Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes - Magnetocaloric Energy Conversion: From Theory to Applications - page 211

Seebeck effect [ 8 ] is taken into account. Here the heat is delivered to start the
motion of the electrons, thus generating electricity. This kind of device is called a
thermoelectric generator. However, if the purpose is to cool or to pump heat, then
the electricity has to be delivered to the device, which starts the transport of heat
from one side of the device to the other. This kind of cooling device is called a
Peltier element or Peltier module (Fig. 6.3 ). Thermoelectric electricity generators
are used in various processes where a suf cient amount of waste heat is available. In

particular, they are used in space technology, for powering satellites and unmanned
space probes. The power plant consists of the radioactive isotopes Uranium 235 or
Plutonium 238 and thermoelectric generators, which use the heat released from the
radioactive core decay [ 9 , 10 ]. The

eld of refrigeration and cooling using Peltier
elements is even wider. From the military and aerospace industries through
everyday consumer applications (food storage, etc.) to industrial and laboratory
needs [ 11 ]. In some special industrial and laboratory processes there is a need for
fast and precise temperature control and management, especially on extremely
small surfaces. This has not been possible until recently, when thin- lm Peltier

elements were designed [ 12 , 13 ]. One of the leading research institutions in this
domain is the German Fraunhofer Institute for Physical Measurement Techniques
[ 14

- 16 ]. The problem of thermoelectrics is primarily its low ef ciency, with

maximum values reaching approximately 15 % [ 17 19 ]. However, at the moment

this is the most accessible commercial technology for thermal diodes. A compre-
hensive review of thermoelectrics and their properties can be found in the topic
written by Nolas [ 20 ].
A simple schematic of a Peltier module is shown in Fig. 6.3 . The electrons move
in the opposite direction to the electric current density, denoted by J. In the warm
part of the p-type semiconductor (at the heat sink), which is doped with excess
holes, the electrons tend to ful

-

l holes (the holes are also carriers, but they migrate

Fig. 6.3 a Example of a Peltier element consisting of an n-type of semiconductor, doped with
electrons, and a p-type semiconductor, doped with holes, both being charge carriers, b Photograph
of a small, thin-lm Peltier module in our laboratory