Active Magnetic Regenerative Refrigeration Research Articles

Development and Validation of a 3-D Analytical Fluidic Model in Cartesian Coordinates for a Magnetic Refrigeration Application

This paper focuses on the development and validation of a three-dimensional (3-D) analytical model based on the formal resolution of the incompressible Navier-Stokes equations in Cartesian coordinates. This analytical fluidic model calculates the evolution of the fluid flow velocity for various pressure gradient shapes (i.e., square, trapezoidal, ramp, triangle, sinusoidal signals, etc.), with or without mean value, using Fourier series. The model can consider square and rectangular channels of any dimensions in terms of height, width, and depth. It allows for the study of different types of fluid flow and geometries. The introduction highlights the significance of analytical modeling, particularly in the context of Magnetic Refrigeration (MR) technologies. The model assumptions and governing equations are presented. The results obtained from the analytical model are validated and show good convergence with those obtained using the COMSOL Multiphysics® software. The comparison demonstrates a high level of agreement between the two models. The analytical model performs well in terms of computation time and accuracy. It will be coupled with an analytical thermic model and used to optimize Active Magnetic Regenerative Refrigeration (AMRR) cycles. The ultimate objective is to determine an optimal pressure gradient evolution in order to maximize heat exchange in a regenerator channel.

The role of external heat exchangers in the performance of active magnetic refrigerator

The static performance of an Active Magnetic Regenerator Refrigerator (AMRR) can be evaluated using various indicators, such as the refrigeration capacity, coefficient of performance, and efficiency, for a given temperature span (that is, the hot-to-cold heat source temperature difference). The two main control parameters (cycle frequency and fluid mass flow rate) must be adjusted according to the imposed temperature span to achieve optimum performance. However, in real-world operations, heat exchangers at both ends of the regenerator thermally connect the device to the surroundings, and the hot and cold temperatures are no more imposed quantities. Under these heat transfer constraints, the device performance, in terms of refrigeration capacity, could be suboptimal, even after adjusting the two control parameters. Indeed, to provide optimal performance, heat exchangers must have precise values of the overall heat transfer coefficient, depending on the desired operating temperature difference. So, if a single pair of heat exchangers are used, it is impossible to characterize the performance of the magnetic refrigerator at different temperature spans. This work compares the systematic procedures for evaluating the performance of a magnetic refrigeration device either with heat exchangers at the cold and hot ends or with an imposed temperature span. Both procedures can be used to obtain the optimal AMRR performance curves. However, this work highlights the necessity to use different (or variable heat transfer coefficient) heat exchangers when the first procedure is adopted. The needed overall heat transfer characteristic of both heat exchangers will be lower for a higher temperature span.

Conceptual design of two novel hydrogen liquefaction processes using a multistage active magnetic refrigeration system

Hydrogen is a promising clean energy carrier. However, its transportation in liquid form faces many challenges because the conventional liquefaction processes for hydrogen are highly inefficient and expensive. Moreover, due to the rising environmental concerns and low energy efficiency of compressor refrigeration technologies, active magnetic regenerator refrigeration technique (AMR) can be used as an alternative. This study presents a novel conceptual design for two hydrogen liquefaction (LH2) processes where the precooling section consists of AMR instead of the traditional compressors technology. The aim is to reduce the specific energy consumption and increase the exergy efficiency of the process while utilizing an AMR pre-cooling section for better environmental safety. Although, the process is considered small scale for LH2 production, it is infact the highest reported capacity for AMR processes. Hence, a comprehensive economic analysis is performed to analyze the costs involved within the processes. The results indicate that the proposed AMR processes provide ∼5.9–15.4% energy savings (SEC 4.15 kWh/kg for process 1 and 8.73 kWh/kg for process 2) as compared to the mixed refrigerants (MR) pre-cooling based processes (SEC 4.41 kWh/kgLH2 for conventional process 1 and 10.32 kWh/kgLH2 for conventional process 2). The Coefficient of performance (COP) of the proposed processes has values of 0.191 and 0.143, which are ∼6.3% and 18.2% more than the conventional processes 1 and 2. Compared to conventional MR processes 1 and 2, the exergy efficiency of process 1 increases by 10.55%, while that of process 2 increases by 18.2%. The economic analysis shows that LH2 can be produced with a competent levelized cost of product (LCOP) ranging between 1.31 and 1.75 US$/kg LH2.

