Environmental Engineering Reference
In-Depth Information
Fig. 6.28 Design concept of a single-stage magnetocaloric device with thermal diodes
magnetocaloric effect of the MC material is what produces the temperature span
between the cold and hot heat exchanger. Therefore, there is no heat regeneration in
the MC material and such a device, depending on the intensity of the magnetic
eld
change, will provide a rather small temperature span between the heat source and
the heat sink.
Figure
6.29
shows the operation for such a single-stage device. It is clear that the
operating temperature span along the length (Fig.
6.28
, y-direction) is mostly
produced by the MC material, implying the single-stage operation. There is a
certain increase/decrease in the temperature span on the hot/cold side of
fl
uid
fl
ows,
respectively. However, this is only due to the
uid absorbing/releasing the heat to
the MC material and should not be regarded as a process of heat regeneration.
Furthermore, two different working
fl
uid loops are applied, one on the hot side and
one on the cold side, which also disregards the option of active magnetic regen-
eration. It is clear that the majority of the temperature span between the hot and cold
sides is done by the MC material with its MC effect. The thermal diodes should not
contribute to the temperature span of the device, or at least this contribution should
be as small as possible, since this decreases the irreversible losses or the required
amount of work for the operation of the thermal diodes. Therefore, the major part of
the overall input work should be magnetic work done on the MC material.
The thermodynamic cycle of the single-stage operation in Fig.
6.29
is as follows.
When the magnetic
fl
eld is applied to the MC material it heats up due to the MC
effect. At this moment, thermal diode 1 becomes operational and starts to transfer
heat from the MC material to the hot heat exchanger. Meanwhile, thermal diode 2
should not operate and should act as a thermal insulator, thus preventing the heat
from being transferred from the MC material to the cold heat exchanger. When the
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