A Harvesting Circuit for Flexible Thin-Film Piezoelectric Generator Achieving 562% Energy Extraction Improvement With Load Screening

An energy-harvesting interface for kinetic energy harvesting from high-voltage piezoelectric and triboelectric generators is proposed in this paper. Unlike the conventional kinetic energy-harvesting interfaces optimized for continuous sinusoidal input, the proposed harvesting interface can efficiently handle irregular and random high voltage energy inputs. An N-type mosfet (NMOS)-only power stage design is introduced to simplify power switch drivers and minimize conduction loss. Controller active mode power is also reduced by introducing a new voltage peak detector. For efficient operation with potentially long intervals between random kinetic energy inputs, standby power consumption is minimized by monitoring the input with a 43 pW wake-up controller and power-gating all other circuits during the standby intervals. The proposed harvesting interface can harvest energy from a wide range of energy inputs, 10 s of nJ to 10 s of µJ energy/pulse, with an input voltage range of 5–200 V and an output range of 2.4–4 V under discontinuous as well as continuous excitation. The proposed interface is examined in two scenarios, with integrated power stage devices (maximum input 45 V) and with discrete power stage devices (maximum input 200 V), and the harvesting efficiency is improved by up to 600% and 1350%, respectively, compared to the case when harvesting is performed with a full bridge rectifier.

Biocompatible and sustainable power supply for self-powered wearable and implantable electronics using III-nitride thin-film-based flexible piezoelectric generator

We present recent development in flexible wide-bandgap semiconductor materials and devices. Especially, we focus on mechanically bendable group-III-nitride (III-N) thin-film heterostructures and their photonic, electronic, and energy-harvesting devices in energy applications. The presentation will cover various topics including (1) multi-functionality of flexible III-N devices, (2) direct growth of high-quality single-crystal-like GaN semiconductor thin films on low-cost flexible metal tapes, and (3) piezoelectric generators and sensors for self-powered wearable systems by harvesting and sensing ambient biomechanical energy. Flexible III-N thin-film heterostructures have an implication of more than just mechanically flexible materials. Bendable devices based on III-N heterostructures can be equipped with new functionalities and even further improved performance characteristics using a new concept of active polarization engineering by controlled external strains. We propose to develop multi-functional and/or further-improved-performance devices by utilizing the interactions between electronic and optical properties and mechanical forces in the flexible III-N heterostructures. The concept will enable new mechano-electro-photonic (MEP) devices. We will show by device modeling that new device concept in flexible transistors based on AlGaInN/GaN heterostructure is possible, including modulation of 2-dimensional electron gas (2DEG) density by bending, strain-effect transistors (Figure 1: I-V characteristics of strain-effect transistors), and high-hole-mobility transistors. Furthermore, photon emitters based on flexible III-N heterostructure can result in higher internal quantum efficiency (IQE), higher wall-plug efficiency (WPE), and color modulation by optimum bending conditions (Figure 2: Concept of color-tunable white light-emitting diodes). We study and develop nearly-single-crystalline GaN thin film on flexible metal tapes by a direct deposition technique for the demonstration of a new wide-bandgap semiconductor platform, targeting high-performance yet economical, flexible, and versatile device technology. Data and analysis are presented for the single-crystal-like film, as representatively shown in Figure 3 (Flexible hybrid tape substrate consisting of GaN thin film and Cu tape with crystallinity-transformational buffer layers). Energy harvesters that scavenge biomechanical energy are promising power supply candidates for wearable and implantable electronics. Of the most widely used energy harvesters, piezoelectric generators can generate more electric charge than their triboelectric counterparts with similar device size, thus are more suitable to make compact wearable devices. We develop a flexible piezoelectric generator (F-PEG) with chemically stable and biocompatible III-N thin film. Data and analysis are presented for the flexible III-N F-PEG. We also demonstrate that the F-PEG can directly power electronics such as light-emitting diodes and electric watches, and charge commercial capacitors and batteries to operate an optical pulse sensor, as representatively shown in Figure 4 (Pulse sensor powered by F-PEG and heart rate measurement on a fingertip). Figure 1

