No matter how efficient electric vehicles get, they still rely on bulky batteries that require extended charging times. Gasoline, which allows drivers to refuel in a matter of minutes and be on their way, has maintained its advantage.
One solution that's often floated is the wireless power transfer. A recent study at Stanford University proposed an electrified highway that would allow a vehicle to recharge on the run , a nice idea in theory but one that would require an enormous infrastructure upgrade that may not be feasible yet. But researchers at Utah State University say they may have an even better answer: A prototype charging system transfers more power with greater range and efficiency than other systems and would lessen the expense dramatically by placing charging pads at select locations rather than all along a road.
A prototype electric coach called the Aggie Bus, developed by Utah State engineers, will soon shuttle students from one end of the Logan, Utah, campus to the other. Rather than lining the bus's route with charging pads, the engineers will place a single pad at one of the stops along the 1.6-mile loop to top off the bus batteries. Hunter Wu, the electrical and computer engineering professor at Utah State University who heads the project, says that this setup not only limits the infrastructure cost, but also allows the bus to use an 80 percent smaller battery. The same idea could be applied to common stopping points for passenger vehicles such as intersections and parking spots, he says, to enable quick top-offs for any type of electric vehicle.
"What our solution supplies is this continuous energy buffer," Wu says. "It can basically maintain the bus to go and go and go and last the whole day."
Wu made wireless power transfer the subject of his Ph.D. thesis while studying at the University of Auckland in New Zealand. He moved to Utah State in 2010 to put his ideas into practice. After two-and-a-half years of effort, his team unveiled the bus in November, demonstrating what they call the best-ever combination of efficiency, amount of power transferred, and distance transmitted for wireless charging.
The technology that makes it possible to charge a car wirelessly relies on the same induction principle that electric toothbrushes and other consumer devices use. A primary coil wired into the power grid generates a magnetic field. That field interacts across a gap with a secondary coil in the device, which then converts the energy in the magnetic field back into electricity.
To increase the power, efficiency, and distance between charger and battery, Wu and his Utah State team used ferrimagnetic material—manganese zinc—to shape the magnetic field generated by the charging pad into a kind of fountain that can be pointed at the device to be charged. They also tuned the two coils to resonate at the same frequency to drive efficiency of power transfer and generated magnetic fields that oscillate at 20,000 times a second to transfer high levels of power.
The system transmits 25 kilowatts of power with 90 percent efficiency across a gap of six inches, with a margin for misalignment of up to six inches, according to Wu. The prototype version of the charging pad is a 56-inch diameter, 160-pound disk of ferrite, copper, and aluminum with the coil of wire on top. For the campus demonstration project, Wu and his team are working to get the diameter down to 36 inches and cut the weight in half. They'll also need to make it waterproof and strengthen it enough to withstand the weight of the bus driving over it. Then they can embed it in the road surface at the bus stop.
The prototype bus can get about 10 minutes of drive time for every five minutes stopped over the charging pad. That charging rate wouldn't be enough for many bus lines. So Wu and his colleagues are working to double the power output of their system every six months for the next year-and-a-half, hoping to arrive at a 200-kilowatt system. Wu says that once the charger reaches 100 watts, it will be closer in line to what a real-world transit agency needs. The 200-kilowatt system should be able to give a bus an hour of driving time for every five minutes of charging, and Wu also wants to increase the gap between charging pad and bus chassis to eight inches. "And that's really suitable for mass transit," he says.
The system is safe, too, as Robert Behunin, Utah State's head of Commercialization and Regional Development, finds himself reminding people. Companies around the world have expressed interest in the technology, Behunin says, and their top concern, along with the system's charging capability, is whether the power transfer is dangerous. "We know that there are people who have concerns about, 'Gosh, what happens if a chihuahua runs under the bus while you're charging?' Well, the answer to that is 'Nothing.' It doesn't even get slightly warm."
If charging pads like Wu's can make electric vehicles more practical for transit, what about everyday driving with cars that aren't traveling a predetermined route. He proposes that pads could go into the places where people most commonly stop, such as intersections and parking spaces. That would give EVs a top-off and without building charging tech into entire highways.
Shanhui Fan, an electrical engineering professor behind the aforementioned Stanford study, thinks power transfer to stationary vehicles is an interesting prospect. But to him, it might be just a first step, and not enough for EVs to catch up to gasoline cars. That would take a more ambitious vision. "You would basically have a way to connect vehicles directly into the grid with some of the major highways being electrified," he says. Getting all those batteries to stay on the grid would be one advantage to the entire system, providing a way to balance electric loads during times of peak power usage. And as for the expense of upgrading the highways, Fan says, the highway system (like most of America's infrastructure) needs a major overhaul anyway and electrifying them would be only a part of that overall cost.
If the new system takes off, it could herald a new era in electric vehicles, one in which range is not an issue and the charging process becomes automatic.