Thursday, August 03, 2017

Self-tuning brings wireless power closer to reality

Cables under my desk.
One of the unlikelier fights I picked while blogging was with an MIT group that aimed to wirelessly power devices – by tunneling:
“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.
They had proposed a new method for wireless power transfer using two electric circuits in magnetic resonance. But there’s no tunneling in such a resonance. Tunneling is a quantum effect. Single particles tunnel. Sometimes. But kilowatts definitely don’t.

I reached out to the professor’s coauthor, Aristeidis Karalis, who told me, even more bizarrely: “The energy stays in the system and does not leak out. It just jumps from one to the other back and forth.”

I had to go and calculate the Poynting vector to make clear the energy is – as always – transmitted from one point to another by going through all points in between. It doesn’t tunnel, and it doesn’t jump either. For the MIT guys’ envisioned powering device with the resonant coils the energy flow is focused between the coils’ centers.

The difference between “jumping” and “flowing” energy is more than just words. Once you know that energy is flowing, you also know that if you’re in its way you might get some of it. And the more focused the energy, the higher the possible damage. This means, large devices have to be close together and the energy must be spread out over large surfaces to comply with safety standards.

Back then, I did some estimates. If you want to transfer, say, 1 Watt, and you distribute it over a coil with radius 30cm, you end up with a density of roughly 1 mW/cm2. That already exceeds the safety limit (in the frequency range 30-300 MHz). And that’s leaving aside there usually must be much more energy in the resonance field than what’s actually transmitted. And 30cm isn’t exactly handy. In summary, it’ll work – but it’s not practical and it won’t charge the laptop without roasting what gets in the way.

The MIT guys meanwhile founded a company, Witricity, and dropped the tunneling tale.

Another problem with using resonance for wireless power is that the efficiency depends on the distance between the circuits. It doesn’t work well when they’re too far, and not when they’re too close either. That’s not great for real-world applications.

But in a recent paper published in Nature, a group from Stanford put forward a solution to this problem. And even though I’m not too enchanted by transfering power by magnetic resonance, it is a really neat idea:
Usually the resonance between two circuits is designed, meaning they receiver’s and sender’s frequencies are tuned to work together. But in the new paper, the authors instead let the frequency of the sender range freely – they merely feed it energy. They then show that the coupled system will automatically tune to a resonance frequency at which efficiency is maximal.

The maximal efficiency they reach is the same as with the fixed-frequency circuits. But it works better for shorter distances. While the usual setting is inefficient both at too short and too long distances, the self-tuned system has a stable efficiency up to some distance, and then decays. This makes the new arrangement much more useful in practice.
Efficiency of energy transfer as a function of distance
between the coils (schematic). Blue curve is for the
usual setting with pre-fixed frequency. Red curve is
for the self-tuned circuits.

The group didn’t only calculate this, they also did an experiment to show it works. One limitation of the present setup though is that it works only in one direction, so still not too practical. But it’s a big step forward.

Personally, I’m more optimistic about using ultrasound for wireless power transfer than about the magnetic resonance because ultrasound presently reaches larger distances. Both technologies, however, are still very much in their infancy, so hard to tell which one will win out.

(Note added: Ultrasound not looking too convincing either, ht Tim, see comments for more.)

Let me not forget to mention that in an ingenious paper which was completely lost on the world I showed you don’t need to transfer the total energy to the receiver. You only need to send the information necessary to decrease entropy in the receiver’s surrounding, then it can draw energy from the environment.

Unfortunately, I could think of how to do this only for a few atoms at a time. And, needless to say, I didn’t do any experiment – I’m a theoretician after all. While I’m sure in a few thousand years everyone will use my groundbreaking insight, until then, it’s coils or ultrasound or good, old cables.


  1. The physicist commentary I've seen on uBeam / ultrasonic charging have similar "It can't work" conclusions; do you disagree?

    See, for example, or

    Thanks for your writing,
    - Tim

  2. A pedantic remark:

    In English, metric units aren't capitalized, even ones named after people like "watt", "joule", "ampere" and "coulomb". Also, it's "kilowatts", not "kilo-Watts": there aren't hyphens in metric units like kilometer, kilogram, and kilowatt.

    The abbreviations like "W", "J", "A" and "C" are capitalized, however!

  3. "magnetic resonance" A million New Yorkers wasting a broadcast charging watt for convenience is megawatt unwise. Placing "off" switches on transformers’ cold sides (washers and dryers; little plug-in power supplies) is stupid.

