O. SARI (1), M. BALLI (1), C. MAHMED (1), CH. BESSON (2), PH. BONHOTE(3), J. FORCHELET(3)

University of Applied Sciences of Western Switzerland

(1)Institute of Thermal Sciences and Engineering

(2)Institute of IESE

(3)Institute of COMATEC

CH-1401 Yverdon-les-Bains, Switzerland

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Abstract

In this paper, a new type of reciprocating magnetic refrigerator working with high remanence permanent magnets as the source of the magnetic field is presented. The simulated and measured magnetic field at the machine air gap is about 1.45 Tesla. Initially, gadolinium metal (Gd) was used as the magnetocaloric refrigerant. Its magnetocaloric performances and its quality were checked experimentally in a developed test bench and confirmed by theoretical calculations based on the mean field theory (MFT). To attain high values of temperature difference between the hot and the cold sources (temperature span), a new kind of the Active Magnetic Refrigeration (AMR) cycle was implemented. However, in order to reduce the energy consumption and then increase the thermodynamic performances of the magnetic system, a special configuration of the magnetocaloric materials is developed. The numerical results of the applied magnetic forces on the new configuration are given and analyzed in details. The developed machine is designed to produce a cooling power between 80 and 100 Watt with a temperature span larger than 20 °C. The obtained results demonstrate that magnetic cooling is a promising alternative to replace traditional systems.

Keywords: Magnetic refrigeration, Magnetocaloric effect, Magnetic refrigerating system, Optimization of system, Active magnetic refrigeration,

1. Introduction

The impact of synthetic refrigerants on the environment as well as the legal safety obligations drive the refrigeration industry to seek for new ways for completely phasing out greenhouse gases or for decreasing their charge in numerous installations. In order to be free from synthetic refrigerants, industries are continuously searching for environmentally friendly and suitable new technologies that will enable high energy savings, therefore reducing indirect CO2 emissions. During the last fifteen years, both, namely the load reduction of the refrigerants in the installations and the use of natural, non-flammable, environmentally friendly refrigerants have been the preferred options by many end-users. Research on future refrigeration technologies orients itself on the indirect cooling technology as e.g. Phase Change Slurry (PCS), CO2 vapor compressor technology, thermo-electric refrigeration, thermo-acoustic refrigeration and magnetic refrigeration (MR).

Since the discovery of the high polarization permanent magnet in Nd-Fe-B, the giant magnetocaloric effect in Gd5Ge2Si2 and the development of the higher performance magnetic cooling systems close to room temperature, intensive studies were motivated on the magnetocaloric materials and magnetic cooling devices. Magnetic refrigeration (MR) is based on the magnetocaloric effect (MCE). This intrinsic property of some magnetic materials was discovered by Warburg in 1881 [1]. It is defined as the response of some magnetic materials to a changing magnetic field which manifests as the isothermal entropy change _S and adiabatic temperature change _Tad (see Fig.1). When a magnetic field is applied to magnetic material close to the phase transition region, the magnetic moments change their ordering state and as consequence the magnetic entropy. Under adiabatic condition, this change in magnetic entropy is compensated by a modification in the lattice (atoms vibration) part of the full entropy which increases or decreases the material temperature depending on the sign of the applied field and the nature of magnetic order in the refrigerant.

Figure 1: Adiabatic temperature change with magnetization for pure Gadolinium

The origin of the MCE was explained independently by Debye and Giauque [2, 3] and pointed out that low temperatures could be reached by using a paramagnetic salt. In 1933 [4], Giauque and MacDougall have achieved successfully temperatures below 1 Kelvin by the use of the demagnetization cooling. Brown was the first to demonstrate the feasibility of MR close to room temperature [5]. In 1976, he obtained a temperature difference of 46 K between the hot and cold end of a simple refrigerator using 158 g of gadolinium metal and an applied field of 7 Tesla. The carrier fluid consisting of a mixture of 80 % water and 20 % ethyl alcohol solution was used as a heat transfer fluid. Compared to the classical refrigeration, magnetic cooling is an environment-safe technique (absence of CFC and HCFC) with many advantages, such as high efficiency, low noise, low pressure and compact configuration.

The modern magnetic cooling technology was born when Zimm et al developed successful operating machines showing that this technology is a feasible and competitive for large scale domestic and industrial applications. The first proof (reciprocating) operated with a magnetic field of 5 Tesla using a superconducting magnet [6]. With 10 K temperature span (between 281 and 291 K), it achieved a cooling power of 600 W, a coefficient of performance (COP) of 10 and maximum of 60 % of Carnot efficiency. The COP represents a ratio between the cooling energy (Qcool) and the total energy input (W). It is worth noting that the COP of the traditional refrigerator is about 30 to 40% of Carnot efficiency [7, 8 and 9].

