Regenerative Refrigeration Research Articles

New thermodynamic cycles for magnetic refrigeration

Most of the existing prototype devices for magnetic refrigeration are based on a thermodynamic cycle with an active magnetic regenerator (AMR) that operates as a Brayton-type regenerative magnetic refrigeration cycle. However, there are several other cycles that may potentially influence not only the efficiency, but also the cost, compactness and simplicity of magnetocaloric devices. In this article we discuss the possibility of introducing new thermodynamic cycles. This is supported by information about, and a comparison of, the corresponding magnetic field sources. We present the results of numerical analyses and compare the characteristics of different thermodynamic cycles under different operating conditions and for different magnetic field intensities. Guidelines for future work on new magnetic thermodynamic cycles are presented.

Design and performance of a room-temperature hybrid magnetic refrigerator combined with Stirling gas refrigeration effect

The experimental prototype described in this work is a hybrid refrigerator that combines the active magnetic refrigeration effect with the Stirling gas regenerative refrigeration effect. In this prototype, gadolinium sheets are packed in the regenerator matrix for both Stirling and active magnetic regenerative refrigeration. Experimental tests were carried out to measure the cooling performance of this hybrid prototype. The influence of the phase angle on the cooling performance was investigated, and a reasonable phase angle of 90° was determined to obtain optimal cooling performance. By combining the two refrigeration effects, a minimum cooling temperature without heat load of 3.5 °C was reached, which is lower than that of 6.5 °C for the pure Stirling refrigeration effect without the magnetic cooling effect. The results of this study show that the cooling performance is improved by 24% for the hybrid effects compared with that exploiting only the Stirling gas refrigeration effect.

Development of the tandem reciprocating magnetic regenerative refrigerator and numerical simulation for the dead volume effect

In this paper, the development of the tandem reciprocating room-temperature active magnetic regenerative refrigerator and the numerical simulation for the effect of the dead volume are presented. The dead volume effect is analyzed by establishing a one-dimensional time-dependent model for the active magnetic regenerator (AMR). The cooling power at the mass flow rate of 5 g s−1 water and a temperature span of 20 K is reduced from 4 W to 2 W when the length of the dead volume (DDV = 12 mm) is increased from 15 mm to 30 mm. The numerical results indicate that the minimization of dead volume facilitates the improvement of the AMR performance. In particular, the components and the parameters of AMR system are demonstrated. The printed circuit heat exchangers (PCHEs) are employed as the warm end heat exchangers in order to minimize the dead volume of the system. The experimental apparatus includes two active magnetic regenerators containing 186 g of Gd spheres. The maximum no-load temperature span of 26.8 K and a maximum cooling power of 33 W at a zero temperature span were obtained with the frequency of 0.5 Hz under the maximum field of 1.4 T.

상온 능동형 자기 재생 냉동기의 개발

In this paper, an investigation of a room temperature active magnetic regenerative refrigerator is carried out. Experimental apparatus includes two active magnetic regenerators containing 186 g of Gd spheres. Four E-type thermocouples are installed inside the Active magnetic regenerator(AMR) to observe the instantaneous temperature variation of AMR. Both warm and cold heat exchangers are designed for large temperature span. The cold heat exchanger, which separates the two AMRs, employs a copper tube with length of 80 mm and diameter of 6.35 mm. In order to minimize dead volume between the warm heat exchanger and AMRs, the warm heat exchangers are located close to the AMRs. The deionized water is used as a heat transfer fluid, and maximum 1.4 T magnetic field is supplied by Halbach array of permanent magnets. The AMR plate, which contains the warm and the cold heat exchangers and the AMRs, has reciprocating motion using a linear actuator and each AMR is alternatively magnetized and demagnetized by a Halbach array of permanent magnet. Since the gap of the Halbach array of permanent magnets is 25 mm and two warm heat exchangers have the motion through it, a compact printed circuit heat exchanger (PCHE) is used as a warm heat exchanger. A maximum no-load temperature span of 26.8 K and a maximum cooling power of 33 W are obtained from the fabricated Active Magnetic Regenerative Refrigerator (AMRR).

