starts with a review of the work, heat and internal energy of a magnetized specimen in a magnetic field, and a list of the thermodynamic potentials is given. These are based on the very recent discovery of an alternative Kelvin force. It is shown that this force is compatible with the internal energy proposed by Landau and Lifshitz. New formulas for the specific enthalpies are presented. Cyclic processes are discussed in detail, e.g. the Brayton, Ericsson and Carnot cycles. Magnetic refrigeration and magnetic heat pump cycles are preferably designed by applying the cascade or/and regeneration principle. Cascade systems allow wider temperature ranges to be obtained. The main objective of this article is to yield a theoretical basis for an optimal design of new magnetic refrigeration and heat pump devices.     

Analysis of room temperature magnetic regenerative refrigeration

Results of a room temperature magnetic refrigeration test bed and an analysis using a computational model are presented. A detailed demonstration of the four sequential processes in the transient magnetocaloric regeneration process of a magnetic material is presented. The temperature profile during the transient approach to steady state operation was measured in detail. A 5 °C evolution of the difference of temperature between the hot end and the cold end of the magnetocaloric bed due to regeneration is reported. A model is developed for the heat transfer and fluid mechanics of the four sequential processes in each cycle of thermal wave propagation in the regenerative bed combined with the magnetocaloric effect. The basic equations that can be used in simulation of magnetic refrigeration systems are derived and the design parameters are discussed.     

Performance optimization of irreversible refrigerators based on a new thermo-ecological criterion

A performance analysis and optimization based on a new thermo-ecological optimization criterion has been carried out for refrigerators. The ecological objective function is defined as the ratio of the cooling load to the loss rate of availability (or entropy generation rate). The maximum of the ecological performance criterion and the corresponding optimal conditions have been derived analytically. The optimum performance parameters which maximize the objective function have been investigated and the effects of irreversibility parameters on the general and optimal performances are discussed detailed. The obtained results may provide a general theoretical tool for the ecological design of refrigerators.     

Detailed numerical modeling of a linear parallel-plate Active Magnetic Regenerator

A numerical model simulating Active Magnetic Regeneration (AMR) is presented and compared to a selection of experiments. The model is an extension and re-implementation of a previous two-dimensional model. The new model is extended to 2.5D, meaning that parasitic thermal losses are included in the spatially not-resolved direction.The implementation of the magnetocaloric effect (MCE) is made possible through a source term in the heat equation for the magnetocaloric material (MCM). This adds the possibility to model a continuously varying magnetic field.The adiabatic temperature change of the used gadolinium has been measured and is used as an alternative MCE than mean field modeling. The results show that using the 2.5D formulation brings the model significantly closer to the experiment. Good agreement between the experimental results and the modeling was obtained when using the 2.5D formulation in combination with the measured adiabatic temperature change.     

A review on Mn based materials for magnetic refrigeration: Structure and properties

Magnetic refrigeration employing magnetically ordered materials is a relatively novel technique, differing in some respects from magnetic cooling by means of adiabatic demagnetization of paramagnetic substances. Magnetic refrigeration has been known for more than a century and is based on the magnetocaloric effect. It has received new impetus recently because it has several advantages over vapor-compression refrigeration. In the last few years the magnetic and magnetocaloric properties of a large number of intermetallic compounds were investigated, in which the magnetic moments are carried by atoms of 3d transition elements. In the present paper we will focus on intermetallic compounds in which one of the components is Mn. The results obtained on several groups of such intermetallic compounds will be reviewed. By far the most promising materials of this group of intermetallics are compounds of the type MnFeP_1−xAs_x. Although it is understood that these compounds are probably nontoxic, the presence of As atoms in them might form a mental barrier to exploit these materials on a commercial basis. Special attention will therefore be paid to efforts attempting to substitute other elements for As in MnFeP_1−xAs_x with the proviso that the favorable magnetocaloric properties be retained.     

