Researcher Strives to Wirelessly Recharge Electric Cars in Motion

Electric vehicles of the future are likely to be able to recharge as they drive along the highway, taking wireless power directly from plates fixed in the road. This would make it possible to drive several thousand miles without having to stop for plugging in for recharge. This idea may seem farfetched but CU Boulder engineers aim to bring it closer to reality.

“We’d like to enable electric vehicles to charge on the go,” said Khurram Afridi, an assistant professor in CU Boulder’s Department of Electrical, Computer and Energy Engineering.

In the two years, Afridi and his colleagues have created a proof of concept for wireless power transfer that conveys electrical energy through electric fields at extremely high frequencies. The ability to transmit large quantities of energy across greater physical distance to in-motion systems from economical charging plates could in the future allow the technology to expand further than small consumer electronics like cell phones and start powering larger things like automobiles.

Presently on a single charge, most electric vehicles can travel between 100 and 250 miles, depending on the brand and model. But charging stations are not many in most parts of the country, requiring drivers to plan their travel strategically. That problem could disappear with this technology, Afridi said.

“On a highway, you could have one lane dedicated to charging,” Afridi said, adding that a vehicle could merely travel in that lane when it required an energy boost and could carry a smaller onboard battery as a result, decreasing the total cost of the vehicle.

The probable applications of wireless power transfer technology are as sensational as they are futuristic, but the questions Afridi is trying to answer date back a lot longer than the present technological age.

An Innovation Long in the Making

The idea of wireless power transfer has captivated scientists for more than a century. In the 1890s, the inventor Nikola Tesla promoted the idea of transferring energy across distance and legendarily demonstrated the idea in public, lighting up a bulb placed at the other end of the stage. The idea led Tesla to initiate work on a wireless power transmission station in Shoreham, New York that he believed would transfer electricity through the air, but the project was never finished because of financial hitches.

The dawn of radio revolutionized long-distance wireless communication happened subsequently, but the high-frequency technology to convey large quantities of electrical energy wirelessly had not been established yet. Wireless power transfer suffered for the most part of the 20th century. The consumer electronics boom of the late 1990s and early 2000s, nevertheless, reawakened interest in the topic again. Today, certain small consumer devices comprise wireless power transfer, which allows the object to draw energy while resting on a specially-designed pad that is plugged into an outlet.

Duplicating this capability for an automobile in motion is a lot more challenging, requiring considerably more power to be transmitted across a greater physical distance from the roadway to the vehicle. A car traveling at highway speeds would zoom past a single charging pad within a fraction of a second, so the pads would have to be positioned every few meters to deliver a continuous charge.

Embracing the Challenge

To solve the scale and in-motion issue, Afridi had to think in a different way about methodology. A smartphone only needs five watts of power for recharging. A laptop may need 100 watts. However, an electric vehicle in motion would surely need tens of kilowatts of power, two orders of magnitude higher.

Most wireless power technology research thus far has concentrated on conveying energy through magnetic fields – the so-called inductive method. Magnetic fields, at strength levels suitable for ample energy transfer, are easier to produce than equivalent electric fields. However, magnetic fields travel in a looping pattern, requiring the use of delicate and lossy ferrites to preserve the fields and the energy directed, resulting in a costly system.

Electrical fields, by contrast, logically travel in comparatively straight lines. Afridi wanted to exploit the more directed nature of electric fields for his innovation and considerably decrease the cost of the system.

The difficulty of using electric fields for wireless power transfer – the capacitive method – is that the large airgap between the electric vehicle and the roadway causes a very small capacitance through which the energy must be conveyed.

“Everybody said that it’s not possible to transfer that much energy through such a small capacitance,” said Afridi. “But we thought: What if we increase the frequency of the electric fields?”

In his laboratory, Afridi and his students arranged metal plates parallel to one another, with a gap of 12 cm. The two bottom plates signify the transmitting plates embedded in the roadway while the two top plates signify the receiving plates within the vehicle.

When Afridi flicks a switch, energy is conveyed from the bottom plates. Immediately, the lightbulb above the top plates lights up—power transmission with no wires necessary. The device has progressively improved to the point where it can convey kilowatts of power at megahertz-scale frequencies.

“When we broke the thousand-watt barrier by sending energy across the 12-centimeter gap, we were just exhilarated,” Afridi said. “There were a lot of high fives that day.”

The Commute of the Future?

Afridi aims to pursue creation of the prototype and scale it for possible real-world applications. He acknowledges funds from the Department of Energy’s ARPA-E division and support from a National Science Foundation CAREER award. A recent seed grant from the Colorado Energy Research Collaboratory, granted to Afridi in collaboration with Colorado State University and NREL, will allow him to explore the viability and development of the in-motion system.

In the near future, Afridi foresees the technology being adapted for warehouse use. Automated warehouse forklifts and robots, for instance, could move along areas enabled for wireless power transfer and never face the necessity to be plugged in, eradicating downtime and boosting productivity. The technology could also be adapted for use in next-generation transportation projects like the Hyperloop, a planned system that could transport passengers from Los Angeles to San Francisco in 30 minutes.

The advent of an electric highway is still years ahead and will unavoidably face many obstacles, both technological and societal. But Afridi is positive and driven to overcome them.

“As a scientist, you feel challenged by things that people tell you are impossible to do,” Afridi said.

Credit: University of Colorado Boulder