A Stanford University research team 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 research is to develop an all-electric highway that
wirelessly charges cars and trucks as they cruise down the road.
The new
technology has the potential to dramatically increase the driving range of
electric vehicles and eventually transform highway travel, according to the
researchers. Their results are published in Applied
Physics Letters (APL).
“Our vision is that you’ll be able to drive onto any highway and charge
your car,” said Shanhui Fan, an associate professor of electrical
engineering. “Large-scale deployment would involve revamping the entire
highway system and could even have applications beyond transportation.”
Driving range
A wireless charging system would address a major drawback of plug-in electric
cars—their limited driving range. The all-electric Nissan Leaf, for example,
gets less than 100 miles on a single charge, and the battery takes several
hours to fully recharge.
A
charge-as-you-drive system would overcome these limitations. “What makes
this concept exciting is that you could potentially drive for an unlimited
amount of time without having to recharge,” said APL study co-author Richard Sassoon, the managing director of the Stanford
Global Climate and Energy Project (GCEP), which funded the research. “You
could actually have more energy stored in your battery at the end of your trip
than you started with.”
The wireless
power transfer is based on a technology called magnetic resonance coupling. Two
copper coils are tuned to resonate at the same natural frequency—like two wine
glasses that vibrate when a specific note is sung. The coils are placed a few
feet apart. One coil is connected to an electric current, which generates a
magnetic field that causes the second coil to resonate. This magnetic resonance
results in the invisible transfer of electric energy through the air from the
first coil to the receiving coil.
“Wireless
power transfer will only occur if the two resonators are in tune,” Fan
noted. “Objects tuned at different frequencies will not be affected.”
In 2007, researchers at the Massachusetts
Institute of Technology used magnetic resonance to light a 60-W bulb. The
experiment demonstrated that power could be transferred between two stationary
coils about six feet apart, even when humans and other obstacles are placed in
between.
“In the MIT experiment, the magnetic field appeared to have no impact
on people who stood between the coils,” Fan said. “That’s very
important in terms of safety. “
Wireless charging
The MIT researchers have created a spinoff company that’s developing a
stationary charging system capable of wirelessly transferring about 3 kW of
electric power to a vehicle parked in a garage or on the street.
Fan and his
colleagues wondered if the MIT system could be modified to transfer 10 kW of
electric power over a distance of 6.5 ft—enough to charge a car moving at
highway speeds. The car battery would provide an additional boost for
acceleration or uphill driving.
Here’s how the
system would work: A series of coils connected to an electric current would be
embedded in the highway. Receiving coils attached to the bottom of the car
would resonate as the vehicle speeds along, creating magnetic fields that
continuously transfer electricity to charge the battery.
To determine
the most efficient way to transmit 10 kW of power to a real car, the Stanford
team created computer models of systems with metal plates added to the basic
coil design.
“Asphalt
in the road would probably have little effect, but metallic elements in the
body of the car can drastically disturb electromagnetic fields,” Fan
explained. “That’s why we did the APL
study—to figure out the optimum transfer scheme if large metal objects are
present.”
Using mathematical
simulations, postdoctoral scholars Xiaofang Yu and Sunil Sandhu found the
answer: A coil bent at a 90-degree angle and attached to a metal plate can
transfer 10 kW of electrical energy to an identical coil 6.5 ft away.
“That’s fast enough to maintain a constant speed,” Fan said.
“To actually charge the car battery would require arrays of coils embedded
in the road. This wireless transfer scheme has an efficiency of 97%.”
Wireless future
Fan and his colleagues recently filed a patent application for their wireless
system. The next step is to test it in the laboratory and eventually try it out
in real driving conditions. “You can very reliably use these computer
simulations to predict how a real device would behave,” Fan said.
The researchers
also want to make sure that the system won’t affect drivers, passengers or the
dozens of microcomputers that control steering, navigation, air conditioning,
and other vehicle operations.
“We need
to determine very early on that no harm is done to people, animals, the
electronics of the car or to credit cards in your wallet,” said Sven
Beiker, executive
director of the Center for Automotive Research at Stanford (CARS). Although a
power transfer efficiency of 97% is extremely high, Beiker and his colleagues
want to be sure that the remaining 3% is lost as heat and not as potentially
harmful radiation.
Some
transportation experts envision an automated highway system where driverless
electric vehicles are wirelessly charged by solar power or other renewable
energy sources. The goal would be to reduce accidents and dramatically improve
the flow of traffic while lowering greenhouse gas emissions.
Beiker, who
co-authored the APL study,
said that wireless technology might one day assist GPS navigation of driverless
cars. “GPS has a basic accuracy of 30 to 40 ft,” he said. “It
tells you where you are on the planet, but for safety, you want to make sure
that your car is in the center of the lane.” In the proposed system, the
magnetic fields could also be used to control steering, he explained. Since the
coils would be in the center of the lane, they could provide very precise
positioning at no extra cost.
The
researchers also have begun discussions with Michael Lepech, an assistant
professor of civil and environmental engineering, to study the optimal layout
of roadbed transmitters and determine if rebar and other metals in the pavement
will reduce efficiency.
“We have the opportunity to rethink how electric power is delivered to
our cars, homes and work,” Fan said. “We’re used to thinking about
power delivery in terms of wires and plugging things into the wall. Imagine
that instead of wires and plugs, you could transfer power through a vacuum. Our
work is a step in that direction.”
