back to article Boffins improve on tech that extracts DC power from ambient Wi-Fi

A research team from the National University of Singapore and Japan’s Tohoku University say they have improved on the use of spin-torque oscillators (STO) to harvest and convert Wi-Fi signals into energy through a series configuration that lit up a battery-less LED. "There is a lot of ambient Wi-Fi wasted, the question is can …

  1. Anonymous Coward
    Thumb Up

    Proof of concept

    First, let me applaud Professor Yang for pointing out that there's a long way to go before this bears fruit, if it ever does.

    I do wonder if it could be improved with a plugged in device broadcasting at a specific frequency to enable over-the-air wireless charging.

    As far as charging wearables and implants, I would think that extracting energy from body heat shows more promise.

    Personally, I can't wait for it to be able to replace my damn smoke detector battery .

    1. Anonymous Coward
      Anonymous Coward

      Re: Proof of concept

      There is nothing here improving on the state of the art in terms of wireless power delivery efficiency in general terms, it just capitalizes on RF that is already nearly universal. You wouldn't want to use the same frequencies as they aren't optimal for power deliver(and would WRECK everyone nears WiFi, and break power level caps on 2.4 transmissions).

      Applications that might be able to leverage the (VERY) limited amount of power are things like alarm contacts for windows and doors. IMHO The smoke detector should be built into the AP sending the signal not a standalone device, and powered off POE like most enterprise APs, which can already do, but for no fathomable reason, don't.

      But all of us wish to avoid the dreaded 3am beep. Believe me :)

      1. the spectacularly refined chap

        Re: Proof of concept

        You wouldn't want to use the same frequencies as they aren't optimal for power deliver(and would WRECK everyone nears WiFi, and break power level caps on 2.4 transmissions).

        That is exactly what the ISM bands are reserved for: delivery of RF energy. Among the applications I've heard of are microwave welding at power levels a thousand times those of an access point. Unlicensed use of the bands for communication is permitted provided they can tolerate interference from other uses. That's a secondary permission though, so if you have someone doing the welding and the whole neighbourhood's WiFi goes down as a result then tough, the welder has first dibs.

        1. Red Ted

          Re: Proof of concept

          The ISM bands at 915MHz and 2.4GHz are there because of the use of those frequencies in microwave ovens. The bands were left clear because of concerns about interference between the ovens and any radio technology using the band. However the quality of the shielding is sufficient that it turns out we can use those bands fine. Indeed my WiFi router sits on top of my microwave and works just fine regardless of whether the oven is on or not.

          However, the 433MHz and 868MHz bands are there as an internationally agreed bands for license free low power radio devices.

    2. Flocke Kroes Silver badge

      Re: Body heat power

      To get energy from heat you need a temperature difference. The inside of a comfortable human is 310K and the outside is about 295K. The maximum possible conversion efficiency is (1-T(cold)/T(hot)) or about 5% for a device with good thermal connections to the inside and outside of a human. The average thermal power available from an entire adult is between 50W and 100W so the upper limit of available power is between 2.5W and 5W. To get all of that someone would have to wear a space suit covered in heat sinks.

      Something a bit more likely so sell is a watch strap that covers only 1/1000 of the available surface area (about 2m²). That gets you at most a couple of milliWatts. Enough for a digital watch (not a smart watch). Anything inside the body would need a good thermal connection to the outside for a supply of cold. For something tiny you would be better off metabolising blood sugar and oxygen to CO₂ and water like the rest of the body. Anything bigger, go for rechargeable batteries and wireless charging.

      1. Spherical Cow

        Re: Body heat power

        > The inside of a comfortable human is 310K and the outside is about 295K

        In the country where I live, for a couple of months each year the outside can be as high as 320K which is higher than the inside (thank goodness for sweat).

        I suppose you could have a watch strap which works with thermal gradient in either direction, but what happens on days when inside and outside are the same temperature?

    3. katrinab Silver badge

      Re: Proof of concept

      A light bulb and some solar panels? I saw a pocket calculator powered that way about 30 years ago.

      1. TimMaher Silver badge

        Re: Proof of concept

        Casio HS-5D, in a kitchen cupboard, as it is used for calculating ingredient weight/ volume.

        Been there for over two decades.

        Works fine.

        1. The Dark Side Of The Mind (TDSOTM)

          Re: Proof of concept

          Oh, Casio... The devices they make are so underrated... I love their reliability and (often apparent) simplicity.

        2. Boothy

          Re: Proof of concept

          I've still got a solar powered Casio fx-451 scientific, which I got for college back in the mid 80s.

