Wind Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 57

supply voltage is tremendously reduced by a factor of 20, from 390 to 15

A, respectively, thus reducing the power consumed. To compensate for the
slow switching frequency, an external PWM generation circuit, which con-
sumes few tens of microwatts, is designed and implemented to multiply the
low-frequency PWM control signal generated from the low-power microcon-
troller to the 10-kHz range to miniaturize the size of the passive magnetic
components. Another approach taken to reduce the energy consumption is
duty cycling the transmission time of the energy-hungry radio module at a
slower rate of every second preset for this chapter.

2.1.3 Experimental Results

The proposed concept of a self-powered WEH wireless sensor node using
an efficient power management circuit, as illustrated in Figure 2.20 , has been
implemented in a hardware prototype for laboratory testing. Several tests
are conducted during the experiments to validate the performance of the
optimized WEH system using an AC-DC active rectifier and MPPT with a
resistor emulation approach in sustaining the operation of the wireless sensor
node.

2.1.3.1 Performance of WEH System with an MPPT Scheme
The experimental tests were conducted in accordance with the wind condi-
tion of the deployment ground illustrated in Figure 2.2 , where the average
wind speed is given as 3.62 m/s. Three experimental tests were conducted as
shown by three different operating regions in Figure 2.21 to differentiate the
performance of the WEH system and its resistor emulation MPPT scheme in
powering the load consisting of a supercapacitor, sensing and control circuit,
and wireless sensor node. The electrical load was first powered with a WEH
system without MPPT, then with a WEH system with MPPT, and last without
a WEH system and MPPT. Referring to Figure 2.21 , it is observed that the su-
percapacitor voltage V cap keeps decreasing during the period when the WEH
system was not equipped with MPPT and even more obvious for the case
where neither WEH system nor MPPT was integrated into the sensor node.
This phenomenon indicates that being solely dependent on the energy stor-
age or the electrical power harvested by the WEH system without an MPPT
scheme is not sufficient to sustain the operation of the sensor node. It is only
when the WEH system was incorporated with the MPPT scheme that suffi-
cient power was provided for both the operation of the wireless sensor node
and to charge the supercapacitor. Whenever the WEH system was operating
in its MPPT mode (see Figure 2.21 ), the generated output voltage of the wind
turbine V in was controlled by the microcontroller to follow the MPPT voltage
( V mppt = 1.15 V) based on the resistor emulation algorithm.
The effect of MPPT on the WEH system was further examined using the
waveforms as shown in Figure 2.22 . First, it is observed that the voltage across
the supercapacitor V cap dropped from 2.9 to 2.75 V after 350 s. In a similar man-
ner, the output DC voltage of the wind turbine V in as shown in Figure 2.22