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Harvesting energy from flow using vortex-induced vibration (VIV) piezoelectric transducers has gained significant attention in recent decades due to their advantages, such as simple structure, blade-less layout, and low maintenance costs. However, most existing studies have focused on designing and analyzing a single piezoelectric energy harvester (PEH), without investigating the fluid-structure interaction and coupling of multiple PEH arrays. Here, we conducted an experimental study using a 2 × 2 PEH array to investigate its dynamic response under different wind speeds and spacings. Results show that the output voltage of the PEH array increases as the vertical spacing decreases, and the maximum average output voltage of 20.6 V per PEH is obtained when the minimum vertical spacing, maximum horizontal spacing, and resonance wind speed conditions are met. Compared to a single PEH, the 2 × 2 array arrangement increases the average output voltage by up to 168%. Additionally, the average output power under the resistance of 1 MΩ increases by 629% to 4.3×10−4 W per PEH, and the maximum output power increases by 792% to 5.3×10−4. Experiments indicate that the vortex shedding coupling can induce higher vibration in a well-defined array, which paves a new way for developing bladeless wind farms. [ABSTRACT FROM AUTHOR]

Piezoelectric energy harvester (PEH) arrays are practical solutions to the low and unreliable power output of a single PEH. However, most PEH circuits are developed for addressing the one-channel ac input from a single PEH. In this article, an extensible rectifier-less synchronized switch harvesting on inductor (ReL-SSHI) array interface with the maximal power point tracking (MPPT) function to manage the multiple ac inputs from the PEH arrays is designed. The ac–dc conversion is performed by the ReL-SSHI topology with passive peak switches featured with self-powered characteristic and fast cold-start. Only one shared inductor is employed, greatly reducing the circuit volume and cost. The MPPT module for each channel is implemented by a simple envelope detector and an ultra-low power hysteresis controller and achieves continuous energy extraction by adapting the fractional normal-operation voltage method. A developed discrete array interface able to handle three PEHs effectively executes the MPPT for each channel with the uphill and downhill response speeds of 2.0 and 7.1 V/s, and a maximal MPPT efficiency of 97.1%. This circuit boosts the power output by 485% compared with that by the traditional full-bridge rectifier, around 33% higher than that from existing synchronous electric charge extraction array interfaces. [ABSTRACT FROM AUTHOR]

Piezoelectric energy harvester (PEH) arrays are promising in many application scenarios. However, few interface circuits have been developed to manage the multiple ac inputs from PEHs. This article proposes an extensible parallel synchronized switch harvesting on inductor array interface scheme to realize a multiinput conversion from PEH arrays. We develop a split-inductor-capacitor topology that can extract electrical energy from multiple PEHs. The proposed circuit topology effectively handles the inductor access conflict in the cases of PEHs with close, identical, and opposite vibration phases, insensitive to ac input phase changes. For demonstrating the capability of dealing with multiple ac inputs from PEHs and the enhancement effect on power output, we construct and test a self-powered three-channel array circuit with the passive peak detection function. Excited by sinusoidal vibrations, our circuit effectively avoids the energy return and efficiently achieves the power combination of the three PEHs. Compared with the classic full-bridge circuit, the proposed circuit helps increase the power output by 4.8 times. Also, to show the universality of the circuit, the circuit's operation under various excitations istested and analyzed. [ABSTRACT FROM AUTHOR]

