Dielectric Elastomers for Energy Harvesting

Dielectric elastomers are a type of electroactive polymers that can be conveniently used as sensors, actuators or energy harvesters and the latter is the focus of this review. The relatively high number of publications devoted to dielectric elastomers in recent years is a direct reflection of their diversity, applicability as well as nontrivial electrical and mechanical properties. This chapter provides a review of fundamental mechanical and electrical properties of dielectric elastomers and up-to-date information regarding new developments of this technology and it’s potential applications for energy harvesting from various vibration sources explored over the past decade

Effect of the nonlinear material viscosity on the performance of dielectric elastomer transducers

As a typical type of soft electroactive materials, dielectric elastomers (DEs) are capable of producing large voltage-induced deformation, which makes them desirable materials for a variety of applications in transduction technology, including tunable oscillators, resonators, biomimetics and energy harvesters. The dynamic and energy harvesting performance of such DE-based devices is strongly affected not only by multiple failure modes such as electrical breakdown, electromechanical instability, loss-of-tension and fatigue, but also by their material viscoelasticity. Moreover, as suggested by experiments and theoretical studies, DEs possess nonlinear relaxation processes, which makes modeling of the performance of DE-based devices more challenging. In this thesis, by adopting the state-of-art modeling framework of finite-deformation viscoelasticity, the effects of nonlinear viscosity of the polymer chains on the oscillation and frequency tuning of DE membrane oscillators are firstly investigated. From the simulation results, it is found that the nonlinear viscosity only affects the transient state of the frequency tuning process of DE oscillators. Secondly, with both finite-deformation viscoelasticity and deformation-dependent viscosity of polymer chains considered, the energy conversion efficiency and harvested energy of dielectric elastomer generators under equi-biaxial loading are also examined. It is found that when a nonlinear viscosity model is used, DE generators appear to reach an equilibrium state faster and the nonlinear viscosity significantly influences the energy harvesting performance. The modeling framework developed in this work is expected to provide useful guidelines for predicting the performance of DE-based oscillators and energy harvesters as well as their optimal design

Flexible and stretchable electrodes for dielectric elastomer actuators

Dielectric elastomer actuators (DEAs) are flexible lightweight actuators that can generate strains of over 100%. They are used in applications ranging from haptic feedback (mm-sized devices), to cm-scale soft robots, to meter-long blimps. DEAs consist of an electrode-elastomer-electrode stack, placed on a frame. Applying a voltage between the electrodes electrostatically compresses the elastomer, which deforms in-plane or out-of plane depending on design. Since the electrodes are bonded to the elastomer, they must reliably sustain repeated very large deformations while remaining conductive, and without significantly adding to the stiffness of the soft elastomer. The electrodes are required for electrostatic actuation, but also enable resistive and capacitive sensing of the strain, leading to self-sensing actuators. This review compares the different technologies used to make compliant electrodes for DEAs in terms of: impact on DEA device performance (speed, efficiency, maximum strain), manufacturability, miniaturization, the integration of self-sensing and self-switching, and compatibility with low-voltage operation. While graphite and carbon black have been the most widely used technique in research environments, alternative methods are emerging which combine compliance, conduction at over 100% strain with better conductivity and/or ease of patternability, including microfabrication-based approaches for compliant metal thin-films, metal-polymer nano-composites, nanoparticle implantation, and reel-to-reel production of μm-scale patterned thin films on elastomers. Such electrodes are key to miniaturization, low-voltage operation, and widespread commercialization of DEA

Wearable Energy Harvesting for Charging Portable Electronic Devices by Walking

Wearable energy harvesting technologies will become an everyday part of future portable electronic devices. By generating the energy where the energy is needed and not relying on a main power source to recharge the portable devices battery, wearable energy harvesters will enable future generations to have even more freedoms, travel further, and never run low on battery again. This will reduce the energy consumption of the mains grid and thus in turn reduce CO² emissions generated by this traditional power source making this research important for the whole plant. This research project aims to take another step towards in helping the development of future technologies by investigating novel wearable energy harvesting designs and showing ability to charge current portable electronic devices such as smart phones and tables. This required research into a broad range of topics including, energies from humans, energy conversion mechanisms, the movement of people and the power demands for charging current portable electronic devices. Background research in the human energy levels and how research to date had gone about exacting different energy sources in different ways was the starting point for this research. This leads on to a more detailed look into the exaction methods and optimization of footfall energy harvester designs. Looking into the human gait cycle gave the information required to replicate human footfall motion for use in scientific experiments. From this background research, two bespoke designs of wearable energy harvester have been created. The first novel design showed a promising way of extracting footfall energy and converting it into useable electrical energy producing Watt-Level of power. The second design is an evolution of the first design but expands the extraction method to both feet and relocated the main harvester unit into a backpack worn by the user. The improved design incorporates a novel approach to energy conversion method by introducing a mechanical energy storage system before transduction into electrical energy. This is shown to increased electrical power output from footfall energy, reduced energy consumption of the wearer and is shown to truly be able to charge current portable electronics. The improved design is shown to produce 2.6 Watts average power from normal walking. The experimental set ups, procedures, and their results are shown throughout this thesis. These experimental results are confirmed by using the wearable energy harvesters on a treadmill at the three main walking speeds showing their real-world capabilities. To demonstrate the wearable energy harvester deigns shown in this research project were truly able to charge current portable technologies, endurance testing was also performed. This confirms the harvesters were able to work for longer periods of time. This longer time frame is needed for the charging times of the current portable devices. After researching into wearable energy harvesting from over the last 20 years it was a struggle to compare all the different forms, designs, types and power outputs. It became clear that the existing methods were unable to provide a clear picture of harvester’s scalability, changeability and useability for future design ideas. This is why a new form of comparison was created and is shown to have strong benefits over the existing methods

