Stanford University researchers have come up with the idea of making an electric car highway in which magnetic fields could wirelessly transmit large electric currents on highways, charging electric cars as they are driven.
The new technology has the potential to dramatically increase the driving range of electric cars and eventually transform highway travel, according to the researchers.
Their results are published in the journal Applied Physics Letters (APL).
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.
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.
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.
To determine the most efficient way to transmit 10 kilowatts of power to a real car, the Stanford team created computer models of systems with metal plates added to the basic coil design.
Using mathematical simulations, postdoctoral scholars Xiaofang Yu and Sunil Sandhu found the answer: A coil bent at a 90-degree angle and attached to a metal plate can transfer 10 kilowatts of electrical energy to an identical coil 6.5 feet away.
To actually charge the car battery would require arrays of coils embedded in the road. This wireless transfer scheme has an efficiency of 97 percent.
The researchers also want to make sure that the system won’t affect drivers, passengers or the dozens of microcomputers that control steering, navigation, air conditioning and other vehicle operations.
Some transportation experts envision an automated highway system where driverless electric vehicles are wirelessly charged by solar power or other renewable energy sources. The goal would be to reduce accidents and dramatically improve the flow of traffic while lowering greenhouse gas emissions.
Sven Beiker, executive director of the Center for Automotive Research at Stanford (CARS), who co-authored the APL study, said that wireless technology might one day assist GPS navigation of driverless cars. “GPS has a basic accuracy of 30-40 feet,” he said. “It tells you where you are on the planet, but for safety, you want to make sure that your car is in the center of the lane.” In the proposed system, the magnetic fields could also be used to control steering, he explained. Since the coils would be in the center of the lane, they could provide very precise positioning at no extra cost.
[source: Stanford University – pdf]
