Search for: All records

Abstract Wind energy harvesters are emerging as a viable alternative to standard, large horizontal-axis wind turbines. This study continues a recent investigation on the operational features of a torsional-flutter-based apparatus, proposed by the author to extract wind energy. The apparatus is composed of a non-deformable, flapping blade-airfoil. A nonlinear torsional spring mechanism, either simulated by a Duffing model or a hybrid Duffing–van der Pol model, installed at equally spaced supports, enables limit-cycle, post-critical vibration. To enhance the output power, stochastic resonance principles are invoked through a novel, negative stiffness mechanism that is coupled to the eddy current device for energy conversion. The output power is explored by numerically solving the stochastic differential equation of the model, accounting for incoming flow turbulence. Three main harvester types with variable configuration are examined; the chord length of the blade-airfoil, used for energy harvesting, varies between 0.5 and 1 m; the spanwise-length-to-chord aspect ratio is four. The flapping frequency varies between 0.10 and 0.25 Hz. The study demonstrates that exploitation of negative stiffness mechanism can improve the performance of the harvester. 

This study examines output energy and efficiency of a torsional flutter harvester in gusty winds. The proposed apparatus exploits the torsional flutter of a rigid flapping foil, able to rotate about a pivot axis located in the proximity of the windward side. The apparatus operates at the “meso-scale”; i.e., the apparatus’ projected area is equal to a few square meters. It has unique properties in comparison with most harvesting devices and small wind turbines. The reference geometric chord length of the flapping foil is about one meter. Energy conversion is achieved by an adaptable linkage connected to a permanent magnet that produces eddy currents in a multi-loop winding coil. Operational conditions and the post-critical flutter regime are investigated by numerical simulations. Several configurations are examined to determine the output power and to study the effects of stationary turbulent flows on the energy-conversion efficiency. This paper is a continuation of recent studies. The goal is to examine the operational conditions of the apparatus for a potentially wide range of applications and moderate mean wind speeds. 

Wind energy harvesters are usually designed to operate in the low wind speed range. They rely on smaller swept areas, as a complement to larger horizontal-axis wind turbines. A torsional-flutter-based apparatus is investigated herein to extract wind energy. A nonlinear hybrid restoring toque mechanism, installed at equally spaced supports, is used to produce energy through limit-cycle vibration. Energy conversion and storage from the wind flow are enabled by eddy currents. The apparatus is used during thunderstorm outflows to explore its efficiency in nonideal wind conditions. The thunderstorm flow model accounts for both nonstationary turbulence and slowly varying mean wind speed, replicating thunderstorm's intensification and decay stages. This paper evolves from a recent study to examine stochastic stability. More specifically, the output power is derived as a random process that is found numerically. Various thunderstorm features and variable apparatus configurations are evaluated. Numerical investigations confirm the detrimental effect of nonideal, thunderstorms on harvester performance with, on average, an adverse increment of operational speed (about +30%). Besides nonlinear damping, the “benign” flutter-prone effect is controlled by the square value of the flapping angle. Since flapping amplitudes are moderate at sustained flutter, activation of the apparatus is delayed and exacerbated by the nonstationary outflow and aeroelastic load features. Finally, efficiency is carefully investigated by quantification of output power and “quality factor.” 

The current trend in offshore wind energy is to design and install systems with larger swept areas that yield unprecedented efficiency. Long and slender blades are needed to achieve this objective. As a result of aerodynamic and structural tailoring, slender blades are particularly susceptible to various dynamic instability phenomena during standard operations. One of these phenomena is the bending-torsion flutter that may lead either to structural failure or system breakdown. The research author has been examining blade flutter under the influence of stochastic perturbations, which include both flow turbulence and aeroelastic load variability. A reduced-order Markov model has been used to describe the effects of the various random perturbations. Mean-square stability has been recently explored; results suggest that perturbations may negatively impact the flutter angular speed and increase the risk of failure. In this study the model is employed to investigate moment stability beyond mean squares, observing that dynamic instability involves nonlinear propagation of the perturbations and may exhibit amplitude dependency. Third-order instability is investigated and compared against previous numerical results. The NREL 5MW reference wind turbine blade is used as a benchmark example. 

This paper investigates the energy production of a “meso-scale”, wind-based energy harvester that exploits the torsional aeroelastic instability of a rigid blade-airfoil, elastically supported at equidistant supports. Torsional flutter is a single mode aeroelastic instability phenomenon, in which a diverging dynamic angular rotation of a body occurs. The apparatus relies on a simple mechanism that uses flow-induced pitch motion to extract and convert airflow kinetic energy to electrical energy. The system is composed by a rigid blade-airfoil, connected to a support structure through a non-linear restoring force (torsional spring-like) mechanism that enables the rotation about a reference pivot axis. The proposed technology is designed to be efficient in the range of low and medium wind speeds (10-13 m/s), in which horizontal-axis wind turbines and other harvesters are not efficient. Deterministic pre-flutter, incipient flutter and post-critical vibrations of the apparatus have been already explored in a previous study. This work aims to further investigate the aeroelastic behavior of the “flapping foil” by examining the effect of turbulence, random experimental error and modeling simplifications of the aeroelastic forces. The analysis is conducted at incipient flutter in the frequency domain using classical unsteady force models. Monte Carlo methods are employed to solve for the probability of incipient flutter speed. Several configurations are considered to improve the efficiency of the energy harvester.