Energy Harvesting Literature Research Articles

The energy extraction potential from pitch-heave coupled flutter

An experimental investigation of a pitch-heave fully-passive system undergoing large amplitude coupled flutter induced limit cycle oscillations (LCO) is conducted. The main objective of this paper is to report and document the large aerodynamic efficiencies that can be experimentally obtained from coupled flutter. This is in contrast to a number of investigations in the energy harvesting literature which i. report relatively very low efficiencies for coupled flutter, and ii. emphasise the case of stall flutter and the related phenomenon of dynamic stall. The elastic axis of the set-up is located at 27% chord. The main parameters varied are the airspeed (Reynolds number) and stiffness coefficient in heave, as expressed by the frequency ratio ranging from below to above one. For a frequency ratio ω¯=ωhωθ=1.22, amplitudes up to 80 deg in pitch and 0.6 heave-to-chord ratio are observed at airspeeds close to the flutter speeds. In conjunction with favourable phase differences, these large LCO amplitudes occurring at low speeds lead to aerodynamic efficiency values hovering around 40%. It is also observed that maximum efficiency calculated solely from the heave motion is 50%. This is a consequence of the negative work being done on the pitch by the aerodynamic moment, which can be related back to the inertial coupling and positive phase difference between pitch and heave. For the vast majority of our observations, the aerodynamic moment does negative work, whereas the lift does positive work.

Conjugate unscented transformation-based uncertainty analysis of energy harvesters.

This article presents a probabilistic approach to investigate the effect of parametric uncertainties on the mean power, tip deflection, and tip velocity of linear and nonlinear energy harvesting systems. Recently developed conjugate unscented transformation algorithm is used to compute the statistical moments of the output variables with multidimensional Gaussian uncertainty in parameters. The principle of maximum entropy is used to construct the probability density function of output variables from the knowledge of obtained statistical moments. The probability density functions for mean power were significantly complicated in shape with two and three distinct peaks for the nonlinear monostable and nonlinear bistable harvesters, respectively. Monte-Carlo simulations with N = 8 × 104 samples for monostable harvester and N = 6.5 × 104 samples for bistable harvester were used for validating the probability density functions. It is concluded that conjugate unscented transformation methodology affords a significant computational advantage without compromising accuracy. In addition, using conjugate unscented transformation method, we show that the dependence of mean power on parameters (excitation frequency, excitation amplitude, etc.), when multidimensional uncertainties are present, is decidedly different relative to a purely deterministic trend. The discrepancy in predicted power between the deterministic and uncertain trends for the monostable harvester, for instance, reach a maximum of 100%, 234%, and 110% for base frequency, base acceleration, and magnet gap, respectively. The deterministic trend consistently overestimates the harvested power relative to the uncertain trends. This work, therefore, may have applications in evaluating "worst case scenario" for harvested power. The major advantage of the presented methodology relative to extant techniques in energy harvesting literature is the accurate and computationally effective applicability to multidimensional uncertainty in parameters.

An electromechanical finite element model for new CNTs-reinforced harvesters subjected to harmonic and random base excitations

Conventional piezoelectric energy harvesters are cantilever beams made of homogenous substructure and piezoelectric layers that produce alternating voltage due to their vibrations. Recently, a class of new emerging composite materials, the carbon nanotubes (CNTs)-reinforced composite, has been proposed making use of CNTs as the reinforcements in a functionally graded (FG) pattern. Harvesters made of functionally graded CNTs-reinforced substructure materials have not been studied in the energy harvesting literature, yet. Thus, in this study, functionally graded piezoelectric CNTs-reinforced harvesters as new energy harvesters subjected to harmonic and random vibrations are analyzed. The harvester is assumed to be comprised of piezoelectric and FG-CNTs-reinforced substructure layers. Five types of CNTs distributions through the substructure thickness direction are considered. The generalized Hamilton’s principle for electromechanical materials based on Euler–Bernoulli beam assumption is adopted in the derivation of governing equations. The finite element formulation of the equations is also presented. Time and frequency domain analyses of the finite element equations are performed. The output power from the CNTs-reinforced harvesters subjected to random and harmonic base excitations is calculated. The harvested energy of the CNTs-reinforced harvesters is compared, and it is concluded that the CNTs distribution has a significant effect on the deflection, produced voltage and harvested power.

