Environmental Engineering Reference
In-Depth Information
ciency of each of the Peltier mechanisms as
thermal diodes we refer to the right-hand side of Fig.
6.5
. The exergy ef
In order to calculate the exergy ef
ciency of
the thermal diode mechanism, which is based on a Peltier module, can be de
ned as
the ratio between the output speci
c exergy
fl
ux and the input speci
c exergy
fl
ux.
In our particular case this means that the output exergy represents the exergy
fl
ux on
the cold side of the Peltier and the input exergy
fl
ux represents the sum of the
exergy
fl
ux from the cold magnetocaloric material, plus the input speci
c electric
power:
e
Ci
_
n
TDi
¼
ð
6
:
11
Þ
w
þ
_
e
Hi
_
Now, for each of the selected thermal diodes A, B and C, we calculate the exergy
ef
ciency as:
e
CA
0
439
_
:
n
TDA
¼
e
HA
¼
437
¼
0
95
ð
6
:
12
Þ
:
w
þ
_
0
025
þ
0
_
:
:
e
CB
0
208
_
:
n
TDB
¼
e
HB
¼
207
¼
0
9
ð
6
:
13
Þ
:
þ
_
þ
w
_
0
:
025
0
:
e
CC
w
þ
e
HC
¼
_
0
:
04
n
TDC
¼
039
¼
ð
6
:
14
Þ
0
:
63
0
025
þ
0
:
:
ciency of
the thermal diode mechanism in this particular case is very high, i.e. 95 % for
thermal diode A and 90 % for thermal diode B. However, much lower, 63 % for
thermal diode C, which operates in the vicinity of the ambient temperature. In the
case that the warm side of the thermal diode corresponds to the ambient temper-
ature, the exergy ef
We can show from the results in Eqs. (
6.12
)
(
6.14
) that the exergy ef
-
ciency of the
Peltier element. This is due to the fact that all the temperature difference between
the ambient temperature and the cold part of the Peltier element is established by
the Peltier element itself and without the exergy
ciency of the thermal diode equals the exergy ef
ux from the magnetocaloric effect.
This example above is similar, as we would compare two heat pumps: air/water
and ground source water/water. In this particular case, the
fl
rst heat pump uses the
heat source as the pure anergy, whereas in the second case, the heat source already
represents a certain exergy. The same is true in the case of Peltier thermal diodes
with an embodied magnetocaloric material. The magnetocaloric material represents
the exergy potential, whereas the Peltier module can somehow be treated in this
case as the heat transporter.
With the above results we prove that Peltier modules, despite their low exergy
ef
ciently applied as the thermal diodes; however, under par-
ticular conditions, where the temperatures of both sides of the Peltier have to be
kept at a small difference (with regard to adiabatic temperature change) and the
thermal diode should still enable a suf
ciency, can be ef
cient transport of heat
fl
ux.
Search WWH ::
Custom Search