Active Magnetic Regeneration - Magnetocaloric Energy Conversion: From Theory to Applications - page 129

Fig. 4.19 The COP at 15 K of temperature span as a function of the utilization factor for different
operating frequencies in the case of a packed-bed AMR with Gd spheres of 0.5 mm for a 1 T
magnetic eld change

(de)magnetization time, in general, increases the heat transfer losses. The time of
the (de)magnetization is usually de eld source (magnet

assembly) and the design of the device itself. On the other hand, the time of the
duration of the

ned by the magnetic

ow period should be carefully optimized. This is also strongly
related to the geometry of the AMR. In the case of thicker walls or larger particles
(spheres) in the AMR, a longer time is required for the

fl uid fl

uid to transfer or absorb the
total amount of energy generated by the magnetocaloric effect from the magnet-
ocaloric material. This means that in this case, the optimum duration of the

fl

fl uid

fl ow period would be longer (and the operating frequency lower) compared to the

case of an AMR with thinner plates or smaller particles.
Figures 4.17 , 4.18 and 4.19 show the impact of the operational parameters
(utilization factor and operating frequency) on the temperature span (at zero cooling
load) and on the cooling power (Eq. 4.16a ) and COP (Eq. 4.17 ) at 15 K of the
temperature span for a packed-bed AMR with Gd spheres of 0.5 mm and a Brayton-
like AMR cycle (with a magnetic uid).

The total mass of gadolinium was assumed to be 0.15 kg, while the hot side
temperature was maintained constant at 293 K. The magnetocaloric properties of
the gadolinium were calculated using the Mean Field Theory (see [ 49 , 77 , 78 ] for
details). By changing the operating frequency, both the

eld change of 1 T and water as a working fl

ow and the (de)
magnetization period were changed, while keeping a constant ratio between those
two periods (

fl uid fl

˄ f = 1:4). The results presented here are reproduced using the
numerical model described in Tu

˄ mag :

ek et al. [ 16 , 17 ]. Here, and also in other
numerical analyses presented in this chapter, the evaluated operating frequency was
limited to 3 Hz. Higher frequencies would in some cases (very

š

ne AMR geometry
as explained later in the text) result in higher cooling powers (but in general in
smaller ef

ciency). The limit of 3 Hz was selected since the great majority of so far
built magnetic refrigerator prototypes operate with frequency below 3 Hz (see
Chap. 7 ) .