Wind Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 39

with this approach of sensing voltage is that if the input voltages of each
of the comparators are too close to each other, excessive oscillations occur,
and the AC-DC rectifier's efficiency is drastically reduced. To enhance the
performance of this active rectifier, a current sensing approach for generat-
ing the gating signals for the MOSFET-based active rectifier is designed and
implemented in the chapter.
Another challenging issue addressed in this chapter is that the electri-
cal power harvested by the WEH system for powering the wireless sensor
nodes is often very low, on the order of the milliwatt range or less. The sit-
uation is even worse if the wind turbine generator is not operating at its
maximum power point (MPP). The primary concern is to develop a high-
efficiency power converter using micropower-associated electronic circuits
to track and maintain maximum output power from the wind turbine gen-
erator to sustain the wireless sensor node operation over a wide range of
operating conditions. Maximum power point tracking (MPPT) techniques
have been commonly used in large-scale WEH systems [66-68] for harvest-
ing a much higher amount of energy from the environment. However, these
MPPT techniques require high computational power to fulfill their objective
of precise and accurate MPPT. Implementation of such accurate MPPT tech-
niques for a small-scale WEH whereby the power consumed by the complex
MPPT circuitry is much higher than the harvested power itself therefore is not
desirable. So far, very limited research works can be found in the literature
that discuss a simple but compatible MPPT algorithm addressing the issue of
a small-scale WEH system. In this chapter, the resistor emulation approach
is investigated for a micro wind turbine generator. The rationale behind the
resistor emulation approach is that the effective load resistance is controlled
to emulate the internal source resistance of the wind turbine generator [69-71]
to achieve good impedance matching between the source and load, and hence
the harvested power is always at its maximum at any operating wind speed.
The emphasis of this chapter is on resolving the two mentioned challenges
associated with a small-scale WEH wireless sensor node using the designed
ultralow-power management circuit with little overhead power consumed.
The rest of the chapter is organized as follows: Section 2.1.1 describes the
details of the wind energy conversion system and determines the output
power available at each stage of the system. Section 2.1.2 discusses the issues
related to the design of an efficient power management circuit to interface
the wind turbine generator and the wireless sensor node. Following that, the
experimental results of the optimized WEH wireless sensor node prototype
are illustrated in Section 2.1.3, with the conclusion reported in Section 2.1.4.

2.1.1 Wind Turbine Generators

Within the WEH system, the energy harvester, which converts the raw wind
energy harnessed from the ambient environment into AC or DC electrical
energy depending on the type of generator used, is first investigated. In this
case, the energy harvester is a horizontal-axis micro wind turbine with a