Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 143

most abundant energy sources in an outdoor environment, so they have been
chosen to be utilized in the proposed HEH system. During the night, when
the sun has set, wind energy that is mostly available throughout the whole
day is present for harvesting. Therefore, it can be seen that the wind and solar
energy sources complement one another. They form an ideal pair for an HEH
system in powering wireless sensor nodes deployed in the outdoor environ-
ment. However, the electrical characteristics of the wind and solar energy
harvesters are different from one another. When these energy harvesters are
combined directly, it is bound to cause a problem of internal impedance mis-
match between them. This will result in very poor power transfer efficiency
between the EH sources and the electrical load.
To overcome this problem, a Type 3 HEH scheme is proposed where each
energy source (i.e., wind and solar) has its own power management circuit
for performing MPPT. This ensures simultaneous charging of the energy stor-
age device as well as powering of the wireless sensor node. The rest of the
section is organized as follows: Section 5.2.1 describes the electrical power
generation from the WEH subsystem. Section 5.2.2 details the design of an
efficient power management circuit to perform MPPT for the SEH subsystem.
Section 5.2.3 illustrates how the WEH and SEH subsystems are interfaced to
achieve the proposed HEH system. Following that, the experimental results
of the optimized HEH wireless sensor node prototype are depicted in Section
5.2.4, with the summary reported in Section 5.2.5.

5.2.1 Wind Energy Harvesting Subsystem

The proposed HEH wireless sensor node is designed to be deployed in a sam-
ple remote sensing area where its environment has four seasons throughout
the year (i.e., spring, autumn, summer, and winter). The environmental con-
dition of the deployment area for each season differs from the other seasons.
Take, for example, during the summer season, there is lots of sunshine, but
only some gentle breezes. Conversely, when it comes to wintertime, the du-
ration of sunlight becomes short, whereas the wind gets stronger. According
to the global solar power map and Canada's wind energy atlas (illustrated by
Solar Power [139] and the Canadian Wind Energy Atlas [140], respectively), it is
observed that the average sun hour and average wind speed in the northern
part of the world like Toronto (Canada) are 1.0 peak sun hour and 4 m/s,
respectively. The term peak sun hour represents the average amount of sun
available per day throughout the year. The total amount of solar radiation en-
ergy can be expressed in hours of full sunlight per square metre (1000 W/m 2 )
or peak sun hours [139]. Over a time span of 12 hours per day, 1 sun hour works
out to be an average solar irradiance level of around 80 W/m 2 .With reference
to the WEH system designed and developed in Chapter 2 , at an average wind
speed of 4 m/s, it can be read from Figure 2.4 that the electrical power har-
vested by the wind turbine generator at its maximal point is around 18 mW.
Based on the power analysis of the WEH system described in Section 2.1.3.2,
it can be seen that the WEH system with a MPPT scheme is able to harvest