Piezoelectric energy harvesters (PEHs) are a reduced, but fundamental, source of power for embedded, remote, and no-grid connected electrical systems. Some key limits, such as low power density, poor conversion efficiency, high internal impedance, and AC output, can be partially overcome by matching their internal electrical impedance to that of the applied resistance load. However, the applied resistance load can vary significantly in time, since it depends on the vibration frequency and the working temperature. Hence, a real-time tracking of the applied impedance load should be done to always harvest the maximum energy from the PEH. This paper faces the above problem by presenting an active control able to track and follow in time the optimal working point of a PEH. It exploits a non-conventional AC–DC converter, which integrates a single-stage DC–DC Zeta converter and a full-bridge active rectifier, controlled by a dedicated algorithm based on pulse-width modulation (PWM) with maximum power point tracking (MPPT). A prototype of the proposed converter, based on discrete components, was created and experimentally tested by applying a sudden variation of the resistance load, aimed to emulate a change in the excitation frequency from 30 to 70 Hz and a change in the operating temperature from 25 to 50 °C. Results showed the effectiveness of the proposed approach, which allowed to match the optimal load after 0.38 s for a ΔR of 47 kΩ and after 0.15 s for a ΔR of 18 kΩ.
Electrochemical methods such as voltammetry and electrochemical impedance spectroscopy are effective for quantifying solid oxide fuel cell (SOFC) operational performance, but not for identifying and monitoring the chemical processes that occur on the electrodes' surface, which are thought to be strictly related to the SOFCs' efficiency. Because of their high operating temperature, mechanical failure or cathode delamination is a common shortcoming of SOFCs that severely affects their reliability. Infrared thermography may provide a powerful tool for probing in situ SOFC electrode processes and the materials' structural integrity, but, due to the typical design of pellet-type cells, a complete optical access to the electrode surface is usually prevented. In this paper, a specially designed SOFC is introduced, which allows temperature distribution to be measured over all the cathode area while still preserving the electrochemical performance of the device. Infrared images recorded under different working conditions are then processed by means of a dedicated image processing algorithm for quantitative data analysis. Results reported in the paper highlight the effectiveness of infrared thermal imaging in detecting the onset of cell failure during normal operation and in monitoring cathode activity when the cell is fed with different types of fuels.
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