Band 4, Heft 3 -

The generation of hydrogen as a chemical energy storage for power generation via fuel cells or for the synthesis of fuels has attained a strong interest in recent years. By way of example this is realized using electrolysis of water with the help of excess electricity of wind power plants. However with low temperature grade waste heat as it could be found in many industrial and household applications, there is another source of usable energy for this purpose. In a first pragmatic experimentation we investigated the pyroelectric effect of ferroelectric BaTiO 3 combined with a temperature cycling to generate hydrogen from water. Therefore, single crystals ground to powder were brought into contact with distilled water and set to a cyclical temperature change from 40 °C to 70 °C. With the help of a highly selective and sensitive measuring system based on a coulometric solid electrolyte detector we could provide a first indication of pyroelectric generated hydrogen by a fraction of 300 Vol.-ppb in the sample gas.

Equivalent circuits are often the first choice for the modeling of piezoelectric systems, as they allow for the consideration of the complete electro-mechanical system with one or even more modes. The parameters of the equivalent circuit model are identified by a measured or simulated frequency response. In this contribution a method for a direct modal condensation of the equivalent parameters for arbitrary FE structures and loads is described and discussed. First the proposed method is demonstrated for a continuous piezoelectric rod and then applied to discrete finite element models. The derived equivalent circuit has an identical appearance to the classical solution, but additionally allows arbitrarily load conditions. Furthermore, the structure of the derived equivalent circuit depends on whether short- or open-circuited modes are used for the modal expansion. The influence of truncated modes is discussed utilizing residual terms, leading to a better understanding of the circuit parameters. Additionally the model based approaches in the third part an experimental parameter identification procedure for many modes is presented as well. The influence of the load and the quality of the model order reduction are discussed for piezoelectric rods. The methods are demonstrated for a base excited energy harvesting system an ultrasonic grubber.

This paper introduces a passive self-tuning energy harvester by applying self-resonating behavior. Under certain operating conditions, self-resonating systems have the capability to passively adjust their dynamical characteristics until the whole system becomes resonant. A clamped-clamped beam with an attached mass sliding freely with a slight gap showed self-resonating behavior. Under a harmonic input excitation and a well-defined operating regime, the mass moved along the beam thus causing a change in the natural frequency of the structure, and then stopped at the position where the natural frequency matched the excitation frequency, resulting in a significant increase in the vibration amplitude. For harvesting energy, a piezoelectric element was glued at one end of the beam. The operating regime of the self-resonating behavior was found experimentally in the two halves of the beam. In the half containing the piezoelectric element, self-resonating behavior was achieved between 126 Hz and 143 Hz. In the other half, it was achieved between 135 Hz and 165 Hz. Maximum power output of 2.5 mW was obtained under an input excitation of 4.92 m/s 2 and 148 Hz. It is to be concluded that applying self-resonating behavior on energy harvesting provides a promising broadband technique.

An energy harvesting device for frequencies of less than 2 Hz with amplitudes of several cm was developed. A pendulum is driving a gear mechanism similar as a clock unit which is driving a wheel carrying 36 magnets. Next to the magnets there are mounted 18 coils generating voltage. This way, a maximum voltage and power of 21.5 V and 114 mW are achieved at 1.25 Hz and 57 mm amplitude.