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.
Comments
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...