Applications of Piezoelectric Micro-Power Generators

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This entry is adapted from the peer-reviewed paper 10.3390/nanoenergyadv3020005

Mechanical power is the most common type of external energy that can be converted into useful electric power. Because of its strong electromechanical coupling ability, the piezoelectric mechanism is a far more successful technique for converting mechanics energy to electrical energy when compared to electrostatic, electromagnetic, and triboelectric transduction systems. The scientific community has maintained a strong interest in piezoelectric micro-power generators because of their great potential for powering a sensor unit in the distributed network nodes. A national network usually has a large mass of sensor units distributed in each city, and a self-powered sensor network is eagerly required.

piezoelectric micro-power piezoelectric effect energy harvesting materials

1. Introduction

Most piezoelectric harvesting systems produce a power output within a range from 1 μw to 1 mw, and the primary use of piezoelectric energy capture is to supply small or micro-energy for microscale electronics such as implantable biomedical devices, wireless sensor nodes, and portable electronics. By utilizing piezoelectric energy capture, these devices can operate on a permanent, self-sufficient power supply without the need for maintenance or replacement. Furthermore, this independent power supply enables electronic equipment to be incorporated in structures or deployed in far-off places. With the recent increase in low-power electronics, piezoelectric energy harvesting has received substantial interest from the worldwide research community.

The voltage generated by an energy harvester exhibits random variations with dual-polarity voltage peaks, which can be attributed to the stochastic characteristics of the input vibration source. To obtain an effective power output, it is necessary to convert random variation voltage to DC voltage with a single polarity and controllable voltage amplitude. Piezoelectric nanogenerators (PENGs) can be used to generate electrical power, which can be stored for extended periods and used to power various devices. To accomplish this, a bridge rectifier is typically used to convert the AC output of the PENG into DC, which is then used to charge a capacitor ^[1]. Once the capacitor has been charged to a sufficient level, it can be connected to an external storage device for the long-term storage of electrical energy. PENGs have been employed in a variety of applications, including self-powered systems for smart homes, as well as deformation detection and human-motion monitoring.

2. Piezoelectric Micro-Power from Wind

Recently, many researchers tried to harvest wind energy using flexible piezoelectric flags. In 2013, Kim et al. first investigated the flapping dynamics of an inverted flag to better meet the requirements of energy harvesting ^[2]. Orrego et al. then studied the wind-energy-harvesting performance of inverted piezoelectric flags in controlled and uncontrolled wind conditions ^[3]. The inverted flag configuration offers several advantages, including the ability to generate sustained power across a wide range of wind speeds, tune resonance by adjusting bending stiffness, and self-oscillate at desired wind speeds by adjusting length. According to a report, piezoelectric flags measuring 60 mm and 100 mm in length produced a peak power output of 1–5 mW/cm³ at a 5–9 m/s wind speed and 0.1–0.4 mW/cm³ at a 2.5–4.5 m/s wind speed, respectively.

Zhang et al. created a rotational piezoelectric-energy harvester that harvests wind energy via the impact-induced vibration of a piezoelectric (PVDF) beam ^[4]. Three piezoelectric cantilever beams are integrated in an exterior casing. To boost energy harvesting conversion efficiency, a flexible piezoelectric material (PVDF) with a significant deformation response and a wide range of vibration frequencies are used as the energy harvesting element. On opposite ends of the rotating shaft, a fan blade and a turntable are affixed. The turntable’s blade count is three. However, it is malleable. The harvester’s functioning is straightforward. The fan blade rotates as air rushes past the harvester, and the turntable is driven by the revolving shaft. At this point, the turntable contacts the piezoelectric (PVDF) beam, causing periodic oscillations in the beam. As a result, an electric field is created. A maximum rms voltage of 160.2 V and a maximum output power of 2566.4 μW were obtained at the wind speed of 14 m/s.

