This paper reviews the state-of-the art in vibration energy harvesting for wireless, self-powered microsystems. Vibration-powered generators are typically, although not exclusively, inertial spring and mass systems. The characteristic equations for inertial-based generators are presented, along with the specific damping equations that relate to the three main transduction mechanisms employed to extract energy from the system. These transduction mechanisms are: piezoelectric, electromagnetic and electrostatic. Piezoelectric generators employ active materials that generate a charge when mechanically stressed. A comprehensive review of existing piezoelectric generators is presented, including impact coupled, resonant and human-based devices. Electromagnetic generators employ electromagnetic induction arising from the relative motion between a magnetic flux gradient and a conductor. Electromagnetic generators presented in the literature are reviewed including large scale discrete devices and wafer-scale integrated versions. Electrostatic generators utilize the relative movement between electrically isolated charged capacitor plates to generate energy. The work done against the electrostatic force between the plates provides the harvested energy. Electrostatic-based generators are reviewed under the classifications of in-plane overlap varying, in-plane gap closing and out-of-plane gap closing; the Coulomb force parametric generator and electret-based generators are also covered. The coupling factor of each transduction mechanism is discussed and all the devices presented in the literature are summarized in tables classified by transduction type; conclusions are drawn as to the suitability of the various techniques.

/pdf/energy-harvesting-vibration-sources-for-microsystems-46t885pwdp.pdf

Energy harvesting vibration sources for microsystems applications

Power consumption is forecast by the International Technology Roadmap of Semiconductors (ITRS) to pose long-term technical challenges for the semiconductor industry. The purpose of this paper is threefold: (1) to provide an overview of strategies for powering MEMS via non-regenerative and regenerative power supplies; (2) to review the fundamentals of piezoelectric energy harvesting, along with recent advancements, and (3) to discuss future trends and applications for piezoelectric energy harvesting technology. The paper concludes with a discussion of research needs that are critical for the enhancement of piezoelectric energy harvesting devices.

/pdf/powering-mems-portable-devices-a-review-of-non-regenerative-30r5zxggdp.pdf

Powering MEMS portable devices—a review of non-regenerative and regenerative power supply systems with special emphasis on piezoelectric energy harvesting systems

Power harvesting refers to the practice of acquiring energy from the environment which would be otherwise wasted and converting it into usable electric energy. Much work has been done on studying the optimal AC power output, while little has considered the AC–DC output. This article investigates the optimal AC–DC power generation for a rectified piezoelectric device. In contrast with estimates based on various degrees of approximation in the recent literature, an analytic expression for the AC–DC power output is derived under steady-state operation. It shows that the harvested power depends on the input vibration characteristics (frequency and acceleration), the mass of the generator, the electrical load, the natural frequency, the mechanical damping ratio and the electromechanical coupling coefficient of the system. An effective power normalization scheme is provided to compare the relative performance and efficiency of devices. The theoretical predictions are validated and found to be in good agreement with both experimental observations and numerical simulations. Finally, several design guidelines are suggested for devices with large coupling coefficient and quality factor.

/pdf/analysis-of-power-output-for-piezoelectric-energy-harvesting-24a8woaetg.pdf

Analysis of power output for piezoelectric energy harvesting systems

This paper presents the latest progress in kinetic energy harvesting for wide applications ranging from implanted devices and wearable electronic devices to mobile electronics and self-powered wireless network nodes. The advances in energy harvesters adopting piezoelectric and electromagnetic transduction mechanisms are presented. Piezoelectric generators convert mechanical strain on the active material to electric charge while electromagnetic generators make use of the relative motion between a conductor and a magnetic flux to induce charge in the conductor. The existent kinetic piezoelectric generators including human-powered and vibration-based devices are comprehensively addressed. In addition, the electromagnetic generators which include resonant, rotational, and ?hybrid? devices are reviewed. In the conclusion part of this paper, a comparison between the transduction methods and future application trends is given.

Kinetic Energy Harvesting Using Piezoelectric and Electromagnetic Technologies—State of the Art

Closed-loop turbulence control is a critical enabler of aerodynamic drag reduction, lift increase, mixing enhancement, and noise reduction. Current and future applications have epic proportion: cars, trucks, trains, airplanes, wind turbines, medical devices, combustion, chemical reactors, just to name a few. Methods to adaptively adjust open-loop parameters are continually improving toward shorter response times. However, control design for in-time response is challenged by strong nonlinearity, high-dimensionality, and time-delays. Recent advances in the field of model identification and system reduction, coupled with advances in control theory (robust, adaptive, and nonlinear) are driving significant progress in adaptive and in-time closed-loop control of fluid turbulence. In this review, we provide an overview of critical theoretical developments, highlighted by compelling experimental success stories. We also point to challenging open problems and propose potentially disruptive technologies of machine learning and compressive sensing.

Closed-Loop Turbulence Control: Progress and Challenges

https://www.princeton.edu/~gasdyn/Eelmovies/eel.pdf

Energy harvesting eel

The wake of a flexible flapping membrane has been investigated using flow visualization and particle image velocimetry. The aspect ratio of the flexible membrane was 0.27. The Strouhal number (St) was varied from 0.44 to 0.16, corresponding to Reynolds numbers between 2.2x105 and 4.1 x105. Under these conditions, a wake structure was observed that resembles the 2P structure characteristic of flapping foils but containing at least four vortex pairs per cycle. For St > 0.35, vorticity contour plots suggest that each multiple vortex pair structure has approximately zero net circulation; whereas for lower St, vorticity of one sign is diminished such that the wake begins to resemble the classical von Karman vortex street. A mean wake velocity profile measured at the mid-span of the membrane suggests a net drag for all flow conditions considered, with the minimum drag occurring for St = 0.35. However, preliminary flow visualization experiments with an increased aspect ratio of 0.45 show a simpler 2S-type structure resembling a reverse von Karman vortex street for which greater efficiency in thrust production is expected.

Flapping Membranes for Thrust Production

Preliminary Velocity Measurements in the Wake of a Submarine Model

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Energy harvesting eel

Flapping Membranes for Thrust Production

Preliminary Velocity Measurements in the Wake of a Submarine Model