Wind Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 54

is more superior than the WEH system without MPPT. It is even more obvious
for higher wind speeds, as seen in Figure 2.17 , where the difference in the har-
vested power between the WEH system with an MPPT scheme and without
an MPPT scheme is significant; up to four times more electrical power can be
harvested from the wind turbine at a wind speed of 8.5 m/s. This exhibits the
importance as well as the contribution of implementing MPPT in the WEH
system.
2.1.2.3 Energy Storage
For long-term deployment of the wireless sensor node, it is required to have
an energy storage device such as a supercapacitor and batteries onboard the
sensor node to sustain its operation throughout the lifetime. It is also cru-
cial to ensure that this energy storage device has an operational life span of
equivalent length or even longer so that the WSN lifetime is prolonged. Com-
paring between the choice of using supercapacitors or batteries as the energy
storage for the WEH system, the supercapacitor has been chosen. The reason
is that the supercapacitor exhibits several superior characteristics over the
batteries that are useful for the WEH system. These characteristics include
numerous full-charge cycles (more than half a million charge cycles), long
lifetime (10 to 20 years operational lifetime), and high power density (an or-
der of magnitude higher continuous current than a battery) to provide high
instantaneous power to the sensor node during burst mode operation such
as radio transmission [34].
Unlike the discrete capacitors, which have very small capacitance values
in the microfarad range and usually are used in supply rails for decoupling
purposes, the supercapacitor has a very large capacitance value in the farad
range suitable for energy storage purposes. When a large capacitor that is in
the farad range is initially attached to the energy source, the component with
minimum energy stored acts as a short circuit to the energy source, and the
supply rail voltage drops to the capacitor voltage level. The same situation
occurs when the large capacitor is attached to the wind turbine generator as
well. Although the wind turbine still charges the supercapacitor under this
condition, it does not do so efficiently. This is because the charging process is
not executed at the MPP, which is the voltage and current combination that
maximizes power output under a given wind speed condition. A supercapac-
itor that is charged in this manner reduces the WEH efficiency by a factor of
two to four as illustrated in Figure 2.18 . Hence, the dynamic response of the
supercapacitor is important to consider in the design of the boost converter to
ensure constant MPPT operation is achieved by having a closed-loop resistor
emulator instead of the open-loop resistor emulation method suggested [70]
where the load impedance is assumed to be constant.
For a time period of 500 s as shown in Figure 2.18 , the supercapacitor
is charged by the WEH system from its discharged stage. At V cap (500 s), the
1.5-F supercapacitor is being charged to voltage levels of 2.14 V with
the MPPT scheme and 0.66 V without the MPPT scheme. Comparing between
the two schemes, it is obvious that the charging performed by the WEH system