Gao et al. introduced a self-assembled film of piezoelectric monolayer BaTiO₃, created using a topochemical conversion procedure to produce BaTiO₃ micro-platelets ^[5]. An interfacial technique was then used to assemble the oriented monolayer BaTiO₃ film, which was subsequently spin-coated with polydimethylsiloxane (PDMS) to create a flexible piezoelectric generator. The generator was tested for its output performance by demonstrating its application in harvesting wind energy. It was fixed onto a support and tapped by a paddle attached to one of the blades of a windmill rotating in the wind. The maximum output power of the piezoelectric generator reached 0.021 mW at an external load of 80 MW, which was sufficient to light an LCD. The generator produced voltage and current outputs of up to 2.3 V and 96 nA, respectively. Table 1 summarizes various windmill-style piezoelectric harvesters, including their transducer type, generator material, dimensions, wind speed, and output power.

Table 1. Summary of various windmill-style piezoelectric energy harvesters.

Device	Transducer Type	Generator Material	Dimensions	Wind Speed (m s⁻¹)	Output Power	References
Fan-type windmill	Piezoelectric bimorph	PZT-5H	60 × 20 × 0.6 mm³	4.47	7.5 mW	^[6]
Vane-type vertical windmill	Piezoelectric bimorph	PZT-5H	96 × 107 × 66 mm³	4.47	5 mW	^[7]
Fan-type windmill	Piezo-composite beam	PZT-5H	72 × 12 × 0.5 mm³	less than 5 m s⁻¹	8.5 mW	^[8]
Contact-less windmill	Piezomagnetic generator	PZT-5A	80 × 80 × 175 mm³	4.47	4 mW	^[9]
Fan-type windmill	Piezomagnetic generator	PZT-5A	31 mm diameter	0.9	363 μW	^[10]

3. Piezoelectric Micro-Power from Liquid Flow

Oceans cover 70% of the Earth’s surface, making wave energy an appealing option for energy harvesting. Notably, the ocean is not only abundant in natural resources such as oil and gas, but also in potential energy, which might be an even more benign source of energy than wind ^[11]. It has been estimated that ocean wave energy can provide as much as 885 TWh of total electricity ^[12].

Taylor et al. developed an energy harvesting system that converts kinetic energy into electricity to power remote sensors and robots using a slender strip of a PVDF piezoelectric polymer that mimics the movement of an eel in water ^[13]. In non-turbulent flow, the system’s bluff body periodically releases vortices on both sides, generating eddy currents that cause the strip to oscillate and deform, producing an AC voltage output. The strip consists of three layers, with a central inactive layer and two active layers of piezoelectric material bonded to each side. The system includes five of these strips, each measuring 132 cm in length, 15.24 cm in width, and 400 µm in thickness. At a flow rate of 1 m/s, the system can generate 1 W of power with an energy conversion efficiency of approximately 33%. The system can charge batteries or capacitors of remote sensor arrays and robot groups, thereby extending their mission life as long as there is flowing water in the ocean.

Mutsuda et al. devised a flexible piezoelectric device (FPED) for harvesting wave energy ^[14]. Using a spray nozzle, the piezoelectric PVDF film was coated on an elastic substrate with electrodes. The elastic material is exceptionally resilient and can survive after experiencing the tremendous bending and deterioration induced by ocean waves and currents. The FPED’s power density can reach 2.6 μW/cm³. Hwang et al. reported a PEH based on sway motion. This is primarily composed of a piezoelectric cantilever and a magnet ^[15]. A catheter with a metal ball is positioned above the magnet and functions at the module’s tip. The system was evaluated using a setting that simulates ocean waves. The maximum output voltage for a device with dimensions of 3.8 cm × 1.9 cm × 0.2 cm at a simulated wave frequency of 0.5 Hz is 21.1 V, and the power density is 47.85 μW m⁻³.

