Wireless Power Transfer for Electric Vehicles Using Cast-Iron Structures as Electrodes in an S-CPT System

In this study, we investigated a shielded capacitive power transmission (S-CPT) architecture that utilizes cast iron covers for roads as transmitting electrodes for charging electric vehicles in motion. We evaluated the coupling capacitance in S-parameters for common electrode materials such as copper, aluminum, ductile cast iron, structural steel, and carbon steel. The surface conditions of ductile cast iron electrodes (casting state, machining, and electrocoating) were also compared. Using a four-plate S-CPT operating at 13.56 MHz, the coupling capacitance gap dependence was characterized. Regardless of the material, the coupling capacitance decreases with increasing electrode separation. However, ductile cast iron reached approximately 70 pF at d = 2 mm, demonstrating a high capacitance comparable to copper and aluminum despite its high permeability and resistivity. This indicates that magnetic properties have a limited influence on the coupling characteristics of the S-CPT system. A prototype incorporating a φ330 mm cast iron cover meeting road load and durability requirements was experimentally verified. The system achieved a 58% power transfer efficiency at 100 W input, maintained 40% efficiency at outputs exceeding 200 W, and maintained 45% efficiency even under 200 mm lateral displacement, demonstrating practical robustness under dynamic operation. Simulations confirmed that the shield electrodes provide a large effective grounding capacity, reduce the impedance of the return path, and contribute to the stabilization of the shield potential. Finite element structural analysis confirmed that the ductile cast iron electrodes can withstand a design load of 25 tons as a structural component, ensuring durability. It was demonstrated that the infrastructure can be used without compromising mechanical safety. The proposed infrastructure concept combines the existing cast iron covers of the vehicle with thin aluminum power receiving plates under the vehicle body, eliminating the need for new large coils and reducing installation costs. Overall, the combination of cast iron structural infrastructure and shielded CPT architecture achieves measured high efficiency (58% at 100 W), robust positional tolerance (45% at 200 mm), EMI reduction, and verified mechanical load-bearing capacity (25 tons), paving the way for a cost-competitive wireless charging system in urban areas.