Optimal Load Resistance Research Articles

Yoyo-ball inspired triboelectric nanogenerators for harvesting biomechanical energy

The yoyo ball is a type of entertainment and fitness toy, that is popular worldwide. Herein, we propose a yoyo-ball-inspired triboelectric nanogenerator (YB-TENG) for biomechanical energy harvesting. The yoyo ball is designed as a rotor, and a string is wound on its inner diameter to form a novel two-in-one mechanism, which converts the periodic hand lifting energy into high-frequency rotation energy of the rotor. Two coaxial stators are installed on both sides of the rotor, and the friction layer and interdigital electrodes are pasted on the sides of the rotor and stator, respectively, to realize a free-standing mode TENG. Based on the fabricated YB-TENG prototype and dynamic model, the relationship between the output voltage waveform, frequency, and rotor speed is examined using time frequency analysis and numerical simulation. After the optimal load resistance is obtained, the effects of the friction layer material and structural parameters (including the number of electrode section pairs, gap length between the rotor and stator, and string length) on the YB-TENG output power are analyzed. By charging the load capacitors and effectively powering a series of LED lamps and micro-power devices, the YB-TENG exhibits potential as a portable energy source.

Bow-type bistable triboelectric nanogenerator for harvesting energy from low-frequency vibration

Triboelectric nanogenerators (TENGs) are emerging, leading-edge, and promising devices for converting ambient vibration energy into electrical energy and power for low-power sensors. However, it is difficult to harvest energy from low-frequency ambient vibrations using conventional TENGs. To solve this issue, in this study, we propose a bow-type TENG with a bistable stiffness feature, which enables the conversion of low-frequency vibration energy into electrical energy with high efficiency. The dynamic model of the bow-type TENG was established based on extended Hamilton’s principle and numerically solved to obtain the dynamic responses. Subsequently, a prototype of the bow-type TENG was fabricated and tested to verify the design concept. Moreover, the dependencies of the energy-harvesting performance on the structural parameters were experimentally investigated. The average power had a maximum value of 0.64 mW at 5 Hz in the case of optimal load resistance. The bow-type TENG could power a number of light-emitting diodes (LEDs) to display the word “HNU” under low-frequency excitation. Hence, the bow-type TENG can substantially extend the function of harvesting vibration energy from high to low frequencies.

Modeling and experimental investigation of magnetically coupling bending-torsion piezoelectric energy harvester based on vortex-induced vibration

This paper presents a magnetically coupling bending-torsion piezoelectric energy harvester based on vortex-induced vibration from low-speed wind. The theoretical model of the energy harvester was formulated and validated by wind tunnel experiments. Numerical and experimental results showed that the power output and bandwidth of the proposed harvester are improved about 180% and 230% respectively compared with the nonmagnetic coupling harvester. Furthermore, the effects of cylinder, piezoelectric layer, load resistance, and magnetic nonlinear parameters on the harvester were investigated based on the distributed parameter model. The results showed that the length of cylinder hardly affect output power, but the diameter of cylinder presented complicated influences. The width of piezoelectric beam was negatively correlated with the torsion angle. With increasing the length of piezoelectric layer, an optimal wind velocity and load resistance can be obtained for the maximum output power. With decreasing of the distance between two magnets, the resonant bandwidth, the optimal power output, and torsion angle can be enhanced, respectively. Besides, the magnetic potential energy increased owing to the magnetically coupling, which led to the improvement of onset speed for the energy harvester. This study provides a guideline on improving the performance of bending-torsion vibration piezoelectric energy harvester.

Vision Analysis of the Influence of Piezoelectric Energy Harvesting on Vibration Damping of a Cantilever Beam

A cantilever beam, manufactured from a steel-carrying substrate and two patches of Macro Fiber Composite of P2 type, was a subject of laboratory research. MFC patches were glued on both sides of the carrying substrate and were parallelly connected. An experimental determination of an optimal resistance for both energy harvesting and vibration passive damping of the cantilever beam was the purpose of the conducted laboratory research. The research contained 10 experiments in which courses of the energy-harvesting process and resistive passive damping of vibration were estimated. Energy harvesting was estimated by measurements of the generated current for the given load-resistance values. Resistive passive damping of vibration was assessed by using a vision method that enabled the displacements’ measurements of 10 selected points in the beam structure for the given shunt-resistance values. Values of both load resistance and shunt resistance were chosen on the basis of analytically calculated optimal load resistance and optimal shunt resistance. On the basis of the conducted experiments, the resistance range for which both the energy-harvesting process and the vibration-damping process are most effective was determined.

