The Thermodynamics of Magnetocaloric Energy Conversion - Magnetocaloric Energy Conversion: From Theory to Applications - page 11

ð T R T amb Þ

D e R ¼ ð T R T amb Þ D s R ¼ q R ð 1 : 35 Þ

T R

Now we can use Eqs. ( 1.29 ) and ( 1.35 )tode ne the exergy ef ciency for cycles

that perform refrigeration by isothermal magnetization:

q R
COP C !

COP
COP C

e R ¼ n ¼ ð 1 : 36 Þ

D

In cases where the refrigeration is not performed isothermally (e.g. in the case of

iso eld cooling in the Brayton process), the exergy ef ciency can be de ned as

follows:

q R
COP C !

COP
COP C

D e R ¼ n ¼ ð 1 : 37 Þ

Note that in the cases shown here, the work or power correspond only to the
magnetic work. However, one should be aware of other exergy inputs, which
should also be taken into account (e.g. pump work, transmission work or losses) for
the de

nition of the exergy ef ciency or the COP of the real device.

1.3.2 Overview of the Basic Thermodynamic Cycles

A comprehensive theoretical description of the different thermodynamic cycles is
given by Kitanovski et al. [ 55 ]. Here, we will present the basic magnetic thermo-
dynamic cycles in T s and H h diagrams. The best-known magnetic thermody-

- -

namic cycles are the Brayton, the Stirling, the Ericsson, the Hybrid

(Ericsson Brayton) cycle and the Carnot cycle.

-

1.3.2.1 The AMR Thermodynamic Cycle

Most of the applications of magnetic refrigeration at room temperature use per-
manent magnets as the magnetic

eld source. Because these are very cost intensive
and limited in their magnetic energy, researchers tend to apply moderate magnetic

elds lead to adiabatic tem-
perature changes in current magnetocaloric materials of up to about 5 K. However,
the required temperature span between the heat source and the heat sink is usually
much larger. Therefore, the most common way to increase the temperature span is
to apply a magnetocaloric thermodynamic cycle that includes regeneration (note
that a cascade system may be applied as well, but this needs to take account of the
irreversible losses of heat transfer [ 37 ]).

elds, mostly between 0.8 and 1.5 T. Such magnetic