Magnetostrictive energy harvesting from steel

Abstract

Energy harvesting is defined as electronic devices acquiring the energy they need to operate from their environment. Potential energy sources in the environment include light, temperature gradients, electromagnetic radiation and kinetic energy. The development of energy harvesting technologies together with a drop in the energy consumption of electronic devices may in the near future make many devices energy independent, which means a drop in the maintenance need of the devices, greater reliability and a reduction in the amount of battery waste. The main aim of the experimental part of this thesis was to present and further develop a new kinetic energy harvesting method, which can harvest energy from elastic waves propagating in steel structures, and to develop harvesting electronics that can be used with the new method. This new method is based on the magnetoelastic effect, which is a natural phenomenon that causes the magnetization of a ferromagnetic material to change when the dimensions of the material are changed. This phenomenon is also known as the inverse magnetostrictive effect. A change in the magnetization of a material will also cause a change in the magnetic flux passing through the material. Since a change in the magnetic flux of a coil will induce and electromotive force in the coil, the magnetoelastic effect can be utilized in energy harvesting. The basic idea behind the new method is harvesting energy from an impact loaded steel bar with a coil wound around the bar. Impact loading a steel bar will cause an elastic wave to propagate along the bar, which will cause alternating compressive and tensile stresses in the steel bar. Since steels are ferromagnetic materials, the strains will alter the magnetization of the bar inducing an electromotive force in a coil around the bar. Thus a part of the kinetic energy carried by the elastic is transduced to electrical energy. In the experimental part a test setup, an experimental harvesting generator and electronic harvesting circuits, to be used with the method, were implemented. The mean power that can be achieved with the experimental method depends on the magnitude and frequency of the impact loading, but during testing mean output power of the harvesting generator was between 1-5 mW, when an elastic wave was propagating in the bar. This is a sufficient mean power for a low power wireless sensor. The thesis also includes a literature survey, which aims to give the reader basic knowledge of existing kinetic energy harvesting methods and magnetostrictive phenomena.