NEWS: Physics promises wireless power

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The tangle of cables and plugs needed to recharge today's electronic gadgets could soon be a thing of the past.

US researchers have outlined a relatively simple system that could deliver power to devices such as laptop computers or MP3 players wirelessly.

The concept exploits century-old physics and could work over distances of many metres, the researchers said.

Although the team had not built and tested a system, computer models and mathematics suggest it would work.

"There are so many autonomous devices such as cell phones and laptops that have emerged in the last few years," said Assistant Professor Marin Soljacic from the Massachusetts Institute of Technology and one of the researchers behind the work.

"We started thinking, 'it would be really convenient if you didn't have to recharge these things'.

"And because we're physicists we asked, 'what kind of physical phenomenon can we use to do this wireless energy transfer?'."

Energy trap

The answer the team came up with was "resonance", a phenomenon that causes an object to vibrate when energy of a certain frequency is applied.

"When you have two resonant objects of the same frequency they tend to couple very strongly," Professor Soljacic told the BBC News website.

Resonance can be seen in musical instruments for example.

"When you play a tune on one, then another instrument with the same acoustic resonance will pick up that tune, it will visibly vibrate," he said.

Instead of using acoustic vibrations, the team's system exploits the resonance of electromagnetic waves. Electromagnetic radiation includes radio waves, infrared and x-rays.

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.

Any energy not diverted into a gadget or appliance is simply reabsorbed.

The systems that the team have described would be able to transfer energy over three to five metres.

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Reply to
John Navas
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Reply to
Axel Hammerschmidt

Reminds me of the rumored weapon "Hammer of Thor" to be used in warfare this decade.

From a high flying altitude, a directed particle beam could be focused tightly enough to reduce a human to a puddle of fat and grease >while inside a car< in a few seconds.

We could combine cell phones and directed energy to eliminate California high speed auto chases. ;-)

Reply to
Thurman

John Navas hath wroth:

(...)

Yawn. Basically, they're building a resonant RF transformer. Coupling efficiency is maximum at resonance so that makes sense. However it's still radio waves and physics limits what can be done at their chosen frequency of 6.4MHz. There's no "wave magnet" available in radio waves. We're working in the near field where field strength varies roughly linearly with the distance. Double the distance and lose half the power. At the claimed 5 meters range, there's very little RF available to charge the device. My guess is about 5% efficiency depending on coil size. I guess energy conservation is of little concern because to deliver typical charger power, a substantial amount of RF must be radiated and lost to nowhere in particular. The device also has to have a relatively high Q coupling coil in order to work. That's difficult to do in a small package and will be seriously detuned by body capacitance. Want more delivered power? Get a bigger coil. Want gross inefficiency? Use a small coil.

The inability to direct the RF will also causes problems. Much of the

6.4MHz signal will get into the cell phone or radio, mix well, and get served as intermod, rectification, and other unwanted circuit malfunctions. Shielding will probably be necessary.

What does make this potentially practical is that the power consumption of portable devices are dropping rapidly. Chances are slim that they can meet FCC Part 15 radiation requirements and still deliver useable amounts of RF, but that can be negotiated (or ignored as in BPL).

While useless for delivering substantial amounts of power, resonant transformer coupling might be suitable for small loads, and only if one doesn't care about efficiency.

Incidentally, the 6.4MHz area is straddled by marine SSB and GMDSS users. I'm sure they'll be thrilled with the new source of RF pollution.

Ok, bring on the investors.

Reply to
Jeff Liebermann

I remember seeing this done with florescent light bulbs. It was an inspiring demonstration for elementary school students in the 70s.

There are issues of concern. In the United States, electricity is transferred in the power grid at 60Hz. 60Hz also corresponds to the frequency of the Lithium Ion (Li+). It has been shown that lithium ions will leave the skin of a human being when exposed to this frequency for an extensive period of time. Some have hypothesized that this may cause depression issues in some people, and others have suggested it may cause cancer. I recall this issue years ago, but it has been largely squashed; however, I was able to find this one resource:

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Now, my question is whether this same effect might exist when using a Tesla coil (that is how this is accomplished). I don't see why it wouldn't. I suspect that to alleviate concerns, they will have to find a frequency that they "determine" is safe and tool standardization along that path. Personally, I think that it is a bad idea from a health perspective and shouldn't be done without many decades of testing on animals. I would hate to be that pig.

Reply to
Thomas T. Veldhouse

On Wed, 15 Nov 2006 18:23:03 GMT, "Thomas T. Veldhouse" wrote in :

While your concern is understandable, I think it's overdone -- the physics make such side effects unlikely.

Reply to
John Navas

On Wed, 15 Nov 2006 10:14:22 -0800, Jeff Liebermann wrote in :

I respectfully suggest you actually read the research instead of making assumptions and leaping from them to all those conclusions.

Reply to
John Navas

At the lower power levels, solar becomes practical as well, it's already used on a lot of calculators. I had a bicycle computer that worked on solar, and an old Sony Walkman (cassette) that used solar to charge an internal NiCad.

Powering a laptop computer would be a real accomplishment, I wouldn't hold my breath waiting for that to happen.

Reply to
SMS

John Navas hath wroth:

What research? There's no link. I did read the BBC article and found it entertaining as I do enjoy reading science fiction. I also indicated that it might be useful if the device being charged consumed very little power. I do the math for resonant coupling for you if you want. Give me a frequency, distance, and delivered power requirement and I'll calculate the size of the coils, coupling efficiency, and approximate radiation. No research required as it's in any electronics textbook that covers RF xformers.

