Professor Shanhui Fan and Sid Assawaworrarit (PhD candidate) recently published "Robust and efficient wireless power transfer using a switch-mode implementation of a nonlinear parity-time symmetric circuit" in Nature Electronics.

They have been working on improving the distance of a wireless charger. Previously they were able to transmit electricity as an object moved, but it wasn't practical.

In their new paper, the researchers show how to boost the system's wireless-transmission efficiency to 92%. The key, Sid Assawaworrarit explained, was to replace the original amplifier with a far more efficient "switch mode" amplifier. Such amplifiers aren't new but they are finicky and will only produce high-efficiency amplification under very precise conditions. It took years of tinkering, and additional theoretical work, to design a circuit configuration that worked.

The new lab prototype can wirelessly transmit 10 watts of electricity over a distance of two or three feet. Shanhui says there aren't any fundamental obstacles to scaling up a system to transmit the tens or hundreds of kilowatts that a car would need. He says the system is more than fast enough to re-supply a speeding automobile. The wireless transmission takes only a few milliseconds – a tiny fraction of the time it would take a car moving at 70 miles an hour to cross a four-foot charging zone. The only limiting factor, says Shanhui, will be how fast the car's batteries can absorb all the power.

Though it could be many years before wireless chargers become embedded in highways, the opportunities for robots and even aerial drones are more immediate. It's much less costly to embed chargers in floors or on rooftops than on long stretches of highway. Imagine a drone, says Shanhui, that could fly all day by swooping down occasionally and hovering around a roof for quick charges.

Excerpted from "Stanford researchers one step closer toward enabling electric cars to recharge themselves wirelessly as they drive"