Micro-energy Harvesting Research Articles

Modeling of enhanced micro-energy harvesting of thermal ambient fluctuations with metallic foams embedded in Phase Change Materials

We present an enhanced micro-energy harvester to transform ambient thermal fluctuations into electricity. The design consists of a Thermoelectric Generator (TEG) joined to a thermal storage unit improved with a Phase Change Material within a metallic foam (pPCM). We show how the augmented effective conductivity of the heat storage unit multiplies the production of electric power through voltage generation in higher and shorter sprouts. The porosity of the metallic foam accelerates the heat transfer and permits higher volumes of the heat storage unit to be effective in harvesting more energy from the surroundings. The pPCM/TEG device is a robust and cost-effective means to optimize the output of TEG based systems to power low-consumption electronics. The potential of this design is demonstrated with examples of micro-energy harvesting under ambient thermal conditions in an aircraft and ground solar irradiation. In these applications, a single TEG module with a moderate merit figure is used and found that pPCM allows a substantial optimization of energy conversion. As an example, the pPCM/TEG devices produce about twenty times more electric energy at even small volume fractions of the foam ε=0.95 than PCM/TEG systems in solar micro-energy harvesting on ground conditions with low thermal gradients.

MEMS Vibrational Power Generator for Bridge Slab and Pier Health Monitoring

Micro energy harvesters (MEH) based on microelectromechanical systems (MEMS) are rapidly developing, providing a green and virtually infinite energy source. The electrostatic vibratory power generator outputs electric power when it vibrates, motivating us to apply it to vibrating civil infrastructures excited by ambient and daily traffic loadings. In this study, an innovative monitoring system utilizing MEH devices was proposed for detecting slab damage and pier scours for bridge structures. Its performance was numerically investigated with finite element models, where the damage in slabs was modeled with a reduced Young’s modulus and scours with fixed boundaries of inclined depth. It was shown that the powers generated at each MEH varied as the target structure’s modal frequency shifted and amplitude changed by damage or scour. A power generation index was proposed to identify slab damage and a reference-free method was introduced to detect uneven pier scours. Utilizing an electrostatic vibration-based MEH (MEMS vibrational power generator), this pioneering study showed that MEMS vibrational power generators can work as sensors for an infrastructure structural health monitoring system.

Thin-film thermoelectric generator based on polycrystalline SiGe formed by Ag-induced layer exchange

SiGe alloys are a promising material for highly reliable, human-friendly thin-film thermoelectric generators for micro-energy harvesting. However, it is difficult to obtain high performances at low thermal budgets in SiGe layers, especially in n-type materials. Ag-induced layer exchange enables the synthesis of Si1−xGex (x: 0–0.3) layers at 500 °C and dynamically controls the Fermi level owing to the self-organizing manner of impurity doping during the layer exchange. Intrinsic, p-type (hole concentration >1019 cm−3), and highly n-type (electron concentration >1020 cm−3) SiGe layers are obtained using pure Ag, B-doped Ag, and As-doped Ag, respectively. Owing to the high carrier concentrations, the thermoelectric power factor at room temperature exhibits high values: 230 μW m−1 K−2 for the p-type and 1000 μW m−1 K−2 for the n-type. The latter value is the highest reported power factor at room temperature for SiGe formed below 1000 °C. The dimensionless figure of merit is determined to be 0.19 from the power factor and the thermal conductivity of 1.6 W m−1 K−1. A thermoelectric generator fabricated with the low-temperature SiGe layers demonstrates a relatively large output for thin films (50 nm): 1.4 nW at room temperature with a temperature difference of 15 °C.

