Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 151

which is equivalent to an open-circuit condition. This is due to the diode at
the output of the SEH subsystem that prevents current flow from the WEH
subsystem as well as the energy storage. The CV MPPT technique used in the
SEH subsystem fixes the input voltage of its boost converter while varying
the output voltage of the boost converter according to the voltage, hence the
energy storage level of the supercapacitor. The MPPT operations of the WEH
and SEH subsystems are independent.
In this HEH system, the two diodes play an important role in insolating
the two subsystems so that maximum power is transferred from the energy
source to the load. However, some amount of power loss would be incurred
in the HEH system. The efficiency of the diode block is on average about
93% over a range of solar irradiance and resistance loading conditions. This
positive outcome is attributed to the very low power loss in the Schottky
diode, where the voltage drop across the Schottky diode is low, in the range
of 0.15 to 0.25 V. As such, it is viable to employ the diode block in the designed
HEH system to ensure MPPT operations are performed by the WEH and SEH
subsystems. Other than the power loss in the boost converter and the diode
block, another investigation was carried out to determine the power loss in
the associated control, sensing, and PWM generation electronic circuits. The
supply voltage of the electronic circuits provided by a voltage regulator is
3V.Based on the current requirement of each individual component in the
sensing and processing circuits, a summary of the total power consumption
of the electronic circuits was calculated and tabulated.
As can be seen in Table 5.1 , the total power consumed by the associated
control, sensing, and PWM generation electronic circuits of the WEH and
SEH subsystems was calculated to be 1.135 mW. As compared to the near-
maximum power harvested by the optimized HEH system seen in Figure
5.11 , which ranged from a few tens to hundreds of milliwatts, the power con-
sumption of the designed electronic circuits was very low and insignificant.
Taking the target deployment area with average wind speed of 4 m/s and
average solar irradiance level of 80 W/m 2 as an example, it can be seen from
Figure 5.11 that the total power harvested by the HEH system was 34 mW. The
power loss of 1.135 mW was only a small fraction, around 5%, of the harvested
power. Even for very low wind speed of 2.3 m/s and low solar irradiance of

TABLE 5.1
Power Consumption of Associated Control, Sensing, and PWM
Generation Electronic Circuits

Component Subsystem Qty Current/ A Power/ W

Comparator WEH 3 7 * 3 = 21 63

2 op-amps WEH 1 90 270

Comparator SEH 1 7 21

Voltage divider SEH — — 85

Oscillator Both 1 12 36

Wireless sensor node/ C Both 1 220 660