Piezoelectric energy harvesting technology is regarded as a remedy to the short-life battery issue in low-power electronic devices, and has been extensively studied in the past two decades. Although a variety of new structures and mechanisms have been proposed for piezoelectric energy harvesters (PEHs), there is not much progress made on the piezoelectric materials that play a determinant role in PEHs performance. Most energy harvesters are still constructed with lead zirconate titanate (PZT). Recently, the new-generation piezoelectric materials PMN-PT and PZN-PT single crystals are proposed and start to be used in transducers. Their electromechanical coupling factor ( k 33 ) is over 90%, and the piezoelectric coefficient ( d 33 ) can reach three times of that of PZT. This study characterizes the PMN-PT and PZN-PT single crystals used in energy harvesters, compared with the polycrystalline PZT, in terms of charging capacitors. Systematic discussions are presented on the voltage, power and energy responses, as well as the reverse coupling effect on the mechanical response and the self-discharge effect of capacitors. Furthermore, a self-powered synchronized switch harvesting on inductor (SSHI) circuit is constructed and tested with the new single-crystal based energy harvesters. Experiments indicate that PMN-PT and PZN-PT can significantly boost the charging capability of energy harvesters, and that performance can be further enhanced by the SSHI circuit. [ABSTRACT FROM AUTHOR]

Piezoelectric energy harvesting is envisioned as an ideal complement to batteries for long-life operation of wireless or remote electronic devices. Most piezoelectric energy harvesters (PEHs) utilize piezoelectric plates or discs as the energy transducing elements, and they cannot generate enough power for most potential applications. In this study, we explore a new way of using arc-shaped piezoelectric patches as the core transducing elements in energy harvesters to improve their performance. An analytical model is developed via the Castigliano’s theorem to elucidate the low stiffness characteristic of the arc-shaped structure. A finite element analysis is conducted, and the results indicate that high and evenly-distributed stress can be easily induced in the arc-shaped PEHs. Prototypes are fabricated with arc-shaped piezoelectric elements and flat piezoelectric plates, respectively, and tested under the same conditions. The experimental data demonstrate that the arc-shaped PEHs are capable of generating 2.55–4.25 times as much power as the equivalent flat-plate PEHs. This study lays the foundation to enhance energy harvester performance by utilizing arc-shaped piezoelectric structures. [ABSTRACT FROM AUTHOR]

Piezoelectric energy harvesters have great potential for achieving inexhaustible power supply for small-scale electronic devices. However, the insufficient power-generation capability and the narrow working bandwidth of traditional energy harvesters have significantly hindered their adoption. To address these issues, we propose a nonlinear compressive-mode piezoelectric energy harvester. We embedded a multi-stage force amplification mechanism into the energy harvester, which greatly improved its power-generation capability. In this article, we describe how we first established an analytical model to study the force amplification effect. A lumped-parameter model was then built to simulate the strong nonlinear responses of the proposed energy harvester. A prototype was fabricated which demonstrated a superior power output of 30 mW under an excitation of 0.3g ( g = 9 . 8 m/s2). We discuss at the end the effect of geometric parameters that are influential to the performance. The proposed energy harvester is suitable to be used in low-frequency weak-excitation environments for powering wireless sensors. [ABSTRACT FROM AUTHOR]

Vibration energy harvesting has a great potential to achieve self-powered operations for wireless sensors, wearable devices and medical electronics, and thus has attracted much attention in academia and industry. The majority of research into this subject has focused on the piezoelectric effect of synthetic materials, especially the perovskite PZT ceramics. Recently the new-generation piezoelectric materials PMN-PT and PZN-PT single crystals have gained significant interest because of their outstanding piezoelectric properties. They can be used to replace the widely-adopted PZT ceramics for improving energy harvesters’ performance substantially. However, there is little research on comparing PMN-PT and PZN-PT energy harvesters against PZT harvesters. In this paper, we present a systematic comparison between vibration energy harvesters using the PMN-PT, PZN-PT single crystals and those using the PZT ceramics. Key properties of the three materials are summarized and compared. The performance of the PMN-PT and PZN-PT energy harvesters is characterized under different conditions (beam length, resistance, frequency, excitation strength, and backward coupling effect), and is quantitatively compared with the PZT counterpart. Furthermore, the effect of the synchronized switch harvesting on inductor (SSHI) circuit on the three harvesters is discussed. The experimental results indicate that energy harvesters using the PMN-PT and PZN-PT single crystals can significantly outperform those using the PZT ceramics. This study provides a strong base for future research on high-performance energy harvesters using the new PMN-PT and PZN-PT single crystals. [ABSTRACT FROM AUTHOR]

