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Stanford researchers designing magnetic resonance coupling system for wireless on-road dynamic charging of EVs

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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.)

Resources

  • 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

Comments

SJC

A lane at a time could be done, but you need a sufficient number of cars using it on a regular basis to make it pay off, this would be the "chicken and egg" problem.

Now, if you could convince auto makers that a wireless charging system could/should be the resonant type, then over time you would have enough cars and break the cycle.

Roger Pham

Let's not count the chicken before they are hatched. This technology is far from proven. Meanwhile, inductive charging is already proven. Simply lower an inductive paddle from the car to the pavement at tne parking spot and automatic wireless charging can be done.

SJC

Basic induction charging is proven and can be done. Whether people will want to pay the money considering battery costs for an EV right now is a question.

Maybe by the time wireless charging becomes popular, the price difference between induction and resonant may not be that great. It would be nice to have the car drive its with GPS and voice commands and never have to recharge at all.

Engineer-Poet
you need a sufficient number of cars using it on a regular basis to make it pay off, this would be the "chicken and egg" problem.
That's where retrofits come in; if thousands of drivers using a particular route can get in-wheel motors installed in a matter of weeks, adoption could be very rapid.
SJC

There was talk about cars with a 20 mile battery selling like hot cakes, but that says these recharge highways are every where. Then the talk turned to interstate highways and then to L.A. county.

If the context of the scenario keeps changing to make the premise fit, then I guess anything fits any time you want it to. Sustainable Mobility means for humanity, not just the U.S. and not just L.A. county.

The question has been asked about what some people are smoking, well perhaps the people asking should ask it of themselves. This pie in the sky cut to fit kind of discussion is pointless.

Define your terms and the limits of the context and then the discussion can be meaningful. This story is about resonance coupling done in the lab and people are already having visions of EVs zooming down wireless super highways...smoking?

Herm

it may be easier to have a fleet of Volts running on CNG.. there, every problem solved!

SJC

I have thought that if Honda combined their Civic GX natural gas car with a Civic hybrid and made it dual fuel, they could have a winner.

High mileage on alternate fuels, if you run out of natural gas it uses gasoline. No range anxiety and a smaller tank because you get better mileage. Might make a fine commuter car, it would clean up the air, save the driver money and reduce oil imports with one move.

Engineer-Poet
There was talk about cars with a 20 mile battery selling like hot cakes, but that says these recharge highways are every where. Then the talk turned to interstate highways and then to L.A. county.
LA county is an example. California in general leans toward the cleanest technologies because of its air-quality issues, so that's a good place to start deploying a promising technology. Of course it would take pretty good penetration of electrified roads to make the 20-mile, sustainer-less BEV attractive to lots of people, but even before that the Volt and plug-in Prius (and maybe Fusion hybrid) could go almost completely electric.

All of these things have to start somewhere, and CARB would probably help. Once it's proven, the world will take notice.

Brotherkenny4

Seems like a non-starter. People are complaining that supplying the infrastructure of simple charging stations is an overwelming task. In order to make these huge infrastructure changes we would have to agree that this is the path forward and then stick to it through a number of administration. That's not likely to happen. I would argue that simple small electric vehicles charged on a home charger would be a better option. The fact that the car companies say that the consumer doesn't want that doesn't convince me. They tend to use marketing studies designed as much to mislead as inform, in order to push the consumer in the direction that is most profitable to them, rather than a true assessment of consumer desire. I don't think anyone has come to market with a low cost electric vehicle with reasonable performance in any quantity yet. When they have, then we will have had a test of the electric vehicle market. Until then it's just a bunch PR spun by marketing guys still trying to get the consumer to spend more money.

E-Biker

" For an operating frequency of 10 MHz, this corresponds to a transfer distance of 2 m."

There goes HF communications.

Engineer-Poet
People are complaining that supplying the infrastructure of simple charging stations is an overwelming task.
And we know it's not true, so why is this a matter of concern?
In order to make these huge infrastructure changes we would have to agree that this is the path forward and then stick to it through a number of administration. That's not likely to happen.
You mean, like the replacement of rail by the Interstate Highway System as the main transport mode of the USA? It seems to me that this is a much smaller task, and all you have to do is prove that it's cheaper and cleaner and it will spread by popular demand. A stretch of I-275 in Michigan was re-paved because of noise complaints regarding grooved concrete, and this is on the same level of difficulty.
I would argue that simple small electric vehicles charged on a home charger would be a better option.
You'd try to sell 25-MPH NEVs to people who commute 50 miles one-way 5 days a week? No wonder you're not getting anywhere.

The new Fusion hybrid's electric system will be able to drive the car at upwards of 60 MPH with the engine off. This car is tailor-made for external electric power. Add a radar cruise control and lane-following for platooning, and it would multiply the carrying capacity of each road lane while being quieter and cleaner. If you can add "cheaper" to that, there are no losers except XOM and OPEC.

Engineer-Poet
There goes HF communications.
You think the ISM bands have destroyed HF communications? There's one centered at 13.560 MHZ and I bet you never noticed.
Zhukova

A 10 mile road with an electrified lane, with 1 car every 300 feet, drawing 10 kW each, would require 1.76 MW of power. But what good is a ten-mile electrified road? Home battery charging would be more practical. The only purpose of an electrified lane would be for long distance like 200 miles. In that case the power requirement would be about 20 times as big or about 35 MW. That's still not that much power. It's widely distributed too. Installing the coils could be done when the road surface is stripped. That's not done very frequently, but roads could be selected for electrification, based on when they need resurfacing.