Enhanced piezoelectric wind-induced vibration energy harvester via the interplay between cylindrical shell and diamond-shaped baffle

Energy harvesting from wind-induced vibrations is considered to be a promising solution for the power requirements of wireless sensor nodes. This paper proposes an enhanced piezoelectric wind-induced vibration energy harvester (EPWEH) via the interplay between the cylindrical shell and diamond-shaped baffle to improve reliability, environmental adaptability, and power generation performance. Different from the most existing hybrid piezoelectric wind energy harvesters where the interaction of vortex-induced vibration and galloping was mainly implemented by altering the cylinder geometry, this EPWEH realized the coupling between vortex-induced vibration and galloping through introducing a downstream diamond-shaped baffle to change the aeroelastic instability of the cylinder. Besides, the pre-bending vibrator only subjected to the unidirectional compressive stress was employed and embedded inside the hollow cylinder, thus avoiding the drawbacks of bidirectional deformation of the traditional piezoelectric vibrator and direct contact between piezoelectric element and fluid. The feasibility of the structure and principle of the EPWEH was proved through a series of experiments. The experimental results demonstrated that the compound-embedded structure led to a coupling phenomenon of VIV and galloping on the cylindrical shell. Besides, it exhibited that the structural parameters brought a significant impact on the vibration characteristics, power generation performance, and wind speed bandwidth of EPWEH as well. Thanks to the performance improvement effect of the baffle, the EPWEH could reach a maximum voltage output enhancement of 910.1% and provide a maximum output power density of 5.493 mW/cm 3 at an optimal load resistance of 200 kΩ. It is expected that this compound-embedded structure can provide a reference for performance improvement of the existing PWEHs. • A novel piezoelectric wind energy harvester with a combined structure is presented. • Sealed piezo-vibrator is indirectly excited by wind-induced vibration. • Reliability is improved by the combination of sealed and pre-bending structure. • High performance characteristics based on coupling of VIV and galloping is realized. • Voltage performance can be enhanced by 910.1% and the power is 5.493 mW/cm 3 .

A magnetoelectric energy harvester for low-frequency vibrations and human walking

This paper proposes a magnetoelectric (ME) energy harvester employing a ME transducer array to capture mechanical energy from low-frequency vibrations and human walking. The use of the double-ring Halbach array helps to enhance the magnetic flux density in the air gap where the ME transducer array is placed. Consequently, larger magnetic field variations are induced on the ME transducer array and more mechanical energy can be converted into electrical energy. A theoretical model is developed and verified by experiments under sinusoidal excitations. The experimental results show that the harvester exhibits broad 3 dB bandwidths for sweep-up and sweep-down conditions. A 3 dB bandwidth of 4.1 Hz is obtained at the acceleration of 0.3 g, indicating that he harvester can operate in a broad bandwidth. The harvester can produce a maximal power of 0.667 mW across an optimal resistive load of 1.52MΩ at 0.3 g. Moreover, the feasibility of the harvester under human-walking induced excitations is experimentally validated at the speed of 3–8 km/h, and the results demonstrate the potential application of the proposed ME energy harvester in powering portable electronic devices.

A High-Resolution Electric Current Sensor Employing a Piezoelectric Drum Transducer.

A high-resolution sensor using a piezoelectric drum transducer is proposed for power frequency current sensing (50 Hz or 60 Hz). The utilization of the magnetic circuit helps to enhance the response to the electric currents in the power cords. The high sensitivity of the sensor originates from the superposition of the Ampere forces and the amplified piezoelectric effect of the drum transducer. The feasibility of the sensor was verified by experiments. The device exhibits a broad 3 dB bandwidth of 67.4 Hz without an additional magnetic field bias. The average sensitivity is 31.34 mV/A with a high linearity of 0.49%, and the resolution of the sensor attains 0.02 A. The resolution is much higher than that of the previous piezoelectric heterostructure for two-wire power-cords. Error analysis shows that the uncertainty reaches 0.01865 mV at the current of 2.5 A. Meanwhile, the device can generate a load power of 447.9 nW with an optimal load resistance of 55 KΩ at 10A (f = 50 Hz) in energy harvesting experiments. The features of high sensitivity, excellent linearity, high resolution, low costs, and convenient installation demonstrate the application prospect of the proposed device for measuring power frequency currents in electric power grids.

