Parallel-plate Active Magnetic Regenerators Research Articles

Experimental study on cooling performance of active magnetic regenerators with different structures

Using the distilled water as the heat transfer fluid, this paper built an active magnetic regenerator test bench to investigate the influence of crucial parameters on the cooling performance of active magnetic regenerators with different geometries, including the relative angle between Gd plates and magnetic field, the utilization factor, the temperature at the hot side of the regenerator, and the operating frequency. In addition, the performance enhancement potential of the newly proposed perforated parallel-plate regenerator was experimentally studied. The impacts of the perforation ratio and perforation on different thicknesses of plates were discussed. For the parallel-plate active magnetic regenerator with a thickness of 0.4 mm and a plate spacing of 0.35 mm, the 3.4% perforation ratio increases the temperature span between the hot and cold ends of the regenerator by 14.7%∼24.7% at no load condition from the baseline none-perforated parallel-plate regenerator.

Numerical simulation of a hybrid system using Peltier thermal switches in magnetic refrigeration

• A 2D transient model of a hybrid magnetic refrigeration system using Peltier modules as the heat switches was established. • The actual transient thermal response of the Peltier modules was considered in the model. • The comparison of the hybrid system with the AMR mode and the Peltier mode was done. • At 4.00 Hz and 0.20 A, the COP and cooling power density of the hybrid system exceeded that of the pure AMR mode. • The influence of operating parameters on the hybrid system performance was evaluated. To overcome the shortcoming of the low cooling power density in the ordinary magnetic refrigerator, a hybrid system using Peltier thermal switches in magnetic refrigeration has been proposed by researchers. In the system, the magnetic material is sandwiched between Peltier modules which are used as the thermal switches to transport the heat to and from the material undergoing the magnetic refrigeration cycle. To analyze the system, a two-dimensional transient model is established by considering the thermal characteristics of the Peltier module, which has not been done before. With the model, the performance of the hybrid system was firstly compared with that using the pure parallel plate active magnetic regenerator (AMR) mode and that using the pure Peltier mode. Compared with pure parallel plate AMR mode, the hybrid system using Peltier thermal switches in magnetic refrigeration can significantly increase the cooling power density at low frequencies (1.60 Hz – 4.00 Hz). In this frequency range, the coefficient of performance (COP) of pure parallel plate AMR mode decreased rapidly with the increase of frequency, while the COP of the hybrid system decreased slowly, and at 4.00 Hz and 0.20 A, the COP of the hybrid exceeded that of the pure parallel plate AMR mode. Compared with pure Peltier mode, the hybrid system can achieve the same cooling power with less power consumption and obtain a higher COP. Furthermore, the influence of operating parameters on the performance of the hybrid system, which included utilization factor, input current, the outlet temperature of the cold-end heat exchanger, frequency, and the number of refrigeration units, was investigated. The results show that, when using Peltier thermal switches, the actual transient thermal response of the Peltier module plays an important role in the hybrid system, which is ignored in previous literature. To achieve efficient operation at high frequency, when using Peltier thermal switches, the thermal response time of the Peltier modules as well as the Joule heating effect, etc, needs to be carefully evaluated.

Electrocaloric cooling over high device temperature span

Summary Electrocaloric cooling demonstrates direct electricity utilization and high energy efficiency, and is often proposed as an environmentally benign, solid-state alternative to the conventional vapor compression cooling technology. As a result, the fabrication of effective electrocaloric materials and engineering of electrocaloric cooling devices have attracted increasing attention in the cooling community. However, the limited material adiabatic temperature change under safe operation field poses a major challenge to practical electrocaloric cooling device development, since real-world applications usually require device temperature span over 20 K. This perspective presents recent efforts devoted to design electrocaloric cooling devices based on active heat regeneration and cascading approaches; both strategies have achieved significant device temperature lift ~10 K with adiabatic temperature change of 2~3 K. The strategies discussed in this work are broadly applicable to the design of efficient and compact cooling systems.

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.

