Electrostatic Vibration Energy Harvester Research Articles

Micro electrostatic energy harvester with both broad bandwidth and high normalized power density

In this paper, an electrostatic vibration energy harvester is proposed and fabricated with both broad bandwidth and high normalized power density (NPD, harvested power/volume/acceleration2). The device is made up of two parts, top movable plate and bottom fixed plate, both of which are fabricated from silicon wafers using advanced micro-electro-mechanical system (MEMS) technology. A CYTOP polymer layer is spray coated for an electret material as well as an adhesive layer for low temperature wafer bonding. The air damping effect in energy harvesting has been investigated. At a low pressure of 3 Pa in a vacuum chamber, maximum power output of 4.95 μW has been harvested at low vibration amplitude of 0.09 g, resulting in a bandwidth of 12 Hz and NPD of 3 mW/cm3/g2, which outperforms most of the previous harvesters. A high harvester effectiveness of 67.9% is therefore achieved. The response on random vibrations is also tested. An average output power of 2.22 μW is harvested when random vibration is applied at a frequency range of 160 ± 12.5 Hz with RMS acceleration of 10.5 m/s2. The excellent overall performance gives promising application for energy harvesting from random sources and multi-device stack for wireless sensor networks.

Analysis of Electrostatic Energy Harvesters Electrically Configured as Bennet’s Doublers

This paper explores electrostatic vibration energy harvesters with power electronic interfaces that configure them as Bennet's doubler. We focus on in-plane overlap generators and consider challenges arising from small capacitance variations due to parasitic capacitances and the influence of diode parameters, such as reverse current and p-n junction capacitance, on system performance. The average charging current and initial bias voltage can be estimated from simulation results. The dependence of the minimum initial voltage on the storage capacitance is determined. When delivering energy into a storage capacitor, the voltage ultimately saturates due to the electromechanical coupling and its value depends on input acceleration, drive frequency, mechanical quality factor, and diode losses. The minimum required initial bias is about ten mVs and can be accomplished by choosing the value of the storage capacitor and utilizing diode-configured transistors in integrated circuits.

Low cost electrostatic vibration energy harvesters based on negatively-charged polypropylene cellular films with a folded structure

Low cost electrostatic vibration energy harvesters based on negatively-charged polypropylene cellular films with a folded structure were designed in this study. Strips of such energy harvesters were excited by applying mechanical stress in length direction. A current in a terminating resistor was generated due to the capacitance variation of the samples. For a typical double-periodic folded-structure electrostatic vibration energy harvester sample whose effective length and width were 30 mm and 10 mm, respectively, the generated power across a matching resistor at a resonance frequency of 36 Hz amounts to 641 μW for a seismic mass of 4 g and an acceleration of 1 g (g is the gravity of the Earth). Similar structures which were designed and fabricated in this study were also tested for energy harvesting and high output power in the order of a few hundred microwatt was gained. Following the presentation of a theoretical model allowing for the calculation of the power generated in a load resistance at the resonance frequency of the harvesters, experimental results are shown and compared to theoretical prediction. It turns out that the experiment results accord well with the theoretical predictions.

A Novel Characterization Method for Accurate Lumped Parameter Modeling of Electret Electrostatic Vibration Energy Harvesters

This letter presents a new method for the characterization of electret transducers, which are typically used in electret electrostatic vibration energy harvesters (electret e-VEHs). This is the first method allowing to accurately measure the value of the equivalent voltage source representing the electret in lumped parameter models of a wide range of electret e-VEHs. An accurate value for this parameter is critical for design, analysis, and optimization, given the increasing complexity of e-VEHs electrical interfaces. Until now, there was no universal method allowing the measurement of this parameter, because of practical difficulties with some geometries, and because of charging non-uniformities. In this letter, the new method is presented, with insights on how to maximize the measurement accuracy. It is then applied to a state-of-the art MEMS electret e-VEH.

Electrostatic vibration energy harvester with 2.4-GHz Cockcroft–Walton rectenna start-up

Abstract In this paper, we propose the design, fabrication and experiments of a macro-scale electrostatic vibration energy harvester (e-VEH), pre-charged wirelessly for the first time with a 2.4-GHz Cockcroft–Walton rectenna. The rectenna is designed and optimized to operate at low power densities and provide high voltage levels: 0.5 V at 0.76 μW/cm 2 and 1 V at 1.53 μW/cm 2 . The e-VEH uses a Bennet doubler as a conditioning circuit. Experiments show a 23-V voltage across the transducer terminal, when the harvester is excited at 25 Hz by 1.5 g of external acceleration. An accumulated energy of 275 μJ and a maximum available power of 0.4 μW are achieved.

