back to article New wonder slab slurps Wi-Fi, converts it into juice for gadgets, boast boffins

Researchers at Duke University's Pratt School of Engineering claim to have created a material that can harvest microwave signals and convert them into electricity. Duke University energy harvesting array So meta ... device boffins claim harvests power from thin air The team developed a "metamaterial", an artificially …


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  1. tom dial Silver badge

    It would be very interesting to see some numbers - say the absolute amount of power extracted from a 100mw broadcast signal at a distance of 30 or 40 meters. My hunch says that at 100% conversion efficiency it would be at least an order of magnitude less than negligible - that if you used the 10 or so wifi sites you can detect from an average urban location, you would get no useful power. I am willing to be proved a fool on this, though.

    The seven I can see right now from my house all are less than one nanowatt.

    1. Chozo

      It would be very interesting to see some numbers

      It would be perhaps more interesting if this 'new' meta-material is actually different to this Chinese work from a couple of years back..

      'An omnidirectional electromagnetic absorber made of metamaterials' by Qiang Cheng, Tie Jun Cui, Wei Xiang Jiang and Ben Geng Cai (Qiang Cheng et al 2010 New J. Phys. 12 063006).

      Capturing RF energy is of course not a new idea and many here have probably played with a crystal radio reciever at some point in their geekery. With the proliferation of WiFi and in particular RFID readers you should be able to light an LED albeit dimly provided you're within 12" or less with a big enough coil using a coventional discrete components.

      1. JamesTQuirk

        Really not new idea ...

        I remember Tesla tying a RF > DC converter using 1 of his coils as antenna, for transmission of energy ....

      2. Suricou Raven

        Crystal radios have an antenna that can double as a washing line.

        1. ChrisB 2

          "Crystal radios have an antenna that can double as a washing line."

          As a boy, my crystal radio antenna was strung up along my Mum's washing line :)

          1. JamesTQuirk

            Metal curtain rod worked too .....

        2. Martin Budden Silver badge
          Thumb Up

          When I was a kid my crystal radio antenna doubled as a bed frame, which was really useful for listening to the radio while in bed.

  2. Herby

    Let's look at this...

    I just looked at my router and its signal is specified as 17dBm, or about 50mw. Given the conversion efficiency of 33% (or so) indicated in the article, that gets you around 17mw. One problem here is that the traffic (and thus the transmission) is at most 50% and more like 10% (on a good day), so that gets you down to a time averaged value of 1.7 mw. Now take into consideration the dropoff (inverse square law) for the signal, and if you are even a little bit away from the originating antenna, you will get (I'm being conservative here) maybe 25%, which gets you down to MAYBE 1/2 milliwatt. If you think that this is 'meaningful' power you are sadly mistaken.

    Moral of the story: Spend the $$$ on a battery. If you use the same amount of power as this system delivers (1/2 milliwatt on a good day) the battery will last quite a long time, and probably cost less as well in the long run.

    1. tom dial Silver badge

      Re: Let's look at this...

      Which works if you surround the antenna(s) with energy collectors and capture the entire signal, making it fairly useless for its intended purpose.

    2. Captain Save-a-ho
      Thumb Up

      Re: Let's look at this...

      Not that it will improve the numbers enough to make it commercial viable, you have to keep in mind that they're proposing arrays of the material, so that 1/2mw would certainly grow larger. You're also underestimating other available signals that would provide additional power. While not world-beating, that enough of a start to work on improving the efficiency and scope such that charging a phone might be possible, even if very slow.

    3. Mage Silver badge

      Re: Let's look at this...

      At any useful distance you are talking uW or nW. Maybe enough to charge an eInk display that wakes up ONCE a day and change it's message, but unlikely even enough for that.

      I don't care if they have 100% conversion or 12V. The power simply isn't there.

      1. Anonymous Coward
        Anonymous Coward

        Re: Let's look at this...

        Unbeliever! Off with his head!

        Just stop it, Mage. When did physics ever Trump economics?

        If people like you carry on, you'll be ruining these "academics" opportunity to fleece the venture capitalists for their next round of funding.

