An impact-based broadband aeroelastic energy harvester for concurrent wind and base vibration energy harvesting

© 2017 This paper proposes a novel broadband energy harvester to concurrently harvest energy from base vibrations and wind flows by utilizing a mechanical stopper. A problem for a conventional wind energy harvester is that it can only effectively harness energy from two types of excitations around its resonance frequency. The proposed design consists of a D-shape-sectioned bluff body attached to a piezoelectric cantilever, and a mechanical stopper fixed at the bottom of the cantilever which introduces piecewise linearity through its impact with the bluff body. The quasi-periodic oscillations are converted to periodic vibration due to the introduction of the mechanical stopper, which forces the two excitation frequencies to lock into each other. Broadened bandwidth for effective concurrent energy harvesting is thus achieved, and at the same time, the beam deflection is slightly mitigated and fully utilized for power conversion. The experiment shows that with the stopper-bluff body distance of 19.5 mm, the output power from the proposed harvesting device increases steadily from 3.0 mW at 17.3 Hz to 3.8 mW at 19.1 Hz at a wind speed of 5.5 m/s and a base acceleration of 0.5 g. A guideline for the stopper configuration is also provided for performance enhancement of the broadband concurrent wind and vibration energy harvester

Performance enhancement of an aeroelastic energy harvester for efficient power harvesting from concurrent wind flows and base vibrations

© 2018 IEEE. In this paper, using a high frequency mechanical stopper as a complementary energy harvester is proposed to improve the performance of energy harvesting from concurrent wind flows and base vibrations. Galloping aeroelasticity of a square-sectioned bluff body is employed to achieve limit-cycle structural oscillations. The analysis demonstrates that the bandwidth for effectively harnessing both aerodynamic and base vibratory energy is substantially widened, and simultaneously, the total power amplitude is significantly enhanced as compared to the original linear galloping energy harvester. It is concluded that the proposed system is viable solution to enhance energy conversion in situations where wind flows and base vibrations are coexisting

Toward Small-Scale Wind Energy Harvesting: Design, Enhancement, Performance Comparison, and Applicability

© 2017 Liya Zhao and Yaowen Yang. The concept of harvesting ambient energy as an alternative power supply for electronic systems like remote sensors to avoid replacement of depleted batteries has been enthusiastically investigated over the past few years. Wind energy is a potential power source which is ubiquitous in both indoor and outdoor environments. The increasing research interests have resulted in numerous techniques on small-scale wind energy harvesting, and a rigorous and quantitative comparison is necessary to provide the academic community a guideline. This paper reviews the recent advances on various wind power harvesting techniques ranging between cm-scaled wind turbines and windmills, harvesters based on aeroelasticities, and those based on turbulence and other types of working principles, mainly from a quantitative perspective. The merits, weaknesses, and applicability of different prototypes are discussed in detail. Also, efficiency enhancing methods are summarized from two aspects, that is, structural modification aspect and interface circuit improvement aspect. Studies on integrating wind energy harvesters with wireless sensors for potential practical uses are also reviewed. The purpose of this paper is to provide useful guidance to researchers from various disciplines interested in small-scale wind energy harvesting and help them build a quantitative understanding of this technique

Enhanced mechanical energy extraction from transverse galloping using a dual mass system

This paper offers a theoretical study of energy extraction through transverse galloping using a dual-mass system. To this end, a two-degree-of-freedom model is developed where fluid forces on the galloping body are described resorting to quasi-steady hypothesis; the model is solved approximately by using the Harmonic Balance Method. Three possible configurations of the dual-mass system have been analyzed. Two of them show an improvement in the efficiency of energy extraction with respect to that of the single mass configuration when the mechanical properties of the dual-mass system are appropriately chosen. In addition, the dual-mass system promotes a broadening of the values of the incident flow velocities at which the efficiency is kept high

