Yes, such a device could work, and no, it's probably not useful
I was over at Network World recently to meet with John Cox, and ran into Matt Hamblen of Computerworld (see his article on the subject at hand) who wanted to talk about RCA’s controversial Airnergy announcement at CES. If you’ve not seen this (don’t look for it on RCA’s Webs site; there’s nothing there), it’s a small box that, so RCA claims, converts Wi-Fi signals into DC energy capable of recharging a mobile device, such as a cellular handset. This isn’t sheer nonsense, along the lines of perpetual motion, but, OK, granted, I’m sure it sounds that way to many. Nonetheless, I think something like this could work, sort of, anyway, but, no, I don’t think it will be practical.
How, then, might something like this work? Simple, really – just build a circuit that captures RF energy (that’s called a radio, BTW) and convert the energy captured into DC current (that’s called a DC power supply) and then store the output of that in something called a battery. Then just take the contents of the battery and use it to charge another battery, via something called a battery charger. Now, really, does that sound so hard? It’s obviously not. All of the technology required exists today, and all of it is cheap.
But the process is also not very efficient. There’s not much Wi-Fi energy to capture in most cases (very weak and infrequent signals that only exist in great numbers regardless in areas with a high population density), and it would take a very long time, I think, to capture enough energy to fully charge a battery capable of subsequently recharging another fairly large battery. So what we have here is practical from a technical perspective, but probably unreliable to the point of being useless in the real world.
Still, it’s got that green cachet, as I noted in my comments to Matt, so the appeal is obvious. I’m a huge fan of environmental efficiency, energy harvesting, and otherwise making use of energy that doesn’t add to pollution or global warming – think hybrid cars, in theory, anyway. But I’m not going to rely on something like Airnergy when my handset’s battery goes dead. I think a spare battery or external battery is still the best option, although one will eventually have to plug in regardless. We need to look into, then, better ways of getting energy onto the grid (solar, wind, geothermal, tidal, and nuclear all come to mind) and also making the grid itself more efficient. But we could also take advantage of a really great approach to wireless energy, one that I also noted to Matt. Nasa’s Tethered Satellite Experiments, which Matt mentioned I worked on but I think he misunderstood me; I didn’t work directly on TSS but I do have a patent with one of the principal investigators on this project, so I did have fairly good knowledge of the concept and implementation, and, OK, I made a couple of suggestions along the way). Anyway, TSS proved, among other interesting developments, that we can indeed generate an electrical current by dragging a wire through Earth’s ionosphere. Imagine lots of these collectors (also called generators; more proven technology), then, in orbit, capturing energy. We then convert this energy to microwaves (along the lines of how a switching power supply works), and beam the result to Earth, subsequently inserting it into the power grid. Clean, green, not cheap yet, but promising. And none of us will feel guilty sucking all the watts we need out of the wall if the source of that energy is, indeed, wireless.
