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the magnets, compared to some other thermodynamic cycles. In a magnetic
refrigerator or heat pump, two aspects
c cost
(e.g. eurokW 1 ) of a device can be strongly affected by the chosen thermodynamic
cycle. Besides the AMR itself, this is also related to the corresponding magnet
assembly and the
the energy ef
ciency and the speci
ow characteristics. In a certain magnet assembly,
researchers usually tend to obtain a homogeneous magnetic
fl
uid
fl
eld (in both high and
low magnetic
eld regions), which is preferable, especially for the Brayton-like
AMR cycle [ 120 , 121 ]. However, such an optimization of the magnetic
eld also
usually leads to more complex permanent magnet assemblies and sometimes also a
reduced magnetic
eld change. Different AMR cycles can therefore lead to dif-
ferent, potentially simpler and less expensive, magnet assemblies.
By choosing the optimum cycle, the performance of the cooling device can be
substantially improved. Therefore, the research community should invest greater
efforts in improving magnetic refrigeration devices with new thermodynamic cycles.
4.6 The Impact of the Heat Transfer Fluid
The heat-transfer (working)
uid and its thermohydraulic properties have an
important role in the performance of the AMR. In order to ensure good cooling
characteristics for the AMR (at high frequency of operation) the applied working
fl
fl
uid should, in general, have high a thermal conductivity and thermal diffusivity,
and a low viscosity.
The majority of the magnetic refrigerator prototypes use water-based heat-
transfer
uids with different alcohol additives. Some earlier prototypes also applied
gases, such as helium, nitrogen or even air [ 5 ]. Similarly, also the majority of
numerical analyses of the AMR performance consider water as a heat-transfer
fl
uid
[ 33 ]. Water is often chosen due to its very good heat transfer properties, non-
toxicity and simplicity of use. However, the majority of modern applied magnet-
ocaloric materials in the AMR corrode when in direct contact with water (for details
see [ 122
fl
125 ] where different corrosion inhibitors are considered). It was shown
[ 28 , 101 ] that a mixture (e.g. ratio of 70:30) of distilled (or deionised) water with
different alcohols (e.g. commercial automotive ethylene glycol) can prevent the
corrosion of the most promising magnetocaloric materials (Gd, La
-
Si).
Furthermore, the alcohol additives decrease the freezing temperature of the mixture
below 0
Fe
Co
-
-
-
C, which will be required in future magnetic refrigeration systems (but
also reduce its thermal diffusivity).
Not many analyses on the impact of the heat-transfer
°
uid on the AMR per-
formance have been performed to date. This subject was somehow neglected,
although it is very important. Petersen [ 119 ] numerically and experimentally, while
Kitanovski et al. [ 15 ] and Wu et al. [ 126 ] numerically, evaluated and compared the
performance of the AMR with different working
fl
fl
uids. They evaluated water,
liquid metals (mercury and Galinstan
a liquid alloy consisting mainly of gallium,
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