The second developed prototype by Zimm et al [10] was a rotating machine working with some rare earth based compound as magnetocaloric refrigerant magnetized and demagnetized through a magnetic field of 1.5 Tesla produced by a magnetic source based on Nd-Fe-B permanent magnets (PM). The obtained cooling power was 50 W at 0 K temperature span and 25 K as the maximum temperature difference between hot and cold source. Later, several demonstrators were reported in the literature. For more details, see Gschneidner et al [11].

The magnetocaloric material is an important key for the development of the magnetic refrigeration technology. However, up to today the principal material used in magnetic cooling prototypes is gadolinium (Gd) metal and its alloys. This is attributed essentially to its good magnetocaloric performances at room temperature, good mechanical properties, low hysteresis, availability in the market and its ability to answer the several engineering requirements. However, the high cost and the chemical instability limit the use of Gd as refrigerant on a large scale application. Aiming to replace Gd, a giant MCE was discovered in the first order transition materials Gd5(Ge1-xSix)4 [12]. A few years later, several other families of MC materials were reported and found to exhibit high level of MCEs on large temperature range: from ambient to low temperatures. These include series such as MnAs1-xSbx [13] MnFeP1-xAsx [14], LaFe13-xSix [15, 16] and their derivatives. From a practical point of view, LaFe13-xSix materials seem to be the most promising in magnetic cooling systems due to their high MCE, low cost and low hysteresis. In our Laboratory, many efforts are focused on the development, improvement and the implementation of this family in collaboration with industrial and academic partners. In this paper, we present the initial results of a preindustrial magnetic cooling system. This machine was designed and developed taking into account the design, market and thermodynamic performance requirements.

2. Magnetic field source

In addition to the magnetic refrigerants, the optimization of permanent magnets to generate high magnetic field is an important key for the development of magnetic cooling technologies. In magnetic refrigeration systems, the magnetic field source is equivalent to the compressor in the conventional compression cycle systems. In magnetic systems, the higher the generated magnetic field is, the higher the temperature and entropy change of working substance and as consequence the more powerful system may be. Considering the magnetocaloric performance of the available materials, an applied magnetic field higher than 1 Tesla is required.

Figure 2: Magnetic field distribution along the axis of the magnetic source given for different air gap height values.

For industrial applications, i.e. supermarket chillers, building air-conditioning, gas liquefaction, etc, superconducting magnets can be used to achieve induction level up to 8-10 Tesla with the restriction to utilize liquid helium or a cryocooler to maintain the superconducting coil near 4 K. However, as pointed out by Gschneidner et al [11] for domestic applications and small cooling systems, the superconducting magnet is out of question and the design of low-cost permanent magnet arrays with high induction is an important aspect of the commercialisation of MRs in the consumer market. With PM machines, the thermal energy is induced without electricity consumption, only an actuator is required to magnetize and demagnetize the magnetocaloric materials. In the literature, several types of magnetic flux sources were reported [17, 18]. For the one developed by Lee et al [17], the magnetic field for a side-opening PM can attend 3 T with an air gap 5.8 mm.

For the machine presented here, an innovative magnetic source is developed and designed. The latter is based on a modified Halbach rotation theorem and can be used for both: reciprocating and rotating magnetic systems. In the first step of the process, we started the optimized design of the new source’s geometry by studying theoretically this structure as a function of the air gap, magnets, remanence flux density, etc.

Due to the complexity of the geometrical structure and the presence of different soft magnetic materials, the analytical formulations are out of question. For this purpose, numerical simulations of the generated magnetic field were carried out.

In this work, the finite-element Flux3D program was used to simulate the magnetic field in the PM circuit. Flux3D is based on a Fortran code running in both operating systems Unix and Windows. It uses Maxwell’s equations as the basis to determine the magnetic potential in static conditions based on this equation:

Where μ is the relative magnetic permeability and Mr is the remanence. The obtained magnetic potential allows the calculations of all the magnetic quantities at any point of space.

In this study, the magnetic field was calculated as a function of the length and the height of the air gap of the magnetic source. The magnet structure is designed on the basis of Nd-Fe-B. The permanent magnets have a highest remanence of about 1.45 Tesla. In order to study and to optimize the structural parameters, the height h of the air gap was varied from 10 mm to 22 mm for a fixed length L = 120 mm and the latter was changed from 120 mm to 200 mm for h = 12 mm. The strength of magnetic induction along y-orientation in the centre of air gap is given in figures 2 and 3. As shown in figures, the geometric structure of permanent magnets can be adapted easily depending on the required application. The induced magnetic field is very sensitive to the air gap height and increases almost linearly when decreasing h. While, the length of the magnetic source influences slightly the magnetic field in the air gap. For the developed here prototype, the adopted field source air gap has a cross of 12 mm x 50 mm and a length of 120 mm. The calculated induction in the centre of the magnet by Flux 3D is about 1.44 Tesla. To check the validity of the magnetic field obtained by 3 D simulations, we have measured the generated magnetic flux density using Hall probe. The measurements results compared with the numerical data are shown in figure 4. The comparison indicates very good agreement of results confirming the ability of Flux 3D evaluating the magnetic field in such systems.

(To be continued)

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