Evanescent Coupling?!? A new way to transfer electrical energy?

[Analog]

From this article:

A phenomenon called "evanescent coupling" could allow electronic gadgets to start charging themselves as soon as their owner walks into their home or office.

Researchers have been looking for a way to make a wireless charger for some time. One idea is to use electromagnetic induction – passing an electric current through a coil to create a magnetic field that induces a current in a neighbouring coil.

This is the way devices like electric toothbrushes are charged, and has been proposed as the basis of a universal recharger pad before (see One charger pad could power up all gadgets).

The snag as far as mobile devices are concerned is that the charger and device must be in close contact with each other for it to work. Alternative schemes - for instance, transmitting electromagnetic waves in all directions to reach any device in a room - would be hugely wasteful.
Trapped at source

Instead, Marin Soljacic at the Massachusetts Institute of Technology wants to use evanescent coupling, which allows electromagnetic energy "trapped" in a charging device to be tapped by a "drain" mobile device if the two have the same resonant frequency.

"The energy is trapped at source, until I bring a device that has the same resonant frequency close to it. Only then can the energy 'tunnel through'," says Soljacic. Crucially, the "charger" only starts powering another device when a compatible gadget comes within range.

Soljacic and colleagues Aristeidis Karalis and John Joannopoulos have carried out numerous computer simulations to see if the idea will work. They discovered that a small circuit, consisting of an inductor loop and a capacitor, could be made to resonate at a frequency of 3 to 4 megahertz, allowing it to trap electromagnetic energy without emitting radio waves to its surroundings.
Inductor loop

In the wireless charger design, alternating current from the mains is converted to this resonant frequency and sent into the circuit. The current travels round the circuit, generating a magnetic field as it passes through the inductor loop and an electric field as it passes through the capacitor. This pulsing magnetic field extends up to 5 metres around the device.

The magnetic field created by the wireless charger is relatively weak, meaning it consumes little power. However, if a mobile gadget fitted with a similar circuit, with the same resonant frequency, is brought into the room, the charger's magnetic field induces an electric current in the gadget's inductor loop.

This current travels round the mobile device's circuit, constantly switching between electrical and magnetic states, just as in the charger's circuit. As a result, the two circuits start to "resonate" together. This increases the transmission of electromagnetic energy via induction and that energy is used to charge up the gadget.

Placing one of these wireless chargers in each room of a home or office could provide coverage throughout the building. Soljacic presented the results at the American Institute of Physics Industrial Physics Forum in San Francisco on 14 November. The team is now trying to develop a prototype device.

**broken link removed**

 

[HarveyH42]

Sounds like something from Nikola Tesla, seventy to eighty years ago... The man was way ahead of his time.

 

[The Mad Professor]

Indeed , if this idea had originated in an inventors garden shed and not the MIT I wonder how the press would be treating the story.

 

[Hero999]

They discovered that a small circuit, consisting of an inductor loop and a capacitor, could be made to resonate at a frequency of 3 to 4 megahertz, allowing it to trap electromagnetic energy without emitting radio waves to its surroundings.

They weren't the first ones to descover this. I stopped reading after this point. Morons.

 

[The Mad Professor]

descover

Upon reflection I feel that the word "descover" is a typographical error created in the heat of the moment, a brief period of synaptic overload
where the authors brain was "thinking" far faster than his nervous system
could implement and by no means should it be implied that said author is by any means illiterate or some low grade moron shooting his mouth off.

On the other hand my personal opinion matters naught and I leave to you the humble reader to make your own informed choice.

Now back to the thread..

Well then "Hero999" save us all the trouble of a google search and share your concealed knowledge with the forum, who was the first to discover it ?

 

[jrz126]

Tesla most likely...He was the man when it came to resonance.

 

[Sceadwian]

"Evanescent coupling" is just a buzz word and doesn't actually mean anything, except for the first part being a word most people don't understand so it sounds impressive and important.
Evanescence means to disappear or fade away acording to the dictionary.
The device described is exactly the same as the inductive method described for toothbrushes. tunneling effects like true evanescent coupling has something to do with optical frequency waves over VERY short distances, and can't be applied on 'human scale' much like the fact that a tunneling diode works doesn't mean we can do the same thing with regular matter through a wall.

 

[evandude]

90% of that article is no better than over-hyped crap to make it sound amazing to laypeople, but there's definitely nothing revolutionary to it.

This is exactly the same kind of thing I ran into doing an RFID system for my senior project a while ago. Hook up an oscillator through a series resistor to a coil tuned with a parallel capacitor and it will resonate quite well and not draw a ton of power. Bring another capacitor-tuned coil close to it, and all of a sudden the coil hooked up to the oscillator draws a lot more current, and is thus presumably generating a much stronger magnetic field than before.