Nonlinear vibration is capable of effectively extending the frequency bandwidth of energy harvesters. Either hardening or softening nonlinearity has been used in various designs to achieve broad-bandwidth energy harvesting. In this paper, we propose a new method to achieve reversible hysteretic responses, i.e., both hardening and softening nonlinear responses, mechanically without additional magnetic interactions. This tunable nonlinearity endows energy harvesters with a great adaptability to environment. The proposed energy harvester is composed of a flexural center and two mass blocks, supported by a pair of elastic rods that are fixed on a vibration base. Different nonlinear responses are invoked by tilting the fixed–fixed elastic rods at different angles. A lumped-parameter model is developed to simulate the nonlinear electromechanical coupling system, and that is analytically solved by virtue of the high-order perturbation technique. The dynamic responses under different frequencies and accelerations are analytically characterized and compared well with the experimental data measured from a fabricated prototype. Furthermore, a nonlinear conditioning circuit (self-powered series synchronized switch harvesting on inductor) is constructed and tested with the proposed nonlinear energy harvester, with which the performance is enhanced about 200% for both resistive loads and capacitive loads. [ABSTRACT FROM PUBLISHER]

We propose a new concept for harvesting vibration energy from multiple directions. Our approach effectively exploits the nonlinear vibration of a doubly clamped elastic rod, converting and amplifying excitations to compressive loads in piezoelectric materials. The proposed multidirectional compressive-mode piezoelectric energy harvester (MC-PEH) is capable of isotropically harnessing vibration energy from any angle in a plane. Meanwhile, the MC-PEH demonstrates a high-voltage output and a wide working bandwidth, along with the distinct softening nonlinear phenomena. Theoretical analysis and experimental testing are performed and exhibit good agreement over a range of excitation frequencies. This study enhances the practicability and adaptability of using energy harvesters in a complex environment. [ABSTRACT FROM PUBLISHER]

Harnessing ambient vibration energy using piezoelectric materials is a promising alternative solution to batteries to inexhaustibly power small-scale mobile devices. This paper presents an innovative energy harvester, named high-efficiency compressive-mode piezoelectric energy harvester (HC-PEH). It consists of a pair of elastic beams and a flex-compressive center where the piezoelectric element bears a dynamic compressive load. The compressive mode is preferred than the commonly-used bending mode and tensile mode due to the superior compressive strength of piezoceramics. To cater to the characteristic of piezoceramics: large ultimate stress, but extremely small ultimate strain, a multi-stage force amplification mechanism is developed in the design. The HC-PEH shows superior capability of high power output and favorable nonlinear phenomena at low frequency range. Under an acceleration of 0.5 g (g = 9.8 m/s 2 ), a maximum power of 19 mW is produced at 21 Hz, which is about one order of magnitude higher than the power output of energy harvesters previously reported. The capability of high power output of the HC-PEH demonstrates the practicability of using energy harvesters to power most of wireless sensors. [ABSTRACT FROM AUTHOR]

Previous droplet-based electricity generator has made breakthroughs in efficient water energy harvesting through taking advantage of the contact electrification, however, the kinetic energy of water remains to be tapped into. In this work, we develop a leaf-mimic rain energy harvester (REH) that allows for the collection of electrostatic and kinetic energy simultaneously based on the synergy of the liquid-solid contact electrification and the piezoelectric effect. Impinged by a water droplet, the REH generates a boosted transferred charge value (101 nC) and high output power density (82.66 W m−2). We also show that such performances enable the construction of self-charged wireless sensor systems with continuous operational capability. The REH is also low-cost and facile for fabrication, paving a new way towards high-efficiency power generation from raindrops as well as other water sources. [Display omitted] • A mechanism of harvesting energy from both the liquid-solid contact and kinetic motion inspired by the leaf-rain interaction. • A new explicit theoretical model that links key parameters on the liquid-solid contact electrification. • Continuous operation of a wireless sensor system powered by the proposed droplet energy harvesters. [ABSTRACT FROM AUTHOR]

