Elsevier

Energy Conversion and Management

Volume 152, 15 November 2017, Pages 166-175
Energy Conversion and Management

An out-of-plane rotational energy harvesting system for low frequency environments

https://doi.org/10.1016/j.enconman.2017.09.042Get rights and content

Highlights

  • An alternative to cantilever beam-type systems for energy harvesting is proposed.

  • The device generates energy in a low frequency rotational environment.

  • It comprises two beams, a spring and two heavy masses joined by the spring.

  • By varying the flexibility of one beam, the device increments output DC power.

  • The generated DC power suffices to feed low power wireless transmitters.

Abstract

We present a novel design of a rotational power scavenging system as an alternative to cantilever beams attached to a hub. The device is meant to provide energy to wireless autonomous monitoring systems in low frequency environments such as wind turbines of 30 kW with rotational speeds of between 50 and 150 rpm. These characteristics define the bandwidth of the rotational energy harvesting system (REH) and its physical dimensions. A versatile geometric configuration with two elastic beams and two heavy masses joined by a spring is proposed. A piezoelectric sheet is mounted on the primary beam while the REH is placed on a rotating hub with the gravitational force acting as a periodic source. This kind of double-beam system offers the possibility to modify the vibration characteristics of the harvester for achieving high power density. An analytical framework using the Lagrangian formulation is derived to describe the motion of the system and the voltage output as a function of rotation speed. Several sets of experiments were performed to characterize the system and to validate the assumed hypothesis. In the experimental setup, a wireless data acquisition system based on Arduino technology was implemented to avoid slip-ring mechanisms. The results show very good agreement between the theoretical and experimental tests. Moreover, the output power of a simple harvesting circuit, which serves as an energy storage device, yields values ranging 26–105 μW over the whole frequency range. This allows us to use the proposed device for the designed purpose, taking into account the power requirements of commercially available wireless transmitter systems.

Introduction

The condition monitoring of structures and rotating machines is highly desirable to improve the health, safety and failure predictability of civil and industrial systems. In this sense, data transmission and sensors are essential elements. Between wired and wireless communication systems, wireless sensing is more appropriate for sensory data acquisition in applications involving rotary motion. The design of piezoelectric energy scavenging systems for low frequencies (<100 Hz) and low accelerations (<1 g) are subjects of current research efforts, mainly because the amplitude of environmental vibration sources is below 0.3 g (or 3 m/s2) and 30 Hz [1], [2].

Vibrating energy harvesting (VEH) systems have been developed largely in this sense, considering different mechanical designs, fields of application and transduction mechanisms [3], [4], [5], [6]. Other mechanisms used in energy harvesting include solar [7], thermal [8], electromagnetic [2] and triboelectric devices for exploiting renewable energy sources as well [9]. However, energy harvesting from rotational motion has been much less investigated in the literature even though there is a large subset of civil or industrial scenarios where rotational kinetic energy is available, for example gas and wind turbines, rotating machines, car tires, wheels, shafts, fans, among others. Thus, powering condition-monitoring systems with rotational energy harvesters sounds promising and attainable.

There are several facts that make harvesting energy from rotations distinct from harvesting energy from vibrations. Firstly, if the harvesting device is properly oriented, Earth’s gravity can act as a source of periodic excitation with the frequency of the rotational speed. Secondly, the centrifugal force, which grows with the square of the rotational speed, induces large constant body forces, while the system is rotating. These issues may represent singular benefits to the mechanical designer to improve energy harvest and adapt a mechanism to scavenge energy in low frequency and low acceleration environments. For example, a passive tuning scheme can be implemented with the centrifugal force acting as an axial force that can change the stiffness of a cantilever beam piezoelectric system. This type of approach has been successfully used by Leland and Wright [10] and Eichhorn et al. [11], who applied a compressive axial preload to a vibration energy scavenger to adjust its resonance frequency.

