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We present the open source micromagnetic framework, MagTense, which utilizes a novel discretization approach of rectangular cuboid or tetrahedron geometry “tiles” to analytically calculate the demagnetization field. Each tile is assumed to be uniformly magnetized, and from this assumption only, the demagnetization field can be analytically calculated. Using this novel approach we calculate the solution to the mag standard micromagnetic problems 2, 3 and 4 and find that the MagTense framework accurately predicts the solution to each of these. Finally, we show that simulation time can be significantly improved by performing the dense demagnetization tensor matrix multiplications using NVIDIA CUDA.

Four different magnet bearing configurations, with varying numbers of combined radially and axially magnetized rings, are studied both experimentally and using the modeling framework MagTense. First the optimal vertical position of the radially magnetized ring is determined using the numerical model. Then the model is validated using experimental data. Finally, we show that a bearing where the rotor and the stator each consist of a single ring of axially magnetized magnets and a single ring of radially magnetized magnets has the smallest radial force per axial lift force at an axial air gap of 1 mm. For this bearing, the ratio of the radial force per axial lift here is 0.383 times that of the same bearing without the radially magnetized ring.

We show that low ramp rate differential scanning calorimetry of the magnetocaloric material La ( Fe 11.47 Si 1.28 Mn 0.25 ) H 1.65 at different applied magnetic fields reveals the presence of distributed phase transitions. Experimentally, we find that with or without an applied magnetic field, samples show a distinct peak pattern in their heat capacity around the transition temperature ( T t ≈ 30 ° C), i.e., multiple heat capacity peaks occur as a function of sample temperature. Additionally, these reproducible patterns occur asymmetrically when heating and cooling. At finite applied fields higher than 0.15 T, we observe clearly distinguishable peaks of identical shape, albeit with different intensities. According to the latter, we re-identify the peaks under seven applied magnetic fields up to 1 T. We find that the peaks shift differently relative to each other as a function of field. In particular, for cooling experiments, the peak temperatures vary linearly in the field, although with different slopes. Through Bean–Rodbell (BR) modeling, we show that the experimentally observed behavior can be simulated by small decoupled variations in the BR parameters η and T 0, indicating a distributed composition of the magnetocaloric material.

A novel freeze-casting device utilizing a thermoelectric element for high precision temperature control allowing for dynamic freezing conditions of freeze-cast materials is presented. Freeze-casting is a processing route for producing materials of anisotropic porosity in the form of aligned and well-defined microchannels. In freeze-casting, particulates of a material are suspended in a fluid and a thermal gradient is applied across for directional freezing. Controlling the thermal gradient across the suspension amounts to controlling the kinetics and freezing direction in the suspension and thus the resulting structural features and dimensions of the microchannels. The performance of the device presented here was evaluated by directional freezing of both water and aqueous ceramic suspension samples using both linear and exponential freezing profiles. The freezing front was successfully tracked by continuously measuring the temperature gradient along the sample using thermocouples directly mounted on the freeze-casting mold. The current minimum operational temperature of the freeze-caster is ∼220 K, with freezing front velocities in the range of ∼5 μm/s to 30 μm/s for sample lengths of 5 mm–25 mm.

We investigate whether field homogeneity of a magnetic assembly can be optimized by varying the remanence of its constituting magnetic segments. We specifically study this hypothesis for a Halbach cylinder using a numerical model, MagTense. We consider a Halbach cylinder consisting of six layers of three concentric rings, each ring made from 16 segments. We show that ideally, the homogeneity can reach close to 1 ppm for a finite magnet. We then proceed to consider a real world set of magnet segments, i.e. non-ideal magnets with a variation in their remanence. This reduces the field homogeneity to about 1000 ppm when considering a Gaussian perturbation of the remanence with a standard deviation of 1%. However, we also show that the reduction in homogeneity may be countered by organizing the magnet pieces found through optimization, which is possible if each magnet segment is well characterized experimentally. We note that the presented method is applicable to any case where homogeneity of the field is important. The results we present are considered for the specific case of nuclear magnetic resonance for concretenes.

