A team at Stanford University has published research results for longer range dynamic wireless energy transfer for potential use in electric vehicle charging.

Both, stationary and dynamic wireless charging has of course already been demonstrated around the world, but Shanhui Fan, a professor of electrical engineering and senior author of the study, thinks about how to enable the continuous flow of electricity by simple active tuning.

"Mid-range wireless power transfer, as developed at Stanford and other research universities, is based on magnetic resonance coupling. Just as major power plants generate alternating currents by rotating coils of wire between magnets, electricity moving through wires creates an oscillating magnetic field. This field also causes electrons in a nearby coil of wires to oscillate, thereby transferring power wirelessly. The transfer efficiency is further enhanced if both coils are tuned to the same magnetic resonance frequency and are positioned at the correct angle.

However, the continuous flow of electricity can only be maintained if some aspects of the circuits, such as the frequency, are manually tuned as the object moves. So, either the energy transmitting coil and receiver coil must remain nearly stationary, or the device must be tuned automatically and continuously – a significantly complex process.

To address the challenge, the Stanford team eliminated the radio-frequency source in the transmitter and replaced it with a commercially available voltage amplifier and feedback resistor. This system automatically figures out the right frequency for different distances without the need for human interference."

The prototype is for now very low power, but works across most of the three-foot range, despite the changing orientation of the receiving coil.

"The group used an off-the-shelf, general-purpose amplifier with a relatively low efficiency of about 10 percent. They say custom-made amplifiers can improve that efficiency to more than 90 percent.

“We can rethink how to deliver electricity not only to our cars, but to smaller devices on or in our bodies,” Fan said. “For anything that could benefit from dynamic, wireless charging, this is potentially very important.”"