Magnetic Refrigeration Devices Research Articles

A new magnetocaloric refrigeration principle with solid-state thermoelectric thermal diodes

In order to bring magnetocaloric technology to new markets in the future several obstacles have to be overcome. One of the most important issues that represent a serious obstacle in the existing prototypes is the low operating frequency, which directly affects the low power density of the device. A potential solution is the introduction of new heat-transport mechanisms. One of such solutions is described in this article. It is the application of a new type of magnetic refrigeration device, based on solid-state thermoelectric thermal diodes with embodied thin-plate magnetocaloric materials. Thermal diodes can provide a very fast heat transport from or to the magnetocaloric material. A quasi 2-D numerical model of the device was developed. This model serves for dynamic heat transfer simulations of magnetocaloric devices, which apply thermal diodes. The analysis considers different operating and geometrical characteristics and gives important information about the performance and operating mechanisms of such a device. Results show capabilities of potential devices' operation at high operating frequencies (one or two orders higher than existing magnetocaloric devices) with corresponding high specific powers (per mass of the MCM) and relatively high efficiencies.

The universal behavior of inverse magnetocaloric effect in antiferromagnetic materials

We report the universal behavior of inverse magnetocaloric effect (IMCE) in antiferromagnetic materials. In contrast to the universal behavior of conventional magnetocaloric effect often observed in ferromagnetic systems, a phenomenological universal master curve can be constructed to describe the temperature dependence of magnetic entropy change for IMCE without rescaling the temperature axis. The proposed universal curve method allows extrapolating the magnetic entropy change of an IMCE material, which would be imperative to judge its suitability in actual magnetic refrigeration devices.

Effects of substitution of Pr, Nd, and Sm for La on the magnetic properties and magnetocaloric effect of La1.4Ca1.6Mn2O7

The effects of Pr, Nd and Sm partial substitution for La on the magnetic properties and magnetocaloric effect near the critical phase transition temperatures of La1.4Ca1.6Mn2O7 are analyzed. Polycrystalline powders of La1.2R0.2Ca1.6Mn2O7 double layered manganites (R=La, Pr, Nd, Sm) with grain sizes of 20–30nm were successfully synthesized by a sol–gel method. The substitution of Pr, Nd and Sm for La ions increases the inter-layer lattice distortion, leading to a decrease of the exchange interaction strength. The magnetic measurements evidence a spin-glass-like behavior at low temperatures for all samples, which may be due to frustration of random competing ferromagnetic and antiferromagnetic interactions together with the anisotropy originating from the layered structure. The Mn 3s XPS spectra show an exchange splitting around 4.75eV from which the Mn formal valence is estimated to be around 3.65+, in excellent agreement with the magnetic data obtained at temperatures above the room temperature. A moderate magnetocaloric effect is found for all samples, with the maximum entropy change located at temperatures near the magnetic transition ones. The high RCP(S) values together with the broadened magnetic entropy curves and low magnetic hysteresis suggest the possibility to use these materials for magnetic refrigeration devices.

Magnetism and large magnetocaloric effect in HoFe2−xAlx

The structural, magnetic and magnetocaloric properties of several HoFe2−xAlx compounds were investigated. The compounds in the Fe-rich region (0.36⩽x⩽0.4) crystallize in the cubic MgCu2 (C15) structure, while for the ones in the intermediate region (0.75⩽x⩽1.125) the hexagonal MgZn2 (C14) structure was observed. Electronic structure calculations were performed, showing a good agreement between theory and experiment. The Curie temperatures were found to decrease with Al content. For the Fe-rich compounds, these are close to room temperature, while for the compounds in the intermediate region, transition temperatures are well below 300K. No magnetic hysteresis was found around the Curie temperature for applied magnetic fields of up to 4T. All of the investigated compounds undergo a second-order magnetic phase transition at the Curie temperature. A maximum magnetic entropy change value of 7.6J/kgK was obtained for the sample with x=1.125, all of the samples displaying rather large RCP values. The possibility of incorporating these materials in magnetic refrigeration devices is discussed.

High performance magnetocaloric perovskites for magnetic refrigeration

We have applied mixed valance manganite perovskites as magnetocaloric materials in a magnetic refrigeration device. Relying on exact control of the composition and a technique to process the materials into single adjoined pieces, we have observed temperature spans above 9 K with two materials. Reasonable correspondence is found between experiments and a 2D numerical model, using the measured magnetocaloric properties of the two materials as input.

Development of a new magnetocaloric material used in a magnetic refrigeration device

Testing directly a magnetocaloric material in a magnetic refrigeration (MR) system is the best way to judge of its applicative potentialities. In this spirit, an oxide expected to show promising magnetocaloric properties around room temperature (Pr0.65Sr0.35MnO3) was produced in large scale and shaped in order to build a regenerator. Magnetization, heat capacity, resistivity, thermal conductivity and a direct test in a MR device were carried out on this manganite. The results were compared to those observed in the reference material which is Gadolinium. The two main conclusions of these preliminary results are: (i) the Pr0.65Sr0.35MnO3 actually displays not only a significant magnetocaloric effect but also a real refrigeration capability at room temperature; (ii) the temperature spans reached in these first experiments are even found to well compare with those obtained with Gd.