Biodegradable cellular polylactic acid ferroelectrets with strong longitudinal and transverse piezoelectricity

Polymers with electrically charged internal air cavities (ferroelectrets) reveal a pronounced piezoelectric response and are regarded as soft electroactive multi-functional materials. This work presents preliminary results on the preparation and piezoelectric effect of ferroelectrets based on the polylactic acid (PLA) polymer. A distinctive feature of the manufactured films is that they are biodegradable. After a microstructure modification of carbon dioxide (CO2) foamed PLA sheets by hot-pressing treatment and corona polarization, these cellular films reveal large piezoelectric d33 and d31 responses in both quasi-static and dynamic modes. For freshly charged films, the maximum quasi-static piezoelectric coefficients are about 600 pC/N (d33) and 44 pC/N (d31) for a relatively thick film of 360 μm and a nominal porosity of about 60%. During the first 20 days after polarization, the piezoelectric activity decreases by half compared to the primary value, but then remains almost unchanged for a long time. Due to an already established inherent biocompatibility of PLA polymers, these eco-friendly ferroelectrets can be potentially used in various biological applications such as biosensors and microenergy harvesters embedded in tissue and artificial muscles.

Figures of Merit for Piezoelectrochemical Energy-Harvesting Systems

Summary Mechanical micro-energy harvesting is a promising technology that could provide energy to small-scale sensors and other microelectronics without the need for traditional power sources. One such mechanism, called the piezoelectrochemical (PEC) effect, is of particular interest because many PEC systems have higher theoretical energy densities than other mechanical energy harvesters. This energy-harvesting mechanism has been demonstrated in multiple systems, including commonly used electrodes for lithium-ion batteries. However, due to the large variety of materials, it is difficult to consistently compare the utility of PEC systems, which makes the selection and optimization of PEC energy harvesters challenging. Here, we propose using figures of merit including short-circuit current ( Δ I S C ) , open-circuit voltage ( Δ V O C ) , fill factor ( F F ), and efficiency ( η E C ), which provide a unifying scheme to quantify the performance of different PEC systems. We experimentally validate this approach and demonstrate ways to increase voltage and current outputs of these harvesters.

Numerical modelling of micro energy harvesting systems based on piezoelectric composites polarized with interdigitated electrodes

This paper focuses on the numerical modelling of micro-energy harvesting systems(MEHSs) based on piezoelectric composites polarised with interdigitated electrodes (PCPIE). The system response and the harvested energy are numerically assessed using a multilayer piezoelectric shell finite element (FE) with a uniform fibre aligned electric field (UFAEF) in each active layer. Circuit and compatibility equations are included to take into account the presence of the electrical network. A state-space (SS) model is derived and used to evaluate the effect of electrical impedance on damping and natural frequencies, as well as dissipated energy/power. An energy harvester beam with a piezoelectric macro fibre composite (MFC) patch is first modelled with the developed tools. Numerical results are found to be in good agreement with experimental results reported in the literature. Finally, a MEHS consisting of a closed-box beam equipped with PCPIE devices bonded to its skin is analysed. The structural system is subjected to dynamic loading imposing oscillating displacements and deformations compatibles with those expected during service-life. Numerical results show the influence of the electrical impedance on system response, damping, natural frequencies, and electrical power.

Microenergy Harvesters Based on Fluorinated Ethylene Propylene Piezotubes

Energy harvesting from vibrations provides power to low‐energy‐consuming electronics for standalone and wearable devices as well as for wireless and remote sensing. In this contribution, compact tubular ferroelectret energy harvesters utilizing a single‐tube design are presented. Such single‐tube harvesters can be fabricated from commercially available fluorinated ethylene propylene (FEP) tubes with wall thicknesses of 25 and 50 μm, respectively, by mechanical deformation at elevated temperature. It is demonstrated that the generated power is highly dependent on parameters such as wall thickness, load resistance, and seismic mass. Utilizing a seismic mass of 80 g at resonance frequencies around 80 Hz and an input acceleration of 1 × g (9.81 m s−2 rms), output powers up to 300 μW can be reached for a transducer with 25 μm thick walls.

