A Critical Look at Wireless Power

OUTER LIMITS: Magnetic-field values [top] and electric-field values [above] for the MIT wireless-power system [yellow and red] exceed ICNIRP limits [green] by a wide margin.

One such application is materials handling. Daifuku Co., based in Osaka, Japan, for example, has licensed patents from the University of Auckland to build tracked conveyor systems with moving platforms that are powered wirelessly. These systems, which make up a significant fraction of Daifuku’s US $2.5 billion in yearly sales, don’t generate the fine particles that would contaminate sensitive processes like chipmaking, as brushed electrical contacts tend to do. Such conveyances are useful in other industrial settings, too. Audi and BMW, for example, both use inductively powered carts on their assembly lines, these systems proving more robust than ones that rely on brushed contacts.

Another well-established application is the charging of electric vehicles. More than a decade ago, GM’s ill-fated EV1 was charged using an inductive paddle, instead of actually plugging it into an outlet. In the ancient port area of Genoa, Italy, you’ll find electric buses that charge themselves wirelessly at the whopping rate of 60 kilowatts for 10 minutes each hour, by parking over flat charging coils built into the road surface. The system was built by Conductix-Wampfler of Weil am Rhein, Germany, which has also licensed patents from the University of Auckland.

These systems have long ago proved themselves able to move power wirelessly, often a lot of it, and with good efficiency. That’s possible because they transfer the power for only a short stretch through the air—a few tens of centimeters at most—nothing like the distances WiTricity and Intel are shooting for. The key question is whether engineering improvements will make greater separations practical.

“I’m skeptical about sending [power] over distances that are larger than the coil diameter,” says Menno Treffers, who works for Royal Philips Electronics and serves as the head of the two-year-old Wireless Power Consortium, which is aimed at establishing industry standards for the wireless charging of consumer electronics. Right now you can get wireless charging pads if you buy a Dell Latitude Z laptop or a Palm Pre smartphone. BlackBerry and iPhone owners can get this feature too, if they purchase special aftermarket charging sleeves.

The idea the Wireless Power Consortium is pushing is that eventually you’ll be able to buy a single charging pad that will recharge whatever device you place on top, regardless of brand. Treffers is keen to bring such interoperability to what he sees as a blossoming consumer technology, but he doesn’t expect it to get to the point where you can recharge your mobile gizmo while using it. “It’s not like you can charge your BlackBerry while sitting on the couch,” he says.

Eberhard Waffenschmidt, a Philips electrical engineer working with Treffers, has examined the question of what distances are possible for resonant inductive charging. His calculations suggest that the prototype systems that Intel and WiTricity have demonstrated are pushing the limits of what can practically be done without efficiency plummeting to ridiculously low levels. And even if poor efficiencies could be tolerated, the RF field levels required to send truly useful amounts of power over even modest distances would be above what you could reasonably expose people to. “All the journalists had missed this,” says Waffenschmidt, adding that “[charging] pads don’t have this problem.”

Is there no way then to increase the distance you can send power wirelessly? Of course there is, but not inductively. If you have a clear path, you could use microwaves or laser beams, as has been demonstrated many times. Or just keep it simple. “Sunlight is excellent for long-distance power transfer,” quips Treffers.

Even if resonant induction ends up being limited to short distances, it may yet have a great influence, particularly for the future of transportation. “We now [have the means to] charge a car safely and efficiently over gaps of 20 to 40 centimeters, and we believe we can build that into a roadway system,” says Covic. “That’s probably a decade away, but you’ve got to have a vision, and ours is roadway-powered systems.”

A small step in that direction is taking place at Berlin-based Bombardier Transportation, which is gearing up to offer an electric streetcar that is inductively powered through the roadway. Although there are other ways to avoid a streetcar’s hard-to-maintain cables [see “Fuel Cells Could Power a Streetcar Revival,” IEEE Spectrum, September 2009], Bombardier’s system, called Primove, avoids many of the problems that some of the competing solutions face.

If this approach for powering streetcars catches on, perhaps wider-ranging electric buses will be next. It’s not unreasonable to think that some decades from now private electric cars may also be able to draw at least some of their power wirelessly from suitably equipped roadways. Who knows? Maybe the early adopters of today’s electric cars will be able to retrofit their rides to get such a boost while cruising. Then even the people who bought their Tesla Roadsters in the 2010s could zoom along inductively powered highways of the future. Such a thing would surely please that roadster’s visionary namesake.