Utilizing the synergy between the inherent contact electrification of the raindrops and the piezoelectric effect will generate considerable energy. Xu et al. designed a leaf-mimic rain energy harvester, which is composed of a PVDF-based piezoelectric energy generator (PEG) and an FEP-based triboelectric energy generator (FEG) ^[16]. When raindrops fall on the surface of the device, the contact electrification energy is first collected by the FEG. Then, the induced vibration energy will be harvested by the PEG. Under synergy, the raindrop energy harvester is able to generate a high-output power density of 82.66 Wm⁻², which is enough to supply the power needed to run a temperature sensor.

4. Piezoelectric Micro-Power from Vehicles

The dissipation of energy in various vehicle components, particularly the suspension system, is a significant factor affecting the fuel efficiency of automobiles. A total of 10–16% of the fuel energy is utilized to propel the vehicle, which overcomes opposition from road friction and air drag, whilst the majority is lost as heat and mechanical energy ^[17]. Researchers are investigating the viability of energy harvesting from the vehicle suspension system and tires, which contain rich vibration and force profiles, as a means of utilizing the unused energy of automobiles.

Singh et al. investigated the use of an inertial vibrating energy harvester as a power source for a tire-mounted sensor module ^[18]. They developed a piezoelectric device using a bimorph cantilever with high-density PZT-ZNN piezoelectric layers and two mechanical stoppers to minimize mechanical strain and generate electricity from the radial vibrations of the tire. During the design process, the team prioritized broadband operation, low weight, and small size. The resulting beam has dimensions of of 25 × 5 × 0.4 mm³ and a tip mass of 11.4 g. During testing, the device produced 31 μW of power across a resistive load of 330 kΩ at 80 Hz and 0.4 g RMS base excitation.

Hong et al. developed a piezoelectric harvester designed for use on roads, called a road-capable piezoelectric harvester (RCPH) ^[19]. The resulting output voltage was measured to be 18 V_max, with an output power of 1150 mW_max and a power density of 1.15 mW/cm², when a load resistance level of 910 Ω was applied. The electrical energy generated by the 1 cm landfilled module was sufficient to power 4 delineators for 40 seconds on a test route. This suggests that the piezoelectric modules could be connected to an emergency lighting system to provide the continuous electricity generated by passing vehicles, making this technology suitable for use on real roads.

Wang et al. developed a PZT-based energy harvester on the jet engine that harnesses the rotational mechanical energy for powering wireless engine monitors ^[20]^[21]^[22]. The designed high-efficiency compressive-mode piezoelectric energy harvester generates energy during the periodic changes in gravity induced by the rotation motion of the engine. The energy harvester demonstrated a high-power output (78.87 mW), a broad bandwidth (22.5 Hz), and strong reliability (2100 RPM).

5. Piezoelectric Micro-Power from Body Movement

Kim et al. created a wearable piezoelectric generator, developed using boron nitride nanosheets, to convert mechanical energy from human body movement into electrical energy ^[23]. The generator was capable of producing a peak output voltage of 22 V and output power of 40 μW under a periodic mechanical push force of 80 kg with a power density of 106 μW/cm³. Moreover, the generator was attached to different human body parts and produced output voltages of 2.5 V at the foot, 1.98 V at the elbow, 0.48 V at the neck, 0.75 V at the wrist, and 1.05 V at the knee in response to differential human movement.

Jung et al. constructed a a piezoelectric generator based on polyvinylidene fluoride (PVDF) that can harvest low-frequency biomechanical energy from body movement for use in wearable devices ^[24]. The device was able to generate an output power density of 3.9 mW/cm², which is sufficient to power 476 LED bulbs. During testing, the generator produced an average output voltage of 45 V and an output current of 225 μA at 35 Hz. The researchers also incorporated the technology into a shoe insole and a watch band. The insole generator generated an average output voltage of 14 V and an average output current of 18 μA, while the watch generator produced an average output voltage of 22 V and an average output current of 50 μA while running. Park et al. presented a nontoxic ultra-flexible piezoelectric energy harvesters based on piezoelectric composite fibers, in which lead-free (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ (BCTZ) nanoparticles (NPs) are dispersed in poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) fibers using a facile electrospinning process ^[25]. Particularly, post-treatments, including post-crystallization and extra-poling, were utilized to improve the piezoelectric performance of the piezoelectric composite fiber-based membrane. Following post-treatments, the V_OC and I_SC produced by the piezoelectric composite fiber membrane rose by 6.6 and 15.7 times, respectively. Regarding the post-treated piezoelectric composite fiber membrane containing 20 wt% BCTZ NPs, the output voltage and current reached 36.5 V and 1.09 μA, respectively, much higher than the previously reported results for lead-free, piezoelectric composite, fiber-type energy harvesters.

