A new drive toward wireless EV charging

  • 27-Jun-2012 12:19 EDT
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Delphi is working with start-up Witricity to develop wireless EV charging technology.

Recharging your electric car at home is a relatively simple process, but grabbing a fast charge on the road is a different matter, at best resembling gassing up at a service station. Most prospective car buyers think that an automated, hands-free charging system would ultimately help sell EVs. After all, drivers would not have to bother to plug in when they get home.

U.S. Department of Energy Secretary Steven Chu recently announced that the agency plans to pay as much as $12 million over three years to as many as four contractors to develop wireless charging technology for EVs. The advanced power-transfer systems might facilitate smaller EV battery pack size, lower weight, higher efficiency, and extended driving range, he said.

The DOE's aim is to commercialize wireless EV charging technology for parked cars during the next decade but also to eventually help spawn roadbed wireless systems that could charge up battery-powered vehicles on the go, said a top technical official at the DOE who asked to go unnamed.

The agency's Office of Energy Efficiency and Renewable Energy will announce the grant recipients later this year. Year one of the technology efforts would be devoted to R&D on a wireless charging system; the second year would involve prototype construction and integration. The third year would be for test and demonstration of the prototypes in vehicles.

“People have been talking a lot lately about wireless charging, but the technology’s been around for a while, though perhaps not always in a useable form,” noted the DOE source.

Noncontact recharging systems, which charge passively in parking spaces and driveways, is really not much more than a transformer, he said. Two wire coils are arranged along a common axis so a change of current in one will efficiently induce voltage change in another.

The principal downsides of inductive charging systems are lower efficiency and greater resistive heating compared to conventional plugs and cables associated with conductive charging. But energy transfer losses can be reduced using optimized magnetic resonance-based coils that could involve higher frequencies, secondary induction coils, capacitors, and specialized drive electronics, according to the DOE source.

DOE researchers, he explained, surveyed the available developmental-technology wireless-charging projects and deemed it ready to provide power transfer efficiency greater than 85%. The prototype should deliver nominal power transfer of at least 3.3 kW (to 5 kW)—that is, the Level 2 ac charging range. The system should accommodate a gap spacing and alignment flexibility over a 3-ft (0.9-m) range, consistent with conditions that would be experienced in the real world. Each contractor would be expected to demonstrate on-road operation of the new technology with at least five vehicles and five wireless charging stations.

The wireless recharging industry has burgeoned of late with the rise of new firms such as Witricity, Momentum Dynamics, and HaloIPT, which was purchased last year by Qualcomm.

Qualcomm has had a wireless charging test under way in London for the past few months. Meanwhile, Hertz is testing Plugless Power's technology in its fleet of Nissan Leafs, Chevrolet Volts, and Ford Transit Connect Electric utility vehicles. Established companies such as Samsung, Delphi, IHI, Hydro-Québec, and Bombardier, as well as automakers including Toyota, General Motors, Audi, and Rolls-Royce, have tested prototypes in the lab.

The SAE International J2954 Wireless Charging Task Force is slated to have a final draft of the guideline completed this year.

The DOE manager also noted some products on the market today such as high-resonance inductive power transfer technology that offer relatively efficient wireless power transfer at rates and involving gap geometries sufficient for recharging cars. A system comprises power and signal distribution systems, a capture resonator, and interface electronics—fitted to the bottom of the vehicle and matched up to a stationary source resonator pad and charging controller.

Compared to standard inductive systems, this new magnetic coupling technology will efficiently transfer power over significantly larger distances and will accommodate greater parking-related vehicle misalignments. The developers claim that their systems can fully charge an EV at rates comparable to those of most residential plug-in chargers—as fast as 4 h.

Wireless charging, particularly if it could be integrated into the roadbed, would help to lessen the need for bulky onboard energy-storage batteries, said the DOE source. Future wireless charging could extend EV range by enabling a driver to charge up during a trip when the vehicle is not in motion, such as when stopped at traffic lights—so-called quasi-dynamic wireless charging. That in turn will help carmakers use a cascade of improvements to produce lighter, less expensive, and more efficient vehicles with a smaller environmental footprint.

A Stanford University research team led by Shanhui Fan, an electrical engineering professor, has designed a high-efficiency charging system that uses magnetic fields to wirelessly transmit large electric currents between metal coils placed several feet apart. The long-term goal of the work is to develop an all-electric highway that wirelessly charges cars and trucks as they cruise down the road.

Large-scale deployment would involve overhauling the entire highway system. Aside from working out technological issues, the cost of burying the system in roadways remains a major obstacle, but cost-effectiveness could be boosted by focusing the system on the most heavily traveled routes and installing the technology during routine road resurfacing efforts.

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