Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes - Magnetocaloric Energy Conversion: From Theory to Applications - page 217

If we consider a realistic temperature pro le (between 273 and 300 K) along the

demagnetized (zero eld) magnetocaloric material in the magnetic refrigerator, it is

possible, based on the approach described above, to de

ne the local exergy ef -

ciency of a particular Peltier thermal diode mechanism. Furthermore, we de ne the

integrated average exergy ef ciency of all the Peltier thermal diode mechanisms

along such a magnetocaloric material (the exergy was de ned at a temperature of

the ambient equal to 303 K). The results for this example are shown in Fig. 6.6 .
As can be seen in Fig. 6.6 , different temperature pro

les, which also correspond

to different mass fl ows of the working fl uid, do not drastically in fl uence the average

exergy ef ciency of all the thermal diode mechanisms embodied in the magnet-

ocaloric device (100 thermal diodes were estimated in this particular case). The
average values of the exergy ef ciency of the thermal diodes are high (above 85 %),

which again points to the high ef

ciency of such a mechanism, despite the fact that

the exergy ef ciency of the Peltier module is very low.

The value 85 % of exergy ef ciency denotes that 15 % of exergy will be

destroyed due to application of the Peltier thermal diodes on the cold side of the
magnetocaloric material (positioned below the magnetocaloric material in Fig. 6.5 ).
Since thermal diodes are placed also on the warm side of the magnetocaloric
material (positioned above the magnetocaloric material in Fig. 6.5 ), one can expect
that Peltier thermal diode mechanism in the magnetocaloric device will reduce its
exergy efciency for about 15 - 30 % (depending on the characteristics of Peltier
modules and their ef

ciency, which in our case was taken to be rather low). Despite
reduction in the exergy ef ciency due to the application of thermal diodes, the same

mechanism avoids exergy losses in other parts of device: valves, seals, dead vol-
ume, bidirectional pumping, etc.

Fig. 6.6 a Temperature proles along the magnetocaloric material in the demagnetized state,
b Corresponding local and average exergy efciency of a thermal diode mechanism on the “ cold ”

side of a magnetic refrigerator