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Highlights • A series of magnetic beds with different geometry and constant mass of MCM is tested. • Results relate the temperature span to bed length, flow rate, and volume of medium. • Dependence of the temperature span on the regenerator bed length is observed. • Advanced geometries of the regenerator bed use the magnet gap more efficiently. Abstract Presently the protection of the natural environment is a key issue. Particular emphasis is placed on the elimination of equipment negatively impacting human surroundings. One modern cooling method that is very efficient and environmentally friendly is based on the magnetocaloric effect. Although numerous prototypes employing this phenomenon have been developed, there is still a need to increase its refrigeration capability and decrease the manufacturing price. The main part in this kind of apparatus is a regenerator bed. Its geometry and the method of filling in with magnetocaloric material impacts the performance of the equipment. The presented paper focuses on the analysis of the influence of the regenerator bed shape on the cooling performance of the system. Six different bed geometries, all with the same amount of gadolinium, were designed, manufactured, and tested on a prototype test stand operating in an AMR cycle. The influence of flow rate and volume of the heat transfer medium on the obtained maximum temperature span was analyzed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this first part of a two-part paper, a new active magnetic regenerator (AMR) laboratory apparatus is presented and evaluated. The setup is composed of a nested Halbach cylinder magnetic circuit (maximum magnetic flux density of 1.69 T) assembled in phase with a double effect displacer that provides the cold and hot blows to the regenerator. A single packed-bed regenerator with 195.5 g of gadolinium spheres is used in a discontinuous (i.e., reciprocating) cycle. The system performance is evaluated in terms of characteristic curves (i.e., cooling capacity as a function of temperature span), coefficient of performance and second-law efficiency as a function of the utilization factor and operating frequency. Maximum values of COP have been identified for a given temperature span. The maximum values of second-law efficiency were obtained at system temperature spans between 15 and 20 K. [ABSTRACT FROM AUTHOR]

In this paper, a novel rotary magnetic refrigerator is described, and initial experimental results are presented. The prototype employs a two-pole magnetic system based on a double U configuration of permanent magnets with an air gap of 43 mm and 1.25 T. The magnetocaloric refrigerant is confined within eight static regenerator enclosures, which are alternatively magnetized and demagnetized with the rotation of the magnets. A rotary valve mechanically coupled with the field generator imparts the direction of heat transfer fluid through the regenerators. Using a total mass of 1.20 kg of gadolinium spheres (of 400-500 microns) as refrigerant and distilled water as regenerating fluid, the device produced a maximum temperature span of 13.5 K under zero applied thermal load with an utilization factor of 1.40 and a heat rejection temperature of 298 K. [ABSTRACT FROM AUTHOR]

A numerical time-dependent model of an active magnetic regenerator (AMR) was developed for cooling in the kilowatt range. Earlier numerical models have been mostly developed for cooling power in the 0.4 kW range. In contrast, this paper reports the applicability of magnetic refrigeration to the 50 kW range. A packed bed active magnetic regenerator was modelled and the influence of parameters such as geometry and operating parameters were studied for different geometries. The pressure drop for AMR bed length and particle diameter was also studied. High cooling power and coefficient of performance (COP) were achieved by optimization of the diameter of the magnetocaloric powder particles and operating frequency. The optimum operating conditions of the AMR for a cooling capacity of 50 kW was determined for a temperature span of 15 K. The predicted coefficient of performance (COP) was found to be ~6, making it an attractive alternative to vapour compression systems. [ABSTRACT FROM AUTHOR]

Abstract: The increasing demand for energy has driven society to explore different sources of energy, seeking to achieve sustainability. In this sense, we designed a thermal machine whose energy source is solar, which was based on the working principle of the Curie thermomagnetic motor. Thus this paper describes the modeling and simulation of a Curie wheel. Two materials were analyzed for the rotor, 30% NiFe alloy and Gd. The heat transfer inherent to the conversion process of solar energy and natural convection were studied, utilizing a one-dimensional finite difference model linked with the rotor magnetization in the magnetic circuit. We verified that for both materials the best performance was achieved with a convection coefficient of 10 W m−2 K−1. Furthermore, Gd offers about one order of magnitude greater efficiency when compared to the 30% NiFe alloy, but unfortunately the actual value is still too small to be commercially implementable. Nevertheless, this result indicates that better magnetic materials with Curie temperatures slightly above ambient are the key to achieving higher efficiencies, and, accordingly, magnetocaloric materials research has much to add. [Copyright &y& Elsevier]

