Magnetic Refrigeration Process

General:-

The main objective of theoretical analysis is to yield a theoretical basis for an optimal design of new magnetic refrigeration devices and to focus on the various parameters of the magnetic refrigeration. Where, the performance of the magnetic refrigeration system can be found in terms of its coefficient of performance (COP).

This chapter presents a comprehensive treatment of the thermodynamics of cyclic magnetic refrigeration processes. It starts with a review of the work, heat and internal energy of a magnetized specimen in a magnetic field, then a list of the thermodynamic potentials be given [65]. Furthermore, described the development of the numerical model based on the magnetocaloric effect. Some Theoretical Background:-

As

Due to Eq. (3.1) holds true for all thermodynamic systems in quasi-equilibrium, an isothermal magnetization process can be compared to the isothermal compression of a gas, as was mentioned.

The magnetocaloric effect for a given material was typically in terms of either an isothermal entropy change or an adiabatic (isentropic, i.e. no irreversible losses) temperature change. So, these two quantities describe the difference in entropy or temperature, respectively, between two lines of constant applied magnetic field on a temperature-specific entropy diagram.

Fig.(3.1) illustrated the temperature-specific entropy diagram for Gd for difference magnetic field and for all temperatures, the entropy of Gd decreases as the magnetic field increased, and the effect be most dramatic near the Curie temperature about 293K. Fig. (3.1): Temperature-volume specific entropy diagram for Gd for different applied Magnetic Fields near the Curie Temperature (293 K)

Thus, the adiabatic temperature change can used as a measurement of the magnetocaloric effect of the material, where, ΔTad (T) ΔH is a function of ΔH and temperature, for a fixed and an arbitrary T and can be defined as:

〖∆T〗_(ad ) 〖(T)〗_(T,∆H,P)= [〖T(S)〗_HI- 〖T(S)〗_H0 ]_(s,p) (3.6)

The change in specific entropy of a magnetic material can write as the change in specific entropy of a magnetic material, and the variations in pressure negligible as: dS(T,H)=(∂s/∂T)_H dT+(∂s/∂H)_T dH (3.7) where, H is magnetic field strength, and the two right terms in Eq. (3.7) can both expressed using the laws of thermodynamics. The first term express as the relationship between specific heat and entropy and given by [7]: c_(μ_0 H )/T=(∂s/∂T)_H (3.8) where c_(μ_0 H ), the specific heat in an isofield