Recharging via radio waves?

[OAQfirst]

I'm not knowledgeable about these things, with all the free energy nonsense I frequently see. So what do you think?

http://tech.yahoo.com/blogs/null/143945
Nokia, however, has taken another baby step in that direction with the invention of a cell phone that recharges itself using a unique system: It harvest ambient radio waves from the air, and turns that energy into usable power. Enough, at least, to keep a cell phone from running out of juice.
Does this sound right? If so, it looks like exciting news. Even for stand-by charging, that's still a good thing.

[mgb_phys]

You have to assume Nokia have done their sums.
But assuming the signal is a couple of milliwatts and you leave it on continually.
My phone battery is 700mAh x 3.6V so stores around 2.5Wh, charging with 1mW would take 3months! It's difficult to believe it's even enough to overcome the leakage in the charging circuit.

[OAQfirst]

It reports their goal is 50 milliwatts. Would that be substantial enough?

[mgb_phys]

My phone lasts a week on standby with a 2500mWh battery so around 15-20mW - if they can pull 50mW out of the air that would certainly help.
I suppose if they didn't just use the phone antenna they could also use the power form lots of other transmitter sources, TV/radio/wifi etc.

[OAQfirst]

Hmm. Well, here's to hoping!

Thanks.

[Greg Bernhardt]

Would this phone technology be useful only to those in major cities?

[russ_watters]

Presumably. I wonder if anyone has actually calculated how dense the radio energy is in various places.

[edit] calculation deleted. I guess you'd want to do a calculation based on surface area....

[Ivan Seeking]

...and over what range of frequencies?

[flatmaster]

Presumably. I wonder if anyone has actually calculated how dense the radio energy is in various places.

[edit] calculation deleted. I guess you'd want to do a calculation based on surface area....

I immagine you would want to make your windings as large as possible to contain the maximum amount of magnetic flux. Probabilly the entire surface area of the phone. Also, if the phone wasn't sitting in the correct orientation with the source, you wouldn't get anything.

[russ_watters]

The surface area of a 1"x2"x4" phone is 8 square inches. At 50mw, that's 900 mw/sq ft. The side surface area of my 19x14x8' living room is 528 square feet, which would mean if I wrapped it with antennas, could generate 475 w. My house is 20'x40'x20', or 2400 square feet side surface area. That's 2.1 kW, which (with storage), would be more than enough to supply all my energy needs.

I find that hard to swallow.

[Moonbear]

It reports their goal is 50 milliwatts. Would that be substantial enough?

Over what time frame?

When would this be most useful? I'd think if you were stranded out in the wilderness without a charger is when you'd need something like this...one would hope the charge would happen fast enough to call for help before you die of dehydration or exposure to the elements. Otherwise, it's nothing more than a gimmick.

[russ_watters]

miliwatts is power, Moonbear - it's got a timeframe in it (seconds).

[Blenton]

Isnt it illegal to 'harvest' energy from radio waves? I could imagine those who live near the transmitters putting up huge coils on their houses to sap up the free kilowatts.

[Moonbear]

miliwatts is power, Moonbear - it's got a timeframe in it (seconds).

D'oh. Sorry, I made the classic mistake of mixing up power and energy. Thanks for the correction.

[Pengwuino]

Isnt it illegal to 'harvest' energy from radio waves? I could imagine those who live near the transmitters putting up huge coils on their houses to sap up the free kilowatts.

What transmitter?

[Blenton]

Radio station transmitters.

[Pengwuino]

Radio station transmitters.

I would think that's somtehing you could do right now. I was thinking he meant that Nokia was planning on building transmitters specifically for recharging cell phones.

[Blenton]

No I believe they meant radio waves currently in the air.

[xykotik]

Before I read the Yahoo! article, I assumed induction, like the contact-free charger for my Braun toothbrush, or the ones used to recharge cardiac pacemakers and ICDs (implanted defibrillators). It doesn't look like that's what they meant.

My immediate image was of holding a four-foot fluorescent tube under an HT power transmission line and seeing it glow dimly. Kinda neat, but not a lot of oomph for being so close to the EM source (albeit 60Hz, not RF). I aint' doin' the math for that, so I'd like to see a white paper or something before I throw away my chargers.

Neat topic