Magnetic Refrigeration Technology at Room Temperature

Modern society largely depends on readily available refrigeration methods. Up till now, the conventional vapor compression refrigerators have been mainly used for refrigeration applications. Nonetheless, the conventional refrigerators – based on gas compression and expansion – are not very efficient because the refrigeration accounts for 25% of residential and 15% of commercial power consumption (Tishin, 1999). Moreover, using gases such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) have detrimental effects on our environment. Recently, the development of new technologies – such as magnetic refrigeration – has brought an alternative to the conventional gas compression technique (Manh, 2007).


Introduction
Modern society largely depends on readily available refrigeration methods. Up till now, the conventional vapor compression refrigerators have been mainly used for refrigeration applications. Nonetheless, the conventional refrigerators -based on gas compression and expansion -are not very efficient because the refrigeration accounts for 25% of residential and 15% of commercial power consumption (Tishin, 1999). Moreover, using gases such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) have detrimental effects on our environment. Recently, the development of new technologies -such as magnetic refrigeration -has brought an alternative to the conventional gas compression technique (Manh, 2007).
The magnetic refrigeration at room temperature is an emerging technology that has attracted the interest of researchers around the world (Bouchekara, 2008). Such a technology applies the magnetocaloric effect which was first discovered by Warburg (Bohigas, 2000;Zimm, 2007). In 1881, Warburg noticed an increase of temperature when an iron sample was brought into a magnetic field and a decrease of temperature when the sample was removed out of it. Thus, the magnetocaloric effect is an intrinsic property of magnetic materials; where it is defined as the response of a solid to an applied magnetic field which appears as a change in its temperature (Bohigas, 2000;Zimm, 2007). Such materials are called magnetocaloric materials. The magnetocaloric effect is present in all transition metals and lanthanide-series elements, which may have ferromagnetic behaviour. When a magnetic field is applied, the magnetic moments of these metals tend to align parralel to it, and the thermal energy released in this process produces the heating of the sample. The magnetic moments become randomly oriented when the magnetic field is removed, thus the ferromagnet cools down (Gschneidner, 1998).
The ultimate goal of this technology would be to develop a standard refrigerator for home use. The use of magnetic refrigeration has the potential to reduce operating and maintenance costs when compared to the conventional method of compressor-based refrigeration. By eliminating the high capital cost of the compressor and the high cost of www.intechopen.com electricity to operate the compressor, magnetic refrigeration can efficiently (and economically) replace compressor-based refrigeration technology. Some potential advantages of the magnetic refrigeration technology over the compressor-based refrigeration are: [1] green technology (no toxic or antagonistic gas emission); [2] noiseless technology (no compressor); [3] higher energy efficiency; [4] simple design of machines; [5] low maintenance cost; and [6] low (atmospheric) pressure (this is an advantage in certain applications such as in air-conditioning and refrigeration units in automobiles).
This chapter is concerned with the magnetic refrigeration technology form the material-level to the system-level. It provides a detailed review of the magnetic refrigeration prototypes available until now. The operational principle of this technology is explained in depth by making analogy between this technology and the conventional one. The chapter also investigates the study of the magnetocaloric materials using the molecular field theory. The thermal and magnetic study of the magnetic refrigeration process using the finite difference method (FDM) is also explained and are presented and discussed in detail.
The chapter is organized as follows. Section 2 introduces the magnetocaloric effect and its application to produce cold. It also introduces active magnetic regenerative refrigeration. Section 3 reviews ten various magnetic refrigeration systems and highlights their pros and cons. In Section 4 and 5, the thermal and magnetic study of the magnetic refrigeration process using the finite difference method are explained and the results from the thermal study are also presented and discussed in detail. Finally, the conclusions are drawn in Section 6.

Definition
The magnetocaloric effect (MCE) is an intrinsic property of magnetic materials; it consists of absorbing or emitting heat by the action of an external magnetic field (Tishin, 1999). This results in warming or cooling (both reversible) the material as shown in Fig. 1.