Research progress of piezoelectrets based micro-energy harvesting

<sec>In this paper, the progress of micro-energy harvesters by using piezoelectret-based transducers as a core element is reviewed, including basic physical principle and properties of piezoelectrets, and their applications in micro-energy harvesting. Piezoelectret is electret-based piezoelectric polymer with a foamed structure. The piezoelectric effect of such material is a synergistic effect of the electret property of the matrix polymer and the foam mechanical structure in the material. Piezoelectret, featuring strong piezoelectric effect, flexibility, low density, very small acoustic impedance and film form, is an ideal electromechanical material for lightweight flexible sensors and mechanical energy harvesters. The piezoelectret prepared by means of grid, template patterning, supercritical CO<sub>2</sub> assisted low-temperature assembly, lithography mold combined with rotary coating and hot pressing has regular voids and good piezoelectric properties. Piezoelectret has been used to harvest vibrational energy, human motion energy and sound energy. </sec><sec>According to the stress direction applied to the piezoelectrets, operating modes of energy harvesters can be divided into 33 and 31 modes. The vibrational energy harvesters based on piezoelectret are utilized to harvest medium frequency vibrational energy generated by factory machines, aircrafts, automobiles, etc. Such energy harvesters can generate considerable power even in a small size. Human motion energy harvesters are generally used to power wearable sensors. The high sensitivity, lightweight, and flexibility of the piezoelectret make such a material a promising candidate for harvesting human motion energy. Owing to very small acoustic impedance, high figure-of-merit, flat response in audio and low-frequency ultrasonic range, the piezoelectrets are more appropriate for acoustic energy harvesting in air medium than conventional PZT and ferroelectric polymer PVDF.</sec><sec>In the future, specific micro-energy harvesters using piezoelectrets as transduction material can be designed and fabricated according to the practical application environment, and their performance can be enhanced by using flexible connections of transduction elements.</sec>

Energy Harvesting from Piezoelectric Cantilever Beam with Different Shapes

This paper reviews the piezoelectric energy harvesting from mechanical vibration. The recent development in the microelectronic devices and wireless sensor networks (WSNs) requires continuous power source for better performance. Many researchers have been done to develop a permanent portable power source for microelectronic devices. Micro energy harvesting (MEH) consists of two basic elements; freely available energy and transducer. Energy is everywhere around us in different forms. The energy conversion ability of piezoelectric energy harvester is high among different MEH techniques. A cantilever type piezoelectric energy harvester under different shapes is mostly studied in the last few years. The output of piezoelectric harvester depends upon the deflection produced, more deflection led to more electrical output. The deflection in cantilever beam under different shapes is different. This review paper presents a comparison of different piezoelectric cantilever beam shapes and output generated analyzed in the last decade.

Optothermally pulsating microbubble-mediated micro-energy harvesting in underwater medium.

Despite the considerable research interest due to practical importance of pervasive wireless sensing systems in a wide range of engineering fields, power management remains an arduous task for further development of pervasive wireless sensing systems due to inherent needs for self-reliant functionality and portability during their operations. To this end, we here propose a new type of energy harvesting strategy in which an optothermally pulsating microbubble is submerged in an underwater medium. The pulsating microbubble gives rise to the periodic vibration of piezocantilevers in contact, which resultantly can produce electrical outputs. On the basis of this simple idea, mechanical power can be extracted from light energy through optothermally pulsating microbubbles in an aqueous medium and subsequently the mechanical power can be converted to electrical power for wireless devices. To elucidate physical factors affecting the performance of the proposed strategy, we thoroughly explore the effect of the intensity and frequency of the laser beam on the pulsation amplitude of optothermally pulsating bubbles and subsequent electrical outputs (e.g., electrical voltage and power). The dependence of electrical output on wetting property of piezocantilevers and electrical resistance is also established. The present work would provide a new framework for fundamental design of bubble-based microactuators as energy harvesters and microsensors in the near future.

Preparation of Nanofibrous PVDF Membrane by Solution Blow Spinning for Mechanical Energy Harvesting.