Thermodynamic analysis of the V-type Stirling-cycle refrigerator

The thermodynamic analysis of a V-type Stirling-cycle refrigerator is performed. The Stirling-cycle refrigerator consists of expansion and compression spaces, cooler, heater and regenerator, and divided into 14 fixed control volumes subjected to a periodic mass flow. The conservation of mass and energy equation are written for each control volume. A computer program is prepared in FORTRAN, and the basic equations are solved iteratively. The mass, temperature and density of working fluid in each control volume are calculated for a given charge pressure, engine speed, and fixed heater and cooler surface temperatures, and the results are obtained from a PC. The heat transfer coefficients are assumed constant. The work, instantaneous pressure and COP of the Stirling-cycle refrigerator are also calculated. The steady cyclic conditions are obtained for temperature after few cycles and the results are given by diagrams.     

Application of an ejector in autocascade refrigeration cycle for the performance improvement

This paper presented a novel autocascade refrigeration cycle (NARC) with an ejector. In the NARC, the ejector is used to recover some available work to increase the compressor suction pressure. The NARC enables the compressor to operate at lower pressure ratio, which in turn improves the cycle performance. Theoretical computation model based on the constant pressure-mixing model for the ejector is used to perform a thermodynamic cycle analysis for the NARC with the refrigerant mixture of R23/R134a. The effects of some main parameters on cycle performance were investigated. The results show the NARC has an outstanding merit in decreasing the pressure ratio of compressor as well as increasing the COP. For NARC operated at the condenser outlet temperature of 40 °C, the evaporator inlet temperature of −40.3 °C, and the mass fraction of R23 is 0.15, the pressure ratio of the ejector reaches to 1.35, the pressure ratio of compressor is reduced by 25.8% and the COP is improved by 19.1% over the conventional autocascade refrigeration cycle.     

Research on performance of regenerative room temperature magnetic refrigeration cycle

On the basis of classical Langevin theory along with statistical mechanics, thermodynamics and magnetism, a new expression of magnetocaloric parameters used for room temperature magnetic refrigeration is proposed, which is briefer and more accurate than the existing one, providing a new way for studying performance of regenerative room temperature magnetic Ericsson refrigeration cycle. Influences of temperature of heat reservoirs and magnetic intensity on cycle refrigeration capacity and coefficient of performance are analyzed. The results show that the maximal temperature span of the cycle increases but its increasing rate decreases with the increase of magnetic field strength. In addition, there exists only one maximum value of effective refrigerating capacity. Two cycles with the same COP can reach a same temperature span under a certain magnetic field strength. A large magnetic field strength can improve COP but the increase rate of COP decreases.     

Numerical investigation of a diffusion absorption refrigeration cycle

A thermodynamic model was developed for an ammonia–water diffusion absorption refrigeration (DAR) cycle with hydrogen or helium as the auxiliary inert gas, manufactured by Electrolux Sweden (currently known as Dometic). The performance of the system was examined parametrically by computer simulation. Mass and energy conservation equations were developed for each component of the cycle and solved numerically. The model was validated by comparison with previously published experimental data for DAR systems. Investigation of cycle performance under different conditions indicated that the best performance was obtained for a concentration range of the rich solution of 0.2–0.3 ammonia mass fraction and that the recommended concentration of the weak solution was 0.1. It was also found that as the degree of rectification decreased, the performance of the DAR cycle decreased. Finally, the study showed that helium was superior to hydrogen as the inert gas: the coefficient of performance of a DAR unit working with helium was higher by up to 40% than a cycle working with hydrogen.     

