Development of Electromagnetic Micro-Energy Harvesting Device

Abstract

The use of energy harvesting devices has generated much research interests in recent years. There are numerous energy harvesters available in the market that are piezoelectric, electromagnetic, electrostatic or combination of piezoelectric and electromagnetic. Many of the harvesters have shown great potential but are either severely limited in power generation since they are actually never optimized to its potential. One of the goals of this thesis is to develop an electromagnetic micro-energy harvester that is capable of working at low frequencies (5-30 Hz) and is capable of producing electrical power for small devices.

Generally, batteries have been used to power low voltage electronics, however the need for self-sustaining and reliable power source have always been a major issue. This project aims to make a harvester of size AA battery that can be used as a reliable and continuous source of power for bio-medical as well as industrial applications.

Firstly, a linear harvester is developed for applications where there is no set natural frequency. The linear harvester consists of a stator and a mover. The stator includes copper coils, outer iron case and delrin holder for the coils while the mover consists of permanent magnets, iron pole and cylindrical rod. The working principles developed are used to optimize and improve the efficiency of energy harvesting system. The linear harvesting system is tested with the permanent magnet to iron pole ratio of 1.25 and permanent magnet to coil ratio of 0.73. The power density of the linear harvester is determined to be 4.44e-4 W/cm3. Thereafter, optimization is done in comsol to improve the performance of the energy harvesting system. The optimized magnet to iron ratio is determined to be 3.175 and permanent magnet to coil ratio of 0.7938. The optimized ratios are used to develop an inertial type non-linear energy harvesting device. The structure of the non-linear harvester is same as the linear one except two stationary magnets are added at the top and bottom of the harvester that act as a non-linear spring. The non-linear harvesting device is tested and the power density of the system is determined to be 2.738e-2 W/cm3. The non-linear harvester was tested at acceleration level of 1g and it was determined that the harvester worked best at natural frequency of 8.66 Hz. The maximum power produced was 38.1 mW. The non-linear type of harvester is easy to assemble and optimize to match ambient natural frequency of numerous vibrating systems. Two frequency tuning methods are looked at for the non-linear energy harvesting system. One is by changing the magnetic air gap and the second is by changing the thickness of the stationary top and bottom magnets. It is determined that changing magnetic air gap is more effective at tuning for a range of natural frequencies. For applications where the natural frequency of the system doesn't exist, such as buoys and beacons at sea, the linear energy harvester works best. For applications where the system vibrates at a certain natural frequency, the non-linear harvester should be used.

Finally, this thesis is concluded with a discussion on the electromagnetic micro-harvester and some suggestions for further research on how to optimize and extend the functionality of the energy harvesting system.