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%."

Comments

Units!

[olsmeister]

7.3V of energy? USB provides 5V of power? Arggh. I think my head just asploded.

Re:Units!

[Penguinisto]

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.)

Resistor

[Anonymous Coward]

Hey, I can get -174 dBm/Hz from a 50 Ohm resistor too. Free energy!

For $4, you can read the paper

[Animats]

Here's the actual paper's paywall. All the paper claims is that "A maximum of 36.8% of the incident power from a 900âMHz signal is experimentally rectified by an array of metamaterial unit cells." So they built a rectenna with a waveguide.

Rectennas have been around for decades, and 82% efficiency (DC watts out / microwave watts into antenna) has been achieved. So 37% is nothing to be excited about.

If you hook up two long wires or plates to a diode, any RF in the vicinity will produce some DC across the diode. This is the principle behind "crystal radios". The problem is that you need big antennas to get much power from ambient RF.

Not enough energy, missing the point!

[foxalopex]

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?