Efficiency boost for wireless charging of moving vehicles

May 08, 2020 //By Nick Flaherty
A wireless charging system for moving vehicles developed at Stanford University has a transfer efficiency of 92 percent
A wireless charging system for moving vehicles developed at Stanford University has a transfer efficiency of 92 percent

Engineers at Stanford University in california have taken a wireless charging system for moving electric vehicles out of the lab and adapted it for real world use.   

Three years ago, Stanford electrical engineer Shanhui Fan and Sid Assawaworrarit, a graduate student in his lab, built the first system that could wirelessly recharge objects in motion regardless of the distance using a technique from quantum machanics. However, this only had a 10 per cent power transfer rate.

Now the two researchers have developed a switched mode amplifier and feedback circuit that can deliver 92 per cent transfer efficiency.

“This is a significant step toward a practical and efficient system for wirelessly re-charging automobiles and robots, even when they are moving high speeds,” said Fan. “We would have to scale up the power to recharge a moving car, but I don’t think that’s a serious roadblock. For re-charging robots, we’re already within the range of practical usefulness.”

The prototype can wirelessly transmit 10W of electricity to a moving receiver over a distance of up to 62cm, but there aren’t any fundamental obstacles to scaling up a system to transmit the tens or hundreds of kilowatts that a car would need, says Fan, and the transmission takes only a few milliseconds. The only limiting factor will be how fast the car’s batteries, or a supercapacitor, can absorb the power.

The design uses a switch-mode amplifier with current-sensing feedback in a parity–time symmetric circuit. The parity–time symmetry guarantees that the effective load impedance on the switch-mode amplifier remains constant, which allows the amplifier maintains high efficiency despite variation of the transfer distance.

The first opportunity is for industrial robots and even aerial drones, say the researchers, as it’s more cost effective to embed chargers in factory floors or on rooftops than on long stretches of highway. This would allow a drone to hover over a


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