          Still works fine, (I just checked, it lives in the drawer of my 'work' desk at home). Although it rarely gets used these days. My only concern is the folding sleeve, which is quite stiff these days.

    4. John Brown (no body) Silver badge

      Re: Proof of concept

      "I do wonder if it could be improved with a plugged in device broadcasting at a specific frequency to enable over-the-air wireless charging."

      I think that defeats the object of this particular exercise. They are looking to take advantage of existing "waste" RF energy, not build yet another power delivery mechanism. They want to see if it's possible to improve on the concept of a crystal radio set to do more than just receive AM radio signals :-)

    5. Psmo
      Thumb Down

      Re: Proof of concept

      Safety equipment is one thing that I hope will never use this tech.

      Spotty home WiFi should not cost lives.

      Besides which, the power for the WiFi router will be one of the first things to go in an electrical fire, so unless the WiFi slowly recharges an internal battery it's going to cut out when you need it.

      And you'll still need to change any internal battery when it reaches EOL.

    6. Persona Silver badge

      Re: Proof of concept

      Yes smoke detectors should be ok. They can beep irritatingly if they aren't getting sufficient power.

      I would also think about using it for smart light switches. Currently these need a little battery which is a pain to replace though some are piezo electric powered. The Kinetic ones employ a clever little magnet and coil solution to generate their working power. Using WiFi power should be an improvement on all three of these methods in the domestic situations where they are used.

      1. John Brown (no body) Silver badge

        Re: Proof of concept

        Something that crossed my mind many years ago was placing a coil around the live feed to light an LED. I never tried it, but then it does pre-suppose a constant live pass-through ring, which it seems most light switches don't have anyway.

        1. Red Ted

          Re: Proof of concept

          Some years ago a farmer was charged with theft of electricity. The BBC noticed a significant radio shadow from one of its transmitters and discovered a cow shed quite close to the transmitter where the farmer had placed large coils of wire on the wall and connected some bulbs across the ends!

  2. Anonymous Coward
    Anonymous Coward

    Neighbor WiFi

    Good for something

    1. Yes Me Silver badge

      Re: Neighbor WiFi

      If my neighbours recuperate energy from my WiFi signal, I certainly expect them to pay me for it. There should be no free lunch here. Also, since they abstract energy, they must also make the S/N ratio worse. I think that's a tort, on top of the stolen energy.

      Lawyers should pay attention...

      1. Wellyboot Silver badge

        Re: Neighbor WiFi

        Stop those freeloading neighbours by covering your house in tinfoil!

      2. electricmonk

        Re: Neighbor WiFi

        Said neighbours might reasonably respond that they never asked you to flood their house with your WiFi radiation. As you have no legal reason to extend your WiFi coverage into their household, you are effectively littering their property with your discarded energy. If you don't want them to make use of it, you should take steps to restrict your unlicensed signal transmissions to within your own property boundary. What they are doing costs you nothing - it won't increase the power consumption of your router.

        This is, of course, different from them piggybacking into your WiFi service to use your Internet connection without your permission - they would be in trouble then, but not because of the energy used.

      3. Nifty Silver badge

        Re: Neighbor WiFi

        Why the joke alert though?

  3. Neil Barnes Silver badge

    Hang on, what about the signal the other side of the harvester?

    If you're extracting energy from the radio wave, surely you're leaving a big hole in the reception behind the harvester? And indeed, in these days of routers steering the WiFi beam pattern to a friendly receiver, is the harvester going to see anything at all?

    1. werdsmith Silver badge

      Re: Hang on, what about the signal the other side of the harvester?

      Receiving data by a WiFi signal is extracting energy from a radio wave.

      To cut down on wasted energy, the base station should be made directional rather than isometric with a method to determine direction and boost gain in that direction, and shouldn’t be broadcasting ssid until a key signal is received, then it should operate at the power it needs to maintain data rate.

      Maybe they do that already. Dunno.

    2. This post has been deleted by its author

    3. hedge

      Re: Hang on, what about the signal the other side of the harvester?

      Not a problem if the yagi antenna is rotating at least twice the speed of the wifi frequency.

    4. The Oncoming Scorn Silver badge

      Re: Hang on, what about the signal the other side of the harvester?

      I remember someone living just down the road from a transmitter, extracting energy from TV\Radio signals to light his house or augment the powering of his household appliances.

      He was eventually caught by investigating engineers looking into the dead zone of TV reception following complaints (He may have got away with it for quite some years, until a new housing development & the owners were wondering why they had no reception despite seeing the transmitter from their back gardens).

      I think he was charged (Pun intended) with theft, not sure what the outcome was.