Electrostatic MEMS vibration energy harvester for HVAC applications with impact-based frequency up-conversion

This paper reports on electrostatic MEMS vibration energy harvesters with gap-closing interdigitated electrodes, designed for and tested on HVAC air ducts. The harvesters were fabricated on SOI wafers with 200 µm device layer using a custom microfabrication process. Designs with aspects ratio (electrodes’ gap versus depth) of 10 and 20 were implemented, while the overall footprint was approximately 1 cm × 1 cm in both cases. In order to enhance the power output, a dual-level physical stopper system was designed to control the minimum gap between the electrodes, which is a key parameter in the conversion process. The dual-level stopper utilizes cantilever beams to absorb a portion of the impact energy as the electrodes approach the impact point, and a film of parylene with nanometer thickness deposited on the electrode sidewalls. The parylene layer defines the absolute minimum gap and provides electrical insulation. The fabricated devices were first tested on a vibration shaker to characterize the resonant behavior. Devices with aspect ratio 10 were found to exhibit frequency up-conversion, which enhances the amount of converted power. Devices with both aspect ratios were found to exhibits spring hardening due to impact with the stoppers and spring softening behavior at increasing voltage bias. The highest power measured on shaker table for sinusoidal vibrations was 3.13 µW (includes enhancement due to frequency up-conversion driven by impact) for aspect ratio 10, and 0.166 µW for aspect ratio 20. The corresponding dimensional figure-of-merit, defined as the power output normalized to vibration acceleration and frequency, squared voltage and device mass, was in the range of 10 · 10−8 m V−2 for both devices, about an order of magnitude higher than state-of-the-art. Testing was carried out on HVAC air duct vibrating with an RMS acceleration of 155 mgRMS, a primary frequency of 60 Hz and a PSD of 7.15 · 10−2 g2 Hz−1. The peak power measured was 118nW with a PSD of 1.91 × 10−9 W Hz−1 at 120 Hz (four times the primary frequency of 60 Hz), some of the highest values reported to date.

Design and test of a Bennet's doubler device with mechanical switches for vibrational energy harvesting

In this work, we demonstrate that the use of self-synchronized mechanical switches in replacement of diodes into electrostatic vibration energy harvesters (e-VEH) can lead to better power generation. Indeed, mechanical switches have the advantage of no leakage current and no threshold voltage. As a proof of concept, we use the Bennet's doubler electrostatic generator. The proposed e-VEH is composed of two variable capacitors triggered by a central electrode taken as an inertial mass. Ambient vibrations induce inertial forces on the central electrode, as a result a voltage doubling is obtained at each operating cycle. The mechanical switches are directly fixed to the moving electrode. In addition, no dedicated pre-charge is required: the system starts with ambient electrical charges. The device is fabricated and tested under harmonic motion. A comparison between the proposed design and those using diodes under the same operating conditions shows an experimental direct increase of the harvested electrical power of around 28%.

Electrostatic vibrational energy harvester with ionic liquid and potassium-ion-electret

This paper introduces an electrostatic vibrational energy harvester with an effective electrode distance of ∼1 μm by utilizing the 1-nm-thick insulating gap that an ionic liquid forms, followed by a proof-of-concept with an AC current of 100 nA (peak-to-peak) and a voltage of 100 mV (peak-to-peak) generated from our prototype.

A Smart Load Interface and Voltage Regulator for Electrostatic Vibration Energy Harvester

This paper presents a new implementation in ams 0.35μm HV technology of a complete energy management system for an electrostatic vibrational energy harvester (e-VEH). It is based on the Bennet's doubler architecture and includes a load voltage regulator (LVR) and a smart Load Interface (LI) that are self-controlled with internal voltages for maximum power point tracking (MMPT). The CMOS implementation makes use of an energy harvester that is capable of producing up to 1.8μW at harmonic excitation, given its internal voltage is kept within its optimum. An intermediate LI stage and its controller makes use of a high side switch with zero static power level shifter, and a low power hysteresis comparator. A full circuit level simulation with a VHDL-AMS model of the e-VEH presented was successfully achieved, indicating that the proposed load interface controller consumes less than 100nW average power. Moreover, a LVR regulates the buffer and discharge the harvested energy into a generic resistive load maintaining the voltage within a nominal value of 2 Volts.

