
Freevolt can power a low-energy device forever, and for free - albybisy
http://www.getfreevolt.com/
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gus_massa
I can't find any details about how much power you can get with this. Let's try
a back of the envelope calculation.

My WiFi router has probably 2W. If you put this device at 1m (3') and assume
that the power is evenly distributed you get 2W/(4 pi 1m^2) = 0.16 W/m^2.
Let's assume that the surface of this device is 10cmx10cm (4''x4'') (quite big
for a small device) and it's perfectly perpendicular, you get 0.16 W/m^2 x
(.1m x .1m) = 0.0016W = 1.6 mW (and this calculation is quite optimistic,
because it assumes no loss in the transmission and in the conversion or
storage)

A typical LED has 30 mW (or more), so this is not enough to keep it light.
Perhaps blink it once every 20 seconds.

And if the device is further away, it's worst. The calculation assumes that
the distance is 1m (3'), with 2m (7') the power is reduced x4.

In the web page they show a fire detection device with a nice led that, so I'm
not sure that they can collect enough power to blink the led. The surveillance
camera is completely out of discussion, specially if it has a motor.

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jepler
Freevolt's "RF Energy Harvesting Whitepaper" figure 1 shows the energy
harvested from RF as .1µW/cm² to 2000µW/cm², but the text gives energy
densities .01µW/cm² to 100µW/cm². This density estimate may be per
transmitter(?) so the harvesting estimates only make sense if you assume
multiple transmitters each contributing to the total harvested energy.

In Figure 3 they have a chart which shows ~30µW harvested from an unspecified,
un-sized "Patch array 1x2" antenna".

Your optimistic estimate is 16µW/cm², so it is in the range of their power
harvesting claims. (though I doubt their unspecified antenna is 100cm²)

What is feasible with an energy budget of 30µW? Well, using off the shelf
microcontrollers and RF modules, you can send a packet every two minutes using
the least optimistic estimates from [http://jeelabs.org/2013/09/08/3-years-on-
one-set-of-batterie...](http://jeelabs.org/2013/09/08/3-years-on-one-set-of-
batteries/) . The power consumption of the radio is the biggest element of the
power budget, so you can add sensors without hurting your power budget too
much. And probably you can do better than this with a better transmitter than
the old RFM12B.

I don't think their claims are impossible for sensors that operate in the µW
range of average power consumption, though though the top end estimate of
2mW/cm² in table 1 seems way too optimistic. But as you point out even a task
as simple as blinking an LED a few times a minute is pushing it. You'll never
power a mobile phone this way.

The whitepaper is
[http://www.getfreevolt.com/downloads/RF%20Energy%20Harvestin...](http://www.getfreevolt.com/downloads/RF%20Energy%20Harvesting%20Whitepaper.pdf)

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ck2
This is somewhat like the demo I've seen of florescent lights working under
high-power transmission lines without connection.

Interesting our bodies absorb all that energy every day without lots of
cancer. Actually, there is lots of cancer.

~~~
DanBC
I accidentally upvoted you. :-/

If I lived near overhead power lines (or had them on my land) I'd definitely
make use of as much of this kind of charging as I could get away with. It's
probably not legal, but it'd make an interesting case. (Assuming it hasn't
already been tested in court.)

~~~
ck2
In the US it is definitely illegal to siphon power from a utility, even
without a physical connection.

Detecting it with casual use is probably impossible though.

But living that close to high power transmission lines can't be good for you
in the long term.