Hydrogen Liquefaction by Active Magnetic Regenerative Refrigeration

The article summarizes the research and development of magnetic refrigeration for hydrogen liquefaction, and the Active Magnetic Regenerative Refrigeration (AMRR) developed by National Institute for Materials Science (NIMS). The NIMS-AMR consists of optimized AMR beds, a superconducting solenoid with correction coils, and a dedicated liquid hydrogen vessel with a liquid level sensor. This article also reports the first successful demonstration of hydrogen liquefaction with the AMRR.

Magnetic Refrigerant Materials for Hydrogen Liquefaction by Active Magnetic Regenerative Refrigeration

Magnetic refrigerant materials are one of the key factors for governing the cooling performance of magnetic refrigeration systems. In particular, not only the physical properties such as magnetocaloric effects, but also the practical form of the magnetic material are of crucial importance to active magnetic regenerative refrigeration （AMRR）. In this JST-mirai project, the development of several types of magnetic materials that show the large magnetocaloric effects in the temperature range from 20 K to 80 K and fabrication of spherical particles of the materials were conducted. The magnetic properties and magnetocaloric effects between the mother-alloy and spherical particles were compared for HoAl2, ErCo2, and HoB2. Moreover, thermal property and electrical property as well as the shape and size of the materials are discussed from the perspective of their application to the AMRR system for hydrogen liquefaction.

Methane liquefaction with an active magnetic regenerative refrigerator

This manuscript reports liquefaction of methane using an active magnetic regenerative refrigerator (AMRR) which cooled from 285 K to 135 K. This refrigerator has two identical regenerators fabricated with adjacent layers of four ferromagnetic refrigerants, each with sequentially lower Curie temperatures and lesser masses from hot to cold temperatures. The refrigerants either heated or cooled by the magnetocaloric effect when moved in or out of a high-field superconducting magnet. The dual multilayer regenerators were assembled in opposition with a common cold region between them. The same mass of helium heat transfer gas was circulated through all layers of the regenerators during the two flow steps of the four-step AMR cycle. A compact coil-fin tube heat exchanger closely fitted around a small liquid storage vessel mounted in the cold region of the assembly was alternatively cooled by cold 2.76 MPa helium gas during hot-to-cold flow steps of the AMRR cycle. In this experiment the cold coil-fin tube heat exchanger was used to cool and liquefy a process stream of methane gas supplied at 295 K from a lecture bottle at different pressures in different runs. By measuring time to liquefy a known volume of methane at three different pressures, cooling powers of the AMRR as a function of temperature were determined. The resultant data were compared to values predicted by the cooling power equation of the AMRR model. Conclusions from these experiments and suggestions for future work are presented.

Improving magnetic cooling efficiency and pulldown by varying flow profiles

Magnetic refrigeration systems are promising cooling solutions that employ the active magnetic regenerator refrigeration cycle to achieve practical temperature spans and environmental benefits. The hydraulic system that ensures a continuous flow of the heat transfer fluid through the system with a reciprocating flow in each regenerator bed is critical to the performance of the refrigeration cycle. Hence, we investigate the characteristics of the parallel flow circuit of a rotary active magnetic regenerator system, which consists of thirteen trapezoid-shaped regenerators, each filled with 295 g of gadolinium spheres. Fluid flow is controlled via electrically actuated solenoid valves (both piloted and direct-acting) connected to the regenerator hot side. By varying the percentage of opening of the control valves, different blow fractions (or fluid flow waveforms) could be investigated. The objective of the study is twofold: (i) assess whether flow imbalances of the heat transfer fluid exist in the cold-to-hot blow (cold blow) and hot-to-cold blow (hot blow) directions, and (ii) determine whether there is an optimal value of the blow fraction both to maximize the cooling performance and realize a rapid temperature pulldown. Flow resistance measurements demonstrate a symmetric flow circuit design and resistances that are similar in the cold and hot blow directions. Moreover, for the studied temperature spans of 6 K and 16 K, the best blow fraction was found to be about 41.6 %. For instance, at a 16 K span, a utilization of 0.32, and at 1.4 Hz, increasing the fluid blow fraction from 25.0 to 41.6 % enhanced the cooling capacity and second-law efficiency from 70 to 330 W and from 2.6 to 17.4 %, respectively. In turn, lower blow fractions favored a more rapid temperature pulldown. The magnetocaloric system was about 30 % faster in establishing approximately 14 K temperature span when the blow fraction was reduced from 41.6 to 30.6 %. Hence, magnetic refrigeration systems can benefit greatly from solenoid valves, which allow the system to operate either in a time-saving mode or an energy-saving mode.

