Environmental Engineering Reference
In-Depth Information
of the thermal contacts and thermal grease resistances residing in the cold and
hot sides of the thermal energy harvester, that is,
R
con
(
H
)
,
R
con
(
C
)
and
R
g
(
H
)
,
R
g
(
C
)
,respectively. To minimize this negative effect, the thermal resistance
R
TEG
of the TEG is made to be as high as possible; conversely, the rest of
the thermal resistances of the thermal energy harvester are designed to be
as small as possible. Taking these design considerations into account, the
miniaturized thermal energy harvester, having a physical size of 20
×
20 mm, is designed in such a way that most of the heat is channelled through
the TEG in order to maximize TEH.
Analysis and characterization works were conducted on the designed ther-
mal energy harvester to evaluate the performance of the TEH subsystem in
powering the wireless sensor node. According to Seebeck's effect, the open-
circuit voltage
V
oc
of the TEG enclosed in the thermal energy harvester, which
is composed of
n
thermocouples connected electrically in series and thermally
in parallel, is given as
×
20
V
oc
=
S
∗
T
=
n
∗
(
T
H
−
T
C
)
(5.8)
where
and
S
represent Seebeck's coefficient of a thermocouple and a TEG,
respectively. When connecting a load resistance
R
L
electrically to the TEG via
the thermal energy harvester as shown in
Figure 5.19
, an electrical current
I
TEG
flows in accordance to the applied temperature difference
T
, which is
given as
V
oc
−
V
TEG
R
s,TEG
n
∗
(
T
H
−
T
C
)
−
V
TEG
I
TEG
=
=
(5.9)
R
s,TEG
where
R
s,TEG
is the internal electrical resistance of the TEG. Based on the
current-voltage (I-V) characteristic of the TEG described in
Equation 5.9
,
the
output power
P
TEG
(
V
TEG
) delivered by the TEG to the load
R
L
can be deter-
harvested by the thermal energy harvester is derived as a function of its out-
put voltage
V
TEG
, which is expressed as
P
TEG
(
V
TEG
)
=
V
TEG
∗
I
TEG
V
TEG
∗
n
∗
∗
(
T
H
−
T
C
)
−
V
TEG
=
(5.10)
R
s,TEG
Based on the technical specifications provided for the Thermo Life TEG [91],
the TEG used in this section is made up of 5200 thermocouples, and each
thermocouple has a Seebeck's coefficient
of 0.21 mV/K. For a given tem-
perature difference
T
C
, between 5 and 10 K, the model illustrated
the simulation results are presented in
Figure 5.20
. Experiments were carried
out to characterize the TEG by applying a temperature difference between
T
=
T
H
−
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