fluttering energy harvester for autonomous powering flehap aeroelastic characterisation and preliminary performance evaluation

Abstract Significant efforts are being devoted in order to develop efficient and reliable energy harvesters based on interactions between structures and environmental fluid flows such as wind or marine currents. In this framework, a fully-passive energy harvester of centimetric size employing an elastically bounded wing has been developed. The system exploits the coupled-mode flutter, leading in certain conditions to finite amplitude and self-sustained oscillations. Electrical output power levels up to 15[mW] have been reached by an experimental prototype within a wind range between 2 and 5 [m/s] by means of electromagnetic coupling as the conversion strategy. Focusing on the aeroelastic point of view, it is crucial to investigate how the kinematics (i.e. flapping amplitude and frequency, phase between the pitch and plunge motion DoFs) varies with the main parameters (e.g. wind velocity and wing geometry), in order to identify the optimal conditions for potential harvesting. With this goal in mind, we present and discuss the results for a representative configuration of the device (first without the extraction mechanism), exploring the behavior within the design wind range, combining wind-tunnel experiments, three-dimensional CFD simulations and the development of a quasi-steady phenomenological model. We find that both the amplitude and the frequency of the flapping motion are maximised for a certain wind velocity. Moreover, the phase between pitch and plunge changes abruptly when close to this condition. Hence, we estimate the mechanical power that the wing is able to collect and the Betz efficiency, e.g. the ratio between the latter and the power available in the flow. The mathematical model is then enriched by additional terms mimicking an electrical resistive circuit and predictions are made regarding the extracted power and global efficiency of the system, showing the presence of optimal conditions for which these quantities are maximised. Finally, we outline future challenges in the harvester development towards a realistic deployment

Uniaxial magnetic anisotropy tuned by nanoscale ripple formation: ion-sculpting of Co/Cu(001) thin films

We have investigated the growth of surface nanostructures on a Co/Cu(001) film and the growth of Co films on a nanostructured Cu(001) substrate as well as the effect of nanoscale pattern formation on the film magnetic properties. Here we demonstrate by scanning tunneling microscopy measurements and magneto-optic Kerr effect hysteresis curves that low-temperature grazing-incidence ion sputtering can be used to induce the formation of nanoscale ripples which reduce the four-fold symmetry of the Co film to two-fold, thus generating a strong in-plane uniaxial magnetic anisotropy. The nanostructures and the associated uniaxial magnetic anisotropy were found to be stable up to room temperature

Monitoring the Air Quality in an HVAC System via an Energy Harvesting Device

The energy consumption of a heating, ventilation, and air conditioning (HVAC) system represents a large amount of the total for a commercial or civic building. In order to optimize the system performance and to increase the comfort of people living or working in a building, it is necessary to monitor the relevant parameters of the circulating air flux. To this end, an array of sensors (i.e., temperature, humidity, and CO2 percentage sensors) is usually deployed along the aeraulic ducts and/or in various rooms. Generally, these sensors are powered by wires or batteries, but both methods have some drawbacks. In this paper, a possible solution to these drawbacks is proposed. It presents a wireless sensor node powered by an Energy Harvesting (EH) device acted on by the air flux itself. The collected data are transmitted to a central unit via a LoRa radio channel. The EH device can be placed in air ducts or close to air outlets

Temperature and time evolution of ripple structure induced by ion sputtering on Cu(110)

The surface morphology generated by ion sputtering on a Cu(110) crystal has been investigated by Scanning Tunneling Microscopy (STM). A ripple structure is observed for all the considered values of the incident ion beam angle theta (0 degrees < theta < 70 degrees). In particular, normal:sputtering produces a well defined ripple structure whose wave vector rotates from (001) to (1 (1) over bar 0) by increasing the substrate temperature. Moreover, for theta = 45 degrees the ripple wavelength lambda increases in time following a scaling law lambda proportional to t(z), with z = 0.26 +/- 0.02. These results are described by a continuum equation which includes, in addition to the surface curvature dependent erosion terms, a diffusion term that takes into account both the surface anisotropy and the effect of an Ehrlich-Schwoebel barrier

Fluttering conditions of an energy harvester for autonomous powering

Flapping states of an energy harvesting device have been investigated by means of experiments, numerical simulations and a phenomenological model. The main aim is to predict the geometrical/physical properties of the system allowing sustained flapping limit cycles to emerge. These latter regimes are interesting when the system is used to harvest energy from flows. The main argument to identify flapping states is based on a simple resonance condition between the characteristic (elastic) time of the system and the flow time-scale. Similar arguments have been successful in other fields of fluid dynamics and fluid-structure interactions including turbulent flows of dilute polymer solutions and interactions between the wake originated by bluff bodies and elastic structures. The predictions of the geometrical/physical properties associated to critical conditions (i.e. those separating stable stages from flapping regimes) have been compared against the results of experiments, numerical simulations and a phenomenological model based on a set of ordinary differential equations. Results clearly confirm the expectations from the resonance condition. Discussions on how to extend our analysis in situations where the extraction stage is taken into account are also provided: this latter is indeed expected to influence the flapping stage and thus the critical conditions for flapping