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Ambient Energy Harvesting-An Electrostatic Approach.

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Title:Ambient Energy Harvesting-An Electrostatic Approach.
Authors:Yu, Jian
Contributors:Civil Engineering (department)
Keywords:Electrostatic Energy harvesting
variable capacitor
exponential growth
positive feedback
contact electrification
show 3 moreelectrostatic induction
water drop
hydrophobic surface
show less
Date Issued:Aug 2018
Publisher:University of Hawaiʻi at Mānoa
Abstract:With the rapid advancement of very-large-scale integration and miniaturization, wireless
micro-sensors, wearable devices and biomedical implants have been implemented all over the
world. Nowadays, batteries are considered as primary power sources for the majority of
electronics. They need to be periodically charged or replaced due to their limited lifetime, which
is inconvenient and may lead to increased costs. One promising solution is to harvest unused
energy sources surrounding those electronic devices into electric energy, which can be considered
as alternative or auxiliary power sources. There is a variety of ambient energy sources, such as
solar, mechanical, thermal and chemical. Among the existing energy harvesting mechanisms,
electrostatic energy harvesting draws our attention because of its various advantages, such as
low-cost fabrication, high achievable energy harvesting efficiency and capability for large-scale
integration and miniaturization. Furthermore, the electrostatic approach can be used to harvest a
variety of ambient energy sources, such as mechanical, thermal and chemical. However, some
challenges also exist, such as relatively small initial charge and capacitance, requirement for an
external power supply, and complex power management control circuits to extract generated
energy at the appropriate time.
Two novel methods have been proposed in this dissertation to solve the current challenges
for electrostatic energy harvesting. The first method utilizes an appropriate, repetitive
reconfiguration process to create a positive feedback mechanism, thus, restoring the generated
energy back to the system. Even a small disturbance on a system could significantly amplify
electric outputs. Because of the exponentially growth rate of energy extraction, this method is
particularly effective for distributed devices to scavenge energy from low-level ambient sources,
thus enabling self-powered operation. As proof of concept, two rotary variable capacitors in
addition with a fixed ceramic capacitor are used to establish a positive-feedback system. To
achieve relatively high capacitance, liquid-contact variable capacitors are developed for the
system. Because of contact electrification and electrostatic induction, the contact variable
capacitors can provide relatively high extra charge in each cycle. In our experiments, the
prototype using three mercury droplets can generate 10.2 J per cycle, corresponding to a
harvesting efficiency of 12.2% and the values for the prototype with three water droplets will be
1.2 J per cycle and 7.9%. The efficiencies of the devices far exceed those of the existing droplet
generators. Since the concept of exponential energy harvesting is not domain specific, it may lead
to new research in directional energy transfer systems in various energy domains, such as salinity
gradients and temperature differences.
An additional proposed method utilizes water droplets that alternate contacts between
CYTOP and PTFE thin films to provide high initial charge for vibration energy harvesting.
Because CYTOP and PTFE develop significantly different surface charge densities during contact
with water, they can be utilized to generate electricity effectively. More importantly, the proposed
method utilizes the strong electrostatic induction in the water droplets due to the electrical double
layer formed at the interface. A harvesting efficiency of 2.5% has been achieved in this study. It is
more effective than existing methods that are based on the much weaker electrostatic induction in
the substrate. Also, unlike existing methods, in which the charge of the drop is not delivered to
external circuits, in our method, the water droplets possesses a dual function as both an electrode
and a passive switch, leading to the direct harvesting of the peak electric potential energy. This
method not only results in simple device architecture but also allows schemes based on variable
capacitors to improve the performance. Using prototype devices, we demonstrate the effectiveness
of this approach in scavenging energy from low-level and low-frequency ambient vibrations.
Description:Ph.D. Thesis. University of Hawaiʻi at Mānoa 2018.
Rights:All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections: Ph.D. - Civil Engineering

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