Performance analysis of a micro-scaled quantum Stirling refrigeration cycle

The cycle model of a general micro-scaled regenerative quantum refrigerator working with an ideal Bose or Fermi gas is established. The combined effects of quantum boundary and degeneracy on the performance of the cycle are investigated based on the thermodynamic properties of a confined ideal Bose or Fermi gas. The inherent regenerative losses of the cycle are analyzed and calculated. Expressions for several important performance parameters, such as the refrigeration load, work input, and coefficient of performance (COP), are derived under the cases of the gas degeneracy, weak gas degeneracy, high temperature limit, and thermodynamic limit. The curves of the refrigeration load and coefficient of performance versus the volume and surface area ratios of the cycle and the refrigeration load versus the coefficient of performance are represented. The effects of the size effect on the refrigeration load and coefficient of performance are discussed. The general performance characteristics of the cycle are revealed. It is found that both the refrigeration load and coefficient of performance of the micro-scaled quantum Stirling refrigeration cycle depend on the surface area of the cyclic system besides the temperature of the heat reservoirs, the volume of cyclic system, and other parameters, while those of the macro-scaled refrigerator are independent of the surface area of a cyclic system. The results obtained here are more general and significant than those in the current literature.

Effect of geometrical shape of the working substance Gadolinium on the performance of a regenerative magnetic Brayton refrigeration cycle

Based on Mean Field Theory (MFT), the entropy of magnetic material Gadolinium (Gd), which is a function of the local magnetic field and temperature, is calculated and analyzed. This local magnetic field is the sum of the applied field H0 plus the exchange field HW=λM and the demagnetizing field Hd=−NM, where the demagnetizing factor N depends on the shape of magnetic materials. Hereby, the impacts of the demagnetizing factor N on the magnetic entropy, magnetic entropy change and main thermodynamics performance of a regenerative magnetic Brayton refrigeration cycle using Gd as the working substance are investigated and evaluated in detail. The results obtained underline the importance of the shape of the working substance used in magnetic refrigerators for room-temperature application; elongated materials provide better thermodynamics performance such as higher COP and net heat absorption. It is pointed out that for low external fields, the magnetic refrigerator ceased to be functional if flat materials were used.

A novel refrigerator attaining temperature below λ point

The present study proposes a novel refrigerator in theory, which uses 4He as working fluid to directly reach 2.3 K and uses a small amount of 3He to attain the temperature below 1.7 K. The compact and highly efficient new refrigerator works with the Vuilleumier cycle. The novel refrigerator is driven by a thermal compressor which creatively uses mix-refrigerants J-T refrigerator alternative to liquid nitrogen as the power source. Furthermore, the Vuilleumier cycle can be used to achieve temperature below liquid helium with the improvement of the ultra-low temperature regenerator material. A new method of reaching the temperature below 1.7 K is proposed on the regenerative refrigerator, which could be an important breakthrough for the cryogenic science and technology.

Performance characteristics of an irreversible regenerative magnetic Brayton refrigeration cycle using Gd0.74Tb0.26 as the working substance

The cycle model of an irreversible regenerative magnetic Brayton refrigerator using Gd0.74Tb0.26 as the working substance is established. Based on the experimental characteristics of iso-field heat capacities of the material Gd0.74Tb0.26 at 0T and 2T, the corresponding iso-field entropies are calculated and the thermodynamic performance of an irreversible regenerative magnetic Brayton refrigeration cycle is investigated. The effects of the irreversibilities in the two adiabatic processes and non-perfect regenerative process of the magnetic Brayton refrigeration cycle on the cooling quantity, the heat quantity released to the hot reservoir, the net cooling quantity and the coefficient of performance are discussed in detail. Some significant results are obtained.

Optimization of the performance characteristics in an irreversible regeneration magnetic Brayton refrigeration cycle

A model of the irreversible regenerative Brayton refrigeration cycle working with paramagnetic materials is established, in which the regeneration problem in two constant-magnetic field processes and the irreversibility in two adiabatic processes are considered synthetically. Expressions for the COP, cooling rate, power input, the minimum ratio of the two magnetic fields, etc., are derived. It is found that the influence of the irreversibility and the regeneration on the main performance parameters of the magnetic Brayton refrigerator is remarkable. It is important that we have obtained several optimal criteria, which may provide some theoretical basis for the optimal design and operation of the Brayton refrigerator. The results obtained in the paper can provide some new theoretical information for the optimal design and performance improvement of real Brayton refrigerators.

