
Electricity From WiFi Signals - ajaimk
http://www.ohgizmo.com/2010/01/09/ces2010-rca-airnergy-charger-harvests-electricity-from-wifi/
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alain94040
On the surface, it looks like a really cheap scam.

The demo is so easy to cook: the device already has a battery, and you can
load a blackberry when you plug it to it? Sure. How do I know the energy is
coming from "ambient wifi signals"?

I'm also suspicious of basic physics. Basic specs for a blackberry charger: 5V
at 800mA.

In theory, I could place 100 of those devices easily in a room, and they would
all be able to generate such power. I'll be nice and assume that I can't stack
them in 3D because they'd block each other's reception, so I'll agree that I
can't stack 10,000 of them in one room and get the same results.

How much power do 100 blackberry chargers generate, and how does it compare to
the power generated by one wifi antenna? If the former is more than the
latter, we have a serious physics impossibility.

~~~
derobert
5V * 0.8A = 4W.

Wireless access points are around 50–100mW. Even if you put a 100% efficient
Dyson sphere around your APs, you'd need 40–80 of 'em.

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ars
Why use WiFi? Don't AM, and TV signals have way more energy? Is WiFi really
the highest energy source available to you?

Or is this just to make it buzzword compliant?

What about visible light? Even indoors, if the lights are on, there is far
more energy available via visible light.

And WiFi barely even transmits if someone is not using it. Does this device
have some sort of busy loop keeping the accesspoint constantly transmitting?
That's really going to be great for interference won't it.

~~~
andyking
And if it works with wi-fi at 2.6GHz, why would it not work with the more
broadly available and higher-powered 3G cell signals at 2.1GHz?

~~~
drhodes
The device would need a power meter to detect and switch channels, which
involves altering the resonance frequency of the antenna, ( I suspect an
amperian loop )
[http://www.physics.upenn.edu/courses/gladney/phys151/lecture...](http://www.physics.upenn.edu/courses/gladney/phys151/lectures/lecture_mar_07_2003.shtml)

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JshWright
Near as I can tell from the video, that little white box is just a little
white box. The Lexan box that guy is leaning on appears to have a much larger
antenna in it, and that's what the cable plugged into the BlackBerry is
plugged in to.

The guy even says "The _goal_ is to get it down to a box this size". That
information seems to be missing from the blog post.

~~~
arfrank
It appears that it's this box that is currently harvesting the energy

[http://www.engadget.com/photos/airnergy-wifi-power-system-
ha...](http://www.engadget.com/photos/airnergy-wifi-power-system-hands-
on/#2603262)

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brk
So now we can have self-powered Wifi access points? ;)

I'm surprised this has the RCA name on it, I don't see how you could
physically make this device work with any degree of reliability in a common
environment.

Given the size and price tag ($40), there can't be a whole lot in this box,
other than a tuned coil and some passive circuitry. It seems like if it was
"that easy" it would have been done by now.

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bioweek
Physics fail. I can't see how they would be allowed to market this?

~~~
Confusion
Kindly demonstrate the alleged 'physics fail'. Harvesting the energy of a
dozen wifi and GSM signals 24/7 seems a reasonable way to yield a significant
amount of energy.

Wait, I'll do the math myself: my accesspoint sends at 100mW. If this
harvesting device is 10x10cm and, on average, 1 meter from the accesspoint, it
will pick up 0.01/Pi ~ 0.3% of that signal, or 0.3 mW. Let's cut that in three
to account for conversion efficiency: 0.1 mW.

My phone has a 0.9 Ah battery. At 3.7V, this is 12kJ. With an uptime of 200
hours, this means it sends out 17 mW to the world. So could I use this device
to power my phone? Only with 170 close-by accesspoints and after 200 hours
would my phone be fully charged. So it indeed sounds pretty bogus.

Edit: updated as per commentary below and I grant this probably overestimates
the actual working of the device. Evenn on an expo, with a couple of hundred
hundred GSM signals in the vicinity, it wouldn't work.

~~~
ableal
You're still off by a power of ten in the areas: 10x10cm = 0.01 m2. (Edit: by
the way, I think you also plugged in the sphere _volume_ formula - surface is
4.pi.r²).

Radius of one meter is a bit close - make that three, and you're off by
another order of magnitude - 10 uW harvestable.

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poub
Is this technology related with spaced-based solar panel satellite as they use
a smilar principle ?

<http://en.wikipedia.org/wiki/Space-based_solar_power>

<http://www.nss.org/settlement/ssp/library/index.htm>

If we can transform a huge wifi-like signal coming from space onto
electricity, surely it’s possible to do the same thing at a smaller scale?

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motters
The plug in charger is a nice idea, but the wifi recharging battery is a
killer app. If they've got a patent on the battery, and they can have it built
into ebook readers and other mobile devices they stand to make a lot of money.

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anApple
If everybody ends up using these devices (and creating black holes for the
signal), the gsm providers have to increase the output power on their antennas
in order to get the same coverage as before.

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nuba
A few months ago MIT's Technology Review
(<http://www.technologyreview.com/communications/22764/>) said this could be
in the market in 3-5 years, so it makes sense to see a product testing the
waters and reaching out for early adopters at this point.

