**1. 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

Magnetic Refrigeration Technology at Room Temperature 227

where S (J K-1) is the entropy (subscripts m and l are respectively for magnetic and lattice

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

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 form -

> *S S dS T B dT dB T B* ∂ ∂

> > *<sup>S</sup> C T T*

> > > *B S C T T*

*C S dS T B dT dB T B*

In the case of an adiabatic process (no entropy change 0 Δ = *S* ) the temperature variation can

= + <sup>∂</sup>

*B T T S dT dB C B*

*T B*

*B B T M dT dB C T*

*S M B T*

*B*

( ) ,

*B T*

*B*

*B*

*T*

∂

= + ∂ ∂ (2)

<sup>∂</sup> <sup>=</sup> <sup>∂</sup> (3)

<sup>∂</sup> <sup>=</sup> <sup>∂</sup> (4)

**<sup>B</sup>** (5)

<sup>∂</sup> = − <sup>∂</sup> (6)

∂ ∂ <sup>=</sup> ∂ ∂ (7)

<sup>∂</sup> = − <sup>∂</sup> (8)

entropies), T (K) is the temperature and B (T) is the magnetic filed induction.

isothermal entropy change or the adiabatic temperature change.

( ) ,

The specific heat capacity CB (J m-3 K) of the material is given as:

can be given as:

This gives:

be written as:

We can write:

From (2) and (4) we can write:

Using the Maxwell relation given as:

where *M* (A m-1) is the magnetization.

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.