Enhanced performance of a flexible and wearable piezoelectric nanogenerator using semi-insulating GaN:Mg/ZnO coaxial nanowires

Flexible piezoelectric nanogenerators (PNGs) with high power-conversion efficiency are of great interest. Here, we propose wearable PNGs based on pristine magnesium (Mg)-doped GaN (GaN:Mg) and GaN:Mg/ZnO coaxial nanowires (NWs) exhibiting state-of-the-art high-voltage and -current outputs. The PNGs were designed to suppress internal screening by a reduction in the diameter of the NWs, successful incorporation of Mg in GaN NWs as a p-type dopant, and deposition of a ZnO shell on the GaN:Mg NWs. The Fermi-level pinning reduces free carriers in thin NWs (diameter up to ~30 nm). By tuning the size of the metal–alloy catalyst, we obtained NWs with diameters ranging from 30 to 50 nm. To deplete the NWs fully of free carriers, Mg is incorporated without activation, which enhanced the resistance of GaN:Mg NWs by forming inactive Mg–H complexes. The flexible PNG fabricated with pristine GaN:Mg NWs exhibited the maximum output voltage and current of 52 V and 23 µA, respectively. The junction current screening was suppressed by depositing a ZnO shell with an optimized thickness of 10 nm on GaN:Mg NWs. The PNG fabricated with GaN:Mg/ZnO coaxial NWs showed an enhanced output voltage and current of 66 V and 40 µA and demonstrated a state-of-the-art high power density of 170 µW/cm2 at an optimum load resistance of 2.5 MΩ. When mounted on the wrist as a wearable healthcare monitoring device, it successfully detected the movement of tendons and muscles as soon as the fingers were moved and exhibited a corresponding change in voltage response. Additionally, the charging of a capacitor is proven by the PNG under continuous foot tapping, demonstrating its potential to charge energy storage devices by walking or jogging.

Energy harvesting from longitudinal and transverse motions of sea waves particles using a new waterproof piezoelectric waves energy harvester

This paper experimentally studied the energy harvesting from the longitudinal and transverse motions of sea waves using a waterproof piezoelectric wave energy harvester (WPWEH). Because of sealing and specific design type of the WPWEH, the piezoelectric cantilever beam was entirely placed inside the water to increase the harvested electrical power. Three orientations of O1,O2 and O3were considered for placing the piezoelectric cantilever beam inside the water channel. In O1orientation, the cantilever beam was vertical and perpendicular to the floor while in O2and O3orientations, it was horizontal and parallel with the floor. The cantilever beam was perpendicular to the flow in O1andO2 and vibrates due to the longitudinal motion of wave particles while it was parallel with the flow in O3 and vibrates due to the transverse motion of wave particles. The influence of orientation, wave rate, and resonant frequency of the cantilever beam on the root mean square of the voltage and harvested electrical power was studied. The O1 orientation was selected as the optimum orientation for energy harvesting, because of having harmonic oscillations with a maximum generated voltage. The optimum electrical load resistance was calculated for the maximum harvested power. The results showed that the maximum density of the harvested electrical power from the WPWEH was increased compared to the other similar works.

A dual impact driven frequency up-conversion piezoelectric energy harvester for ultralow-frequency and wide-bandwidth operation

In the view of the limitation of traditional frequency up-conversion piezoelectric energy harvesters (FUC-PEH) which has a narrow operating frequency range and is difficult to adapt to the ultralow frequency excitation, this study presents a dual impact driven FUC-PEH composed of two piezoelectric energy harvester units (PEHUs). The electromechanical model of the FUC-PEH is established based on the single degree of freedom system and piezoelectric coupling factor. An experimental prototype is made and tested under different excitation frequencies and accelerations. The dynamic characteristics and energy output characteristics of the system are analyzed by theoretical calculation and experimental tests. The results show that the proposed energy harvester can provide efficient energy output in a wide frequency range of 3.5 Hz–15 Hz under different excitation accelerations. When the excitation acceleration is 6 m/s2 and the excitation frequency is 9.8 Hz, the average output power of the lower PEHU across optimum load resistance can reach 1.17 mW. When the excitation frequency is 15 Hz, the average output power of the upper PEHU across optimum load resistance can reach 1.86 mW. The theoretical and experimental results show that the proposed energy harvester has potential for powering miniaturized wireless portable devices.

