Wind Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 49

always at a maximum for different wind speeds. However, for other loading
conditions, shifting away from the internal resistance of the wind turbine
generator, either very light or heavy electrical loads, the electrical output
power being generated by the generator drops significantly. This exhibits
that the MPPT technique based on resistor emulation is a possible option to
assist the small-scale WEH system to achieve maximum power harvesting
from the wind turbine generator.
Khouzam and Khouzam [69] discussed the direct-coupling approach for
optimum load matching between the energy harvester and its load by care-
fully selecting the harvester's rated parameters with respect to the load pa-
rameters. Another resistor emulation approach proposed by Paing et al. [70] is
to operate the boost converter as an open-loop resistor emulator with proper
selection of the components to naturally track the MPP to match the opti-
mal load impedance for the energy harvester. Both approaches require some
form of initial tuning as well as the load impedance needs to be fixed. How-
ever, in practice, this may not be the case as the load impedance tends to
change just like the charging and discharging process of the supercapacitor;
therefore, the direct-coupling method as well as the simple open-loop resistor
emulation method may not be suitable in this context. To overcome that, a
microcontroller-based resistance emulator with a closed-loop feedback resis-
tance control scheme is proposed as the MPP tracker of the WEH wireless
sensor node for various dynamic conditions. The proposed scheme does not
require any initial tuning, unlike those two existing approaches, because there
is a microcontroller, together with its feedback resistance, to automatically
tune the WEH system to its MPPT points. In addition, the proposed MPP
tracker is embedded with the closed-loop control feature to continuously
track and emulate the reference optimal resistance as the load impedance
changes. The designed boost converter circuitry with the resistor emulation
MPPT approach is depicted in Figure 2.13 . It is essentially composed of three
main building blocks: (1) a boost converter to manage the power transfer
from the wind turbine to the load (i.e., power management unit, superca-
pacitor, and wireless sensor network [WSN]); (2) an MPP tracker based on
aresistor emulation approach and its sensing and control circuit that ma-
nipulates the operating point of the wind turbine to keep harvesting power
at the maximum power point; and (3) a PWM generation circuit. Using the
voltage and current-sensing circuit, the feedback resistance signal R fb is ob-
tained and compared with the reference resistance signal R opt,ref in a micro-
controller to perform the closed-loop MPPT control of the boost converter via
the PWM generation circuit. The PWM generation circuit is used to multiply
the low-frequency PWM control signal ( 100 Hz) generated from the low-

power microcontroller to a much higher switching frequency (10 kHz) so that
smaller filter components are used in the boost converter to miniaturize the
overall WEH system.
To experimentally verify the concept of the resistor emulation approach to
perform MPPT for small-scale WEH, the input resistance of the boost con-
verter, which is known as the emulated resistance of the wind turbine R em ,is

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