EV Charging Through Wireless Power Transfer: Analysis of Efficiency Optimization and Technology Trends

This paper presents the technology trends for wireless power transfer (WPT) for electric vehicles (EV), and analyzes the factors affecting its power transfer efficiency (PTE) and describes the techniques currently used for its optimization. The review of technology trends encompasses both stationary and moving vehicle charging systems. The study of the stationary vehicle charging technology is based on developments at WiTricity and Oak Ridge National Lab (ORNL). The moving vehicle charging technology is primarily described through the results achieved by the Korea Advanced Institute of Science and Technology (KAIST). The factors affecting the PTE are determined through the analysis of the equivalent circuit of magnetic resonant coupling. The air gap between both transmitting and receiving coils along with the magnetic field distribution and the relative impedance mismatch between the related circuits are the primary factors affecting WPT efficiency. Currently the industry is looking at an air gap of 20 cm or below. To control the magnetic field distribution, KAIST has recently developed the Shaped Magnetic Field In Resonance (SMFIR) technology that uses optimized shaped ferrite material to provide a low reluctance path or maximum exposure of the magnetic field to the resonant coils. The PTE can be further improved by means of impedance matching. As a result, Delphi’s current implementation of WiTricity’s technology exhibits WPT efficiency above 90 % for stationary charging with power capacity of 3.3 kW, while KAIST has demonstrated a maximum efficiency of 83 % for moving vehicle with its On-Line Vehicle (OLEV) project with the power capacity of 100 kW.