Active Magnetic Regeneration - Magnetocaloric Energy Conversion: From Theory to Applications - page 134

On the other hand, the parallel-plate AMR performs slightly better COP values,
mostly due to the smaller viscous losses. However, shorter packed-bed AMRs
(below 40 mm with an optimum corresponding sphere diameter) can operate with a
similar ef

ciency as the parallel-plate AMR.
The development of a parallel-plate AMR with a plate thickness of 0.1 mm or
less and a spacing of approximately 0.05 mm, and packed-bed AMRs with spheres
below 0.1 mm (with short lengths), also from other magnetocaloric materials (not
just Gd) is one of the major future challenges for magnetic refrigeration at room
temperature. This is very important, especially for operations at higher frequencies,
which enables higher cooling loads and high temperature spans (and consequently
higher compactness). Furthermore, it is necessary that the AMRs are constructed
with none or only a minor maldistribution, since this can drastically affect the
AMR

' s performance (see Sect. 4.3.4 ).

4.5 The Analysis of Different AMR Thermodynamic Cycles

In this section, we present numerical and experimental analyses of thermodynamic
cycles for an AMR. For this purpose, we have evaluated a Brayton-like, Ericsson-
like, and a Hybrid Brayton - Ericsson-like AMR cycle. The Carnot-like AMR
thermodynamic cycle was numerically investigated; however, it was not tested
experimentally, since this was not possible with the experimental device and its
magnetic eld distribution applied.

4.5.1 Numerical Investigation and Comparison of Different
AMR Thermodynamic Cycles

The numerical programme used for the comparison of the thermodynamic cycles
with an AMR was based on a model developed by Tu š ek et al. [ 16 , 17 ].

rst analysis, the AMR was assumed to have the shape of a coaxial
cylinder with a length of 8 cm, a porosity of 39 %, and a total mass of magnet-
ocaloric material equal to 2.05 kg. This kind of ring can be, for instance, applied in
a rotary magnetic refrigerator. It was assumed that the ring consists of packed beds
of gadolinium spheres with a diameter of 0.5 mm. The magnetocaloric properties of
the gadolinium were calculated using the Mean Field Theory [ 49 , 77 , 78 ]. The
working

In the

uid was assumed to be water. The simulations were performed for a 15 K
temperature span between the heat source (293 K) and the heat sink.
The working regime of the magnetic cooling device for a particular AMR cycle
was de

fl

ned by the characteristics of the fl uid fl ow pro le and the magnetic eld

pro le. The regimes for the Brayton-like, Ericsson-like, and Carnot-like AMR

cycles are shown in Table 4.5 .