Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 178

converters, the performance of the designed HEH system for enhanced per-
formance in the indoor wireless sensor node was evaluated. In indoor appli-
cations like hospitals and factories, say the ambient condition is as follows:
solar irradiance of 1010 lux and temperature difference of 10 K, referring to
Figures 5.17 and 5.20 with operating conditions of 1010 lux and 10 K, the
maximum power obtained by summing the individual MPPs of the thermal
energy harvester P TEG and solar panel P pv is 727 W, and the actual har-

vested power P HEH,actual measured from the two paralleled energy sources is
690 W. The power difference between the calculated and measured pow-

ers, due to impedance mismatch between two paralleled energy sources, is
35

W, as shown in Figure 5.29 . Taking into consideration both the power dif-
ference and the power losses in the voltage-regulating and MPPT converters
as shown in Figure 5.30 , the net harvested power output to power the indoor
wireless sensor node through the boost converter with efficiency of 90% is
621 W. This harvested power from the HEH system is more than what is

harvested by each individual energy source (i.e., ambient light of 432

Wor

thermal energy source of 223 W), hence the significance of the proposed

HEH system is exhibited.

5.3.5.3 Performance of the Designed HEH System for an Indoor Wireless
Sensor Node
In order to evaluate the designed HEH system for sustaining the operation
of the wireless sensor node used in an indoor environment, the MPPT per-
formance of the boost converter based on a fixed reference voltage scheme
was investigated as shown in Figure 5.31 . During the performance evalua-
tion process, all the three main contributors of the power losses in the HEH
system were also included in the power analysis. A 68-k fixed resistor was

used instead of a supercapacitor, which requires a long time to charge and
discharge, to be the load so that the dynamic and steady-state responses of
the MPP tracker could be examined for use in the power analysis. The load
resistance represents the power consumed by the wireless sensor node for
sensing and communicating operations as well as the power losses incurred
by the electronic circuitries associated with the voltage-regulating and MPPT
converters at a voltage level of 4.2 V.
Referring to Figure 5.31 , it can be seen that from the initial start time t until
the seventh second, there was only the presence of the heat source applied
to the HEH system. The power harvested by the thermal energy harvester at
a temperature difference Wat3.6 V, which was

less than the power required by the load. In order to sustain the operation
of the wireless sensor node, an additional energy source from the artificial
lighting with illumination of 380 lux was inserted at t =7s.Itcan be seen from
Figure 5.31 that the output voltage of the boost converter V o connected to the
load increased from 3.6 to 5 V in around 5 s after which it settled down at 5 V.
In the midst of that, the input voltage of the boost converter V in surged and
the HEH system shifted away from its MPP. Since a microcontroller-based
MPP tracker with a closed-loop voltage feedback control was implemented

T of 10 K was about 190