Alternative Caloric Energy Conversions - Magnetocaloric Energy Conversion: From Theory to Applications - page 417

where P s is the spontaneous polarization, X is the elastic stress and E is the electric

eld. Furthermore, if the pyroelectric material is short-circuited and subjected to a
temperature change we can write:

p dT

dt

I p ¼ A ð 10 : 16 Þ

where A is the surface area of the pyroelectric material and t is the time.
The pyroelectric current generated due to the temperature uctuations of the

pyroelectric material can be harvested using the different thermodynamic cycles and
devices that are presented later in this chapter. However, it is already obvious, that
in order for a pyroelectric material to convert as much thermal energy as possible
into electrical energy, the pyroelectric material should possess a large pyroelectric
coef

fl

cient is a property of dielectric materials
whose polarization changes intensively by changing their temperature. The same
conclusion was already made for the electrocaloric effect, i.e. the larger the rate of
change of the polarization with respect to the temperature, the larger will be the
electrocaloric effect. Such a behaviour of the polarization can be found in ferro-
electric materials around their phase transition (transition from the paraelectric to
the ferroelectric phase) temperature. To

cient. A large pyroelectric coef

nd out more about the properties of fer-
roelectric materials, the reader is referred to Sect. 10.1.1 .

10.1.5 Pyroelectric Materials for Energy Harvesting

The pyroelectric effect was probably already observed by the Greeks more than
2,400 years ago in the mineral tourmaline; however, the pyroelectric effect was

rst

introduced to the scienti c community in 1,717 by the physician and chemist Louis

Lemery [ 57 ]. Intensive investigations of the properties of pyroelectric materials
began in 1960 and during the period 1960 2003 there were more than 8,500

publications [ 58 ]. Nowadays, pyroelectric materials can be found in many

-

elds of

applications, such as thermal imaging, light, motion and re detection, etc. In this

chapter the focus will be on pyroelectric materials that could be applied for energy
harvesting applications. The pyroelectric materials used in the rst generation of

prototypes for energy harvesting devices can be divided between ceramic and
polymer materials. In general, it is desirable for the pyroelectric material to possess
a large pyroelectric effect. However, other properties should be taken into account
when choosing the appropriate pyroelectric material, such as a high electrical
resistivity (to minimize the leakage current through the pyroelectric element), a low
heat capacity (to minimize the heat input needed to change the temperature of the
pyroelectric material) and a small hysteresis [ 59 , 60 ]. Furthermore, for some
applications, the high dielectric strength of the pyroelectric material is important
[ 61 , 62 ].