A common practice in the design of VEH devices to maximize power is matching the resonant frequency of the harvester to the excitation frequency. This approach appears to be applicable also to rotational devices. However, a main drawback of this methodology is that the physical dimensions of the devices (especially at low frequencies) are such that it is impossible to fit them given the size and weight limitations of practical applications. In rotating devices such as car-tires, wheels or shafts, the reduced availability of space is crucial and determines the actual design of the harvesting device. In the following paragraphs, a brief survey of the literature on rotational energy harvesting (REH) devices is presented. Manla et al. [12] proposed a system of power generation from rotating vehicle wheels using a piezoelectric generator and a ball bearing. The system is designed to be a tire pressure monitoring system (TPMS). The ball impacts the piezoelectric transducer inside a tube by means of the centrifugal force. Experiments showed that the device can produce 4 mW of electrical power at 800 rpm in a volume of 2 cm3. They also demonstrated that increasing both the tube length and the mass of the ball bearing increases the output power. Also Zheng et al. [13] developed a TPMS based on a novel asymmetric air-spaced piezoelectric cantilever. They reported that the device has several desirable advantages. Firstly, the voltage generated is increased due to the much larger distance between the piezo layers and the neutral plane; and secondly, the asymmetric structure makes the device more robust since the piezo layer is operating in the compression mode. The prototype was capable of generating 47 μW with a 21.6 gr. proof mass at a driving speed of 80 km/h. More recently, Roundy and Tola [14] presented an energy harvester using the dynamics of an offset pendulum along with a nonlinear bistable restoring spring to improve the operational bandwidth of the system. Depending on the speed of the rotating environment, the system can act as a bistable oscillator, a monostable stiffening oscillator, or linear oscillator. Simulation and experimental tests showed that the prototype generator is capable of directly powering an RF transmission system every 60 s or less over a speed range of 10–155 km/h. With the aim to powering a TPMS, Manla et al. [15] developed an off-axis piezoelectric device consisting of pre-stresses piezoelectric beams and a magnet, which is capable of generating energy in a large range of rotational speeds due to the implementation of a levitating magnet that generates nonlinear magnetic over a wide range of centrifugal forces. The prototype occupies a volume of approximately 17.74 cm3 and generates an output power ranging from 0.2 to 3.5 µW when the rotating speed changes from 180 rpm to 330 rpm.

A passive self-tuning REH device was presented by Gu and Livermore [16]. Their harvester comprises a radially oriented beam mounted at a distance r from the axis of rotation. As the rotational speed varies, the corresponding tension due to centrifugal force on the beam adjusts the beam’s resonant frequency so that the harvester always works at or near its resonant frequency. In this way, and under a proper design, the resonant frequency of the harvester can match the frequency of the rotation over a wide frequency range, significantly improving its performance compared with an untuned REH. The same authors also presented an impact-based REH that also works as a self-tuning device with an optimized design [17]. The system comprises two beams: a rigid piezoelectric generating beam and a narrow, flexible driving beam with a tip mass at the end. The mass impacts the generating beam repeatedly under the influence of gravity to drive generation, while the centrifugal force from the rotation modifies the resonant frequency of the flexible driving beam and the frequency response of the harvester. Thus the generation is improved due to the self-tuning mechanism. As a result, the system is capable of generating a power density of 30.8 μW/cm3 over a wide frequency range. Khameneifar et al. [18], [19] proposed and tested a REH system consisting of a rotating piezoelectric cantilever beam with a tip mass mounted on a hub. The gravitational force generates mechanical excitation while the hub is in rotary motion. Expressions for the optimum load resistance and maximum power were obtained and experimentally validated using PDVF and PZT transducers. A maximum power of 6.4 mW at a rotational speed of 1320 rpm been achieved with a 0.25 cm3 PZT device. This is about 44 times higher than when a PVDF film is used. Thus, their proposal could be used as a power generator for a wireless communication system. For low frequency (<1 Hz) and low size (<10 cm), a hybrid electromagnetic-tribolectric nanogenerator consisting of four units of freestanding triboelectric nano generators (TENG) and four electromagnetic generators (EMG) [9] can be used as a self-powered sensor for road traffic monitoring. With an optimization of the geometry of the electromagnetic component and with the combination of TENG and EMG, Askari et al. [9] showed that the proposed device is capable of the power and voltage generation even with very small displacements and low frequencies. Moreover, depending on the triggering frequency, TENG or EMG dominates the power generation considering different mechanical loads.

More recently, Hsu et al. [20] presented a self-frequency tuning REH consisting of a cantilever beam mounted on a rotating axis, similar to the configuration in [18]. The difference is that, in this case, the numerical results obtained by a commercial software package which implements FEM calculations take into account shear deformation, piezoelectric plate, and the centrifugal force in the piezoelectric beam. The beam is oriented in the radial direction, so that the tensile stress induced by the centrifugal force stiffens the beam to passively tune the resonance frequency. The results show a relatively good power generation, verifying the frequency-tuning, but they fail to predict a precise voltage peak generation. Recently, Guan and Liao [21] reported a very promising generation about 80–800 μW on a REH with a cantilever piezoelectric beam at rotating frequencies of 7–13.5 Hz. Their proposal is based on rotating cantilever beams where the mass center of the rotating beam coincides with the center of rotation. This allows to harness the full drive of the gravitational force and reduce the amplitude of the centrifugal force. The generated high output voltage proved to be enough to power low-power wireless communication nodes.