Shi et al. fabricated a flexible PENG from electrospun fiber mats containing 15 wt% BaTiO₃ NPs, 0.15 wt% graphene nanosheets, and PVDF ^[26]. During the finger pressing-releasing operation, the PENG created a peak voltage of 112 V, which powered an electric timepiece and illuminated 15 LEDs. In addition, it gathered energy from human movements such as finger tapping, wrist flexion, and foot stepping. Under the conditions of wrist flexion and finger tapping, the output voltage reached 7.7 V and 7.5 V, respectively. When the PENG was placed under the foot hell and toe, the PENG generated a maximum voltage of 7.8 V and 2.8 V, respectively. The heel exerted greater pressure than the toe, resulting in an increased output voltage. Through an appropriate structural design, the hard ceramic block can also be used as a wearable device for energy harvesting. Inspired by two branches of the hyoid bone of the woodpecker, Wang et al. designed a two-layer band piezoelectric energy harvester for powering smartwatches and wristbands via the movement mode and the impact mode. The energy harvester is assembled by integrating multiple PZT ceramic bulks onto the watchband. The maximum output power can reach 12.25 mW during normal walking, while the maximum output power can reach 15.4 mW in impact mode, which can meet the basic working requirements of watches and bracelets.

6. Implantable Devices

Piezoelectric energy harvesting has emerged as a promising option to eliminate the need for batteries in wearable electronics, thanks to the development of innovative methods and materials, and advancements in low-power electronic technology. In addition, implantable active medical devices such as cardiac pacemakers, cardioverter defibrillators, cardiac monitors, and neurological brain stimulators can also benefit from this technology ^[27]. By incorporating energy harvesting into these devices, the need for later maintenance operations can be significantly reduced, which can, in turn, lower the associated costs and minimize potential risks. This demonstrates the potential of piezoelectric energy harvesting in powering a range of devices, from portable electronics to implantable medical devices.

Jiang et al. built a flexible piezoelectric array for ultrasonic energy harvesting using a PZT/epoxy composite ^[28]. Under ultrasonic excitation, the designed apparatus generated a constant power output. There was an output voltage of 2.1 V_p-p and a current of 4.2 µA. The produced electricity might be stored in capacitors and utilized to power commercial LEDs. In vitro experiments revealed that the output signals exhibit a weak attenuation of about 15% after penetrating a simulating implanted tissue with 14 mm thickness.

Ansari et al. exploited heartbeat vibrations to power a lead-free pacemaker using a fan-folded structure of piezoelectric beams ^[29]. Multiple piezoelectric beams were piled on top of one another. The created energy-collecting gadget, which measured at 2 cm × 0.5 cm × 1 cm, could operate at extremely high frequencies. The fan-folded shape allowed for the reduction in frequency to the appropriate levels, and the power production was more than 10 µW, which was sufficient to power a lead-free pacemaker without external power.

Deterre et al. introduced a very compact piezoelectric energy harvester that generates electricity from normal blood pressure fluctuations to operate a pacemaker without leads ^[30]. The gadget was a spiral-shaped piezoelectric beam contained within an ultra-flexible package with a 10 μm diaphragm. The diameter of the harvester was 6 mm and its volume was 21 mm³. The experimental results revealed that a gadget with an optimal design generates a power density of 3 μJ cm⁻³ per heartbeat. Hong et al. developed a wood-templated transmuscular piezoelectric energy harvester that transfers the ultrasonic energy to power implantable wireless devices. Using wood as a template, unidirectionally aligned layered piezoelectric ceramics are obtained, and the flexibility of the device can be improved by adding polymers into the matrix. Under the ultrasonic stimulation conducted ex vivo, the wood-templated piezoelectric ultrasonic energy harvester exhibits a maximum open-circuit voltage of 21 V, and a short-circuit current of 2 mA output. While in vivo, the device will generate 4.5 V output, which can supply power for most implants.

