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Wireless Recharging EVs on the Go

April 8th 2018

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Researchers at the University of Colorado Boulder (Boulder, CO) have developed a proof of concept for wireless power transfer that could potentially allow electric vehicles to be charged while they are moving.

The researchers' wireless power concept is based on transferring electrical energy through electric fields at very high frequencies. Future electric vehicles, they envision, could be able to recharge while driving down the highway by drawing wireless power directly from low-cost charging plates installed in the road.

Most electric vehicles today can only travel between 100 and 250 miles on a single charge, and charging stations are still far from ubiquitous in the U.S., requiring drivers to carefully plan their trips. That problem, say the researchers, could go away with this technology.

"On a highway, you could have one lane dedicated to charging," says Khurram Afridi, an assistant professor in CU Boulder's Department of Electrical, Computer and Energy Engineering. Afridi notes that a vehicle could simply travel in that lane when it needed an energy boost and could carry a smaller onboard battery as a result, reducing the overall cost of the vehicle.

While wireless power technology currently exists for low-power mobile devices, applying it to a vehicle, especially one in motion, would require a far more significant amount of power - on the order of tens of kilowatts of power - to be sent across a greater physical distance. In addition, a car traveling at highway speeds would not be in range of any single charging pad for more than a fraction of a second - so such pads would need to be placed every few meters to provide a continuous charge.

Most wireless power technology research to date has focused on transferring energy through inductive charging - i.e., using magnetic fields. Magnetic fields - at strength levels appropriate for substantial energy transfer - are easier to generate than equivalent electric fields. However, magnetic fields travel in a looping pattern, requiring the use of fragile and lossy ferrites to keep the fields and the energy directed, resulting in an expensive system.

Electrical fields, however, naturally travel in relatively straight lines, and the researchers saw an opportunity to take advantage of their more directed nature to substantially reduce the cost of the system. The challenge, however, of using electric fields for wireless power transfer – i.e., the capacitive approach – is that the large airgap between the roadway and the electric vehicle results in a very small capacitance through which the energy must be transferred.

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

To test such an approach, the researchers set up parallel metal plates separated by 12 centimeters. The two bottom plates represent the transmitting plates within the roadway while the two top plates represent the receiving plates inside a vehicle.

When the researchers flip a switch, energy is transmitted from the bottom plates and a lightbulb above the top plates instantly lights up. The researchers steadily improved the system to the point where it can transmit 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 says. "There were a lot of high fives that day."

The researchers plan to continue to develop the prototype and scale it for potential real-world applications. They have received funds from the Department of Energy's ARPA-E division and support from a National Science Foundation CAREER award, as well as a recent seed grant from the Colorado Energy Research Collaboratory to further explore the feasibility and optimization of the in-motion system.

In the near term, Afridi envisions the technology being adapted for warehouse use. Automated warehouse robots and forklifts, for example, could move along areas enabled for wireless power transfer and never need to be plugged in. The technology could also be adapted for use in next-generation transportation projects like the Hyperloop, a proposed system that could take passengers from Los Angeles to San Francisco in 30 minutes.

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