Magnetic Field Sources

Abstract

In this chapter we describe some of the most important issues that relate to the sources for magnetic fields. We will try to provide information for engineers about the different possible magnetic field sources with respect to their application in magnetocaloric energy conversion. However, the main emphasis will be on permanent magnets and their assemblies. This is due to the fact that other kinds of magnetic field sources, for instance, electric resistive coils (electromagnets) operate with a rather low efficiency. The efficiency in electromagnets in terms of thermodynamics 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 field source will, in most cases, also require cooling, especially in cases where a high magnetic flux 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 superconducting 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][2][3][4][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 field 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 magnetic field source is crucial for obtaining a cost-effective and energy-efficient device.