Magnetic properties and magnetocaloric effect in La1.4−x Ce x Ca1.6Mn2O7 perovskites synthesized by sol–gel method

The magnetic properties and magnetocaloric effect near the critical phase transition temperatures of La1.4−x Ce x Ca1.6Mn2O7 (x = 0, 0.2) double layered manganites are analyzed. Polycrystalline nanopowders of La1.4Ca1.6Mn2O7 and La1.2Ce0.2Ca1.6Mn2O7 were successfully synthesized by a sol–gel method. The magnetic measurements suggest an antiferromagnetic alignment of the Ce and Mn moments in La1.2Ce0.2Ca1.6Mn2O7. Both Ce3+ and Ce4+ ions are present in the doped sample, as indicated by the XPS spectra. A moderate magnetocaloric effect was found for both samples, with the maximum entropy change located at temperatures near the magnetic transition ones. The high RCP(S) values together with the broadened magnetic entropy curves suggest the possibility to use these materials for magnetic refrigeration devices.

Classification proposal for room temperature magnetic refrigerators

Magnetic refrigeration promises to be a breakthrough technology in the field of refrigeration at room temperature. Several magnetic refrigerator prototypes have been developed by research groups, with different embodiments and process thermodynamics. This short paper follows an idealized line connecting, from a conceptual rather than historical point of view, the different geometries, and proposes a classification based essentially on the type of magnetic source, the type of magnetocaloric material, and the relative motion of the active elements of the device. The proposed taxonomy is then applied to some prototype presented in literature, showing its potential to give a comprehensive description of magnetic refrigeration devices by means of a twelve digit string.

Design concepts for a continuously rotating active magnetic regenerator

Design considerations for a prototype magnetic refrigeration device with a continuously rotating AMR are presented. Building the active magnetic regenerator (AMR) from stacks of elongated plates of the perovskite oxide material La 0.67Ca 0.33− x Sr x Mn 1.05O 3, gives both a low pressure drop and allows grading of the Curie temperature along the plates. This may be accomplished by a novel technique where a compositionally-graded material is tape cast in one piece. The magnet assembly is based on a novel design strategy, to create alternating high- and low magnetic field regions within a magnet assembly. Focus is on maximising the magnetic field in the high field regions but also, importantly, minimising the flux in the low field regions. The design is iteratively optimised through 3D finite element magnetostatic modelling.

Magnetic refrigeration at room temperature – from magnetocaloric materials to a prototype

Based on the magnetocaloric effect, magnetic refrigeration at room temperature has for the past decade been a promising, environmentally friendly new energy technology predicted to have a significantly higher efficiency than the present conventional methods. However, so far only a few prototype refrigeration machines have been presented worldwide and there are still many scientific and technological challenges to be overcome. We report here on the MagCool project, which spans all the way from basic materials studies to the construction of a prototype. Emphasis has been on ceramic magnetocaloric materials, their shaping and graded composition for technological use. Modelling the performance of a permanent magnet with optimum use of the flux and relatively low weight, and designing and constructing a prototype continuous magnetic refrigeration device have also been major tasks in the project.

A Monolithic Perovskite Structure for Use as a Magnetic Regenerator

A La0.67Ca0.26Sr0.07Mn1.05O3 (LCSM) perovskite was prepared for the first time as a ceramic monolithic regenerator used in a regenerative magnetic refrigeration device. The parameters influencing the extrusion process and the performance of the regenerator, such as the nature of the monolith paste and the influence of the sintering on the adiabatic temperature change, were investigated. Comparisons between the extruded monolithic structure before and after the sintering showed that an increase of the adiabatic temperature change was seen after the sintering. Furthermore, calculations show that the performance of the monolithic structure is potentially superior to a parallel plate regenerator, indicating the potential cost and structural benefit of using such structure, i.e. a mechanically stable ceramic thin wall structure, which can be produced in one processing step.

Experimental results for a magnetic refrigerator using three different types of magnetocaloric material regenerators

Magnetic refrigeration is a potentially environmentally-friendly alternative to vapor compression technology because it has a potentially higher coefficient of performance and does not use a gaseous refrigerant. The active magnetic regenerator refrigerator is currently the most common magnetic refrigeration device for near room temperature applications, and it is driven by the magnetocaloric effect in the regenerator material. Several magnetocaloric materials with potential magnetic refrigeration applications have recently been developed and characterized; however, few of them have been tested in an experimental device. This paper compares the performance of three magnetocaloric material candidates for AMRs, La(Fe,Co,Si) 13, (La,Ca,Sr)MnO 3 and Gd, in an experimental active magnetic regenerator with a parallel plate geometry. The performance of single-material regenerators of each magnetocaloric material family were compared. In an attempt to improve system performance, graded two-material regenerators were made from two different combinations of La(Fe,Co,Si) 13 compounds having different magnetic transition temperatures. One combination of the La(Fe,Co,Si) 13 materials yielded a higher performance, while the performance of the other combination was lower than the single-material regenerator. The highest no-load temperature span was achieved by the Gd regenerator.