Performance characteristics of magnetic Brayton refrigeration cycles using Gd, Gd0.74Tb0.26 and (Gd3.5Tb1.5)Si4 as the working substance

Based on the experimental characteristics of the iso-field heat capacity changing with temperature for the room-temperature magnetic refrigeration materials Gd, Gd0.74Tb0.26, and (Gd3.5Tb1.5)Si4, the corresponding entropy versus temperature curves are calculated and presented, the regenerative magnetic Brayton refrigeration cycles, using these magnetic materials as the working substances, are established. The non-perfect regenerative heat quantity, net cooling quantity, released heat quantity, coefficient of performance (COP) and other performance parameters of these magnetic Brayton refrigeration cycles are analyzed and calculated. Furthermore, the performance characteristics of the Brayton refrigeration cycles employing Gd, Gd0.74Tb0.26, and (Gd3.5Tb1.5)Si4 as the working substance are evaluated and compared, the influence of non-perfect regenerative heat on the performance characteristics of these magnetic Brayton refrigeration cycles is revealed.

Modelling an active magnetic refrigeration system: A comparison with different models of incompressible flow through a packed bed

Abstract Active Magnetic Regenerative refrigeration system (AMR) is an environmentally attractive refrigeration alternative to vapour compression plants. A magnetic refrigerator is composed of a regenerator that is a solid packed bed made of a magnetic material and a secondary heat transfer fluid flowing in the porous matrix. The secondary fluid can be a liquid (water or water anti-freezing mixture). Modelling is a particularly cost-effective method for studying, designing and optimizing a magnetic refrigeration system. In this paper a comparison between three different models to simulate the thermo-fluidodynamic behaviour of an AMR cycle is carried out. Each model simulates both the magnetic material and the secondary fluid of an AMR operating in conformity with a Brayton regenerative cycle. In the reported models Gd x Dy 1−x alloys are the constituent materials for the regenerator over the temperature range 260–280 K. The heat transfer medium is a water-glycol mixture (50% by weight).With these models, the refrigeration capacity, the power consumption and consequently the coefficient of performance of the cycle can be predicted.

A Monolithic Perovskite Structure for Use as a Magnetic Regenerator

A La0.67Ca0.26Sr0.07Mn1.05O3 (LCSM) perovskite was prepared for the first time as a ceramic monolithic regenerator used in a regenerative magnetic refrigeration device. The parameters influencing the extrusion process and the performance of the regenerator, such as the nature of the monolith paste and the influence of the sintering on the adiabatic temperature change, were investigated. Comparisons between the extruded monolithic structure before and after the sintering showed that an increase of the adiabatic temperature change was seen after the sintering. Furthermore, calculations show that the performance of the monolithic structure is potentially superior to a parallel plate regenerator, indicating the potential cost and structural benefit of using such structure, i.e. a mechanically stable ceramic thin wall structure, which can be produced in one processing step.

Cooling Characteristics of Regenerative Magnetic Refrigeration With Particle-Packed Bed

The aim of our study was to elucidate the fundamental cooling characteristics and to improve the cooling characteristics of a room-temperature magnetic refrigerator operated under an active magnetic regenerator (AMR) cycle. The AMR refrigeration cycle, which includes a thermal storage process and a regeneration process, is used to realize a practical magnetic refrigerator operating near room-temperature. The basic components of the target AMR system are a magnetic circuit, test section, fluid-displacing device, and associated instrumentation. Spherical gadolinium particles are packed in the test section as the magnetic working substance, and air and water are used as heat transfer fluids. The cooling characteristics of the target AMR system under various operating conditions are investigated. The results show that the AMR cycle is very effective in improving the cooling performance of the room-temperature magnetic refrigerator.

Numerical modeling on a reciprocating active magnetic regenerator refrigeration in room temperature

Considering the unignorable factors in practice, a new time independent, 2-dimensional porous media model of room-temperature Active Magnetic Regenerative Refrigeration (AMRR) has been proposed. The 2-D model improved the existing 1-dimensional model by introducing the influence of heat transfer effect though the regenerator wall and conduction for y-axis inside the regenerator. This study compared the previous 1-D model with the 2-D model and concluded that the system can lose 22% of cooling capacity caused by air convection and the conduction loss in y can reach to 10% of cooling capacity. It is concluded that the new model will be useful to predict the performance of room AMRR for more practical conditions.

Experimental results for a magnetic refrigerator using three different types of magnetocaloric material regenerators

Magnetic refrigeration is a potentially environmentally-friendly alternative to vapor compression technology because it has a potentially higher coefficient of performance and does not use a gaseous refrigerant. The active magnetic regenerator refrigerator is currently the most common magnetic refrigeration device for near room temperature applications, and it is driven by the magnetocaloric effect in the regenerator material. Several magnetocaloric materials with potential magnetic refrigeration applications have recently been developed and characterized; however, few of them have been tested in an experimental device. This paper compares the performance of three magnetocaloric material candidates for AMRs, La(Fe,Co,Si) 13, (La,Ca,Sr)MnO 3 and Gd, in an experimental active magnetic regenerator with a parallel plate geometry. The performance of single-material regenerators of each magnetocaloric material family were compared. In an attempt to improve system performance, graded two-material regenerators were made from two different combinations of La(Fe,Co,Si) 13 compounds having different magnetic transition temperatures. One combination of the La(Fe,Co,Si) 13 materials yielded a higher performance, while the performance of the other combination was lower than the single-material regenerator. The highest no-load temperature span was achieved by the Gd regenerator.

