The vision of an MIT physicist: Getting rid of pesky rechargers

The age of convenient portable electronics is also the age of annoying rechargers. Every iPod, cellphone, and digital camera needs to be plugged in regularly or the battery dies and is useless.

But Marin Soljacic, a physicist at the Massachusetts Institute of Technology, has a plan that would mean the end of rechargers.

In a paper awaiting publication, he suggests a way to deliver power "wirelessly." He has shown that it is possible to use a carefully designed magnetic field to deliver power to anything within about 10 or 15 feet.

To recharge a device, the person would just have to leave it within the field -- say, in a home office -- where it would pick up power using a built-in antenna without harming anything else in the room.

"It's a great idea," said Sir John Pendry, a physics professor at Imperial College London who has discussed the work with Soljacic but is not involved in it.

Wireless power could have a dramatic effect on how people use technology. Soljacic said he is setting up a test of the idea in his laboratory and cautioned that he won't know how practical it is until he has done extensive testing. For example, he said, he doesn't know how much energy would be wasted when it operates. The magnetic field is not powerful, he said, so it would likely be safe, but it's still untested.

Soljacic said he was inspired in part by the rise of wireless Internet access, which encourages laptop users to wander far from electrical outlets. But he was also inspired after being awakened by the warning sounds of a cellphone that was low on power. There has to be a better way, he recalls thinking.

It was not a new thought. The inventor Nikola Tesla explored wireless energy and in the early 1900s began construction on an enormous tower in New York to transmit it. (The project was abandoned.) Today, physicists can transmit power without wires -- such as with a high-powered laser -- but such ways would be impractical, or downright dangerous, for recharging electronics in someone's home.

The trick is to find a way of delivering power to just the right places so that the cellphone recharges but the nearby couch is not incinerated.

Soljacic's idea makes use of a concept in physics called resonance in which an object reacts to a force only at certain frequencies. To understand resonance, imagine getting on a swing. If you lean back and forth at the wrong speed, the swing won't go. At just the right speed, the so-called "resonant frequency," the swing and you are in synch, and you push the swing higher and higher.

Soljacic realized that he could build a simple antenna that would resonate with a particular kind of magnetic field, allowing the antenna to draw power while the other objects in the room would not. The field is about as powerful as the earth's natural magnetic field, he said.

The next step is to set up a transmitter and receiver in the lab and see if the set-up works as the theory in his paper suggests. If the results are promising, he said, he will do more work on optimizing the system, such as finding the best design for the antennas and making sure that power is not delivered to other objects.

His work has so far been funded by the National Science Foundation, and his patent application is pending.

Soljacic hopes to put the device to work for consumers eventually. But he said that it would probably be used initially for a more specialized military or industrial application, such as powering mobile robots on a factory floor.

"I am very optimistic," he said, "that it will find some use."

A paper describing the work is available at: http://arxiv.org/abs/physics/0611063.