Conclusions and Future Works - Energy Harvesting Autonomous Sensor Systems - page 215

load. However, only a small fraction of the harvested energy has been ex-
tracted by the vibration energy harvester for use. As such, for future research
work, the energy extraction process can be optimized using dedicated power
management techniques like synchronized electric charge extraction (SECE),
parallel or series synchronized switch harvesting on inductor (SSHI), and
so on. In addition, further investigations are necessary for commercially vi-
able, cheaper piezoelectric material with similar high performance. This step
is necessary for the commercialization of this impact-based VEH research
work.
Hybrid energy harvesting from two renewable energy sources has been
proven with a hardware prototype to yield more electrical power than a sin-
gle EH source. The augmenting process of two energy sources is done by
either using an individual power converter for each EH source or directly
connecting the energy sources in parallel. Other than the parallel configura-
tion, for future research, the renewable energy sources can be stacked in series
to produce higher voltage but at the same time yield more electrical power.
In addition, it is also worthwhile to explore EH from more than two energy
sources available in the ambient environment. With more electrical energy
harvested from different energy sources, the reliability of the wireless sensor
nodes can be improved.
Last, in this topic, the concept of magnetic energy harvesting via inductive
coupling has been presented and demonstrated. An adequate power manage-
ment circuit was developed to bridge between the magnetic energy harvester
and the electrical load. However, there was no proper matching between the
impedances of the source and the load. As such, in future research work, an
intelligent control of the power electronic converter to match the source and
load impedances can be developed. With this proper impedance-matching
scheme in place, the harvested power from the magnetic energy harvesting
would be maximum. This surplus in harvested energy would be useful for the
operation of the electrical load. For the research work on wireless power trans-
fer (WPT) via strongly coupled magnetic resonances, it is clear that evanescent
resonant coupling is able to transmit a watt level of electrical power from the
transmitting end to the receiving end over a distance of 1 to 2 m. In order to
further increase the power levels beyond a few watts, there are two factors to
be considered: (1) The power rating of the matching capacitors has to be in-
creased. Capacitors using suitable material like tantalum that can withstand
high current and operate at the megahertz frequency range can be used to
replace ceramic capacitors. (2) High-frequency, high-power amplifiers to gen-
erate an AC electrical power in the megahertz range and watt level of power
can be used.