Wrist-worn smart devices such as smartwatches and wristbands are rapidly penetrating our daily life for health, security and fashion purposes. However, the battery-powered wearables have a limited lifetime and need to be frequently recharged, which degrades user experience and further poses environmental problems. Inspired by the structure of the woodpecker's head, this paper reports a bionic two-layer band piezoelectric energy harvester for self-powered smartwatches and wristbands. An inner band and an outer band of the harvester mimic two branches of the hyoid bone of the woodpecker. The energy harvesting unit employs a piezoelectric array structure. A finite element analysis is conducted to identify the optimal positions of piezoelectric patches in the array and an improved rectifier circuit is developed to manage the multiple AC inputs. We design two operation modes, i.e., movement mode and impact mode, to accommodate energy harvesters in different human activities. A maximum power output of 15.41 mW is achieved in experiments when tapping the prototype, corresponding to an equivalent power density of 1049.01 μW/cm3, around five times higher than previous wrist-worn harvesters. The design demonstrates its capability of continuously powering a watch and a screen during a normal walking process. The concept of shifting from the watch body to watch bands provides the design of energy harvesters a larger space and higher user-friendliness, which, fused with the piezoelectric array method, shows the potential to merge the gap of the power generation of energy harvesters and the power consumption of wrist wearables. [Display omitted] • Woodpecker-inspired two-layer band design for wrist-worn wearables. • A piezoelectric array structure as an energy harvesting unit. • Two working modes including movement mode and impact mode. • The maximum power output of 12.25 mW in the movement motion and 15.41 mW in the impact mode. • Able to continuously power a watch or a screen during normal walking. [ABSTRACT FROM AUTHOR]

The performance of vibration-based energy harvesters deteriorates when used in rotational environments with an offset distance. To address this issue, we propose a method of using magnets to improve the performance of the high-efficiency compressive-mode piezoelectric energy harvester (HC-PEH) with the offset configuration in rotational environments. The performance of the magnetically coupled rotational HC-PEH system with offset distances is experimentally studied. The results show the proposed method is feasible and significantly improves the performance (peak-peak voltage) by 254.2%. To attain comprehensive insight into the electromechanical behaviors of the system, we develop a theoretical model with consideration of nonlinear stiffness, nonlinear damping, and nonlinear piezoelectricity. The governing equations of the system are obtained by using Hamilton's principle and their approximate analytical solution is derived via the harmonic balance method. The simulation results closely render the experimental data and are used to characterize the system. Based on the developed theoretical model, a parametric study is performed and the results indicate that the best performance improvement can be obtained via tuning the parameters related to the magnetic force, the centrifugal force, and the initial deformation of the elastic beams. This work helps overcome a major obstacle for the application of rotational energy harvesting. • Verification on adverse effects of the offset distance in rotational environments. • A brand-new solution by using magnetic repulsion and its experimental verification. • The peak-peak voltage of the energy harvester shows a maximum increase by 254.2%. • Model of the magnetically coupled rotational HC-PEH system with offset distances. • Discussions on the dynamical characteristics of the system. [ABSTRACT FROM AUTHOR]