Energy harvesting from a floor structure based on multiple piezoelectric transducer beams

In this paper, an energy harvesting floor based on piezoelectric transducers is studied. A structure with multiple layers of piezoelectric beams is designed. Different striking frequencies are studied. Fourier transform is applied to analyze the frequency characteristic of the output voltage. Experiments are explored to find out the optimal external load resistance for maximum output power. The experimental results show that, with three piezoelectric transducers in a beam array, and a stroke of 4 mm at the striking frequency of 1.263 Hz, the maximum output power is 733.5 µW.

A hybrid piezoelectric device combining a tri-stable energy harvester with an elastic base for low-orbit vibration energy harvesting enhancement

How to efficiently harvest the low-orbit vibration energy over a wide frequency range is one main challenge of the piezoelectric energy harvester. In this paper, combining the advantages of elastic base (EB) and the interaction of coupled elastic system, a hybrid piezoelectric device combining a tri-stable piezoelectric energy harvester (TPEH) with an EB is presented to enhance the harvesting ability of the low-orbit vibration energy. The EB composed of a mass and spring can amplify the low-orbit vibration and provide the TPEH enough kinetic energy to overcome the potential barrier and jump to the high-orbit oscillation, resulting in an even better operating bandwidth and higher power generation. The nonlinear electromechanical model describing the dynamic responses of the presented harvester is derived. The effects of the mass ratio and stiffness ratio on the dynamic performances of the hybrid energy harvester are numerically investigated with dynamic bifurcation diagrams method. The results show that the presented harvester has wider frequency bandwidth and higher power generation than those of the TPEH with rigid base by properly selecting the mass ratio and stiffness ratio, and it can more easily snap-through from low-orbit oscillation to high-orbit oscillation to reach larger dynamic response at lower excitation levels. Experiments are conducted to validate the simulations, and the experimental results are in good agreement with the theoretical results. Finally, fluencies of the load resistance on the power generation are also experimentally studied, and an optimum load resistance to achieve the peak power is obtained.

Analytical Modeling of a Doubly Clamped Flexible Piezoelectric Energy Harvester with Axial Excitation and Its Experimental Characterization.

With the rapid development of wearable electronics, novel power solutions are required to adapt to flexible surfaces for widespread applications, thus flexible energy harvesters have been extensively studied for their flexibility and stretchability. However, poor power output and insufficient sensitivity to environmental changes limit its widespread application in engineering practice. A doubly clamped flexible piezoelectric energy harvester (FPEH) with axial excitation is therefore proposed for higher power output in a low-frequency vibration environment. Combining the Euler–Bernoulli beam theory and the D’Alembert principle, the differential dynamic equation of the doubly clamped energy harvester is derived, in which the excitation mode of axial load with pre-deformation is considered. A numerical solution of voltage amplitude and average power is obtained using the Rayleigh–Ritz method. Output power of 22.5 μW at 27.1 Hz, with the optimal load resistance being 1 MΩ, is determined by the frequency sweeping analysis. In order to power electronic devices, the converted alternating electric energy should be rectified into direct current energy. By connecting to the MDA2500 standard rectified electric bridge, a rectified DC output voltage across the 1 MΩ load resistor is characterized to be 2.39 V. For further validation of the mechanical-electrical dynamical model of the doubly clamped flexible piezoelectric energy harvester, its output performances, including both its frequency response and resistance load matching performances, are experimentally characterized. From the experimental results, the maximum output power is 1.38 μW, with a load resistance of 5.7 MΩ at 27 Hz, and the rectified DC output voltage reaches 1.84 V, which shows coincidence with simulation results and is proved to be sufficient for powering LED electronics.

