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The All-Electric Car You Never Plug In

Wireless power transmission would let EVs draw their power from the road

Picture an all-electric vehicle cruising down the highway, emitting little noise and no noxious fumes. It’s such an improvement that you have to wonder why only a handful of all-electric vehicles are now available on the mass market. 

Here’s a big reason: Picture the driver of that same car getting a call from a relative living far away who needs immediate help. Suddenly, the driver’s eyes become riveted on the most important indicator on the dashboard: the estimated number of kilometers that the car can go on the remaining battery charge. Will he make it to his relative’s house? Even if he does, will he find a charging station so he can get back home?

There’s a name for this modern misgiving: range anxiety, a new form of disquiet experienced by drivers of all-electric cars. The Nissan Leaf, for example, can be driven on the highway for only about 120 kilometers on a single charge, and fully charging up its batteries takes 8 hours or more.

But maybe there’s a way to relieve this fear forever and make drivers’ lives much easier as well. If we embed transmitting coils in roadways, electric cars carrying receiving coils could charge themselves as they zoom down the road. An e-car owner would never have to search for a charging station or plug in the car. That is the goal of our research team at the Korea Advanced Institute of Science and Technology (KAIST), in Daejeon, which has developed what we call the on-line electric vehicle (OLEV) system.

Wireless power transmission isn’t a new idea: Nikola Tesla built a 57-meter-tall tower behind his lab in Shoreham, N.Y., in the first years of the 20th century, partly to beam power to remote equipment. But only in the past decade have researchers begun to make the breakthroughs that can allow for commercially practical wireless charging, not only for portable electronic products like smartphones but even for industrial robots and electric cars. 

The technology depends on the same principle of electromagnetic induction that enables a transformer to change the voltage of an alternating current. This current flows through one coil of wire, creating a magnetic field whose polarity reverses with each cycle and inducing a corresponding alternating field in a secondary coil. The ratio of the number of turns in the two coils determines whether the transformer steps voltage up or down. Transformers usually include an iron-rich core, which links the coils and increases the field strength, but you don’t really need it. If the two coils are separated by air, current flowing through the first coil will still create a magnetic field, which will still be picked up by the second coil—it just won’t be picked up as well. The greater the air gap, the less efficient the transfer of power will be.

More than 90 years after Tesla began building his tower, an ambitious project in California tried to apply his concept of wireless power transmission to automobiles. In 1994, the Partners for Advanced Transit and Highways project, led by researchers at the University of California, Berkeley, demonstrated the transfer of power from coils buried in the road to the cars above. It worked whether the cars were at rest or in motion. The receiving coils were on the underside of the test vehicles and were separated from the transmitting coils by an air gap of only 7.5 centimeters. They captured 65 percent of the injected power, an impressive achievement at the time. But still, a scheme that wasted a full 35 percent of the power could not be brought to market, and an air gap that narrow would have required hanging the receiving coils so low that a bump or a pothole could have sheared them right off. 

How, then, to increase the efficiency of the power transfer without having to make the low-slung receivers even more vulnerable? The answer that we and other researchers have recently settled on is called magnetic resonance coupling.

A familiar illustration of the power of resonance is that of the opera singer and the wine glass. As the diva warbles, the acoustic waves hit the glass, causing it to vibrate; if she hits the note with the same natural frequency as the glass, each cycle will amplify the vibrations until finally they are strong enough to shatter the glass. The singer’s voice enters into a special relationship with that glass, passing more energy to it than to other, nonresonating items in the room. Similarly, when a transmitting coil sends electromagnetic waves tuned to a frequency matching the resonance of a circuit holding a receiving coil, it will transfer energy to it very efficiently. 

Researchers around the world have begun applying this principle over the past decade. In 2007, MIT professors caught the world’s attention by powering a lightbulb suspended in space, 2 meters away from the transmitting coil. Those researchers went on to found a Massachusetts start-up, WiTricity Corp., which is working with several auto companies on wireless charging stations for household garages. Quebec’s Bombardier is developing its Primove system in Europe to transmit power to public buses and trams. 

Our research at KAIST began in 2009 when one of us (Suh) identified battery-powered electric cars as a technology that would work far better if these cars didn’t have to lug around such huge, expensive batteries. Suh met with Myung-bak Lee, then the president of South Korea, and proposed that KAIST develop wirelessly powered vehicles. The National Science and Technology Council allotted the enterprise US $25 million, and our OLEV project was ready to roll.

On a cool March day in 2010, a distinguished group of scientists and politicians gathered in Seoul Grand Park to witness the inaugural run of a bright green-and-blue tram wreathed in flowers, with the words “Seoul Zoo” painted on its nose. Se-hoon Oh, then the mayor of Seoul, declared that he expected wirelessly powered buses and trams to be “the environment-friendly transportation in the future.” 

Today the OLEV tram is still zipping around the park on a 2.2-km loop of roadway, 370 meters of which has transmitting coils embedded in the asphalt. As the tram rolls along, magnetic sensors in the road detect its approach and activate the transmitters to send 62 kilowatts to the receiving coils on the underside of the tram. Meanwhile, the tram operator keeps an eye on a monitor that shows how well the tram is aligned with the transmitting coils it passes over, and thus how efficiently it’s receiving power. (We are developing a system that will align the vehicle automatically by measuring the strength of the magnetic field.) The bus still contains a battery, but it carries 40 percent less energy than it would have to otherwise. It’s also 6 percent lighter, at 1100 kilograms, and significantly cheaper, costing $88 500. 

The key advances our team made are in the design of the electromagnetic field. The first decision had to do with the relative strength of the two component fields—electric and magnetic. An engineer can choose which field to favor. For example, a dipole antenna generates a stronger electric field, and a loop antenna generates a more powerful magnetic field. We chose a dual loop design for the buried transmitter because magnetic fields are better at transmitting power through asphalt, dirt, and other substances that may come between the transmitter and the receiver. 

The second decision was to increase the efficiency of this power transmission with a shaped magnetic field—one whose path from the transmitter coil to the receiver coil is guided by the presence of ferrite cores on both sides. This arrangement minimizes field leakage and hence waste. In recent experiments with an OLEV bus, we charged the bus while it was being driven, its body 20 cm above the road, and achieved an average transfer efficiency of 75 percent. 



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