Thursday, May 1, 2008
Power goes wireless
A new system for transmitting power could get rid of the tangle of cables that keep alive our cell phones, laptops and other devices, researchers report.
Physicists at the Massachusetts Institute of Technology in Cambridge, Mass. found that power could be transmitted without wires using special “resonant” antennas. The researchers used the system to power a 60-watt light bulb more than two meters (about two yards) from a wireless transmitter at 40 percent efficiency.
Two images of a 60-watt bulb lit from 2 meters away by a power-transmitting coil. Note the obstruction in the lower image.One known method uses electromagnetic radiation, like radio waves. More commonly used for wireless transmission of information, these can also transmit power. But not very effectively. Since radiation spreads in all directions, almost all the power would end up being wasted into space. An alternative strategy is to beam the radiation specifically toward the electronic device to be charged—but then problems can arise if some other object gets in the way, or if you move the device.
The MIT concept, called “WiTricity” for wireless electricity, involves using so-called coupled resonators. These are objects that, if struck or disturbed, tend to naturally oscillate at a definite rhythm. If two of them tend to have matching rhythms, they actually enhance each others’ oscillations.
One example is a child on a swing. If she swings her legs in synch with the natural rhythm of the swing itself, the swing will soon be briskly in motion.
The type of resonance behind such a push-pull system is called mechanical, but other types of resonances are possible. There are acoustic resonances, for example. Imagine a room with 100 identical wine glasses, each filled with different amounts of wine. This gives each glass a different “resonant frequency,” or natural rhythm of vibration. If a singer then sings a loud enough note in the room, a glass of the corresponding frequency might accumulate enough energy to explode, while the other glasses sit undisturbed.
The MIT team focused on yet another type of resonance, magnetic.
They set up two copper coils, each a self-resonant system. One coil, attached to a power source, is the “sending” unit. Instead of sending out electromagnetic waves, it fills its surroundings with an oscillating magnetic field. This leads to a power exchange with the other, “receiving” coil. Because the magnetic field, unlike radio waves, never gets too far from the sending unit, the energy isn’t lost into space. And extraneous objects entering the field have no impact because they normally don’t resonate along with the system.
With such a design, power transfer has a limited range, and the range would be shorter for smaller-size receivers. Still, for laptop-sized coils, power levels more than enough for a laptop can be transferred over room-sized distances nearly omni-directionally and efficiently, regardless of what’s between the objects, researchers said.
“As long as the laptop is in a room equipped with a source of such wireless power, it would charge automatically, without having to be plugged in,” said MIT’s Peter Fisher.
Although the power transfer efficiency remains below the ideal, team member Andre Kurs said in an email that he’s optimistic it can be improved. He also acknowledged that inefficiency raises environmental concerns, but argued that the new system on balance might actually help the environment. That’s because the batteries that it would replace also tend to lose efficiency over time, and contain toxic chemicals.
