Active Magnetic Regenerator Cycle Research Articles

Simulation of an operation of nested Halbach cylinder arrays in regenerative magnetic cooling cycles: The way to maximum thermal span

In this study, a numerical model of the Active Magnetic Regenerator (AMR) cycle, implemented in the reciprocal demonstrator, was developed using COMSOL Multiphysics. A nested Halbach cylinder (NHC) array served as the magnetic field source. Additional simulation of an operation of the NHC array was carried out. To eliminate the discrepancies between the heat exchange duration of the heat transfer medium (HTM) and the hot and cold ends of the regenerator, an adequate time dependence of the inner cylinder rotation angle was calculated. The latter provides the symmetrical sinusoidal form of time dependence of the magnetic flux density in the gap of NHC array, which is important for enhancing the performance of a magnetic refrigerator. It was established that in order to achieve a maximal temperature span, it is necessary to shift the phases of the magnetic field insertion/removal and heat transfer fluid pumping processes by nearly half of the operating cycle period. The latter brings the simulated cycle closer to the ideal AMR cycle.

1D models of an active magnetic regeneration cycle for cryogenic applications

In investigation of an active magnetic regenerator (AMR) cycle operating at room temperatures, 1D models have been extensively used to accurately computing its performance metrics. However, extending these models to simulate an AMR cycle at cryogenic temperatures introduces inherent complexities and challenges. The broad temperature span and low operating temperatures required for cryogenic applications, such as hydrogen liquefaction, lead to significant density variations of the working fluid within the AMR that cannot be overlooked. In this work, two 1D AMR models assuming a compressible working fluid operating at cryogenic temperatures are demonstrated which address the large density variations and the numerical stiffness of the equations. The models exhibit good agreement with experimental and 2D numerical results of an AMR configuration designed for hydrogen liquefaction. A comparative study is conducted between the developed models and an incompressible AMR model at cryogenic temperatures shows that the incompressible model predicts cooling powers that are higher by a factor of up to 10 at high values of utilization, highlighting the error of assuming an incompressible fluid on estimating the performance metrics.

MnxFe5−xSi3 for active magnetic regenerative refrigeration at room temperature

Magnetocaloric properties of MnxFe5−xSi3 with x = 1, 1.1, 1.2, 1.3 were investigated for use in an active magnetic regenerator (AMR) for room temperature refrigeration. The Mn content was adjusted to tailor their Curie temperature and, hence, the operating range of the magnetic refrigerant. The peak magnetic entropy change and the maximum adiabatic temperature change for MnxFe5−xSi3 decreased with increasing Mn substitution. An in-house AMR testbed was built to investigate the suitability of the refrigerant in an AMR cycle. The maximum temperature span between cold and hot ends achieved by the AMR testbed using Mn1.2Fe3.8Si3 was 4.75 K at 300 K compared to 7.5 K for Gd. It was also demonstrated that the morphology of the refrigerant can have a measurable effect on the heat exchange between the refrigerant and the cooling fluid, which highlights that designing a solid magnetic refrigerant also requires appropriate technology to process the material to the desired size and shape. The magnetic refrigerant characteristics of MnxFe5−xSi3 were substantially inferior to those of Gd, but MnxFe5−xSi3 remains attractive in terms of material cost and processability.

Mn5−xGe3Nix refrigerant for active magnetic refrigeration

Mn5−xNixGe3 (x = 0, 0.025, 0.075, 0.1, 0.15, 0.2, 0.25, and 0.4) alloys were synthesized to demonstrate that the magnetocaloric (MCE) properties [Curie temperature, magnetic entropy change (ΔSM), and adiabatic temperature change (ΔTad)] can be fine-tuned by changing the Ni fraction. An active magnetic regenerative (AMR) cycle testbed was built in-house to investigate the suitability of Mn5−xNixGe3 as a solid magnetic refrigerant operating at room temperature and as a composite refrigerant for an AMR cycle. The maximum temperature difference between the hot and cold ends, ΔTspan, measured from the AMR testbed was 7.5 K and 5.6 K for Gd and Mn5Ge3, respectively. The ΔTspan values were reasonable considering the inferior ΔSM and ΔTad for Mn5Ge3 compared to those of Gd. A two-layer composite refrigerant consisting of Mn5Ge3 and Mn4.975Ni0.025Ge3 produced ΔTspan of 6.3 K, which is 13% higher than that of the single Mn5Ge3 refrigerant, indicating the effectiveness of a multi-layer refrigerant selected to closely match the temperature gradient in an AMR cycle. This work highlights the importance of tailoring the MCE properties as required to build a composite refrigerant.