Thermodynamic approach
The absolute entropy, which is a function of temperature and induction in the magnetocaloric material is a combination of the magnetic entropy, the entropy of the lattice and the entropy of the conduction electrons (assumed negligible). It is given by the following equation: where S (J K -1 ) is the entropy (subscripts m and l are respectively for magnetic and lattice entropies), T (K) is the temperature and B (T) is the magnetic filed induction.
In magnetocaloric materials, a significant variation of the entropy can be observed by the application or removal of an external magnetic field. For a given material, MCE depends only on its initial temperature and the magnetic field. The MCE can be interpreted as the isothermal entropy change or the adiabatic temperature change.
The separation of entropy into three terms given in (1) is valid only for second order phase transition materials characterized by a smooth variation of the magnetization as a function of temperature. For first order transitions (abrupt change of magnetization around the transition temperature), this separation is not accurate (Kitanovski, 2005). For most applications, it is sufficient to work with the total entropy which -in its differential formcan be given as: The specific heat capacity C B (J m -3 K) of the material is given as: This gives: From (2) and (4) we can write: In the case of an adiabatic process (no entropy change 0 S Δ= ) the temperature variation can be written as: Using the Maxwell relation given as: where M (A m -1 ) is the magnetization.
We can write: The magnetocaloric effect (the adiabatic variation in temperature) can then be expressed as follows: In the case of an isothermal process, the temperature does not change during the magnetization and we can express the entropy as: Using the Maxwell relation given by (7), the magnetic entropy change can be expressed as: and the heat saved in this way is transferred to the lattice thermal motion.

Theoretical approach of MCE: molecular field theory
The theoretical calculation of the MCE is based on the model of Weiss (MFT: Molecular Field Theory) and the thermodynamic relations (Huang, 2004). To interpret quantitatively the ferromagnetism, Weiss proposed a phenomenological model in which the action of the applied magnetic field B was increased from that of an additional magnetic field proportional to the volume magnetization density B v as: The energy of a magnetic moment is then: The magnetic moments will tend to move in the direction of this new field. Adapting the classical Weiss-Langevin classical calculations to a system of quantum magnetic moments, one finds: where: where: J (N m s) is the total angular momentum, n (mol −1 ) is the Avogadro number, J g is the Landé factor, B µ (J T −1 ) is the Bohr magnetron, B k (J K-1) is the Boltzmann constant, () J Bx is the Brillouin function, λ is the Weiss molecular field coefficient and 0 µ (T m A -1 ) is the Permeability of vacuum.
The magnetic entropy is given by the relationship of Smart (Allab, 2008): The lattice contribution can be obtained using the Debye model of phonons (Allab, 2008). It is given by the following equation: where: D T (K) is the Temperature of Debye and R (J K −1 mol −1 ) is the universal gas constant.

Application of MFT to gadolinium (Gd)
In this section the theoretical study based on the MFT developed in the previous section is applied to the gadolinium. Table 1 gives the parameters used to calculate the magnetocaloric properties.  The numerical solution of equations (14), (15) and (16) allows getting the isotherms of magnetization and its evolution as a function of temperature calculated by the method of Weiss as shown in Fig. 2 (a) and Fig. 2

Application of MCE to produce cold
The magnetic cycles are generally composed of the process of magnetization and demagnetization, in which heat is discharged or absorbed in four steps as depicted by Fig. 3. From thermodynamic point of view, the magnetic cooling can be realized by: Carnot, Stirling, Ericsson and Brayton, where the Ericsson and Brayton cycles are believed to be the most suitable for such medium or room temperature cooling. Such cycles are predisposed to yield high cooling efficiency of the magnetic materials (Bouchekara, 2008).
www.intechopen.com Fig. 3(a) shows the conventional gas compression process that is driven by continuously repeating the four different basic processes shown while Fig. 3 (b) shows the magnetic refrigeration cycle comparison. The steps of the magnetic refrigeration process are analogous to those of the conventional refrigeration. By comparing (a) with (b) in Fig. 3, one can see that the compression and expansion are replaced by adiabatic magnetization and demagnetization, respectively. These processes change the temperature of the material and heat may be extracted and injected just as in the conventional process. Fig. 3. Analogy between magnetic refrigeration and conventional refrigeration.

