Energy of the device can be used

Energy
harvesting relates to the practice of scavenging small amounts of energy from
ambient environmental sources (e.g. wind, water, heat, vibration) in order to
power either some small, low power electronic system directly, or to charge an
electrical storage reservoir (usually a rechargeable battery or capacitor) that
can be used to power a higher power application at time intervals.

 Much of what has been learnt about energy
harvesting has been learnt in the last fifteen years or so, and it is enough to
fuel the promise that many electronic systems can have built-in energy
harvesting functionality in the future. However, at present the low amounts of
power that can be delivered from energy harvesting devices is proving a barrier
to adoption of the technology. Device optimisation is one way in which the
power density of a harvesting device can be significantly improved. Another way
in which the power output can be enhanced is through the use of the ‘harvesting
circuitry’; i.e. the circuitry that is usually connected to the output of the
harvesting device to condition and/or manage the electrical power output. In
this thesis, both of these avenues are explored. In regard to device
optimisation, an analytical model of a piezoelectric cantilever-based vibration
energy harvesting device is developed whose resulting expression for the power
output of the device can be used as an objective function in a computer-based optimisation
algorithm. The required inputs to the optimisation problem are the target
resonant frequency for the device and constraints for the volume of the device.
The output is a design (i.e. the dimensions) for a device that is geometrically
optimised for maximum power output. In this thesis, the developed model is used
in conjunction with a conjugate gradient optimisation algorithm, provided by
Mathcad 2000 Professional software (Parametric Technology Corporation/Mathsoft,
MA, USA). The resulting 2 harvesting device design was found, through
experiment, to be capable of producing a maximum power output of 370.37µW for a
volume of 1cm3 and a resonant frequency of 87Hz. This achieved power level is
amongst the highest of power densities reported in the literature to date. In
regard to the harvesting circuitry, this thesis also proposes a novel
harvesting circuit concept for the purpose of obtaining an enhanced power
output. The suggested circuit is based on a combination of a charge pump-type
circuit and the Synchronised Switch Harvesting on Inductor (SSHI) technique,
which is a technique that is known to increase the power output of
piezoelectric energy harvesters by as much as 900% 1. By combining these two
functions in the manner that has been conceived in this thesis, the functions
become mutually conducive to the aim of enhancing the power output of a
piezoelectric generator. Aside from taking advantage of the voltage boosting
effect the SSHI technique is known for, the particular implementation of the
technique adopted in this thesis also enables a charge pump-type circuit to
collect charge during the whole of each AC cycle of the piezoelectric generator
output, rather than for just part of it as occurs with the commonly-used bridge
rectifier circuit. Therefore, in the new harvesting circuit concept suggested
in this thesis, at least two mechanisms are in place for enhancing the power
output of a piezoelectric vibration energy harvester. In addition,
consideration has been given to allow conditioning of the harvested power such
that a regulated DC supply, which is the preferred format for most electronic
device and systems, might be obtained. The result is a circuit that is capable
of producing over three times the amount of DC power than the standard bridge
rectifier circuit for the same input vibration conditions, with additional
provision for easy formatting of the power to form a regulated supply. 3 This
first chapter of the thesis presents the motivation for carrying out the
research, describes the context in which the research fits, specifies the aims
and objectives, and presents the contribution of the research. The chapter ends
by presenting an overview of the methodology used and describing the rest of
the thesis structure.

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