Experimental discovery and verification of the third type of DC gas flow in pulse tube cryocooler

DC gas flow in pulse tube cryocooler (PTC) is a crucial problem both in theory and application which considerably affects the refrigeration performance. We have experimentally discovered and verified the third type of DC gas flow in PTC which is formed due to hydrodynamic and thermodynamic asymmetry of the regenerator and other flow channels. This new type of DC gas flow is possible to be identified in other regenerative engines or refrigerators. We also introduced a highlighting method which can suppress this kind of DC gas flow effectively in most cases, with the best result of 30 K temperature drop at the cold end of the PTC.

A numerical analysis of an active magnetic regenerative refrigerant system with a multi-layer regenerator

Magnetic refrigeration is an emerging technology based on the magnetocaloric effect in solid-state refrigerants. The active magnetic regenerative refrigeration (AMRR) cycle is a special kind of regenerator for the magnetic refrigerator, in which the magnetic material matrix works both as a refrigerating medium and as a heat regenerating medium, while the fluid flowing in the porous matrix works as a heat transfer medium. The performance of an AMRR cycle depends strongly on the behaviour of the adiabatic magnetization temperature change as a function of material temperature in the flow direction of the regenerator. In the present paper, a practical model for predicting the performance and efficiency of an AMRR cycle has been developed. The model simulates both the ferromagnetic material and the entire cycle of an AMRR operating in conformity with a Brayton regenerative cycle. The model simulates different kinds of layered regenerators operating at their optimal operation point. The program study the Gd − x Tb 1− x alloys as constituent materials for the regenerator over the temperature range 275–295 K, and Gd x Dy 1− x alloys in the temperature range 260–280 K. With this model, the refrigeration capacity, the power consumption and consequently the coefficient of performance can be predicted. The results show a greater COP for the refrigerator based on the magnetocaloric technology compared with the COP of a classical vapour compression plant working between the same thermal levels.

Numerical analysis of active magnetic regenerators for hydrogen magnetic refrigeration between 20 and 77 K

A model active magnetic regenerator refrigerator (AMR) with the Brayton-like operation cycle was analyzed by numerical cycle simulation in the temperature range between 20 and 77 K. In order to study the performance using magnetic material with various transition temperatures T c , entropy of magnetic material with second order phase transition was calculated using mean field theory and Debye approximation. The cooling performance is shown to be high when the heat exhaust temperature is close to the transition temperature. It is shown that the optimized operation condition depends on both T c and operation temperatures. Multi-layered AMR beds were shown to improve the performance of AMR. Multi-stage AMR was also discussed.

Dimensionless numerical model for simulation of active magnetic regenerator refrigerator

In order to obtain a better reliability, consistency and accuracy of results obtained with a numerical simulation of an AMRR (active magnetic regenerator refrigerator), a dimensionless numerical model was developed, which can equally be used for determination of regenerator’s heat transfer coefficient and simulation of passive heat regenerators or AMRR operation. Regenerator’s heat transfer coefficient α f, is a crucial input parameter in the simulation of AMRR operation and has a primal effect on the outcome of a solution. This paper deals with a derived dimensionless model and discusses errors involved when using different models for heat transfer coefficient and AMRR operation simulation.

Influence of Micro-Channel Shape and Magnetic Material on the Magneto-Refrigeration Process of Integrated Circuits

We developed a two dimensional transient numerical model that solves the first step of heat transfer of an active magnetic regenerative refrigerator (AMR) using the heat conduction equation for an adiabatic system. For micro-refrigeration, an AMR device is constituted by a magnetic material, placed on a silicon wafer containing micro-channels where a heat exchanging fluid flows. The magnetic materials used in the simulations are the promising the Gd5Si2Ge2, La(Fe0.88Si0.22)13 and La0.66Sr0.33MnO3 compounds, because they exhibit a giant magnetocaloric effect near room temperature. We considered different initial conditions, namely different micro-channel shapes, sizes and separations, aiming to increase the performance of the micro-cooler device. The influence of the thickness of the magnetic material on refrigeration power is also studied.