Active magnetic regenerative refrigeration using superconducting solenoid for hydrogen liquefaction

A magnetic refrigerator that makes use of the magneto-caloric effect realizes a highly efficient cooling device. Since the cooling power of magnetic refrigerators depends largely on the strength of the magnetic field, the use of a superconducting magnet is essential. Using magnetic refrigeration, achieving a liquefaction efficiency of larger than 50% is theoretically possible, which is twice that of conventional gas expansion refrigerators. In this study, an active magnetic regenerative refrigerator, one of the magnetic refrigerators using a superconducting solenoid, was built and hydrogen liquefaction was successfully demonstrated.

Review of Multi-Physics Modeling on the Active Magnetic Regenerative Refrigeration

Compared to conventional vapor-compression refrigeration systems, magnetic refrigeration is a promising and potential alternative technology. The magnetocaloric effect (MCE) is used to produce heat and cold sources through a magnetocaloric material (MCM). The material is submitted to a magnetic field with active magnetic regenerative refrigeration (AMRR) cycles. Initially, this effect was widely used for cryogenic applications to achieve very low temperatures. However, this technology must be improved to replace vapor-compression devices operating around room temperature. Therefore, over the last 30 years, a lot of studies have been done to obtain more efficient devices. Thus, the modeling is a crucial step to perform a preliminary study and optimization. In this paper, after a large introduction on MCE research, a state-of-the-art of multi-physics modeling on the AMRR cycle modeling is made. To end this paper, a suggestion of innovative and advanced modeling solutions to study magnetocaloric regenerator is described.

Caloric Solid-State Magnetocaloric Cooling: Physical Phenomenon, Thermodynamic Cycles and Materials

Magnetic refrigeration is a promising and ecologic technology, alternative to the conventional vapor-compression refrigeration by the employment of solid-state materials as refrigerants instead of the fluid-state ones, own of vapour compression refrigeration. This emerging technology bases its operation on the MagnetoCaloric Effect (MCE), which is a physical phenomenon, related to solid-state materials with magnetic properties. For materials displaying simple magnetic ordering (i.e. paramagnetic to ferromagnetic phase transformations) a rapid increase in magnetic field causes a temperature rise in the material; likewise, a rapid reduction in the field causes cooling. This variation in temperature is called adiabatic temperature change. In 1982 the Active Magnetic Regenerative refrigeration cycle, well known as AMR cycle was introduced. The innovative idea considers a magnetic Brayton cycle but the main innovation consists of introducing the AMR regenerator concept, i.e. the employment of the magnetic material itself both as refrigerant and as regenerator. A secondary fluid is used to transfer heat from the cold to the hot end of the regenerator. Substantially every section of the regenerator experiments its own AMR cycle, according to the proper working temperature. Through an AMR one can appreciate a larger temperature span among the ends of the regenerator.

Mathematical Modelling of Active Magnetic Regenerator Refrigeration System for Design Considerations

A magnetic refrigeration system has the potential to alternate the compression system with respect to environmental compatibility. Refrigeration systems currently operate on the basis of the expansion and compression processes, while active magnetic refrigeration systems operate based on the magnetocaloric effect. In this study, a single layer of Gd was used as the magnetocaloric material for six-packed-sphere regenerators. A one-dimensional numerical model was utilized to simulate the magnetic refrigeration system and determine the optimum parameters. The optimum mass flow rate and maximum cooling capacity at frequency of 4 Hz are 3 L·min−1 and 580 W, respectively. The results show that the maximum pressure drop increased by 1400 W at a frequency of 4 Hz and mass flow rate of 5 L·min−1. In this study, we consider the refrigeration system in terms of the design considerations, conduct a parametric study, and determine the effect of various parameters on the performance of the system.

