Environmental Engineering Reference
In-Depth Information
material, which has very high thermal conductivity
K
for good thermal con-
duction. To maintain the temperature difference across the thermal energy
harvester, the Teflon material is inserted between the hot and cold sides of the
housing structure shown in
Figure 3.1
to prevent the adverse effect of parallel
parasitic thermal resistance [92].
3.1.2.2 Electrical Analysis
It should be noted that the effective temperature gradient
T
TEG
depends
not only on the thermal and electrical properties of the TEG but also on the
resistive load that is connected. When connecting a load resistance
R
L
to the
et al. [93].
V
oc
R
s,TEG
+
T
R
s,TEG
+
S
∗
I
TEG
=
R
L
=
(3.3)
R
L
Depending on the dimension of the TEG, that is,
h
and
A
leg
, which are the
height and the area of a single thermocouple leg, respectively, and the elec-
trical resistivity
of the materials used, the TEG internal electrical resistance
R
s,TEG
, which is composed of
n
thermocouples of p-type and n-type semicon-
ductor materials, is given by Dalola et al. [93].
h
A
leg
R
s,TEG
=
n
∗
2
∗
(3.4)
The output power
P
L
delivered by the TEG to the load
R
L
can be expressed
as
I
TEG
R
L
P
L
=
R
L
(
R
s,TEG
S
2
T
TEG
=
(3.5)
+
R
L
)
2
dent on both the TEG electrical resistance
R
s,TEG
and the electrical resistance
of the external load
R
L
. Under an impedance-matching condition where the
load resistance
R
L
is equal to the internal electrical resistance
R
s,TEG
, the TEG
is generating the maximum output power given by
S
2
T
TEG
4
R
s,TEG
P
L,MPPT
=
(3.6)
During operation of the TEG, the output voltage is reduced by the ohmic
voltage drop across its internal resistance
R
s,TEG
. Consequently, the voltage
at its maximum power
V
MPPT
is about half that of the open-circuit voltage
V
oc
(
V
oc
=
S
T
≈
2
V
MPPT
), and the maximum power changes with temperature
T
2
.
difference
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