Mark F. posted an interesting piece on Boing Boing, skeptical of the RCA “Airnergy WiFi Hotspot Power Harvester,” a device that can allegedly charge its internal battery via Wi-Fi radio signals. They definitely get endless demerits for a groaningly-bad product name. How can you say “Airnergy” without choking on your own bile?

Mark quotes from a piece on OhGizmo!

From OhGizmo!:

The Airnergy has a battery inside it, so you can just carry it around and as long as you’re near some WiFi, it charges itself. Unlike a solar charger, it works at night and you can keep it in your pocket. Of course, proximity to the WiFi source and the number of WiFi sources is important, but at the rate it charges, if you have a home wireless network you could probably just leave anywhere in your house overnight and it would be pretty close to full in the morning.

A commenter on OhGizmo! offers the following:

Here’s some math. Long story short, by my calculations, 100% efficiency and absorption at 5 feet away from a 100mW home router, (reasonable figures), it would take 34.5 years to charge that blackberry battery.

It’s not a Dyson Sphere, so you only get the power that hits the antenna.

Surface of a sphere = 4pir^2, r = 60″ (5 feet).

Surface area of a 5′ sphere = 45,216 square inches.

The device appears about 2″ x 3″ = 6 square inches.

The device then picks up, best case, 0.000133 of the power out from the router, which is 100mW, so.. 0.0133mW

If you leave it there for 24 hours, 0.0318 mWh are stored.

According to Will’s battery, it has ~4,000 mWh capacity.

So, it would take 12,579 days, or 34.5 years, to charge your blackberry battery once, presuming 100% absorption, no losses.

RCA’s Wifi “power harvester”

## 32 thoughts on “Alt.CES: RCA’s Wi-Fi “Power Harvester” Snakeoil?”

1. Alan Parekh says:

The salesman obviously doesn’t have a clue, when asked how long it will take to charge (same exact question I would have asked) he talks about the device charging something instead of the actual charging of the internal batteries.

Let me guess the unit was “topped up” last night or in the morning to supplement the free energy harvesting. Even if the device did work how dare he call it green because it uses free energy. Guess what the hot spot is plugged into the wall.This energy is just as free as the plugs in your local cafe.

Someone needs to get one of these and crack it open to prove that their claims are jacked up.

2. Greg says:

The altoids mint tin enclosure has gone mainstream. Wait, that might actually be a repainted altoids tin, with an awl punch in the center for disguise.

On second thought, this is a ripoff of ladyada’s “Mintyboost”. Sure it performs the exact same function, but it has the exact same enclosure too? Throw in a magic antenna and RCA has a truckload of unsellable crap.

Im gunna by that truck, hook all of them up to my phone, and park it outside of a tardbucks for a day. If I can get 12,579 of them at a cut rate I’ll be able to charge my phone in a day.

3. paolo- says:

If it actually worked, I don’t see how it wouldn’t be a green. Sure the router uses power but it does weather or not you’re there. Kind of how a volcano produces a lot of CO2 and heats up a lot, weather or not you put a kettle near it to heat up your tea.

I’m thinking of a remake for this, make an antenna that picks up signals at different frequencies where power might be more plenty full AM, FM, TV… any idea how much power you could get from something like that ?

1. Stephen says:

The same logic would suggest that using a forest fire to make tea on is also an environmentally friendly thing to do. It’s there whether or not you’re using it, after all.

It’s also important to remember to not use more energy travelling to the volcano and harnessing its heat than you save from not boiling the kettle. The thing that makes tech green is in using energy and materials efficiently, which is where this device falls down. Really badly. Here you replace an increasingly efficient wired recharging process with something that barely functions and that’s not a green choice.

4. Stephen says:

It’s just a clearly impractical idea. It sounds like a brilliant product and that’s why everyone likes it but it just won’t work like you want it to.

Antennae work by using radio waves to excite their constituent molecules to create a voltage between the two ends. We’ve always known that’s how they works. The reason that no one’s really bothering to power things from the signals is that it’s a) pretty hard to collect high frequency signals for power rather than communication purposes (and low frequency signals don’t really carry enough energy to bother with) and b) it’s not really worth it if you did for efficiency reasons.

The Open University did an experiment to see if they could do this. What they did was attach a 1KW microwave (1,000,000x more powerful than a wireless router) to a parabolic director and then built a wall sized array of compatible dipoles. They then connected the array to a toy helicopter, chosen for its power efficiency. They pointed the converted microwave, which was now classed as an restricted energy weapon at the array and discovered that 1000 W of focused microwave radiation could produce just enough power to let the helicopter hover but couldn’t even lift the power cable. If at least a million times more power, a focused emitter and a wall sized collector can’t power a toy this device isn’t going to power your phone.

Mankind really has a choice to make – it can ramp up the power output and add parabolic dishes on its WiFi routers to the point where it’s practical to charge gadgets using captured WiFi or it can choose not to make everyone sterile.

5. Paul says:

OhGizmo! commenter seemed to assume quite a few things.

Just recently I was in a coffee shop & went to see if there was wifi… at least a dozen hotspots (only one open, much to my surprise.) Many of the hotspots had pretty strong singnals.

So I’m really thinking you might even be able to cut that time in half with lots of routers. And, really, 25% battery life is really enough for me to get through my email. So there you go… Blackberry charged up by the middle of 2014! Sweet!

1. Stephen says:

Due to the way that energy is radiated into the environment you’d actually see more significant improvements from moving the charger nearer to a single router than setting it between a large number of distant routers. In fact, if you had a large number of networks on slightly different frequencies (channels) around you you’d quickly see interference sapping its effectiveness even further.

