An Experimental Study on Energy Harvesting with Piezoelectric Inverted Flags

Abstract

Growing concerns over environmental pollution and the increasing need for continuous operation of portable electronics and wireless sensors have been major forces deriving the recent hype in developing centimeter-scale energy harvesters. This thesis contributes to this emerging field via provision of an improved understanding of the operation of piezoelectric inverted flag energy harvesters based on systematic experimentation of the impact of their planform geometry, quantifying their lifespan, and examining interaction effects between multiple harvesters. The thesis starts with a thorough literature review on the available piezoelectric wind energy harvester designs. Specific focus is given to the so-called 'inverted flag' configuration, featuring a free leading-edge and a fixed trailing-edge. The potential of this relatively new configuration as a promising approach to power small electronics and remote wireless sensors from wind excitation is thoroughly discussed. A parametric experimental investigation is conducted to assess the influence of planform geometry on the dynamics and energy harvesting performance of composite PVDF (polyvinylidene difluoride) inverted flag harvesters. It is shown that, for a given mass ratio, parameters such as amplitude, flapping frequency, power, and power density are all inversely proportional to aspect ratio and directly proportional to second moment of planform area. In fact, an important contribution of this thesis lies in proposing the second moment of planform area as a more reliable parameter, compared to aspect ratio, for evaluating the dynamics and power generation performance of these flags. Additionally, it is shown that higher power densities and broader operational velocity ranges are achievable when increasing the mass ratio. However, this comes at the expense of the flag's sensitivity to variations in operating conditions. To quantify the lifespan of PVDF inverted flag harvesters, a first of its kind endurance test is carried out. It is shown that such a harvester can operate reliably around half a million cycles before reaching its failure point, which indicates that the energy produced over its lifespan is enough to power a standard low-power temperature sensor for several months, with a sampling rate of one sample per minute. Finally, a systematic experimental investigation is conducted to assess the interactions between a flag and the wind tunnel wall, or between multiple inverted flags when vertically aligned on top of each other. Both bare metal flags and flags with PVDF elements attached were considered for this investigation. For bare metal flags, results demonstrated that as the gap distance (either between a flag and a wall or between multiple flags) decreases, the operational wind speed range for flapping gradually decreases and shifts to lower velocities. Moreover, the flapping frequency decreases proportionally with wind speed and gap distance, whereas the flapping amplitude remains relatively unaffected. For flags with PVDF elements attached, the narrowing of the flapping wind speed range becomes even more significant. The flapping frequency is substantially reduced, and the power reduction becomes clearly pronounced, reducing by up to one order of magnitude in comparison to a single flag. These findings suggest that vertical alignment of inverted flags can be significantly detrimental to energy harvesting.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Alistair Revell (Supervisor) & M Nabawy BSc, MSc, PhD, MRAeS, SMAIAA, FHEA (Supervisor)