Active Magnetic Regeneration - Magnetocaloric Energy Conversion: From Theory to Applications - page 97

Fluid fl ow from the ' hot side ' (HHEX) through the heated magnetocaloric

￿

material to the ' cold side ' (CHEX) in the absence of the magnetic eld (the same

fl ow direction and therefore

displacing it). During this process of heat transfer, the

uid as in rst phase (Fig. 4.2 a), but in the counter- fl

uid cools down (while
the material heats up) and in the CHEX it absorbs heat from the surroundings
(Fig. 4.2 d).

fl

We can, in general, distinguish between rotary and linear (reciprocating) AMRs
that further result in rotary and linear magnetic refrigeration devices, respectively
(see Chaps. 7 and 8 ) . The difference (with regard to AMR) is only in the kinetics of
the movement between the AMR and the magnetic

eld source. The operational
phases and the performed thermodynamic cycles of a particular AMR can be treated
equally for both types.
It should be noted that the term Brayton-like (and later also Ericsson-like and
Carnot-like) is used since the AMR cycle differs from the conventional Brayton or
even the Brayton regenerative cycle, as explained later in this chapter. Furthermore,
the conventional Brayton magnetic thermodynamic cycle is based on an adiabatic
magnetization and demagnetization. However, in a practical device, regardless of the
magnetic eld source, it is rather dif cult to ensure an instantaneous increase and

decrease of the magnetic eld. Since there is an unavoidable heat transfer between the

magnetocaloric material and the working

uid (although static) situated in the AMR
during the time needed to increase or decrease the magnetic eld from the minimum
to the maximum value, or vice versa, we can only talk about quasi-adiabatic (de)
magnetization. One should note that a quasi-adiabatic (de)magnetization process
should be as fast as possible and much faster than that of the convective heat transfer
between the magnetocaloric material and the working

fl

fl uid during the fl uid fl ow

period in order to reduce the heat transfer losses (see Sect. 4.4 ).
An AMR can also be applied for other thermodynamic cycles, for example the
Ericsson-like, Carnot-like, Stirling-like or some hybrid cycles (e.g. a combination
of a Brayton-like and an Ericsson-like cycle). However, different AMR thermo-
dynamic cycles will be explained later in this chapter, while the basic operation of
the AMR, establishing the temperature pro le along its length and its unique

thermodynamic cycle will be explained in detail for the most widely applied
Brayton-like AMR cycle (see also Sect. 1.3 ).
Figure 4.3 shows a Brayton-like AMR cycle, schematically represented in a

T s diagram for periodic steady-state conditions (Fig. 4.3 a, b) and the required

magnetic

-

ow regime (Fig. 4.3 d), together with an example of
a magnet assembly that is suitable for a Brayton-like AMR cycle (Fig. 4.3 c).
Generally, the AMR cycle is similar to the cascade system, where several
thermodynamic cycles (connected in series) are used to increase the temperature
span. As explained in Hall et al. [ 11 ], the major difference between an AMR and a
cascade cycle is in the fact that the AMR cycle does not pump heat directly between
the next-neighbour particles, but all the particles accept or reject heat to the heat-
transfer fluid at the same time and are coupled indirectly through the working fluid.
So, in the AMR cycle, there is no overlapping of the internal cycles at the same

eld pro le and fl uid fl