Wind Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 60

also dropped from 2.9 to 2.75 V. Based on the characteristic curve of the wind
turbine generator shown in Figure 2.11 , the amount of electrical power har-
vested by the WEH system at a wind speed of 3.62 m/s and V in of between
2.75 and 2.9 V was around 1 to 2 mW. Since the power consumption of the
wireless sensor node, including the sensing and control circuit ( 0.4 mW), of

around 3.6 mW at 1 s per transmission was more than the harvested power
of 1 to 2 mW, the power harvested by the WEH system alone was not suffi-
cient to maintain the operation of the sensor node. However, when the source
and load impedances were nicely matched using the boost converter and its
closed-loop resistance emulator, the harvested power from the wind turbine
increased tremendously, generating sufficient power to charge the superca-
pacitor V cap from 2.75 back to 2.9 V in 150 s, as shown in Figure 2.22 , which
is less than half of the previous discharge time. At a wind speed of 3.62 m/s,
the electrical power harvested by the WEH system with its MPPT mode was
approximately 7.86 mW, even if we consider the losses in the converter where
3.6 mW of power were consumed by the wireless sensor node, the rest of the
harvested power of 4.26 mW was supplied to charge the supercapacitor. As
such, the WEH system with the MPPT scheme was definitely able to sustain
the sensor node's operation.
Another experimental test was carried out to compare the performance of
a conventional sensor node, which operates solely on the energy storage, and
a WEH sensor node as shown in Figure 2.23 . As the conventional sensor node
consumes 3.6 mW of average power from the supercapacitor, the voltage
across the supercapacitor dropped from 2.8 to 2.55 V in around 275 s, which
is calculated to be 1 J of energy transferred to the load. Compared to the
WEH system without MPPT, as shown in Figure 2.22 , the discharge rate of
the supercapacitor was much higher in this case because there was no extra
power generated from the WEH system to supplement the sensor node's
operation. Once the WEH system with MPPT was activated, the harvested
power of 7.86 mW was used to power the sensor node as well as to charge
the supercapacitor back to 2.8 V in 225 s.
Among the three testing options, the WEH wireless sensor node with the
MPPT scheme yielded the most superior performance. This is because the
WEH sensor node was incorporated with a wind energy supply designed to
harvest power at its optimal point, to charge the supercapacitor and sustain
the operation of the sensor node. To further validate the superior performance
of the WEH wireless sensor node with the MPPT scheme incorporated, the
sensor node was tested in a light wind condition of 3-m/s wind speed where
5mWofpower was available at the output of the active rectifier as observed in
Figure 2.9 . After taking the losses of the DC-DC converter into consideration,
the available power of 3.74 mW had a surplus after taking into account the
power consumption of the wireless sensor node. Hence, the charging voltage
V cap waveform as shown in Figure 2.24 illustrates that the WEH system with
the MPPT scheme, operating at lower wind speeds, was still able to supply
power to the sensor node as well as to charge the supercapacitor.

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