        1. Martin

          Re: Let's look at this...

          "When did physics ever Trump economics?"

          It never did.

          But I can remember it being theoretically impossible to get more that 9600 Baud down a phone line on a normal modem.

          I remember when it wasn't theoretically possible to get more than 10M Ethernet.

          Wireless in the home was always going to be very slow, and only useful for emails - if you wanted to stream music, you'd have to use wired access - and films would work, but only short ones in low definition.

          True Internet over a mobile phone was never going to work at a reasonable speed....

          I know that your theoretical calculations say this'll never work. But I wouldn't mind betting that you're wrong.

      2. noem

        Re: Let's look at this...

        There are quite a few 8-bit AVRs (ATtiny series, etc) that are more than happy on a couple uWatts and I even heard that some ARM variants could too. Paired with a super capacitor or small coin cell an embedded sensors life could well out last your lifetime. However, I think it's important to note that this seems to be more of a demonstration for metamaterials in manufacturing than an actual attempt. As devices become more efficient and power harvesting techniques improve, wireless power could become viable in the consumer arena.

    4. This post has been deleted by its author

  3. Anonymous Coward
    Anonymous Coward

    Welcome to 2009

    "The difference with Nokia's prototype is that instead of harvesting tiny amounts of power (a few microwatts) from dedicated transmitters, Nokia claims it is able to scavenge relatively large amounts of power — around a thousand times as much — from signals coming from miles away. Individually the energy available in each of these signals is miniscule. But by harvesting radiowaves across a wide range of frequencies it all adds up, said Rouvala."

    Nokia used more than just microwave bands to harvest the energy from.

  4. Anonymous Coward
    Anonymous Coward

    I'd prefer thermal conversion. Why can't I charge my phone by leaving it in my pocket or under my black Labrador :)

    1. Anonymous Coward
      Anonymous Coward

      "Why can't I charge my phone by leaving it ....under my black Labrador?"

      Get the dog to lie on a cold floor, and stick a peltier generator under the dog. The dog will be emitting a guessed 20 watts, the temperature difference is going to be about 10C, the efficiency of the peltier chip will be diddly squat, so the output will be diddly squat squared. Still better than this nonsense of harvesting radio waves...

  5. Stephen 2

    Saying it generates 7 volts means nothing without telling us the amp/ma.

    volts x amp = watts

    7 volts with 1 amp = 7 watts. Nice!

    7 volts with 0.1amp = 0.7 watts :/

    1. Rampant Spaniel

      Thank you, I was just thinking the same thing!

    2. JeffyPoooh

      It's more likely 7 volts into a DMM with 10Mohm input Z. On the order of five MICRO-watts.

      This article is built on a foundation of ignorance.

      I'm surprised it doesn't say "7 volts per day", as seems to traditional for Arts grads.

  6. btrower

    Just adding my vote

    This article, as written, does not really make much sense. I presume they are just reporting the news as given to them, but as has been hinted by others here:

    1) The claim is unremarkable. EM Induction is 19th Century technology. This is the 21st Century.

    2) The amount of power has to be very small. This is just a straight up arithmetic exercise.

    People here gave best case scenarios. In practice you won't have it that good.

    You can, as we have for many years, tease out useful energy to, for instance pass on a radio signal. However, this is hardly ground-breaking. If there is something truly special here, it is not clear from the Reg Article and the claims are too modest to bother looking it up.

  7. Anonymous Coward
    Anonymous Coward


    <pedant>Just wondering why the same quote was mentioned twice.</pedant> ("Until now, a lot of work with metamaterials[...]")

  8. Tromos

    Don't cancel that new nuclear power station just yet.

    Back in the 1970s I remember visiting a friend who lived about half a mile from the Crystal Palace transmitting aerial. Just holding apart the test leads on an AVO meter set to a high AC voltage range got a reading around 80 volts. Those old moving coil AVOs required a current of 50 microamps to achieve full scale deflection, so there was definitely a milliwatt or two of power being picked up. With modern voltmeters and their huge input impedances it requires, at most, nanoamps to get a reading. There is no useful power to be got from harvesting normal Wi-Fi signals without a collector the size of a dustbin lid and so close to the transmitting aerial that it would blot out a goodly chunk of the coverage area. You'd be better off with a coil of wire around your hat generating power as you move through earth's magnetic field.

