Duke Univ. Device Converts Stray Wireless Energy Into Electricity For Charging 216
Lucas123 writes "Engineers at Duke University say they've constructed a device that can collect stray wireless signals and convert them into energy to charge batteries in devices such as cell phones and tablets. The WiFi collection device, made of cheap copper coils and fiberglass, can even aggregate energy from satellite signals and sound waves (abstract). The researchers created a series of five fiberglass and copper energy conductors on a circuit board, which was able to convert microwaves into 7.3V of electrical energy. By comparison, Universal Serial Bus (USB) chargers for small electronic devices provide about 5V of power. The device, the researchers say, is as efficient as solar cells with an energy conversion rate of 37%."
Cell phones? (Score:5, Interesting)
The FCC limits wireless access point RF power to 1 watt.
From the image, I would guess that the metal thingy is 2 feet square, or about 1/3 square meter. I can't tell from the image whether the capture aperture is the profile or the end of the wedge, but let's give it the benefit of the doubt.
Standing 10 meters from a WAP is a sphere with area 4*M_PI*R^2 = 1256 m^2. A 1/3 meter capture aperture would eclipse 0.3/1256 of this, for about 240 microwatts. At 37% efficiency, that's about 80 microwatts. (Am I doing this right?)
Maybe possibly this could power micropower sensors (note: with a 2-foot square antenna on each one).
But a cell phone?
Re:Units! (Score:5, Interesting)
Way back in the 70's (early 80s?) I recall a guy who wrapped his whole house in copper wire making large coils to tap the energy from the overhead power cables. He powered his whole house off this which was a mistake. The authorities charged him with theft.
Re:Units! (Score:5, Interesting)
Nope, it didn't, since induction was the key, not resonance. ;)
IIRC, the rig involved the peculiar way the tension lines ran in parallel to his roof peak-line. This allowed him to wrap a shitload of long, large wooden dowels with copper wire, then hang them in his attic, orienting them all parallel to the overhead lines. The results would be captured, cleaned-up, and then presented to his home circuitry as household power (120VAC, 60Hz, etc).
Pretty simple, really - but yeah, I remember his being charged with theft as well (though technically, I think nowadays that wouldn't fly as easily, since there have since been plenty of legal precedents made that allow you to make free use of any and all magnetic and radio energy that falls on your property, even if you get it through induction.)
Not enough energy, missing the point! (Score:5, Interesting)
This doesn't even pass the common sense logic rules if you understand physics. The issue is there's not much energy in these types of radio waves. A cellphone transmits a maximum of around 1 watts, a wifi router 50 milliwatts if you're lucky. By the time the radio waves have reached you their effective power has already dissipated by the square of the distance. Sure you might get a voltage potential that's in the 7 volt range but how's that useful if there's next to no current to do anything. Short of standing under a high voltage power line or next to some high power transmitter which probably wouldn't be safe for your health, this isn't going to work.
People also misunderstand Tesla's work. Tesla's work wasn't that you could just pop up an antenna and get free power. His plans involved putting up a massive transmission tower that would dump power into the air at an efficient frequency. A coil and antenna could then be used to pick up this power wirelessly. Great idea but the issue then is how exactly would you charge for this power when anyone with some know how could build a receiver to grab the "free" power?
Re:For $4, you can read the paper (Score:5, Interesting)
Actually, the difference here is that they built a rectenna out of metamaterials, specifically a split-ring resonator (SRR) design. I presume their point here is that they came up with a compact rectenna design that can work fairly well at 900 MHz. The paper you referenced with the 82% efficiency used a dipole antenna for 5.8 GHz. The wavelength at 5.8 GHz is something like 50 mm, and they used a 1/2 wave dipole antenna (their length was around 25 mm). The wavelength at 900 MHz is 333 mm, but their SRR design was only 40 mm on a side (a 1/2 wave dipole would have to be 150 mm or so).
I don't think they were making any claims of new physics here, but probably pointing out a design that would be fairly compact and leverage all the 900 MHz EMI flying around. For what its worth, their max efficiency occurred for a resistive load of 70 Ohms, which is a reasonable load for something that you want to power with an energy harvester.
Re:Units! (Score:4, Interesting)
Exactly. For the win? If he had a few solar panels parked on his roof (even if they were never hooked up), it would easily explain why his usage patterns were screwy at times, explain a battery bank, and even (in states with solar tariff credits) allow him to sell the power company their own juice back.
Attenuating waves and generating harmonics. (Score:5, Interesting)
This device will also interfere with the radio signals. It will both attenuate them and create harmonics due to the rectifiers.
"Raising ground resistance" by having radio-energy-utilizing devices pull power from the air is a non-trivial issue.
Example: A former colleague had, previously, been a plant manager for a factory in a small African country. The plant was in the country's capital, home to their "voice of the fearless leader" high-powered radio station.
One day, while touring the plant, he found a collection of burned-out fluorescent tubes, and had them hauled away. Shortly after he was contacted by his maintenance head, who asked him not to do it again. It seems there was a black market in burned out fluorescent tubes.
The radio station was so strong that, if you put three feet of wire on each end of a burned-out tube it would light up quite nicely from the radio power. A lot of people couldn't afford electricity and light fixtures. But a burned out tube and six feet of wire was readily available. So much of the town's houses were illuminated this way.
So many were, in fact, that the radio signal would no longer reach the edges of the country. So Fearless Leader would send his troops through town when the attenuation got to be a problem, and they'd confiscate and smash the tubes of all the improvised radio-powered lights they found. After each such raid, the people would be down at the plant to buy more "dead" tubes, creating a profitable side-business for the maintenance guy.