Flow energy harvesting using piezoelectric fluttering plates – FLUTTENER

This project combines a theoretical analysis of the fluid-solid-electric coupling mechanisms, a numerical study of the nonlinear dynamics of the flapping plate coupled to the output circuit, and an experimental study of flapping plates in a wind-tunnel with one or more piezoelectric patches connected to various output circuits. Two different elements are essential to the success of this project, starting with its pluridisciplinarity (with participants of different backgrounds and research interests). As a result the second major challenge resides in the modeling of the nonlinear fluid-solid dynamics, in particular the representation of the fluid forces on the structure: a fully-coupled model must be obtained that is able to reproduce the physical mechanism and main properties of a flapping flag while remaining simple enough to be used jointly with state-of-the-art power electronics techniques and algorithms.

Research performed during this project has already identified the key role of the piezoelectric coupling between the output circuit and the fluid-solid system on the coupled dynamics (stability, frequency, amplitude) and on the amount of harvested energy. Indeed, using a simple model for the piezoelectric coupling (a continuous distribution of small piezoelectric patches on the structure) and for the output circuit (a purely resistive circuit or a resonant circuit), we showed that a tuning of the characteristic time-scale to the fluid-solid dynamics leads to a destabilization of the structure (i.e. a reduction of the critical flow velocity above which flapping and energy harvesting can develop) and to an increase in the amount of energy transferred to the output circuit. This frequency tuning is the result of a complex interaction between the mechanical and electrical properties: the flapping frequency of the flag is indeed influenced by the feedback forcing of the output circuit through the piezoelectric coupling.

This project further identified the role of the detailed piezoelectric coverage of the flag, and how it can be optimised for maximum efficiency.
Finally, the interactions of multiple piezoelectric flags were considered, both through their hydrodynamic coupling via the surrounding fluid, but also through their electrodynamic coupling via the output circuit. New power electronics strategies were designed to power a single circuit using multiple vibrating structures, potentially with different mechanical properties.

This project is based on the close collaboration of two different scientific communities (namely, mechanical studies of fluid-solid interactions and power electronics) on a topic of great technological, economical and societal importance: assessing the flow energy harvesting potential of piezoelectric flags, a design that has recently received an increasing attention. A better understanding of the fluid-solid-electric coupling mechanism will provide the scientific knowledge and methods, that will be necessary to industries and governments in order to properly assess the technological and economical feasibility of such systems. This better understanding will also help identifying potential optimization and improvement strategies for such systems, and more generally for flow energy harvesting systems based on fluid-solid interactions and instabilities.

``Influence and optimization of the electrodes position in a piezoelectric energy harvesting flag'', par M. Pineirua, O. Doaré et S. Michelin, J. Sound Vib., \textbf{346}, 200--215, 2015

``Fluid-solid-electric lock-in of energy harvesting piezoelectric flags'' par Y. Xia, S. Michelin et O. Doaré, \emph{Phys. Rev. App.}, \textbf{3}, 014009, 2015

``Electrical Interfacing Circuit Discussion of Galloping-Based Piezoelectric Energy Harvester'' par Y.-Y. Chen et D. Vasic, \emph{Phys. Procedia}, \textbf{70}, 1017--1021, 2015

``Resonance-induced enhancement of the energy harvesting performance of piezoelectric flags'' par Y. Xia, S. Michelin et O. Doaré, \emph{Appl. Phys. Lett.}, \textbf{107}, 263901, 2016

``Synchronized flutter of two slender flags'' par J. Mougel, O. Doaré et S. Michelin, \emph{J. Fluid Mech.}, \textbf{801}, 652--669, 2016

``Synchronized switch harvesting applied to piezoelectric flags'' par M. Pineirua, S. Michelin, D. Vasic et O. Doaré, \emph{Smart Material Struct.}, \textbf{25}, 085004, 2016

``Electro-hydrodynamic synchronization of piezoelectric flags'' par Y. Xia, O. Doaré et S. Michelin, \emph{J. Fluids Struct.}, \textbf{65}, 398--410, 2016

``Fluid-solid-electric energy transport along piezoelectric flags'', \emph{Eur. J. Comp. Mech., Special Issue on “Fluid Flows with Interactive Boundaries”} (sous presse), 2017

Above a critical flow velocity, a flexible plate becomes unstable to flutter and enters large-amplitude self-sustained flapping oscillations. This instability and the resulting oscillations can develop over a large range of flow velocity, making this mechanism attractive to harvest energy from a geophysical flow (wind, tidal or river currents,...): a fraction of the energy associated with the spontaneous solid deformation can be converted into electrical form through piezoelectric patches placed on the plate's surface. Such a system is particularly promising in the domain of small-power generation, when the connection to the electrical grid is technically or economically prohibitive.
The present research project will study theoretically, numerically and experimentally the dynamics of such a piezoelectric plate in a steady flow to evaluate the energy harvesting potential of this technology. A critical element of this project's success will be the assessment of the system's efficiency through the development of a fully-coupled fluid-solid-electrical model. The proper understanding and representation of this strong coupling between the fluid-solid dynamics and the output electrical circuit is an original element of the present proposal, that thereby distinguishes itself from existing studies in fluid-structure interactions (with a realistic representation of the harvesting mechanism and output circuit) and from the traditional approach in power electronics (with a realistic model for the fluid-solid dynamics taking into account threshold effects, non-linearities and spatial variations associated with the flag dynamics).
The objectives of this project are: (i) to evaluate the system's efficiency, (ii) to optimize the output electrical circuit to maximize the harvested energy and (iii) to understand the potential couplings between multiple neighboring harvesting units. Hence, the project will be organized around three main tasks.
The first task will study numerically and experimentally the dynamics of such a piezoelectric flag with a purely dissipative output circuit. A coupled model for the fluid-solid-electric dynamics will be proposed, taking into account the piezoelectric coupling and the non-linear fluid-solid dynamics associated with the self-sustained plate's oscillations. The efficiency of the system and its dependence with its main characteristics (inertia, stiffness, coupling, etc...) will also be determined.
The second task will focus on the optimization of the output circuit to maximize the harvested energy. Linear propagative and/or resonant circuits will be considered, but the bulk of this task will be dedicated to the application of state-of-the-art non-linear active techniques recently developed. A new approach will also be proposed, that is better suited to the non-linear dynamics of the fluttering plate.
The third task will study potential electro- and hydrodynamic interactions between two or more neighboring units, by identifying the constructive or destructive nature of these interactions in terms of harvested energy.
This project is characterized by its pluri-disciplinarity, including three young researchers working at three different research institutions and with complementary backgrounds and sensitivities (fluid-mechanics and fluid-solid interactions on one hand, and electrical engineering/power electronics on the other hand).