Environmental Engineering Reference
In-Depth Information
as noted already before the resistive magnets, especially at higher magnetic
ux
densities, will require in most cases also cooling associated with the Joule heating
due to the electrical resistance in the coil. Despite this, if such a system is built, one
has to keep control of the electrical circuit in order to provide fast switching of the
magnet. The Joule heating can be avoided by the application of superconducting
magnets. However, for fast switching it makes sense to apply in both cases a
tandem system of two magnets, interconnected into the serial electrical circuit and
the single cryocooler. In this manner, the electric current and the associated mag-
netic energy could be shifted from one part of the system to another part. There is
no evidence that such a tandem system has been ever built for the purpose of the
magnetocaloric energy conversion.
fl
8.4 AMR Devices with Thermal Diode Mechanisms
The application of the AMR principle is restricted by the frequency of the opera-
tion, associated mostly with the convective heat-transfer between the magnetoca-
loric material and the working
uid. For the 40 K temperature difference between
the heat source and the heat sink, and at least the same ef
fl
ciency of the device as
that of the compressor-based refrigerator, the future maximum frequency of the
operation of the advanced AMR with the working
uid based on water and addi-
tives, will most probably not exceed 5 Hz. In the case of the application of liquid
metals, we could consider the limiting frequency to be 10 Hz (see Sect.
4.6
).
However, this might not represent a suf
fl
cient power density of device. Therefore,
the research community should also focus on developments of the AMR devices
with the application of thermal diode mechanisms. Depending on the particular
mechanism or the design of the device, the frequency of the operation can be very
high, even 100 Hz. Our experimental and numerical analyses show that the max-
imum performance when applying thin-
lm Peltier modules, for the thermal diode
mechanism, will be reached at a frequency of about 25
30 Hz. However, other
thermal diode mechanisms, especially if they are based on a micro-scale design, can
boost such a frequency to be higher.
Although we have previously dedicated a whole chapter to the thermal diode
mechanism we show here one example of the application of the thermal diode
mechanism, combined with the AMR principle (Fig.
8.25
).
In Fig.
8.25
, a closed, rotating, four-pole magnet assembly provides the mag-
netization and demagnetization processes to the magnetocaloric material. This is in
contrast to other rotary systems consisting of a tiny plate and not a porous structure.
The magnetocaloric material is embodied between two thermal diode mechanisms.
We show in Fig.
8.25
eight such segments. When the magnetic
-
eld is rotating,
then in the case of segments that are under the high magnetic
eld region (mag-
netization), all the upper thermal diode mechanisms in four of such segments are
switched on, providing the heat
ux to be transferred to the upper micro-heat
exchanger. In these segments, the lower thermal diode mechanisms have to be
fl
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