Environmental Engineering Reference
In-Depth Information
Fig. 6.4 a The coefcient of performance (COP), b The specic cooling power of thin-lm Peltier
modules (calculation based on information in [
7
])
refrigerator. This is provided by the magnetocaloric effect. The thermal diode
mechanism is therefore applied only for the heat transport.
In our particular case we assume the operation of a magnetic refrigerator that is
based on AMR with thermal diodes (Fig.
6.5
). In this particular case, thin-
lm
Peltier modules serve for the heat transport to and from the magnetocaloric material.
For the purpose of analysis, we
x the temperature difference on the hot and cold
sides of the Peltier module to be only 0.1 K. During the operation a temperature
pro
le is built up along the magnetocaloric material with the thermal diodes. The
coldest part (during the demagnetization) of the magnetocaloric material in our
particular case is estimated to be 273.1 K, and the hottest part (during the
demagnetization) is estimated to be 300.1 K. We assume now an iso
eld cooling
process, during which the magnetocaloric material is in a demagnetized state and
the thermal diodes, which are connected to the heat source (via the
ow), are
active. From these, the evaluated thermal diode mechanisms (activated Peltier
modules) are represented by A, B and C. The T
fl
uid
fl
−
x diagram in Fig.
6.5
also presents
an approximate temperature pro
le established over selected thermal diodes in the
steady-state condition.
We estimate that the temperatures of the cold parts of the selected Peltier thermal
diodes A, B, C correspond to 273, 288 and 303 K, respectively. Each of these
thermal diodes has a temperature span between the heat source and the heat sink
equal to 0.1 K. Therefore, the temperatures of the hot parts of the selected Peltier
thermal diodes A, B, C correspond to 273.1, 288.1 and 300.1 K, respectively.
In Fig.
6.5
(right), the exergy
ow diagram is shown. The element A in this case
represents the Peltier module. The exergy of cooling energy from the cold mag-
netocaloric material
fl
ows into the element A. If this is not considered to be Peltier
element, but an element with the in
fl
nite thermal conductivity, the exergy
fl
ux
ux which exits element A (i.e. Q
H
¼
Q
C
and E
H
¼
E
C
). However, since the Peltier module in this particular case is con-
sidered for transport of heat, the example in Fig.
6.5
(right) becomes more complex.
In order to provide the operation of the Peltier module, this requires certain input of
which enters element A equals the exergy
fl
Search WWH ::
Custom Search