The hardest part of wireless power over any appreciable distance, is to prevent power showing up where power is not permitted. The regulations limiting incidental RF radiation are much more stringent than are the RF-safety regulations.
Any system claiming to transmit watt s of power over distances of feet in an unshielded room, is likely to grossly exceed the far-field RF-interference limits in the regulations. The higher the operating frequency, the worse the problem.
In 2007, Kurs etal reported wirelessly transmitting power at 6.4 MHz using 0.6-meter diameter loops for transmitting and receiving loops 3 meters apart. Kurs etal reported capturing 4 watts of useful power from the receiving loop, while emitting 1.5 watts of far-field radiated power. This amount of radiated power exceeds the FCC limits by a factor of 800000 = 59 dB, and is easily enough for transatlantic communications under favorable conditions.
Energous claims similar power-transfer levels at similar distances. Energous claims to localize power to decimeter-sized regions hanging in free space (if their graphics are to be believed). This implies operation at decimeter wavelengths, so they may be operating at 2.4 GHz. I expect that their unwanted-radiated-power problems will be severe to fatal.
Andre Kurs, Aristeidis Karalis, Robert Moffat, J. D. Joannopoulos, Peter Fisher, and Marin Soljacic; "Wireless Power Transfer via Strongly Coupled Magnetic Resonances"; Published online June 7, 2007; 10.1126/science.1143254 (Science Express Research Articles). U.S. patent application 20080278264
@Jessica Lipsky The easiest way to ensure efficient charging would seem to be a gauge on the personal device that reports the charging strength being received combined with information from the charger regarding how much "effort" is being expended. If maximum output is being transmitted by the charger but minimal signal is being received by the personal device then there must be excessive distance or blockage of the signal.
Agreed, I imagine the objects in a room could potentially prevent easy charging. If the technology could be as accurate in sending a signal as wifi (and we all know wifi is never 100%), perhaps this could really take off. What do you see as ways around this conundrum?
Besides the safety issues regarding exposure, I wonder what objects will block the signal and prevent charging. Most people probably don't want to be carefully checking the line-of-sight to their charging home base. I'd think it would be less trouble to connect the phone to a charging cord or a charging dock than to seek out the compatible remote charger (only 15 feet away) and ensure that the devices are coordinating properly to recharge the phone.
I didn't find anywhere that it says that the power being broadcast is going out at 2.4GHz (which would seem to be a less-than-ideal frequency, agreed). It does, however, use the two aforementioned frequencies (2.4/5 GHz) to locate participating devices.
For example, PowerCast has been at this for a few years, and they use 915MHz.
RF for powering remote sensor networks is an extended topic. IMEC in Belgium, for example, are driving research since quite a bit, now.
WSN can work with less than mW, in duty cycle mode. Anyway, charging-up your mobile telephone is another story, you need Watts for decent recharge time, and since there is FDA regulation, you can't send RF anywhere, so you need to focus in a small spot, avoiding humans and pets.
Further, 2.4 GHz is the frequency of WiFi, but maybe other frequencies would be better for remote powering. Your WiFi EIRP has legal limitation (500mW EIRP MAX or 4W at 15% duty-cycle). To charge your phone-up you need to broadcast Watts over distance, so focusing becomes paramount to cope with health regulation.
RF power transfer is the way to go, no doubt, anyway I'd like to carefully see at how good we are in focusing energy in a very limited spot, not touching at people walking around. Any additional data would be appreciated.