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The team consists of (top to bottom, left to right) Andre Kurs, Prof. John Joannopoulos, Aristeidis Karalis, Prof. Marin Soljacic, Prof. Peter Fisher, and Robert Moffatt. (Source: MIT, Aristeidis Karalis)
A 60-watt bulb illuminates for the future of wireless power

"Wireless" isn't exactly a new concept to computing. Network connectivity, USB devices and even displays had their cords cut in recent years.  Researchers from the Massachusetts Institute of Technology took the final steps towards cutting the last tether of the laptop user: the power cord.

Transmitting power wirelessly is traditionally limited to line-of-sight methods such as microwave or laser, which have a "significant negative effect" on anyone or anything unfortunate enough to be caught in the middle.

Playfully dubbed "WiTricity" by the team, the researchers were able to power a sixty-watt lightbulb from seven feet away using the principle of magnetically coupled resonance. The basic concept is similar to existing electromagnetic inductive chargers, but does not suffer the massive drop in efficiency when distance is increased.

The experiment works as follows. Two magnetic coils resonate at the same frequency.  When one of these coils is attached to a power source, the resonant magnetic field produced by the coil increases dramatically.  The second, unpowered coil "couples" with the resonating magnetic field.  The resonance from the second coil is then converted back to electricity for the device.

The MIT researches are quick to tout magnetically coupled resonance over electromagnetic induction.  Aristeidis Karalis, an MIT graduate student that worked on the project, states, "Here is where the magic of the resonant coupling comes about. The usual non-resonant magnetic induction would be almost 1 million times less efficient in this particular system."

In addition to increased efficiency, the WiTricity project does not transmit biologically harmful electromagnetic radiation during operation.  Additionally, line-of-sight issues present in microwave technology disappear with WiTricity; magnetic fields are more-or-less unaffected by non-metallic materials in most environments. 

The most current WiTricity experiments use coils approximately 20" in diameter and operate at distances of approximately two meters.  The team hopes to eventually power a notebook from a several meters away.

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RE: Multiple Recivers
By bolders on 6/12/2007 12:52:40 PM , Rating: 2
I Dont know if you could run multiple receiver coils all tuned to the transmitter coil but if it was possible each reciever would act as a load on the transmitter. I.E. there is a finite amount of power which would be distributed across the receiver network - The more recievers the less power each one would receive.

The power from the output of a system can never be greater than the power applied to the system input. In fact it will always be less because some energy is lost due to inefficiencies within the system. If things were not this way perpetual motion would be a possibility.

Also, it appears to me that that you have to apply current to the transmitter coil in order to generate the magnetic field. If this assumption is correct, in order to transmit fairly large amounts of power (e.g. a couple of kilowatts and above) then the coils are going to have to start getting quite large due to the coils DC resistance or, the supply voltage is going to have to be increased significantly.
However this is based on my understanding of how transformers work …perhaps there is something I am missing!!!

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