Freevolt can power a low-energy device forever, and for free | Hacker News Reader

Freevolt can power a low-energy device forever, and for free

getfreevolt.com

2 pointsalbybisy7 comments

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.

There is PDF buried in the developers section that says that they can get 100µW with a single antenna and multiple antennas can be combined to increase the power. I think a continuously powered device would not be a good application. Instead a device that powers on occasionally where the FreeVolt is trickle charging a battery would be a better application.

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... . 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...

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.

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.)

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.

If you drink a cup of tea (250ml) at 80°C, you have a temperature difference of 80°C-37°C=43°C. If you drink it in 5 minutes (300s) then the power you get from the temperature is

  P = 250ml * 1g/ml * 43°C * 1cal/g°C * 4.18J/1cal / 300s = 150W

If you add one tablespoons of sugar (12g), you get approximately 3.8 kcal/g so in the same 5 minutes you get

  P = 12g * 3.8kcal/g * 4.18J/1cal / 300s = 635W

but this is not absorbed instantly, but you have to average with all the other food you eat during the day (the average is approximately 100W).

So you get much more energy from a cup of tea than using a WiFi router inside your mouth (anyway, I'd not recommend it).

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