The heat transfer and flow resistance of a novel freeze-cast porous regenerator of the magnetocaloric ceramic La0.66Ca0.27Sr0.06Mn1.05O3 was experimentally characterized. Such a porous architecture may be useful as a regenerator geometry in magnetic refrigeration applications due to the sub-millimeter hydraulic diameters that can be achieved. Here the heat transfer effectiveness and friction losses are characterized using experiments and processed with a 1D numerical model. Empirical correlations of the friction factor and Nusselt number are reviewed and chosen for modelling the specific geometry. The experimental results show that the freeze-cast regenerator has increased heat transfer effectiveness and pressure drop compared to reference packed bed regenerators made from epoxy bonded spherical and irregular particles, as well as packed, unbonded spheres. Fixing the pressure drop and regenerator size, the freeze-cast regenerator achieves 10-15% higher heat transfer performance compared to packed bed regenerators.

Micromagnetic simulations are employed for predicting the behavior of magnetic materials from their microscopic properties. In this paper we focus on hysteresis loops, which are computed by assuming quasistatic conditions: i.e. the magnetization distribution remains at equilibrium while the applied magnetic field is slowly varied. The dynamic behavior of micromagnetic systems is governed by the Landau-Lifshitz equation. In order to apply the dynamic equation to a quasistatic problem, it is necessary to artificially decouple the relaxation dynamics from the time-scale of the variation of the applied field. This decoupling is normally done in an iterative fashion: the field is considered fixed until the equilibrium point is reached, and subsequently updated. However, this approach is indirect and also has the potential issue that a system might switch to a different equilibrium configuration before the previous equilibrium becomes unstable, which is a behavior not possible in the quasistatic regime. Instead, here we derive the differential equation, which directly describes the evolution of the equilibrium states of the Landau-Lifshitz equation as a function of the external field, or any other externally varied parameter. This approach is a more rigorous description of quasistatic processes and inherently enforces the system to follow a given equilibrium configuration until this disappears or becomes unstable. We demonstrate this approach with simple examples and show it to be as or more stable than the previously used approaches.

The optimal segmentation of three-dimensional permanent-magnet systems into uniformly magnetized blocks to generate a desired field is a question that has been studied extensively in recent years. We present a procedure that generates the theoretically optimal shape and magnetization direction of all the magnet segments in the system, given only the total number of blocks as a constraint. Materials of arbitrary magnetic permeability can be included in this optimization framework. As an optimization objective we consider any functional that is linear with respect to the magnetic field distribution. We furthermore assume that all magnetic materials obey a linear constitutive relation. We show that with these assumptions calculating an optimal three-dimensional segmentation is equivalent to determining a centroidal Voronoi tessellation. We use the most-well-known and most-straightforward algorithm for the generation of centroidal Voronoi tessellations, known as "Lloyd's method," which can also be thought of as a fixed-point iteration. We show that the procedure is guaranteed to lead to a configuration that is at least locally optimal with respect to the assumed linear objective functional. However, we present results providing a strong indication that for most design problems applying our technique a few times starting from different random configurations is very likely to lead to the globally optimal solution.

The stray- and demagnetization field tensor for a homogeneously magnetized tetrahedron is found analytically. The tetrahedron is a special case of four triangular faces with constant magnetization-charge surface density, for which we also determine the tensor field. The tensor field is implemented in the open source micromagnetic and magnetostatic simulation framework MagTense and compared with the obtained magnetic field from an FEM solution, showing excellent agreement. This result is important for modeling magnetostatics in general and for micromagnetism in particular as the demagnetizing field of an arbitrary body discretized using conventional meshing techniques is significantly simplified with this approach.

We present the engineering of directional porosity in the form of lamellar micro-channels in the magnetocaloric ceramic of La0.66Ca0.27Sr0.06Mn1.05O3 (LCSM) by freeze-casting. Freeze-casting is a templating technique based on the anisotropic growth of ice crystals in aqueous suspensions upon directional freezing, which, when applied to a suspension of LCSM results in hierarchical structures of aligned porosity in the form of micro-channels with widths of 5 μm to 20 μm. Channel sizes and tortuosity are measured and calculated from analysis of SEM images obtained at cross sections perpendicular and parallel to the freezing direction, respectively, while freezing conditions are monitored by temperature measurements. We propose that freeze-casting demonstrate apparent applicability within processing of ceramic materials for application as regenerator for magnetic refrigeration.

