Taking wireless power further
Coming onto the market this year, WiPower also uses near-field inductive charging flexible coupling; it can travel a little over two inches from the transmitting antenna and cover an area about 5”x3” (so one charging area can charge multiple devices - depending on size). One Qualcomm demo powers miniature hovercrafts wirelessly; because they don’t need to touch the transmitter, they can fly over the whole charging zone, but this would also work for medical devices that need to stay sterile – or for charging your phone without taking it out of its case.
Still in the labs is the plastic sheet developed by researchers at the University of Tokyo. This has copper coils to detect chargeable devices and micro-electromechanical system (MEMS) switches that turn on the power when a device is in range. This is done using pentacene, an organic molecule whose electrical conductivity can be changed, in addition to more copper coils to transmit power a short distance above the sheet (demonstrated by powering a small light in a fishbowl of water). All of the layers can be printed onto the plastic; ultimately that could be simpler and cheaper than making the silicon transistors in current charging pads.
There are ways to send power further, but you have to use different forms of wireless. The RF wireless power system developed by Powercast harvests energy from radio waves; it can travel long distances but it only generates 3-4W of power – enough to run a sensor or recharge a AA battery. Another option is to convert the power to and from an optical signal; that’s what PowerBeam is developing. You can send higher levels of power wirelessly if you make the two coils resonate at the same frequency (called resonant inductive coupling); the energy released from the first coil is picked up by the other, even at a distance. Unfortunately, this tech is still in the labs (at MIT, WiTricity, Intel and Sony).
What you often trade for the convenience of wireless power – both systems shipping today and those in development – is efficiency. Especially with a tightly-coupled system this can be high, but it’s rarely as efficient as a cable (and there are few independent measurements). On the other hand, if you add intelligence to wireless charging and you can save power by not charging devices that don’t need more power (more advanced smartphone chargers do that already but many power adapters don’t).