Active Magnetic Regeneration - Magnetocaloric Energy Conversion: From Theory to Applications - page 137

Fig. 4.23 a The maximum
speci c cooling power as the
function of the frequency of
the operation for three
different magnetic
refrigeration thermodynamic
cycles. b The maximum COP
as a function of the frequency
of the operation for three
different magnetic
refrigeration thermodynamic
cycles

In all three cases, both the speci c cooling power and the COP increase with an

increase of the magnetic eld change

leads to a higher cooling power and also higher COP values (although to a smaller
extent, since with a larger magnetic

eld. It is well known that a higher magnetic

eld change the performed magnetic work is
increased) [ 37 ]. The reason for this is that a higher magnetic eld change leads to a

higher adiabatic temperature change, which compensates the irreversible heat losses
between the magnetocaloric material and the working

uid. For example if the
adiabatic temperature change during the magnetization equals, e.g. 4 K, this also
represents theoretically the maximum possible temperature difference between the
magnetocaloric material and the working

fl

uid.
The temperature difference between the magnetocaloric material and the work-
ing

fl

fl uid during the fl uid fl ow period strongly depends on the heat transfer coef -

cient and the available heat transfer area. In the case that the heat transfer coef cient

or the area is too small, then the temperature difference between the magnetocaloric
material and the working uid is too large (or the heat cannot be transferred for such

a small temperature difference). Literally, the irreversible heat losses in terms of
temperature difference will overcome the

fl

“ ”

generated adiabatic temperature change,

and the device will not be able to cool any more.