Output characteristics of thin-film flexible piezoelectric generators: A numerical and experimental investigation

A fully integrated piezoelectric energy harvesting interface is proposed for harvesting energy from irregular human motion. To handle irregular pulse inputs generated by the piezoelectric transducer (PZT), the proposed harvesting interface includes a wake-up controller that activates the harvesting interface only when human motion is detected and deformation is applied on the piezoelectric material, thereby keeping static power loss low. The PZT output voltage is increased to its peak voltage by removing any type of external load capacitance seen by the PZT during its deformation. Once the peak voltage is detected, a multi-voltage conversion-ratio-based switched-capacitor circuit is activated to transfer PZT-generated energy to the battery in multiple ratio steps to maximize the conversion efficiency, with the help of a carefully designed harvesting controller. To deal with open-circuit voltages (VOCS) higher than the maximum voltage tolerated (VMAX) by available technology, capacitive partial electric charge extraction is activated every time the PZT output voltage approaches the VMAX. The proposed harvesting interface extracts 3.37 times more energy than a conventional full-bridge rectifier-based harvesting scheme.

Abstract

Battery life is a major concern in wireless sensing applications, as it causes a trade-off between system size and power consumption of the electronic circuits connected to it. Even if electronic circuit power consumption is steadily decreasing, the energy density of common energy storage systems is still extremely low in space-constrained applications. In this scenario, energy harvesting is a valuable solution to extend, in theory indefinitely, the autonomy of ubiquitous sensing systems. In particular, vibrational energy harvesters are an excellent solution to power sensors in industrial and automotive applications. This paper presents an electrostatic energy harvester (EEH) interface. Recently, electret-based EEHs have attracted considerable attention because of their capability to generate large powers, even at low accelerations [1]. Unfortunately, these devices are characterized by extremely high internal impedances and their interfacing circuits need to be simultaneously ultra-low-power and capable of working reliably with several tens of Volts applied to the input. To the best of our knowledge, only one solution has been proposed to efficiently interface high-voltage energy harvesters [2]. However, that circuit did not allow fully autonomous battery-less operation and did not work under 25μW available power.

Flexible piezoelectric generators have attracted a great deal of attention recently for their promising applications in mechanical energy harvesting and portable electronic devices. The significant properties of 2D MXene (Ti3C2TX) inspired us to develop a nanocomposite with poly(vinylidene fluoride) (PVDF) by varying the Ti3C2TX filler using tape casting techniques. The structural and microstructural properties of the composite film were examined through X-ray diffraction and field-emission scanning electron microscopy. FTIR studies revealed that an increase in the electroactive phase from 73 to 82% was observed by adding the Ti3C2TX filler into the PVDF matrix. The frequency-dependent dielectric constant, dielectric loss, and RT impedance spectra were studied. The leakage current density and ferroelectric behavior of the Ti3C2TX-added PDVF composite films showed significant results as compared to the pure PVDF film. A bipolar strain with an electric field (S–E) curve was used to investigate the reverse piezoelectric coefficient (d33*). The piezoelectric response of the Ti3C2TX-added PDVF composite films showed a significant enhancement of the output power density (56.9 μW/cm3) in comparison with that of a PVDF-based piezoelectric generator having a power density of 5.6 μW/cm3. Finally, the flexible piezoelectric generator was employed to efficiently detect human body movements as a self-poled wearable sensor and self-powered device.

