Environmental Engineering Reference
In-Depth Information
in the direction of the electric current density). When the electrons pass the junction
of a metal/p-type semiconductor, the energy of the electrons drops and the heat is
released. This is because they charge the metal with negative charges, which then
act by repelling of the electron
ow. When the electrons enter a, e.g. copper,
conductor on the cold side of a p-type semiconductor (p-n metal junction) they are
excited to a higher energy level. They absorb heat from the metal, which conse-
quently cools down. Then, when the electrons leave the n-type semiconductor and
enter the copper, their energy level drops (the heat is released).
In the following description we will try to avoid the common use of Peltier
modules as coolers or heat pumps. Their broad application in thermal management
(i.e. electronic devices) and also in most other cases represents only a cooling
device, and not really a real thermal switch or a heat valve. So, how do we
distinguish between the two cases?
It is well known that the ef
fl
ciency of a Peltier module strongly depends on the
temperature range in which it operates. This also holds true for the exergy ef
-
ciency. In thermal management, the Peltier modules usually provide the necessary
cooling power with a certain larger temperature span between the heat source and
the heat sink.
However, a larger temperature difference will in
uence the COP of the Peltier
module, as can also be seen in Fig.
6.4
. So in order to operate with the highest
possible efciency of the magnetocaloric device, the Peltier module needs to
operate at small currents as well as small temperature differences between the heat
source and the heat sink. In this particular case, the cooling power of the module is
rather low. It also demonstrates the operation where a thermal diode plays a role.
A Peltier thermal diode does not necessarily require a high cooling power (as in
thermal management), which also relates to high currents. In particular, when
applied in combination with other solid-state refrigeration technologies (magnet-
ocaloric, electrocaloric, barocaloric, elastocaloric, etc.), the large temperature span
of the Peltier thermal diode drastically in
fl
ciency of the
cooling device. So in such a case it should represent a solid-state heat transporter
rather than a heat pump. Comprehensive information about the necessity for such
operating characteristics can be found in references [
4
fl
uences the overall ef
6
].
A Peltier thermal diode can provide fast switching (depending on whether we
have bulk ceramics or thin
-
ux as well as operating as a heat
valve, with certain losses, which in the case of the closed heat valve relates to the
thermal mass and the thermal conductivity of the Peltier module.
lms) of the heat
fl
6.2.1.1 Why Must the Ef
ciency of a Thermal Diode
with Peltier Modules Be High?
With regard to the calculation of the exergy ef
ciency from the data in Fig.
6.4
we
may wrongly estimate that the ef
ciency of a thermal diode, based on Peltier
modules, will be very low. Note again that it is not the Peltier element that creates
the temperature difference between the heat source and the heat sink of the magnetic
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