About eco-friendly magnetic refrigeration- The Future

One of the difficulties with conventional vapour-compression refrigeration cycles is that most of the better refrigerants are ozone depleting substances consisting of chlorinated fluorocarbons (HCFCs) like freon gas. 'Freon' is a trade name for a family of  haloalkane refrigerant manufactured by DoPont and other companies. These refrigerants are commonly used due to their superior stability and safety properties: they are not flammable nor obviously toxic as are the fluids they replaced, such as Sulphur dioxide. Unfortunately, these chlorine-bearing refrigerants reach the upper atmosphere when they escape. In the stratosphere, CFCs break up due to UV-radiation, releasing their chlorine atoms. These chlorine atoms act as catalyst in the breakdown of ozone, thus causing severe damage to the ozone layer that shields the Earth's surface from the Sun's strong UV radiation. The chlorine will remain active as a catalyst until and unless it binds with another particle, forming a stable molecule. So the major risk involved with this refrigerator is that the manufacturers have to be careful with not to let the harmful freon gas leak out. Newer refrigerants, currently being the subject of research, have reduced ozone depletion effect that include HCFCs (R-22, used in most homes today) and HFCs (R-134a, used in most cars) and have replaced most CFC use. HCFCs in turn are being phased out under the Montreal Protocol and replaced by hydrofluorocarbons (HFCs), such as R-410A, which lack chlorine. However, CFCs, HCFCs, and HFCs all have large global warming potential. Supercritical carbon dioxide, known as R-744 have similar efficiencies compared to existing CFC and HFC based compounds, and have many orders of magnitude lower global warming potential . Although modern refrigerators have replaced freon with a less harmful liquid, other environmental cooling techniques are being actively explored. One novel possibility is to use magnets to extract heat away, where rather than going into the expansion of a gas—as in conventional refrigerators—the thermal energy goes into disordering the aligned spins of a magnet. Magnetic refrigeration has three prominent advantages compared with compressor-based refrigeration. First, there are no harmful gases involved; second, it may be built more compactly as the working material is a solid; and third, magnetic refrigerators generate much less noise.

    Magnetic refrigeration utilizes the magnetocaloric effect (MCE). This effect causes a temperature change when a certain metal is exposed to a magnetic field. All transition metals and lanthanide series elements obey this effect. These metals, known as ferromagnets, tend to heat up as a magnetic field is applied. As the magnetic field is applied, the magnetic moments of the atom align. When the field is removed, the ferromagnets cool down as the magnetic moments become randomly oriented. A magnetic field can easily align the spins on the manganese sites so that if the magnetized material is allowed to come into thermal contact with a ‘hot’ object, then heat can depolarize the spins as per the scheme suggested in the flow chart. Soft ferromagnets are the most efficient and have very low heat loss due to heating and cooling processes. Gadolinium, a rare-earth metal, exhibits one of the largest known magnetocaloric effects. Also one can employ arc-melted alloys of gadolinium, silicon, and germanium that provide greater temperature ranges at room temperatures in the design of most modern magnetic refrigeration system.

The flowchart of magnetic refrigeration; Here, H is the magnetic field, Q is the heat transfer, T is the temperature and ΔT_ad is the temperature change when the spins depolarize (with no heat transfer).

   Keeping this principle in mind, hypothetically one can design a magnetic refrigerator as follows: The heat transfer fluid for the magnetic refrigeration system may be a liquid alcohol-water mixture having a given freezing point, assuring the mixture does not freeze at the set operating temperatures. This heat transfer fluid shall be cheaper than traditional refrigerants and also eliminates the environmental damage produced from these refrigerants. During the operation the heat transfer fluid gets cooled to desired lower level of temperature by the non-magnetized cold set of beds that contain the small spheres of magnetocaloric material. This cooled fluid is then sent to the cold heat exchanger where it absorbs the excess heat from the freezer. This fluid leaves the freezer at 0°F. The warm fluid then flows through the opposite magnetized set of beds, where it is heated up to the desired higher level. This hot stream is then cooled by room temperature air in the hot heat exchanger. The cycle then repeats itself after the beds have switched positions back and forth to the field while still keeping them in contact with the heat transfer plates. Thus there will have an eco-friendly, non-compressor, noise free and highly compatible refrigeration system for house hold and automobile cooling applications. In this perspective, our group is engaged in developing such an eco-friendly solid state system based on superpapramagnetic nanoferrites especially for applications at near room temperature magnetic refrigeration.