We present a flexible piezoelectric generator, capable to harvest energy from human arterial pulse wave on the human wrist. Special features and advantages of the flexible piezoelectric generator include the multi-layer device design with contact windows and the simple fabrication process for the higher flexibility with the better energy harvesting efficiency. We have demonstrated the design effectiveness and the process simplicity of our skin- attachable flexible piezoelectric pulse wave energy harvester, composed of the sensitive P(VDF-TrFE) piezoelectric layer on the flexible polyimide support layer with windows. We experimentally characterize and demonstrate the energy harvesting capability of 0.2~1.0μW in the Human heart rate range on the skin contact area of 3.71cm2. Additional physiological and/or vital signal monitoring devices can be fabricated and integrated on the skin attachable flexible generator, covered by an insulation layer; thus demonstrating the potentials and advantages of the present device for such applications to the flexible multi-functional selfpowered artificial skins, capable to detect physiological and/or vital signals on Human skin using the energy harvested from arterial pulse waves.

Recently, environmentally friendly and flexible piezoelectric generators have attracted considerable interest in powering wearable and implantable devices. In our present work, an environmental-friendly, flexible and efficient piezoelectric generator is demonstrated, based on composite film of 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BZT–BCT) nanoparticles embedded into polyvinylidene fluoride–trifluoroethylene P(VDF–TrFE) polymer. The effect of the different doping contents of BZT–BCT on the output performance and flexibility were investigated. As a result, the composite film with 30% contents BZT–BCT exhibited more excellent performance, which generated the maximum open-circuit voltage of 8.11 V and short-circuit current of 2.83 µA. Moreover, the output voltage of the device shows good linear relationship with the applied force. Owing to the sensitivity of microforce, this developed piezoelectric generator could open a new application field for using as force sensor.

Environment friendly, flexible, and robust sensors have attracted considerable research attention due to their potential for a wide range of devices in energy generation and harvesting, sensing, and biomedical applications. In this manuscript, we demonstrate a lead-free, solution processed flexible piezoelectric energy generator based on a nanocomposite film, consisting of MgO nanoparticles of sizes around <50 nm, embedded in poly(vinylidene difluoride) [PVDF] and its copolymer with trifluoroethylene, that is, P(VDF-TrFE) matrix. Piezoelectric, ferroelectric, and leakage current measurements made on samples with various concentrations of MgO nanoparticles revealed a dramatic improvement in these characteristics at 2 wt % MgO with nearly 50% increase in the piezoelectric coefficient as compared to pure P(VDF-TrFE), attributed to the preferred conformation of P(VDF-TrFE) chain, improved crystallinity of the P(VDF-TrFE) matrix, and uniform distribution of nanoparticles. Assessment of the interactions between -OH groups attached to MgO surface and P(VDF-TrFE), carried out using Fourier-transform infrared spectroscopy (FTIR), suggested weak van der Waals forces between -OH groups and P(VDF-TrFE) being responsible for the observed improvement. This flexible nanocomposite device exhibits superior energy harvesting performance with over two-times improvement in the voltage output (2 V) compared to device using P(VDF-TrFE) films alone. Along with superior electrical properties, nanocomposites also exhibit excellent endurance against electrical as well as mechanical fatigue, with piezoelectric coefficient remaining unchanged even after 10 000 bending cycles, supporting their suitability in flexible energy harvesting applications.

Energy harvesting from the environment becomes a valuable technology, especially for sea wave applications, in which it usually ends up wasted despite its potential to be harvested. Due to its wide availability and high energy density, piezoelectric energy harvesting (PEH) is becoming popular for flexible energy harvesting. This paper presents a flexible horizontal piezoelectric (FHP) energy harvester to harvest energy from the surface of sea wave. The harvester is made of bimorph piezoelectric devices; they are utilised to amplify and convert the collected mechanical vibrations into electrical power. A finite element model is established from ANSYS simulations to solve the iteration method by generating resonance frequency (fr). Then, Taguchi method, SN ratio and the ANOVA approach were used by considering the input variable of fr to estimate the optimum performance through control factors; number of blade, length and thickness. From the performance test result, it is proven that the higher numbers of blade including length, and minimum numbers of thickness significantly improve the significant level, α = 0.05% of ANOVA. Three prototypes are developed with approximate body dimensions through the resonance frequency perform and generate a 160.3 Hz on blade dimensions of 10 × 300 × 0.2 mm, with a piezoelectric (PZT) on its surface. This particular study shows that the potential of output power is generated from sea wave surface through a significant relationship between length, thickness, and blade design. This research develops a novelty for energy harvesting from flexible piezoelectric generator on sea wave application that could be easily install on offshore platform.