Epoxy bonded regenerators of both spherical and irregular La(Fe,Mn,Si)13Hy particles have been developed aiming at increasing the mechanical strength of active magnetic regenerators (AMR) loaded with brittle magnetocaloric materials and improving the flexibility of shaping the regenerator geometry. Although the magnetocaloric properties of these materials are well studied, the flow and heat transfer characteristics of the epoxy bonded regenerators have seldom been investigated. This paper presents a test apparatus that passively characterizes regenerators using a liquid heat transfer fluid with an oscillating flow at low Reynolds numbers, simulating the hydraulic working conditions in AMRs. Dimensionless parameters, including friction factor, effectiveness and overall Nusselt number, are presented for the epoxy bonded La(Fe,Mn,Si)13Hy regenerators and reference packed particle beds. Moreover, a five-layer AMR based on spherical particles is tested actively in a small reciprocating magnetic refrigerator, achieving a no-load temperature span of 16.8 °C using about 143 g of epoxy-bonded La(Fe,Mn,Si)13Hy materials. Simulations based on a one-dimensional (1D) AMR model are also implemented to validate and analyze the results from the active test.

The maximum efficiency for photovoltaic (PV) and thermoelectric generator (TEG) systems without concentration is investigated. Both a combined system where the TEG is mounted directly on the back of the PV and a tandem system where the incoming sunlight is split, and the short wavelength radiation is sent to the PV and the long wavelength to the TEG, are considered. An analytical model based on the Shockley-Queisser efficiency limit for PVs and the TEG figure of merit parameter zT is presented. It is shown that for non-concentrated sunlight, even if the TEG operates at the Carnot efficiency and the PV performance is assumed independent of temperature, the maximum increase in efficiency is 4.5 percentage points (pp.) for the combined case and 1.8 pp. for the tandem case compared to a stand alone PV. For a more realistic case with a temperature dependent PV and a realistic TEG, the gain in performance is much lower. For the combined PV and TEG system it is shown that a minimum zT value is needed in order for the system to be more efficient than a stand alone PV system.

We present a numerical model that can simulate a magnetocaloric sample on the grain size level, including magnetostatics, heat transfer, local hysteresis and spatial variation of stoichiometry expressed as a variation in Curie temperature, . Grain structure of a sample is realised as a number of regions each having a uniform and defined through a Voronoi-map. We show that demagnetising effects, caused by a finite sample size, and spatial variation in can account for the previously experimentally observed 'virgin' effects in the adiabatic temperature change and isothermal entropy change, respectively and first order reversal effect as a function of temperature. We conclude that even a very little variation in local stoichiometry of less than a percent, corresponding to a standard deviation in of for has a significant impact on the overall properties and history dependence of a sample.

We present a combination of numerical models that can together simulate the initial packing of particles, followed by sintering and finally the resulting microstructural properties. For the latter we here focus on the magnetism of a sintered sample, and the associated coupling between heat and magnetism known as the magnetocaloric effect. We present a 3-dimensional time-dependent numerical model that spatially resolves samples down to the grain size, and includes the demagnetizing field, chemical inhomogeneity realized as a spatial variation of Curie temperature across the sample, local hysteresis and heat transfer. We can thus model how particle size, packing, sintering and chemical inhomogeneity affect the observed properties of magnetocaloric samples. For example, we show that even a modest distribution in Curie temperature (TC) across the sample results in a significant broadening and lowering of the total entropy change of the sample around TC. We discuss how clustering of grains with similar values of TC across the sample influences the results.

Room temperature magnetic refrigeration has attracted substantial attention during the past decades and continuing to increase the performance of active magnetic regenerators (AMR) is of great interest. Optimizing the regenerator geometry and related operating parameters is a practical and effective way to obtain the desired cooling performance. To investigate how to choose and optimize the AMR geometry, a quantitative study is presented by simulations based on a one-dimensional (1D) numerical model. Correlations for calculating the friction factor and heat transfer coefficient are reviewed and chosen for modeling different geometries. Moreover, the simulated impacts of various parameters on the regenerator efficiency with a constant specific cooling capacity are presented. An analysis based on entropy production minimization reveals how those parameters affect the main losses occurring inside the AMR. In addition, optimum geometry and operating parameters corresponding to the highest efficiency for different geometries are presented and compared. The results show that parallel plate and micro-channel matrices show the highest theoretical efficiency, while the packed screen and packed sphere beds are possibly more practical from the application point of view.