The Active Magnetic Regenerative Refrigeration (AMRR)
The direct exploitation of the giant MCE around the room temperature is limited by the fact that existing MCE materials do not achieve high temperature differences (Lebouc, 2005). For example, a sample of gadolinium around room temperature produces an MCE of approximately 10 K in a magnetic field of 5 T. www.intechopen.com Step 1: Magnetization of the material from an initial state where the entire system is at temperature Ta. Each point of the regenerator material sees its temperature increase by ΔT following the application of the magnetic field.
Step 2: Flow of the fluid from the cold source to the hot source. The heat produced by the magnetization step is removed by the fluid flowing from the cold source T c to the hot source T h . This creates a temperature gradient along the bed.
Step 3: demagnetization of the material. The temperature of Each point of the regenerator decreases by Δ T due to the demagnetization.
Step 4: Flow of fluid from the hot source to cold source. The flow of the fluid from the hot source T h to the cold source T f transfers its heat to the regenerator. The temperature gradient is amplified. Fig. 4. Representation of AMRR cycle and temperature profile along the MCE material.
Since the gadolinium is considered as one of the best magnetocaloric materials currently available (Lebouc, 2005), the MCE corresponds to the absolute maximum value that can be obtained between the hot tank and cold tank. Thus it is obviously hard to imagine the exploitation of the MCE in most refrigeration applications (Engelbrecht, 2005).
This technical barrier has been overcome by the application of the Active Magnetic Regenerative Refrigeration (AMRR) (Engelbrecht, 2005;Lebouc, 2005;Tura, 2002). Regeneration in magnetic refrigeration systems allows the heat rejected by the network in any step of the cycle to be restored and returned to the network in another step in the same cycle (Yu, 2003). Thus, the capacity used for cooling the network load can be used effectively to increase the actual change of entropy and the obtained temperature difference (Yu, 2003).
AMRR cycles are illustrated in Fig. 4. The regenerative bed consists of plates of MCE material that initially have a quasi-linear temperature profile between the hot and cold tanks.
The bed itself acts as a regenerator. The different solid parts of the regenerator are connected by the fluid, so the heat does not need to be transferred between two solid parts separated, but on the same block.
Each particle of the bed undergoes a regenerative Brayton cycle and the entire bed undergoes a cascade Brayton cycle (Yu, 2003). This cycle is repeated 'n' times and the T Δ generated is amplified at each cycle to reach the temperatures limits of hot and cold sources (steady state). This T Δ is higher than the adiabatic temperature change of refrigerant material (MCE). In addition, the regenerator bed can be achieved by superposing different materials of different composition to expand the temperature's range of variation and thus to extend the utilization range of the system.

Magnetic refrigeration systems
Since the first magnetic refrigeration system manufactured by Brown in 1976, many researchers around the world have paid considerable attention to the magnetic refrigeration around room temperature and consecutively developed some interesting systems (Bouchekara, 2008) (Yu, 2010) (Bjørk, 2010. This section reviews -in detail -some of the magnetic refrigeration systems available until now.

The magnetic system of Brown
The system of Brown is a rotating system and employs an Ericsson cycle (Yu, 2003). The magnetic field is produced by an electromagnet (water cooled) with a maximum magnetic field of 7 T. The MCE material used is the Gd in the form of plates with 1 mm thickness, separated by stainless steel wires with 1 mm intervals to allow the regenerator fluid to flow vertically. The fluid is composed of 80% of water and 20% of alcohol. Without load and after 50 cycles, the temperatures reached were 46 ° C for the heat source and -1 ° C for the cold source, thus 47 C T Δ=°. However, the cooling power obtained was not rely important, this is due to the large T Δ obtained. Moreover, the cycle can operate only at low frequencies; the temperature gradient is reduced because both warm and cold sides have time to interact.

The magnetic system of Steyert
An alternative system with a rotating refrigerant, implementing a Brayton's cycle has been designed by Steyert (Yu, 2003). In this system, the porous magnetocaloric material has a form of rings. This wheel (the regenerator with a ring form) rotates through a first area of low magnetic field and a second area of high magnetic field as shown in Fig. 5. The exchange fluid enters the wheel (regenerator) at the temperature hot T and exits at the temperature cold T , having transferred its heat to the coolant located in the area of weak field. After receiving the heat of the load to cool cold Q the fluid enters the wheel again at a temperature cold T +Δdue to heat exchange with the wheel which is at this instant at the temperature hot T +Δ. The temperature of the fluid increases to hot T +Δ. Finally, the fluid transfers heat hot Q to the reservoir of the hot source completing one cycle at the same time. Fig. 5 describes schematically the magnetic system of Steyert.