In this study, a proof of concept for seamless energy flow is demonstrated by converting light energy into electrical energy and then storing it. A simple heterojunction structure of an FTO/ZnO/NiO/AgNWs/ZnO array transparent photovoltaic (TPV) device is employed to ensure an excellent average visible transmittance value of 67.7% while storing light energy as electrical energy in a capacitor bank. By simple and stable array connection of unit cell devices, the power leakage is minimized while maximizing output voltage. In the array TPV device, an open‐circuit voltage of 1.4 V is achieved under 365 nm illumination, with a voltage of 1.26 V stored in the capacitor bank, accumulating to over 6 V. The stored electrical energy is successfully converted for use by an light‐emitting diode (LED) light source, demonstrating sustained light‐up for over 30 s. This work explores facile energy generation, storage and utilization through TPVs, with a good potential for transparent energy harvesting and human interface applications.

Piezoelectric energy conversion that generate electric energy from ambient mechanical and vibrational movements is promising energy harvesting technology because it can use more accessible energy resources than other renewable natural energy. In particular, flexible and stretchable piezoelectric energy harvesters which can harvest the tiny biomechanical motions inside human body into electricity properly facilitate not only the self-powered energy system for flexible and wearable electronics but also sensitive piezoelectric sensors for motion detectors and in vivo diagnosis kits. Since the piezoelectric ZnO nanowires (NWs)-based energy harvesters (nanogenerators) were proposed in 2006, many researchers have attempted the nanogenerator by using the various fabrication process such as nanowire growth, electrospinning, and transfer techniques with piezoelectric materials including polyvinylidene fluoride (PVDF) polymer and perovskite ceramics. In 2012, the composite-based nanogenerators were developed using simple, low-cost, and scalable methods to overcome the significant issues with previously-reported energy harvester, such as insufficient output performance and size limitation. This review paper provides a brief overview of flexible and stretchable piezoelectric nanocomposite generator for realizing the self-powered energy system with development history, power performance, and applications.

In a piezoelectric energy harvesting (PEH) system, the dynamics of the device as well as the energy flow within the system vary with different harvesting interface circuits connected. Meanwhile, the impedance matching theory is regarded as theoretical base for harvesting power enhancement, and hopefully could provide guidance for harvesting interface optimization. Most previous literatures on impedance matching for PEH started their analyses by assuming that the harvesting interface, which is nonlinear in nature, can be equalized to resistive load, or linear load whose impedance value can be arbitrarily set, so that the output impedance of the piezoelectric structure can surely be matched. Yet, after investigating the equivalent impedances of the existing harvesting interfaces, including standard energy harvesting (SEH), parallel synchronized switching harvesting on inductor (P-SSHI), and series synchronized switching harvesting on inductor (S-SSHI), we found that, their ranges are in fact limited. Therefore, to optimize the harvesting power, constrained matching instead of free matching should be adopted. In addition, we also clarify some confusing points in the previous literatures on impedance matching for energy harvesting. With the understanding on energy flow within piezoelectric devices, we know that only a portion of the extracted energy is able to be harvested, while the other is dissipated throughout the harvesting process. So even the extracted power from the source is maximized by matching the impedance; there is no guarantee that harvesting power is surely improved. The harvesting power also depends on the ratio between harvested energy and dissipated energy. These two issues discussed in this paper are crucial to improve the harvesting power and efficiency in piezoelectric energy harvesting systems.