Electricity From WiFi Signals | Hacker News Reader

Electricity From WiFi Signals

ohgizmo.com

52 pointsajaimk21 comments

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.

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.

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.

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

Per photon, according to Plank's equation:

     E = hf

So the higher the frequency, the higher the energy carried.

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

For the same amount of photons light carries a lot more energy. That is why pocket calculators can work just form a couple of photo-voltaic cells.

But of course if you live next to a powerful TV transmitter or radio station you are surely better of harvesting that.

> Or is this just to make it buzzword compliant?

Yes.

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?

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.technologyreview.com/communications/22764/

The link is not necessarily the same tech, but it says that current levels in prototypes are at 3-5 mW from a wide range of EM frequencies.

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.

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

http://www.engadget.com/photos/airnergy-wifi-power-system-ha...

Oh...

And it's not just a matter of miniaturizing electronics. There is a matter of the energy density of signals in the air...

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.

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

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.

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.

Thanks for clarifying. I figured it was obvious.

Still forgot a factor of 4 while dividing through the area over which the EM radiation has been spread.

Simple solution to the RF energy density problem: pop the phone and "harvester" into your microwave and nuke it on high for 3 hours. Bingo, full charge.

Demo: http://www.youtube.com/watch?v=uu-oroNqAbs

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?

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.

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.

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.

Reply on news.ycombinator.com