Design and Optimization of a Perfectly Symmetric Planar Spiral Receiving Coil with Low E-field Exposure for Large-space WPT

This paper introduces a perfectly symmetrical planar spiral receiving coil (PS-coil) for large-space wireless power transfer (WPT) systems, designed to significantly reduce electric field (E-field) exposure. The coil’s structure utilizes multiple symmetrically wound spiral wires, replacing traditional single-wire coils. This symmetry facilitates distributed and precise capacitor compensation, effectively mitigating the effects of parasitic capacitance and associated E-field exposure. The PS-coil preserves the magnetic field (H-field) distribution while significantly reducing E-field emissions, allowing for greater power reception within the limits set by the International Commission on Non-Ionizing Radiation Protection guidelines (2020). Finite element simulations indicate that at an effective current of 6 A, the PS-coil achieves a local specific absorption rate (SAR) of 0.72 W/kg at a 1 cm distance—80% lower than traditional designs and well below the 2 W/kg guideline. Finally, a prototype model, comparable in size to an iPad, confirms that the calculated inductance is within 2% of the measured values, with a self-resonance frequency 38% higher than that of traditional coils. Tests in a large space, 3 m in diameter and 2.4 m high, demonstrate that the PS-coil can transmit over 200 W with 54.8% efficiency. Additionally, comparisons of traditional coils with the proposed PS-coil under the same conditions indicated an 18.9 dB reduction in E-field exposure.