Temperature dependent thermal conductivity of magnetocaloric materials: Impact assessment on the performance of active magnetic regenerative refrigerators

Due to the dynamic nature of the active magnetic regenerative mechanism in magnetocaloric refrigeration, the thermal conductivity of the refrigerant is a critical parameter. Experimental studies have shown how the thermal conductivity of high-performance magnetic refrigerants can drastically change around their Curie temperatures (TC). However, this fact has been largely ignored in the numerical simulation of devices, raising the need to assess the impact of this approximation, particularly when the simulations are aimed at optimizing or dimensioning a particular device geometry. In this paper we show how, by employing a unidimensional numerical model of a magnetic refrigerator with parallel plates, two different temperature dependent thermal conductivity scenarios of the refrigerant affect the resulting temperature span and cooling power. By considering a gadolinium-like material as the refrigerant with thermal conductivities varying 50% near its TC, a change of the resulting device temperature span of ∼15% is reached. The cooling power is also affected, changing also ∼15% when the considered systems are at half their respective maximum temperature span. Our results are also discussed in terms of other geometries where the impact of these effects can be even larger, namely in cases where the axial thermal conduction in the AMR element is not negligible, or the time-scale of longitudinal thermal processes has a larger impact on the optimum operating frequency.

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.

Preparation of La0.7Ca0.3-xSrxMnO₃ Manganites by Four Synthesis Methods and Their Influence on the Magnetic Properties and Relative Cooling Power.

Manganites of the family La0.7Ca0.3−xSrxMnO3 were fabricated by four preparation methods: (a) the microwave-assisted sol-gel Pechini method; (b) sol-gel Pechini chemical synthesis; (c) solid-state reaction with a planetary mill; and (d) solid-state reaction with an attritor mill, in order to study the effect of the preparation route used on its magnetocaloric and magnetic properties. In addition, the manganites manufactured by the Pechini sol-gel method were compacted using Spark Plasma Sintering (SPS) to determine how the consolidation process influences its magnetocaloric properties. The Curie temperatures of manganites prepared by the different methods were determined in ~295 K, with the exception of those prepared by a solid-state reaction with an attritor mill which was 301 K, so there is no correlation between the particle size and the Curie temperature. All samples gave a positive slope in the Arrot plots, which implies that the samples underwent a second order Ferromagnetic (FM)–Paramagnetic (PM) phase transition. Pechini sol-gel manganite presents higher values of Relative Cooling Power (RCP) than the solid-state reaction manganite, because its entropy change curves are smaller, but wider, associated to the particle size obtained by the preparation method. The SPS technique proved to be easier and faster in producing consolidated solids for applications in active magnetic regenerative refrigeration compared with other compaction methods.

Performance investigation of a high-field active magnetic regenerator

Regenerative magnetic cycles are of interest for small-scale, high-efficiency cryogen liquefiers; however, commercially relevant performance has yet to be demonstrated. To develop improved engineering prototypes, an efficient modeling tool is required to screen the multi-parameter design space. In this work, we describe an active magnetic regenerative refrigerator prototype using a high-field superconducting magnet that produces a 100 K temperature span. Using the experimental data, a semi-analytic AMR element model is validated and enhanced system performance is simulated using liquid propane as a heat transfer fluid. In addition, the regenerator composition and fluid flow are simultaneously optimized using a differential evolution algorithm. Simulation results indicate that a natural gas liquefier with a 160 K temperature span and a second-law efficiency exceeding 20% is achievable.

Energy performances and numerical investigation of solid-state magnetocaloric materials used as refrigerant in an active magnetic regenerator

Magnetic is the most diffused, developed and evolved technique among solid-state cooling, a class of ecofriendly refrigeration systems employing solid-state caloric materials. Active Magnetic Regenerative refrigeration cycle (AMR), a Brayton based thermodynamical cycle, is the benchmark cycle for magnetic refrigeration. This paper aims to provide a map of energetic performances of an Active Magnetic Refrigerator by the development of a two-dimensional numerical model, whom replies the behaviour of one of the regenerators mounted in 8Mag, the experimental prototype of the first Italian Rotary Permanent Magnet Magnetic Refrigerator (RPMMR), mounting gadolinium. To this hope, through the model a performance map has been delineated and it has been investigated the behaviour of the AMR regenerator, mounting gadolinium, in order to explore the limit conditions of prototype working, in terms of cold-hot heat exchanger range, fluid flow rate and frequency. In a second step, the performance map has been extended to other MCE materials, possible candidates for magnetic refrigeration at room temperature.