Using numerical methods to screen magnetocaloric materials in an active magnetic regenerative cycle

• A parametric convective heat transfer term is used to assess t span & cooling power. • Four major magnetocaloric materials are screened. • Optimal fluid velocity depends on the magnetic transition temperature. A 2-dimensional numerical model of a room temperature operating Active Magnetic Regenerator (AMR) that comprises of a regenerator, hot and cold heat exchangers, heat transfer fluid is developed. The regenerator is made of a magnetocaloric material (MCM) which heats up upon applying a magnetic field, H, and cools down when the field is removed; thus, making it the most essential part of an AMR. The model takes experimentally measured ∆T ad (H,T) and the C p (H,T) data as input and provides quantitative performance metrics of the magnetic cooling system, such as ∆T span and the cooling load, as output. With this model, it is possible to assess a wide range of MCMs in an AMR. 4 different MCMs were investigated using this model in terms of their ∆T span and the cooling loads—LaFe 10.96 Co 0.97 Si 1.07 , MnFeP (1-x) As (x) and AlFe 2 B 2 and Gd. During the screening of MCMs, all the important operating conditions of the device were fixed such as the fluid flow rate, ambient temperature, cycle duration, magnetic field strength. Our results indicate that Gd exhibits the maximum ∆T span with respectable cooling load, while AlFe 2 B 2 generates the lowest ∆T span . Even though LaFeCoSi compound did not perform as well as Gd, it could be the MCM choice for the realization of magnetic refrigeration on a global scale due to its much lower cost.

Propane liquefaction with an active magnetic regenerative liquefier

Magnetic refrigeration is a well-known cooling technique based on the magnetocaloric effect (MCE) of certain solids as they enter or leave a high magnetic field. An active magnetic regenerator (AMR) uses certain ferromagnetic materials simultaneously as MCE refrigerants and as a regenerator. An effective active magnetic regenerative refrigeration cycle consists of four steps: adiabatic magnetization with no heat transfer gas flow; heat transfer gas flow at constant high field; demagnetization with no heat transfer gas flow; and heat transfer gas flow at constant low field. The first heat transfer gas flow step from a cold-to-hot temperature in this cycle rejects heat from the magnetized regenerator to a hot sink and the second reverse heat transfer gas flow step from a hot-to-cold temperature absorbs heat from a cold source. Our primary objectives of the present work were to demonstrate an AMR-cycle liquefier, determine the cooling power of a magnetic refrigerant executing an AMR cycle, and understand the impact of intermittent cooling of the AMR cycle of a reciprocating, dual regenerator design with continuous liquefaction and parasitic heat leaks. This article describes how an AMR-cycle refrigerator using Gd regenerators moving through ∼2.7 T changes at 0.25 Hz was used to liquefy pure propane at two different supply pressures. The measured rates of liquefaction and elapsed times were measured and used to determine the volume collected and derive cooling power at liquefaction conditions for both runs. These results were compared to those obtained from cool-down temperature vs. time data during the same run. The agreement between the two, independent cooling-power results was excellent after the duty cycle of the AMR cycle cooling was properly treated. No direct measurements of the efficiency were made.

Research progress of physical model of full-solid-state magnetic refrigeration system

Magnetic refrigeration is a kind of energy-saving, environment-friendly and intrinsically-high-efficient refrigeration technology, which has a wide application prospect. At present, the magnetic refrigeration systems based on active magnetic regenerator cycle have been widely studied and many prototypes of refrigerators have been developed. However, fluids and gases are mainly applied to heat exchange in these systems, which brings some problems such as low operating frequency, large regenerative loss, and complicated sub-component design. These problems increase the cost and reduce the efficiency of magnetic refrigerators. In view of the above problems and challenges, researchers try to introduce the solid-state heat transfer enhancement mechanism, and to design and optimize the full-solid-state magnetic refrigeration system model. In this paper, the development process of magnetic refrigeration technology at room temperature is briefly introduced at first. And the reasons for the low operating frequency and efficiency of the magnetic refrigerator, caused by using fluids for heat exchange, are analyzed. Then, two types of solid-state heat exchange media are briefly described, which are thermal diodes (i.e., electric-field-controlled thermal diode and magnetic-field-controlled thermal diode) and high thermal-conductivity material elements. In this paper we review the research progress of the full-solid-state magnetic refrigeration model based on thermal diodes and high thermal-conductivity material elements. Some key items for these models are described in detail, such as the architectural design concept, physical mechanism and working principle, the main performance simulation results of these systems and their physical change rules. Then, the main performances (i.e. operating frequency, specific cooling power, temperature span, and coefficient of performance) of the full-solid-state magnetic refrigeration model and the AMR model are summarized and comparatively analyzed. It shows that the full-solid-state magnetic refrigeration system can work at high frequency and has greater specific-cooling-power. Meanwhile the design of full-solid-state magnetic refrigeration system is more compact and simpler. The characteristics and problems of the two types of solid heat exchange media are also analyzed. Due to the strong thermal transport capability, easy access and integration of thermoelectric elements, the full-solid-state magnetic refrigeration technology based on thermoelectric thermal diodes has greater application potential. Finally, the main research directions and key scientific problems for further studying the full-solid-state magnetic refrigeration field are discussed and analyzed.