The magnetic system of Kirol
This system was designed by Kirol (Yu, 2003) on the principle of a rotating machine and Ericsson's cycle. The magnetic field is produced by permanent magnets NdFeB and reaches a maximum value of 0.9 T in the air-gap. The refrigerant rotor is composed of a flat disk of 270 g of gadolinium as magnetocaloric material. During one rotation of the rotor, the four thermodynamic cycles are operated and a T Δ of 11 K is obtained.

The Spanish device
The device shown in Fig. 6 was developed by the team of the Polytechnic University of Catalonia in Barcelona (Allab, 2008). The magnetocaloric material is a ribbon of gadolinium (Gd 99.9%) fixed on a plastic disc and immersed in a fluid (olive oil). The magnetic cycle of magnetization / demagnetization is provided by the rotation of the plastic disc and its interaction with a magnet. The temperature span is obtained respectively: 1.6 and 5 K for a magnetic field of 0.3 T and 0.95 T. This corresponds to 2.5 times the MCE of Gd. Even if obtained performances of this system were weak, this device is the first that has shown the feasibility of magnetic refrigeration with fields accessible by permanent magnets.

Japanese system
Okamura et al. have constructed a magnetic refrigeration system, as shown in Fig. 7-a (Okamura, 2006). The yoke has an outer diameter of 27 cm and a length of 40 cm. The magnetic field is produced by rotating permanent magnets, producing a maximum field of 0.77 T. The bed regenerator is composed of 4 blocks. Each block is composed of a different alloy GdDy (sphere shaped) to enhance the range of variation of temperature. The fluid circulation is ensured by a pump and a rotary valve. The power obtained is about 60 W. The initial system has been improved as shown in Fig. 7-b (Okamura, 2007). The stator used was a laminated yoke and the magnetic field source was improved (the maximum field is 0.9 T). This helped to obtain a power of 100 W (using Gd as MCE material). (a) www.intechopen.com  Okamura et al., 2006Okamura et al., , 2007.

The magnetic system of Zimm
The ACM (Astronautics Corporation in Madison) has led many researches on magnetic refrigeration and achieved several patents in this field (Engelbrecht, 2005). In this corporation, an AMRR system was designed; it consists of a wheel with 6-bed regenerators composed of gadolinium powder. This wheel is rotating inside an area of high magnetic field of 1.5 T. The regenerative beds exchange with the fluid. The flow of the fluid is adjusted according to the relative position of each bed inside the magnetic field. Fig. 8 shows the photography of this prototype.
For cycles rotating with a frequency varying from 0.16 to 2 Hz and water rate flows varying from 0.4 to 0.8 l/min, the temperature spans obtained between the heat source and the cold source are from 4 to 20°C and the cooling power values are from 50 to 100W. Fig. 8. ACM prototype (Zimm, 2002). www.intechopen.com

Canadian system
At the University of Victoria in Canada, Tura and Rowe (2007) constructed a magnetic refrigerator containing permanent magnets for a testing of all sorts of magnetic refrigerants in different configurations. This machine is shown in Fig. 9. A nested Halbach array of NbFeB permanent magnets was applied and led to a magnetic field of 0.1-1.47 T strength.
Water was the heat transfer fluid with a heat rejection temperature range of 253-311 K, and the operation frequency was between 0 Hz and 4 Hz. The prototype showed cylindrical magnetocaloric regenerators (with a porosity of 57%) whose volume, diameter and length were 20 cm 3 , 16 mm, and 110 mm, respectively. The void in the regenerator of the hot heat exchanger and the cold heat exchanger was 0.83 cm 3 and 0.4 cm 3 , and the parallel flow paths in the heat exchangers were optimized with a computational fluid dynamics (CFD) approach. The system which is designed to be flexible showed many advantages: for example, a simple design, easy accessibility to all the components and very low heat leakages. This machine reached a maximum temperature span of 13.2 K (Yu, 2010).

