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Radio Energy Harvested With Inkjet-Printed Antenna
judgecorp writes "Everlasting green energy for RF tags and other low-power devices could be possible as scientists have harvested energy from ambient radio waves using cheap antennas printed by an ordinary inkjet. The scientists, from Georgia Tech, started at 100MHz but have now produced systems which scavenge power at up to 60GHz, allowing them to draw power from most of today's major radio technologies."
It's called a crystal radio.
A diode does it too.
http://lkml.org/lkml/2005/8/20/95
Because it seems like if you want to power these things, they need to use power from a radio source. Which doesn't make them green at all.
The radio source is there all the time anyway, It is there for other uses.
But as should be obvious, the vast majority of radio waves are never used, being disparate over vast distances or absorbed by the earth itself. Utilizing this "wasted" energy costs nothing, because we are already emitting that energy, and utilizing it costs no more. At the emitter you can't measure if a radio wave hits one antenna or a million antennas. Its no different to you as the sender of that wave.
So by using freely available wasted energy these devices obviate the need for ANOTHER power source and are therefor green.
Sig Battery depleted. Reverting to safe mode.
Because it seems like if you want to power these things, they need to use power from a radio source. Which doesn't make them green at all.
They do, indeed, consume some energy from the RF broadcast(in principle, if you really chaffed the place with them, the reduction in SNR might actually be noticeable by devices trying to communicate...) However, there are two other considerations:
1. Particularly in classic broadcasting(less your fancy 802.11-draft-whatever-with-beamforming-and-a-line-of-sight-yadda-yadda smart antenna nonsense) a substantial amount of broadcast power just floats away into the aether, never to be snagged by any receiver. So long as you are(by making receivers super cheap) just burning through some of this formerly wasted power, the energy counts as "free". Not until your piggybacking requires the towers to start cranking it up is their a cost.
2. If the deployment of some distributed-sensor net widgetry is an inevitability(there are legimitate grounds for question at this point; but we generally don't take advantage of them) it has to be powered somehow. The major contenders are A. Lithium primary cells: unless somebody plans on cleaning the whole thing up a decade from now, the delightsome battery goo is going straight into the environment. B. Photovoltaics(in suitably sunlit locations that are OK with sporadic power): the energy generation itself is clean, the manufacturing and some of the components are rather less so. C. Piezoelectrics: not all of the suitable candidates contain lead; but a lot of the common ones really ought to be collected after use.
In our brutally entropic universe, nothing is truly "green"; but it is quite possible that RF harvesting will prove to be green-er and/or more convenient in some applications.
It will degrade the signal of downstream recipients. So does absolutely every radio receiver, with no exceptions.
However, please consider that the only downstream recipient may well be the earth or space, considering that the path between a transmitter and receiver often does not pass particularly close to another receiver. How much one of these would impact the downstream signal quality anyway depends on just how much power this is extracting, and just how weak the signal would have been at the downstream receiver without this being present.
Also keep in mind that radio waves can be rather fickle. Placing these devices in certain locations may actually increase the received signal strength downstream, perhaps by absorbing an interference source, or by attenuating a secondary path of the signal which would have interfered with the primary signal.
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