Active Magnetic Regeneration - Magnetocaloric Energy Conversion: From Theory to Applications - page 149

c cooling power (per mass of
magnetocaloric material) and the maximum COP (obtained at the optimum utili-
zation factor

Figures 4.32 and 4.33 show the maximum speci

uids as a
function of the operating frequency for packed-bed and parallel-plate AMRs,
respectively. The temperature span between the heat source (293 K) and the heat
sink was set at 15 K.
The trend of dependency for the speci

— mass fl ow rate for each case) for all the analysed working fl

c cooling power and the COP with the
operating frequency presented in Figs. 4.32 and 4.33 is in general expected and
explained in more detail in Sects. 4.4 and 4.5 (for water as a heat-transfer

uid).
It was also expected that due to the better heat transfer geometry the packed-bed
compared to parallel-plate AMR would have a higher speci

fl

c cooling power and a
higher optimum operating frequency (the exception here is Galinstan in the parallel-
plate AMR, as explained below), but also smaller COP values due to higher viscous
losses.
In the case of the packed-bed AMR the best cooling characteristics were
obtained with water. The additives of ethanol signi cantly reduce the performance

of a device, especially the cooling power at frequencies above 1 Hz (mostly due to
the lower thermal diffusivity and thermal conductivity). Even though Galinstan has
by far the highest thermal diffusivity of all the evaluated

uids, it does not perform
as well as some might expect in the packed-bed AMR. This is mostly due to its very
high density and the viscous losses, which prevent a better performance. A similar
situation is also true for ethylene glycol, which suffers from a very high viscosity

fl

Fig. 4.32 The maximum
speci c cooling power and
the maximum COP for the
analysed working uids as a

function of the operating
frequency for a packed-bed
AMR at 1 T of magnetic eld
change and 15 K of
temperature span

fl