Combined numerical and analytical optimization of coil geometry in an axial flux permanent magnet energy harvester

Inhalt:
This paper provides mechanical and electrical design guidelines for axial flux permanent magnet generators used in energy harvesting applications. The chosen harvester design features a pendulum mass directly attached to the shaft of an axial flux permanent magnet machine. This direct drive approach allows high efficiency, not only due to absence of a gear but also due to lower friction losses in the bearings. On account of the aperiodic excitation of this pendulum, energy harvesting generators require specific design guidelines and procedures. The aim of this research is to optimize the (energy) efficiency of the generator, i.e. the electrical energy obtained from a single, well-defined mechanical excitation. Key optimization parameters are the number of pole pairs, the coil geometry and the ratio of the individual coil’s inner and outer radius. Additionally, the impact of the wire diameter is investigated. The aforementioned parameters strongly affect not only the induced voltage, but also the winding resistance - unfortunately in opposite ways. The analysis does not only rely on analytical equations, but also include numerical simulations to overcome some of the simplifications inherent in analytical calculations. These simulations primarily allow a more detailed consideration of effects originating from different coil manufacturing technologies with their impact on coil shape and total wire length. The careful combination of all findings allows the derivation of optimum design guidelines for axial flux generators specifically tailored for energy harvesting. This way they contribute significantly to the ongoing development of high-efficiency autonomous power supply systems. Seen from the practical side, the paper focuses on generators with a peak output power of up to 100 mW and a maximum diameter of 100 mm. Finally, it was found that an important design constraint was the peak open circuit output voltage. This value was set at a minimum of 2 V to ensure acceptable efficiency in the downstream energy conditioning circuitry.