Multiple points of equilibrium for active magnetic regenerators using first order magnetocaloric material

First order transition material (FOM) usually exhibits magnetocaloric effects in a narrow temperature range which complicates their use in an active magnetic regenerator (AMR) refrigerator. In addition, the magnetocaloric effect in first order materials can vary with field and temperature history of the material. This study examines the behavior of a MnFe(P,Si) FOM sample in an AMR cycle using a numerical model and experimental measurements. For certain operating conditions, multiple points of equilibrium (MPE) exist for a fixed hot rejection temperature. Stable and unstable points of equilibriums (PEs) are identified and the impacts of heat loads, operating conditions, and configuration losses on the number of PEs are discussed. It is shown that the existence of multiple PEs can affect the performance of an AMR significantly for certain operating conditions. In addition, the points where MPEs exist appear to be linked to the device itself, not just the material, suggesting the need to layer a regenerator in a way that avoids MPE conditions and to layer with a specific device in mind.

Experimental investigation of different fluid flow profiles in a rotary multi-bed active magnetic regenerator device

A rotary multi-bed active magnetic regenerator (AMR) device was modified to allow testing different fluid flow waveforms, with different blow fractions (i.e., the fraction of the AMR cycle when there is fluid flow in the regenerators). The different values of blow fraction were generated using different cam rings that actuate the poppet valves at the inlet and outlet of the regenerators, controlling how long the valves stay open and the number of valves open at the same time. Results showed that smaller blow fractions yield higher values of temperature span for fixed flow rate and cooling capacity, but lower values of coefficient of performance for the same conditions. An analysis of the shaft and pumping powers showed that shorter blow fractions cause higher pressure drop and higher torque oscillations and mechanical vibrations. The highest value of second-law efficiency of 19.1% was obtained for the largest blow fraction tested (80%). Designs for magnetic refrigerators where the fluid flow waveform can change during operation are also discussed in this paper.

Thermal analysis of a multistage active magnetic regenerator cycle for hydrogen liquefaction

The present paper deals with thermal analysis of cascade Active Magnetic Regenerator (AMR) cycle for the liquefaction of hydrogen starting from room temperature. For this purpose, the energy equations for fluid and solid within the regenerator bed of the AMR cycle have been considered. To solve the resulting mathematical model implicit finite difference method has been used. Thermal energy and mass balances are performed for several liquefaction systems composed of different number of cascade cycles. A simulation method using Hysys simulation commercial code has been presented. The multistage system operates with an ideal magnetic material as refrigerant and hydrogen gaseous as carrier fluid. First, the coefficient of performance (COP) of the AMR cycle and the required volume of magnetic material as functions of the number of cascades have been investigated. Then, the required volume is optimized by using the relationship between the COP and the volume. It has been found that a number of 6 AMR cycles operating in series is the optimal number of cascades required to liquefy 1 kg/h of hydrogen supplied at 25 °C. The system can operate between two volumes of magnetic material; namely, the minimum required volume (2.96 L) and the most efficient volume (7.44 L), corresponding to COP values of 1.23 and 4.7 respectively.

Review of magnetic refrigeration system as alternative to conventional refrigeration system

The refrigeration system is one of the most important systems in industry. Developers are constantly seeking for how to avoid the damage to the environment. Magnetic refrigeration is an emerging, environment-friendly technology based on a magnetic solid that acts as a refrigerant by magneto-caloric effect (MCE). In the case of ferromagnetic materials, MCE warms as the magnetic moments of the atom are aligned by the application of a magnetic field. There are two types of magnetic phase changes that may occur at the Curie point: first order magnetic transition (FOMT) and second order magnetic transition (SOMT). The reference cycle for magnetic refrigeration is AMR (Active Magnetic Regenerative cycle), where 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. Regeneration can be accomplished by blowing a heat transfer fluid in a reciprocating fashion through the regenerator made of magnetocaloric material that is alternately magnetized and demagnetized. Many magnetic refrigeration prototypes with different designs and software models have been built in different parts of the world. In this paper, the authors try to shed light on the magnetic refrigeration and show its effectiveness compared with conventional refrigeration methods.

