Look, Ma – no wires!
Electricity broadcast through the air may someday run your home.
More than a century ago, Thomas Edison and Nicola Tesla dreamed of broadcasting electrical energy through the airwaves. Instead, to their surprise, a grid of metallic wires sprang up and encircled the globe, distributing power to homes and businesses nearly everywhere.Skip to next paragraph
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Now, as more and more people carry portable gadgets, from cellphones to laptops, iPods to PDAs, they want to eliminate any need for wires. The problem: Their devices need to be recharged, eventually, by plugging them into a wall socket.
But an age when wireless gadgets never run down may be dawning.
At a physics conference in San Francisco Wednesday, a group of researchers proposed a method that would allow cellphones, laptops, industrial robots, and other gadgets to be recharged simply by being within a few meters of an energy source. A system of "midrange" energy nodes, akin to the wireless "hot spots" that give computers wireless access to the Internet, wouldn't replace power lines. But they could someday result in entire buildings or other large areas in which wireless devices are automatically charged when they come into range. Other applications include sending power to electric buses along a highway, or charging microscopic nanorobots as they work, perhaps inside the human body.
The idea was conceived by Marin Soljacic, an assistant professor of physics at the Massachusetts Institute of Technology (MIT). A metaphorical light bulb lit up over his head in the middle of the night, he says, when a beeping cellphone woke him because it needed recharging. Why, he wondered, couldn't a cellphone – or his four Roomba robotic vacuum cleaners, for that matter – be constantly recharged wirelessly?
"In the last few years, our society has witnessed a dramatic transformation," Professor Soljacic explains by phone. "Seven years ago, the only battery-operated thing I used was a flashlight, which I didn't use that often." Today, he says, we're flooded with portable battery-powered devices that need recharging.
His idea involves "electromagnetic resonances," in which energy jumps between two locations, as it does for very short distances inside an electric motor or transformer. "It's a very general phenomenon in nature," Soljacic says. "In quantum mechanics, it's called 'tunneling.' In electromagnetics, it's called 'evanescent coupling.' "
A paper putting forth the concept, "Wireless Non-Radiative Energy Transfer," written by Soljacic and fellow physicists Aristeidis Karalis and J.D. Joannopoulos, is under review at the scientific journal Nature Physics. In it, the researchers propose two ways to design devices for practical wireless energy transfer. The designs, using either "dielectric disks" or "conducting-wire loops," show 30 to 60 percent efficiency in transferring power, Soljacic says. "Potentially, this could really be useful for certain applications, at least."
The paper also contains calculations that simulate what would happen in the real world. "The numbers that we got from these calculations are encouraging," he says. "We fairly strongly believe in our theory, based on previous experience. But, of course, experiments will be the ultimate judge."
Soljacic and his team have begun designing experiments to confirm their calculations.
"He's a very creative guy," says Erich Ippen, a physics professor at MIT who's talked with Soljacic about his work. "These are not unknown physics phenomena," Professor Ippen says. "But what's innovative here is ... the idea of using [them] for practical purposes."
The concept of wireless energy transfer, even over relatively short distances, is an attractive one, says Paul Saffo, a Silicon Valley technology forecaster. "We're all just dying to cut the cord," he says. When he goes on the road, Mr. Saffo says, he's always mindful that his electronic devices may run down. "I'm always looking for opportunities to suck in some more electrons," he says.