Sensors that provide critical information about jet engine performance are widely installed on static components but are rarely found on rotors because of their inaccessibility and extremely high rotation speeds. We present a new monitoring method, integrating energy harvesting technology with wireless sensors to achieve real-time self-powered engine monitoring. Energy harvesters, used to generate power from ambient vibration, are sustainable alternatives to batteries for achieving self-sustained long-term operation of electronic devices. By utilising structural nonlinearity, force amplification mechanism, and the piezoelectric effect, we show a 22.52-g energy harvester capable of high power output (78.87 mW), broad working bandwidth (22.5 Hz), and strong reliability (2100 RPM). Our approach breaks limitations from wired connections that are weighty and vulnerable to failures. We theoretically and experimentally analyse the nonlinear responses and demonstrate the harvester by constantly lighting 112 LEDs and a self-powered wireless sensor system in a jet engine. This work paves a new way for developing future monitoring systems for advanced jet engines and other rotating machinery applications. The self-powered wireless sensor system consists of an energy harvester as the power supply and a wireless sensor node as the communication device. When the jet engine operates, the energy harvester that installs on the rotational component generates power for the wireless sensor node and then the real-time monitoring information of the engine components will be sent wirelessly to a local area network (LAN) in the aircraft for working condition analysis and failure prediction. The energy harvester of the monitoring system exhibits great compatibility in the rotational environment of jet engines and excellent power generation capability (max. to 78.87 mW) and working bandwidth (22.5 Hz in 0–2100 RPM). A combination of such characteristics guarantees the realization of the self-powered and wireless operation of sensor systems in jet engines. Image 1 • A brand-new sensor architecture for real-time and self-powered jet engine monitoring. • A maximum output power 78.87 mW and efficiency 36.43%, capable of powering 112 LEDs simultaneously. • A demonstration of a self-powered wireless sensor system in jet engine components for the first time. • A comprehensive study on the compressive-mode piezoelectric energy harvester used in the rotational environment. • A comparative study the compressive-mode and bending-mode PEHs under vibration and rotational excitations. [ABSTRACT FROM AUTHOR]

• Multiple nonlinear energy harvesters are developed based on compliant orthoplanar springs and piezoelectric materials. • Experiments with both harmonic and random excitations are performed. The proposed COPS harvesters could show an extended operational bandwidth in both conditions. • The study quantitatively demonstrates the effectiveness of interaction of multiple nonlinear vibration modes on harvester performance enhancement, and which effect can be tuned by the addition of multiple masses and spring legs. • The quad-leg design shows a broad bandwidth of 35 Hz while maintaining a voltage response of 4 V. This paper compares the performance of three nonlinear compliant orthoplanar springs, namely, bi-leg, quad-leg and pent-leg designs to study the vibration mode interaction effect for widening the operational bandwidth of piezoelectric vibration energy harvester. All the designs have three or more vibration modes below 150 Hz with the addition of multiple masses. Finite element analysis results of spring designs and vibration mode shapes are presented and compared with experiments. The prototypes were manufactured and experimentally tested to analyze their dynamic voltage-frequency responses and operational bandwidths under sinusoidal and band-limited excitations. The experimental results demonstrate that the operational bandwidth is significantly broadened by the geometric nonlinearity of the structure. The addition of multiple masses brings the vibration modes of the system closer, and thus further helps to extend the operational bandwidth. In addition, we introduce multiple piezoelectric plates into harvesters to enable energy harvesting in all vibration modes. The quad-leg design shows the largest frequency bandwidth of 35 Hz, maintaining a peak-to-peak voltage of 4 V under the forward frequency-sweep excitations. Under the reverse frequency-sweep excitations, the voltage response and the operational bandwidth are comparable to those of the forward-sweep ones as a result of continuous overlapping of linear and nonlinear voltage peaks. Random excitation experiments demonstrate the effectiveness of the proposed mutli-leg structure in harvesting energy from wideband environmental vibrations. [ABSTRACT FROM AUTHOR]