Design enhancement and non-dimensional analysis of magnetically-levitated nonlinear vibration energy harvesters

Over the past decade, there has been special interest in developing nonlinear energy harvesters capable of operating over a wideband frequency spectrum. Chief among the nonlinear energy harvesting techniques is magnetic levitation–based energy harvesting. Nonetheless, current nonlinear magnetic levitation–based energy harvesting approaches encapsulate design challenges. This work investigates some of the design issues and limitations faced by traditional magnetic levitation–based energy harvesters such as damping schemes and stiffness nonlinearities. Both experiment and model are used to quantify and evaluate damping regimes and stiffness nonlinearities present in magnetic levitation–based energy harvesters. Results show that dry friction, mostly ignored in magnetic levitation–based energy harvesting literature, contributes to the overall energy dissipation. Measured and modeled magnetic forces–displacement curves suggest that stiffness nonlinearities are weak over moderate distances. An enhanced design utilizing a combination of mechanical and magnetic springs is introduced to overcome some of these limitations. A non-dimensional model of the proposed design is developed and used to investigate the enhanced architecture. The unique potential energy profile suggests that the proposed nonlinear energy harvester outperforms the linear version by steepening the displacement response and shifting the resonance frequency, resulting in a larger bandwidth for which power can be harvested.

Three-Degree-of-Freedom Hybrid Piezoelectric-Inductive Aeroelastic Energy Harvester Exploiting a Control Surface

Aeroelastic energy harvesting by transforming wind energy into low-power electricity via low-profile and geometrically scalable devices has received growing attention in the literature of energy harvesting for wireless electronic components. The goal is to harvest flow energy available in high-wind areas toward enabling self-powered systems, such as sensor networks employed in structural health and usage monitoring of aircraft and rotorcraft. Other than bluff-body-based energy harvester configurations using vortex-induced vibrations, the use of an aeroelastic typical section with a proper transduction mechanism is a popular and convenient approach to create instabilities and persistent oscillations for flow energy harvesting. In this work, a hybrid three-degree-of-freedom airfoil-based aeroelastic energy harvester that simultaneously uses piezoelectric transduction and electromagnetic induction is analyzed based on fully coupled electroaeroelastic modeling. This particular configuration exploits a control surface for enhanced design flexibility as compared to its well-explored two-degree-of-freedom counterparts. The two transduction mechanisms are added to the plunge degree of freedom in the presence of two separate electrical loads, and dimensionless electroaeroelastic equations are obtained. The effects of aeroelastic parameters and load resistance values on the overall electroaeroelastic behavior (total power generation and linear flutter speed) are discussed in detail.

Design Optimization of Linear and Non-Linear Cantilevered Energy Harvesters for Broadband Vibrations

In much of the vibration-based energy harvesting literature, resonant energy harvesters are designed around a single base excitation frequency, whereas many applications comprise broadband, time-varying vibrations. Since many naturally occurring vibrations are low frequency, a relatively large mass or beam length is required to resonate at the driving frequencies. This article presents a modeling and optimization procedure for designing vibration energy harvesters for maximizing power generated by vibrations recreated from real-world sources at low frequencies. It is shown that the device coupling coefficient, a significant parameter in determining the energy transduction performance, can be decoupled into terms related to the stiffness and mass distribution of the device, each of which can be optimized independently. To demonstrate the use of this design optimization procedure, measured accelerations are used to provide time-varying, broadband inputs to the energy-harvesting system. Under various size and mass constraints, optimal linear resonant harvesters are presented for human walking and automobile driving scenarios. The frequency response functions are presented alongside time histories of the power harvested using the experimental base acceleration signals. Finally, these results are compared to a non-linear device that utilizes spatially periodic magnetic excitation, a feature that is particularly suited to low-frequency, time-varying excitation.

