Duke Univ. Device Converts Stray Wireless Energy Into Electricity For Charging

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%."

microwaves at what field strength?

[swschrad]

news flash: any antenna provides voltage. usually in the microvolt range. to get enough voltage like they did, say, enough to blow a FET in the front end of a receiver at basically no current, you have to put the antenna in one hell of a strong RF field. a field strong enough to produce enough current to charge batteries or operate CMOS circuits is a field too strong to stay in, according to FCC emission guidelines. so I see this as a project for a grade, and not a "discovery."

7.3V? Psh!

[Schrockwell]

I can build up a couple kilovolts by scuffing my shoes on the carpet.

Also, sure it might be 37% efficient, but do you realize how SMALL the density of RF energy is? The Friis transmission equation [wikipedia.org] gives you some idea: it decreases by the square of the distance away from the source, due to that power spreading out in a sphere. When you start off with only a couple mW of power and an omnidirectional antenna, there isn't much power left to harvest when these tiny receiving "metamaterial" antennas are even just a few feet from an access point.

Re:Resistor

[Anonymous Coward]

Jokes like that are intended for people who get the joke and shouldn't need explanation.
I can tell you got it though :)

Re:radiation too?

[Entropius]

Nuclear radiation doesn't work that way. We have gizmos that turn nuclear radiation into power; they're called radiothermal generators, and work by absorbing the radiation with some material that heats up, then capturing the thermal energy as it flows across a Peltier junction. We power spacecraft with 'em.

But this doesn't make the plutonium less radioactive any faster. Those plutonium nuclei are still going to take their sweet time decaying.

Nuclear power plants take advantage of this, too; heat in the reactor core is heat in the reactor core, and it doesn't matter whether it comes from fission directly or from secondary decay of fission products. But we can't do anything magic to fission products to make them decay into something stable any faster; eventually they get far enough down the decay chain to something long-lived enough that it's not worth trying to harvest the heat they release any more.

Re:Units!

[Cloudy Wheat Beer]

He was not discovered for that reason. A device known as a TDR ( Time Domain Refelectrometer ) is used by the industry to determine where, and what kind of faults, lie on the transmission line. It sends a pulse down the line, and then analyzes the reflection that comes back to determine the location ( distance down the line) and fault type ( open or short circuit ). This guy's unauthorized 'tap' on the transmission line would show up on the results. The lines company could then easily find out where the 'extra' transformer is located, and the guy is busted. Ive heard of farmers using overhead transmission lines to power their electric fences, and were discovered by this method.