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EEVblog #55 – RCA Airnergy WiFi Hotspot Energy Harvesting Marketing BS

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jahonen:
Out of curiosity, I measured a local cell base station power level with a spectrum analyzer and got about 140 nW using 500 kHz bandwidth, just using some wire as an antenna. Here is the result:



With that kind of power, one would need for charging the 5.6 Wh battery, about 5.6 Wh/140 nW = 40 million hours of charging time (assuming no losses anywhere), which is about 4500 years. I suspect that the battery is long gone before the charging is complete :)

Note that 140 nW is actually quite high received power level, since it is equal to -38.5 dBm. IIRC, GSM spec requires -102 dBm (63 fW). It is left as an exercise for the reader to calculate how long the charging would take with that minimum power level ;)

Slightly similar topic, I have also seen in my antenna book, "Antennas For All Applications", mentioned that it would make interesting solar cell, if one could rectify the sunlight directly (after all, it has an electric field component). Again, with perfect diode it would reach 50% efficiency, which would be a record-breaking figure compared to conventional solar cells. But, unfortunately in reality rectifying light is quite impossible with current technology, since there is nowhere near fast enough diode available to do that (electron relaxation time etc).

Regards,
Janne

luky315:
In theory you can draw out a lot more power if you place a resonant circuit (Qc = Qantenna) in the near-field of the antenna.
Still not enought to charge a cell Phone battery, but 10% efficiency is possible.

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