    1. Frumious Bandersnatch

      Re: Don't cancel that new nuclear power station just yet.

      You'd be better off with a coil of wire around your hat generating power as you move through earth's magnetic field.

      Yoink. I'm off to the patent office to register my new "power-generating cycle helmet". Ta muchly.

    2. btrower

      Re: Don't cancel that new nuclear power station just yet.


      Re: "You'd be better off with a coil of wire around your hat"

      Amateur. I have had one directly on my head for years. Get with the program!

    3. RobHib

      @Tromos -- AVOs and their response to RF power.

      Are you sure it was a genuine AVO meter?

      The traditional AVO Meter uses a copper oxide rectifier as the AC rectifying diode NOT a point-contact germanium or junction silicon diode etc. Copper oxide rectifiers have a very bad frequency response which makes them cut off in the upper audio frequencies, perhaps 100kHz or so.

      This fact is very useful (and well known to techies). If one had to measure [any other] voltage near VHF transmitter antennae etc. (i.e.: when one's up the mast near the antenna etc.), one used an AVO as it would NOT respond to the transmitted power. (I've used an AVO specifically for this purpose myself on many occasions.)

      Most other multimeters use a point-contact diode which DO respond very well up to microwave frequencies and thus it would likely have indicated as you've stated.

      Moreover, as the copper oxide rectifier as used in the AVO (Model 8s and earlier) was a hard-to-get replacement, when it was damaged some slack techies just replaced it with point-contact types (OA91 etc.) which worked but the calibration was somewhat buggered.

      I'd suggest either the AVO wasn't a genuine AVO brand (and you're using the term generically) or it had its diode changed for the wrong type by improper service.

      The AVO Meter was pretty unique in using a copper oxide rectifier (at least in multimeters made since WWII). I still own three of them which I still use when working in RF environments, they're invaluable (it's a shame they're still not made).

  9. Anonymous Coward
    Anonymous Coward

    Nice idea, but...

    Under UK law, unless the power was actually being used to run a radio whose purpose was to receive that particular signal anyway, you may be committing theft.

    1. DavCrav

      Re: Nice idea, but...

      And if you use this for TV signals, you might need a TV licence as well.

  10. Anonymous Coward
    Anonymous Coward

    Re. Nice idea but...

    Heard of someone using a homemade (miles of wire) tuned antenna to power lighting from the field of a nearby multikilowatt MW transmitter.

    Turns out that they only found him when investigating a weird directional dead spot in their transmission pattern.

    In theory this would be a great way to send power to an electric bicycle or solar garden lantern in order to keep the batteries charged, 13.56 MHz is the band used for most ISM equipment and the kit to send power is trivial to build.

  11. Anonymous Coward
    Anonymous Coward

    The other stray RF band

    This is less powerful than the harvesting of an RF band that most people have abandoned for gadgets - light. A pocket device covered with practically priced solar cells would produce about 2W in direct sunlight, less than 0.5W indoors near a window during the day, or less than 0.001W with efficient artificial lighting. That's perfect for ultra-low power devices but it's no help at all for power-hundry devices that must be plugged in for a charge every night.

    1. Anonymous Coward
      Anonymous Coward

      Re: The other stray RF band

      Surely a pocket device covered in solar cells wouldn't be much use at all, unless you have some spectacularly luminous testicles.

  12. Anonymous Coward
    Anonymous Coward

    Watch out there's a lawyer about

    If a council can prosecute someone with solar panels on their roof slurping the light emitted from streetlights there can't be a lawyer far away from threatening to sue anyone who has the temerity to use one of these.

    The council completely ignores the fact that the light that hits the solar panels would be totally wasted is beyond them. It is their light (ignoring the fact the we pay for it) and want a cut from anyone not using it for its intended purpose.

