Alternative Caloric Energy Conversions - Magnetocaloric Energy Conversion: From Theory to Applications - page 391

simplify, as the electrocaloric material is subjected to a positive electric eld change

the material heats up, yet as the electric eld is turned off the opposite occurs and

the material cools down. Speaking thermodynamically, the electrocaloric effect is
analogue to the magnetocaloric effect, though instead of the magnetic

eld change

an electric eld change is required to induce the caloric effect in the material.

However, the electrocaloric energy conversion has some potential advantages over
the magnetocaloric energy conversion, such as higher power density and higher
compactness of the energy conversion devices, no dependence on rare-earth
materials, no moving parts of the device, operation of the devices with less
vibrations, silent operation, etc. Nevertheless, the area of the electrocaloric energy
conversion only recently attracted more attention by the scienti c community and

therefore the development of the electrocaloric energy devices is at its early stage.
When the pyroelectric material is subjected to a variation of the temperature and
short circuited, an electrical current named the pyroelectric current will be gener-
ated. Therefore, the pyroelectric energy conversion is a way to directly convert
thermal energy into electrical energy. The materials used for pyroelectric power
generation are of the same group as those used for electrocaloric energy conversion.
However, they are exposed to different thermodynamic cycles and operating con-
ditions and therefore the desired characteristics of the materials used for pyro-
electric power generation can be, to some extent, different from those used for
electrocaloric energy conversion.
In this chapter, the physics behind the electrocaloric and pyroelectric effect are
discussed and some materials with the electrocaloric and pyroelectric effect are
reviewed. The emphasis, however, is on the rst concepts and prototypes of the

electrocaloric and pyroelectric energy conversion devices.

10.1.1 Introduction to the Electrocaloric Effect

The electrocaloric effect (ECE) is a physical phenomenon that occurs in some
dielectric materials under the in eld. It is expressed as

the adiabatic temperature or isothermal entropy change of the material. To explain
this phenomenon let us consider a polar dielectric material in which charged par-
ticles are initially randomly distributed (Fig. 10.1 a).
When the material is exposed to an electric

fl uence of a varying electric

eld, an electric force acts on the
charged particles. These tend to move or shift in the direction of the eld and

undergo a transition from the disordered to the ordered state (Fig. 10.1 b). A con-
sequence of this orientation is an apparent accumulation of negative charges on the
top surface, and positive charges on the bottom surface, of the dielectric material.
The amount of charge accumulated per surface area is de

ned as the polarization.

However, this is a simpli ed explanation of polarization, which is known to be a

combination of a number of different effects. For a detailed description of polari-
zation and the properties of dielectric materials, the reader is referred to Kao [ 1 ].