Self-powered nanogenerators composed of poly(vinylidene fluoride) (PVDF) have received much attention. Solution blow spinning (SBS) is a neoteric process for preparing nanofiber mats with high efficiency and safely, and SBS is a mature fiber-forming technology that offers many advantages over conventional electrospinning methods. Herein, we adopted the SBS method to prepare independent PVDF nanofiber membranes (NFMs), and successfully employed them as nanogenerators. Finally, we tested the change in the output current caused by mechanical compression and stretching, and studied its durability and robustness by charging the capacitor, which can drive tiny electronic devices. The results show that the PVDF nanogenerators by using this SBS equipment can not only be used in wearable electronic textiles, but are also suitable for potential applications in micro-energy harvesting equipment.

Porosity-tuned thermal conductivity in thermoelectric Al-doped ZnO thin films grown by mist-chemical vapor deposition

The potential of thermoelectric thin films lies in wide range of applications from micro-energy harvesting to the sensors. For this, it is essential to have high power factor and ultra-low thermal conductivity which have been reported in thin films produced by expensive vacuum techniques. However, for practical applications, it is essential to use inexpensive technique to grow thin film in large area. In this direction, we report the use of mist-chemical vapor deposition (CVD) technique to develop oxide thin films for thermoelectric application. We grow c-axis oriented nano-porous thin films of 2% Al-doped ZnO (AZO). These nano-porous films have enhance phonon scattering which results in the depression of thermal conductivity (κ) while maintaining similar order of magnitude of power factor as reported in dense films prepared by vacuum techniques. For example, κ300K decreases from 6.5 W/m.K for dense thin film (porosity = 7.9%) grown by pulsed laser deposition to 5.54 W/m.K for porous film (porosity = 24.2%) grown by mist-CVD while maintaining the power factor of similar order of magnitude for AZO film deposited on SrTiO3. The depression of thermal conductivity in porous films may lead to higher figure of merit which is promising for practical applications of thermoelectric oxide films.

Micro-Energy Harvesting System Including a PMU and a Solar Cell on the Same Substrate With Cold Startup From 2.38 nW and Input Power Range up to 10 $\mu$W Using Continuous MPPT

This paper presents a power management unit (PMU) powered by a 1 mm $^{\boldsymbol{2}}$ solar cell on the same substrate to rise up the harvested voltage above 1.1 V. The on-chip solar cell and the PMU are fabricated in standard 0.18 $\mu$ m CMOS technology achieving a form factor of 1.575 mm $^{\boldsymbol{2}}$ . The PMU is able to startup from a harvested power of 2.38 nW without any external kick off or control signal. The PMU features a continuous and two-dimensional maximum power point tracking (MPPT) working in open-loop mode to handle a harvested power range from nW to $\mu$ W, by modifying both the charge pump topology and the switching frequency. The MPPT is based on four voltage level detectors that define five working regions depending on the illumination and on a self-tuning reference current for a fine adjustment of the switching frequency. The chip also includes an auxiliary charge pump to generate the voltage level necessary for the control circuit, implemented as a Pelliconi charge pump of eight stages with NMOS transistors in P-well as diodes. A Dickson charge pump with transmission gates as switches and with variable gain and capacitance per stage is also designed as the main charge pump. Finally, two relaxation oscillators are implemented to drive both charge pumps. This paper is accompanied by a video file demonstrating the PMU operation by powering an off-chip nor gate.

