Environmental Engineering Reference
In-Depth Information
the magnetocaloric
can be shaped in a simple plate, foil or tape,
not a complex porous structure, and the small mass of magnetocaloric material
is compensated by the very high frequency,
“
regenerator
”
smaller
ow
can be performed through an expanded surface of a heat exchanger, which is not
made of magnetocaloric material and other materials can be processed in much
better and more precise ways (e.g. Cu and similar materials). Thus frictional
losses can be decreased and the heat-transfer area with the
fl
uid-
fl
ow viscous losses and larger heat-transfer surface
—
the
fl
uid
fl
fl
uid
fl
ow can be
enhanced,
prevented losses due to the
fl
uid oscillatory or bi-directional
fl
ow (
fl
uid
switching), since no
fl
uid switching is required anymore and no
“
dead
”
volume
of the
fl
uid occurs. Namely,
the working
fl
uid always
fl
ows in the same
direction,
prevented internal
fl
uid leakage and prevented friction by dynamic seals and
valves
—
by the introduction of the proposed concept the rotation of the magnetic
eld source enables the use of
“
static
”
valves (if any), so neither mechanical
friction nor the related
uid leakage is expected. The thermal diode mechanisms
actually overtake the role of the valve system.
fl
8.7.3 Magnet Assembly and Related Motor Drive
In the case of superconducting or electric resistive magnets, a motor is not required.
The system can simply operate with an ON-OFF operation. However, it would
make sense to perform a design of a tandem magnet, so the electric current could
switch from one to system another. As we have noted already before, very high
magnetic
eld changes (e.g. above 3 T) will require application of a rather different
magnetic thermodynamic cycle than the AMR Brayton-like one.
In the case of permanent magnets, one way to develop magnet assemblies is that
they consist of multiple poles in similar way as permanent-magnet motors. Such a
structure will most probably be embodied with a shield (and yoke), and therefore no
additional need for shielding will be required. Anyway, a good magnet design for
the magnetocaloric device should lead to the minimum possible leakage of the
magnetic
ux into the environment. The multiple-poles feature can be applied for
AMR principles as well as principles based on a thermal diode mechanism. One
should pay attention to the torque as well as eddy currents, which might be pro-
duced as well. The torque issue is also very important for the selection of the motor,
and therefore one should try to keep it as low as possible. We also did a note on
solving the cogging torque, related to rotating magnet designs (see the chapter on
magnetic
fl
eld sources). One of the solutions for preventing eddy currents is by
using laminates of material with electrical insulation or by choosing materials with
good magnetic but low electric conductivity.
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