Sure, it's a rather interesting phenomenon, and was surprising and interesting to us when we first noticed it (and to a number of other people we demonstrated it to), but it's nothing revolutionary, and making it out to be "the future of wireless charging" and calling it something fancy like "evanescent coupling" is ridiculous. and it STILL doesn't solve the problem of directionality. When the device is charging, the coil is STILL radiating power to the whole room (thus requiring a ton of power use if you expect to charge any device at a range of more than an inch or two), this would just minimize the power use when there were no devices in the area to charge. By using words like "tunneling" they make it sound like this 'technology' is somehow emitting a focused beam of magnetic field toward the target device, or sending it through a wormhole, or some other crap like that.

 

[Grant Fleming]

A bit hard on the guy you were!

Upon reflection I feel that the word "descover" is a typographical error created in the heat of the moment, a brief period of synaptic overload
where the authors brain was "thinking" far faster than his nervous system
could implement and by no means should it be implied that said author is by any means illiterate or some low grade moron shooting his mouth off.

On the other hand my personal opinion matters naught and I leave to you the humble reader to make your own informed choice.

Now back to the thread..

Well then "Hero999" save us all the trouble of a google search and share your concealed knowledge with the forum, who was the first to discover it ?

I wasn't aware this was a spelling/grammar forum! I don't profess to being a perfect speller, I am now a little afraid of posting but I will give it a go still. The Mad Professor has overlooked an apostrophe in the above quote and also in his previous quote (am I doing o.k so far in picking up mistakes in this spelling forum?) so I will be watching all his future posts very carefully (and his posting history) for any errors, I will make it a hobby of mine! I'm sure others will watch also to keep with the spirit of things. Anyway I apoligize for any grammatical mistakes I may have made in any of my posts.
Cheers,
Grant Fleming (a real name-how about that!)

 

[Oznog]

It's not just something that others have already "discovered", it's rubbish.

He's talking about broadcasting radio energy. A radio tower doesn't draw less power if nobody turns on a receiver tuned to that frequency. The receiver has basically no effect on the transmitter unless:
1. The RX and TX are very close (near touching), or
2. The RX and TX are in a resonant cavity (microwave oven)

Neither is true for the scenario listed, or in fact ANY real-world scenario for "wireless power".

 

[evandude]

It's not just something that others have already "discovered", it's rubbish.

He's talking about broadcasting radio energy. A radio tower doesn't draw less power if nobody turns on a receiver tuned to that frequency. The receiver has basically no effect on the transmitter unless:
1. The RX and TX are very close (near touching), or
2. The RX and TX are in a resonant cavity (microwave oven)

Neither is true for the scenario listed, or in fact ANY real-world scenario for "wireless power".

Well, yes and no... driving a tuned LC circuit isn't the same as driving an antenna; the idea of another resonant LC circuit being introduced and absorbing energy causing the amount of power transmitted to change isn't crazy (or new) - see "Grid dip oscillator" on wikipedia: https://en.wikipedia.org/wiki/Grid_dip_oscillator

Of course, when they talk about ranges of feet or 10's of feet, it once again becomes BS as you say, because the effect on transmitted power will be insignificant at that kind of range. If the receiver is far enough away to only be receiving 1/1000 of the transmitted power (which, by the inverse square law, is probably not unrealistic), even if the tuned coil in it is absorbing as much energy as possible, it would still only be absorbing a miniscule fraction of the total transmitted power and thus be a negligible load on the TX coil.

The real BS is that they make it sound like this is somehow "focusing" magnetic field at the device to be charged, when in fact it is little more than a 'regular' inductive charging system (radiating energy with no real directionality) with a sort of automatic on-off switch (or more accurately, an on/less-on switch) added. It would still require insane amounts of power to be pumped through the transmitting coil for even a small amount of power to be received at the device end, if the device was feet (or 10's of feet) from the charger, rather than sitting right on top of it.

Whatever. I say the same thing to this that I say when I think of all the free-energy schemes that are always popping up. Let them toot their horns and wow the press with fancy jargon and all that, but if they're basing everything upon a bunch of BS that just sounds good to the uneducated, they'll never get anywhere with it since they can't capitalize on it if they never get it working. Let capitalism sort it out...

 

[Analog]

Here's More from Scientific American

Tired of your laptop's battery dying during presentations or bursting into flames? Take heart: scientists are perfecting a new method for transmitting electrical energy from a base station using a technique that resembles a wireless Internet connection. Researchers say that a specially designed device should be able to draw power from a strong magnetic field permeating a room.

The effect, which has not yet been demonstrated, would take advantage of the stationary magnetic field that surrounds a charged loop of metal. This so-called near-field can be powerful--it is what makes an electric motor turn. And in principle its oscillations can induce an electric current in another nearby loop, because dynamic magnetic fields create electric fields and vice versa. The second loop could act as a battery or recharger, but it would normally receive only a slight current because the near field fades rapidly over distance.