Abstract: A laboratory test stand for magnetocaloric effect investigations has been developed. The test stand is compact and easily reconfigurable. Gadolinium in the form of particles is used as a refrigerant. The material is magnetized/demagnetized due to the reciprocating motion of a magnetic bed. A Halbach array of permanent magnets is employed as a magnetic field source. It generates a magnetic field of about 1 T. In order to decrease the distance along which the magnetic bed has to move, a magnetic shield was used which limits the range of external magnetic field influence. The effectiveness of the shielding and the decrease in magnetic field intensity were shown in the form of magnetic field distribution maps. The paper presents first experimental results which are measured as the temperature difference between the two outermost reservoirs. The achieved results are promising – the temperature span between the heat exchangers amounts to about 2 °C. [Copyright &y& Elsevier]

Abstract: The paper presents the numerical optimization of a packed-bed and a parallel-plate AMR with gadolinium as the magnetocaloric material. The diameter of the spheres and the length of the packed-bed AMR, and the plate thickness, the spacing between the plates (porosity) and the length of the parallel-plate AMR, were varied and optimized. The results reveal that for the considered conditions the optimum length of the packed-bed AMR should be 20 mm or less at higher operating frequency (3 Hz) and 40 mm at lower operating frequency (0.5 Hz) and the corresponding optimum sphere diameter should be between 0.07 mm (with regard to the cooling load) and 0.17 mm (with regard to the COP). The parallel-plate AMR should consist of plates that are as thin as possible, with a spacing between the plates of 0.035 mm (with regard to the cooling load) and 0.075 mm (with regard to the COP). [Copyright &y& Elsevier]

Abstract: In this paper, a reciprocating magnetic refrigeration prototype (MR) with a novel and simple design is presented using permanent magnets for operation at room temperature. The design, installation, operation and preliminary results are reported. The experimental machine is characterised by the implementation of a force compensation system applied on the displacement of the active magnetic regenerator (AMR). Preliminary calculations based on experimental measurements show that approximately 90% of the displacement force is reduced with the proposed design. Using a total of 180 g of Gd as magnetocaloric refrigerant in a magnetic field of 1 T, the device achieved a maximum temperature span under no thermal load of 3.5 K, rendering a cooling power of about 3 W under a span approaching 0 K. [Copyright &y& Elsevier]

Abstract: The primary objective of this paper is to quantify the impact of the internal magnetic field and reversibility of the magnetocaloric effect on the numerical evaluation of the thermal performance of a parallel-plate active magnetic regenerator. To this end, direct measurements of the adiabatic temperature change of commercial-grade gadolinium samples were carried out for an applied field change of 1.65 ± 0.03 T (for both magnetization and demagnetization) and temperatures between 283 and 303 K. The tests were carried out in a specially constructed apparatus and the results were in good agreement with the literature. The data were incorporated into a mathematical model for solving the fluid flow and conjugated heat transfer in the parallel-plate regenerator. It was found that if demagnetization phenomena (i.e., the demagnetization factor and the reversibility of the magnetocaloric effect) are not take properly into account, the thermal performance of the regenerator can be severely over-predicted. [Copyright &y& Elsevier]

Abstract: The simulation of magnetocaloric refrigerator behaviour needs a good description of the material properties. Magnetocaloric material data are often given as a function of few values of the external magnetic field applied on a sample, but active material reacts to its internal field. The simulating of the magnetocaloric effect using experimental data can then create some artefacts. In this paper we present an alternative way to obtain material data expressed as a function of the internal magnetic field. This characterization is built on the knowledge of the zero magnetic field heat capacity of second-order phase transition materials such as gadolinium, as well as on the direct measurements of magnetocaloric data. An inverse approach is performed to calculate the data as a function of the internal magnetic field with an improved level of detail. The obtained data allow a better modelling of the magnetocaloric effect in an active magnetic regenerator. [Copyright &y& Elsevier]