Electrostatic Vibration Energy Harvester Pre-charged Wirelessly at 2.45 GHz

This paper reports the design, fabrication and experiments of an electrostatic vibration harvester (e-VEH), pre-charged wirelessly for the first time by using an electromagnetic waves harvester at 2.4 GHz. The rectenna uses the Cockcroft-Walton voltage doubler rectifier. It is designed and optimized to operate at low power densities and provides high voltage levels: 0.5 V at 0.5 μW/cm2 and 0.8 V at 1 μW/cm2 The e-VEH uses the Bennet doubler as conditioning circuit. Experiments show 23 V voltage across the transducer terminal when the harvester is excited at 25 Hz by 1.5 g of external acceleration. An accumulated energy of 275 μJ and a maximum power of 0.4 μW are available for the load.

Electrostatic Energy Harvesting Circuit with DC-DC Convertor for Vibration Power Generation System

This paper presents an interface circuit with power control features for electrostatic vibration energy harvesting. A DC-DC convertor is used to control the output voltage of a diode-based charge pump circuit. Therefore, the maximum and minimum voltage across the variable capacitor of the energy harvester may be adjusted to track the maximum power point of the system. The power conversion function of the DC-DC convertor depends on the switches configuration. An example of Maximum Power Point Tracking (MPPT) for different conversion function is presented in this paper. Simulation results show that at least 10 μW is generated.

Characterization of the capacitance variation of electrostatic vibration energy harvesters biased following rectangular charge-voltage diagrams

This paper presents for the first time a method to measure the capacitance variation of electrostatic vibration energy harvesters (e-VEHs) that employ conditioning circuits implementing a biasing scheme that can be represented by a rectangular charge-voltage diagram. Given the increasing number of e-VEHs using such complex conditioning circuits and the complex dynamics that are induced from this type of biasing, a mean to assess this measurement is of primary importance for the analysis of e-VEHs. The proposed method is based on the inspection of the voltage evolution across two simple conditioning circuits implementing a rectangular charge-voltage diagrams biasing scheme. After the method is presented, it is carried out for the characterization of a state-of-the-art MEMS e-VEH.

Interface circuit with adjustable bias voltage enabling maximum power point tracking of capacitive energy harvesting devices

The operation analysis of a new interface circuit for electrostatic vibration energy harvesting with adjustable bias voltage is carried out in this paper. Two configurations determined by the open or closed states of an electronic switch are examined. The increase of the voltage across a biasing capacitor, occurring when the switch is open, is proved theoretically and experimentally. With the decrease of this biasing voltage which occurs naturally when the switch is closed due to imperfections of the circuit, the bias voltage can be maintained close to a target value by appropriate ON and OFF control of the switch. As the energy converted by the variable capacitor on each cycle depends on the bias voltage, this energy can be therefore accurately controlled. This feature opens up promising perspectives for optimization the power harvested by electrostatic devices. Simulation results with and without electromechanical coupling effect are presented. In experimental tests, a simple switch control enabling to stabilize the bias voltage is described.

Power optimization and effective stiffness for a vibration energy harvester with displacement constraints

This paper presents experiments on how to approach the physical limits on power from vibration energy harvesting under displacement-constrained operation. A MEMS electrostatic vibration energy harvester with voltage-control of the system stiffness is used for this purpose. The power saturation problem, when the proof-mass displacement reaches a maximum amplitude for sufficient acceleration amplitude, is shifted to higher accelerations by use of load optimization. In addition, we demonstrate the effect of varying the electromechanical coupling k2. Measurement results show that harvested power can also be made to follow the optimal power of the velocity-damped generator for a range of accelerations, which implies displacement constraints. Compared to the saturated power, the power increases 1.5 times with the optimal load for electromechanical coupling at k2 = 8.7%. This is improved 2.3 times for a higher coupling of . The obtained system effectiveness exceeds 60%. This work shows a first demonstration of reaching optimal power in the intermediate acceleration-range between the two extremes of maximum efficiency and maximum power transfer. The experimental results follow the theoretical results for a device with both load and stiffness tuning surprisingly well, despite only optimizing the load here. We compared a linearized lumped-model of the device with the same augmented by end-stop nonlinearities. The comparison shows that an effective stiffness due to end-stop impacts in the latter model closely matches the optimal stiffness for the former model, and therefore can explain why the experimental output power is close to optimal despite the lack of deliberate stiffness tuning.

