Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 175

comprised of (1) sensing some external analog signals from sensory devices
(such as temperature, humidity, etc.) and (2) communicating and relaying the
sensed information to the gateway node every 5 s. On receiving the data at the
base station, the collected data is then postprocessed into usable information
for any follow-up action.

5.3.5 Experimental Results

The near-optimal HEH wireless sensor node has been successfully imple-
mented into a hardware prototype for laboratory testing. Several experimen-
tal tests have been conducted to analyze the performance of the HEH system
and its simple and ultralow-power fixed reference voltage MPPT scheme in
powering the connected load consisting of the supercapacitor; the sensing,
control, and PWM generation circuitries; and the wireless sensor node.

5.3.5.1 Performance of a Parallel HEH Configuration
As mentioned, when the solar and thermal energy sources of different char-
acteristics are combined, it is bound to have impedance mismatch among the
integrated energy sources. As such, the performance of the parallel hybrid en-
ergy harvester, which contains the combined characteristics of the solar panel
as well as the thermal energy harvester, was investigated. With reference to
Figures 5.24 to 5.27 , it is illustrated that the fixed reference voltage method is
able to operate the hybrid energy harvester near its MPPs for different light
intensities and temperature differences, but at the expense of some percent-
age of power loss in the harvested power. It is thus important to examine the
significance of these power differences between the actual harvested power
P HEH,actual with respect to the MPPs P HEH,mppt of the hybrid energy harvester
as recorded in Figure 5.29 .
Considering an extreme operating condition, which is at low light illumina-
tion of 380 lux and small temperature difference of 5 K as seen in Figure 5.29a ,
for example, the power harvested at the fixed reference MPPT voltage of 3.6 V,
P HEH,actual , and the maximum obtainable power P HEH,mppt of the hybrid energy
harvester is 252 W and 260 W, respectively. The power difference is only

8 W, which is about 3% of its harvested power as shown in Figure 5.29b .

The power loss of 8 Wisdue to the impedance mismatch issue between the

solar panel and thermal energy harvester when they are connected directly
without the use of separate power converters.
Similarly, the power differences between the actual harvested power
P HEH,actual with respect to the MPPs, P HEH,mppt , for all the other operating
conditions range between 8 and 35

W (see Figure 5.29a ), which are 3%
to 6% of the harvested power (see Figure 5.29b ). Although the proposed
HEH system (i.e., hybrid energy harvester) and its power management unit
would incur power loss (i.e., 8 to 35 W) in the overall harvested power,

this power loss is small and marginal compared to those MPPT techniques
that require high computational power and cost to fulfill their objective of
precise and accurate MPP tracking. It is thus justifiable to utilize the simple