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Wireless charging could widen efficiency edge over cords, suggests DOE lab

Bengt Halvorson


Oct 26, 2018

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ORNL wireless charging demonstration

ORNL wireless charging demonstration

Researchers at the Department of Energy’s Oak Ridge National Laboratory last week announced they've managed to transfer 120 kw of power at an efficiency of 97 percent—more efficiently than any currently available physical charging connection.

A version of the 20-kw wireless charging system that the same laboratory previously designed is being used for commercial delivery trucks. In pushing power to 120 kW, the ORNL team was able to achieve this with a six-inch gap—a distance that’s about normal for the ground clearance of a passenger car.

The demonstration is seen as a step on the way to the goal of providing a system that could deliver up to 350 kw and reduce electric-vehicle charging times to about 15 minutes—or potentially less. Before that, researchers will increase the power level of the system to 200 kw and refine the design.

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Today’s DC fast chargers have reached about 94 percent efficiency, but as we reported earlier this week, a new design claims a 97.6-percent efficiency at full power.

ORNL wireless charging demonstration

ORNL wireless charging demonstration

In either case, it would be a lot more efficient than Level 1 charging from a 120-volt household AC socket or Level 2 charging via a commercial or home charging station running on 240-volt AC. Those systems, in a best-case scenario, according to the charging-hardware maker ABB, have about an 88-percent or 93-percent efficiency, respectively, at best. And that’s not counting losses at the pack, which may total another 5 to 8 percent according to some studies.

While wireless charging for vehicles is certainly not a new idea, the power levels are. The SAE established J2954, a specification guideline for wireless charging up to 7.7 kw, aimed at light-duty vehicles and operating in a common frequency band (85 kilohertz), in May 2016. Based on tests conducted by another DOE lab, Idaho National Laboratory, it was verified that at up to those power levels even unmatched hardware could achieve a 93-percent efficiency.

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The ORNL researchers boast that they’ve achieved their efficiency level with a new coil design—which really isn’t what the fledgling wireless-charging business needs right now. Part of what complicates matters more is that companies aiming at commercializing wireless charging—Qualcomm, WiTricity, and Samsung are examples—continue to each have competing coil designs that impede progress.

ORNL wireless charging demonstration

ORNL wireless charging demonstration

In November 2017 the SAE made some ‘recommended practice’ revisions to J2954 aimed at interoperability between brands, using a single coil setup to 11 kw. But at the 22 kw power level, increasingly useful as battery packs get larger, interoperability remains a work in progress.

Concurrently, the ORNL researchers are continuing to refine the dynamic wireless charging technology, with the eventual goal tech, with the eventual goal of also achieving lower-power charging at highway speeds, via charging pads embedded in roadways.

READ MORE: Using Plugless Power's wireless charging with Nissan Leaf: video

Last year, Qualcomm demonstrated a dynamic version of its Halo wireless charging system that could provide 20 kw of continuous power through a dynamic system good for freeway speeds—enough to move an aerodynamically efficient compact or even mid-size vehicle. Qualcomm, which is looking to license multiple suppliers, credited its proprietary technology and coil design for that system’s dynamic energy transfer at an 80-percent efficiency.

The barrier even for static wireless charging continues to be cost, however—an estimated $1,500 to $2,500 per vehicle, according to Navigant—for a benefit that automakers haven’t been able to justify yet. If researchers and companies can make higher power levels and cross-compatibility work (at efficiency advantages), then the barriers might quickly dissolve.

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