DEVELOPMENT OF PIEZOELECTRIC ENERGY HARVESTING SYSTEM FOR LOW-FREQUENCY VIBRATIONS

Harvesting energy from vibration sources has attracted the interest of researchers for the past three decades. Researchers have been working on the potential of achieving self-powered MEMS scale devices. Piezoelectric cantilever harvesters have caught the attention in this field because of the excellent combination of high-power density and compact structure. The main objective of this thesis is to develop a novel and optimum piezoelectric harvester system using lumped parameter model (LPM) for given vibration sources. The finite element model (FEM) is used in this work as an original approach to be utilized for optimal design optimization. Three types of validations are accomplished to solidify the use of FEM in mimicking the distributed parameter model (DPM) for linearly tapered piezoelectric cantilevers. The first two validations are accomplished using beam deflection and relative transmissibility functions. Comparisons between the FEM and the DPM developed by the literature are performed. The third validation is carried for an electromechanical piezoelectric cantilever in FEM. Results confirmed the effectiveness of the developed FEM. A number of significant contributions are achieved while fulfilling the aim of this work. First, a dimensionless parameter, Power Factor (PF), is derived and used to understand the impact of the geometry on the piezoelectric harvester performance. The PF showed an optimum performance at a taper ratio of 0, taking the full length of the cantilever and thickness ratio of 0.7. Second, the accuracy of the LPM for linearly tapered piezoelectric harvesters and optimal design are investigated. Results indicated that the percentage of the deflection error between the LPM and the FEM reaches 9% when the taper ratio is zero. However, when tip-mass to cantilever ratios are larger than 2, the error decreases to less than 0.5% leading to more accurate results in the vibrational response of the beam. Further studies on the accuracy are accomplished using the relative transmissibility function. Results showed that as the taper ratio decreases towards zero, the percentage error of using the LPM to predict the vibration response increases significantly to 55%. These results lay the foundation for the third contribution of developing correction factors for tapered and optimal piezoelectric cantilever harvesters using FEM. Comparisons of the corrected LPM and FEM for different configurations are examined. Results indicated that as the taper ratio decreases, the surface power density increases. However, the developed optimal design exhibits the highest surface power density of 1.40×104 [(mW/g2)/ m2] which is 16.4% more than the best following shape of a taper ratio 0.2 and 58% more than the taper ratio 1. Furthermore, a parametric study of the optimal design is performed to scrutinize the effect of various parameters on the harvester performance. Finally, detailed criteria for designing the optimal piezoelectric harvester for different conditions are structured

Electroactive poly(vinylidene fluoride) based materials: recent progress, challenges and opportunities

A poly(vinylidene fluoride) (PVDF) and its copolymers are polymers that, in specific crystalline phases, show high dielectric and piezoelectric values, excellent mechanical behavior and good thermal and chemical stability, suitable for many applications from the biomedical area to energy devices. This chapter introduces the main properties, processability and polymorphism of PVDF. Further, the recent advances in the applications based on those materials are presented and discussed. Thus, it shown the key role of PVDF and its copolymers as smart and multifunctional material, expanding the limits of polymer-based technologies.FCT (Fundação para a Ciência e Tecnologia) for financial support under the framework of Strategic Funding grants UID/FIS/04650/2019, and UID/QUI/0686/2019 and project PTDC/FIS-MAC/28157/2017, PTDC/BTMMAT/28237/2017, PTDC/EMD-EMD/28159/2017. The author also thanks the FCT for financial support under grant SFRH/BPD/112547/2015 (C.M.C.), SFRH/BPD/98109/2013 (V.F.C.), SFRH/BD/140698/2018 (R.B.P.), SFRH/BPD/96227/2013 (P.M.), SFRH/BPD/121526/2016 (D.M.C.), SFRH/BPD/97739/2013 (V. C.), SFRH/BPD/90870/2012 (C.R.). Financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) through project MAT2016-76039-C4-3-R (AEI/FEDER, UE) (including FEDER financial support) and from the Basque Government Industry and Education Departments under the ELKARTEK, HAZITEK and PIBA (PIBA-2018-06)

Small, fast, and tough: Shrinking down integrated elastomer transducers

We review recent progress in miniaturized dielectric elastomer actuators (DEAs), sensors, and energy harvesters. We focus primarily on configurations where the large strain, high compliance, stretchability, and high level of integration offered by dielectric elastomer transducers provide significant advantages over other mm or μm-scale transduction technologies. We first present the most active application areas, including: tunable optics, soft robotics, haptics, micro fluidics, biomedical devices, and stretchable sensors. We then discuss the fabrication challenges related to miniaturization, such as thin membrane fabrication, precise patterning of compliant electrodes, and reliable batch fabrication of multilayer devices. We finally address the impact of miniaturization on strain, force, and driving voltage, as well as the important effect of boundary conditions on the performance of mm-scale DEAs