Environmental Engineering Reference
In-Depth Information
It is evident from Figs.
4.17
,
4.18
and
4.19
that there is an optimum operating
frequency and utilization factors that ensure the optimum operation of the AMR.
However, both optimum operating parameters strongly depend on the optimization
criterion. The question is which AMR cooling characteristic is the most crucial for a
particular application: a high temperature span, a high cooling power or a high
ef
ciency (COP)? In general, the cooling power increases with the operating fre-
quency and the utilization factor, but only up to frequencies and utilization factors
at which the required temperature span along the AMR can be established and
exceeded. In the particular case shown in Fig.
4.18
, the cooling power is increasing
with the operating frequency for all the analysed frequencies, since also at the
highest analysed frequency the required temperature span of 15 K can be exceeded
(see Fig.
4.17
). In general, this depends strongly on the AMR geometry and its heat
transfer characteristics: the better they are the higher the optimum frequency of
operation will be. On the other hand, the cooling power is not monotonically
increasing with the utilization factor, since it starts to decrease for utilization factors
between 1 and 1.5 (depending on the operating frequency) and reaches negative
values at utilization factors of about 2, at which the required temperature span of
15 K cannot be exceeded anymore (see Fig.
4.17
).
The largest temperature spans are, in general, obtained at signi
cantly smaller
operating frequencies and smaller utilization factors compared to the highest cooling
powers. The situation is similar for the COP values, which are, in general, decreasing
with the operating frequency (see Fig.
4.19
). Higher frequencies are related to higher
optimum mass
ow rates and therefor larger viscous losses as well as a higher degree
of overlapping of the local thermodynamic cycles in the T
-
s diagram and thus a
higher input magnetic work, which reduces the overall ef
fl
ciency. This is also
explained in detail in, e.g. [
16
,
43
,
45
,
119
]. However, an ef
cient AMR that is related
to a
ne AMR geometry would also be able to establish a large temperature span at
higher frequencies, which further results in a higher cooling power as well as higher
COP values.
Figures
4.18
and
4.19
show the AMR
s performance at 15 K of temperature
span. The reason that this analysis and the analysis later in this chapter are based on
a relatively low temperature span (not applied in practical applications) is in the
limited maximum temperature span that can be reached by some of the evaluated
AMRs, their operating conditions and regimes (thermodynamic cycles). Therefore,
a more comprehensive comparison can be performed for a lower temperature span.
The dependence of the cooling power on the temperature span and further on the
COP is shown in Sect.
4.5
for different AMR thermodynamic cycles. The theo-
retical results usually show a linear-like dependence of the cooling power and
temperature span (for magnetocaloric materials with the second-order phase tran-
sition), while the experimentally measured dependence is more complex mostly due
to losses to the surroundings [
92
,
110
].
Another crucial parameter that strongly in
'
'
s performance is the
geometry of the magnetocaloric material in the AMR. It is in fact bene
fl
uences the AMR
cial to have
as
ne an AMR structure as possible (thin plates or small particles and a small
hydraulic diameter) in order to be able to operate at a higher optimum operating
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