Chemistry Reference
In-Depth Information
The COP can be optimized by adjusting the input current. The optimized
current I
max COP
that maximizes COP, and the corresponding maximum COP
are expressed in eqn (6.22) and eqn (6.23), respectively:
SDT
R
d
n
3
r
4
n
g
|
7
I
max COP
¼
p
(6
:
22)
1
þ
ZT
avg
1
p
1
þ
ZT
avg
T
H
T
C
COP
max
¼
COP
C
(6
:
23)
p
1
þ
ZT
avg
þ
1
where COP
C
¼
T
C
/(T
H
T
C
) is the Carnot coecient of performance. The
maximum COP depends on the thermoelectric figure of merit ZT. Note that
the COP can be negative when the temperature of the hot side is much
higher than that of the cold side, in which the heat energy dissipated to the
cold side by thermal conduction is larger than the Peltier cooling, so that
the net cooling power becomes negative. When one wants to maximize the
cooling power Q
in
instead of COP, the optimal current I
max Qin
is obtained
when dQ
in
/dI
¼
0, such that
I
max Q
in
¼
ST
C
R
(6
:
24)
and the corresponding maximum cooling power, and the COP are
obtained to be
Q
in
;
max
¼
KDT
þ
S
2
T
C
2R
.
(6
:
25)
1
2
ZT
C
DT
ZT
H
T
C
COP
max Q
in
¼
(6
:
26)
:
6.5.3 Fabrication of Thermoelectric Devices
Conventional thermoelectric devices are fabricated using bulk thermo-
electric materials. Most of the thermoelectric materials with high figures of
merit are semiconducting alloys
51,58,69
or nanostructured bulk materials.
20,21
These materials need to be scalable for a large scale production, so bulk
materials are ideal.
21
However, in applications that require a low-power re-
frigeration for a very small area, e.g. hotspot cooling for micro-processors,
thin-film materials are ideal for thermoelectric device fabrication since thin-
film based TE can be batch-fabricated with the chip and can produce large
cooling power density.
74
Organic thermoelectric materials composed of
conducting polymers such as poly(3,4-ethylenedioxythiophene)
70-72
are
usually made in the form of thin films. These organic materials can be useful
for thin-film thermoelectric devices,
73
but their very low figures of merit have
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