Alternative Caloric Energy Conversions - Magnetocaloric Energy Conversion: From Theory to Applications - page 422

Self-Driven Pyroelectric Energy Harvesters

Cuadras et al. [ 65 ] constructed an experiment in which the pyroelectric effect was
used to charge a capacitor. A special electrical circuit was developed in order to
store the electrical charge in the capacitor (generated due to the pyroelectric effect)
during the heating and cooling of the pyroelectric material. The pyroelectric
material was heated up from room temperature to 62 C using a hair dryer and then

cooled down, based on free cooling, back to the initial temperature. Moreover, to
achieve a higher voltage on the capacitor and to store more energy in the capacitor,
two pyroelectric elements (in this case two PZT ceramics with a thickness of
100

°

m) were electrically connected in parallel. After a number of cycles (one cycle
was de

µ

ned as a process during which the pyroelectric material heats up and cools
back down once), in approximately 1,000 s, a maximum voltage of 31 V was
reached on the charged capacitor, with an available energy of 0.5 mJ.
Zhang et al. [ 68 ] investigated the possibility of harvesting electrical energy from
the temperature

uctuations of a pyroelectric material exposed to solar radiation and
changing wind conditions during daytime. The experiment was conducted under
laboratory conditions, where solar radiation was simulated using a spotlight with
20 W of power and a centrifugal fan was used to simulate the changing wind
conditions. As the pyroelectric material, a 140-

fl

m-thick PZT ceramic in the shape
of a disc with a diameter of 24 mm was used. The pyroelectric material was
exposed to a constant solar radiation of 1000 Wm − 2 . On the other hand, the fan was
periodically switched on and off, creating different air

µ

ow velocities around the
pyroelectric material and changing the heat-transfer rate due to the heat convection.
The tests lasted for approximately 430 s and every 100 s the fan was turned on for
30 s, thereby cooling down the pyroelectric material for 16 K. Due to the tem-
perature variation of the pyroelectric material, an average electric power density of
4.2

fl

Wcm − 3 (per volume of the pyroelectric material) was produced.
A similar experiment to the one described above was conducted by Mane et al.
[ 63 ]. They investigated the possibility of harvesting electrical power by exposing a
pyroelectric material to changing levels of radiation. Three different pyroelectric
materials, a single-crystal PMN-0.3PT, a commercially available PZT ceramic and
a pre-stressed composite PZT ceramic, were considered. A lamp was used as the
light source and a rotating disc with an aperture was positioned between the lamp
and the sample (the pyroelectric material). By rotating the disc, the sample was
periodically exposed to increased levels of radiation, and therefore a temperature
variation was induced in the sample. Different cyclic frequencies of the disc were
investigated. The results showed that

µ

the maximum peak power density of

Wcm − 3 (per volume of the pyroelectric material) was achieved in the PMN-
30PT single-crystal ceramic material at an angular velocity of 0.64 rad

8.64 µ

s − 1 of the
rotating disc and at a heating rate of the pyroelectric material of 8.5 Ks − 1 . Under the
same conditions, a peak power density of 4.48

·

Wcm − 3 was measured in the
commercially available PZT ceramic and a peak power density of 6.31

µ

Wcm − 3 in
the prestressed composite PZT ceramic. However, the authors did not provide

µ