Conjoined piezoelectric harvesters and carbon supercapacitors for powering intelligent wireless sensors

Doctoral thesis, 2018

To achieve total freedom of location for intelligent wireless sensors (IWS), these need to be autonomous. To achive
this today there is a need of broadband piezoelectric energy harvesting and a long-lasting energy. The Harvester need
to be able to provide sufficient amount of energy for the intelligent wireless sensor to perform its task. The energy
storage needs to fulfill the requirement of a large number of charge discharge cycles and contain sufficient power for
the intelligent wireless sensor.
The biggest issue with piezoelectric energy harvesting today is the bandwidth limitation. Solutions today to achieve
larger bandwidth make a tradeoff where the output is decreased. The biggest issue for energy storage today is the
limitation of energy density for supercapacitors and the lack of sufficient life cycles for batteries.
This thesis aims to realize piezoelectric energy harvesters with broad bandwidth and maintained power output.
Moreover, for energy storage in the form of supercapacitors realize an electrode material that has a high effective
surface area, good conductivity not dependent on a conductive agent and can be used without a binder. This thesis
cover background and history of the two fields, discussion of technologies used and presents solutions for piezoelectric
energy harvesting and carbon based supercapacitor storage.
A Backfolded piezoelectric harvester was made of two conjoined piezoelectric cantilevers, one placed on top of a
bottom cantilever. By the backfolded design this thesis show that by utilizing the extended stress distribution of the
bottom cantilever a maintained power output is achieved for both output peaks. By introducing asymmetry where the
top cantilever have 80% length compared with the bottom cantilever the bandwidth was increased. An effective
bandwidth of 70 Hz with voltage output above 2,75 V for 1 g is achieved.
To achieve further enhanced bandwidth a piezoelectric energy harvester with selftuning was designed. The
selftuning was achieved by a sliding mass on a beam, which is conjoined, to two piezoelectric cantilevers in a
backfolded structure. By introducing length asymmetry, the effective bandwidth was enhanced to 38 Hz with a power
output above 15 mW, for 1 g, which is sufficient for an intelligent wireless sensor to start up and transmit data.
To utilize the positive output effect from conjoined cantielvers a micro harvester was fabricated. The design was
based on the same principle as for the backfolded, but for fabrication reasons the design was made in one plane. The
harvester contain two outer cantilevers conjoined to a backfolded middle cantilever. Due to fabrication difficulties,
only a mechanical characterization of the harvester was possible. The result from the characterization looks promising
from a harvesting point of view, by showing a clear peak that seems to be somewhat broadband.
Energy storage for an autonomous wireless intelligent sensor (IWS) needs to be able to charge and discharge during
the lifetime of the IWS. Therefor the choice fell on supercapacitors instead of batteries. Over time the supercapacitor
due to its superior amount of charge and discharge cycles, outperform a battery when energy density is compared.
Increasing the energy density for supercapacitors gives the advantage to prolong the providing of power to the
IWS. One such electrode material is conjoined carbon nanofibers and carbon nanotubes. The material is not dependent
on conductive agents or binders. The effective surface area can be expanded through a denser structure of CNF, where
more CNT can grow. In combination with activation, which will yield more micropores, hence an increased
capacitance for the presented synthesized material yielded 91 F/g with an effective surface area of 131 m2.
There is many challenges to power an IWS on a gasturbine. This thesis cover challenges like vibrations on cables,
placement issues and the charge of a supercapacitor by harvested energy that comes in small chunks. Solutions for
these challenges are offered.
The presented work in this thesis shows how the bandwidth for piezoelectric energy harvesters can be broader by
asymmetric implementation of conjoined resonators. In addition, the advantages of conjoined carbon electrode
materials to be implemented as electrode material in supercapacitors. Both harvester and storage are intended to be
used as energy sources for intelligent wireless sensors.