Active Magnetic Regeneration - Magnetocaloric Energy Conversion: From Theory to Applications - page 142

Hybrid AMR cycles is longer (as it is also performed during the (de)magnetization
process), smaller velocities are required for the same utilization factor. Conse-
quently, a smaller pressure drop occurs.
It has been reported in Bj

ø rk and Engelbrecht [ 18 ] that changing the magnetic

le can have a strong impact on the performance of the AMR. A further
investigation was performed in order to study the impact of the magnetization
pro

eld pro

le on the performance of a particular thermodynamic cycle. In this particular
case, four different magnetic

eld pro les were considered (Fig. 4.27 ). These were

de ned as the ratio ( ˄ mag : ˄ const ) between the time periods of a variable magnetic

eld (equal to the magnetization period) and the time period during which the
magnetic eld remains constant. The simulation was carried out for ˄ mag : ˄ const ratios

between 0.125 and 1. The temperature span was taken to be 15 K.
Figure 4.28 shows the results of a numerical analysis that was performed for a
gadolinium packed-bed AMR with different magnetic

les and different
AMR thermodynamic cycles. Note again that the utilization factor was varied in
order to obtain the maximum values of the COP and the maximum values for the
speci

eld pro

c cooling power per mass of magnetocaloric material.
Based on the results in Fig. 4.28 , we can conclude that for a particular geometry
of the AMR (Fig. 4.24 ), a higher cooling power or COP can be obtained with a
smaller ratio

˄ const , since a long (de)magnetization time increases the irre-
versible heat transfer losses.

˄ mag :

Fig. 4.27 The magnetic eld proles as a function of the time period