An optimized magnet for magnetic refrigeration

A magnet designed for use in a magnetic refrigeration device is presented. The magnet is designed by applying two general schemes for improving a magnet design to a concentric Halbach cylinder magnet design and dimensioning and segmenting this design in an optimum way followed by the construction of the actual magnet. The final design generates a peak value of 1.24 T, an average flux density of 0.9 T in a volume of 2 L using only 7.3 L of magnet, and has an average low flux density of 0.08 T also in a 2 L volume. The working point of all the permanent magnet blocks in the design is very close to the maximum energy density. The final design is characterized in terms of a performance parameter, and it is shown that it is one of the best performing magnet designs published for magnetic refrigeration.

Co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature in nanocrystalline La 0.7Te 0.3MnO 3

Sol–gel prepared nanocrystalline La 0.7Te 0.3MnO 3 has rhombohedral crystal structure (space group R3¯C) at room temperature and orders ferromagnetically at ∼280 K ( T C ). A large magnetic entropy change of ∼12.5 J kg −1 K −1 is obtained near T C for a field change of 50 kOe. This magnetocaloric effect could be explained in terms of Landau theory. The temperature dependence of electrical resistivity shows metal–insulator transition at T C and a giant magnetoresistance of ∼52% in 50 kOe. The co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature makes nanocrystalline La 0.7Te 0.3MnO 3 a promising material for magnetic refrigeration and spintronic device applications.

Active magnetic regenerator performance enhancement using passive magnetic materials

Magnetic refrigeration devices using permanent magnets are currently limited to useful field strengths of less than 2 T, and more practically less than 1.5 T. In this range, the useful magnetocaloric effect is less than 6 K and limits the cooling power of active magnetic regenerator (AMR) devices. Maximizing the useful magnetocaloric effect is critical in enabling commercially viable permanent magnet devices, and methods of increasing the net change in magnetic field would be beneficial. It has been shown [O. Peksoy, A. Rowe, J. Magn. Magn. Mater. 288 (2005) 424] that the geometry of a regenerator and the magnetic properties of the refrigerant can alter the local magnetic field, H. This is called demagnetization. A numerical model is used to study demagnetizing effects in a single-material AMR. The use of additional passive magnetic material to reduce demagnetization is examined and suggests that augmenting the effective field, H, is possible. Numerical results are validated with experiments in near room temperature using AMRs consisting of Gd and Gd 0.74Tb 0.26 adjacent to layers composed of 1010 carbon steel. Experimental data show an increase in the no-load temperature span for certain operating conditions and confirm the beneficial impact of using passive magnetic material to reduce demagnetization effects at low fields.

Magnetocaloric effect in the intermetallic compound Gd 7Pd 3

As a part of a current research concerning the physical characterisation of Rare Earth intermetallic compounds for magnetic refrigeration devices, the magnetocaloric properties of the Gd 7Pd 3 phase, which shows a ferromagnetic transition at 318 K, are presented. Heat capacity measurements have been performed in the 5–360 K temperature range and for applied magnetic fields up to 5 T (Tesla). Basing on the heat capacity data the isothermal entropy change Δ S M and the adiabatic temperature rise Δ T ad have been calculated ( Δ S M =2.5 and 6.5 J/K kg and Δ T ad=3.0 and 8.5 K at 320 K and respectively at 2 and 5 T). From the Δ S M vs. T plot the refrigerant capacity in the 260–360 K temperature range has been calculated to be 100 J/kg for an applied magnetic field of 2 T and 380 J/kg for an applied magnetic field of 5 T.

Tunable magnetocaloric effect in ceramic perovskites

A large entropy variation (magnetocaloric effect) has been discovered in ceramic perovskites with the formulas La0.65Ca0.35Ti1−xMnxO3−z and La0.5+x+yLi0.5−3yTi1−3xMn3xO3−z. Both Curie temperature and entropy change were studied from 4.2 to 400 K for different stoichiometric compositions and applied magnetic fields. Our conclusion is that these materials are excellent candidates for working materials in magnetic refrigeration and liquefaction devices in a wide temperature range.

4536258 Distilling apparatus operating on the thermocompressor principle

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 lifetime cost comparison between the magnetic refrigeration device and such a unit, we find similar costs, the former being slightly cheaper, assuming the cost of the magnet can be recuperated at end of life.

The tasks of geographical science in the era of the scientific and technological revolution

In this paper, numerical and experimental investigations on a magnetic refrigeration device based upon the active magnetic regeneration (AMR) cycle operating near room temperature are presented. A numerical 1D model based on the transient energy equations is proposed for modelling the heat exchange between the magnetocaloric material and the carrier fluid in the regenerator bed. The validity of 1D AMR-numerical model is investigated through the recently developed magnetic cooling demonstrator by Clean Cooling Systems SA (CCS) at the University of Applied Sciences of western Switzerland (HES−SO). The obtained results including the temperature span, the coefficient of performance and the cooling power are presented and discussed. In general, good agreements have been noted between the experimental and numerical results.