Parametric optimization of a new combined refrigeration cycle – active thermal potentiostatting system

The active thermal potentiostatting system proposed by Martinovskii and Tsirlin is directly generalized to a more practical case, in which one intermediate chamber, besides a thermal potentiostatting chamber, and two irreversible refrigeration cycles are included and the influence of the thermal resistance between the working fluid and the reservoirs, the heat leakages from the environment to the intermediate chamber and from the intermediate chamber to the potentiostatting chamber are taken into account. Expressions for the main parameters of the system are derived. By using the optimal control theory, the minimum total power input of the system with non-zero cooling rates is calculated and the temperatures of the working fluid in the isothermal processes of the refrigeration cycles are optimized. The optimal allocation of the heat-transfer areas of the heat-exchangers in the refrigeration cycles is discussed in detail. The results obtained here are more general and useful than the relevant results in literature and can provide some valuable guidance for the optimal design and operation of real active thermal potentiostatting systems.     

不可逆回热式玻色布雷顿制冷循环性能分析

基于玻色气体的热力学性质和回热式布雷顿制冷循环的不可逆模型,导出循环的一些重要性能参数,如循环的制冷量、回热量、输入功和性能系数的一般表达式。通过数值计算获得了循环的一些重要的性能特性曲线,分析了循环的不可逆性和回热特性对玻色布雷顿制冷机性能的影响。     

Discussion of the feasibility of the Einstein refrigeration cycle

A careful modelling of the thermodynamic properties of the water–ammonia–butane system, the working fluid mixture used in the Einstein cycle, with the Patel–Teja cubic equation of state is performed. Numerical simulation is used to investigate the feasibility limits of this refrigeration cycle. Two modified configurations of the cycle are considered. A conflict between the evaporator and the condenser/absorber operating conditions is noted. The condenser/absorber operation needs a higher system pressure, which limits the refrigeration temperature in the case of air-cooling. On the other hand, the condensation of ammonia and the presence of a small quantity of water in the evaporator limit also the refrigeration temperature. In the case of a water-cooled machine, with a condenser/absorber temperature of 30 °C, the cycle COP reaches 0.19 which is still low.     

A magnetic field source system for magnetic refrigeration and its interaction with magnetocaloric material

The magnetic field source system constitutes an important component of magnetic refrigeration. A specific methodology for its' dimensioning is proposed in this paper. It is based on analytical calculation models and takes into account the geometry of the system, the magnetic properties of the magnetocaloric material and the magnetothermal cycle (direct or active magnetic regenerative refrigeration). The analytical calculation of the field is first developed and applied to usual permanent magnet-based field sources with and without soft magnetic materials. Then the forces generated by the interaction between the field and the magnetocaloric effect material are analytically evaluated considering the real field distribution. All calculations are validated thanks to two- or three-dimensional finite element method simulations.     

Thermodynamic analysis of a liquid-flooded Ericsson cycle cooler

A novel approach to implementing a gas Ericsson cycle heat pump was developed. The concept, termed a liquid-flooded Ericsson cooler (LFEC), uses liquid flooding of the compressor and expander to approach isothermal compression and expansion processes. Analytical models of liquid-flooded compression and expansion processes were developed using ideal gas, constant specific heat, and incompressible liquid assumptions. Special considerations for use of positive displacement compressors with fixed volume ratios are detailed. The unique behavior of a liquid-flooded compressor was explored, including the discovery of an optimum liquid flooding rate that minimizes compression power. A computer model of the LFEC cycle was developed using ideal gas, incompressible liquid, and constant specific heat assumptions. The model was used for a thorough parametric study. The purpose of the study was to explore the feasibility of the concept, identify the optimum operating parameters, and to provide a basis for the design of an experimental system.     

Performance analysis of air cycle refrigerator integrated desiccant system for cooling and dehumidifying warehouse

In this paper, the performance of air cycle refrigerator integrated desiccant system used to cool and dehumidify warehouse is analyzed theoretically. Simulation analysis is carried out to calculate the system coefficient of performance, cooling effects and the humidity change under different values of pressure ratio, storage zone temperature inside dock and outdoor air conditions. Also, the effect of the air cycle and the rotor parameters on the system performance is evaluated. From the simulation result it is found that, the desiccant system has the ability to supply air to the dock area at very low humidity. The system coefficient of performance increases due to the exhaust heat recovery on the desiccant system, and this enhancement can be more than 100%. The coefficient of performance of the proposed system is greater than that of a conventional system under the same operating conditions.     

