[jacquesm]

Yes, I mentioned phased arrays in my earlier comment. Even so that's a lot less focused than what you might believe it does, it's more like a directional wavefront than a focused beam. So you'll still lose lots of power due to the expanding wavefront (conveniently left out of the wikipedia drawings but I can see why they did that).

[watmough]

Back when my dad still worked at Aberdeen University, they had a phased array of maybe 7 or 8 transducers of about 8 cm each, that at a range of 6 inches or so, could deliver a focused beam that would penetrate a phantom / lab animal enough to cook a 2 - 3 cm sphere several cm below the skin.

The research was aimed at basically heating carcinomas above the 44 oC needed to kill the cells.

So you can focus an ultrasound beam, but it seems like a heck of a way to charge a phone.

[jacquesm]

If you pump enough energy into the source, sure. But your average coffeeshop is not going to replace their ceilingtiles with what would be really inefficient space heaters in order to charge some phones (and cook the inhabitants in the process).

That's the whole point of the exercise here: if the efficiency isn't there the whole thing is dead because you can't be pumping kilowatts into space in order to get a few watts (or milliwatts) back out. The difference between the two will get converted into heat!

So the only way this will work is if the efficiency is really high, much higher than seems to be feasible right now.