Cooltech systems
The company Cooltech Applications in France built a rotary magnetic refrigerator composed of eight pieces of supporting discs positioned in synthetic material (see Fig. 10), which were mechanically stable and thermally isolated Muller, 2005, 2006).. These inserts were interchangeable for the test of different magnetocaloric materials, different sensors for temperature, pressure, air velocity, hydrometry and electrical power. Each insert was packed with 165 g Gd. The rotating axes were made of stainless steel, where four pieces of NdFeB permanent magnets were rotating to provide a magnetic field of 1 T. However, the authors reported on a new type (open Halbach) magnetic assembly, which yielded a magnetic field between 1 to 2.4 T. The flow of fluid was controlled to improve the cooling capacity, which was obtained in the range of 100W to 360 W (Yu, 2010).  Muller, 2005, 2006).

A rotary magnetic refrigerator prototype was developed in collaboration between the National Institute of Applied Sciences INSA of Strasbourg and the company Cooltech
Applications in France (Muller et al., 2007). The system was composed by a rotary magnet assembly and of four static blocks of magnetocaloric material performed by gadolinium. The maximum magnetic field was 1.3 T and water was the working fluid. Unfortunately, there is no more information available. However, from Fig. 11, one may easily verify the manner how the magnetic field is produced in the magnetocaloric material (Yu, 2010).

Fig. 11. A rotary magnetic refrigerator of the joint collaboration action between Cooltech
Applications and INSA in France (Muller et al., 2007). Fig. 12. The AMR bed was composed of 37 parallel plates of Gadolinium of 0.6 mm thickness, showing a spacing of the heat transfer fluid channels of 0.1 mm and 0.2 mm, respectively. The Halbach arrays, which produced a magnetic field intensity between 0.8 T and 1.1 T in the air gap, consisted of an assembly of three sets of NdFeB magnets of 50 mm thickness. The French experts obtained experimentally the evolutions of the average temperatures at the hot end and the cold end reservoirs for different initial temperatures and operation frequencies. The device led to a maximum temperature span of 16.1 K (Yu, 2010).

The G2Elab prototypes
The first device constructed at G2Elab (Grenoble Electrical Engineering Laboratory) is an alternating device type as shown in Fig. 13. The regenerator is composed of parallel plates of gadolinium with 1 mm in thickness and 50 mm in length. The magnetic field is produced by a permanent magnet (Halbach cylinder) creating a magnetic field of 0.8 T. The fluid used is water. Its circulation is ensured by a peristaltic pump operating in both directions (Clot, 2002). The pneumatic actuator produces the movement of the refrigerant blocs and provides magnetization / demagnetization phases. The controller is programmed to manage the Halbach cylinder and the flow of fluid to perform the four phases of the cycle. The system is closed and there is no exchange with the outside. It was designed to study the Active Magnetic Regenerative Refrigeration (AMRR) cycles and exploit different materials. Fig. 13. The G2Elab first device (Clot, 2002).
A second prototype was developed at G2Elab (Allab, 2008), (Bouchekara, 2008), (Dupuis, 2009). This structure is quite similar to a rotating machine. It is also similar to some existing prototypes (Okamura, 2005(Okamura, , 2007. It consists of a permanent magnet which forms the rotor and of a stator made of magnetic yoke and four refrigerant beds (see Fig. 14).
The yoke is composed of four poles which are aimed to better conduct the magnetic flux within the refrigerant bed. The magnetization and demagnetization phases are obtained by a simple rotation of the permanent magnet. The beds undergo an active magnetic regenerative refrigeration AMRR cycle and operate two by two in the opposite way.

MCE material
Fig. 14. The second prototype of the G2Elab, Components of the prototype (left) and the Prototype in its actual environment (right) (Bouchekara, 2008).

The Slovenian system
An interesting prototype of a rotary magnetic refrigerator (Tusek et al., 2009) has been built on the basis of permanent magnets at the University of Ljubljana in Slovenia. Their rotary magnetic refrigerator consisted of a rotating drum (cylinder) that rotated around an internally positioned stationary soft iron core and externally positioned stationary permanent magnets. As shown in Fig. 15, the magnetic structure was composed of four NdFeB permanent magnets and low carbon 1010 steel used as a soft ferromagnetic material, and two magnetic circuits existed to allow the rotary movement of the AMR's. After optimization of the magnet structure geometry, a range of magnetic field intensities from 0.05 T to 0.98 T was obtained in the air gaps. There were 34 AMR's in the rotary drum and each AMR had the dimensions 10 mm × 10 mm × 50 mm. Gd plates, with a thickness of 0.3 mm, were filled in the AMR's and the total mass of Gd was approximately 600 g. The prototype could operate up to a frequency of 4 Hz. This reference mainly focused on the experience in development of such a rotary magnetic refrigeration prototype and no experimental results were reported. However, first predictions according to the researchers are that approximately a 7 K temperature difference will be achieved (Yu, 2010). www.intechopen.com