The aim of this work is to assess the influence of a magnetic sample on the applied magnetic field inside the air gap of a magnetic circuit. Different magnetic sources including an electromagnet, a permanent magnet in a soft ferromagnetic toroidal yoke, as well as 2D and 3D Halbach cylinders are considered, using a numerical model. Gadolinium is chosen as magnetic material for the sample, due to its strong magnetocaloric properties and its wide use in magnetic refrigeration prototypes. We find that using uniform theoretical demagnetizing factors for cylinders or spheres results in a deviation of less than 2% in the calculation of internal magnetic fields at temperatures above the Curie point of gadolinium. Below the Curie point, a stronger magnetization of the cylinders and spheres leads to a larger deviation which can reach 8% when using uniform demagnetizing factors for internal magnetic field calculations.

A promising solid‐state refrigeration technology, known as magnetic refrigeration, has reached a groundbreaking result. The potential for environmentally friendly cooling using the magnetocaloric effect is disadvantaged by small temperature windows of effective cooling and the requirement of expensive, high magnetic field producing Nd‐Fe‐B permanent magnets. Researchers from Imperial College London, Ames Laboratory (USA) and DTU Energy, Technical University of Denmark (see article no. 1700143) show through utilising a larger portion of the phase diagram of so‐called soft first order magnetic materials that they can reduce the maximum field required to within that attainable with cheap ferrite‐based permanent magnets (around 0.5 T) and significantly broaden the working temperature range. Traditionally, magnetocaloric materials are controlled through temperature and applied magnetic field, but with the addition of applied hydrostatic pressure it is possible to move around in the phase diagram of the La(Fe,Mn,Si)13Hz material series, taking advantage of its so‐called multicaloric properties. By careful choice of magnetic field and pressure controlled cooling cycles this work shows that a significant bottleneck towards commercially competitive refrigeration devices based on the magnetocaloric effect can be overcome.

Magnetic refrigerators can theoretically be more efficient than current vapor compression systems and use no vapor refrigerants with global warming potential. The core component, the active magnetic regenerator (AMR) operates based on the magnetocaloric effect of magnetic materials and the heat regeneration processes of periodic fluid blows. Magnetocaloric materials with a first order phase transition (FOPT) are suitable to realize a higher cooling capacity than commonly used gadolinium, but layering such materials is necessary, due to a large isothermal entropy change (Δ푆푚) in a narrow region around their Curie temperature. Simulations are implemented to investigate how to layer the FOPT materials for obtaining higher cooling capacity. Moreover, based on entropy generation minimization, optimization of the regenerator geometry and related operating parameters is presented for improving the AMR efficiency. In addition, simulations are carried out to investigate the potential of applying nanofluid in future magnetic refrigerators.

First order magnetic phase transition materials present a large magnetocaloric effect around the transition temperature, where these materials usually undergo a large volume or structural change. This may lead to some challenges for applications, as the material may break apart during field change, due to high internal stresses. A promising magnetocaloric material is La(Fe, Mn, Si)13Hy, where the transition temperature can be controlled through the Mn amount. In this work we use XRD measurements to evaluate the temperature dependence of the unit cell volume with a varying Mn amount. The system is modelled using the Bean-Rodbell model, which is based on the assumption that the spin-lattice coupling depends linearly on the unit cell volume. This coupling is defined by the model parameter η, where for η > 1 the material undergoes a first order transition and for η ≤ 1 a second order transition. We superimpose a Gaussian distribution of the transition temperature with a standard deviation σT, in order to model the chemical inhomogeneity. Good agreement is obtained between measurements and model with values of η ∼ 1.8 and σ(T) = 1.0 K.

