iPhone 5 has a 5.45wh battery. If you can place a "speaker" in your room and add 0.3w * 8 h while you sleep that's ~44% charge every night. Which could be fairly useful. Up the power or assume people spend more than 8h a day in there bedroom and plenty of people would spend up to say 200$ to have one less thing to worry about.

Orders of magnitude, for me, starts at 1.5 log unit. That would make it ~15% charge overnight, assuming 100% power transfer efficiency.

Let’s read a bit more:

”the power levels beamed are more than 50 times lower than the lowest ultrasound imaging exposure limits set by the FDA for medical imaging”

The lowest limits I could find in http://www.fda.gov/RegulatoryInformation/Guidances/ucm070856... is 20 mW/cm^2 (“A simple conservative approach for pulsed Doppler FHR monitors is to use 20 mW/cm2 as a guide for the maximum spatial-average pulse-average intensity at the transducer face”)

Dividing by 50 for the ”more than 50 times lower than the lowest ultrasound imaging exposure limits”, and multiplying for (ballpark) 100 cm^2 receiver area (about what one can get in a mobile phone), I get 40 mW of power received.

http://www.fda.gov/downloads/MedicalDevices/DeviceRegulation... gives 17 mW/cm^2 for ophthalmic use, so 20 mW/cm^2 isn’t even strictly the lowest limit set by the FDA.

There may be a useful product here, but if so, I don’t see how all of what they say can be true.

Yea, ~20% efficency and 17mW/cm^2 * 100cm^2 = 0.34w which is in the ballpark for useful power levels. 1/50th of that seems useless.

Sure, as long as it's right in the focus of the beam. If that means putting it in one particular spot, is there any benefit over inductive chargers? That method would be faster and more efficient.

Also, not to be the old guy...cables don't bug me THAT much. I'll take USB C and charge with 100W instead, thank you very much!

I'm with you. Investors/companies have to get into their heads that power transmission will never be as easy as data transmission. There's no power Wi-Fi, only some crude expensive approximations over mediocre distances with mediocre power levels. We could instead focus on improving cables (and confined power in general) -- have them available everywhere, retractable, higher power. There's hardly any physical limit to confined power transmission; if you look at trends into the future you might expect device power consumption to actually rise a little, only furthering the divide. Cables are just so more elegant.

Part of the problem not discussed here is the 'Absorption conflict': for efficient power transmission you need materials that are able to absorb very well the given form of power; but at the same time you want nothing to interact in the path of the unconfined flux. This can only be dealt with up to a point, since our everyday objects' configuration and atomic composition are not so different from the composition of a receiver. This applies to all technologies: even for resonant EM power transfer you are relying on a specific conductor geometry; thankfully there's no strong conductor in our bodies, but you will have to deal with metal parts everywhere.

Wireless information gets around this kind of limitation because power ceases to important for the capacity of a channel [1] past a threshold (roughly when the signal PSD equals the noise PSD), while the determining factor is bandwidth. So you just use a few Ghz+ radiation and you have enormous bandwidth available; once that becomes insufficient we can move to Thz+ light; and so on without much environmental concerns (just medium concerns -- atmosphere gets opaque), since power levels will be kept constant or lower.

[1] https://en.wikipedia.org/wiki/Channel_capacity#Example_appli...

I can think of a single highly popular counter example which used 100% wireless power over several miles.

https://en.m.wikipedia.org/wiki/Crystal_radio

RFID are often a short range example of the same idea.

So, it's less that wireless power is useless As it is modern devices needing lots of power.

Crystal radio is an interesting concept -- but note I didn't claim it was impossible, just highly inefficient and with mediocre power/range (I couldn't find sources, but apparently it's typically 40uW with a ~1m? antenna).

Remember that's for a (usually) 50kW station -- that's 1e-9 efficiency, or 0.0000001%. Realistically I'd expect you to be able scale this to 10k's receivers to up to about 0.001%. It's not realistic to increase this power significantly due to health/electronics concerns for persons living close to the source and massive consumption -- bringing the power near a single station to 4W would consume 5GW, about 1% of total US power consumption!

Interesting to imagine a system with sightly higher frequencies (20Mhz?) dedicated to power transmission. It'd essentially be a hugely wasteful system (and expensive with high powered antennas everywhere to mitigate invsq law) which however might be able to power ultra low power sensors and gadgets. Some clever phase distribution (pseudorandom modulation should do it) system would allow location with ~15m accuracy too (even working indoors, although building refraction and reflections might be a little troublesome).

Hehe, well, ubeam says it literally has mechanical aiming mechanisms that locate power recipients and focus directly at them. So, practical and maintenance limitations aside, they have definitely answered that criticism.

If you read the FAQ on eevblog, 100 KHz frequency ultrasound will lose 90% of its energy at 2 meter range or just over 6 feet. That energy goes to heating the air that the sound travels through.

Then you have to contend with any orientation offset. If the receiver is off by 90 degrees or more, there is zero energy received. If your hand blocks half the surface area, that's another 50% power loss.

If you boost the power above 140 dB safety limit, you can burn your hands or cause other problems. At 140 dB at 2 meters, the theoretical best case with 100% efficiency transmitter and receiver will deliver 0.85 watts. You obviously lose even more than that.

http://www.eevblog.com/forum/projects/the-ubeam-faq/

you would also be spending $200 extra on your electric bill

At 1% efficiency that's 30w or ~= 6 cents per day if your paying 10c/kwh. So, efficiency is only really important if it's crazy terrible.

PS: I suspect the real issue is making a receiver fit into a reasonable cellphone case.