Magnetocaloric effect (MCE)
Certain materials heat up, when brought into a magnetic field. This phenomenon is called the magnetocaloric effect (MCE). Materials endowing this effect are called magnetocaloric materials (MCM). Using the magnetocaloric effect, a magnetic cooling cycle near room temperature is possible.
The magnetocaloric effect (MCE) manifests itself as a change of thermodynamic state of a magnetic material in an external alternating magnetic field H. Depending on the conditions (isothermal or adiabatic) under which the magnetic field H is applied, either the isothermal entropy change ∆st or the adiabatic temperature change ∆Tad are commonly used as numerical parameters characterizing the magnitude of the magnetocaloric effect.
An increasing field H causes the magnetic moments/spins of the magnetic material to align parallel to H decreasing the magnetic entropy Smag. Due to adiabatic conditions within the system, the total energy of the system stays constant and the material is heated by a temperature ∆Tad with the net temperature of the material now being Tstart + ∆Tad due to an increase in the lattice entropy Slat.
Magnetocaloric materials are metals endowing the magnetocaloric effect (MCE). Such materials include pure elements such as Gadolinium or alloys such as Lanthanum-iron-silicon (LaFeSi) or Iron-phosphorus (Fe2P).
This effect was first theoretically postulated in the works of Langevin in 1905. He predicted that the temperature of a paramagnetic material should change as a function of magnetic field. The later called magnetocaloric effect was first observed in experiment by Weiss and Picard in the year 1917 where they could measure a temperature change on Ni among application of a magnetic field.
Magnetic Cooling Cycle
By using the magnetocaloric effect (MCE) intelligently, a magnetic cooling cycle is possible. The magnetocaloric cycle is performed analogous to the refrigeration cycle by Carnot. In contrast to the conventional compressor-based Carnot cycle not the pressure but the magnetic field is increased and decreased. Similar to a gas compression cycle, the magnetic cooling process includes four steps.
Four steps of magnetic cooling cycle
At the starting point (1) the magnetic material is in its paramagnetic state. The magnetocaloric material is placed in a thermally insulated (adiabatic) environment and the magnetic field is applied. The material heats up due to the above-mentioned magnetocaloric effect (2).
The added heat Q is then extracted by fluid or gas heat exchange media. The magnetic field hereby is held constant. Once the material has cooled down to the temperature Tstart (3), it is brought back to adiabatic conditions and the magnetic field H is removed.
As the total entropy of the system is again constant and the spin entropy of the system increases and the lattice entropy decreases, the material must expel energy in the form of heat, whereas the material is cooled to a temperature of Tstart – ∆Tad.
The material is then put in contact with a thermal transfer fluid which is in contact with the environment to be refrigerated.
In an iterative process a cooling cycle can then be realized and can be used in a magnetic cooling device for example a refrigerator or air conditioner.
Active Magnetic Regeneration (AMR)
Magnetic cooling has the capability to be up to 40% more efficient than conventional gas compression cooling and heating solutions.
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