Energy Harvesting with Permanent Magnets

With the recent advances in emerging technologies such as the internet of things, wireless sensor networks and wearable devices; and the need to power them efficiently, environmentally friendly and with less e-waste, research communities turned faces towards harvesting energy from ambient sources. One of them is converting kinetic energy from vibrations into electrical energy through an electromechanical transduction process with the help of piezoelectric, electrostatic or electromagnetic devices. The working principle of the latter is based on Faraday’s law, which states that exposing a conductor to a time-varying magnetic flux induces a voltage.
This dissertation contributes to the research in electromagnetic vibration energy harvesters as a supply source of electrical energy for low-power devices. The project aims to clearly understand the conditions and parameters that a harvester must fulfil to ensure maximum power transfer for a given motion frequency and amplitude. The study starts by describing the mechanical and electrical domains of the harvester system, likewise the essential background to follow the report. Then, the concept of shape function is introduced to derive mathematical expressions for the restoring magnetic force that results from the magnet-magnet interaction and the magnetic flux on a one-loop coil. These equations are then employed in a parametric analysis and mathematical model of one-dimensional electromagnetic energy harvesters. Their study includes component sizing and mounting, optimal restoring magnetic force, friction mitigation and nonlinear dynamics.
Following that, four novel two-dimensional electromagnetic energy harvesters are designed and tested in a 2D motion shaker. The study intends to show how magnetic stiffness determines the harvester’s performance. Some methods to modify its stiffness are drawn to match the harvester effective resonance frequency with an external motion frequency. In addition, required simulations in a finite element method software are realized to collect the information of the magnet-magnet interaction and induced magnetic flux on a coil winding so than solving a mathematical model of these two-dimensional harvesters.
Lastly, a simple design of the harvester electrical domain is examined. First, the rectification stage finds a low power-loss solution with a rectifier bridge composed of NMOSFETS. Furthermore, the necessary conditions of the storage stage are listed. Then, the entire harvester system is tested with two prototypes with a sweeping load to find the maximum power transfer, obtaining a peak value of around 10 mW.