Analysis of an electrostatic energy harvester with variable area, permittivity and radius

This paper reports on an electrostatic vibration energy harvester (e-VEH) system with variable area, variable permittivity, and variable radius. Nonlinear oscillator equations are established for each case, and solved analytically and numerically. The power produced by each configuration of the harvester is presented in terms of resistive load, frequency and external excitation.

A batch-fabricated electret-biased wideband MEMS vibration energy harvester with frequency-up conversion behavior powering a UHF wireless sensor node

This paper reports a batch-fabricated, low-frequency and wideband MEMS electrostatic vibration energy harvester (e-VEH), which implements corona-charged vertical electrets and nonlinear elastic stoppers. A numeric model is used to perform parametric study, where we observe a wideband bi-modality resulting from nonlinearity. The nonlinear stoppers improve the bandwidth and induce a frequency-up feature at low frequencies. When the e-VEH works with a bias of 45 V, the power reaches a maximum value of 6.6 μW at 428 Hz and 2.0 grms, and is above 1 μW at 50 Hz. When the frequency drops below 60 Hz, a ‘frequency-up’ conversion behavior is observed with peaks of power at 34 Hz and 52 Hz. The −3 dB bandwidth is more than 60% of its central frequency, both including and excluding the hysteresis introduced by the nonlinear stoppers. We also perform experiments with wideband Gaussian noise. The device is eventually tested with an RF data transmission setup, where a communication node with an internal temperature sensor is powered. Every 2 min, a data transmission at 868 MHz is performed by the sensor node supplied by the e-VEH, and received at a distance of up to 15 m.

Nonlinear restoring force of spring with stopper for ferroelectric dipole electret-based electrostatic vibration energy harvesters

Previously, we succeeded in developing a new electret [termed a ferroelectric dipole electret (FDE)] having an extremely high electric field using a polarized ferroelectric material. However, the pull-in, in which an oscillator sticks to the FDE under its strong electrostatic force, poses a problem for practical vibration energy harvesters. In this study, we propose use of nonlinear restoring force of a spring with a stopper in order to prevent pull-in for FDE-based vibration energy harvesters. The spring with a stopper was designed using a finite element method (FEM) analysis such that the restoring force of the spring will exceed the electrostatic force of the FDE. The proposed harvester combines the FDE and the spring successfully, and generated electricity without the pull-in. It also showed the highest figure of merit of output power and wide frequency band when compared with other available electret-based vibration energy harvesters.

A 100nW Power Overhead Load Interface for Electrostatic Vibrational Energy Harvester with a High Biasing Voltage

Electrostatic vibrational energy harvester (e-VEH) appeals to the ever expanding applications of self powered sensors. The capacitive transduces and their conditioning circuits are extensively studied in literature, yet usually lacking a load interface that manages that energy extraction from the harvester and efficiency delivers it to a load. A load interface, as well as its controller, designed for Bennet's Doubler conditioning circuit is presented in this work. The energy extraction from the harvester is achieved through a multiple energy-shot transfer process to minimize the resistive energy loss. The load interface controller simulation shows a low power overhead of less than 100nW making it adequate for practical implementation.

静電気力を考慮した静電型エナジーハーベスタのSPICE等価回路モデル

In this paper, we present a SPICE equivalent circuit model of an electrostatic vibration energy harvester. The SPICE model consists of a generation part and a mass-spring system part considering electrostatic force. The change in capacitance between electrodes with the position of the mass is analyzed by FEM analysis and is applied for the generation part. For investigating the characteristics of the actual harvester, the harvester was fabricated and charged by a -200 V monopolar potential. The maximum output power of the monopolar-charged harvester is 1.1 µW with applied vibration amplitude of 350 µmpp at 323.3 Hz with 0.8 MΩ optimum load resistance. We confirmed that the examined and analyzed results indicate high correspondence.

Electrostatic vibration energy harvester using an electret-charged mems transducer with an unstable auto-synchronous conditioning circuit

This paper reports for the first time experiments using an electrostatic vibration energy harvester comprised of a low voltage electret-charged MEMS transducer joined to an unstable autosynchronous conditioning circuit with rectangular charge-voltage characteristic, also known as the Bennet's doubler conditioning circuit. The experimental results show that the electret voltage, even if of low value, can be used as the necessary pre-charge for these type of electrostatic vibration energy harvesters. Also, the use of such a conditioning circuit with a low-voltage electret capacitive MEMS tranducer instead of the previously-reported conditioning circuits with direct connection to load or through a rectifier, can be advantageous in terms of maximal harvested power for a low-voltage electret, showing up to 95% higher converted power.