Thermal Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 92

T H

R con ( H )

R g ( H )

R S, TEG

i TEG

Hot Side, T HJ

+

P

TEG

V oc =
α*∆ T TEG

∆ T TEG =
P * R TEG

+
-

V TEG R L

R TEG

-

Cold Side, T CJ

R g ( C )

R con ( C )
T C

FIGURE 3.2
An equivalent electrical circuit of the thermal energy harvester.

3.1.2.1 Thermal Analysis
Referring to Figure 3.2 , it can be observed that the TEG is connected to the hot
and cold reservoirs via the thermal contact and thermal grease resistances,
which are given by R con ( H ) , R g ( H ) and R g ( C ) , and R con ( C ) ,respectively. Consid-
ering all these thermal resistances R Total (see Figure 3.2 , left circuit) residing
in the housing structure of the thermal energy harvester and comparing them
with the TEG's internal thermal resistance R TEG , the actual temperature drop
across the thermoelectric generator T TEG may then be expressed as

R TEG
R Total

T TEG = T ∗

R TEG

= ( T H − T C ) ∗ (3.2)

R con ( H ) + R g ( H ) + R TEG + R g ( C ) + R con ( C )

Due to the finite thermal resistances of the thermal energy harvester, the
temperature difference

T TEG across the junctions of the TEG is lower than

the temperature gradient T that is externally imposed across the thermal

energy harvester. To minimize this negative effect, the thermal resistance R TEG
of the TEG must be as high as possible, or in other words, the rest of the thermal
resistances of the thermal energy harvester must be minimized.
The unwanted thermal resistance of the thermal energy harvester, which is
defined as R Thermal = KA , can be minimized by carrying out some appro-

priate hardware design on the thermal energy harvester, such as: (1) increasing
the contact surface A of the heat transfer area, (2) reducing the thickness of the
material

x /

x used like the fins of the heat sink, and (3) selecting aluminium