As for nonlinear effects harnessing, nonlinear magnetic interactions have been proposed to increase the bandwidth of VEHs. The pioneering works of Cottone et al. [22] and Erturk et al. [23] presented some of the first evidence in the use of nonlinear phenomena for this purpose. Cottone et al. [22] studied a piezoelectric inverted pendulum where a small magnet was added on top of the pendulum. The dynamics of the inverted pendulum tip is controlled with the introduction of an external magnet determining the mono-stable or bistable characteristics of the interaction. The results showed that, under stochastic excitation for the bistable potential condition, the harvested power increased between 400% and 600% compared to the monostable case. Erturk et al. [23] presented a ferromagnetic cantilever beam with piezoelectric layers attached to the root of the cantilever, and two permanent magnets located symmetrically near the free end and subjected to harmonic base excitation. The system exhibited a strange attractor motion as a mechanical structure. They reported that this generator yields large-amplitude periodic oscillations for excitations over a frequency range. Comparisons were made against a conventional case without magnetic buckling and the superiority of the piezomagnetoelastic structure as a broadband electric generator was demonstrated. It resulted in a 200% increase in the open-circuit voltage amplitude.

In rotating environments, Ramezampour et al. [24] presented an interesting work. They proposed a frequency up-converting REH acting through the magnetic interaction provided by several magnets placed on a rotating shaft, while passing near a PZT cantilever beam fixed to a base. They theoretically and experimentally demonstrated that by applying an appropriate number of rotating magnets the extracted power can be enhanced even more than ten times.

Our proposal presents the following original contributions:

  • An alternative to cantilever beam-type systems for energy harvesting in low frequency rotational environments.

  • A versatile geometrical configuration with two elastic beams and two heavy system joined by a spring with the possibility to modify several parameters of the harvester for achieving high power density.

  • A wireless data acquisition system based on Arduino technology to avoid slip-ring mechanisms.

  • Generation of sufficient DC power to feed low power wireless transmitters.

The paper organization of the paper is as follows. After an introductory section, the REH design is presented and the analytical model describing its dynamics and the electrical output voltage is derived in Sections 2 and 3. Section 4 presents the experimental setup and the experiments conducted to validate the theoretical findings. After a discussion of the advantages of the proposed REH device, some concluding remarks are presented in Section 5.

Section snippets

Physical considerations and proposal

The most common REH devices consists of cantilevered beams attached to the axis of a rotating system oriented in such a way that they have centrifugal forces proportional to the beam length [14], [17]. These conceptual designs may have the disadvantage that these forces are much larger than the gravitational force, and only small transverse vibrations could be generated. An alternative approach to overcome this problem was reported by Guan and Liao [21] who proposed a system to reduce the

Mathematical model

Fig. 1 shows a schematic view of the proposed system, which is connected to a rotating cylindrical hub at a certain distance from the axis. As mentioned above, the axis of rotation is perpendicular to Earth’s axis, with gravity acting as an alternating excitation on the REH system for one cycle of rotation. Therefore, the frequency of the excitation force will correspond to the rotating frequency of the hub and the voltage generated in the piezoelectric patch will be a function of the

Experiments and results

The aim of this section is to validate the previous theoretical findings and show the advantages of the proposed REH system. Experimental tests were performed in the following way: an electric motor with a variable speed controller provided the rotational motion of the system, which was rigidly mounted to the hub as observed in Fig. 4. Instead of using a more common slip-ring system, an Arduino board with a Bluetooth connection was used, which connects the collected data to a PC at a rate of 100

Conclusions

This paper presents an alternative to simple cantilever piezoelectric beams attached to a rotating hub for scavenging energy. The device is developed to power wireless autonomous monitoring systems of wind turbines of 30 kW, whose operating bandwidth is defined by the rotation speeds of the aerogenerator, ranging 50–150 rpm. The REH comprises two flexible beams and two heavy masses joined by a spring, with a piezoelectric sheet mounted on one of the beams. Based on the Lagrangian formulation, an

Acknowledgments

The authors wish to thank CONICET (PIP N°: 11220120100346), UNS, UTN FRBB and ANPCyT under grant PICT 2013-2065 for their financial support.

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