      Story or my memory may be partially apocryphal, or at least wildly inaccurate, having heard the story from the Daily\Sunday Express & restated one of my former lecturers in C&G 224 on TV\Radio theory in the 70's/80's

      1. Neil Barnes Silver badge

        Re: Hang on, what about the signal the other side of the harvester?

        The story was used as an example during my technical training at the BBC back in the late seventies. Sadly after forty years I recall no details... but it was the inspiration for my original comment.

      2. Nifty Silver badge

        Re: Hang on, what about the signal the other side of the harvester?

        2 apocryphal stories here

        - Farmer in Iowa and

        - Engineer living at a military base in England

        Pure urban legend IMO.

      3. Paul Hovnanian Silver badge

        Re: Hang on, what about the signal the other side of the harvester?

        "I think he was charged (Pun intended) with theft, not sure what the outcome was."

        Actual theft would depend on whether the energy harvester was operating in the near field or far field[1]. near field involves a receiver close enough to the transmitter to result in parasitic coupling between the two and additional energy being drawn from the transmitter final stage. Near field would be stealing. Far field is intercepting r.f. energy with no effect on the transmitter. The same energy would have gone out anyway and been absorbed (and converted to heat) by some other object[b]. Of course, if it can be shown that the energy might have been delivered to a customer but was intercepted, it could be tortious interference.

        [1] Assuming a jury could understand the difference.

        [b] Aesop's(?) fable of the man caught 'stealing' the scent of baking bread would be a useful analogy.

  4. Conundrum1885

    Re. Harvester

    Yes, this might indeed be a problem.

    The fix would be just running the DC-DC when the capacitor actually gets low.

    A smoke alarm battery powered by WiFi would be useful, however the reason for using an alkaline is that when actually sounding the peak

    current can get surprisingly high (amps)

    Incidentally does anyone know how those WiFi boosting bulbs work?

    It looks like they receive one channel and then retransmit on another, but how does it compensate for latency and for that matter multipath


    Would make more sense to receive on 2.4 and transmit at 5.2 so devices can lock on to the "better" signal thus freeing up the lower bands.

  5. Wellyboot Silver badge


    >>>the resultant green energy could eventually eliminate the need for batteries<<<

    Only if the original source is green, this is recovering a small % of the energy originally transmitted.

    1. Anonymous Coward
      Anonymous Coward

      Re: Green?


      Greta might hear you.

    2. Roland6 Silver badge

      Re: Green?

      >Only if the original source is green

      Does harvesting CMBR satisfy the requirement...

      Now whoever achieves that would have something worth shouting about!

  6. Chris G

    Medical lmplants

    Since 2017 there has been work ongoing developing a bio-supercapacitor at UCLA, using the charge from ions in bodily fluids.

    That makes far more sense than relying on a body spending enough time in range of a wifi source.

    Think of instances like the Texas cold spell and the consequent failure of all services, using energy sourced from the host body is a more obvious route.

    1. Anonymous Coward

      Re: Medical lmplants

      Chris G> using the charge from ions in bodily fluids.

      Sound like a load of wank.

      1. Anonymous Coward
        Anonymous Coward

        Re: Medical lmplants

        "Sound like a load of wank."

        Is it possible to overcharge a battery? Just asking.

      2. Trigonoceps occipitalis

        Re: Medical lmplants

        Great, a way to prevent my battery running out at "interesting" times during my pornography "research".

        Of course, relying on bodily ions may present some timing issues.

    2. TRT Silver badge

      Re: Medical lmplants

      We don't know who struck first - us, or them. But we know it was us that scorched the sky. At the time they were dependent on solar power and it was believed that they would be unable to survive without an energy source as abundant as the sun. Throughout human history, we have been dependent on machines to survive. Fate, it seems, is not without a sense of irony.

      The human generates more bio-electricity than 120-volt battery and over 25,000 BTUs of body heat. Combined with a form of fusion, the machines have found all the energy they would ever need. There are fields...endless fields, where human beings are no longer born. We are grown. For the longest time, I wouldn't believe it...and then I saw the fields with my own eyes. Watch them liquefy the dead, so they could be fed, intravenously, to the living.

    3. skeptical i

      Re: Medical lmplants

      re: "using the charge from ions in bodily fluids"

      So THAT's what General Ripper's obsession with precious bodily fluids was about.

  7. Hotears

    We can put a quick upper bound on this, as it is being sold to the press. Everyone here probably have wifi analyzer or something similar on their phone:

    Right now, I have -55dBm from my closest (8m) access point, and -70 dBm from next closest. -70dBm is 30x weaker in linear power than -55dBm, so we can ignore that, and the neigbours.

