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Piezoelectric microelectromechanical systems (PiezoMEMS) are attractive for developing next generation self-powered microsystems. PiezoMEMS promises to eliminate the costly assembly for microsensors/microsystems and provide various mechanisms for recharging the batteries, thereby, moving us closer towards batteryless wireless sensors systems and networks. In order to achieve practical implementation of this technology, a fully assembled energy harvester on the order of a quarter size dollar coin (diameter=24.26 mm, thickness=1.75 mm) should be able to generate about 100 μW continuous power from low frequency ambient vibrations (below 100 Hz). This paper reviews the state-of-the-art in microscale piezoelectric energy harvesting, summarizing key metrics such as power density and bandwidth of reported structures at low frequency input. This paper also describes the recent advancements in piezoelectric materials and resonator structures. Epitaxial growth and grain texturing of piezoelectric materials is being developed to achieve much higher energy conversion efficiency. For embedded medical systems, lead-free piezoelectric thin films are being developed and MEMS processes for these new classes of materials are being investigated. Non-linear resonating beams for wide bandwidth resonance are also reviewed as they would enable wide bandwidth and low frequency operation of energy harvesters. Particle/granule spray deposition techniques such as aerosol-deposition (AD) and granule spray in vacuum (GSV) are being matured to realize the meso-scale structures in a rapid manner. Another important element of an energy harvester is a power management circuit, which should maximize the net energy harvested. Towards this objective, it is essential for the power management circuit of a small-scale energy harvester to dissipate minimal power, and thus it requires special circuit design techniques and a simple maximum power point tracking scheme. Overall, the progress made by the research and industrial community has brought the energy harvesting technology closer to the practical applications in near future.

In this study, a piezoelectric wind energy harvester was vibrated that aims to convert high-speed wind energy into electrical energy using vibrations caused by centrifugal force. Vibrations induced by centrifugal force enabled effective distortion of the piezoelectric clamped beam and thus produced electric charge through the piezoelectric effect. A clamped beam was used rather than a conventional thin cantilever to harvest the wind energy in the proposed harvester. The centrifugal force was introduced by a pair of rotating eccentric turbines that are installed on two ball bearings on both sides of the piezoelectric unimorph. Benefiting from the rotating eccentric masses of these turbines, the harvester is capable of capturing wind energy in high speed wind environments. A prototype was set up to examine the effects of the wind speed and the structural parameters on the electrical output of the harvester. It is found that the harvester worked efficiently with wind applied from the axial directions in a 20–55 m/s speed range and produced a maximum open-circuit voltage of 47.2 V. When connected to an external load of 50 kΩ, the harvester showed a peak output power of 3.69 mW at a wind speed of 55 m/s.

Many researchers introduced an array of generators for broadband energy harvesting. The array has been studied in comparison to a single element from this array, but never compared to a single reference harvester with same volume as the whole array. This paper presents a theoretical study of evaluating the performance of the array harvester in comparison to the reference harvester. Power from the reference harvester as well as from the array is analytically calculated. The array is compared to the reference harvester when loaded by their optimal resistances which provide maximum power capability. The comparison is divided into two sections: firstly when the elements of the array are tuned to resonate at matching frequencies and secondly when they are tuned to non-matching resonance frequencies. The comparisons lead to two significant limits of the working bandwidth of the array: the lower and the upper limit. Between the two limits, the power produced from the array is less than the reference harvester, but with a small additional bandwidth. Below the lower limit, the array has no advantage over the reference harvester. Above the upper limit, output power of the array is inconsistent. Hence, design guidelines for the array are provided.

The magneto-thermoelectric generator (MTG) converts wasted thermal energy into electrical energy in two steps. The first step involves thermal to mechanical energy conversion through balance of magnetic and elastic forces and the second step involves mechanical to electrical energy conversion through piezoelectric effect. The requirements for soft magnetic material in improving the efficiency of first step were identified and met through the design of a composite architecture. The Curie temperature of La (1–x) Sr x MnO 3 can be engineered to be near room temperature by modifying the Sr content. Composite of La 0.85 Sr 0.15 MnO 3 (LSMO) and Ni 0.6 Cu 0.2 Zn 0.2 Fe 2 O 4 (NCZF) was found to exhibit high saturation (M s ) and remnant (M r ) magnetization magnitude while maintaining the soft magnetic nature. Two-step sintering was found to prevent the inter-diffusion of LSMO and NCZF phases and provided high density without grain growth. The LSMO-NCZF (70:30 wt %) composite exhibited a large variation in M s with respect to the change in temperature near Curie temperature which meets the requirements for efficient operation of MTG. The fabricated MTG using LSMO-NCZF (70:30 wt %) composite reached 0.2 Hz operational frequency and generated electrical output voltage of 2 V p–p and peak power of 17 µW under the thermal gradient of 80 °C (0 °C/80 °C).