Transcritical CO2 refrigeration cycle with ejector-expansion device

An ejector expansion transcritical CO₂ refrigeration cycle is proposed to improve the COP of the basic transcritical CO₂ cycle by reducing the expansion process losses. A constant pressure mixing model for the ejector was established to perform the thermodynamic analysis of the ejector expansion transcritical CO₂ cycle. The effect of the entrainment ratio and the pressure drop in the receiving section of the ejector on the relative performance of the ejector expansion transcritical CO₂ cycle was investigated for typical air conditioning operation conditions. The effect of different operating conditions on the relative performance of the ejector expansion transcritical CO₂ cycle was also investigated using assumed values for the entrainment ratio and pressure drop in the receiving section of the ejector. It was found that the COP of the ejector expansion transcritical CO₂ cycle can be improved by more than 16% over the basic transcritical CO₂ cycle for typical air conditioning operation conditions.     

Modeling of a domestic frost-free refrigerator

In the present study, a comprehensive thermo-fluidic model is developed for a domestic frost-free refrigerator. The governing equations, coupled with pertinent boundary conditions, are solved by employing a conservative control volume formulation, in the environment of a three-dimensional unstructured mesh. Experiments are also conducted to validate the results predicted by the present computational model. It is found that the computational and experimental results qualitatively agree with each other, although certain discrepancies can be observed in terms of the exact numerical values obtained. For the freezer compartment, the computationally predicted temperatures are somewhat higher than the experimental ones, whereas for the refrigerating compartment, the computed temperatures are lower than the corresponding experimental observations. The difference between experimental and computational results may be attributed to the lack of precise data on the airflow rates and the unaccounted heat transfer rates through the door gaskets and the compressor. From the heat transfer and fluid flow analysis, certain modifications in the design are also suggested, so as to improve the performance of the refrigerator.     

Mass recovery adsorption refrigeration cycle—improving cooling capacity

The study investigates the performance of two-bed, silica gel-water adsorption refrigeration cycle with mass recovery process. The cycle with mass recovery can be driven by the relatively low temperature heat source. In an adsorption refrigeration cycle, the pressures in adsorber and desorber are different. The chiller with mass recovery process utilizes the pressure difference to enhance the refrigerant mass circulation. Cooling capacity and coefficient of performance (COP) were calculated by cycle simulation computer program to analyze the influences of operating conditions. The mass recovery cycle was compared with conventional cycle such as the single stage adsorption cycle in terms of cooling capacity and COP. The results show that the cooling capacity of mass recovery cycle is superior to that of conventional cycle and the mass recovery process is more effective for low regenerating temperature.     

Performance characteristics of a small-capacity directly cooled refrigerator using R290/R600a (55/45)

In this study, the performance of a small-capacity directly cooled refrigerator was evaluated by using the mixture of R290 and R600a with mass fraction of 55:45 as an alternative to R134a. The compressor displacement volume of the alternative system with R290/R600a (55/45) was modified from that of the original system with R134a to match the refrigeration capacity. Both systems with R290/R600a (55/45) and R134a were tested, and then optimized by varying the refrigerant charge and capillary tube length under experimental conditions for both the pull-down test and the power consumption test. The refrigerant charge of the optimized R290/R600a system was approximately 50% of that of the optimized R134a system. The capillary tube lengths for each evaporator in the optimized R290/R600a system were 500 mm longer than those in the optimized R134a system. The power consumption of the optimized R134a system was 12.3% higher than that of the optimized R290/R600a system. The cooling speed of the optimized R290/R600a (55/45) system at the in-case setting temperature of −15 °C was improved by 28.8% over that of the optimized R134a system.