Direct measurement of the magnetocaloric effect (ΔTad) of Mn5−x (Fe,Co)xGe3

Magnetic field–induced adiabatic temperature change, ΔTad, measures the temperature gradient that enables a magnetic refrigerant to transfer heat from reservoirs. The ΔTad of Mn5Ge3 was evaluated by directly recording the temperature of a Mn5Ge3 sample while subjecting it to an external field. The peak ΔTad of 0.89 ± 0.006 at 0.8 T for Mn5Ge3 at its Curie temperature was consistent with the value previously obtained from specific heat capacity measurements. In addition, the peak values of ΔTad for Mn4.75Co0.25Ge3 and Mn4.75Fe0.25Ge3 were measured to be 0.77 ± 0.009 and 0.93 ± 0.011, respectively, at 0.8 T near their respective Curie temperatures. ΔTad measurements under cyclic application of a 0.8 T field showed that the alloys, whose magnetocaloric effect stems from the second-order magnetic phase transition, exhibited no hysteresis in ΔTad and maintained their peak ΔTad within 1.3% under cyclic field. The ΔTad of sandwiched Mn4.75Co0.25Ge3 and Mn4.75Fe0.25Ge3 plates was also measured to demonstrate that a combination of the alloys can be used to form a graded regenerator for the active magnetic regenerator cycle.

Material screening metrics and optimal performance of an active magnetic regenerator

A variety of metrics to rank the magnetocaloric materials can be found in the literature, but a quantitative assessment showing their efficacy has not been reported. A numerical model of an active magnetic regenerator cycle is used to assess the predictive ability of a set of material metrics. The performance of eight cases of known magnetocaloric material (including first order MnFeP1-xAsx and second order materials Gd, GdDy, Tb), and 15 cases of hypothetical materials are considered. Using a fixed regenerator matrix geometry, magnetic field, and flow waveforms, the maximum exergetic cooling power of each material is identified. Several material screening metrics such as relative cooling power (RCP) are tested and a linear correlation is found between maximum RCP and the maximum exergetic cooling power. The sensitivity of performance to variations in the hot side and cold side temperatures from the conditions giving maximum exergetic power are determined.

Magnetocaloric properties of La0.666Sr0.373Mn0.943Cu0.018O3

Magnetocaloric properties of La0.666Sr0.373Mn0.943Cu0.018O3 (LSMCO) perovskite (such as magnetic entropy change, full-width at half-maximum, relative cooling power and magnetic specific heat change) at applied magnetic field of 0.05 T were calculated using the phenomenological model. The results indicate the prospective application of LSMCO due to high magnetocaloric effect near the Curie temperature. Furthermore, the magnetocaloric properties of LSMCO sample are comparable with magnetocaloric properties of MnAs film, La1-xCdxMnO3 and La1.25Sr0.75MnCoO6, and significantly larger than that of Gd1-xCaxBaCo2O5.5 and Ge0.95Mn0.05. It is recommended that magnetocaloric effect of LSMCO can be used as a promising practical material of an apparatus based on the active magnetic regenerator cycle.

A two-dimensional investigation about magnetocaloric regenerator design: parallel plates or packed bed?

Magnetic Refrigeration (MR) is a novel refrigeration technique based on eco-friendly solid materials as refrigerants, whom react to the application of magnetic fields, with warming and cooling by magnetocaloric effect. The thermodynamical cycle which best suits the magnetic refrigeration is Active Magnetic Regenerator cycle (AMR). Regenerator is the core of a magnetic refrigerator, since that it plays a dual-role: it operates both as refrigerant and regenerator in an AMR cycle. An AMR cycle consists of two adiabatic stages and two isofield stage. In this paper an investigation is conducted about the magnetocaloric refrigerator design through two-dimensional multiphysics numerical models of two different magnetocaloric regenerators: (1) a packed bed and (2) a parallel plates magnetic regenerators made of gadolinium, operating at room temperature under a 1.5T magnetic field induction. Both models employ water as secondary fluid. The tests were performed with variable fluid flow rate at fixed AMR cycle frequency. The results obtained are presented in terms of temperature span, cooling power, coefficient of performance and mechanical power of the circulation pump, and they indicate under which operating conditions packed bed configuration is to be preferred to parallel plates and vice versa.