2. torchtech.judgementgaming.com says:

Even though there are a dozen WiFi APs there is not even one watt of energy being radiated to the ~6^2 inch area that this device takes up. And if the signals are out of polarization then there could be 50 time less power.

3. amasci.com says:

> OhGizmo! commenter seemed to assume quite a few things.

Yes, chief among them is the assumption that antennas have broadside absorbing area like a solar cell. Nope.

In reality, radio antennas can absorb waves which pass quite a distance from the metal. In other words, the broadside area of a dipole antenna has little to do with the area of absorbed RF energy …and the “virtual size” of a receiving antenna can extend well outside a plastic enclosure.

More important is the operating wavelength. In the ideal case, a receiving antenna can absorb all the radio energy in a circular region about 1/3-lambda diameter. 2.4GHz WiFi has quite small wavelength, so its antenna acts like a “solar cell” which is about 4.2cm or 1.6″ diameter. Pretty close to the Altoids area? Perhaps they included more than one antenna placed at opposite ends of the box, so the total area might be larger still. But don’t forget that we’re assuming an ideal case, and a practical antenna might absorb 10x less power.

Brief explanation: fields produced by a receiving antenna superpose with the incoming wave to create an interference pattern having a downstream ‘node’ or RF shadow. This shadow can be much larger than the antenna itself. And since a thin wire can produce roughly the same RF fields as a thick wire or a foil plate, the broadside area of a metal antenna is not a good guide for estimating the Effective Aperture of the antenna. A better guide is the antenna length alone.

6. Chris says:

I have been playing with radios for several years, and have even built a few so called free energy devices. The most I was able to power up was a 1.5 Vdc wall clock at 20mA.

The same can be accomplished with a very large antenna a couple capacitors and a couple diodes, very impressive when coupled with a LED. Only useful as an RF detection device and even then it wasnt very good at that for most frequencies, unless with in close proximity.

I built a wi-Fi antenna and designed my home system a few yrs ago,and still couldnt see acquiring anything close to what the claims are for this device.

Much like the solar charged pathway lights, they work great when new and out of the box, but no one ever tells you the rechargeable battery inside was mostly charged when installed, and no matter how long the days are on this planet you wont achieve a full charge from these, though they do work better then RF powered Flashlights

1. Richard says:

My solar lamps will charge a dead flat nimh cell put into them up to give a full nights worth of light after one day of direct sun. Perhaps you are getting the cheap ones where there are only a few active strips in the panel and most of it is just black backing material?

In anycase, this device being powered off “wifi” is a joke. I dont think anyone will disagree.

7. japroach says:

Using energy from a forest fire to make tea could very well be green.

Its not like your driving miles to find a wireless router, we are assuming you pass by these energy sources during your daily routine.

Say theoretically this worked, and router manufacturers started boosting their output power to allow faster charging. Then it would no longer be green.

1. Stephen says:

If green means environmentally friendly and ecological etc then a forest fire is not a green source of energy. Common sense should tell us this. It’s a forest and it’s on fire. It’s also very inefficient – very little energy released in the fire can be put to any use.

You may not go out of your way to travel to a wireless router but you do go out of your way in building a device which can convert the signal into energy. That doesn’t just happen by accident. It’s a disproportionate input of energy and materials for the amount of output so it’s not green. You need to compare all inputs and output before you can say if a power source is worthwhile. Driving to a volcano to gather energy from it is just a clear image of one input.

If it actually worked like the makers say it does then it wouldn’t matter how much power is used by routers because it would be collected and used. That would be perfectly fine, there’s little/no wasted energy and you’d get longer ranged WiFi, it’s all good. The problem is that it doesn’t work, it’s too inefficient.

8. Mark says:

Even if it’s inefficient, i like the idea. When i step out of my door i can see 20 (!) networks on my mobile, and there are GSM and 3G and DECT phones and bluetooth etc. around so why not make electricity from it besides having my genes damaged by all the microwaves?
I got an “ambient power module” from futurehorizons.net some years ago which is just a diode network and it gave some milliamps at 1.5V, using a long cable as antenna. The promised 9 watts output are ridiculous but it might work better now with a proper antenna. There wasn’t any WIFI at all at the time i bought it. I’ll give this another try.

9. Mostly Cajun says:

Along with dozens of other pesky physical laws.

What about the energy you need to build this little tribute to tree-hugging?

The numbers in the original post quoted wherein actual SCIENCE is used, and those pesky calculations and stuff sort of let the air out of this idea.

In a somewhat similar fashion, this is how electric toothbrushes recharge, but the tow components are close enough for actual magnetic coupling.

The fact is that if you have an old, leaky battery, this scheme won’t keep up with the self-discharge rate.

10. Wayne S says:

Yes, this product is 100% snake oil. I am an Electrical Engineer and designer of RFID power harvesting circuits. In my opinion (<-for legal reasons) what they claim is not remotely possible. Here are the definitive calculation.... assuming the WiFi antenna is a single dipole with with a gain of 1.4 dBi and the AirEnergy device antenna is a patch antenna with a gain of about 6dBi, and the WiFi transmitter is radiating the regulation 100mW at 2.4 GHz, then the Friis formula predicts that at 10 ft the Airenergy received power is 7.5 micro-watts. If we have a 3.3 volt 1000 mAHr battery, and a 100 % conversion efficiency of 2.4GHz AC to 3.3V DC then it would take 51.6 years to charge the battery. If you reduce the distance from the WiFi transmitter to 1 ft, the charge time is still 2 years. There is just no abundance of energy floating around in the air to grab, no matter how clever the "reciever". Is this company really "RCA" or did they just license the name?