The key to boosting the induced current is resonance, says photonics researcher Marin Soljacic of the Massachusetts Institute of Technology. Soljacic and his colleagues propose introducing a short gap in a metal loop and attaching two small disks at each end. When electrified, such an object has a natural, or resonant, frequency that results from current flowing back and forth along the loop from one disk to the other.

If two loops have the same frequency, one should be able to receive energy from the other through the magnetic near field--much like two identical tuning forks brought into close proximity, the group reasoned. From a few meters away, the rate of energy transferred in this way might reach tens of watts, or enough to power a laptop, according to simulations Soljacic presented November 14 at a meeting of the American Institute of Physics. "The results are strongly encouraging but the real test of the thing will be experiments, which we are working on now." Soljacic says.

"It's a great idea. I think a lot of people will be very surprised by it," says photonics expert John Pendry of Imperial College London. It could be very useful, he says, in specialized applications such as space exploration, say to power a rover within a few meters of a power supply. He questions whether people would accept it in their homes, though, given concerns over the effect of power lines and cell phones on the body. "I think they'll have a problem convincing people it's okay," he says. --JR Minkel

**broken link removed**

 

[Grant Fleming]

Not again!

Anal og,
What is it with you? Do you get paid for re-distributing this garbage?
Cheers,
Grant

 

[Roff]

Bill Beaty has some interesting things to say on the subject of 'Energy-sucking' Radio Antennas. Here's an excerpt:

Iron-core transformers are examples of tight magnetic coupling, and significant power can be transferred between the coils of a 60Hz transformer. Capacitors are similar: they are examples of tight electrostatic coupling. Resonant circuits give us two new options for tightly-coupled power systems: pairs of high-amperage resonant loop-antennas, and pairs of high-voltage resonant dipole antennas. The spacing of each of these must be below 1/4 wavelength for the phenomenon to appear, and the e- or b-field strength must be very high. Now that I'm speaking of this, I know I've seen such things in common use. Air-core transformers in high-power VHF radio transmitters employ this effect. If both sides of an air-core transformer are tuned to the same frequency, then the b-field surrounding the transformer will build up to a very high level, and the throughput of energy will be very high, even though there's no closed iron-ring magnetic circuit, and coupling between the coils is *apparently* very loose.

Bill Beaty is not a quack. He does look at electronics and physics from a different angle than many of us. If you're not familiar with his web site, browse it for a while. Read some of his explanations of how stuff "really" works. Interesting stuff.

 

[Analog]

Analog,
What is it with you? Do you get paid for re-distributing this garbage?
Cheers,
Grant

Actually, no. I am a professor of electrical engineering, and I happen to teach electromagnetics as part of my curriculum. I am curious as to why this is being so touted, even though no working models have been demonstrated.

Scientific American is a relatively respected publication, and MIT is a respected university. I have seen no articles shooting it down so far, and if you do, please post them.

 

[Hero999]

Bill Beaty has some interesting things to say on the subject of 'Energy-sucking' Radio Antennas. Here's an excerpt:

Bill Beaty is not a quack. He does look at electronics and physics from a different angle than many of us. If you're not familiar with his web site, browse it for a while. Read some of his explanations of how stuff "really" works. Interesting stuff.

I've read that artical and I respect him and agree that it is possible. However I don't have the same admiration for the idiot who wrote the artical at newscientisttech.com, the terminology used is wrong and claiming that resonance is a newly descovered phenonimum has only cemented my oppinion that it's total crap.

 

[Analog]

http://www.physorg.com/news100445957.html

 

[eblc1388]

Just ask any radio armateur how many of them have got a lamp attached to one or a few turns of cable loop which they often used to check the tank circuit of a transmitter. Many do.

The MIT picture did bring back memory of the old days when hobbyist were using 807 or 813 as the transmitter PA stage.

Edited: The News and reporters just told us that MIT did it "seven feet away" without telling us the million dollar information. i.e. the actual coils are over three feet in diameter(seen from the picture). I reckon a 10 meter diameter coil would probably do the same thing with another 10 meter diameter coil placed 20 meter away.

So, I did it in 66 feet !!!!

 

[Roff]

Just ask any radio armateur how many of them have got a lamp attached to one or a few turns of cable loop which they often used to check the tank circuit of a transmitter. Many do.

The MIT picture did bring back memory of the old days when hobbyist were using 807 or 813 as the transmitter PA stage.

Edited: The News and reporters just told us that MIT did it "seven feet away" without telling us the million dollar information. i.e. the actual coils are over three feet in diameter(seen from the picture). I reckon a 10 meter diameter coil would probably do the same thing with another 10 meter diameter coil placed 20 meter away.

So, I did it in 66 feet !!!!

I think resonance in the receiving coil is key to efficient coupling.

 

[justDIY]

man I'd love to have a huge coil thing hooked up to charge my laptop, from another huge coil thing a few feet away. that'd be awesome, much better than plugging my pocket sized 70 watt smps based charger into a multistrip.

some of the whiz kids at MIT do neat stuff, this isn't it.