An electromechanical finite element model for piezoelectric energy harvester plates

Vibration-based energy harvesting has been investigated by several researchers over the last decade. The goal in this research field is to power small electronic components by converting the waste vibration energy available in their environment into electrical energy. Recent literature shows that piezoelectric transduction has received the most attention for vibration-to-electricity conversion. In practice, cantilevered beams and plates with piezoceramic layers are employed as piezoelectric energy harvesters. The existing piezoelectric energy harvester models are beam-type lumped parameter, approximate distributed parameter and analytical distributed parameter solutions. However, aspect ratios of piezoelectric energy harvesters in several cases are plate-like and predicting the power output to general (symmetric and asymmetric) excitations requires a plate-type formulation which has not been covered in the energy harvesting literature. In this paper, an electromechanically coupled finite element (FE) plate model is presented for predicting the electrical power output of piezoelectric energy harvester plates. Generalized Hamilton's principle for electroelastic bodies is reviewed and the FE model is derived based on the Kirchhoff plate assumptions as typical piezoelectric energy harvesters are thin structures. Presence of conductive electrodes is taken into account in the FE model. The predictions of the FE model are verified against the analytical solution for a unimorph cantilever and then against the experimental and analytical results of a bimorph cantilever with a tip mass reported in the literature. Finally, an optimization problem is solved where the aluminum wing spar of an unmanned air vehicle (UAV) is modified to obtain a generator spar by embedding piezoceramics for the maximum electrical power without exceeding a prescribed mass addition limit.

Modeling of Piezoelectric Energy Harvesting from an L-shaped Beam-mass Structure with an Application to UAVs

Cantilevered piezoelectric energy harvesters have been extensively investigated in the literature of energy harvesting. As an alternative to conventional cantilevered beams, this article presents the L-shaped beam-mass structure as a new piezoelectric energy harvester configuration. This structure can be tuned to have the first two natural frequencies relatively close to each other, resulting in the possibility of a broader band energy harvesting system. This article describes the important features of the L-shaped piezoelectric energy harvester configuration and develops a linear distributed parameter model for predicting the electromechanically coupled voltage response and displacement response of the harvester structure. After deriving the coupled distributed parameter model, a case study is presented to investigate the electrical power generation performance of the L-shaped energy harvester. A direct application of the L-shaped piezoelectric energy harvester configuration is proposed for use as landing gears in unmanned air vehicle applications and a case study is presented where the results of the L-shaped — energy harvester — landing gear are favorably compared against the published experimental results of a curved beam configuration used for the same purpose.

Issues in mathematical modeling of piezoelectric energy harvesters

The idea of vibration-to-electric energy conversion for powering small electroniccomponents by using the ambient vibration energy has been investigated by researchersfrom different disciplines in the last decade. Among the possible transduction mechanisms,piezoelectric transduction has received the most attention for converting ambient vibrationsto useful electrical energy. In the last five years, there have been a considerable number ofpublications using various models for the electromechanical behavior of piezoelectricenergy harvester beams. The models used in the literature range from elementarysingle-degree-of-freedom (SDOF) models to approximate distributed parameter models aswell as analytical distributed parameter solution attempts. Because of the diverse nature ofresearchers working in energy harvesting (including electrical, mechanical and materialsengineers), several oversimplified and incorrect physical assumptions have been propagatedin the literature. Issues of the correct formulation for piezoelectric coupling, correctphysical modeling, use of low fidelity models, incorrect base motion modeling, andthe use of static expressions in a fundamentally dynamic problem are discussedand clarified here. These common indiscretions, which have been repeated in theexisting piezoelectric energy harvesting literature, are addressed and clarifiedwith improved models, and examples are provided. This paper aims to providecorrections and necessary clarifications for researchers from different engineeringdisciplines interested in electromechanical modeling of piezoelectric energy harvesters.