Engineer-Poet

Ten miles? Try 50 miles. Think "platoons" of cars running 20 feet apart; fitting 24 platoons of 4 cars each spaced 100 feet apart per mile of road at 60 MPH is 96 vehicles per minute, more than 5700 vehicles per hour. (Typical capacity for a freeway is 1500-2000 vehicles/lane/hour. The "platooned" system can put 6-8 lanes' worth of traffic onto 2 lanes.)

96 cars/mile * 50 miles * 15 kW/vehicle = 72 megawatts. One GE gas-fired generator at 300 megawatts could power 4 such roads at once.

What does this get you? The commuter car needs just enough battery power to get to the freeway; instead of a 100-mile battery like a Leaf, perhaps 20 miles would do for many drivers. Once on the freeway the car draws power from the road and recharges its battery with any surplus. Instead of paying $10,000 for a 25 kWh battery, you pay $2000 for a 5 kWh battery. The car is cheaper and lighter, but it's still clean, almost silent and smooth as silk. Also, Volts and Fusion hybrids and plug-in Priuses can be retrofitted to run off the same power source. This gives you a base of vehicles which can go into service much more quickly than making new EVs to order.

MG

@ EP,
BTW how compatible is traveling in "platoons" of cars with the American belief in freedom of choice? If in a communist country they did it, western propaganda would likely use it as 'proof' of restricted freedoms.
I personally wouldn't like to drive in a platoon, (and never use cruise control). It could be too tiring to keep the same pace all the time, requires too much concentration. Depending on mood, freshness, company, music, I drive faster or slower. If I see something interesting beside the road, I may slow down naturally.

In terms of reducing congestion, what do you think of this idea:
Have a class of car, not wider than 1.25 m, single seat, or tandem seats (one behind the other). Divide one or more street and highway lane(s) (say current HOV lane) into two, with some new color, say blue. I found that minimum highway lane width on interstates is about 3.70 m, divided in two, it's 1.80 m, enough for a car of 1.25 m (including mirrors - some solution for mirrors to be found). The car similar to that size was VW 1.0L used by VW CEO on a highway trip about 10 years ago. If such a car is a hybrid or an EV, heavy batteries could be put in the floor to lower the centre of gravity and make them more stable. It's true that such cars would be easy to overturn by vandals, when parked.

Zhukova

Platooning is dumb. It's never been demonstrated with more than two or three cars on on a test track road and never with real conditions, like rain, snow, traffic jams, obstacles, heavy merging traffic. How is it going to work with random cars with wide variation in cars with different braking distances? Even when cars have fully automated drivers, reaction time and brake variation will require separation of more than 50 feet at 60 miles per hour on real roads. With real drivers, without anything like platooning, the reaction time of two seconds, 190 feet at 60 mph, should be increased to account for all the people talking on cell phones.

Engineer-Poet
how compatible is traveling in "platoons" of cars with the American belief in freedom of choice?
Is drafting the car in front of you compatible with choice? That's all it is. You leave the platoon when you want to.
It could be too tiring to keep the same pace all the time, requires too much concentration.
Since you obviously haven't read the GCC articles on platooning and the SARTRE project, I will refer you here to get started.

Zhukova, I'll just refer you to the article:

The project is being driven by seven European partners and is the only one of its kind to focus on the development of technology in which platooned traffic operates in a mixed environment with other road users.
Also see Google's self-driving car.

Zhukova

Engineer-Poet, you quoted it "focus on the development ". This means they haven't done anything outside of the proving ground with a handful of vehicles in a very controlled environment.
I've read all the articles about SARTRE and seen videos and looked at their web site. Nowhere is mentioned how cars with braking distances of 110 ft and 175 ft, on dry pavement, can be mixed in a train. If the lead car has to panic stop, cars are going to crash into each other unless they all have the same braking distance (which can't be automated), or the gap is increased to 100 ft or more. All they've demonstrated is that the cars can follow a lead car. When it's tested with lots of emergency situations, obstacles, heavy traffic, then I might believe it. Same with Google's driverless car. I've read about the platoon concept for twenty years and haven't seen anything real yet, and they never talk about braking distances. Real trains are mechanically coupled, so individual cars can have different braking distances. It won't work for cars virtually connected with radio waves.

Engineer-Poet
This means they haven't done anything outside of the proving ground with a handful of vehicles in a very controlled environment.
Google was also at that stage once. Since then they've run quite a few miles on public roads in traffic. SARTRE seems to be progressing nicely.
Nowhere is mentioned how cars with braking distances of 110 ft and 175 ft, on dry pavement, can be mixed in a train.
That's trivial: sort the vehicles in descending order of stopping distance.
cars are going to crash into each other unless they all have the same braking distance (which can't be automated), or the gap is increased to 100 ft or more.
Even in your own example the worst-case would be 65 feet. Besides, platoons can warn following platoons of hazards, and it makes no sense to try to avoid all collision scenarios. Vehicles under computer control will have faster reactions than those driven by humans and will have more time to brake anyway.
When it's tested with lots of emergency situations, obstacles, heavy traffic, then I might believe it.
Given that the payoff includes large increases in road capacity and substantial fuel savings, there's ample incentive to do that testing.

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