Abstract: The present paper describes the design, operation and preliminary results of an active magnetic regenerator test machine. The regenerator consists of twenty eight 0.85 mm thick parallel gadolinium (Gd) plates that form 0.1 mm thick parallel channels with an effective length of 126 mm. The working fluid is water and the magnetic field source is a Halbach array with a maximum flux density of 1.65 T. A maximum temperature span between the hot and cold ends of the regenerator (∆T REG) of 4.45 K and a maximum cooling capacity of 3.9 W were measured in tests without and with a thermal load, respectively. The behavior of ∆T REG as a function of the utilization factor and of the temperature of the hot source is explored in the manuscript. The experimental results are discussed in the light of the fluid flow and heat transfer phenomena taking place in the regenerator. [Copyright &y& Elsevier]

Abstract: A new, fast and flexible, time-dependent, one-dimensional numerical model was developed in order to study in detail the operation of an active magnetic regenerator (AMR). The model is based on a coupled system of equations (for the magnetocaloric material and the heat-transfer fluid) that have been solved simultaneously with the software package MATLAB. The model can be employed to analyze a wide range of different operating conditions (mass-flow rate, operating frequency, magnetic field change), different AMR geometries, different magnetocaloric materials and heat-transfer fluids, layered and single-bed AMRs, etc. This paper also presents an optimization of the AMR’s geometry, where the AMR consists of a packed-bed of grains (spheres) of gadolinium (Gd). The optimization of the mass-flow rate and the operating frequency of the AMR were performed by studying five different diameters of Gd spheres. [Copyright &y& Elsevier]

Abstract: The active magnetic regenerator (AMR) is an alternative refrigeration cycle with a potential gain of energy efficiency compared to conventional refrigeration techniques. The AMR poses a complex problem of heat transfer, fluid dynamics and magnetic field, which requires detailed and robust modeling. This paper reviews the existing numerical modeling of room temperature AMR to date. The governing equations, implementation of the magnetocaloric effect (MCE), fluid flow and magnetic field profiles, thermal conduction etc. are discussed in detail as is their impact on the AMR cycle. Flow channeling effects, hysteresis, thermal losses and demagnetizing fields are discussed and it is concluded that more detailed modeling of these phenomena is required to obtain a better understanding of the AMR cycle. [Copyright &y& Elsevier]

Abstract: The commercialization of magnetic refrigerators depends upon the ability to meet performance targets while having acceptable equipment costs. This paper links device design parameters and performance of magnetic refrigerators to the cost of cooling delivered. A device configuration parameter, D, is defined that links the field volume to the volume of magnetocaloric material. Combined with the magnet performance parameter, efficiency, and specific exergetic cooling, the cost structure of a magnetic refrigerator is determined. Some magnetic refrigerators reported in the literature are classified using their configuration parameters, and are then compared in terms of demonstrated performance using results available in the literature. The required improvement in performance is calculated such that the cost of a magnetic refrigerator would be equivalent to a conventional compressor-based device. Finally, some of the reasons for different performance are discussed with a focus on opportunities for improvements. [ABSTRACT FROM AUTHOR]

Abstract: A team of researchers have developed and built a prototype of a rotary magnetic refrigerator (MR). The principle of the operation of the presented magnetic refrigerator is based on the rotary movement of active magnetic regenerators (AMRs) with a magnetocaloric material and on the magnetic field generated by permanent magnets Nd–Fe–B. The first part of the paper presents the development and analysis of the structure for generating the magnetic field and the basic operational principle of the magnetic refrigerator concerned, and the second part gives a description of the development process for certain key elements of the complete magnetic refrigeration system. [Copyright &y& Elsevier]