Investigation of Concurrent Energy Harvesting from Ambient Vibrations and Wind

In recent years, many new concepts for micro-power generation have been introduced to harness wasted energy from the environment and maintain low-power electronics including wireless sensors, data transmitters, controllers, and medical implants. Generally, such systems aim to provide a cheap and compact alternative energy source for applications where battery charging or replacement is expensive, time consuming, and/or cumbersome. Within the vast field of micro-power generation, utilizing the piezoelectric effect to generate an electric potential in response to mechanical stimuli has recently flourished as a major thrust area. Based on the nature of the ambient excitation, piezoelectric energy harvesters are divided into two major categories: the first deals with harvesting energy from ambient vibrations; while the second focuses on harvesting energy from aerodynamic flow fields such as wind or other moving fluids. This Dissertation aims to investigate the potential of integrating both sources of excitation into a single energy harvester. To that end, the Dissertation presents reduced-order models that can be used to capture the nonlinear response of piezoelectric energy harvesters under the combination of external base and aerodynamic excitations; and provides approximate analytical solutions of these models using perturbation theory. The analytical solutions are used, subsequently, to identify the important parameters affecting the response under the combined loading and to develop an understanding of the conditions under which the combined loading can be used to enhance efficacy and performance. As a platform to achieve these goals, the Dissertation considers two energy harvesters; the first consisting of a piezoelectric cantilever beam rigidly attached to a bluff body at the free end to permit galloping-type responses, while the second consists of a piezoelectric cantilever beam augmented with an airfoil at its tip. The airfoil is allowed to plunge and pitch around an elastic axis to enable flutter-type responses. Theoretical and experimental studies are presented with the goal of comparing the performance of a single integrated harvester to two separate devices harvesting energy independently from the two available energy sources. It is demonstrated that, under some clearly identified conditions, using a single piezoelectric harvester for energy harvesting under the combined loading can improve its transduction capability and the overall power density. Even when the wind velocity is below the cut-in wind speed of the harvester, i.e. galloping or flutter speed, using the integrated harvester amplifies the influence of the base excitation which enhances the output power as compared to using one aeroelastic and one vibratory energy harvesters. When the wind speed is above the cut-in wind speed, the performance of the integrated harvester becomes dependent on the excitation\u27s frequency and its magnitude with maximum improvements occurring near resonance and for large base excitation levels

Characterisation of aeroelastic harvester efficiency by measuring transient growth of oscillations

With growing demand for small autonomous electrical devices, such as those in wireless sensor networks, energy harvesting has attracted interest with the promise of low maintenance and sustainable power sources. Galloping energy harvesters utilise the fluid-structure interaction to transform kinetic energy in fluid flow into electrical energy. The performance of galloping energy harvesters depends on the geometry of the tip, with the structure of the flow around the tip defining the nature of fluid-structure interaction and hence the potential efficiency of the device. In this work the curved blade tip geometry is investigated. To experimentally characterise the performance of the harvester, a method utilising the free oscillation transient is developed. The method avoids implementation of a transduction mechanism and hence optimisation of the associated parameters. The developed method is generic and can be applied to other energy generators. The power coefficient of curved blades of different curvatures is measured and the optimal range identified. The maximum coefficient of performance of the curved blade harvester occurs at tip speed ratios from to and reaches , which is 3 to 4 times lower than Savonius turbines, the best performing devices at similar Reynolds numbers. The square prism geometry is used as a comparator and found to have a coefficient of performance 10 times less than the curved blade. Flow visualisations confirm the curved blade to act as an airfoil in the highest performing cases, hence future tip shapes should be developed to promote flow attachment

