The field of power harvesting has experienced significant growth over the past few years due to the ever-increasing desire to produce portable and wireless electronics with extended lifespans. Current portable and wireless devices must be designed to include electrochemical batteries as the power source. The use of batteries can be troublesome due to their limited lifespan, thus necessitating their periodic replacement. In the case of wireless sensors that are to be placed in remote locations, the sensor must be easily accessible or of a disposable nature to allow the device to function over extended periods of time. Energy scavenging devices are designed to capture the ambient energy surrounding the electronics and convert it into usable electrical energy. The concept of power harvesting works towards developing self-powered devices that do not require replaceable power supplies. A number of sources of harvestable ambient energy exist, including waste heat, vibration, electromagnetic waves, wind, flowing water, and solar energy. While each of these sources of energy can be effectively used to power remote sensors, the structural and biological communities have placed an emphasis on scavenging vibrational energy with piezoelectric materials. This article will review recent literature in the field of power harvesting and present the current state of power harvesting in its drive to create completely self-powered devices.

Energy harvesting technologies have been explored by researchers for more than two decades as an alternative to conventional power sources (e.g. batteries) for small-sized and low-power electronic devices. The limited life-time and necessity for periodic recharging or replacement of batteries has been a consistent issue in portable, remote, and implantable devices. Ambient energy can usually be found in the form of solar energy, thermal energy, and vibration energy. Amongst these energy sources, vibration energy presents a persistent presence in nature and manmade structures. Various materials and transduction mechanisms have the ability to convert vibratory energy to useful electrical energy, such as piezoelectric, electromagnetic, and electrostatic generators. Piezoelectric transducers, with their inherent electromechanical coupling and high power density compared to electromagnetic and electrostatic transducers, have been widely explored to generate power from vibration energy sources. A topical review of piezoelectric energy harvesting methods was carried out and published in this journal by the authors in 2007. Since 2007, countless researchers have introduced novel materials, transduction mechanisms, electrical circuits, and analytical models to improve various aspects of piezoelectric energy harvesting devices. Additionally, many researchers have also reported novel applications of piezoelectric energy harvesting technology in the past decade. While the body of literature in the field of piezoelectric energy harvesting has grown significantly since 2007, this paper presents an update to the authors’ previous review paper by summarizing the notable developments in the field of piezoelectric energy harvesting through the past decade.

Multifunctional material systems combine multiple functionalities in a single device in order to increase performance while limiting mass and volume. Conventional energy harvesting systems are designed to be added to a host structure in order to harvest ambient energy surrounding the system, but often cause undesirable mass loading effects and consume valuable space. Energy harvesting systems can benefit from the introduction of multifunctionality as a means of improving overall system efficiency. This paper presents the investigation of a novel multifunctional piezoelectric energy harvesting system consisting of energy generation, energy storage, and load bearing ability in a single device. The proposed self-charging structures contain piezoelectric layers for power generation, thin-film battery layers for energy storage, and a central metallic substrate layer, arranged in a bimorph configuration. Several aspects of the development and evaluation of the self-charging structure concept are reviewed. Details are provided on the fabrication of a piezoelectric self-charging structure. An electromechanical model is employed to predict the response of the harvester under harmonic base excitation. Experimentation is performed to confirm the ability of the device to simultaneously harvest and store electrical energy. Finally, both static and dynamic strength analyses are performed to determine the load bearing ability of the structure.

The recent proliferation of microscale devices has raised the issue of energy harvesting for replacing batteries that present maintenance and recycling problems. Particularly, piezoelectric seismic microgenerators offer the advantages of easy maintenance and high power output, but are very sensitive to frequency drifts that can dramatically decrease their performance. The purpose of the present article is to expose a technique to ensure that the harvester resonance frequency matches the base motion frequency, without any external intervention. The principles of the proposed method rely on ultralow-cost frequency sensing combined with an energy-efficient stiffness tuning, through the use of an additional actuator. Experimental results carried out to validate the model show that such an approach permits increasing the effective bandwidth of the structure by a factor of 4 in terms of mechanical vibrations and having a 100% frequency band gain in terms of total power output of the device (i.e., taking into account the energy spent by the actuation). The total energy produced by the harvesting device, taking into account the actuation cost, is discussed as well.

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