Introduction
Magnetic refrigeration has become an emerging technology that has the potential for high energy efficiency. It makes use of the magnetocaloric effect found in solid state refrigerants. Sold state refrigerants are instruments used either for heating or cooling, although the main application here is cooling. The combination of solid state rrefrigerants, water based heat transfer fluids and high efficiency fluids leads to environmental freindly products, which minimizes contribution to current global warming issue.
The magnetocaloric effect was originally found in the 18th century, by E. Warburg (1881) in pure iron, where the cooling effect only varied between 0.5 to 2 K/T. Then in the late 1920s, major advances appeared when two scientists: Debye (1926) and Giauque (1927) proposed cooling via adiabatic demagnetization. A few years later in 1933, Giauque and MacDougall demonstrated the process for cryogenic purposes and managed to achieve a temperature of 0.25K (Cryogenic meaning the study of the production of very low temperatures and behavior of materials at those temperatures). Advances occurred through the years up to 1997, until the first near room temperature proof of magnetic refrigerator concept was demonstrated by Professor Karl A. Gschneidner, Jr. from Iowa State University. This discovery drew attention and interests from scientists and companies worldwide. Since then new kinds of room temperature materials and magnetic refrigerator designs have developed.
In this modern era, reliable cooling technology is highly demanding. Refrigeration plays an important role in the societies’ health as well as the global economy. The modern refrigeration is based on a compression/ expansion refrigeration cycle, as they are quite reliable and relatively low cost technology. Over the years, such conventional refrigerator were considerably improved due to throughout research and development efforts. However some liquids that are used as refrigerants are hazardous chemicals, which are quite harmful to the environment. One of the effect would be the well known ozone layer depletion caused by the widely used refrigerant, chlorofluorocarbon.
Magnetic refrigeration technology is based on the magnetocaloric effect, an intrinsic property of magnetic materials near their magnetic ordering temperature (the temperature at which the thermal energy becomes large enough to destroy the macroscopic magnetic ordering within the material). Such material heats up when magnetized and cool down when demagnetized. In other words, magnetocaloric effect is the reduction in entropy as the magnetic moments of the atoms are aligned upon exposed to a magnetic field, while increase in entropy when demagnetized and magnetic moments become randomly oriented. Materials that obey this effect are known as the Ferromagnets. Soft Ferromagnets, which can be magnetized but do not tend to stay magnetized, are the most efficient and have very low heat loss due to heating or cooling processes.

Figure 1.1 Schematic representation of a magnetic refrigeration cycle. Yellow depict material in low magnetic field while green depict material in high magnetic field.
Figure 1.1
At first, randomly oriented magnetic moments are aligned once exposed to a magnetic field, resulting in heating of the magnetic material. The heat is then removed from the material to the ambient through heat transfer fluids. When the magnetized magnetic material is having the same temperature as its surrounding, it will be removed from the magnetic field. On removing the field, the magnetic moments in the material randomizes, resulting in cooling of the material below the ambient temperature. Thus, heat from the refrigeration system can then be transferred to the magnetic material via the heat tranfer fluid. This concludes one complete magnetic refrigeration cycle.
Figure 1.2 shows the Basic Principle of working Magnetocaloric Effect.
Figure 1.2
The material which exhibits the strongest magnetocaloric effect is Gadolinium. It is a rare earth ferromagnetic material, acts as a coolant. For the magnetic refrigeration system, a mixture of water and ethanol can be used as the heat transfer fluid for the system as such mixture has a freezing point of -40°F, so that the mixture will not freeze at operating temperatures. Furthermore this type of heat transfer fluid is cheaper than traditional refrigerants and environmental friendly.
One of the advantages of magnetic refrigeration would be energy saving. Energy consumption can be reduced by up to 50% when using a unit with magnetic refrigerating technology instead of a conventional refrigerator. Other than that, the efficiency of magnetic refrigeration is 60% to 70% as compared to Carnot efficiency, which leads to a higher C.O.P. (coefficient of performance) as compared with conventional refrigeration. Moreover it is as mentioned above that magnetic refrigeration is environment friendly because unlike conventional refrigerators, there is no production of CFC, or hazardous chemicals like NH3 (Ammonia) and greenhouse gases that will affect the ozone layer. Furthermore, magnetic refrigeration does not require maintenance and it is mechanically simple in construction.
On the other hand, the initial investment for such technology is higher as compared with conventional. Magnetocaloric materials like gadolinium are rare earth materials, hence the availability of it will be limited, while materials that are at higher availability would give weaker magnetocaloric effect, and thus lower efficiency.