Design and optimization of piezoelectric MEMS vibration energy harvesters

Low-power electronic applications are normally powered by batteries, which have to deal with stringent lifetime and size constraints. To enhance operational autonomy, energy harvesting from ambient vibration by micro-electromechanical systems (MEMS) has been identified as a promising solution to this universal problem. In this thesis, multiple configurations for MEMS-based piezoelectric energy harvesters are studied. To enhance their performances, automated design and optimization methodologies with minimum human efforts are proposed. Firstly, the analytic equations to estimate resonant frequency and amplitude of the harvested voltage for two different configurations of unimorph MEMS piezoelectric harvesters (i.e., with and without integration of a proof mass) are presented with their accuracy validated by using finite element method (FEM) simulation and prototype measurement. Thanks to their high accuracy, we use these analytic equations as fitness functions of genetic algorithm (GA), an evolutionary computation method for optimization problems by mimicking biological evolution. By leveraging the micro-fabrication process, we demonstrate that the GA can optimize the mechanical geometry of the prototyped harvester effectively and efficiently, whose peak harvested voltage increases from 310 mV to 1900 mV at the reduced resonant frequency from 886 Hz to 425 Hz with the highest normalized voltage density of 163.88 among the alternatives. With an intention of promoting uniform stress distribution along the piezoelectric cantilever and providing larger area for placing proof masses, in this thesis a T-shaped cantilever structure with two degrees-of-freedom (DOF) is proposed. Thanks to this special configuration, a considerable amount of stress/strain can be obtained from the tip part of the structure during the vibration, in addition to the anchor region. An analytic model for computing the frequency response of the proposed structure is derived, and the harvester performance is studied analytically, numerically and experimentally. The conventional MEMS energy harvesters can only generate voltage disadvantageously in a narrow bandwidth at higher frequencies. Therefore, in this thesis we further propose a piezoelectric MEMS harvester with the capability of vibrating in multiple DOF, whose operational bandwidth is enhanced by taking advantage of both multimodal and nonlinear mechanisms. The proposed harvester has a symmetric structure with a doubly-clamped configuration enclosing three proof masses in distinct locations. Thanks to the uniform mass distribution, the energy harvesting efficiency can be considerably enhanced. To determine the optimum geometry for the preferred nonlinear behavior, we have also used optimization methodology based on GA. The prototype measurements demonstrate that our proposed piezoelectric MEMS harvester is able to generate voltage at 227 Hz (the first mode), 261.8 Hz (the second mode), and 286 Hz (the third mode). When the device operates at its second mode frequency, nonlinear behavior can be obtained with extremely small magnitude of base excitation (i.e., 0.2 m/s²). Its normalized power density (NPD) of 595.12 (μW·cm⁻³·m⁻²·s⁴) is found to be superior to any previously reported piezoelectric MEMS harvesters in the literature. In this dissertation, we also propose a piezoelectric MEMS vibration energy harvester with the capability of oscillating at ultralow (i.e., less than 200 Hz) resonant frequency. The mechanical structure of the proposed harvester is comprised of a doubly clamped cantilever with a serpentine pattern associated with several discrete masses. In order to obtain the optimal physical aspects of the harvester and speed up the design process, we have utilized a deep neural network, as an artificial intelligence (AI) method. Firstly, the deep neural network was trained, and then this trained network was integrated with the GA to optimize the harvester geometry to enhance its performance in terms of both resonant frequency and generated voltage. Our numerical results confirm that the accuracy of the network in prediction is above 90%. As a result, by taking advantage of this efficient AI-based performance estimator, the GA is able to reduce the device resonant frequency from 169Hz to 110.5Hz and increase its efficiency on harvested voltage from 2.5V to 3.4V under 0.25g excitation. To improve both durability and energy conversion efficiency of the piezoelectric MEMS harvesters, we further propose a curve-shaped anchoring scheme in this thesis. A doubly clamped curve beam with a mass at its center is considered as an anchor, while a straight beam with proof mass is integrated to the center of this anchor. To assess the fatigue damage, which is actually critical to the micro-sized silicon-based piezoelectric harvesters, we have utilized the Coffin-Manson method and FEM to study the fatigue lifetime of the proposed geometry comprehensively. Our proposed piezoelectric harvester has been fabricated and its capability in harnessing the vibration energy has been examined numerically and experimentally. It is found that the harvested energy can be enlarged by a factor of 2.66, while this improvement is gained by the resonant frequency reduction and failure force magnitude enlargement, in comparison with the conventional geometry of the piezoelectric MEMS harvesters

A bio-mimicking aeroelastic energy harvester

The deployment of small autonomous electrical devices is set to revolutionise many industries with applications from wearable devices to structural monitoring of bridges. However, current developments of small autonomous electrical devices are limited by the restrictions of energy storage, such as finite lifespan and environmental impact. Energy harvesters aim to solve this problem by converting energy from readily available ambient sources. The work presented in this thesis relates to the development of an alternative geometry for an aeroelastic energy harvester, which was initially inspired by the trembling of aspen leaves in barely noticeable winds. The geometry, known as the curved-blade, forms oscillations due to the galloping instability, which can be exploited for energy harvesting. The dynamics of a prototype device are investigated resulting in the discovery of two distinct branches of oscillations separated significantly in amplitude. Flow visualisations demonstrate the flow to become attached in the higher amplitude branch, allowing the curved-blade to act similarly to an aerofoil, rather than the bluff bodies which have most commonly been studied. This regime presents the opportunity of improved harvesting efficiencies. To aid in the further investigation of the device, a method is developed which enables the energy harvesting performance to be characterised from the free oscillation transient. The method avoids the implementation and optimisation of a transduction mechanism and could be applied to many other energy generating devices. The method was applied to curved-blades of varying curvatures and the optimal curvature range found to coincide with the range in which the flow becomes attached, illustrating that the attachment of the flow acts to enhance the performance. Additionally, the cyclic formation and shedding of a leading edge vortex was observed, however further work is required to investigate whether these unsteady flow structures are beneficial to performance