Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 154

Wind Turbine-
Generator

12 cm

11 cm

10 cm

9 cm

8 cm

7 cm

6 cm

5 cm

Solar Panel

4 cm

Power
Management
Circuit

3 cm

Supercapacitor

2 cm

1 cm

Wireless
Sensor Node

FIGURE 5.13
Hardware prototype of a hybrid energy harvesting wireless sensor node.

and SEH subsystems and their MPPT schemes in powering the electrical load
consisting of a supercapacitor, sensing and control circuits and wireless sensor
node.
In region A of Figure 5.14 , the electrical load is at first powered with the
SEH subsystem with a constant voltage-based MPPT scheme. It can be seen
from Figure 5.14 that the output voltage of the solar panel V solar was fixed at its
MPPT reference voltage V mppt,re f of 2.58 V. This tracking profile verifies that
the SEH subsystem was operating near its MPP. During this time duration of
90 s, the 1.5-F supercapacitor's voltage V cap was charged from 3.1 to 3.35 V,
and around 13 mW of electrical power was harvested by the SEH subsystem
at an average solar irradiance level of 80 W/m 2 . However, in region B1 of
Figure 5.14 , when the SEH subsystem was disconnected from the load, the
output voltage of the supercapacitor V cap fell back to 3.1 V in 100 s. In region
C, V cap continues to decrease at a slower rate. This is because solar energy
was again harvested but not at the MPP. The output voltage of the solar
panel V solar is seen in Figure 5.14 as 2.94 V, which is higher than the MPPT
voltage reference of 2.58 V. Without MPPT control for the SEH subsystem, the
harvested solar energy was not able to sustain the entire unit.