Micro-energy harvesting based on vortex-induced vibration of cross-flow hydroturbine with various cantilever beam configurations

The cross-flow hydroturbine is a water impulse turbine with a low efficiency relatively, but it is easy to maintain and can adjust to a wide range of flow rates. Also, vortex fields are formed downstream of the cross-flow hydroturbine when the working fluid passes through an impeller, and these vortex fields can cause vibrations during electrical energy production. However, these vortex fields could gradually disappear and become a wasted, unused energy source. Therefore, amplifying and causing vortex fields continuously could result in vortex-induced vibration (VIV), which can be used as an energy source to enhance the performance of micro-energy harvesting (MEH) system. The vibration is typically converted into piezoelectric energy using a piezoelectric energy harvester, such as cantilever beam, membrane or other structure. In this study, the downstream of the cross-flow hydroturbine was equipped with a cantilever beam to harvest the vibration energy of the vortex fields. Numerical analysis was conducted using the commercial CFD code, ANSYS CFX 17.1 with a shear stress transport (SST) turbulence model. A 2-way fluid-structure interaction (FSI) analysis was also used to investigate the motion of the cantilever beam for the feasibility of using it in piezoelectric energy harvesting. As a result, the velocity vector and deformation of the cantilever beam were graphically depicted.

Acoustically Excited Oscillating Bubble on a Flexible Structure and Its Energy-Harvesting Capability

When a bubble oscillates under the action of an acoustic field, it generates a cavitational microstreaming flow around it. We here explore oscillation dynamics of a bubble hanging on a flexible structure (i.e., piezocantilever) by acoustic excitation, and assess the suitability of the proposed method to micro-energy harvesting systems. We preferentially investigate the characteristics of bubble oscillation, such as the maximum amplitude and resonant frequency by varying the applied frequency and bubble size. The amplitude of the oscillating bubble is found to be maximized at the resonant frequency depending on the bubble size. Additionally, we measured the electrical outcome generated from bubble oscillation-induced microstreaming and resultant vibration of the piezocantilever, as functions of the applied frequency, bubble size, and distance between the bubble and piezoactuator. The generated voltage is considerably dependent of the applied frequency and bubble size. Meanwhile, it is inversely proportional to the distance between the bubble and piezoactuator. Finally, we found that the electrical output can be can be improved by increasing the number of bubbles. This work will provide a new framework for the fundamental design of bubble-mediated micro-energy harvesters and microsensors.

Effect of a-Si thin film on the performance of a-Si/ZnO-stacked piezoelectric energy harvesters

In this letter, we present the fabrication and characterization of a zinc oxide (ZnO)-based nanogenerator for piezoelectric micro-energy harvesting by combining thin films of amorphous silicon (a-Si) and ZnO. We utilized the a-Si thin film as an interlayer to assemble several a-Si/ZnO-stacked piezoelectric nanogenerators (SZPNGs) on indium tin oxide (ITO)-coated polyethylene naphthalate substrates. We investigated the influence of the a-Si layer thickness on the output voltages of the SZPNGs and demonstrated the existence of an optimal a-Si thickness for maximizing the output voltage. Overall, the SZPNGs generated higher output voltages than a conventional ZnO-based piezoelectric nanogenerator (ZPNG) lacking an a-Si interlayer, indicating enhanced performance. In particular, the SZPNG based on the optimal a-Si thickness exhibited a sixfold higher output voltage compared with the conventional ZPNG. This improved performance was ascribed to a combination of the Schottky barrier at the ITO/a-Si interface, preventing the screening effect and the relatively high dielectric constant (εr≈13) of a-Si, minimizing the loss of the piezoelectric potential induced in the ZnO layer. The results herein are expected to assist the development of even more advanced ZnO-based piezoelectric nanogenerators in the future.

An experimental study of a-Si/ZnO-stacked hetero-structures for potential thermoelectric energy harvesting applications

Here, we present a study of thermoelectric devices with amorphous silicon/zinc oxide (a-Si/ZnO)-stacked hetero-structures fabricated using both radio frequency magnetron sputtering and rapid thermal annealing techniques. Overall, the Seebeck coefficient (S) and power factor (S2σ, where σ is the electrical conductivity) of the a-Si/ZnO-stacked hetero-structures were found to be superior to those of pure a-Si structures. In particular, the Seebeck coefficient and power factor of the a-Si/ZnO (9/8 layers)-stacked hetero-structures were about 1.6 and 23.8 times those of the pure a-Si structures, respectively. These improvements can be attributed to hole blocking by the a-Si/ZnO potential barriers formed at the interface between the ZnO layer with a wide energy bandgap and the a-Si layer with a relatively narrow bandgap. In addition, the a-Si and ZnO materials used in this work are non-toxic, earth-abundant, and cheap, and the fabrication processes were simple and cost-effective, making the hetero-structures suitable for use in non-toxic and biocompatible thermoelectric devices and also for micro-energy harvesting applications.

