Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes - Magnetocaloric Energy Conversion: From Theory to Applications - page 246

unit of time). This low frequency means higher ef ciency, but low power density of

the device. Because of the expensive permanent magnet materials, as well as
expensive magnetocaloric materials, an ef

cient magnetic refrigerator operating
with the basic principle of AMR will be very expensive [ 145 ] (see also the chap. 9 ) .
Most of the losses related to devices that apply the basic AMR principle are related
to the low manufacturability of magnetocaloric materials, high heat-transfer irre-
versible losses, viscous losses, mechanical friction losses, losses related to valve
systems (dead volume and internal leakage) [ 4 6 ].

-

culties can be overcome by the introduction of thermal diode
mechanisms [ 4 ]. After the publication by Kitanovski and Egolf in 2010 [ 4 ] others
also began with research on thermal diodes in magnetic refrigeration at or near
room temperature. Almost all these investigations were directed towards solid-state
thermal diodes. Since there were not many investigations, we describe the different
approaches of the various authors.
Silva et al. [ 146 , 147 ] theoretically investigated the use of solid-state thermal
recti

Most of these dif

cators (with controllable thermal conductivity) to be used in magnetic
refrigeration. They proved that such a principle can be applied to magnetic
refrigeration as the cascade system of thermal diodes. They showed the great
potential of the proposed technology with high cooling-power densities and very
high operating frequencies (above 100 Hz) [ 146 ].
Olsen et al. [ 148 ], together with Tasaki et al. [ 149 - 151 ], evaluated the principle
of the solid-state thermal diode mechanism in which they also evaluated a kind of

cator or solid contact thermal switch. The work was
focused on the use of such mechanisms for a magnetic cooling system in vehicles.
In the concept of a solid-state magnetocaloric refrigerator the device consisted of
several layers of magnetocaloric material and thermal diodes (which formed
together a cascade system). Based on the concept a number of numerical simula-
tions were performed. The results of Olsen et al. [ 148 ] revealed that the proposed
concept could operate with a high volumetric cooling power even for a temperature
span of 60 K. When they compared an ideal gadolinium regenerator with plates of
50 microns thickness (which is not yet achieved in magnetic refrigeration, and
therefore the authors referred to such an AMR as a

ctive type of solid recti

) and the concept
of cascaded embodied gadolinium between thermal diodes, the last showed about
3.5-times higher cooling power density [ 148 ].
Most of the work relating to applications of thermal diode mechanisms in
magnetic refrigeration has been done in the

“ dream pipe ”

eld of solid-state Peltier thermal
diodes. For instance, Egolf et al. [ 152 , 153 ] presented a detailed study of the
operation of thin- lm thermoelectric modules with Ni-nanowires and concluded

that they could be a potential solution to be used as thermal diodes for magnetic
refrigeration; however, they pointed out certain aspects that have to be taken into
consideration, i.e. the thermal and electrical resistance, durability and low-cost
production.