Two weeks ago, I read several articles on proposed wireless power transfer, e.g. on CNN News or this older one from Physorg. Since I find the idea to have power transmitted wirelessly for home use really exciting, I tried to dig into the topic, you can read the full text here. A brief summary of what I found:
There are currently two ideas on the market that rely on different schemes.
A) The one has been proposed by a group of physicists from MIT, Marin Soljacic (assistant professor of physics), Aristeidis Karalis, and John Joannopoulos (professor of physics). They have a paper on the arxiv about it: physics/0611063. I could also find these slides from a talk one of them (M. Soljacic) gave. Besides the news article on BBC, the story has been echoed with slight alterations, e.g. here, here, or here.
B) The other one is the technology used by a company called Powercast, and seems to build up mainly on work done by Dr. Marlin Mickle and collaborators from Pittsburgh. The company has a website that you find here. It it almost void of any information. They have a form that you can fill out and they send you some pdf-files. These again hardly contain any information. They apparently have presented their device on the consumer electronics show 2007, see e.g. here, here, here, or here.
Proposal A uses a source and receiver that resonate. From their documented work, I could not find out how sensitive their setup is to the orientation between source and receiver. It seems to me, if the orientation between both is not ideal, then the power transfer could be damped. But more importantly, I could not find any analysis on the energy flow (Poynting vector) that takes place. For their setup to work, they assume only a small fraction of the energy (~1/1000) is extracted from the total field, this means the total field that is present has to be pretty strong. From what I estimated, this easily exceeds FCC bounds on the power density for a useful amount of transmitted power (~2 Watt in a couple of hours, for details, see full text). One can avoid this by making the sender and receiver larger (this lowers the power density), but then the devices become pretty unhandy.
Proposal B essentially uses a sender that broadcasts the energy into space, the 'power-beacon' and a receiver that they call 'power harvester'. This technology is FCC approved, and consequently transfers only very small amounts of power. As they say
"Our wireless systems can recharge batteries in any consumer device smaller than a cell phone, from up to a meter away." [source]
My concern here is that they essentially emit radiation undirected, say, like a light bulb does, but only a small fraction of it can be used by the receiver. This mean, a whole lot of the energy is lost. I could not find any data on the efficiency of that power transfer, but it seems to me it is a huge waste of energy.