Experiments with a recently constructed rotary multi-bed active magnetic regnenerator (AMR) prototype have revealed strong impacts on the temperature span from variations in the resistances of the flow channels carrying heat transfer fluid in and out of the regenerator beds. In this paper we show through numerical modeling how unbalanced flow in the beds decreases the cooling power and COP for a dual bed device. Furthermore, it is shown how resistance variations in multi-bed devices give rise to unbalanced flow in the individual beds and how this decreases cooling powers and COPs of the machines by approximately 30% and 50%, respectively.

We use powder X-ray diffraction to evaluate the temperature dependence of the crystalline properties during the magnetic phase transition of La(Fe,Mn,Si)13Hz as a function of the Fe/Mn/Si ratio. Both the paramagnetic and ferromagnetic phases were observed as peak overlaps in the patterns around the Curie temperature (TC) occurring continuously in a temperature range of about 5 K around TC. Using the Williamson-Hall method, we evaluate the strain developing in the crystallites during the transition and find that it is associated with the growth of the paramagnetic phase as the transition occurs. Based on our measurements and microstructure analyses, we propose that cracking during the phase transition is due to or aggravated by the small content of a La-rich phase.

The total cost of a 25 W average load magnetic refrigerator using commercial grade Gd is calculated using a numerical model. The price of magnetocaloric material, magnet material and cost of operation are considered, and all influence the total cost. The lowest combined total cost with a device lifetime of 15 years is found to be in the range $150-$400 depending on the price of the magnetocaloric and magnet material. The cost of the magnet is largest, followed closely by the cost of operation, while the cost of the magnetocaloric material is almost negligible. For the lowest cost device, the optimal magnetic field is about 1.4 T, the particle size is 0.23 mm, the length of the regenerator is 40-50 mm and the utilization is about 0.2, for all device lifetimes and material and magnet prices, while the operating frequency vary as function of device lifetime. The considered performance characteristics are based on the performance of a conventional A+++ refrigeration unit. In a rough life time cost comparison between the AMR device and such a unit we find similar costs, the AMR being slightly cheaper, assuming the cost of the magnet can be recuperated at end of life.

We measure the magnetocaloric effect of the manganite series La0.67Ca0.33-xSrxMnO3 by determining the isothermal entropy change upon magnetization, using variable-field calorimetry. The results demonstrate that the field dependence of the magnetocaloric effect close to the critical temperature is not given uniquely by the critical exponents of the ferromagnetic-paramagnetic phase transition, i.e., the scaling is nonuniversal. A theoretical description based on the Bean-Rodbell model and taking into account compositional inhomogeneities is shown to be able to account for the observed field dependence. In this way the determination of the nonuniversal field dependence of the magnetocaloric effect close to a phase transition can be used as a method to gain insight into the strength of the spin-lattice interactions of magnetic materials. The approach is shown also to be applicable to first-order transitions.

We report a thorough study of the thermal hysteretic behaviour of a single phase sample of the magnetocaloric material La0.67Ca0.33MnO3. Previous reports in the literature have variously found hysteretic and non-hysteretic behaviour. We show the importance of measuring under carefully defined heating and cooling procedures. Careful analysis of the specific heat, measured at five different temperature ramp rates, and the magnetic entropy change indicates that there is no observable hysteresis, even though the behaviour of both quantities is consistent with a first-order phase transition. We discuss the reasons for this and for the differing results previously found.

We compare the cost of a high temperature superconducting (SC) tape-based solenoidwith a permanent magnet (PM) Halbach cylinder for magnetic refrigeration.Assuming a five liter active magnetic regenerator volume, the price of each type ofmagnet is determined as a function of aspect ratio of the regenerator and desiredinternal magnetic field. It is shown that to produce a 1 T internal field in theregenerator a permanent magnet of hundreds of kilograms is needed or an area ofsuperconducting tape of tens of square meters. The cost of cooling the SC solenoidis shown to be a small fraction of the cost of the SC tape. Assuming a cost ofthe SC tape of 6000 $/m2 and a price of the permanent magnet of 100 $/kg, thesuperconducting solenoid is shown to be a factor of 0.3-3 times more expensive thanthe permanent magnet, for a desired field from 0.5-1.75 T and the geometrical aspectratio of the regenerator. This factor decreases for increasing field strength, indicatingthat the superconducting solenoid could be suitable for high field, large coolingpower applications.

