Thermal Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 105

nor its integrated MPPT is connected to the sensor node. The conventional
sensor node, which operates solely on the supercapacitor, consumes 200 W

of average power from the 0.1-F supercapacitor; hence, the voltage across the
supercapacitor drops from 4.65 to 4.5 V in around 330 s, which is calculated to
be 69 mJ of energy transferred to the load. Once the TEH system with MPPT
is activated, the harvested power of around 450

Wisused to power the
sensor node as well as to charge the supercapacitor back to 4.6 V in 170 s.
This indicates that solely depending on the energy storage to sustain the
operation of the sensor node is not sufficient; it is only when the TEH system
with its MPP tracker is employed that sufficient power is provided for both
the operation of the wireless sensor node and charging the supercapacitor.
The designed buck converter with the resistor emulation MPPT approach
has already been demonstrated to yield good performance in extracting maxi-
mum power from the thermal energy harvester, but this comes at the expense
of additional power losses in the converter and its associated control and
PWM generation circuits. It is thus necessary to investigate the significance
of these power losses as compared to the total harvested power. The first
investigation is to determine the efficiency of the buck converter

conv as a
function of its output load power P load over its input DC power P dc under
different temperature differences and loading conditions. Take, for example,
at a temperature difference and output load resistance of 20 ◦ C and 10 k

,

respectively, the efficiency of the buck converter is given by

P out
P in ∗

conv = 100% (3.11)

7 V 2

2 . / 10 k

= ∗ 100% = 92%

9 V ∗ 88 A

For all other temperature differences and loading conditions, the efficien-
cies of the buck converter are calculated using Equation 3.11 to be on an
average of 90%, and the computed results are shown in Figure 3.14 . This
high-efficiency buck converter is very favourable and desirable in a very low
power rating condition of milliwatts or even lower. Another investigation
being carried out is to determine the power consumption of the associated
control and PWM generation electronic circuits and its significance as com-
pared to the harvested power. Based on the voltage and current requirements
of each individual component in the TEH system shown in Figure 3.10 , the
total power consumption of the electronic circuits can be calculated as follows:

P consumed @20 K, 10 k = P PMOS @ V MPPT + P comparator @ V MPPT + P filter @2 . 8 V

+

P oscillator @2 . 8 V (3.12)

= 9 V ∗ (4 A + 21 A) + 2 . 8 V ∗ (3 A + 20 A)

= 289 W

These power losses associated with the resistor emulation-based MPP tra-
cker at different temperature differences are illustrated in Figure 3.15 . Taking