Refrigeration is the process of removing heat from an enclosed space or from a substance and transferring it to a place where it is unobjectionable. Magnetic refrigeration, based on the magneto caloric effect (MCE), has recently received increased attention as an alternative to the well-established vapor compression refrigerators for room-temperature applications. Conventional refrigeration systems are causing a great threat to the nature. We all are aware of the impact of refrigerants such as cfcs, hfcs, etc. but we still continue to use it. Why? It is mainly because of the non-availability of an alternative technology that is cost efficient.
Magnetic refrigerators were in use for the last 15years, those were focused on research and industrial purposes. MR is highly efficient over conventional refrigerating systems, but its cost and intense magnetic field used made it impractical for home uses. But the recent developments in this field assure that-it can be commercially implemented.
Compared to traditional systems, magnetic refrigerators are safer, doesn't make noise and are compact. The working principle of magnetic refrigeration is magneto thermodynamic phenomenon. This causes temperature changes in certain materials when exposed to dynamic magnetic field. This is also known as adiabatic demagnetization.
Thermo dynamic cycle of magnetic refrigeration
Following is the thermodynamic cycle involved in the magnetic refrigeration process.
- Adiabatic magnetization: Magnetic dipoles of a magnetocaloric material gets aligned when placed in an insulated environment surrounded by changing magnetic field thereby decreasing the material's magnetic entropy and heat capacity. Thus the substance gets heated
- Isomagnetic enthalpic transfer: The added heat is removed by a coolant and the magnetic field is kept constant to prevent the dipoles from reabsorbing the heat
- Adiabatic demagnetization: Now the surrounding magnetic field is reduced, the thermal energy causes the magnetic moments to overcome the field, and thus the sample cools
- Isomagnetic entropic transfer: Now the material is placed in contact with the environment to be cooled.
Selection of refrigerant
A good magnetic refrigerant should present a high entropy change at the working temperature of the refrigerator device under the application of a magnetic field. Above 40 K, paramagnetic materials cannot be used because the temperature variation involved in the magnetization process is small. Magnetic refrigeration was restricted to low temperatures because the heat capacity of a material increases with temperature. Thus, at high temperatures, the entropy change produced by an external magnetic field on paramagnetic materials is not enough to obtain a substantial temperature variation. The magnetization of a ferromagnetic material changes very rapidly near its Curie temperature, which means that at this temperature a high entropy change is produced with a magnetic field variation. Two important classes of magnetic refrigerants are paramagnetic materials for low-temperature ( T<20K) systems and ferromagnetic materials for higher temperature operation.
Commercial magnetic refrigeration/cooling units will be a reality in the near future. Magnetic refrigeration is an economic and environmentally sound alternative to vapor-cycle refrigerators and air conditioners. It offers considerable operating cost savings by eliminating the most inefficient part of the existing refrigerators – the compressor. It uses a solid refrigerant and a common heat transfer fluids (e.g. water, air or liquid helium) with no ozone-depleting and global-warming effects. The stronger the MCE the higher the efficiency of the device . One step away commercialization. But whether or not it will be just a niche market or a full-blown growth market 5 years from now is difficult to predict.