Wireless Power for Moving Electric Vehicles Closer to Reality

Two men stand in front of and beneath a pair of large white wheels trimmed with copper foil. Electronic equipment hangs from the bottom of the wheels and more equipment is on a table behind one wheel.
Photo: Stanford University

Wireless charging of moving electric vehicles is one step closer to hitting the road, Stanford University researchers say. Such technology could also help charge mobile devices, medical implants, and factory robots, the scientists add.

Wireless charging via a technique known as magnetic induction is currently used to power electric toothbrushes, power tools, robotic vacuum cleanersmedical implants, and other devices. Inductive charging depends on at least two coils—one in the power transmitter, and one in the power receiver. When the transmitter coil is energized, it generates an electromagnetic field that can transfer energy to the receiver coil.

In 2007, MIT researchers showed it was possible to wirelessly power a light bulb more than 2 meters away, raising the possibility that coils buried in a road could help charge electric vehicles on the move above them. Such a strategy could in theory let an electric vehicle drive for an unlimited amount of time without having to stop to recharge, says study senior author Shanhui Fan, an electrical engineer at Stanford University.

However, to deliver wireless energy in an efficient manner, techniques generally have to precisely tailor the electrical properties of the transmitters and receivers to account for their position and orientation with respect to one another. So these methods either require that both transmitters and receivers stay nearly still, or “you need to constantly adjust the parameters of your circuits if transferring power to a moving device, which adds quite a bit of complexity to the system,” Fan says.

Now Fan and his colleagues have developed a technique that can efficiently deliver constant power to a nearby moving device using simple electronics. The scientists detailed their findings this week in the journal Nature.

Typically, wireless power techniques use radio waves to excite the transmitter coil. Fan and his colleagues replaced this radio-wave source with an off-the-shelf general-purpose amplifier.

If the coils are identical, the whole system will automatically select the frequency that maximizes energy transfer efficiency from the transmitter to the receiver no matter the distance between the coils because of a concept from quantum physics known as parity-time symmetry, the researchers say. This eliminates the need for complex tuning electronics for the transmitters and receivers as they move.

In experiments, this new technique could power an LED bulb on a receiving coil. Its brightness stayed constant as the receiver moved up to about one meter away from the transmitter. “We really think we've taken an important step forward with dynamic charging, with wireless power transfer to a moving object,” Fan says.

The new technology should charge wireless devices no matter how fast they are traveling, Fan says. “The system very quickly adjusts itself within basically a few hundred microseconds,” he says.

The transmitter delivers energy to a receiver with an efficiency “on the order of 90 percent or so on the scale of a meter,” Fan says. However, the commercial amplifier they used is not efficient at generating the oscillating electric field needed for wireless power transfer. “Going forward, we're designing an amplifier specifically for wireless energy transfer,” Fan says. “The amplifier we used works over a very broad range of frequencies, but we only need a narrow range of frequencies, so we can design an amplifier specifically for energy transfer that is more efficient.”

The experiments involved only one milliwatt of power, whereas electric cars can require tens of kilowatts. Still, “the underlying physics should be able to scale up to a much higher amount of power,” Fan says. “The high power electric vehicles need present a challenge, but interesting applications with mobile devices and medical implants might be available before reaching such high power scales.”

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