Charging Electric Vehicles on the Road
A Stanford University research team has designed a high-efficiency charging system that uses magnetic fields to wirelessly transmit large electric currents between metal coils placed several feet apart. The long-term goal of the research is to develop an all-electric highway that wirelessly charges cars and trucks as they cruise down the road.
The new technology has the potential to dramatically increase the driving range of electric vehicles and eventually transform highway travel, according to the researchers. Their results are published in the journal Applied Physics Letters (APL).
“Our vision is that you’ll be able to drive onto any highway and charge your car,” said Shanhui Fan, an associate professor of electrical engineering. “Large-scale deployment would involve revamping the entire highway system and could even have applications beyond transportation.”
A wireless charging system would address a major drawback of plug-in electric cars – their limited driving range. The all-electric Nissan Leaf, for example, gets less than 100 miles on a single charge, and the battery takes several hours to fully recharge.
A charge-as-you-drive system would overcome these limitations. “What makes this concept exciting is that you could potentially drive for an unlimited amount of time without having to recharge,” said APL study co-author Richard Sassoon, the managing director of the Stanford Global Climate and Energy Project (GCEP), which funded the research. “You could actually have more energy stored in your battery at the end of your trip than you started with.”
The wireless power transfer is based on a technology called magnetic resonance coupling. Two copper coils are tuned to resonate at the same natural frequency – like two wine glasses that vibrate when a specific note is sung. The coils are placed a few feet apart. One coil is connected to an electric current, which generates a magnetic field that causes the second coil to resonate. This magnetic resonance results in the invisible transfer of electric energy through the air from the first coil to the receiving coil.
“Wireless power transfer will only occur if the two resonators are in tune,” Fan noted. “Objects tuned at different frequencies will not be affected.”
In 2007, researchers at the Massachusetts Institute of Technology used magnetic resonance to light a 60-watt bulb. The experiment demonstrated that power could be transferred between two stationary coils about six feet apart, even when humans and other obstacles are placed in between.
“In the MIT experiment, the magnetic field appeared to have no impact on people who stood between the coils,” Fan said. “That’s very important in terms of safety. “
The MIT researchers have created a spinoff company that’s developing a stationary charging system capable of wirelessly transferring about 3 kilowatts of electric power to a vehicle parked in a garage or on the street.
Fan and his colleagues wondered if the MIT system could be modified to transfer 10 kilowatts of electric power over a distance of 6.5 feet – enough to charge a car moving at highway speeds. The car battery would provide an additional boost for acceleration or uphill driving.
Here’s how the system would work: A series of coils connected to an electric current would be embedded in the highway. Receiving coils attached to the bottom of the car would resonate as the vehicle speeds along, creating magnetic fields that continuously transfer electricity to charge the battery.