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.
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.
Wireless energy transfer could help compensate "for the fact that our batteries are still pretty awful," Saffo says. Technologies that try to pack even more energy inside small devices have a drawback: They could become ticking "bombs" if their energy were suddenly released by accident. Recent troubles with some Dell laptop batteries have focused attention on the safety of storing energy inside portable devices.
Such concerns would seem to make transferring energy to gadgets wirelessly an attractive alternative. Devices running on wireless energy transfer could have no batteries and simply receive their power from the energy source directly – by staying in range of their power source.
Because the energy would jump only to a receiving device that resonates with the originating source, wireless energy transfer would be harmless to people, Soljacic says. "The only energy that would go into free space is magnetic energy of the kind that human beings are around all the time," he says, such as that given off by Magnetic Resonance Imaging (MRI) machines in hospitals or when high-speed magnetic levitation ("maglev") trains are suspended slightly above their tracks to cut friction.
"We are fairly optimistic that the safety issues will be OK," Soljacic says.
But proving a new technology is safe can be a tall task, Saffo notes. "Even if it's harmless, someone's got to test it and confirm it's harmless," he says. "We have a long history of consumers being very sensitive about things that the experts tell them are very safe, like food irradiation."
Soljacic grew up in Croatia and attended high school there before eventually moving to the United States, just as did Nicola Tesla (1856-1943), the eccentric genius and electrical pioneer who was at times both an employee and a rival of Thomas Edison.
The recent Hollywood movie "The Prestige" depicts Tesla using a form of wireless energy transfer around 1899 to light hundreds of electric light bulbs planted in the ground in an open field some 25 miles from where the energy was being generated. The scene is based on contemporary accounts of such an incident. Tesla also speculated that ships at sea might someday be powered by onshore electrical plants, with the energy being conducted through the atmosphere.
A form of wireless energy transfer called magnetic induction is already in use in products such as electric toothbrush chargers. But such devices require the energy source and device to be in very close proximity in relation to their sizes, meaning that in small devices, the source and device must nearly touch one another. By employing the principle of "resonance," Soljacic's technique allows much greater distances, such as across a room, and thus offers a much wider range of potential applications.
Perhaps his idea is coming forth now because a need for it has developed, Soljacic speculates. "If I came up with this seven years ago, or if Tesla came up with it 100 years ago, he'd probably discard it as being completely useless," he says.
The principles that his team is applying are widely known, he acknowledges. They have just spotted the possibilities first. "I have no illusions. If we hadn't discovered this, in one or two or three years, somebody else would," he says.
But while the need may be apparent, gadget lovers shouldn't expect to see this new technology for some time, Saffo says. For one thing, the first applications are likely to be tested away from large numbers of humans, such as powering mobile robots on a factory floor.
"I wouldn't hold my breath," he says. If you're going to build a new house, "plan on putting in wall plugs."