Alternative Caloric Energy Conversions - Magnetocaloric Energy Conversion: From Theory to Applications - page 404

ow is, however, not the only medium that could be used to transfer heat
from the AER to the heat sink and heat source. Instead of a

A fl uid fl

uid a matrix made out
of a solid material could be used, and such a solution was proposed by Gu et al.
[ 49 ]. The solid matrix can simply be a set of parallel plates that can

fl

t into the gaps
between the electrocaloric plates. The working principle of a device stays the same,
only this time, instead of a uid, a solid matrix is moved in and out of the AER by a

mechanical system. Gu et al. [ 49 ] analysed the performance of a 10-mm-long AER
with a solid-state matrix. The electrocaloric plates were assumed to be made of P
(VDF-TrFE-CFE) terpolymer. They predicted that such a device with an AER
length of 10 mm could provide 9 Wcm − 3 of cooling-power density for a 20 K
temperature span. Moreover, they estimated that it could reach more than 50 % of
the Carnot ef

fl

ciency.

Devices with Thermal Diodes

In order to control the heat- fl ux direction from the electrocaloric material in a

speci c working step of an electrocaloric device (the heat should be transferred to a

heat sink during or after the polarization and to the heat source during or after the
depolarization) an element, called a thermal diode, could be applied. A detailed
description of different thermal diodes and their application in the case of mag-
netocaloric energy conversion can be found in Chap. 6 . Due to the analogy between
magnetocaloric and electrocaloric energy conversion, the concepts presented in
Chap. 6 could also be applied for electrocaloric refrigeration and heat pumping. In
this section, some of the concepts of electrocaloric energy conversion devices with
thermal diodes are discussed.
For example, Epstein et al. [ 50 ] presented the concept of an electrocaloric device
that is constructed from a thin-

lm multilayer capacitor (constructed from layers of

thin- lm electrocaloric material and layers of electrically conductive material)

sandwiched between two thermal diodes. The device is schematically illustrated in
Fig. 10.7 . The operation of the device consists of two main steps and two sub-steps.
First, the electrocaloric material in the multilayer capacitor is polarized. During the

rst half of the polarization, the thermal diodes are not active (no heat is transferred
from the multilayer capacitor) and the multilayer capacitor heats up. In the second
half of the polarization, the thermal diode in the contact with the heat sink is active
and the heat is isothermally transferred from the multilayer capacitor to the heat
sink (Fig. 10.7 a). Next, during the

rst part of the depolarization, the multilayer
capacitor cools down (both thermal diodes are inactive and no heat is transferred).
During the second part of the depolarization, the thermal diode that is in contact
with the heat sink is activated and the heat can be absorbed by the multilayer
capacitor (Fig. 10.7 b). This process repeats many times and after a number of
repetitions the steady state is achieved. Based on a numerical simulation, the

authors estimated that if a thermal diode with a conductivity contrast (de ned as the

ratio between the thermal resistances of the active and inactive thermal diodes) of