Highlights • Hybridization of piezoelectric and triboelectric energy generators. • Bifurcation induced superharmonic resonances for low-frequency energy harvesting. • High power outputs with force amplification effect by a truss mechanism and impact. • Capability of harnessing energy from a broad frequency range. • Frequency up conversion approach for higher output power. Abstract In this paper, we report a hybrid generator to harness energy from low-frequency ambient vibrations. The generator is designed with a piezoelectric energy harvester (PEH) patch, a triboelectric nanogenerator (TENG) patch, a spring-mass system and an amplitude limiter. The spring-mass system receives energy from excitations and applies forces to the piezoelectric element and the triboelectric layers. The unique amplitude limiter is deliberately introduced into the system, not only achieving the desired frequency up-conversion effect, but also boosting the voltage responses significantly. The limiter also causes hardening nonlinearity and dynamic bifurcation triggering superharmonic resonance so that the generator resonates at a frequency of about 3 Hz. Furthermore, the proposed PEH adopts the strong compressive operation mode, and employs a truss mechanism to effectively amplify the impact forces. In experiments, open-circuit voltages are 58.4 V from PEH and 60 V from TENG under an excitation of 1.0 g at resonance. The hybridized generator is capable of achieving a maximum power of 19.6 mW from the two sources with matched impedances. The working bandwidths of the PEH and the TENG reach up to 5.39 Hz and 7.25 Hz, respectively, out of our targeted frequency domain [2.5 Hz, 10 Hz] due to the multiple resonances and nonlinearity. In the applications to charge capacitors, the high saturation voltage and relatively short charging time validate the effectiveness of our power management technique. Furthermore, the generator proves to be able to effectively scavenge energy from human body motions and charge a capacitor of 4.7 μF to 7.6 V in around 50 s, which indicates a great potential of practical applications in wearable devices. [ABSTRACT FROM AUTHOR]

• The RMS voltage response of the energy harvester is increased by up to 1225%;. • We present a new quasi-zero stiffness mechanism for energy harvesters;. • We derive a stable solution to the adverse effect of the offset distance;. • The nonlinear dynamics of the proposed systems are analyzed comprehensively. The self-powered sensing technology based on gravity-rotation-excited vibration energy harvesting (GRE-VEH) enables information acquisition directly from rotatory machines online. However, the centrifugal effect due to the distance between the mass centroid of the energy harvester and the rotating axis (i.e., offset distance) seriously limits its broad engineering applications. To address this issue, we propose methods of using a tensile force provided by pre-deformed springs to counter the constant centrifugal force around the target rotation speed for improving the performance of the energy harvester affected by offset configurations. The GRE-VEH system with a linear spring is first studied by experiments and simulations, where a distributed-parameter model is developed and experimentally validated. The results well validate the proposed mechanism, where the performance of the harvester prototype significantly increases (by up to 1225%). A parametric study is also performed theoretically. The results indicate that the best performance improvement can be obtained by tuning the parameters related to the tensile length, the stiffness, and the damping of the spring. To enable the proposed mechanism in large centrifugal force cases, a pair of negative springs is introduced to the system for constructing a quasi-zero stiffness (QZS) structure together with the linear spring. The new GRE-VEH system is theoretically studied by an updated model, where the system's main characteristics are revealed. The simulation results well indicate the feasibility of the method using the QZS mechanism. This work paves a new way of solving the offset distance problem for the application of the GRE-VEH and also provides new insight into the application of the QZS mechanism. [Display omitted] [ABSTRACT FROM AUTHOR]

