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Wireless highway charges electric cars as they go

About the author

Jon is the presenter of Science in Action on the BBC World Service. He trained as a mechanical engineer (with automotive and aeronautic design) before becoming a journalist. He has worked at the BBC for over a decade and has reported from areas as diverse as war zones and technology shows.

(Copyright: SPL)

(Copyright: SPL)

Researchers are close to developing a system that allows electric cars to recharge while they are moving… via a recharging road.

Road works. Inconsiderate drivers. Congestion. Today’s drivers have their fair share of stress already. But now there is a new malaise for the modern motorist: range anxiety.

That is the term given to drivers of electric cars that are struck by the sudden fear that their vehicle does not have enough charge to reach its destination. Most of us have experienced that sinking feeling when the little orange indicator light comes on to tell us we are low on petrol, but there is not a gas station in sight. Imagine that, combined with the feeling that you get when your cellphone starts beeping because the battery is low, and you are nowhere near a plug. That gets you close to the feeling of range anxiety.

It is an interesting phenomenon, particularly when you begin to look at how many of us actually use our cars. According to the US Bureau of Transportation Studies, 78% of drivers do less than 40 miles (65km) a day – a trivial distance for many of today’s electric cars. In fact, the poster child of electric cars – the Tesla – has a range of 300 miles (485km) using some batteries.

According to, Dr Richard Sassoon, of Stanford University, there are “three main reasons” that many of us choose the internal combustion engine over its cleaner, quieter alternative.

“One is the short range that an electric vehicle can travel between charges, and that’s based on the size of the battery,” he said. “The second is the lack of a sufficient charging infrastructure, and the third is that even if you can charge, it takes a long time to charge – several hours. That means you’re going to have to take a break in your trip in order to charge your vehicle.”

Researchers and firms are trying to tackle all of these problems. Firms, such as  Better Place, have started building battery “switching stations” that allow drivers to pull in and swap their batteries as easily as filling up with gas, whilst countless researchers are developing more efficient batteries.

But Dr Sasson believes there may be another answer: recharging roads.

Engineers in his lab are developing a way to wirelessly charge electric cars from magnetic coils embedded into the road. The car would pick up the power via another coil, meaning – in theory – that you would never have to make a charging stop again.

The system works using a technique called “magnetic resonance coupling”. You can think about resonance as the phenomenon that allows an opera singer to smash a glass using only the power of their voice. In that case, when the singer hits a note that has the same resonant frequency as the glass, they couple and energy begins to build up in the glass, eventually causing it to smash.

Instead of using acoustic resonance, the Stanford team use the resonance of electromagnetic waves. A coil in the road that is connected to a power line is made to vibrate with the same resonance frequency as the coil on the bottom of the car, allowing energy to flow between them.

Traffic charge

It builds on pioneering work done at MIT in 2006 which showed the technique could be used in stationary situations, to power televisions and other gadgets. The Stanford system now claims to have upped the efficiency dramatically. They have come up with designs of coil that allow 97% efficient transmission of power over a distance of about 2m (6ft). Using models, they estimate they can transfer up to 10kW of power.

“That number is about the number we’d probably want to transfer to vehicles” says Dr Sasoon.

And to turn this principle into a practical “recharging road” is not as difficult as it seems, he says.

"Road beds are made of asphalt or concrete, but there is often a lot of steel in the roads - a lot of rebar, a lot of ties between the segments of the road and so on,” he said. "What we want to do is use that to our advantage." 

He believes they could use much of the metal in the roadbed as part of the transmitter, and then the receiver would use the metal of the car body, again avoiding too many extra structural components. 

It may take years, if not decades, until roads are retrofitted in this way. But various firms, including an MIT spin-out called WiTricity, are already taking the first steps by building charging stations for car parks, garages and beyond.  And it has already caught the attention of car firms, including Toyota, Mitsubishi and Audi.

“We aim to offer our customers a premium-standard recharging method – easy to use and fully automatic, with no mechanical contacts,” said Dr. Bjorn Elias of Audi Electronics Venture GmbH (AEV), a subsidiary of the car company that is working with WiTricity, recently. “Wherever you park the car, its battery will be recharged – perhaps even at traffic signals.”

Audi – and others – are working to create a public standard and believe that the first units – for use in garages – will go into production in a few years’ time. At that time, Dr Sasoon believes, electric cars will become the technology of choice, displacing our current love of gas guzzlers and banishing the concept of range anxiety forever.

“You never need to worry about stopping and filling up,” he said.

Of course just because our cars can carry on forever does not mean that we will want to. I do not think I can go more than a couple of hundred miles without snacks and a bathroom break. A whole different kind of range anxiety…

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