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Wireless Power
Jon Lebkowsky, 16 Nov 06
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Tesla never quite pulled it off, but a group of contemporary physicists are developing wireless energy transmission using "'resonance,' a phenomenon that causes an object to vibrate when energy of a certain frequency is applied." They're considering something similar to a phenomenon you can see in musical instruments with the same acoustic resonance – play a tune on one, and the another with the same resonance will pick it up. Exploit the resonance of electromagnetic waves, and "energy can tunnel from one object to the other." Focusing the energy and getting viable efficiency are still challenges, and it's not clear that this will be a long distance solution. [Link to "Wireless Non-Radiative Energy Transfer," a paper establishing "that such a non-radiative scheme could indeed be practical for medium-range wireless energy transfer."]


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As a former radio frequency design engineer and 30+ ham radio licensee, I think this is largely hype.

Another reason it smells like hype is the touting of resonance, as if it's something new. We've known about resonance for well over a century. Nearly every electronic device in your environment uses it in some manner.

The problem is efficiency -- or the trade-off between efficiency and portability.

For maximum portability, the transmitter needs an omni-directional pattern. This results in severely attenuated power level at any one point inside a given radius from the transmitter. Think of it -- the transmitter doesn't know where a receiver is going to be, so it has to put energy out in all directions at once.

As an alternative, as the beamwidth decreases, receiver power increases. But at some point, you either need the transmitter tracking the receiver -- expensive to implement -- or your receiver and transmitter must remain stationary, taking on most of the disadvantages of wires. And you'll still get no more than a few percent of the transmitted power.

The only application where I see this as useful is for very low powered devices that are stationary -- things like remote sensors for indoor-outdoor thermometers.

Otherwise, the LAST thing we need is a "new" energy transmission medium that loses 99% of the energy put into it.

Then there's the issue of all that power bouncing around. Some studies show increased rates of brain cancers in ham radio operators, but such studies have not been performed for cell phone users. Anyone feel like being a guinea pig?

Posted by: Jan Steinman on 17 Nov 06


Having briefly skimmed the paper (and not being a professional physicist) it seems that they have foreseen and avoided the issues mentioned in your comment. Namely they are talking about creating non-radiative electromagnetic fields which are not damaging to humans and also avoid the issues of loss of energy assocaited with radiative methods. Worth reading the paper (brief and clearly written) to understand the specifics.

Posted by: Hana Loftus on 17 Nov 06

Actually, I *am* a professional physicist, and this seems like a very interesting article. I hadn't thought of looking at these large-scale quasi-static modes extending outside the actual object; from what I remember of my old Jackson E&M classes it seemed like everything was either a resonant cavity with only internal losses considered, or an antenna that radiated only in a narrow bandpass.

I assume that in this case, of the two objects (disks or loops) mentioned, one is the "transmitter" and the other the "receiver"? If that's the case, then it seems like their quality factor k/gamma for delivering power is quite strongly dependent on the distance between the two objects -- e.g. p. 6 for the disks: their k/gamma is 40 when the separation is 3 radii, but drops to only 6 if you increase it to 5 radii! And their figures in the back seem to show that in realistic cases where k/gamma ~ 5, you end up dissipating as much power as you successfully transmit. So in order to get efficiencies near 100% you really want to keep the resonant objects close to each other -- but then you might as well just use a cord, right?

It seems like an interesting direction in which to take things, but they really need to find ways of controlling the amount of power dumped off to nowhere. For example, what if they made the wire loops superconducting (at room temperature, ha), to cut down on the losses due to internal resistance?

Posted by: Richard Scalzo on 17 Nov 06

Tesla actually did pull it off. However, he was never funded after Colorado Springs and so spent his life trying to make money (through the invention of the rotary engine, etc.) to finish his wireless dream and make it available to the public.

We used to say Da Vinci never pulled off his flying machine - only in the last few years did we realize the opposite. Will it take five hundred years to recognize Tesla's genius properly?

Posted by: Tod Brilliant on 17 Nov 06

Reasons for not using a cord:

The object is moving. A supply running beside or beneath a road is an obvious case.

There is a risk of damage to the cord. Factory floor, outdoors, near livestock...

There is water present. Bathrooms, kitchens, outdoors.

In truth all the paper reads as to me is a mathematical description of primitive transformers and capacitors. Yes, electronic engineers regularly use short range wireless power transmission, it's just that the two plates, or coils are physically fixed together. Sometimes these are resonant cicuits but the trouble with those is that the slightest movement or bringing some other object close changes the resonant frequency, but maybe that doesn't matter. Perhaps Armstrong should be our guide in these matters rather than Tesla, see

Posted by: Michael Saunby on 18 Nov 06

Jan, I'd suggest taking Hana's advice and reading the paper. I have to say, as a former physicist, it sounds pretty legit.

You're right that the BBC article's talk about "resonance" is stupid. However, that's just the reporter not understanding what's going on, the actual paper is describing something novel. What they're describing is _not_ like normal electromagnetic broadcasting, which is traveling waves; they're talking about an emitter setting up standing waves around itself (where the wavelength is significantly larger than the device size), and bringing the receiving device (the portable thing you want to charge) within that standing wave. This apparently means optimizing for different conditions than normal broadcasting (they talk about how their method is different from a cell phone's antenna, though it went over my head. Their math doesn't look too hard to check, though, if you had the time & inclination.)

The paper claims they have power-coupling to power-loss ratios of up to 40:1, meaning it's up to 98% efficient. That's awfully damn good. Although in a more realistic scenario that they described (in a room with walls, with a person around) it was more like 60%. And they did modeling of "what if a human is sitting in the field", and it looks like the person doesn't absorb much energy at all.

You're right that any significant inefficiency just to avoid the hassle of electrical cords is not necessarily worth it, but if the efficiencies can be high, it'd be cool.

Richard, you're right that the efficiency varies greatly with distance; but even when distance between objects was about 3x the size of objects, efficiency was still good. But yes, might be better off sticking with inductive-charging surfaces like Splashpower (except have it built into tabletops & desktops, rather than a dish that you have to place stuff in.) Still, this is very much worth investigating. All sorts of interesting applications could be enabled by it.

And Tod, you're right that Tesla did get power transfer to work, but it was broadcast-style traveling waves, which are horribly inefficient, as Jan was talking about. Tesla never did stuff like this.

It'll be interesting to see where this goes!

Posted by: Jeremy Faludi on 18 Nov 06



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