• T-phage-inspired multilegged design for microrobot. • A rotationally asymmetric structure. • Multiple working modes: forward motion, backward motion, steering, climbing, and swimming. • The maximum velocity of 120 mm/s under a driving voltage of 50 V p-p and the maximum carrying weight of 10.6 gs, 6 times more than the weight of the microrobot. • Miniaturization capability and ability to work in various environments, i.e., a narrow tube and water. T-phages use contractile tails to recognize host bacterial and move on the surface of bacterial. This virus demonstrates remarkable locomotive capabilities in recognition and infection progress. The field of microrobotics focuses on achieving these functions in mobile robotic systems of centimeter size. However, owing to the structure size and weight, it has been a challenge to satisfy high robustness, fast motion, adequate carrying capacity, low driving voltage, and versatile environmental adaptability in one robot. In addition, functional microrobots require multimodal locomotive strategies that reconcile the constraints imposed by different applications. Inspired by the unique structure of T-phage in the micro-world, we develop a T-phage Mimic MicroRobot (TMMR), which is composed of a triangular prism core with piezoelectric elements and legs attached on bases, mimicking the core and the tail fibers of the T-phage, respectively. The triangular prism structure avoids rollover while moving, indicating the robustness of the design. TMMR of 2.5 × 2.5 × 2.6 mm3 weighs 1.74 gs and can be easily further miniaturized. It is capable of moving forward, backward, and steering at a low voltage of 25 V. Working mechanisms for each type of motion are analyzed using the finite element method. Results show that TMMR reaches 120 mm/s, 4.8 body lengths per second (BL/s), and carry an object weighing 10.6 gs, 6 times more than its weight. By sealing the body and adding passive flaps, TMMR achieves a "swimming" motion with a velocity of 16 mm/s. TMMR shows potential for versatile applications in a narrow and constrained environment. [Display omitted] [ABSTRACT FROM AUTHOR]

• Distributed-parameter model for a doubly-clamped energy harvester affected by the misalignment-induced torsions in vibration environments. • Experimental validation for the developed model and the effects of the bending-torsional coupling vibration on the harvester. • Characterizations of the bending-torsional coupling vibration for the doubly-clamped beam structures. • Investigations on the effects of the key parameters on the torsion-induced jump-down phenomenon. This article presents a theoretical and experimental study on the unique bending-torsional coupling vibration of a doubly-clamped nonlinear piezoelectric energy harvester due to the misalignment of the centroid and the torsional axis in vibration environments. Based on the Euler-Bernoulli beam theory, the extended Hamilton's principle, and the Galerkin discretization, a distributed-parameter model considering the misalignment is developed, and the corresponding governing equations are formulated. The developed model is validated against experimental data and Finite Element Analysis, where good agreements are achieved. Effects of the key parameters on the system's responses, and the characteristics and the causes of the bending-torsional coupling vibration are studied numerically based on the developed model. The results show that the torsion-induced chaotic motion can activate the jump-down phenomenon, largely reducing the electrical output of the harvester. Additionally, the occurrence of the torsion-induced jump-down phenomenon is sensitive to the system's initial state in some cases and can be reduced or even avoided by effectively tuning parameters that have slight effects on the bending resonance, such as the preloaded axial force and the distance between the torsional axis and the centroid. [ABSTRACT FROM AUTHOR]

• A comprehensive electrode coverage design principle is developed. • Equivalent stiffness and damping are introduced to evaluate coupling coefficients. • Strongly coupled harvesters are compatible with a wide range of electrode coverage. Cantilevers are widely used for piezoelectric vibration energy harvesters (PEHs), thanks to the low resonance frequency and large mechanical strain under weak excitations. Electrode coverage optimization is essential for maximizing output power. In this study, bimorph cantilevers with segmented electrodes are developed to investigate the correlation between effective electrode coverage (EEC) and the harvesting behavior of piezoelectric cantilevers. A nonlinear electromechanical model is established using Hamilton's principle, numerically solved with the method of harmonic balance, and further validated by experimental measurements. Equivalent linear stiffness and equivalent mechanical damping ratio are firstly introduced for evaluating the equivalent effective electromechanical coupling coefficients and identifying strong and weak coupling harvesters. Both harvesters fabricated with and without proof mass behave mainly as strongly coupled systems and can obtain the maximum possible power at a very low EEC. Variation of EEC in higher range acquires almost constant maximum power on dramatically changing resistance. A comprehensive EEC design principle for acquiring the maximum output power is obtained for cantilevered PEHs. An optimal EEC obtaining the maximum effective electromechanical coupling coefficient is needed for acquiring the unique maximum power pick for weakly coupled harvesters, whereas a wide range of EEC can be used to maximize output power for harvesters those mainly behave as strongly coupled systems. [ABSTRACT FROM AUTHOR]

