Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 147

V solar to charge the energy storage device and (2) to perform near-MPPT so
that maximum power transfer takes place. Referring to Figure 5.6 , there is a
voltage-sensing circuit, essentially a simple resistive voltage divider, to sense
and divide the output voltage of the solar panel into two for the microcon-
troller to process. The power loss in this voltage divider circuit is very small,
a few tens of microwatts, and it is quite insignificant as compared to the har-
vested power on the order of milliwatt level. The feedback voltage signal
V fb obtained from the terminal of the solar panel is compared with the ref-
erence voltage signal V mppt,ref in a microcontroller to perform the closed-loop
MPPT control of the boost converter via the pulse width modulation (PWM)
generation circuit.
The PWM generation circuit, as seen in Figure 5.6 , is used to convert the
low-frequency PWM control signal, about 100 Hz, generated from the low-
power microcontroller to a much higher switching frequency, 10 kHz, so that
smaller filter components can be used in the boost converter to miniaturize the
overall SEH subsystem. Depending on the voltage, hence the energy storage
level of the supercapacitor, V cap or V out , the output voltage of the solar panel
V solar is manipulated to transfer maximum power to the supercapacitor by
adjusting the duty cycle of the PWM gate signal of the boost converter such
that V solar is as close as possible to V mppt,ref , the reference MPPT voltage of
2.58 V at which the harvested power is near its maximum. As the energy level
of the supercapacitor increases or decreases, the output voltage of the boost
converter V out varies with the supercapacitor's voltage. However, at the input
voltage of the boost converter V in ,itisfixed to the solar panel's reference
MPPT voltage V mppt,ref of 2.58 V; hence, the optimal duty cycle of the boost
converter D opt , which ensures near MPPT takes place for the solar panel, is
expressed as

V out
V in

D = 1 − (5.4)

V cap
V mppt,ref

D opt = 1 − (5.5)

To experimentally verify the concept of a constant voltage approach to
perform MPPT for small-scale SEH, a set of experiments was conducted.
During the experiments, the MPPT capability of the designed boost converter
circuitry was tested for different solar irradiance, and the experimental results
are shown in Figure 5.7 .
Initially, for the first 20 s, it is observed in Figure 5.7 that the boost converter
was not controlled to operate the SEH subsystem at its MPP. After this, the
MPP tracker utilizes the closed-loop proportional integral (PI) controller to
manipulate the duty cycle of the boost converter according to Equation 5.4 ,
which in turn controls the input voltage of the boost converter towards the
optimal reference voltage value of 2.58 V. Once the MPP of the power curve
for a wind speed of 80 W/m 2 was reached, the closed-loop MPP tracker
control the boost converter to maintain power harvested from the solar panel