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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

February 2, 2012 in Infrastructure, Plug-ins, Smart charging, Vehicle Systems | Permalink | Comments (44) | TrackBack (0)


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The question is can they transfer power to a car (or a series of cars) moving at 60 mph.
They might need to alter the frequency as the distance between the car and charging plate varys.

It would be phenomenal if they could get it working, although building an electric lane on a motorway would be very expensive.

And they would want to get a standard hammered out so all the cars could use the same charging infrastructure (due to the cost).

Also, 10 KW might not be enough for Motorway use, you might need 15 Kw or more to allow >= 70 mph travel.

It might be worth building electric roads in cities to encourage people to use EVs in densly populated areas, rather than try and build hundreds of miles of electric motorway.

This is a very interesting topic, and could be a way around the "battery problem".

The battery lobby will bury this!

The battery lobby would love it. People won't buy an EV with a 100-mile battery because they can't afford it, but if they only need a 20-mile battery because it's fully charged every time they leave the parking lot or get off the freeway, they'll sell like hotcakes.

There is enough room for both: 1) EVs with expensive large batteries (100+ KWh) with cable/plug to quick charge it every 500/700 Km or so. 2) much smaller battery (5 to 10 KWh?) with wireless charging embedded into streets/roads for continuous 'on-the-move' charging for infinite e-range.

It would rather easy to equip EVs with a sealed energy metering and automated charge system to pay/charge for energy used. Automated charge to the owner's credit card would not be much of a challenge. It is already done for many pay roads and bridges.

Of course the charging system installation + energy cost would be factored in the users charge. Many private firms would be more than happy and willing to supply and install 1,000,000+ miles of it as soon as equipped EVs are there in large enough numbers.

What a beautiful way to create 5 million new jobs. Common America, lets get creative again.

The transmitters would need to be switched on only if cars are in its vicinity otherwise the electrical losses would make the whole idea impractical. Just imagine a 200 mile interstate highway with 10 Kw. 10 Mhz. transmitters operating continuosly.

Didn't Tesla invent this thing back in the 20's or so? Called it the magnetic coil transducer or some such?

Large chunks of steel passing through magnetic fields at 60mph.

How are they keeping eddy currents from developing in every car body that passes through the magnetic field of these chargers? Might give quite a few people the old "hot foot".

Compared to the 10 MHz or so drive frequency, the variation due to the motion of the car is nothing.

Skin effect will keep current from going very deep in metal floor pans. Switch to plastic and things might get interesting.

It would be relatively easy to use aluminium or copper screening in plastic floor to block radio frequencies from wireless changing system, much the same way as waves are blocked from your home micro wave ovens.

Installing the energy transmitting equipment in streets/roads beds with appropriate machinery should not be much more complicated as laying underground power/communication cables. One hundred such machines could cover most roads in a few years.

Energy used charging sub-systems will be developed and incorporated at the same time.

A cost-benefit calculation must be done to see which would be more cost-effective overall:
1)PHEV's with 20-mile AER to be charged twice daily, using an ICE for range extension, or,
2)20-mile AER BEV's that will depend on massive wireless charging infrastructure that will cost a lot to build and to maintain.

For long-distance inter-city travels, a PHEV is much more desirable, since smaller highways will likely not have the wireless electrical transmission. In winters, the waste heat of the PHEV's ICE will provide free cabin heating and defrosting.

On a second thought, wireless charging and PHEV's can co-exist, making it much more convenient for PHEV and BEV's owners not having to get their hand dirty nor risking electrical shock. Just park the vehicle at designated locations and it will be charged and billed automatically and wirelessly. Those not wanting to pay extra for the convenience of wireless charging can still reach out for the plugs and pay less.

It seems to me that in-motion charging would require a huge infrastructure investment as opposed to still charging. Consider a parking garage, or better yet a suburban mall or factory lot. That kind of upgrade would have a more reasonable infrastructure cost. The key here is standardization as was mentioned before. Given RFID systems used in parking garages and expressway exits now, the pay systems can be easily handled, and the hour or more of mall/lot parking can give the needed boost for the trip home. You need to add a switch in each capable car to allow the driver to say no, but that can be tied to the shut down process easily with today's console screens and speakers. Neat tech and practical very soon if not now.

I don't think any rational person is advocating tearing up the streets and spending trillions of dollars to have wireless EVs, at least I hope not.

This could make for more robust wireless charging methods in stationary applications. Perhaps higher power, quicker charging and more tolerant to some misalignment. Cordless charging could be big, but it has to be reliable and cost effective.

I think safety is one of the reasons the EV1 and RAV4V had inductive charger paddles. You could plug them in while it was raining outside in a parking lot without fear of shock, but you still got wet. This is safe and dry.

Many decades ago, the world was faced with similar costly infrastructure required for regular old fashion phone lines. Many million Km were (hand) laid on wooden-cement-steel poles and/or buried in most countries at a very high labor and material cost just to be able to talk to each other.