• Segmented arc-shaped piezoelectric energy harvester is proposed. • A new distributed-parameter electromechanical coupling model is developed. • The analytical solutions are validated with FEM and experiments. • Energy harvester characteristics with optimal parameters to generate maximum output voltage of 50 V are fully investigated. Advances in energy harvesting techniques, along with low-power electronic devices open up the possibility of developing self-powered engineering systems. Most existing energy harvesters are constructed with beams such as cantilevers, fully-clamped beams and buckled beams. Here we present an attempt to add a new design dimension to the piezoelectric energy harvesters (PEHs) by exploiting a curved-beam structure. This design will be shown to dramatically improve the contemporary harvester performance. To establish an analytical base for curved PEHs, we take a variable-curvature unimorph as the general modelling object and derive its governing equations based on the Timoshenko beam theory. Using the mode expansion method with the consideration of boundary conditions and continuous conditions, we present analytical solutions of the electrical and mechanical response of the PEH. The model developed is verified via the finite element method and it can accurately estimate the modal response of the curved PEH. Prototypes are fabricated and tested to validate the analytical model and solutions. Furthermore, we comprehensively analyzed the key parameter effects including the length of the straight beam, thickness ratio and Young's modulus on the performance of the PEH. The proposed analytical model provides an analytical guidance for composite arc-shaped PEHs with variable curvature and it also helps in the design and optimization of new PEHs. [ABSTRACT FROM AUTHOR]

Researchers have recently focused on improving the efficiency of piezoelectric energy harvesters by redesigning their substrate beams. Two approaches have been explored, including changing the shape of the cross-section area, i.e., width variation, and changing the thickness of cantilever beams. The width changing approach suffers from the deduction of output power from the piezoelectric elements of the trimmed-off regions, while performance deterioration does not occur in the thickness-variable approach. The benefit of thickness-variable beams on energy harvesting has been analyzed theoretically, but its detailed experimental study is still absent. In this study, we for the first time fabricate a thickness-variable harvester based on the Garolite FR-4 epoxy laminate, and validate the superiority of the variable thickness beam on the harvester performance enhancement. A theoretical model is developed based on the Euler-Bernoulli beam theory and numerically solved via the Rayleigh-Ritz method. We compare the thickness-variable composite beam configuration with a conventional uniform-thickness counterpart and unveil the beneficial effect of evenly-distributed strain on performance enhancement. Experiments indicate that the thickness-variable method increases on the energy harvester efficiency by 78%. [ABSTRACT FROM AUTHOR]

Ocean, as a natural system containing a tremendous amount of energy, can be used for either the large-scale power grid network or small-scale distributed off-grid electronic devices via the energy harvesting technology. As one low-cost and effective way to capture flow energy, the vortex induced vibration (VIV) energy harvesting is attracting more and more attention. However, the direction of water flow in a natural water environment is changeable while most existing VIV harvesters are limited by their directional sensitivity. These harvesters only respond to flow excitations from one fixed direction and become insufficient once the flow direction varies. In this paper, we take the lead to address the unidirectional sensitivity issue and propose a novel direction-adaptive energy harvester. We establish theoretical models to analyze the Kármán vortex street, the torque excitation, and the vortex-induced pressure oscillations. Prototypes are fabricated and tested to characterize the direction-adaptive capability of the proposed design under different flow conditions. The experiments demonstrate that the energy harvesting angle span is extended by the guide wing from 40° to 360° under a wide flow velocity range. The guide-wing method endows harvesters with an all-around multidirectional sensitivity, and thus will accelerate energy harvesters' applications in oceans. • Introduce the directional self-adaptability to VIV energy harvesters. • Develop the first all-around 360° direction-adaptive flow energy harvester. • Design and analyze a unique guide wing theoretically and experimentally. • Can be adopted by other harvesters to improve their environmental adaptability. [ABSTRACT FROM AUTHOR]