High-Performance Piezoelectric Energy Harvesters and Their Applications

The last decade has witnessed significant advances in energy harvesting technologies as a possible alternative to provide a continuous power supply for small, low-power devices in applications, such as wireless sensing, data transmission, actuation, and medical implants. Piezoelectric energy harvesting (PEH) has been a salient topic in the literature and has attracted widespread attention from researchers due to its advantages of simple architecture, high power density, and good scalability. This paper presents a comprehensive review on the state-of-the-art of piezoelectric energy harvesting. Various key aspects to improve the overall performance of a PEH device are discussed, including basic fundamentals and configurations, materials and fabrication, performance enhancement mechanisms, applications, and future outlooks.

A comprehensive review on piezoelectric energy harvesting technology: Materials, mechanisms, and applications

Superelastic conducting fibers with improved properties and functionalities are needed for diverse applications. Here we report the fabrication of highly stretchable (up to 1320%) sheath-core conducting fibers created by wrapping carbon nanotube sheets oriented in the fiber direction on stretched rubber fiber cores. The resulting structure exhibited distinct short- and long-period sheath buckling that occurred reversibly out of phase in the axial and belt directions, enabling a resistance change of less than 5% for a 1000% stretch. By including other rubber and carbon nanotube sheath layers, we demonstrated strain sensors generating an 860% capacitance change and electrically powered torsional muscles operating reversibly by a coupled tension-to-torsion actuation mechanism. Using theory, we quantitatively explain the complementary effects of an increase in muscle length and a large positive Poisson’s ratio on torsional actuation and electronic properties.

Hierarchically buckled sheath-core fibers for superelastic electronics, sensors, and muscles

Soft robotics inspired by the movement of living organisms, with excellent adaptability and accuracy for accomplishing tasks, are highly desirable for efficient operations and safe interactions with human. With the emerging wearable electronics, higher tactility and skin affinity are pursued for safe and user-friendly human-robot interactions. Fabrics interlocked by fibers perform traditional static functions such as warming, protection, and fashion. Recently, dynamic fibers and fabrics are favorable to deliver active stimulus responses such as sensing and actuating abilities for soft-robots and wearables. First, the responsive mechanisms of fiber/fabric actuators and their performances under various external stimuli are reviewed. Fiber/yarn-based artificial muscles for soft-robots manipulation and assistance in human motion are discussed, as well as smart clothes for improving human perception. Second, the geometric designs, fabrications, mechanisms, and functions of fibers/fabrics for sensing and energy harvesting from the human body and environments are summarized. Effective integration between the electronic components with garments, human skin, and living organisms is illustrated, presenting multifunctional platforms with self-powered potential for human-robot interactions and biomedicine. Lastly, the relationships between robotic/wearable fibers/fabrics and the external stimuli, together with the challenges and possible routes for revolutionizing the robotic fibers/fabrics and wearables in this new era are proposed.

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Functional Fibers and Fabrics for Soft Robotics, Wearables, and Human-Robot Interface.

Promoting smart cities into the 5G era with multi-field Internet of Things (IoT) applications powered with advanced mechanical energy harvesters

Based on contact heat transfer, a novel method to increase power density of piezoelectric transformers is proposed. A heat transfer structure is realized by directly attaching a dissipater to the piezoelectric transformer plate. By maintaining the vibration mode of the transformer and limiting additional energy losses from the contact interface, an appropriate design can improve power density of the transformer on a large scale, resulting from effective suppression of its working temperature rise. A prototype device was fabricated from a rectangular piezoelectric transformer, a copper heat transfer sheet, a thermal grease insulation pad, and an aluminum heat radiator. The experimental results show the transformer maintains a maximum power density of 135 W/cm3 and an efficiency of 90.8% with a temperature rise of less than 10°C after more than 36 h, without notable changes in performance.

Power density of piezoelectric transformers improved using a contact heat transfer structure

This letter reports a piezoelectric vibration energy harvester that uses the local lateral resonant modes of a folded structure to widen the operation frequency band. In addition, energy conversion efficiency is improved. A prototype energy harvester was fabricated and tested. The output power achieved two power peaks: 0.43 mW at 97 Hz and 6.64 mW at 120.9 Hz. The output power remained above 20 μW within the operation frequency band that ranged from 88 Hz to 177 Hz when the energy harvester was driven with a vibration of 0.7 g peak acceleration. The output power remained higher than half of one of the maximum power peaks (0.43 mW) between 95 Hz and 101 Hz. Meanwhile, it remained higher than half of the other maximum power peak (6.64 mW) between 120.5 Hz and 123.8 Hz.

Harvesting vibration energy using two modal vibrations of a folded piezoelectric device

We demonstrate a new electromagnetic resonant cavity wind energy harvester combined with dual-branch reed and turning fork vibrator to harvest wind energy for wireless sensor nodes. Compared with electromagnetic structures in the past, the proposed magnetic circuit has a better linearity and a magnetic loop that more effectively increases the rate of the change of magnetic flux. Moreover, the motion of the free magnet is directed vertically to the direction of the magnetic pole for a zero resistance startup mode and a compact construction. We also designed a tuning fork-like reed structure to reduce system losses. To achieve maximum output power, we defined a virtual resistance to deal with the decrease of the electromotive in the circuit caused by the Lorentz force between the magnet and the induced magnetic field of the coils when the circuit is turned on. The dimensions of the wind inlet were 60 mm × 60 mm. The maximum output power and conversion efficiency can reach 56 mW at wind speed of 20.3 m/s and 2.3% at wind speed of 4 m/s, respectively, with wind speed from 3.2 m/s to 21.2 m/s. The experiments show that this electromagnetic wind energy harvester has high output power and wide wind speed range.

Electromagnetic resonant cavity wind energy harvester with optimized reed design and effective magnetic loop

Small piezoelectric fibers wound with helical electrodes on their outer surface will produce torsional displacement. A piece of piezoelectric tube with an outer diameter of 1mm, a wall thickness of 0.1mm, and an effective axial length of 40mm was used as a prototype actuator’s body, while two pieces of copper wires of 40μm diameter were used as the electrodes. After poled with 2kV∕mm electric field strength, the prototype actuator produces 1.7° torsional angle when it was driven from −500to500V. A resonant frequency as high as 10kHz was observed on this actuator.

Small torsional piezoelectric fiber actuators with helical electrodes

A major deficiency in piezoelectric actuator performance is caused by hysteresis and creep effects. Switched capacitor charge pump control was previously proved to be an effective way to reduce hysteresis. In this work, creep character of a piezoelectric stack under charge pump control is investigated and modeled. Experiments show that the creep can be reduced by about 77% compared with the one under voltage control. The creep factor denoting the degree of creep shows hysteresis loops similar to displacement outputs under voltage control.

Yong Bin Liu

Papers

Power density of piezoelectric transformers improved using a contact heat transfer structure

Harvesting vibration energy using two modal vibrations of a folded piezoelectric device

Electromagnetic resonant cavity wind energy harvester with optimized reed design and effective magnetic loop

Small torsional piezoelectric fiber actuators with helical electrodes

Note: creep character of piezoelectric actuator under switched capacitor charge pump control.