    1. Frumious Bandersnatch

      Re: Watch out there's a lawyer about

      prosecute someone with solar panels on their roof

      Just goes to show what a scam electricity generation is. You know those electrons the electric companies supply on one wire? They suck them back to their plant on the other wire! (then sell them back to you again--the cheek!)

      1. JamesTQuirk

        Re: Watch out there's a lawyer about

        So playing "light sabres" with fluro tubes under High Power Lines, would incur a penatly, theft by induction !!!!!

        As Tesla demostrated, I believe we could send energy "wirelessly" through air, but do we really want to drown in a sea of "RF" energy emmissions ?

      2. Anonymous Coward
        Anonymous Coward

        Re: Watch out there's a lawyer about

        Maybe as electricity is provided as alternating current, they only provide it to us for half the time. The other half of the time they are pulling it back again. (lol)

        So we aren't getting to keep any of the electrons they provide, just the electrons' energy? Or something?

        I am sorely in need of 10 minutes on WiKi to refresh my O level physics knowledge :)

    2. Adrian 4

      Re: Watch out there's a lawyer about

      [citation needed]

    3. Martin Budden Silver badge

      Re: Watch out there's a lawyer about

      My bullshit detector is twitching. Solar panels won't get enough energy from street lights to switch the inverter from "sleep mode" to "be useful mode" (inverters need a certain minimum power from the solar panel for them to decide it's worth their effort doing anything: this is because the act of 'inverting' uses some power).

  13. RobHib

    So what!

    The team developed a "metamaterial", an artificially assembled material not found in nature and built for specific purpose

    Yawn, Yagi and Uda's 'Yagi' antenna, invented in 1926, essentially acts as a metamaterial because its reflector's phase changes the propagation direction from the forward to backward (sends the wave back from where it came). So there's little new here, the principle is well known.

    Real innovation with metamaterials will come when a metamaterial is able to replace existing materials such as the glass in lenses (the limitations of which are a real problem and require new thinking and a solution), especially if we're to make cameras significantly smaller).

    But there's little in this area on the horizon at this stage. Alas, the best metamaterial solutions can achieve are some benefits at microwave frequencies.

  14. Chris G

    I think the answer is in the College name

    Duke University's 'Pratt' School of Engineering.

    As soon as I began to read the article I thought ; inverse square law, 7v at what amperage etc.

    Unless they have a free method to amplify this energy that the rest of physics doesn't know about there is little to crow about.

    Antennas have been converting radio waves to low voltage and even lower current since at least Heinrich Hertz' experiments with spark transmitters.

    Maybe the article is missing something!

  15. Ben 54

    Next news article...

    And a patent troll who don't even know what this is, never invented anything useful in their lives, and have no scientific knowledge nor a team swoops in with a patent claim.

  16. Ted 3

    Is the definition of "metamaterial" a bit wonky?

    The article defined a "metamaterial" as "an artificially assembled material not found in nature and built for specific purpose".

    Is this precise enough to be correct? I mean, simple nylon is artificially assembled (via a chemical reaction), is a material not found in nature (is a "synthetic" fibre) and is built for a specific purpose (if not stockings, then as a strong tensile bearing material). By the El Reg definition, nylon would be a metamaterial. The same could be said for kevlar or carbon fibre. I am sure the metamaterial boffins would like to think they are working on is a bit more "special" than just plain old nylon.

    I suppose my question (out of ignorance) is when does a material stop being a simple synthetic material and start becoming a super-sexy thing worthy of the designation "metamaterial"?

  17. Sandpit

    Legal defense

    Did they ask permission from home owners before transmitting energy across their airspace?

  18. Neoc

    Missing numbers

    I want to see how much power is consumed by the WiFi transmitter before and after these receptors are introduced. And for kicks, both while nascent, quiescent and during heavy data transmission.

  19. AgeingBabyBoomer

    What about mobile phones?

    Plenty of spare radiation floating around form those towers.

    I'll wire up me tinfoil hat..

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