Magnetic Field Sources - Magnetocaloric Energy Conversion: From Theory to Applications - page 37

Chapter 3
Magnetic Field Sources

In this chapter we describe some of the most important issues that relate to the
sources for magnetic elds. We will try to provide information for engineers about

the different possible magnetic eld sources with respect to their application in
magnetocaloric energy conversion. However, the main emphasis will be on per-
manent magnets and their assemblies. This is due to the fact that other kinds of
magnetic

eld sources, for instance, electric resistive coils (electromagnets) operate
with a rather low ef ciency in electromagnets in terms of thermo-

dynamics must be considered as the ratio of the energy output (magnetization
energy) and the energy input to the coil. The current passing through the coiled
wires in an electromagnet will induce Joule heating, due to the electric resistance.
Not only will a loss of valuable energy occur, but the magnetic

ciency. The ef

eld source will, in
most cases, also require cooling, especially in cases where a high magnetic ux

density is required. In superconducting magnets, which can be treated as a special
class of electromagnets, the current may be kept circulating through the wires of a
coil without electrical resistance losses. However, in order to keep the supercon-
ducting magnet operating, a cryogenic system is required to provide cooling for the
superconducting electrical coils. Namely, there is no practical application of a
room-temperature superconducting material. There are, however, some indicators
that this could be possible [ 1

fl

5 ]. And if this happens in future, this chapter will
unfortunately (or fortunately) not be of much use, but the application will open up
an unbelievable range of applications, which will certainly change our way of life,
including magnetocaloric energy conversion.
The reader should note that the magnetic

-

eld source is the most expensive part
of a magnetic refrigerator or a heat pump, especially if those are based on
permanent or superconducting magnets. Therefore, the optimal design of a mag-
netic eld source is crucial for obtaining a cost-effective and energy-ef cient

device.