Environmental Engineering Reference
In-Depth Information
respond to the temperature changes in the MC material. In the next process, shown
in Fig.
6.30
d, the passive thermal diode 2 becomes operational. A substantial
increase in its effective thermal conductivity makes it more susceptible to the
temperature levels of the cold
uid and MC material. A fast heat transport via
thermal conduction occurs, which is, as an example, presented as a linear tem-
perature pro
fl
le of thermal diode 2. Meanwhile, thermal diode 1 is non-operational
with low effective thermal conductivity (acts as non-perfect heat insulator), which
leads to a slower temperature response to the surrounding temperature changes. In
this case most of the heat
uid to the MC material.
Figure
6.31
presents the operation of active thermal diodes coupled with a
magnetocaloric material. All four phases of the operation are practically the same as
in Fig.
6.30
.
However, the difference is in the operating temperature pro
fl
ux is transported from the cold
fl
le of the active
thermal diodes. The notion
in this case means that the active thermal
diodes should operate as a sort of heat pump. When they are non-operational, they
should exhibit only a low heat transfer due to their low thermal conductivity.
Whereas in operating mode, when their characteristics are in
“
active
”
uenced directly by
some energy input (e.g. electric energy), they operate as heat pumps. This re
fl
ects in
rapid heat transport in the direction of a temperature gradient, from the cold side to
the hot side of the thermal diode (similar to the heat pump). This can be seen from
Figs.
6.31
b, d.
Figure
6.31
b shows the operation of an active thermal diode 1, while the active
thermal diode 2 is non-operational. Furthermore, the operation of thermal diode 1 is
shown with the opposite temperature pro
fl
le (with respect to analogue passive
thermal diodes shown in Fig.
6.30
) since they operate in a similar way to heat
pumps; therefore, the heat in a thermal diode is transported from a lower temper-
ature (MC material side) to a higher temperature (hot
uid side). Meanwhile, the
non-operational active thermal diode 2 responds to the temperature changes only by
its low thermal conductivity. Figure
6.31
d shows similar operation to that in
Fig.
6.31
b. However, now the active thermal diode 2 is operational. Thus it works
as a heat pump, and rapidly transports heat from the lower temperature (cold
fl
fl
uid
side) to the higher temperature (MC material side).
At this point it has to be emphasized that even though the active thermal diodes
can operate as heat pumps (i.e. Peltier), thus having the ability to cool/heat, they
should not be used to increase the temperature span in a magnetocaloric device.
Their sole purpose is to work as fast heat-transport mechanisms between the
uid
and the MC material. The temperature span of the device should be, in any case,
performed by the MC material. The active thermal diode should just help to
transport the heat faster from/to the MC material at the lowest possible temperature
differences. In this case the ef
fl
ciency of the active thermal diodes can be kept high,
which will not substantially affect the total ef
ciency of a device.
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