Environmental Engineering Reference
In-Depth Information
In a recent study by Egolf et al. [
154
], which was focused on thermal switches
built with Ni-nanowire Peltier modules, the performance of these modules was
evaluated on a theoretical and an experimental basis. An approximate estimate of
the performance of a magnetic refrigerator of the types built so far was compared to
that of a magnetic refrigerator with nanowire thermal switches. The authors com-
pared the operation of the AMR-based magnetic refrigerator with a refrigerator
based on a thermal diode mechanism. The results reveal that for the same cooling
power of 50 W and the same exergy ef
ciency of 52 % (for the given operating
parameters and a magnetic
eld change of 2 T), the AMR-based refrigerator would
require
ve times more magnetocaloric material than a refrigerator based on thermal
diodes. Consequently,
the mass of permanent magnets would be substantially
smaller.
In the study of Tomc et al. [
5
,
6
,
155
], thin-
lm Peltier thermal diodes were
considered to be applied with a thin plate of gadolinium. The value for the magnetic
eld change was taken to be 1 T. The results of their study reveal that the appli-
cation of thin Peltier thermal diodes in combination with a magnetocaloric material
could lead to high exergy ef
ciency for the whole device, e.g. up to 65 % for an
operating frequency of about 20 Hz (corresponding to a speci
c cooling power of
about 2 kW per kg of gadolinium). This would strongly depend on the thickness of
the magnetocaloric material, the operating frequency (the higher the frequency is,
the lower efciency will be), as well as the temperature span between the heat
source and the heat sink.
The only work that relates to the application of micro
uidic thermal diodes in
magnetic refrigeration at room temperature was reported in a diploma thesis at the
University of Ljubljana [
156
]. The results of this particular work showed that
fl
uidic
thermal switches can be successfully applied for very high operating frequencies
(above 100 Hz).
fl
6.6 Potential Congurations of Thermal Diodes
in Magnetic Refrigeration
As noted before, the reason that today
s state-of-the-art magnetocaloric technology
is not competitive with conventional refrigeration technologies is based on one of
the most important characteristics of a magnetocaloric device, i.e. the power den-
sity. Not having a suf
'
ciently high power density directly affects the cost of a
device. However, to decrease the costs of the device (which is predominantly a
consequence of its large mass and the high costs of the magnetic and magnetoca-
loric materials) the power density should be increased. And the best way to achieve
this is to increase the operating frequency of the device. However, an increase in the
operating frequency usually leads to low ef
ciency due to the poor heat-transfer
characteristics, which the classical active magnetic regenerator has to face. In the
preceding chapters on AMR research and development a number of suggestions
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