Entropy generation in a parallel-plate active magnetic regenerator with insulator layers

This paper proposes a feasible solution to diminish conduction losses in active magnetic regenerators. Higher performances of these machines are linked to a lower thermal conductivity of the Magneto-Caloric Material (MCM) in the streamwise direction. The concept presented here involves the insertion of insulator layers along the length of a parallel-plate magnetic regenerator in order to reduce the heat conduction within the MCM. This idea is investigated by means of a 1D numerical model. This model solves not only the energy equations for the fluid and solid domains but also the magnetic circuit that conforms the experimental setup of reference. In conclusion, the addition of insulator layers within the MCM increases the temperature span, cooling load, and coefficient of performance by a combination of lower heat conduction losses and an increment of the global Magneto-Caloric Effect. The generated entropy by solid conduction, fluid convection, and conduction and viscous losses are calculated to help understand the implications of introducing insulator layers in magnetic regenerators. Finally, the optimal number of insulator layers is studied.

Application of magnetic cooling in electric vehicles

The features of an active magnetic regenerator refrigerator are determined for its application in mobile air-conditioning systems. The thermal requirements of an electric vehicle have first been obtained and result in a cooling demand of 3.03 kW at a temperature span of 29.3 K. A comprehensive parametric study has been conducted in order to find the active magnetic regenerator refrigerator design and working parameters that fulfill the vehicle needs with a minimum electric consumption and device mass. Specifically, a permanent-magnet parallel-plate active magnetic regenerator refrigerator made of Gd-like materials is considered. According to the possibilities of current prototypes, in the study the cycle frequencies have been limited to 10 Hz and the applied magnetic fields, to 1.4 T. The results show that an active magnetic regenerator refrigerator made of plates between 30 and 40 μm thick and channels between 20 and 40 μm high could meet the vehicle demand with a coefficient of performance between 2 and 4 and a total mass between 20 and 50 kg. Compared to vapor-compression devices for mobile air-conditioning systems (coefficient of performance = 2.5 and mass 12 to 15 kg), the active magnetic regenerator refrigerator works optimally with fluid flow rates at least three times larger. In order to integrate active magnetic regenerator refrigerators into mobile air-conditioning systems, the hydraulic loops should be consequently redesigned.

An efficient numerical scheme for the simulation of parallel-plate active magnetic regenerators

A one-dimensional model of a parallel-plate active magnetic regenerator (AMR) is presented in this work. The model is based on an efficient numerical scheme which has been developed after analysing the heat transfer mechanisms in the regenerator bed. The new finite difference scheme optimally combines explicit and implicit techniques in order to solve the one-dimensional conjugate heat transfer problem in an accurate and fast manner while ensuring energy conservation. The present model has been thoroughly validated against passive regenerator cases with an analytical solution. Compared to the fully implicit scheme, the proposed scheme achieves more accurate results, prevents numerical errors and requires less computational effort. In AMR simulations the new scheme can reduce the computational time by 89%.

Experimental comparison of multi-layered La–Fe–Co–Si and single-layered Gd active magnetic regenerators for use in a room-temperature magnetic refrigerator

We present experimental comparison of different parallel-plate active magnetic regenerators (AMRs) with two different groups of magnetocaloric materials. First, a gadolinium-based single-layered AMR was tested and analysed under different operating conditions. In the next step, three different multi-layered AMRs with different compositions and different Curie temperatures made from LaFe13 − x − yCoxSiy materials were constructed and tested. Seven-, four- and two-layered La-based AMRs were evaluated. The measurements were performed with respect to the maximum measured temperature span under different operating conditions and the cooling load under different temperature spans. In order to find the optimum operating temperature range the AMRs were further compared for different hot-side temperatures. The Gd-based AMR produced a larger temperature span, especially at higher operating frequencies and higher mass-flow rates. Among the multi-layered La–Fe–Co–Si AMRs, the seven- and four-layered AMRs showed very similar characteristics, while the two-layered AMR was much poorer.

A numerical comparison of a parallel-plate AMR and a magnetocaloric device with embodied micro thermoelectric thermal diodes

This article introduces a novel heat-transfer mechanism for magnetocaloric energy conversion based on the introduction of thermoelectric thermal diodes. These can provide the very rapid transport of heat from/to the magnetocaloric material. For the purposes of the study, a new mathematical-numerical model has been developed. The model serves for dynamic heat-transfer simulations and an estimation of the cooling characteristics of magnetocaloric devices, which apply thermal diodes. Micro Peltier elements were considered as the thermal diodes. A theoretical analysis was performed in order to compare the performance of a parallel-plate active magnetic regenerator with a magnetocaloric device that uses thermal diodes. The results show the great potential for magnetocaloric devices with thermal diodes, especially in terms of an increased operating frequency and specific cooling powers.