An experimental device that can accurately measure the magnetic entropy change, Δs, as a function of temperature, T, and magnetic field, H, is presented. The magnetic field source is in this case a set of counter-rotating concentric Halbach-type magnets, which produce a highly homogeneous applied field with constant orientation. The field may be varied from 0 to 1.5 T in a continuous way. The temperature stability of the system is controlled to within ±10 mK and the standard range for the current setup is from 230 K to 330 K. The device is under high vacuum and we show that thermal losses to the ambient are negligible in terms of the calorimetric determination of the magnetic entropy change, while the losses cannot be ignored when correcting for the actual sample temperature. We apply the device to two different types of samples; one is commercial grade Gd, i.e., a pure second-order phase transition material, while the other is Gd5Si2Ge2, a first order magnetic phase transition material. We demonstrate the device’s ability to fully capture the thermal hysteresis of the latter sample by following appropriate thermal resetting scheme and magnetic resetting scheme. © 2015 AIP Publishing LLC.

The performance of a combined solar photovoltaic (PV) and thermoelectric generator (TEG) system is examined using an analytical model for four different types of commercial PVs and a commercial bismuth telluride TEG. The TEG is applied directly on the back of the PV, so that the two devices have the same temperature. The PVs considered are crystalline Si (c-Si), amorphous Si (a-Si), copper indium gallium (di) selenide (CIGS) and cadmium telluride (CdTe) cells. The degradation of PV performance with temperature is shown to dominate the increase in power produced by the TEG, due to the low efficiency of the TEG. For c-Si, CIGS and CdTe PV cells the combined system produces a lower power and has a lower efficiency than the PV alone, whereas for an a-Si cell the total system performance may be slightly increased by the TEG.

Experimental results for an active magnetic regenerator (AMR) are presented. The focus is on whether or not it pays off to partly substitute soft magnetic material with non-magnetic insulation in a flux-conducting core in the magnet system. Such a substitution reduces losses due to heat conduction and eddy currents, but also reduces the magnetic field. Two different cores were tested in the AMR system with different cooling loads and it is shown, that in the present case, replacing half of the iron with insulation lead to an average reduction in temperature span of 14%, but also a small decrease in COP, hence the substitution did not pay off. Furthermore, it is shown experimentally, that small imbalances in the heat transfer fluid flow greatly influence the system performance. A reduction of these imbalances through valve adjustments resulted in an increase in the temperature span from approximately 16 K to 27.3 K.

We quantify the effect of hysteresis on the performance of the magnetocaloric first order material Gd5Si2Ge2 undergoing an ideal active magnetic regenerator (AMR) cycle. The material is carefully characterized through magnetometry (VSM) and calorimetry (DSC) in order to enable an accurate model description of the phase transition at varying magnetic fields and temperatures. Using detailed experimental property data, a Preisach type model is used to describe the thermal hysteresis effects and simulate the material under realistic working conditions. We find that the adiabatic temperature change is limited by a significant fraction of the thermal hysteresis.

A magnetic material in an externally applied magnetic field will in general experience a spatially varying internal magnetic field due to demagnetizing effects. When the performance of active magnetic regenerators (AMRs) is evaluated using numerical models the internal field is often assumed to be spatially constant and equal to the applied field, thus neglecting the demagnetizing field. Furthermore, the experimental magnetocaloric properties used (adiabatic temperature change, isothermal entropy change and specific heat) are often not corrected for demagnetization. The demagnetizing field in an AMR is in general both a function of the overall shape of the regenerator and its morphology (packed particles, parallel plates etc.) as well as the magnetization of the material. Due to the pronounced temperature dependence of the magnetization near the Curie temperature, the demagnetization field is also temperature dependent. We propose a relatively straightforward method to correct sufficiently for the demagnetizing field in AMR models. We discuss how the demagnetizing field behaves in regenerators made of packed spheres under realistic operation conditions.