Maximum power point tracking control for mechanical rectification wave energy converter

As a clean and renewable energy, wave energy has abundant reserves, and its reasonable development and utilization can effectively alleviate the problems of energy shortage and environmental pollution. This paper proposes a mechanical rectification wave energy converter. The reciprocating linear motion of the buoy is directly converted into the rotary motion through the ball screw. The magnetic coupling force transmission system is adopted to ensure the sealing of the device. Using mechanical rectification mechanism to ensure the service life of the generator. In addition, a dynamic model of the mechanical rectification wave energy converter is established. Through simulation analysis, the curve of power with load resistance is obtained. And there is an optimal load resistance value corresponding to the maximum power in different wave conditions. In order to improve the power generation efficiency, the maximum power point tracking control algorithm is designed based on the admittance differentiation method. By controlling the duty cycle of the switch tube in the Buck-Boost chopper circuit, the variable load is realized, and then the maximum power point tracking control is realized. Finally, simulation analysis verifies the feasibility of the algorithm under different wave conditions.

A piezoelectric wind energy harvester excited indirectly by a coupler via magnetic-field coupling

Energy harvesting from wind-induced vibration energy using piezoelectric materials has received great attention because of the self-powered demand for wireless sensor networks. To improve the reliability, output performance and environmental adaptability (or designability of the structure), a piezoelectric wind energy harvester excited indirectly by a coupler via magnetic-field coupling (MC-PWEH) is proposed in this paper. The MC-PWEH is mainly composed of a piezoelectric transducer and a coupler consisting of a flexible beam, a cylinder, some exciting magnets and added coupler mass. By introducing the interaction of the piezoelectric transducer sealed in the chamber and the cylinder via the magnetic coupling, the interactive vortex-induced vibration (VIV) and galloping is realized and the composite vibration energy is transformed into electric energy. The feasibility of the structure and principle of the MC-PWEH is proved through theoretical simulations and experiments. The results indicated that the piezoelectric transducer began to oscillate strongly and generate fairly high output voltage when the wind speed exceeded a low critical wind speed, accompanied by a coupling phenomenon of VIV and galloping, and then finally converged to a stable value when the wind speed exceeded a high critical value. Moreover, the critical wind speeds, bandwidth of wind speed, natural frequency and power generation performance of the MC-PWEH could be adjusted by changing the structural parameters. Under the transducer proof mass of 50 g, the optimal wind speed bandwidth for the MC-PWEH to output voltage greater than 3.5 V obtained was 25.84 m⋅s−1 that occupies 86.12% of the experimental wind speed bandwidth. The achieved maximum output power was 4.73 mW at the optimal external load resistance of 1000 kΩ and wind speed of 24 m⋅s−1.

A novel high-power density, low-frequency electromagnetic vibration energy harvester based on anti-phase motion

The paper proposes a novel electromagnetic vibration energy harvester based on the concept of anti-phase vibration. Anti-phase motion is desirable in electromagnetic vibration energy harvesting applications as it results in a higher power output due to the increase in the relative velocity between the coil and the magnet components. The proposed device achieves anti-phase motion at any driving frequency due to a developed new design. The harvester was experimentally tested under a base input of 0.49 g, 0.38 g and 0.36 g, recording a power density of 765.3 Wm−3, 420.8 Wm−3 and 587.2 Wm−3 respectively under a resonant frequency of 11.8 Hz. Although the device has not yet been optimised, these values obtained are already comparable, if not higher than the recent previous works on vibration energy harvesting. In addition, the observed experimental results have well agreed with the results obtained through the mathematical model, derived in this study. The original design was then optimised to determine the optimum load resistance, mechanical arm length and component dimensions that would result in the maximum power output. Considering a base input of 0.36 g and the same friction and damping forces as obtained from the experiment, a power density of 1708.5 Wm−3 could theoretically be achieved under the same natural frequency.

