Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes - Magnetocaloric Energy Conversion: From Theory to Applications - page 250

Fig. 6.29 Temperature
distribution of a single-stage
magnetocaloric device with
thermal diodes along the
length (y-direction in
Fig. 6.28 ) of the device

process of demagnetization occurs, the temperature of the MC material decreases
(due to the MC effect).
At this point thermal diode 2 should start to operate, transporting the heat from
the cold heat exchanger to the MC material. During this process thermal diode 1
should not operate and should act as a thermal insulator, preventing heat from the
hot heat exchanger from being transferred back to the MC material. All four phases
of the magnetization, thermal diode 1 ON, demagnetization, thermal diode 2 ON
represent a single thermodynamic cycle for such a device. The ow through

the heat exchangers may be continuous or periodic. In the last case, this is dif

fl uid fl

cult

to perform without very large losses. Furthermore, the periodic fl uid fl ow should be

well synchronized with the operation of the device.

6.6.1.1 Active and Passive Thermal Diodes in a Single-Stage Operation

According to the previous subsections, these two types of thermal diodes should
operate in a different manner regarding their temperature pro

le of operation.
Figures 6.30 and 6.31 show the steady-state operation of the magnetocaloric
device with passive and active thermal diodes, respectively. Both gures show all

four phases of a single-stage thermodynamic cycle, regarding the temperature
pro ow (see Fig. 6.28 , x-direction).

Figure 6.30 presents the operation of passive thermal diodes coupled with
magnetocaloric material. In Fig. 6.30 a the magnetization phase occurs, which leads
to a temperature increase in the MC material. Both thermal diodes (TD1 and TD2)
are at that point non-operational and just a weak temperature response of the
thermal diodes can be seen due to the MC material temperature increase. Both
thermal diodes have,

le transversal to the fl uid fl

in this example, low effective thermal conductivities.