Electromagnetic Energy Harvesting

Abstract

This chapter focuses on the use of electromagnetic transducers for the harvesting of kinetic (vibration) energy. The chapter introduces the fundamental principals of electromagnetism and describes how the voltage is linked to the product of the flux linkage gradient and the velocity. The flux linkage gradient is largely dependent on the magnets used to produce the field, the arrangement of these magnets, and the area and number of turns for the coil. The characteristics of wire-wound and micro-fabricated coils, and the properties of typical magnetic materials, are reviewed. The scaling of electromagnetic energy harvesters and the design limitations imposed by micro-fabrication processes are discussed in detail. Electromagnetic damping is shown to be proportional to the square of the dimension and analysis shows that the decrease in electromagnetic damping with scale cannot be compensated by increasing the number of turns. For a wire wound coil, the effect of increasing coil turns on EM damping is directly cancelled by an increase in coil resistance. For a planar micro-coil increasing the number of turns results in a greater increase in the coil resistance, resulting in an overall decrease in damping. Increasing coil turns will, however, increase the induced voltage which may be desirable for practical reasons. An analysis is also presented that identifies the optimum conditions that maximise the power in the load. Finally, the chapter concludes with a comprehensive review of electromagnetic harvesters presented to date. This analysis includes a comparison of devices that confirms the theoretical comparison between conventional wound and micro-fabricated coils and the influence of device size on performance.