The All-Electric Car You Never Plug In

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?

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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.

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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.