Environmental Engineering Reference
In-Depth Information
4.4 The Impact of the Operational Parameters
and Geometry on the Performance of the AMR
The operation and performance of the AMR have been the subject of various
researchers, from the theoretical (numerical) [
33
] as well as the experimental
[
5
,
116
] points of view. Among others, those studies showed that the performance
of the AMR (with a particular magnetocaloric material) strongly depends on the
operational properties, e.g. [
43
,
45
,
50
,
117
] and the geometrical characteristics, e.g.
[
15
17
,
118
]. In particular, the operating conditions (utilization factor and opera-
tional frequency) must be carefully chosen. If, for example the mass
-
ow rate and
the related utilization factor (Eq.
4.26
) are too small or too high this can disable the
heat regeneration process and prevent a temperature span from being established.
The utilization factor is de
fl
ned as:
m
f
c
f
s
f
m
MCM
c
MCM
_
U
¼
ð
4
:
26
Þ
where
m, c
f
,
s
f
, m
MCM
,
_
c
MCM
represent the mass
fl
ow rate of the heat-transfer
fl
uid,
the speci
ow period, the mass of magnetocaloric
material in the AMR and its average speci
c heat of the
fl
uid, the
fl
uid
fl
c heat, respectively. If the amount of
fl
uid displaced (pumped) through the AMR (utilization factor) is too small (also
regarding the operational frequency and the AMR geometry), the fluid does not
have the capability to transfer or absorb the whole amount of energy generated
during the magnetocaloric effect, as explained in [
16
,
119
]. If, on the other hand,
more than the optimum amount of
fl
uid is pumped through the AMR, this can cause
fl
uid from the hot end of the AMR (or even from the HHEX) to be transported into
the CHEX. This of course reduces the temperature span as well as the total cooling
power of the AMR [
52
]. Under such conditions, the temperature span between the
heat source and heat sink will be small and the operation will be similar to one
without regeneration (single-stage device). The impact of the utilization factor on
the AMR
s performance is clearly shown in Figs.
4.17
,
4.18
and
4.19
. It was shown
by various researchers [
17
,
43
,
45
,
50
,
117
] that in most cases the optimum utili-
zation factor is between 0.2 and 0.8, which means that in the optimum case the
entire
'
fl
uid in the AMR is not displaced during the
fl
uid
fl
ow period. In other words,
the
uid entering the AMR will, in most cases, only move until, e.g. the middle of
the AMR, and displace the rest of the
fl
uid out of the AMR at the other side.
Furthermore, a higher utilization factor also results in a higher degree of overlap-
ping of the local (internal) thermodynamic cycles (see Fig.
4.3
a, b), which further
increase the input magnetic work needed to run the cycle. This should, therefore, be
carefully optimized.
Another crucial operational parameter of the AMR is the operating frequency,
which is usually de
fl
ned as the number of performed thermodynamic cycles per unit
of time:
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