Design optimization of microfabricated coils for volume-limited miniaturized broadband electromagnetic vibration energy harvester

Broadband electromagnetic vibration energy harvesters (EMVEHs) can effectively convert mechanical energy into electricity from human motions with low and random frequencies. However, such applications require a small device footprint which limits the power density of the energy harvester, making it hard to generate sufficient energy for wearable electronics. Here, we present a design of a nonresonant broadband EMVEH based on a magnet array with alternating magnetic polarities and microfabricated planar coils. To enhance the power output and power density of the EMVEH, we propose a geometric optimization method for determining the coil geometry. First, the magnetic flux density (MFD) distribution and the power output under different types of excitation motions are studied by finite element analysis and numerical simulation, respectively. Based on these results, a design of a stackable microfabricated double-sided planar coil sheet consisting of 10 coil groups is chosen. Through the optimization based on the trade-off between induced voltage and coil resistance, four coil turns in each coil group is determined as the optimal design for maximizing the output power, which proved independent of the excitation conditions. Based on the optimized design, a prototype consisting of 20 layers of coil sheets connected in series on each side is fabricated, occupying only 1.73 cm3. The device is tested to have a wide half-power bandwidth of 8 Hz, a maximum output power of 346.7 μW, and a maximum power density of 200.4 μW/cm3, when connected to a matched load of 69.46 Ω under sinusoidal excitations of 2.0 g acceleration. Furthermore, we propose a preliminary optimization methodology to determine the optimal number of stacked coil layers.