    0dBm is one milliwatt coming from the antenna in the phone. -30dBm is one microwatt. -50dBm is 10 nanowatt. -55 is 3 nanowatt.

    As a sanity check we can take 100mW EIRP and divide by the area of the sphere, we get an upper bound of 12nW/cm2 with no walls.

    Ahem. And that assumes 100% tx duty cycle on the access point. Reality is 5%.

    for the led demo, they'd need in the order of 100uW after losses, 200uW into the antenna, for a 10x10cm antenna we need 20mW/m^2, we are 0.7m from a continuous 100mW transmitter.

    (I am not going to try to convert to sheep in vacuum * norris )

  8. Anonymous Coward
    Anonymous Coward

    Tesla's dream...

    ...finally becomes a low-voltage reality.

    Wonder what this does to signal quality of people actually using the Wifi.

    1. Neoc

      Re: Tesla's dream...

      Ya beat me to it.

  9. manford

    Crystal radio?

    This reminds me of the old crystal radios that grabbed enough power from AM radio transmitters to power a earphone with the broadcast. WiFi is nowhere near as powerful as a 50000 kilowatt transmitter several miles away. As a kid, it struck me that if you could link multiple circuits together in phase, you could likely power a light. Unfortunately, that was long before efficient LEDs came on the scene.

    1. Hull

      Re: Crystal radio?

      50 MW? That would be very impressive.

      1. Mnot Paranoid

        Re: Crystal radio?

        IIRC - Radio Luxembourg cooked the ionosphere when they went above 2MW, resulting in far worse reception, and DUGA-3 (The Woodpecker OTH radar) went up to 10MW, jamming the entire planet in several HF bands - in an age when people actually used Shortwave radio.

  10. Anonymous Coward
    Anonymous Coward

    In the old days . .

    More years ago than I should be able to remember, there was a case of someone who lived near a TV transmitter, who installed an array of large aerials in his loft all pointed at said transmitter. He "harvested" sufficient power to run all of the lights in his house - and this was with incandescent lamps!. Unforntunately for him the broadcaster did some signal strength measuments and found a hole behind his house. It ended up costing him quite a lot for his "free" electricity!

  11. David Pearce

    Old wives tales

    There is no way that an array of antennas more than a few wavelengths from a transmitter can extract much power. At long wave you might stand a chance of a usable power coupling, enough to be noticed. At UHF or above you will always be in the Far Field.

    The old Crystal Palace TV transmitter used to radiate at 1 MW ERP, which at 100 m works out as 8 W/m2

    This is 55 V/m, well above safety limits.

    You would need several antennas to run a single light bulb

  12. Disgusted Of Tunbridge Wells Silver badge

    I don't understand how he would have been causing a dead spot any more than any grounded metal object would.

  13. Ace2 Silver badge

    “As a scientist…”

    “We’re getting lots of inquiries from different people, but as a scientist I don’t want to overcommit.“

    Mad respect for that. If only the battery scientists respected the same principle!

  14. ITMA Silver badge

    BT and their WiFi

    Who remembers BT's ad banging on about "UK's most powerful WiFi"?

    Ignoring the fact it was banned as "misleading" (c'mon this was BT, what do you expect?), maybe one should encourage all of one's neighbours to switch to BT broadband, commence large scale harvesting "tests" and tell your electrickery company to take a hike and take their (now) useless "dumb" leccy meter with them as you no longer need their very expensive "juice"

    1. Roland6 Silver badge

      Re: BT and their WiFi

      >Ignoring the fact it was banned as "misleading"

      I'm a little surprised the "Pixelated Paula" advert is still running...

  15. You aint sin me, roit

    Tin foil question

    Does this mean I need to invest in higher grade tin foil? Or would that increase the spin torque resulting in brain melting overheating? Would lead be a better alternative?

    Asking for a friend...

  16. Cragganmore

    Just a shame the antenna is as big as a tennis court...

    1. ITMA Silver badge

      Ah you've used some of BT's "discs"? LOL


    ref Bro GO's + others contributions:

    And that, Boys & Girls, is why the Radio Amateur Licence gives you permission to receive radio transmissions of limited (receipt) wattage for the purposes of operating a loudspeaker / headphones / similar announcing device up to a sufficient wattage* for the interpretation by said devices of such transmission signals . . . blah blah blah - - - - - - contd page 95 etc.

    (* Don't ask watt (yo-ho-ho) it is; I've forgot, and I can't be bottomed to look it up - Consider it your initiative test for cold Covid lockdown afternoons)

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