Conclusions and Future Works - Energy Harvesting Autonomous Sensor Systems - page 214

(source), power management circuit, energy storage device, and wireless sen-
sor node (load). Power output per unit mass or volume (i.e., power/energy
density) is a key performance unit for the energy harvester. Based on the
characteristic and capability of each of the ambient energy sources, the EH
system is designed to suit the target application and its ambient conditions
and event/task requirements. For the EH-WSN, various types of ambient
renewable EH systems, based on wind energy harvesting (WEH), thermal
energy harvesting, vibration energy harvesting (VEH), solar energy harvest-
ing, hybrid energy harvesting (HEH), and magnetic energy harvesting are
designed and then implemented into hardware prototypes for proof of con-
cept. To optimize these EH systems, the harvested power is conditioned to an
appropriate form for either charging the system batteries or powering the con-
nected load directly. Proper load impedance matching between the EH source
and the electrical load is carried out to maximize the usage of the harvested
energy. Several different types of power-electronic-based management cir-
cuits, such as an active alternating current-direct current (AC-DC) converter,
DC-DC converter with maximum power point tracking, energy storage and
latching circuit, and others, have been introduced and implemented.
7.2 Future Research Works
In this EH research work, starting from scrap to what has been achieved so far,
a wide breadth of studies and depth investigations on critical issues have been
carried out. However, there is still room for more research, improvements,
and further optimization. Some possible future research works include those
discussed next.
For the direct WEH research, the efficiency of the electric generator is rela-
tively low because the generator is not optimized to operate at its rated speed.
To overcome that, several ways have been suggested to improve the efficiency
of the electric generator, like introducing a gearbox between the wind turbine
generator (WTG) hub and the generator and modifying the two-pole gen-
erator to a multipole generator. Other than the electric generator, the wind
turbine itself is another possible area for future research. With adequate de-
sign of the blades of the wind turbine to suit the wind profile at the point
of deployment, the aerodynamic efficiency can be improved. As for indirect
WEH research, the analysis for the conversion process from wind power to
electrical power through the piezoelectric cantilever beam effect can be en-
hanced using computer-aided engineering software such as Ansys. By doing
so, with prior knowledge of the incoming wind speed, the power throughput
of the vibration-based piezoelectric wind energy harvester can be estimated.
In addition, the mechanical resonance of the harvester's structure can be op-
timized to suit the needs of a target application.
In the VEH system, the energy extracted by the impact-based piezoelectric
energy harvester is sufficient to power the radio-frequency (RF) transmitter