Stanford researchers designing magnetic resonance coupling system for wireless on-road dynamic charging of EVs
2 February 2012
|Simplified schematic of the wireless energy transfer system in free space. Yu et al. Click to enlarge.|
A Stanford University research team is designing a high-efficiency wireless charging system using magnetic resonance coupling (earlier post) 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.
Their proposed design, as published in the journal Applied Physics Letters (APL), would transfer up to 10 kW of electrical energy to a coil 6.5 feet away with an efficiency of up to 97%.
Resonant coupling wireless power transfer uses two copper coils tuned to resonate at the same natural frequency. 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 transfer of electric energy through the air from the first coil to the receiving coil.
In 2007, researchers at the Massachusetts Institute of Technology used magnetic resonance to light a 60-watt 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. The MIT researchers created a spinoff company—WiTricity (earlier post)—that is 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.
The power transfer efficiency of a WiTricity solution depends on the relative sizes of the power source and capture devices, and on the distance between the devices. Maximum efficiency is achieved when the devices are relatively close to one another, and can exceed 95%. WiTricity has entered partnerships with Toyota and Delphi.
Shanhui Fan, an associate professor of electrical engineering, and his colleagues wondered if the MIT system could be modified to transfer 10 kW of electric power over a distance of 6.5 feet—sufficient to charge a car moving at highway speeds.
To determine the most efficient way to transmit 10 kilowatts 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. That’s why we did the APL study—to figure out the optimum transfer scheme if large metal objects are present.—Shanhui Fan
Using mathematical simulations, postdoctoral scholars Xiaofang Yu and Sunil Sandhu found that a coil bent at a 90-degree angle and attached to a metallic plane can transfer 10 kW of electrical energy to an identical coil 6.5 feet away.
In conclusion, we study the wireless energy transfer in a complex electromagnetic environment and propose an optimal system design for the case when a metallic ground plane needs to be in a close proximity of the receiver resonator. Transfer efficiency as high as 97% can be achieved when the transfer distance is about λ/15. For an operating frequency of 10 MHz, this corresponds to a transfer distance of 2 m. We believe that the transfer efficiency can be further increased by fine tuning the system design, for example, increasing the size of the metallic plane will result in a slightly higher transfer efficiency.—Yu et al.
Fan and his colleagues recently filed a patent application for their wireless system. They next plan to test it in the laboratory and eventually try it out in real driving conditions. 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.
Although a power transfer efficiency of 97% is high, Sven Beiker, executive director of the Center for Automotive Research at Stanford (CARS) and his colleagues want to be sure that the remaining 3% is lost as heat and not as potentially harmful radiation.
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.
The SAE Taskforce on wireless charging and positioning of electric vehicles (SAE J2954), which is slated to have a final draft of a guideline this year, is currently not tackling on-road dynamic charging. (Earlier post.)
Xiaofang Yu, Sunil Sandhu, Sven Beiker, Richard Sassoon, and Shanhui Fan (2011) Wireless energy transfer with the presence of metallic planes. Appl. Phys. Lett. 99, 214102 doi: 10.1063/1.3663576
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