Unless it’s your smoke alarm saving your life, mysterious electronic beeping in the middle of the night is highly annoying.
It certainly annoyed Marin Soljacic a few years ago when he found himself standing in his kitchen in his pajamas in the middle of the night for about the sixth time in a month, staring at his cell phone, beeping to tell him he’d forgotten to plug it in.
Most of us would probably either plug the thing in or stuff it under a pillow and go back to bed. Most of us, however, aren’t assistant professors of physics at the Massachusetts Institute of Technology (MIT).
“It occurred to me,” he said, “that it would be so great if the thing took care of its own charging.”
Last week, Soljacic and fellow team members from MIT’s Department of Physics, Department of Electrical Engineering and Computer Science and Institute for Soldier Technologies reported they successfully lighted a 60-watt light bulb from a power source more than two meters away, with no physical connection between the source and the appliance.
With “WiTricity,” cell phones, MP3 players, laptop computers and many other portable electronic devices could soon charge themselves without ever being plugged in–or even operate without batteries at all.
Various methods of wireless power transmission have been known for decades. Radio, after all, is nothing more than wirelessly transmitted power: sound is converted to a varying electrical current in the transmitter, which generates electromagnetic waves that set up a matching varying electrical current in the receiver, which amplifies it and uses it to drive a speaker, recreating the original sound.
But radio radiates in all directions, meaning very little power is actually transmitted to any one receiver, and most of it is altogether wasted.
WiTricity doesn’t use electromagnetic radiation at all: instead, it’s based on magnetism and the physics of coupled resonant objects.
Two objects resonating at the same frequency exchange energy extremely efficiently. My daughter is just learning to keep a playground swing going by pumping her legs in time with the swing–that is, by matching the swing’s natural frequency, she’s become more efficient at imparting energy to it.
Another example: I love to hum inside stairwells (doesn’t everyone?). If I hum at just the right pitch, the whole stairwell resounds like a pipe organ–because I’ve managed to find the stairwell’s resonant frequency, at which point the small amount of energy I’m imparting to the air to make it vibrate is most efficiently transferred to stairwell itself, making the whole stairwell vibrate.
The MIT design consists of two copper coils. One, attached to the power source, is the sending unit. Rather than send out electromagnetic waves, it fills the space around it with a magnetic field oscillating at a particular frequency. The second copper coil is designed to resonate with that oscillating magnetic field. A copper coil within an oscillating magnetic field generates a current, enough, in MIT’s case, to power a light bulb.
Power transformers make use of something similar, called magnetic induction, to transmit power between coils over short distances. But those coils aren’t designed to resonate with each other. Resonant coupling makes the transfer of energy almost a million times more efficient.
Since the magnetic field doesn’t radiate, most of the power that isn’t picked up by the receiving unit is bound to the originating coil, rather than being lost into the environment. That also means that this system has a limited range, and the smaller the receiver, the smaller that range is.
Still, the MIT team says power sufficient to run a laptop, can be easily transferred over room-sized distance even if line-of-sight is obstructed: in other words, as long as a coil-equipped laptop is in a WiTricity equipped room, its battery could charge automatically, or it could operate without a battery or being plugged in.
Considering I live in fear of getting my feet trapped in the maze of power cables underneath my desk and having to call the fire department to get myself free, I’m all in favor of wireless power, wireless data, wireless anything.
WiTricity? Why not?