Practical Maximum-Power Extraction in Single Microbial Fuel Cell by Effective Delivery through Power Management System

Power management systems (PMSs) are essential for the practical use of microbial fuel cell (MFC) technology, as they replace the unstable stacking of MFCs with step-up voltage conversion. Maximum-power extraction technology could improve the power output of MFCs; however, owing to the power consumption of the PMS operation, the maximum-power extraction point cannot deliver maximum power to the application load. This study proposes a practical power extraction for single MFCs, which reserves more electrical energy for an application load than conventional maximum power-point tracking (MPPT). When experimentally validated on a real MFC, the proposed method delivered higher output power during a longer PMS operation time than MPPT. The maximum power delivery enables more effective power conditioning of various micro-energy harvesting systems.

The macro-fibre composite–bonded effect analysis on the micro-energy harvester performance and structural health–monitoring system of woven kenaf turbine blade for vertical axis wind turbine application

The application of vertical axis wind turbine is suitable in a low wind speed environment. Nevertheless, vertical axis wind turbine with kenaf turbine blade will promote an additional green concept by utilising biocomposite materials. The innovation in turbine blade via macro-fibre composite as structural health–monitoring system and micro-energy harvester can enhance the vertical axis wind turbine technology application. Hence, this research’s objective is to evaluate the factors influencing the performance of micro-energy harvester and to assess the feasibility of structural health–monitoring application in biocomposite turbine blade. There are two methods to attach the macro-fibre composite used in this study, which are surface bonded and embedding into the turbine blade. Vibration simulation experiment and modal testing approach are conducted on the kenaf turbine blade, and further analysis was performed via the Taguchi statistical analysis to determine the factors affecting the micro-energy harvester and structural health–monitoring performance. The results show that by bonded to the surface as proposed is the best technique in promoting higher micro-energy harvesting at the vibration range of 10–90 Hz. Furthermore, the structural health–monitoring system was proved to operate simultaneously with the micro-energy harvesting system in turbine blades.

Formation and Characterization of Various ZnO/SiO2-Stacked Layers for Flexible Micro-Energy Harvesting Devices

In this paper, we present a study of various ZnO/SiO2-stacked thin film structures for flexible micro-energy harvesting devices. Two groups of micro-energy harvesting devices, SiO2/ZnO/SiO2 micro-energy generators (SZS-MGs) and ZnO/SiO2/ZnO micro-energy generators (ZSZ-MGs), were fabricated by stacking both SiO2 and ZnO thin films, and the resulting devices were characterized. With a particular interest in the fabrication of flexible devices, all the ZnO and SiO2 thin films were deposited on indium tin oxide (ITO)-coated polyethylene naphthalate (PEN) substrates using a radio frequency (RF) magnetron sputtering technique. The effects of the thickness and/or position of the SiO2 films on the device performance were investigated by observing the variations of output voltage in comparison with that of a control sample. As a result, compared to the ZnO single-layer device, all the ZSZ-MGs showed much better output voltages, while all the SZS-MG showed only slightly better output voltages. Among the ZSZ-MGs, the highest output voltages were obtained from the ZSZ-MGs where the SiO2 thin films were deposited using a deposition power of 150 W. Overall, the device performance seems to depend significantly on the position as well as the thickness of the SiO2 thin films in the ZnO/SiO2-stacked multilayer structures, in addition to the processing conditions.