What is unique is this section: 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.

I've never heard of this phenomenon and would be genuinely interested in reading the research report. Last time I checked, there's no such thing as a static or potential RF field but I'm open to new ideas. It might be done with superconductors, where the field isn't dampened, but not on a piece of wire.

Reply to
Jeff Liebermann

SMS hath wroth:

Minor trivia: Ever wonder how much power your laptop sucks? No need to measure it. Just look at the rating on the battery charger. Multiply volts times amps to get watts. The idea is that the battery charger is suppose to run the laptop with the battery removed. It can only do that if it has sufficient power capacity. Most of mine are around 40 watts. However, I've seen Toshiba desktop replacements with

120 watt power adapter and blow enough hot air to suspect all of it is being used.
Reply to
Jeff Liebermann

On Wed, 15 Nov 2006 11:13:52 -0800, Jeff Liebermann wrote in :

Google and all the other search engines down? Took me all of about 30 seconds to find:

  • "Wireless Non-Radiative Energy Transfer", Marin Soljacic, Aristeidis Karalis, and J.D.Joannopoulos. 2006 AIP Industrial Physics Forum in San Francisco (CA), USA, 2006. [Key words there are "non-radiative"]
  • physics/0611063 (November 2006) Wireless Non-Radiative Energy Transfer Aristeidis Karalis, J. D. Joannopoulos and Marin Soljacic Received. 07 November 2006 Last updated. 07 November 2006 Abstract. We investigate whether, and to what extent, the physical phenomenon of long-lifetime resonant electromagnetic states with localized slowly-evanescent field patterns can be used to transfer energy efficiently, even in the presence of extraneous environmental objects. Via detailed theoretical and numerical analyses of typical real-world model-situations and realistic material parameters, we establish that such a non-radiative scheme could indeed be practical for medium-range wireless energy transfer. Subject. Optics; Classical Physics Comment. 17 pages, 6 figures Article Options Full Text [link]

You're accusing MIT, these scientists, and a professional society of promulgating fiction, without having read the paper, based simply on your own quick assessment of a BBC article?

Again, I respectfully suggest you actually read the research instead of making assumptions and leaping from them to all those conclusions. I've hopefully now got you started.

Reply to
John Navas

The charger needs to be able to both charge the battery, and run the laptop

The only way to determine the power is to measure it.

I've instrumented a lot of laptops to measure all the different power levels of each supply. In reality, the core, the hard drive, and the screen consume magnitudes more power that all the other components.

You also really don't know the maximum power the laptop consumes until you run a power virus program. You have to run these when you're doing worst case design of the laptops thermal solution and power supply components.

Reply to
SMS

On Wed, 15 Nov 2006 11:10:41 -0800, SMS wrote in :

These are more entertainment than practicality, since they are very low power, and still depend on batteries.

With hours of exposure needed for mere minutes of operation; i.e., not practical.

No kidding, because solar power density is way too low to be practical ever outdoors, and isn't available indoors.

Do your homework before ranting.

Reply to
John Navas

The local grid is 60 Hz, but the long distance power transmission is high voltage DC.

Reply to
SMS

On Wed, 15 Nov 2006 11:22:35 -0800, Jeff Liebermann wrote in :

A solar panel in the contiguous United States on average delivers 19 to

56 W/m² or 0.45-1.35 kWh/m²/day. [National Renewable Energy Laboratory]

In other words, it would take a 4 sq meter solar panel to power my ThinkPad reliably. That's a 6-1/2 foot square panel. Maybe Steven has a hat that big, but I don't. :)

Reply to
John Navas

High voltage AC.

Reply to
Gordon Montgomery

On Wed, 15 Nov 2006 12:02:08 -0800, SMS wrote in :

In fact most current long distance power transmission is high voltage AC, although HVDC is slowly gaining traction. Because of the high cost of conversion between AC and DC, the breakeven for HVDC is about 600-800 km.

Reply to
John Navas

This negativity coming from someone who thinks they'll be beaming power wirelessly to that same laptop soon?

They'll probably be beaming your laptop from one place to another a la Star Trek before they'll ever be beaming power to it OTA! ;-)

Reply to
Todd Allcock

On Wed, 15 Nov 2006 17:05:41 -0700, Todd Allcock wrote in :

I'm guessing we'll first see wireless recharging of low power devices like cell phones, cordless phones, digital cameras, wireless remotes, MP3 players, etc. Cell phones in particular would benefit from this technology, seamlessly charging whenever brought within range of a base station.

Reply to
John Navas

SMS hath wroth:

Agreed. I forgot about the battery. However, it charges at perhaps C/10 or C/20 which would not contribute a huge increase to the drain on the power supply. Still, you're correct and it should be added into the guestimation.

Sure, but my point was that one could obtain a fair estimate of how much the laptop will burn from the charger ratings. At the least, it will not be more than the charger ratings or it will go into foldback, shutdown, or burn out. The idea was to use the nameplate numbers to estimate how much power would need to be delivered by a solar, inductive, cazapitive, or wireless power solution.

Agreed (again). However, I'm just suggesting using the rating for an initial estimate on laptop power consumption. In your experience, how close is the power rating to the actual measured maximum power drain?

Reply to
Jeff Liebermann

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