The Thermodynamics of Magnetocaloric Energy Conversion - Magnetocaloric Energy Conversion: From Theory to Applications - page 3

1.2 Heat, Work and the Basic Thermodynamic Relations

The thermodynamics in this chapter relates to the magnetocaloric material as the
observed system. Because of this, we deal with the thermodynamics described with
the internal magnetic

eld in the magnetocaloric material. This should not be
misunderstood as the external magnetic eld, which is related to the magnetic eld

source (e.g. the magnetic air gap of a permanent magnet).

One way to understand magnetic energy is to consider it as a form of potential
energy. Imagine a rock on a mountain, having a potential energy (a magnetocaloric
material in a magnetic

eld in the “ empty ”

eld, produced by a permanent magnet or induced by an
electrical coil). In order to put the rock on the mountain, work has to be performed
on it (work is performed on the magnetocaloric material when it is magnetized;
therefore, the magnetocaloric material receives magnetic work from the permanent
magnet or electric

eld source). When the rock is rolled downhill, its potential
energy decreases and the rock does work, e.g. through kinetic energy (the mag-
netocaloric material does work during the demagnetization process). Actually,
during the demagnetization the magnetocaloric material will have to be pulled out
of the magnetic

eld. Looking at the magnetocaloric material, which in our case is
the observed system, the magnetocaloric material performs the work.
In the following text, we will assume conditions of constant pressure p and
volume V for a solid magnetocaloric material. For a simpler presentation we have
written all the equations in their one-dimensional form and apply notation for the
exact differential for speci c work (dw instead of d w) and speci c heat (dq instead

of d q).
Figure 1.1 shows an example of a thermodynamically closed system, which is
analogous to a piston compressing a gas in a cylinder. In such a system, there is no
transfer of mass over the system boundaries (the magnetocaloric material represents
the system boundaries). In a thermodynamically opened system, however, there is a
mass ow of the magnetocaloric material in and out of the system boundaries. For

instance, as shown in Fig. 1.2 ,axed system boundary is shown around the
magnetocaloric material. According to Fig. 1.2 , the magnetocaloric material rotates
(

fl

fl ows) through such a boundary. The rst law of thermodynamics for a closed

thermodynamic system states that:

du ¼ d q d w ð 1 : 1 Þ

The internal energy of the magnetocaloric material will increase if heat is added
to a system or if work is performed on the magnetocaloric material. According to
Fig. 1.1 , the work is performed on the magnetocaloric material by moving the
material into a magnetic eld, produced by a permanent magnet or by the induction

of a magnetic eld using an electrical coil. Because of this, the magnetocaloric

material is magnetized (Fig. 1.1 b), and its internal magnetic

eld increases.

c work required to magnetize the magnetocaloric material in a ther-
modynamically closed system is equal to (see also [ 26 , 37 , 39

The speci

46 ]):

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