As Electric and Hybrid Electric Vehicles (EVs and HEVs) become more prevalent, there is a need to change the power source from gasoline on the vehicle to electricity from the grid in order to mitigate requirements for onboard energy storage (battery weight) as well as to reduce dependency on oil by increasing dependency on the grid (our coal, gas, and renewable energy instead of their oil). Traditional systems for trains and buses rely on physical contact to transfer electrical energy to vehicles in motion. Until recently, conventional magnetically coupled systems required a gap of less than a centimeter. This is not practical for vehicles of the future.
Power transferred by standing electromagnetic waves induced by the magnetic dipole of a loosely coupled air core transformer1,2,3,4 can theoretically provide the mechanism for this power transfer over a significant fraction of a meter without physical contact, eliminating the need for precision alignment between a vehicle and its power source. The air core is responsible for the loose coupling of this transformer's primary (transmitter) and secondary (receiver) and defines Evanescent Power Transfer (EPT). By comparison the wireless power transfer technology used by most systems on the market today is Inductive Power Transfer (IPT)5,6,7,8,9,10,11, which employs a fair amount of iron to increase the coupling.
This paper describes the EPT approach at the Oak Ridge National Laboratory (ORNL). It has demonstrated power transfer greater than 4 kW in a full-scale laboratory apparatus with an efficiency of 95% from transmitting antenna to a battery-like load with an antenna separation of 25.4 cm. Higher power transfer levels will only require components with higher voltage ratings. Since optimum values for any one parameter do not line up with the others, the final set of operating conditions will depend more on economics than on the constraints of physics.