Characteristics analysis and parameter optimization of a micro-combustion based thermoelectric generator

Micro combustion based thermoelectric generator (micro-CTEG) has great potential to provide reliable electricity for portable electronic devices. Previous works on micro-CTEG has focused on the micro combustion issue, here we explore the heat transfer and thermoelectric (TE) coupling mechanism and give the guidelines for optimizing the performance of micro-CTEG. An analytical model was derived considering heat exchange inside the combustor, TE generator structure and temperature dependent material properties. Based on the optimization results, the temperature drop across the TE leg remains nearly constant for filling fraction approaching 1.0 and decreases remarkably with decreasing input fuel power for filling fraction approaching 0.1 due to the excessive gap heat loss. High filling fraction and high input fuel power is preferred to balance the power density and total efficiency. With identical thermoelectric figure of merit (ZT), increasing power factor under low filling fraction and decreasing thermal conductivity under high filling fraction are preferred respectively. The optimal load resistance factor, which represents the ratio of load resistance to internal resistance of TE generator, shows a decreasing trend with increasing temperature drop across TE leg. The optimal leg length remains nearly unchanged as thermal resistance of the heat sink is lower than 1 K/W.

Planar magnetoelectric voltage transformer based on a composite ferromagnet-piezoelectric heterostructure

A planar magnetoelectric voltage transformer based on a layered composite structure containing layers of amorphous ferromagnet and piezoelectric lead zirconate-titanate has been fabricated and investigated. The transformer uses a coilless system (the so-called "magnetic capacitor") to excite the direct magnetoelectric effect at the frequency of the acoustic resonance of the structure, which made it possible to significantly simplify the design. The voltage transformation ratio of the magnetoelectric transformer, in contrast to the known electromagnetic or piezoelectric transformers, can be tuned by an external magnetic field from zero to 13.2 with an increase in the field from zero to 30 Oe. The operating range of the output voltage of the transformer is 0−400 mV, and the output power reaches 9 μW at optimal load resistance. The technique for calculating the characteristics of the transformer is described.

Comparative Study of Achievable Efficiency Between Three- and Four-Coil Wireless Power Transfer Systems

In magnetic resonant wireless power transfer (WPT), intermediate resonant coils (i-RCs) can be deployed between a transmitter and a receiver as a means of improving performance, but this entails a high additional cost. In the search for a cost-effective strategy for the use of additional components of WPT, we provide a comparative analysis of the achievable transmission efficiency (TE) between three-coil (3C) and four-coil (4C) systems. In particular, we find optimal control parameters, e.g., load resistance and inductive coupling coefficient, to derive the achievable TE for each system in closed-form expressions. In comparing the achievable TEs of the two systems, we also remark on the effective use of additional coils in two main ways. First, by optimizing the load resistance, the position of the i-RC is determined to be close to the transmitter or the receiver in a 3C system for improving the achievable TE depending on the strength difference between inductive and resonant couplings. Moreover, if the i-RC is placed near the transmitter, the 3C system shows the same achievable TE as the 4C system. Second, by optimizing the inductive coupling coefficient, the achievable TE of the 3C system is not affected by the location of the i-RC, whether it is placed nearer to the transmitter or the receiver, and the 4C system always outperforms the 3C system. Through a series of experiments conducted in a variety of environments, we verify both the validity of these observations and the accuracy of the theoretical analysis.

Load Resistance Optimization of a Magnetically Coupled Two-Degree-of-Freedom Bistable Energy Harvester Considering Third-Harmonic Distortion in Forced Oscillation.

In this study, the external load resistance of a magnetically coupled two-degree-of-freedom bistable energy harvester (2-DOF MCBEH) was optimized to maximize the harvested power output, considering the third-harmonic distortion in forced response. First, the nonlinear dynamic analysis was performed to investigate the characteristics of the large-amplitude interwell motions of the 2-DOF MCBEH. From the analysis results, it was found that the third-harmonic distortion occurs in the interwell motion of the 2-DOF MCBEH system due to the nonlinear magnetic coupling between the beams. Thus, in this study, the third-harmonic distortion was considered in the optimization process of the external load resistance of the 2-DOF MCBEH, which is different from the process of conventional impedance matching techniques suitable for linear systems. The optimal load resistances were estimated for harmonic and swept-sine excitations by using the proposed method, and all the results of the power outputs were in excellent agreements with the numerically optimized results. Furthermore, the associated power outputs were compared with the power outputs obtained by using the conventional impedance matching technique. The results of the power outputs are discussed in terms of the improvement in energy harvesting performance.