I found a more detailed article at the IEEE Spectrum web site. It discusses power efficiencies and magnet field strengths and provides some encouraging numbers. Seems as if pacemaker wearers will even be able to be in the same room and the power radiator. An excerpt is below. You can view the full article here:
http://spectrum.ieee.org/nov06/4735
To understand how the MIT idea works, we first have to look at how a regular omnidirectional radio transmitter works. Electrical energy is pumped into such a transmitter, and the energy is then carried away by radio waves that radiate in every direction. So the amount of energy that can be picked up by a receiver located at any given point away from the transmitter’s antenna is only a fraction of the total amount of energy being put into the transmitter.
Now, in the MIT scheme, instead of familiar radio waves, energy is carried by “evanescent waves,” which owe their existence to a wrinkle in the laws that govern electromagnetism. The most important feature of evanescent waves is that although they carry energy, they don’t radiate it away. Rather, they borrow energy from the transmitter and then promptly return it. The reason evanescent waves are unfamiliar to most people—though they do have applications in the fiber-optic cables that carry most data today—is because the laws of physics dictate that they must typically have short ranges; their intensity decays exponentially with distance. That makes them unsuitable for many uses, such as carrying data signals over long distances through the air.
However, the MIT team claims that it’s possible to build a transmitter capable of setting up a field of evanescent waves with an effective range of several meters. The evanescent field doesn’t get absorbed by nearby objects, because only objects that are precisely tuned to resonate with the emitted field can absorb energy from it. An analogy is to “imagine a hundred glasses filled with different levels of water,” Karalis says, and then turn on a speaker set to “generate sound at a particular frequency. Most of the glasses won’t feel anything—but one [if it happens to be at the resonance frequency] might even break.”
A suitably resonant receiver “senses the field and literally sucks it, drains it out,” says Karalis, who estimates that over a distance of a couple of meters, the scheme could approach a power transmission efficiency of 50 percent. “So if I want to feed something with 10 watts, I just supply 20 watts from my source,” he says.
“Even if that’s too optimistic, and the efficiency is as low as 10 percent,” he adds, “for any practical purpose, that’s very good—but we expect much more than that.”
Despite the potential for high efficiencies, the strength of the magnetic fields involved is very low. Initially, the MIT team believed that the magnetic fields required would be similar to those used in MRI medical imaging machines, with field strengths of about 1 tesla. But when they finished their calculations, they were pleasantly surprised to find that to transmit a few watts over a few meters (enough to power a cellphone or to recharge a laptop), the required magnetic field should be about 10 000 times less, around the same strength as the earth’s magnetic field.
The exact design and size of the transmitter has yet to be worked out, but for home applications, a room could be energized with a loop antenna, about a meter across, mounted on the ceiling.
No really, This is great! I’ve been saying someone needed to develop a wireless power solution for years - every time I try to set up a new development lab and I need to re-wire it for more amperage and then buy over 100 power strips for all the computers, routers, and other hardware I need. A total nightmare.
The concept is based on resonance.
“When you have two resonant objects of the same frequency they tend to couple very strongly,” Professor Soljacic told the BBC News website. Typically, systems that use electromagnetic radiation, such as radio antennas, are not suitable for the efficient transfer of energy because they scatter energy in all directions, wasting large amounts of it into free space.
To overcome this problem, the team investigated a special class of “non-radiative” objects with so-called “long-lived resonances”.
When energy is applied to these objects it remains bound to them, rather than escaping to space. “Tails” of energy, which can be many metres long, flicker over the surface.
“If you bring another resonant object with the same frequency close enough to these tails then it turns out that the energy can tunnel from one object to another,” said Professor Soljacic.
Hence, a simple copper antenna designed to have long-lived resonance could transfer energy to a laptop with its own antenna resonating at the same frequency. The computer would be truly wireless.
How it works:
1) Power from mains to antenna, which is made of copper
2) Antenna resonates at a frequency of 6.4MHz, emitting electromagnetic waves
3) ‘Tails’ of energy from antenna ‘tunnel’ up to 5m (16.4ft)
4) Electricity picked up by laptop’s antenna, which must also be resonating at 6.4MHz. Energy used to re-charge device
5) Energy not transferred to laptop re-absorbed by source antenna. People/other objects not affected as (they are) not resonating at 6.4MHz
Can’t wait.
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