Development of the active magnetic regenerative refrigerator operating between 77 K and 20 K with the conduction cooled high temperature superconducting magnet

The experimental investigation of an active magnetic regenerative refrigerator (AMRR) operating between 77 K and 20 K is discussed in this paper, with detailed energy transfer analysis. A multi-layered active magnetic regenerator (AMR) is used, which consists of four different rare earth intermetallic compounds in the form of irregular powder. Numerical simulation confirms that the AMR can attain its target operating temperature range. Magnetic field alternation throughout the AMR is generated by a high temperature superconducting (HTS) magnet. The HTS magnet is cooled by a two stage Gifford-McMahon (GM) cryocooler. Helium gas was employed as a working fluid and its oscillating flow in the AMR is controlled in accordance with the magnetic field variation. The AMR is divided into two stages and each stage has a different mass flow rate as needed to achieve the desired cooling performance. The temperature variation of the AMR during the experiment is monitored by temperature sensors installed inside the AMR. The experimental results show that the AMRR is capable of achieving no-load temperature of 25.4 K while the warm end temperature is 77 K. The performance of the AMRR is analyzed by observing internal temperature variations at cyclic steady state. Furthermore, numerical estimation of the cooling capacity and the temperature variation of the AMR are examined and compared with the experimental results.

Exergy Analysis of a Parallel-Plate Active Magnetic Regenerator with Nanofluids

This paper analyzes the energetic and exergy performance of an active magnetic regenerative refrigerator using water-based Al2O3 nanofluids as heat transfer fluids. A 1D numerical model has been extensively used to quantify the exergy performance of a system composed of a parallel-plate regenerator, magnetic source, pump, heat exchangers and control valves. Al2O3-water based nanofluids are tested thanks to CoolProp library, accounting for temperature-dependent properties, and appropriate correlations. The results are discussed in terms of the coefficient of performance, the exergy efficiency, and the cooling power as a function of the nanoparticle volume fraction and blowing time for a given geometrical configuration. It is shown that while the heat transfer between the fluid and solid is enhanced, it is accompanied by smaller temperature gradients within the fluid and larger pressure drops when increasing the nanoparticle concentration. It leads in all configurations to lower performance compared to the base case with pure liquid water.

Performance of the Fast-Ramping High Temperature Superconducting Magnet System for an Active Magnetic Regenerator

Fast magnetic field alternation is indispensable for continuous magnetic refrigeration. An active magnetic regenerative refrigerator (AMRR) utilizes magnetocaloric effect of magnetic materials which occurs during magnetization and demagnetization processes. A conduction cooled high temperature superconducting (HTS) magnet can be one of the prospective candidates as an alternating magnetic field generator. This paper describes the development effort of the cryogen-free HTS magnet for the AMRR. The magnet consists of twelve double pancake GdBCO coils which are insulated with polyimide tape. A two-stage GM cryocooler was employed to cool down the magnet. The critical current of the magnet was measured at the operating temperature before alternating current (AC) operation. In order to remove heat produced by AC loss with small temperature difference, thermal links between the cryocooler and the magnet were carefully designed. This paper presents the test results, AC loss analysis of the HTS magnet in the AMRR system. Maximum central magnetic field of 3 T (150 A) was achieved with the maximum ramping rate of 1 T/s (50 A/s). The AC loss was measured as 11.2 W at the operating conditions and the generated heate was effectively removed by the cryocooler. The AC loss was predicted by the numerical simulation and the simulation results were compared with the experimental results.

Sensitivity analysis and multiobjective optimization of a parallel-plate active magnetic regenerator using a genetic algorithm

This paper analyzes the energetic and exergetic performance of an active magnetic regenerative refrigerator. A 1D numerical model has been developed to simulate a system composed of a parallel-plate regenerator, magnetic source, pump, heat exchangers and control valves. The effects of several parameters like the mass flowrate and the plate and fluid thicknesses are studied. Various types of refrigerants are tested thanks to the CoolProp library, accounting for temperature-dependent properties. A multiobjective optimization based on a genetic algorithm is used to enhance the system performance regarding the Pareto efficiency. Parameters like mutation and crossover fraction are considered in order to determine the optimal results in a fair computational time. The main goals are to maximize the coefficient of performance, the exergetic efficiency, and the cooling power while respecting precise constraints. The best tuning leads to a solution 9 times faster than mapping all possibilities using a rough grid size.