7. Piezoelectric Micro-Power Generator for Sensing

Piezoelectric materials have applications beyond energy harvesting, including self-powered sensing. The piezoelectric equation provides a quantitative relationship between the electrical output of the material and the applied strain or stress, which means that piezoelectric nanogenerators (PENGs) can be used as sensors to detect and measure stimuli such as pressure, vibration, acceleration, and acoustic waves. Unlike resistive or capacitive sensors, PENGs are positive devices for which batteries are not required.

According to Deng et al., a robotic arm could be remotely controlled by combining PENG sensing, signal transmission, and executive control ^[31]. The functional layer of the PENG utilized cowpea-structured PVDF/ZnO nanofibers, which increased mechanical flexibility and electrical output, allowing the sensors to respond quickly and sensitively to bending angle motions. When the sensors were attached to the finger knuckle, they converted the bending action of the finger into electrical impulses, allowing the robotic hand to replicate the same gesture as a human hand through a peripheral circuit module.

To achieve a personalized healthcare system, it is crucial to monitor vital signs. This allows for an accurate assessment of an individual’s physiological state and provides a baseline for identifying related medical conditions. Hong et al. developed a flexible piezoceramic composite sensor that utilizes a kirigami structure to detect joint motions and differentiate between different motion modes ^[32]. They employed a modified, template-assisted, sol-gel technique for the fabrication of the composite, and used a two-dimensional honeycomb piezoceramic kirigami as the main sensor element. The developed kirigami structure improves the piezoelectric characteristics and provides high-dimensional anisotropy. This technique has potential applications in the development of flexible electronics for health-monitoring devices, as well as the prevention and rehabilitation of upper-extremity musculoskeletal problems.

Signals related to the biomechanics of the human body could also provide vital medical information. Yao et al. recently developed a wearable piezoelectric nanogenerator (PENG) that can measure biomechanical signals related to the human body, providing valuable medical information. The PENG is made of 3D-printable piezoelectric nanocomposites manufactured using additive manufacturing techniques and has exceptional responsiveness and compliance ^[33]. To measure punch force during boxing activities, the researchers embedded the PENG lattice with microarchitectures and placed it inside a boxing glove. The signals generated by the PENG were transmitted to a terminal using an embedded WiFi module. By analyzing the electrical signal collected from the specific PENG electrode, the researchers were able to plot the spatial distribution of force corresponding to a direct blow or a right hook on the receiving device. This application demonstrated the PENG’s ability to monitor the hand’s reaction force during boxing activities. In another study, Fuh et al. successfully demonstrated foot pressure mapping using a sensor array composed of PENGs ^[34]. The researchers used 2D electrode patterns and thin films of piezoelectric polymers to create complex 3D piezoelectric microsystems. The self-powered fabricated device is capable of significantly boosting piezoelectric output and can be directly applied to foot pressure measurement and human motion monitoring. When the device was subjected to pressure at various locations and levels, it exhibited a variety of performances.

Existing piezoelectric elements are fabricated in fixed geometries such as planar sheets, disks, or tubes within strict laboratory environment conditions and high-quality substrates. They cannot conformally fit 3D free-form surfaces, which significantly confines the practical application value of piezoelectric films. Liu et al. developed a facile method to conformally fabricate piezoelectric thin films onto 3D free-form objects. By using flame treatment, the surface energy is significantly reduced, and the films can easily be assembled onto any surface. Thus, the piezoelectric thin films can be fabricated onto any surface on-site.

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