Thermal study
Heat exchanges play an important role in magnetic refrigeration systems, both in the cold production cycles, and in the interaction with external environments, including the substance to be cooled. Thus, a thermal study is needed to determine the performance of a magnetic refrigeration system and optimize it. The aim of this section is to focus on the thermal modeling of magnetic AMRR systems.
Most of heat exchanges operating in the magnetic refrigeration are via convection. The convection represents transfer processes performed by the motion of fluids (Bianchi, 2004). In a solid (index 's') in contact, with a fluid (index 'f'), the flow through the wall (index 'w') can be written as: where : n is the normal to the wall and where: 2 hWK m   represents the coefficient of heat transfer by convection or simply the convection coefficient.
Using the first law of thermodynamics, by subtracting the mechanical energy, we get the balance of internal energy that gives us the heat equation governing the temperature field at any point in the domain (Janna, 2000) ( ) where: the dissipation function and P [ ] W is the local thermal power produced or absorbed.

Application to AMRR
Governing equations for AMRR system have been developed throughout the years with the objective of to analytically or numerically describe the thermal behaviour at specific time and for a given set of boundary conditions. They consist of a system of two equations, one for the fluid and the other for the solid matrix. These equations are derived from the energy balance expression for each phase. Since they are coupled they must be solved simultaneously (Bouchekara, 2008). The model of an AMRR cycle has been developed in (Bouchekara, 2008). Fig. 16 illustrates the concept of an AMRR regenerator modelled using one dimensional (1D) approximation. Fig. 16. Conceptual drawing of a 1D AMRR model.
The system of equations given by the energy balance (explained above) for both the magnetocaloric material and the fluid by neglecting the axial conduction (this approximation can be justified for different conditions: low thermal conductivity, very thin plates, etc.) can be summarized by the following system of equations: To solve this system we use the finite difference method. We use a grid of elements that range from 0 to L for the space and from 0 to τ for the time. Thus, the derivatives with respect to the time are calculated using forward formulas, and those with respect to the space are calculated using backward formulas. This gives a centered discretization scheme. Thus the system (25) becomes: The AMRR model has been implemented using Matlab ® commercial software.

Results
We will now apply the model developed earlier to a regenerator in the form of plates, as shown in Fig. 17(a). The equivalent cell of the whole regenerator is given in Fig. 17 (b). This cell has the same parameters as the regenerator except the width that is eqp lN l = (where eq l represents the equivalent width, p N represents the number of plates and l is the width of one plate). The model parameters (for this simulation case) are shown in Table 2. The magnetocaloric material used is gadolinium, the coolant used is water and the magnetic field is generated by permanent magnets B = 1 T.  Fig. 18 (a) shows the temperature evolution of both sides (hot and cold) of the material versus time. After a transient phase, the two curves reach their steady state. In addition, we note that the final value is greater than the initial MCE. From this curve we can extract the evolution of the temperature at the end of each cycle ( Fig. 18 (b)). The small delay between the two curves of this figure is due to programming constraints, i.e. the magnetization phase has been introduced (programmed) before the demagnetization phase.

Magnetostatic study
From the definition of the MCE, it is obvious that the performance of a magnetic refrigeration system depends mainly on the efficiency of the MCE material and the strength of the magnetic field. Thus the study of magnetic field sources dedicated to magnetic refrigeration systems is of a paramount importance. In this section we will pay a particular attention to the design of these sources using the finite element method.
The field sources described throughout this section are built with permanent magnets (Neodymium Iron Boron) with a remanent magnetization of 1.46 r B = T and a magnetic permeability of 1.064 The MCE material used is gadolinium with isotropic magnetic permeability 2 r µ = . In this study, MCE solid blocks are considered. However, in actuality to ensure a better heat exchange between the MCE material and the exchanging fluid, other forms are considered (plates, powder, etc). The yoke, when it exists, is made of XC10 steel.

