Over the last couple of decades, numerous piezoelectric footwear energy harvesters (PFEHs) have been reported in the literature. This paper reviews the principles, methods, and applications of PFEH technologies. First, the popular piezoelectric materials used and their properties for PEEHs are summarized. Then, the force interaction with the ground and dynamic energy distribution on the footprint as well as accelerations are analyzed and summarized to provide the baseline, constraints, potential, and limitations for PFEH design. Furthermore, the energy flow from human walking to the usable energy by the PFEHs and the methods to improve the energy conversion efficiency are presented. The energy flow is divided into four processing steps: (i) how to capture mechanical energy into a deformed footwear, (ii) how to transfer the elastic energy from a deformed shoes into piezoelectric material, (iii) how to convert elastic deformation energy of piezoelectric materials to electrical energy in the piezoelectric structure, and (iv) how to deliver the generated electric energy in piezoelectric structure to external resistive loads or electrical circuits. Moreover, the major PFEH structures and working mechanisms on how the PFEHs capture mechanical energy and convert to electrical energy from human walking are summarized. Those piezoelectric structures for capturing mechanical energy from human walking are also reviewed and classified into four categories: flat plate, curved, cantilever, and flextensional structures. The fundamentals of piezoelectric energy harvesters, the configurations and mechanisms of the PFEHs, as well as the generated power, etc., are discussed and compared. The advantages and disadvantages of typical PFEHs are addressed. The power outputs of PFEHs vary in ranging from nanowatts to tens of milliwatts. Finally, applications and future perspectives are summarized and discussed.

Today, unmanned undersea vehicles (UUVs) are taking an increased number of U.S. Navy mission roles. These vehicles will most likely be launched within a large distance from its assigned mission area. At times it may be desirable for these vehicles to loiter and await tasking. One of the major limiting factors for these UUV activities is endurance. Currently, UUVs are limited to the range provided by one single battery charge. However, a reliable and renewable power resource to charge the battery trays is a big challenge issue for the U.S. Navy. Which is why the U.S. Navy is interested in developing a renewable seabed power source that will allow UUVs to charge while deployed and remain undetected. UUVs are generally far away from terrestrial power with no power source available to charge a battery. A covert, submerged, renewable power source that utilizes wave energy, wind, solar, temperature differences, etc., to charge a submerged power center is highly desirable. Working with the Navy Science Advisor, we obtained the requirements for a typical UUVs battery, which includes i) the high-charge-rate for two battery trays has about ~11kWh, ii) takes about 7 hours to charge from 0 to 100 percent, iii) the charge voltage is 32VDC, iv) and the power charge station is preferably located on ocean bed, which is typically under 60 meters of ocean water surface.Ocean wave energy offers a renewable resource that is predictable several days in advance and is significantly higher in its energy density comparing to wind and solar energies. But current tactics of harvesting energy from the ocean have a lot of flaws [1] –[7]. Therefore, a new and innovative technology must be invented and developed for a renewable rechargeable submerged ocean generator for U.S. Navy UUVs. One of our proposed submerged ocean wave power generators is shown in the Figure 1(a). The rotating mass can be designed to let all the power take off components be housed in a nondeformed water isolated housing, which solves the water leakage challenge of a submerged WEC. However, no existing power take-off and conversion technology is suitable for this submerged WEC.We have successfully developed a portion of the submerged WEC, the power take-off and conversion system, as shown in Figure 1(b), which can be isolated in a solid shell without motion or deformation interaction with outside seawater. The system can pick up ocean wave energy with submerged buoy which then transfers the vibrational energy absorbed by the buoy to the deep-water generator through a rope-spring system. Centrifugal rotational mass converts wave vibration energy to arbitrary direction rotational energy. During this, a gear system converts low speed rotation to high-speed rotation to the alternator/motor which then converts more rotational energy to electrical energy. In the power-take-off system, three groups of gear systems are uniformly distributed on a circular plate to make a stable and balanced 3D system. Each group of gear systems is ended with two-side gears and one driving gear to make a more stable gear sub-system. In order to reduce friction, ball bearings are used in combination with holding shifts on circular plate. In addition, one-way clutches are used to block resistance from backward rotation of motors. The system has the potential to generate kWs of electrical power underwater. In this presentation, we will discuss i) the innovative design concept and work principle, ii) the mechanical power take-off mechanism, iii) mechanical energy transportation, and iv) the mechanical to electrical power alternators. In this paper/poster, both analytical modeling and experimental studies will be presented. In addition, we are planning to demonstrate the system in the exhibition hall of this conference to let anyone approach and test this system for themselves.

This paper reports the comprehensive investigations of the environmental temperature influencing on dynamic impedance and energy conversions behaviors of a flextensional piezoelectric transducer under mechanical and electrical excitations. The displacement, capacitance, electrical impedance, and inductance spectra were evaluated under electrical stimulation to elucidate the frequency-dependent electromechanical characteristics, whereas mechanical excitation was employed to evaluate the resultant voltage, piezoelectric coefficient, electrical energy and power, impedance, and displacement. At the off-resonance frequency, the generated voltages and capacitance increased slightly with an increase in temperature during mechanical excitation, i.e., the piezoelectric coefficient increase due to the polarization increasing with temperature elevation. In contrast, with the temperatures increase at the resonance frequency, the natural frequency of the transducer, the generated peak voltage, electrical energy and power, and internal impedance decreased, significantly, due to i) the reduction of stiffness of the PZT material, and ii) both the damping ratio and damping coefficient increasing owing to intensified internal loss processes. In addition, two distinct electrical power peaks were observed near the resonance and anti-resonance frequencies under mechanical excitation. As the temperature increased from 20 to 40 °C, the corresponding output peak powers decreased by 16.1% and 31.8%, respectively, while their optimal load resistances exhibited opposite trends. The highest power peak, occurring near anti-resonance and demonstrated greater temperature sensitivity. These findings provide valuable insights into the temperature-dependent electromechanical behavior of piezoelectric devices for sensors, actuators and acoustic transducers applications, especially, offer practical guidance for optimizing energy conversion and load matching in piezoelectric energy-harvesting systems.

This study explores the impact of proof mass on i) impedance behaviour, ii) natural frequency, iii) generated resonance peak voltage, and iv) peak power for a piezoelectric multilayer stack (PM)-based flextensional piezoelectric energy harvester (PM-FPEH). Both finite element modelling and experimental validations were conducted in this study. For a weighted 23.7-gram PM-FPEH, proof masses of 10, 20, 30, 40 and 50 g were applied on its top, respectfully. The simulation results agreed well with the experimental validations. It indicates that the generated voltage increases linearly with the weight ratio of the proof mass to the PM-FPEH by a factor of 2.4. However, the resonance frequency ratio for the PM-FPEH with proof mass to without proof mass decreases linearly with the square root of the weight ratio of proof mass to the PM-FPEH by a factor of 0.36. The ratio of power with proof mass to without proof mass increases linearly with the weight of the proof mass to the weight of the PM-FPEH by a factor of 8.28. Therefore, this study demonstrated that proof mass not only lower resonance frequency, but also significantly increase power output for a PEH. In addition, the PM-FPEH with a 50-gram proof mass can charge a 6,600 µF super-capacitor from 0 to 6.9 V in approximately one minute under 0.2 g acceleration at 283 Hz.