How magnetism might recharge an electric car as it drives

Imagine a futuristic freeway that charges electric cars wirelessly as they drive along it. While this may sound like science fiction, engineers are taking steps to bring this concept closer to reality.

Wireless charging pads already exist for smartphones, but they only work if the phone is sitting still. Using this technology for cars would be just as inconvenient as the current practice of plugging a car in for an hour or two at a charging station or overnight at home. So what if a car could recharge as it drives?

Engineers from the Stanford School of Engineering have demonstrated a practical way to use magnetism to transmit electricity wirelessly to recharge electric cars, robots and even drones.

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. However, the technology was too inefficient to be useful outside the lab.

Now, in Nature Electronics, the two engineers demonstrate a technology that could one day be scaled up to power a car moving along a road. In the nearer term, the system could make it practical to wirelessly recharge robots as they move around in warehouses and on factory floors — eliminating downtime and enabling robots to work almost around the clock.

“This is a significant step toward a practical and efficient system for wirelessly recharging automobiles and robots, even when they are moving at high speeds,” Fan said.

“We would have to scale up the power to recharge a moving car, but I don’t think that’s a serious roadblock. For recharging robots, we’re already within the range of practical usefulness.”

Wireless chargers transmit electricity by creating a magnetic field that oscillates at a frequency that creates a resonating vibration in magnetic coils on the receiving device. The problem is that the resonant frequency changes if the distance between the source and receiver changes by even a small amount.

In their first breakthrough three years ago, the researchers developed a wireless charger that could transmit electricity even as the distance to the receiver changed. They did this by incorporating an amplifier and feedback resistor that allowed the system to automatically adjust its operating frequency as the distance between the charger and the moving object changed.

But that initial system wasn’t efficient enough to be practical. The amplifier uses so much electricity internally to produce the required amplification effect that the system only transmitted 10% of the power flowing through the system.

In their current paper, the researchers boosted the system’s wireless-transmission efficiency to 92%. The key, 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 W of electricity over a distance of 0.5–1 m. Fan 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 resupply a speeding automobile. The wireless transmission takes only a few milliseconds — a tiny fraction of the time it would take a car moving at 110 km an hour to cross a 1.2 m charging zone. The only limiting factor, Fan said, will be how fast a car battery can absorb all the power.

Assawaworrarit explained that the wireless chargers shouldn’t pose a health risk because even chargers powerful enough for cars would produce magnetic fields that are well within established safety guidelines.

Though it could be many years before wireless chargers become embedded in highways, the opportunities for robots and 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 that could fly all day by swooping down occasionally and hovering around a roof for quick charges,” Fan said.

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