    "I’m a theoretician after all" Thank you for doing the heavy lifting. "8^>)

  4. Just today, Innocentive sent out a "Challenge #9934010" to propose a wireless power source for wall-mounted flat screen TV, with a 15k USD reward. (As a fallback plan, they will even accept power cables if you can make them small and transparent...) I loath the idea of continuously beaming couple hundred watts across a bedroom, like in a microwave oven

  5. The key point about wireless power transfer is that this is *near field*, i,e, much nearer than wavelength.
    Frequency for witricity is 9.9MHz, and for more general NFC comms 13.56MHz. Not 30-300MHz.
    It's definitely not radiative, power-transfer falls off as r^3

    Does this change your view on tunnelling? Because I *would* have called this tunnelling. I'm still prepared to accept if you think that's wrong, but your input facts are wrong.
    That also changes the perspective on safety. The human body just can't absorb anything at 10MHz, you'd have to be 7meters tall to be a decent antenna even if you were made of metal.
    Plus, I would still challenge that 1W at 300 MHz is "dangerous". An old 2G mobile used to transmit at up to 2W at 900 MHz. And people put them against their heads. Despite "fears" there is *no* peer-reviewed RCT evidence of harm. I will agree with you that is wasn't ideal, and a modern value of 0.3W max at 2.4GHz seems rather more sensible

  6. Oops, typo, mobile @2.1GHz. Of course it's WiFi at 2.4 GHz

  7. There's a huge body of literature on the subject, most of it is in the engineering publications from IEEE, conferences like APEC, etc. I wouldn't expect "science-y" publications like Nature to be at the forefront.
    The technology is in early stages of commercial deployment.
    -- Tom

  8. TelecomsGuy,

    As I said, I'm a theoretician. For what I'm concerned, 10 MHz is the same as 30 MHz. And, yes, it's non-radiative, near-field. I read the paper.

    My issue with the "tunneling" is that it suggests the energy doesn't pass through the space in between. Which it definitely does. It's misleading and it's not a standard nomenclature, definitely not.

    I don't know anything about the impact of radiation on tissue. But really, take or give a factor 10, it's not going to cut it. This technology isn't anywhere close to hitting big is all I'm saying.

  9. Hi Tim,

    Thanks for the reference, I'll give this a read!

  10. Bee,
    Theoreticians are allowed to ignore the properties of human tissue, but not the wavelength. If you put the correct numbers in and calculate, I believe you may change your view.

    You mention 30cm resonators. They aren't. Typical resonators are more like 2-3cm in size. To get a feel for this, contactless payment cards use precisely this technology to both power and communicate, and that's the size of the antenna. Smart engineering to efficiently couple to EM with "wavelength" 30 meters.

    As to "the energy must pass through the space in between", I believe you are strong on phenomenology. I believe the correct approach is to calculate the outcome of some experiment, even with perfect theoretical elements. A numerical EM solver of a realisable experiment, the result simply doesn't agree that "the energy must....." . Separate 2cm resonant emitter/receivers by 10 cm, and transfer power at 10MHz, 1kW. Now tile the space between them with non-resonant "perfect antennas". Turns out the interposing objects sink only Milliwatts, not 1kW.
    We can do better.....even interposing a sheet of copper conductor between the two, doesn't prevent energy transfer. That does seem analogous to tunnelling through a barrier to me.

    We shouldn't find this surprising, because as you note if this weren't true then beaming a kilowatt would be Unsafe. There are fairly strict ITU (government, standardised) limits on this, and it all gets tested and verified. Technologies like Witricity and others on the same principle, are routinely deployed and pass those standard limits.

  11. Bee,
    A possibly equivalent corollary - would you count as tunnelling the evanescent wave of light near a boundary of total internal reflection?

    For that case, I believe that the experimental evidence is zero tunnelling delay of photons. I remember having seen a reputable reference, can't recall it, but am sure you do.

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  13. Bee, I luv ya, but stick to particle physics. :^)
    Yeah as Tim said the ultra sonic thing is going no where.
    You can do other types of transfer, but it's just hard to beat a piece of metal conducting electric fields. It would be nice if magnetic things worked better.. but it's mostly (like tunneling) a distance thing. (Sure, bigger distances.)

  14. Don't worry, I have no intention to go into engineering.

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  16. @TelecomsGuy,

    What is frequently overlooked in these schemes is that resonance is simply a trick to get high transmission coil currents. The bottom line is the power received is V^2/R where V = dPhi/dt is the voltage induced in the receiving coil. So for high power you need high magnetic field oscillating quickly. A weakly coupled transformer might have a k of 0.01 or so: so to get the same power transmission as for k=1 you need 100x the current in the primary and/or 100x the frequency. Bang the primary into an LC circuit with a high Q and voila you have super high currents with a demand on your drive circuit similar to a k=1 tranformer.

    It gets more complicated once you factor in the secondary: if your secondary demand has a frequency that is a bit off it will have a back-reaction that will tend to cancel out the efforts of your drive circuit, think of two coupled oscillators working against each other. So to get the best performance you want your secondary to be at the same frequency. That's the only reason for tuning, to maximize the power cycling in your primary.

    As for efficiency, that in principle has nothing to do with range or resonance; your losses are simply from coil resistance, capacitor losses, eddy currents in nearby conductors etc. Add these all together and they reduce the Q of your primary. If your Q losses are much lower than your power transmission then great, you have an efficient system.

    So no "tunnelling" involved, just brute force finessed with a resonant  oscillator.


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