Tomorrow, the world will rise to the challenge to (machine) install on-the-move charging systems on all major roads and streets for future much lower cost EVs with unlimited e-range with very small on-board batteries. That will spell the end of ICEVs, HEVs and PHEVs.

Of course, Oil lobbies, gasoline distribution lobbies, ethanol and corn lobbies, EV batteries lobbies, non-believers and naysayers lobbies, extreme right lobbies, ICE manufacturers lobbies, and all groups and individuals financially interested into the status quo will fight it to the last blood drop.

Sorry, I meant the Range-Extender lobby will bury this!

It is a matter of who will pay for it and is it in the public good. When car companies in the early 1900s wanted cities to build paved roads, they said no because taxes would be higher and the only people to benefit would be the car companies.

Perhaps this COULD happen some day, but it would take a MAJOR transformation in the way the U.S. population looks at government and public investment. Considering the trend has been going the other direction for 30+ years I doubt it will turn around without a huge force for change occurring.

SJC....this is an ideal 'users pay' application with very high benefits potential. Governments, who want to make money with it, could participate with interested private industries, but it would not be an obligation.

Other interesting side benefits would be (easy) automated steering and speed control of all EVs on the wireless charging equipped (left?) lane. That should reduce driver fatigue, accidents and insurance cost. EV drivers, on the automated lane, could use their smart cell phones/tablets without endangering the life of others.

Beyond the futuristic talk, WHO is going to do this, toll roads? Where are they going to do this, how many people will it serve and what will it cost?

This is just a lab idea right now, it is no where near deployment, so all the talk about super highways is a bit premature to say the least.

Wouldn't you pay a mileage fee to use a road if it was a quarter the price of liquid fuel? How much would you be willing to pay if it came with the bonus of guiding your car so you could sleep or read? Facilitating platooning so that the capacity of the road is multiplied and congestion becomes a bad dream?

Placing coils in a roadway means major reconstruction, but Hanazawa's capacitive system could be placed in asphalt pavements by digging up a strip, laying down mesh and rolling the re-heated asphalt back over it.

Advocate that to the public and private sector decision makers, I wish you luck.

Consider those same decision-makers looking at the pricetag for electrifying 4 lanes of a freeway vs. trying to widen it.

Consider them looking at the difference between electrifying the truck lanes with rail, and putting up noise walls to block the engine noise from diesel trucks.

Consider them trying to keep employers when they can't meet air-quality standards, and wondering what they can change.

Not only would you have to retrofit millions of miles of roads but hundreds of millions of vehicles. Now ask yourself, what is the probability of THAT happening?

Not only would you have to retrofit millions of miles of roads
The entire Interstate system is under 43,000 miles. The most heavily-trafficed roads would supply the bulk of the payoff; leaving rural interstates like I-70 west of Kansas City (except for the Denver metro area) and doing major California surface roads instead would grab the low-hanging fruit first.
hundreds of millions of vehicles
There are only about 6 million passenger vehicles in Los Angeles county. If a third of them are retrofitted over 5 years, that's about 400k/year. A similar number would be replaced over the same period.

In-wheel motors are good candidates for reuse. Shops which work on tires and brakes would appear to have much of the required skill set for retrofits. Dealers too, naturally.

what is the probability of THAT happening?
It depends on the payoff, doesn't it?

Commuters driving 15,000 miles per year in vehicles getting 25 MPG in traffic burn 600 gallons/year in commuting at a cost of about 16¢/mile. If this mileage is instead driven on electricity at 4¢/mile, it would save about $1800/year. That's a lot of money. It would justify at least $3500 in retrofits for a vehicle with a remaining 5-year lifespan. For a vehicle like a Prius or Leaf, only the pickup and charging system would need to be added; given a few kilowatts from the road, a Prius could cruise at low speeds and run its climate control indefinitely with the engine off. Worth $1000? If the car is commuting every day, I suspect so.

The numbers depend on the specifics, but there is definitely potential there.

Selectively electrifying (one lane at a time) busy highways used by wireless charging equipped vehicles, should not be a major challenge for USA's industry. Of course, users would be charged for the energy used + a surcharge to cover installation cost + a fair profit. If tight enough standards are applied, many service providers could be involved through the open bidding process (with distance limits). Early licenses could be limited to 100 - 300 miles etc.

A properly equipped electrified lane could effectively handle about 2X the level of traffic because every vehicle in the controlled lane would travel at the same speed (110 Km?) and much closer together.

I have a better idea:

Very short (1m) sections of "rail" get automatically electrified only when there is a car above them.

The "rail" doesn't include a u shaped guide, the car stays on-rail thanks to computer control.

A car doesn't need a driver when on a "rail".

A car is able to run 10-20 miles to get outside of the electrified ways and pass other cars on its own electric power w/o rail.

Car "convoys" get automatically formed and managed so that when in a convoy there is no need for a driver.

In case of bad weather, rain and so on, only the 1m rail *sections* with problems get deactivated.

"e-rails" can be "nailed" to existing roads quickly from rolls carried by specially built "seeding" trucks.

Excess energy recovered from braking can be returned to the rail if the onboard batteries are full.


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