One way to extend the range of electric vehicles may be to provide power wirelessly through coils placed under the surface of a road. But charging moving vehicles with high-power wireless chargers below them is complex.
Researchers at North Carolina State University have developed a method to deliver power to moving vehicles using simple electronic components, rather than the expensive power electronics or complex sensors previously employed. The system uses a specialized receiver that induces a burst of power only when a vehicle passes over a wireless transmitter. Initial models indicate that placing charging coils in 10 percent of a roadway would extend the driving range of an EV from about 60 miles to 300 miles, says Srdjan Lukic, an assistant professor of electrical engineering at NCSU.
Wireless charging through magnetic induction—the same type typically used for electric toothbrushes—is being pursued by a number of companies for consumer electronics and electric vehicles (see “Wireless Charging—Has the Time Finally Arrived?”). Such chargers work by sending current through a coil, which produces a magnetic field. When a car with its own coil is placed above the transmitter, the magnetic field induces a flow of power that charges the batteries.
Stationary inductive chargers for electric vehicles typically use sensors to ensure that the receiver coils on the vehicle are aligned above wireless charging pads correctly. The NCSU researchers’ system operates without position sensors in an attempt to simplify the design and make it more efficient. When there are no vehicles, the transmitter coil gives off a weak field. But when a vehicle with a receiver passes by, electronics in the receiver trigger a strong magnetic field and an accompanying flow of power, says Lukic.
Precisely controlling when the roadway coils produce a magnetic field is important for safety reasons; if the field misses the car’s receiving coils, it could attach to parts of the car or attract stray objects. “Somehow we have to channel or contain the magnetic field produced by the transmitter to always be right below the receiver. We cannot just beam out a strong field into the environment,” he says. Some designs have a series of coils that are always energized, but that approach is not energy-efficient, Lukic says.
In a stationary induction charger, the power receiver is made with a simple coil. The NCSU device is more sophisticated. It uses capacitors and inductors to manipulate the power transfer and magnetic field, says Lukic. The coupling between transmitter and receiver could be done with power electronics, but such a system would be more expensive than the NCSU device, he says.
The researchers have made a low-power prototype and intend to reach a rate of 50 kilowatts, which is equivalent to direct-current fast chargers, which work more efficiently than conventional alternating-current chargers.
Commercial interest in wireless charging systems for moving vehicles is growing. Qualcomm is working on a “dynamic” charging system that builds off its current stationary wireless EV charger. The University of Utah has tested a wireless charging infrastructure for city buses and has spun out a company called Wireless Advanced Vehicle Electrification to build commercial products. With the Utah system a bus could charge from coils placed under the road surface where passengers load or at traffic lights. Dynamic wireless power transfer could also be used for robots.
The techniques that the NCSU researchers used for dynamic EV charging have already been applied in some consumer electronics, says Katie Hall, the chief technology officer of WiTricity, a company that makes wireless charging equipment. But the electronic tooling used for small electronics, such as switches, isn’t readily available for high-power applications. “That kind of technology doesn’t seamlessly scale to kilowatts or hundreds of kilowatts,” she says.
The Oak Ridge National Laboratory is also working on ways to automatically match the wireless power transmitter and receiver, says Omer Onar, a researcher who works on wireless vehicle charging there. The new work addresses only one of the barriers of the dynamic charging, he says: “Most of the [commercial] barriers are associated with cost and infrastructure.”