Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes - Magnetocaloric Energy Conversion: From Theory to Applications - page 209

6.1 Introduction

The operation of a thermal diode mechanism can be managed by the application of
an external energy source (active thermal diodes) or simply by in fl uencing the

characteristics of materials or uids (passive thermal diodes) without the need for

an additional power source. The thermal diode can have the function of a thermal
switch or valve,

fl

ow. One of the most basic
mechanisms that can be applied as a thermal diode is a thermosyphon [ 1 ]. Another
possibility is the heat pipe [ 2 , 3 ], which can be found in numerous different systems,
from space applications, electronic devices, to applications in buildings. Despite the
fact that a heat pipe can provide ef

i.e. to block or enable heat to fl

cient heat transfer, it cannot operate as a fast
switching mechanism or thermal diode for both the heat- fl ux direction and the heat-

fl ux intensity (i.e. changes that need to be implemented in a certain very short time

interval between a millisecond and approximately a second).
Figure 6.1 shows an example of the application of a thermal diode mechanism in
the case of an application in magnetocaloric, barocaloric, electrocaloric or elast-
ocaloric energy conversion. In the case of Fig. 6.1 a material is sandwiched between
two thermal diodes. When the material is being magnetized (magnetocaloric),
polarized (electrocaloric), pressurized (barocaloric) or stretched (elastocaloric), it
heats up. For a thermodynamic cycle we need to remove this generated heat. In this
particular case the thermal diode ' s operation is set to ON in order to guide the

generated heat s temperature will

decrease. Another thermal diode, which is connected with the heat source at the
same time, needs to be set OFF (or it should represent a closed heat valve). When
the material is being demagnetized (magnetocaloric), depolarized (electrocaloric),
depressurized (barocaloric) or released (elastocaloric), it cools down. For a ther-
modynamic cycle we need to bring the heat from the heat source. In a continuous
process these steps will form a thermodynamic cycle.
This is only one example where a thermal diode mechanism operates as a
thermal switch or heat valve. However, thermal diodes may also possess other
mechanisms or a combination of these mechanisms. Figure 6.2 shows an example

to the heat sink. Consequently, the material '

Fig. 6.1 Thermal diode
operation in magnetocaloric,
electrocaloric, elastocaloric or
barocaloric energy conversion