Magnetic refrigeration: an eco-friendly technology for the refrigeration at room temperature

Magnetic refrigeration is an emerging, environment-friendly technology based on a magnetic solid that acts as a refrigerant by magneto-caloric effect (MCE). In the case of ferromagnetic materials MCE is a warming as the magnetic moments of the atom are aligned by the application of a magnetic field, and the corresponding cooling upon removal of the magnetic field. There are two types of magnetic phase changes that may occur at the Curie point: first order magnetic transition (FOMT) and second order magnetic transition (SOMT). The reference cycle for magnetic refrigeration is AMR (Active Magnetic Regenerative cycle) where 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. Regeneration can be accomplished by blowing a heat transfer fluid in a reciprocating fashion through the regenerator made of magnetocaloric material that is alternately magnetized and demagnetized. In this paper, attention is directed towards the near room-temperature range. We compare the energetic performance of a commercial R134a refrigeration plant to that of a magnetic refrigerator working with an AMR cycle. Attention is devoted to the evaluation of the environmental impact in terms of a greenhouse effect. The comparison is performed in term of TEWI index (Total Equivalent Warming Impact) that takes into account both direct and indirect contributions to global warming. In this paper the AMR cycle works with different magnetic refrigerants: pure gadolinium, second order phase magnetic transition (Pr0.45Sr0.35MnO3) and first order phase magnetic transition alloys (Gd5Si2Ge2, LaFe11.384Mn0.356Si1.26H1.52, LaFe1105Co0.94Si110 and MnFeP0.45As0.55). The comparison, carried out by means of a mathematical model, clearly shows that GdSi2Ge2 and LaFe11.384Mn0.356Si1.26H1.52 has a TEWI index always lower than that of a vapor compression plant. Furthermore, the TEWI of the AMR cycle working with FOMT materials is always better than that of SOMT materials. Gd5Si2Ge2 is the best FOMT material.

A comparison between rare earth and transition metals working as magnetic materials in an AMR refrigerator in the room temperature range

This paper describes a two-dimensional (2D) multiphysics model of a packed bed regenerator made of magnetocaloric material. The regenerator operates as a refrigerant for a magnetic refrigerator operating at room temperature on the strength of an active magnetic regenerator (AMR) cycle. The model is able to simulate the thermofluidodynamic behavior of the magnetocaloric material and the magnetocaloric effect of the refrigerant. The model has been validated by means of experimental results. Different magnetic materials have been tested with the model as refrigerants: pure gadolinium, second order phase magnetic transition Pr0.45Sr0.35MnO3 and first order phase magnetic transition alloys Gd5(SixGe1−x)4, LaFe11.384Mn0.356Si1.26H1.52, LaFe11.05Co0.94Si1.10 and MnFeP0.45As0.55. The tests were performed with fixed fluid flow rate (5 l/min), AMR cycle frequency (1.25 Hz) and cold heat exchanger temperature (288 K) while the hot heat exchanger temperature was varied in the range 295–302 K. The results, generated for a magnetic induction which varies from 0 to 1.5 T, are presented in terms of temperature span, refrigeration power and coefficient of performance. From a global point of view (performances and cost), the most promising materials are LaFeSi compounds which are really cheaper than rare earth compounds and they give a performance sufficiently higher than gadolinium.

Influence of manganite powder grain size and Ag-particle coating on the magnetocaloric effect and the active magnetic regenerator performance

The magnetocaloric performance of La0.67Ca0.27Sr0.06Mn1.05O3 is investigated as a function of the powder grain size and also as a function of decoration of grains with highly conductive silver particulates as a coating layer. We demonstrate that the thermal and electrical conductivities can be significantly modified by the Ag-particle coating when the material is examined in sintered pellet form and we compare results with a second manganite composition La0.67Ca0.33MnO3 with significantly smaller grain size. However, we find that this microstructural engineering does not improve the performance of the active magnetic regenerator cycle using the silver decorated material in powder form. The regenerator performance is improved by the reduction of the powder grain size of the refrigerant which we attribute to improved thermal management due to increased surface to volume ratio.

能動的蓄冷式磁気冷凍の基礎的研究

能動的蓄冷式磁気冷凍の基礎的研究

Lanthanum Concentration Effect of Magnetocaloric Properties in La x MnO3−δ

Magnetocaloric properties of La xMnO 3−δ films in the composition range 0.75 ≤ x ≤ 1 near phase transition from a ferromagnetic to a paramagnetic state were investigated. For x > 0.75 composition, it is showed that the increasing of La concentration improves magnetocaloric properties. It is also showed that post-annealing films in O 2 improves magnetocaloric effect. The magnetocaloric properties are affected by Mn +3/Mn +4 ratios, which can be varied either by changing La concentration or varying the oxygen content in the La xMnO 3−δ system. Moreover, La xMnO 3−δ films can be used as a working material of an apparatus based on the active magnetic regenerator cycle that cools hydrogen gas.