Improved modelling of a parallel plate active magnetic regenerator

Much of the active magnetic regenerator (AMR) modelling presented in the literature considers only the solid and fluid domains of the regenerator and ignores other physical effects that have been shown to be important, such as demagnetizing fields in the regenerator, parasitic heat losses and fluid flow maldistribution in the regenerator. This paper studies the effects of these loss mechanisms and compares theoretical results with experimental results obtained on an experimental AMR device. Three parallel plate regenerators were tested, each having different demagnetizing field characteristics and fluid flow maldistributions. It was shown that when these loss mechanisms are ignored, the model significantly over predicts experimental results. Including the loss mechanisms can significantly change the model predictions, depending on the operating conditions and construction of the regenerator. The model is compared with experimental results for a range of fluid flow rates and cooling loads.

Geometrical optimization of packed-bed and parallel-plate active magnetic regenerators

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).

The effect of flow maldistribution in heterogeneous parallel-plate active magnetic regenerators

The heat transfer properties and performance of parallel-plate active magnetic regenerators (AMRs) with heterogeneous plate spacing are investigated using detailed models previously published. Bulk heat transfer characteristics in the regenerator are predicted as a function of variation in plate spacing. The results are quantified through a Nusselt number scaling factor that is applied in a detailed 1D AMR model. In this way, the impact of flow maldistribution due to heterogeneous parallel plate stacks on AMR performance is systematically investigated. It is concluded that parallel-plate stacks having a standard deviation greater than about 5% on their plate spacing are severely penalized in terms of both cooling power and achievable temperature span due to the inhomogeneity of the stacks.

A comprehensive experimental analysis of gadolinium active magnetic regenerators

The main goal of this study was an experimental comparison of six different active magnetic regenerators (AMRs) with gadolinium as the magnetocaloric material. The analysis was carried out for three different parallel-plate AMRs (with different porosities and different orientations of the plates in the magnetic field) and three different packed-bed AMRs (filled with spheres, powders and cylinders). Since the operation of an AMR is strongly affected by the operating conditions, the experiments were performed at different mass-flow rates and at different operating frequencies. These were required in order to define the optimum corresponding operating conditions for the analyzed AMRs. As comparative criteria the maximum temperature span, the cooling capacity and the experimentally predicted COP were taken into consideration.The experimental analysis was performed on a new prototype of magnetic refrigerator designed as an experimental device. Its operation is based on the linear movement of a permanent-magnet assembly over a static AMR. The magnet assembly provides a measured magnetic field of about 1.15 T.The results reveal that the geometry of the AMR (the form of the magnetocaloric material) has a crucial impact on the performance of the magnetic refrigerator. The best overall cooling characteristics (temperature span, cooling capacity and COP) were obtained for the parallel-plate AMR with the smallest porosity (∼25%) and the orientation of the plates parallel to the magnetic field. This particular AMR generated a temperature span of 20 K, which is also the largest, so-far measured and published temperature span with a parallel-plate AMR for a given magnetic field change generated by permanent magnets. With respect to the comparison of the experimentally predicted COP values, the parallel-plate AMRs show higher efficiencies than the packed-bed AMRs.

The influence of demagnetizing effects on the performance of active magnetic regenerators

Active magnetic regenerators (AMR) comprise an involved, multi-physics problem including heat transfer, fluid flow, magnetocaloric properties, and demagnetizing fields. In this paper a method is developed that combines previously published models that simulate a parallel-plate AMR and the magnetostatics of a stack of parallel plates, respectively. Such a coupling is non-trivial due to the significant increase in computational time, and a simplified scheme is thus developed and validated resulting in little extra computational effort needed. A range of geometrical and operating parameters are varied, and the results show that not only do demagnetizing effects have a significant impact on the AMR performance, but the magnitude of the effect is very sensitive to a range of parameters such as stack geometry (number of plates, dimensions of the plates and flow channels, and overall dimensions of the stack), orientation of the applied field, and the operating conditions of the AMR (such as thermal utilization).

Assessment of demagnetization phenomena in the performance of an active magnetic regenerator

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.