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(c) Xiaofang Yu 1 , Sunil Sandhu 1 , Sven Beiker 2 , Richard Sassoon 3 and Shanhui Fan 1 1 Department of Electrical Engineering and 2 Center for Automotive Research 3 Global Energy and Climate Project, Stanford University Wireless energy transfer to moving vehicles Electric vehicles offer superior energy efficiency while offering an enormous potential for reducing CO 2 emissions if the electricity is supplied from a renewable or nuclear source. However, they are presently neither range- nor cost-competitive compared to conventional vehicles, due to limited options for recharging, and expensive energy storage (batteries). We explore the feasibility of wireless power transfer directly to vehicles cruising at highway speed, via magnetically-coupled resonating coils located in the roadbed and in the vehicles. Abstract Introduction In recent years there has been renewed interest in wireless power transfer. In particular, a recent experiment, conducted at MIT, has demonstrated that two resonant circuits, with their magnetic fields strongly coupled in the near-field regime, allows highly efficient power transfer over a distance of approximately two meters. Power transfer between the resonators occurs in the near-field regime through the magnetic field. The use of magnetic fields is crucial for safety reasons since they interact very weakly with biological organisms. Challenges • Can 20kW be transferred wirelessly over a 0.5m distance? • Does the relative velocity of the transmitter and receiver have any impact? • What would the geometry of the coil look like? • To what extent will a vehicle body influence the power transfer? Theory The mechanism for the wireless energy transfer can be described by the coupled mode theory. For a system consists of a source resonator (denoted with subscript s) and a receiver resonator (denoted with subscript r), we have the following equations Where corresponds to the energy stored in the resonator. is the resonant frequency. is the intrinsic loss (absorption, radiation, etc) rate. is the work extraction rate. is the coupling coefficient. The transfer efficiency Simulation results 2m 0.8m 2m 0.8m Blue line – FDTD simulation; Red line – Coupled mode theory -- Transfer efficiency; -- Transfer time Wireless energy transfer with the presence of metal planes. Transfer efficiency as a function of distance for different structures. (a) (b) (d) Keys to maximize the transfer efficiency: •Resonant mode with a high quality factor. •System symmetry (resonant frequency matching). •Couple through aligned magnetic field. •Capacitor orientation also affects the efficiency. Acknowledgement This work is supported by the Global Climate and Energy Project (GCEP) at Stanford. Conclusion We numerically demonstrated that efficient wireless energy transfer can be achieved between two high Q resonators in a complex electromagnetic environment. In particular, in the close proximity of metallic planes, efficient wireless energy transfer can be achieved with proper system designs. The time scale of the energy transfer is in microseconds, which is much smaller than the moving time scale of a car. Static result can be applied to moving vehicles. [A. Kurs et al. Science, vol. 317 (2007) pp 83-86] Magnetic filed as a function of time at the source resonator and receiving resonators
