The process of collecting low-level kinetic energy, which is present in all moving systems, by using energy harvesting principles, is of particular interest in wearable technology, especially in ultra-low power devices for medical applications. In fact, the replacement of batteries with innovative piezoelectric energy harvesting devices can result in mass and size reduction, favoring the miniaturization of wearable devices, as well as drastically increasing their autonomy. The aim of this work is to assess the power requirements of wearable sensors for medical applications, and address the intrinsic problem of piezoelectric kinetic energy harvesting devices that can be used to power them; namely, the narrow area of optimal operation around the eigenfrequencies of a specific device. This is achieved by using complex numerical models comprising modal, harmonic and transient analyses. In order to overcome the random nature of excitations generated by human motion, novel excitation modalities are investigated with the goal of increasing the specific power outputs. A solution embracing an optimized harvester geometry and relying on an excitation mechanism suitable for wearable medical sensors is hence proposed. The electrical circuitry required for efficient energy management is considered as well.

With the aim of increasing the efficiency of maintenance and fuel usage in airplanes, structural health monitoring (SHM) of critical composite structures is increasingly expected and required. The optimized usage of this concept is subject of intensive work in the framework of the EU COST Action CA18203 “Optimising Design for Inspection” (ODIN). In this context, a thorough review of a broad range of energy harvesting (EH) technologies to be potentially used as power sources for the acoustic emission and guided wave propagation sensors of the considered SHM systems, as well as for the respective data elaboration and wireless communication modules, is provided in this work. EH devices based on the usage of kinetic energy, thermal gradients, solar radiation, airflow, and other viable energy sources, proposed so far in the literature, are thus described with a critical review of the respective specific power levels, of their potential placement on airplanes, as well as the consequently necessary power management architectures. The guidelines provided for the selection of the most appropriate EH and power management technologies create the preconditions to develop a new class of autonomous sensor nodes for the in-process, non-destructive SHM of airplane components.

The development of wearable devices and remote sensor networks progressively relies on their increased power autonomy, which can be further expanded by replacing conventional power sources, characterized by limited lifetimes, with energy harvesting systems. Due to its pervasiveness, kinetic energy is considered as one of the most promising energy forms, especially when combined with the simple and scalable piezoelectric approach. The integration of piezoelectric energy harvesters, generally in the form of bimorph cantilevers, with wearable and remote sensors, highlighted a drawback of such a configuration, i.e., their narrow operating bandwidth. In order to overcome this disadvantage while maximizing power outputs, optimized cantilever geometries, developed using the design of experiments approach, are analysed and combined in this work with frequency up-conversion excitation that allows converting random kinetic ambient motion into a periodical excitation of the harvester. The developed optimised designs, all with the same harvesters’ footprint area of 23 × 15 mm, are thoroughly analysed via coupled harmonic and transient numerical analyses, along with the mostly neglected strength analyses. The models are validated experimentally via innovative experimental setups. The thus-proposed f = 50 mm watch-like prototype allows, by using a rotating flywheel, the collection of low-frequency (ca. 1 to 3 Hz) human kinetic energy, and the periodic excitation of the optimized harvesters that, oscillating at their eigenfrequencies (~325 to ~930 Hz), display specific power outputs improved by up to 5.5 times, when compared to a conventional rectangular form, with maximal power outputs of up to >130 mW and average power outputs of up to >3 mW. These power levels should amply satisfy the requirements of factual wearable medical systems, while providing also an adaptability to accommodate several diverse sensors. All of this creates the preconditions for the development of novel autonomous wearable devices aimed not only at sensor networks for remote patient monitoring and telemedicine, but, potentially, also for IoT and structural health monitoring.

One of the most common ambient energy sources suitable for energy harvesting is waste heat emitted by machines, from hot pipes, from human and animal bodies (i.e., resulting from metabolism) or from radioactive materials. Heat emanated from humans is particularly interesting for energy harvesting applications, since it is possible to use it as a power source for wearable devicesand in particular for medical sensors. On the other hand, thermoelectric generators (TEGs) allow converting waste heat into electrical energy via the Seebeck effect. An experimental setup is developed in this work with the aim of charactering the performances of three types of commercially available TEG devices. Heat is hence brought into the system on the hot side of the TEGs, while it is dissipated via a heatsink on their cold side. Temperatures on both sides are measured with thermocouples and by using thermal imaging, while, concurrently, various electrical loads are connected to the harvesters. Multimeters are employed to measure the resulting electrical parameters (voltage and current), enabling the determination of the power output and the efficiency of the studied devices. The acquired data provide thus means of matching the analysed TEGs to the respective working conditions in adequate applications.

Energy harvesting is the process of collecting low-level ambient energy and converting it into electrical energy to be used for powering miniaturized autonomous devices, sensor networks, wearable electronics or Internet-of-Things components. The use of the pervasive kinetic energy, converted into electrical energy, is of special interest in this frame. The possibility to use bimorph piezoelectric cantilevers to convert ambient vibrations to electrical energy is therefore thoroughly analyzed in this work. A reliable modelling tool for optimizing the design of the miniature harvesters to be used in a broad frequency range, while maximizing the obtained powers, is hence needed. The problem complexity is induced by the necessity to simulate the dynamic response of the considered harvesting devices via a coupled electromechanical model. The recently developed comprehensive coupled analytical model based on distributed parameters is thus used as a benchmark to verify and tune suitable finite element (FE) numerical models. Modal (allowing to determine the mechanical dynamic response and the respective eigenfrequencies), harmonic (resulting in coupled frequency response functions) as well as linear and nonlinear transient FE analyses (resulting in dynamic responses under forced excitation at discrete time steps, including geometric nonlinearities) are therefore performed and complex dynamics effects are observed.