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%."
Re:Too bad (Score:5, Informative)
Ugh. And the unit we want is missing...
Look at Watt they make you give... (Clive Owen)
Re:Resistor (Score:5, Informative)
Your joke is too subtle without a reference.
http://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise [wikipedia.org]
For $4, you can read the paper (Score:5, Informative)
Here's the actual paper's paywall. [aip.org] 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 [wikipedia.org] have been around for decades, and 82% efficiency [ieee.org] (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.
Re:Too bad (Score:4, Informative)
I'm resisting the urge to cry. TFA on the Duke University web site makes the same mistake.
The whole article smells of bullshit. It's easy to generate 7V from radio waves, I have done it myself, but the amount of current is tiny. I could run a small LCD clock or ultra low power microcontroller, but never charge a phone from it. Even an old Nokia dumbphone needs far more power than this or the small solar panel it is compared to can provide. We are in battery backed solar calculator territory here.
It's s shame because there are genuine uses for this kind of technology. Sensors that need to operate in the dark but are very low power, for example. No-one will be charging their smartphone this way unless they get in the order of 100,000x more efficient though.