We present an experimental setup recently developed at DTU Energy Conversion for measuring specific heat and direct isothermal entropy change in a varying magnetic field (DSC device) using calorimetry. The device operates in high vacuum (~1e-6 mbar) and measurements are fully automated with respect to magnetic field and temperature control. A magnetic field source comprised of two concentric Halbach type magnets that are fixed with respect to each other through a mechanical gear supply the applied magnetic field. The applied field range is 0.001 to 1.57 T with a minimum field step size smaller than 0.01 T. The magnet control is fully integrated in software allowing measurement scans to be automated. This device is an upgrade of an existing device where it is now possible to install a sample and then run temperature scans at different magnetic fields (specific heat measurement) as well as magnetic field scans under isothermal conditions (direct isothermal entropy change measurements).

In active magnetic regeneration (AMR) systems, not only the magnetocaloric properties of materials, but also the regenerator geometry plays an important role in the system performance. Packed sphere regenerators are often employed in existing prototypes, however, the characteristics such as relatively large pressure drop and almost fixed porosity make loss reductions and further optimization challenging. This paper proposes and focuses on packed screen regenerators, which may exhibit lower pressure drop and equivalent heat transfer performance to packed sphere regenerators. A 1D AMR model is improved and applied to simulate the regenerators. The performance of the new regenerators is studied and compared with that of the packed sphere regenerators. Possible fabrication methods of the packed screen regenerators are also discussed.

The magnetocaloric effect of a magnetic material is characterized by two quantities, the isothermal entropy change and the adiabatic temperature change, both of which are functions of temperature and applied magnetic field. We discuss the scaling properties of these quantities close to a second order phase transition within the context of critical scaling theory. In the critical region the isothermal entropy change will exhibit universal scaling exponents. However, this is only true close to Tc and for small fields; we show that for finite fields the scaling exponents in general become field dependent, even at Tc. Furthermore, the scaling exponents at finite fields are not universal: Two models with the same critical exponents can exhibit markedly different scaling behaviour even at relatively low fields. Turning to the adiabatic temperature change, we argue that it is not determined exclusively by the scaling part of the free energy and its derivatives. This means that the field dependence of the adiabatic temperature change in the critical region depends on the regular part (background) of the specific heat associated with the lattice and conduction electrons. The field dependence can still be fitted to a power-law expression but with non-universal exponents as we show explicitly both within mean-field theory and using the so-called Arrott-Noakes equation of state. Finally, we discuss the implications of these observations for the interpretation of a widely used phenomenological scaling procedure.

MAGGIE, a new AMR prototype, is presented. It has been designed to produce a temperature span and cooling power relevant to commercial refrigeration applications combined with an attractive COP and a compact design. Concepts and design considerations are described. Initial non optimized tests show a COP of 3.6 at a temperature span of 7.2K with 103 W of cooling power.

We discuss how magnetic domains influence the magnetic entropy change calculated from magnetisation data. In a simple qualitative model we show that the effect is to change the shape of the apparent isothermal entropy change curve compared to the true curve determined by the entropy. We further show that failure to correct for the magnetostatic demagnetisation will augment the apparent effect of domains.

The magnetocaloric effect of a magnetic material is characterized by two quantities, the isothermal entropy change and the adiabatic temperature change, both of which are functions of temperature and applied magnetic field. We discuss the scaling properties of these quantities close to a second-order phase transition within the context of the theory of critical phenomena. Sufficiently close to the critical temperature of a second-order material, the scaling of the isothermal entropy change will be determined by the critical exponents and will be the same as that of the singular part of the entropy itself. However, this is only true in the critical region near Tc and for small fields; for finite fields, scaling with constant exponents, in general, break down, even at Tc. The field dependence can then be described by field-dependent scaling exponents. We show that the scaling exponents at finite fields are not universal, showing significant variation for models in the same universality class. As regards the adiabatic temperature change, it is not determined exclusively by the singular part of the free energy and its derivatives. We show that the field dependence of the adiabatic temperature change at the critical temperature depends on the nonsingular part of the specific heat. The field dependence can still be fitted to a power-law expression but with nonuniversal exponents, as we show explicitly both within mean-field theory and using the so-called Arrott-Noakes equation of state. Within the framework of the Bean-Rodbell model, we briefly consider the scaling properties of the magnetocaloric effect in first-order materials. Finally, we discuss the implications of our findings for a widely used phenomenological scaling procedure for magnetocaloric quantities.