Structure A: monobloc linear system
This first structure is suitable for linear magnetic refrigeration systems with direct cycles. It consists of two magnets (to create the magnetic field), a soft magnetic material yoke (to canalize the magnetic flux) and a block of MCE material (to create the cold) as shown in Fig.  19 (a). The MCE material has a linear alternating movement along the 'y' axis (to achieve magnetization and demagnetization phases). The magnetic characteristics (induction and magnetic force profiles) are shown in Fig. 19 (b) and Fig. 19 (c) while Fig. 19 (d) represents the distribution of the magnetic induction B in Tesla.

Structure B: Halbach cylider
The structure B is a Halbach cylinder (Fig. 20). It is a magnetized cylinder composed of 'N' segments of ferromagnetic material producing (in the idealized case) an intense magnetic field confined entirely within the cylinder with zero field outside. This second structure can be used in an AMRR system. The MCE material, which can be in the form of plates stacked in a cylinder, is guided by a linear motor or actuator to create the phases of magnetization (material located inside the cylinder) and the demagnetization phase (material located www.intechopen.com The projection of the geometry of the structure on the plane (oxy).
(a) Geometry of the first structure modeled by Flux 3D. outside the cylinder). The opposite way works also; i.e. the material remains fixed and the magnet is connected to the linear motor. In this case, the magnetic behavior is the same as in the first case. We modeled this structure with the commercial software 'Flux 3D'. The parameters used in this simulation are given in Table 3.  Table 3. The dimensions of the Halbach cylinder used in the simulation. Fig. 21 (a) shows the induction at the center of the material according to the movement. It shows clearly the phases of magnetization and demagnetization produced by this structure. Fig. 21 (b) represents the magnetic forces exerted by the cylinder on the block of MCE material. The distribution of the magnetic induction is shown in Fig. 21 (c).

Structure C: double Halbach cylinder
Structure C is a double Halbach cylinder; two cylinders are concentric and have the same number of segments (Fig. 22). Using this structure for magnetic refrigeration systems allows having the phases of magnetization and demagnetization simply by rotating one of the cylinders while the active material remains stationary at the center of the structure. The magnetic field produced at the center is the sum of the two fields produced by each cylinder. When the magnetizations of the first cylinder segments are in the same direction of those of the second cylinder, the magnetic field produced is high and the magnetization phase is achieved (this position is taken as a reference, i.e.°= 0 θ ). However, when the www.intechopen.com magnetizations of the first cylinder are in opposition with those of the second cylinder the magnetic field produced is low (°= 180 θ ) and the demagnetization phase is achieved.

Structure D: rotating multiblock system
The structure D presented here has a configuration adapted to rotary magnetic refrigeration systems with a direct thermal cycle. It has two magnets to create the field; a yoke of soft material to canalize the magnetic flux and N MCE material blocks for the creation of the cold (Fig. 23). Table 4 below shows the parameter values used in our simulation for the rotating multiblock system.

Conclusion
The conventional gas compression refrigerators have been mainly used for refrigeration applications. Generally, such refrigerators are not power-efficient. In addition, gases used in these refrigerators causes harmful effects on the environments. This has led to the development of magnetic refrigeration technology. Over the last decade or so, magnetic refrigeration at room temperature has become the subject of considerable attention. This technology is based on the use of magnetocaloric effect: that is the response of a solid to an applied magnetic field which emerges as a change in its temperature. This technology is www.intechopen.com ultimately aimed at developing a standard refrigerator for home use. Unlike conventional refrigerators, magnetic refrigerators are cost-effective and environmentally friendly. Moreover, it is noiseless and power-efficient and requires low maintenance cost and atmospheric pressure besides that its machines are easy to design.
This chapter introduced the magnetic refrigeration technology. It began by providing some essential definitions and provided a detailed review of ten magnetic refrigeration prototypes which are available until now. The operational principle of this technology was explained in depth through the comparison with the conventional one. The chapter then moved on to investigate the study of the magnetocaloric material using the Molecular Field Theory. The thermal study of the magnetic refrigeration process using the finite difference method (FDM) was then explained with providing some useful simulation results. Finally, the magnetic study of magnetic refrigerators using the finite element method (EFM) was presented with some practical results.